dependence of the abdominal wall-mesh interfacial strength on the fixation method for ventral hernia...
DESCRIPTION
Hernia occurrence is on the rise. The most common approach to repair today is a hernioplasty repair using a surgical mesh for permanent reinforcement after repairing the hernia defect. Different fixation techniques using materials such as tacks, staples, sutures, or adhesives are utilized to provide initial fixation until tissue ingrowth occurs. Currently, regarding ventral hernia mesh repair operations there is inadequate amount of information available regarding the efficiency of a mesh repair using adhesives. Consequently, this study compares the interface strength between mesh and tissue when mesh is fixed with either of the two following techniques: a) adhesives or b) sutures. Lap shear test conducted on excised tissue specimens determined the fixation strength of the interface between tissue and mesh at 24 hours, 1 week, and 2 weeks post recovery. Uniaxial experiments were used to obtain nonlinear material properties of mesh and tissue. The material properties were then utilized toward building a computational model of the mechanical experiments.TRANSCRIPT
DEPENDENCE OF THE ABDOMINAL WALL-MESH INTERFACIAL STRENGTH
ON THE FIXATION METHOD FOR VENTRAL HERNIA REPAIR
by
Hummad Mohammad Tasneem
A Thesis
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Master of Science
Major: Biomedical Engineering
The University of Memphis
May 2014
ii
Copyright © 2014 Hummad Tasneem
All rights reserved
iii
ACKNOWLEDGEMENTS
Without the support of a number of people, completion of this research project
would not have been possible. First and foremost, I would like to thank my friends and
family for being the best support group a person could ask for, cheering me on every step
of the way. I also want to thank Dr. Elaina Paulus, Dr. Nate Stoikes, Dr. John Sharpe,
Mr. Samir Rustom, and the rest of the UT team for leading the animal study and surgical
portion of the experiments. I want to extend gratitude to Robert Jordan and Rick Voyles
for providing maintenance and technical support with the Instron machine. In addition, I
would like to thank my fellow graduate students, in particular Thien-Chuong Phung,
Corey Holt, Adentoun Komolafe, Jie Gao, and Dema Assaf, all of whom advised and
assisted me through my thesis work. Similarly, I want to extend thanks to Jenina Madrid,
Phillip Nuvue, Hadiya Khan, Bilal Tasneem, and Alex Richardson for reviewing my
writing and helping me to develop my final report. Finally, I would like to thank my
thesis committee members: Drs. Eugene Eckstein, Nate Stoikes, and Esra Roan. Their
guidance and support during the course of this thesis study has been indispensable. A
special thanks goes out to my head advisor, Dr. Esra Roan, for keeping me motivated
during this process.
iv
ABSTRACT
Tasneem, Hummad. M.S. The University of Memphis. May 2014. Dependence of the
Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia
Repair. Major Professor: Esra Roan, Ph.D.
Hernia occurrence is on the rise. The most common approach to repair today is a
hernioplasty repair using a surgical mesh for permanent reinforcement after repairing the
hernia defect. Different fixation techniques using materials such as tacks, staples, sutures,
or adhesives are utilized to provide initial fixation until tissue ingrowth occurs. Currently,
regarding ventral hernia mesh repair operations there is inadequate amount of
information available regarding the efficiency of a mesh repair using adhesives.
Consequently, this study compares the interface strength between mesh and tissue when
mesh is fixed with either of the two following techniques: a) adhesives or b) sutures. Lap
shear test conducted on excised tissue specimens determined the fixation strength of the
interface between tissue and mesh at 24 hours, 1 week, and 2 weeks post recovery.
Uniaxial experiments were used to obtain nonlinear material properties of mesh and
tissue. The material properties were then utilized toward building a computational model
of the mechanical experiments.
v
Table of Contents
List of Tables ................................................................................................................................. vii
List of Figures ............................................................................................................................... viii
Chapter 1: Introduction .................................................................................................................... 1
Chapter 2: Background .................................................................................................................... 5
2.1 Surgical Repair....................................................................................................................... 5
2.1. A. Surgical Mesh ............................................................................................................ 6
2.1. B. Adhesive Fixation Technique .................................................................................... 8
2.2. Hernia Repair Complications ................................................................................................ 9
2.3. The Mesh-Tissue Interfacial Strength ................................................................................. 13
2.4 Interfacial Strength Measurements ...................................................................................... 14
2.4. A. Lap Shear Test ......................................................................................................... 16
2.5. Material Properties .............................................................................................................. 17
2.5. A. Linear Material Properties ....................................................................................... 18
2.6 Finite Element Method in Biomechanics ............................................................................. 20
2.6. A. Non-Linear Mechanical Properties .......................................................................... 21
Chapter 3: Methods and Materials ................................................................................................. 24
3.1 Mechanical Testing Instrument and Software ..................................................................... 28
3.2 Biomechanical Evaluation ................................................................................................... 29
3.2. A. Uniaxial Test Method and Procedure ...................................................................... 29
3.2. B. Nonlinear Material Properties .................................................................................. 29
3.2. C. Lap Shear Test ......................................................................................................... 31
3.2. D. Data Analysis ........................................................................................................... 31
3.3 Finite Element Analyses of the Uniaxial Extension and Lap Shear Experiment ................. 33
3.3. A. Simulation of Uniaxial Extension Using FEA ......................................................... 34
3.3. B. Lap Shear Simulation Using FEA............................................................................ 36
Chapter 4: Results and Discussion ................................................................................................. 39
4.1 Uniaxial Extension Experiments .......................................................................................... 39
4.1. A. Average Normalized Force ...................................................................................... 40
4.1. B. Nonlinear Mechanical Properties for Mesh and Tissue ........................................... 42
vi
4.2 Lap Shear Tests for Obtaining Interfacial Strength ............................................................. 45
4.3 Computational Study using FEA to Simulate Mechanical Experiments ............................. 53
4.3. A. Uniaxial Simulation with FEA ................................................................................ 53
4.3. B. Lap Shear Simulation with FEA .............................................................................. 57
Chapter 5: Conclusions .................................................................................................................. 61
5.1. Conclusions ......................................................................................................................... 61
5.2. Clinical Significance ........................................................................................................... 61
Chapter 6 Future Work .................................................................................................................. 63
6.1 Mechanical Experiments and Sample Preparation ............................................................... 63
6.1. A. Future Work ............................................................................................................. 63
6.1. B. Limitations that need to be addressed in Future Studies .......................................... 64
6.2 Finite Element Models for Computational Simulations....................................................... 65
6.2. A. Next Step toward Developing Full Robust Model of a VHMR .............................. 65
6.2. B. Limitations in Current Models that should be addressed in Future Work ............... 66
References ...................................................................................................................................... 67
Appendices ..................................................................................................................................... 71
A. Experiment Results ............................................................................................................... 71
B. Matlab Code: F-D Data Evaluation ....................................................................................... 75
C. Matlab Code: Box Plots ........................................................................................................ 77
D. ABAQUS INP File: Tissue Uniaxial Model ......................................................................... 78
E. Strain Energy Model vs. Uniaxial Tissue Experiments ......................................................... 80
F. FEA Uniaxial Simulation vs. Uniaxial Tissue Experiments .................................................. 83
G. ABAQUS INP File: Surgical Mesh Uniaxial Model ............................................................ 86
H. Strain Energy Model vs. Uniaxial Surgical Mesh Experiments ............................................ 88
I. FEA Uniaxial Simulation vs. Uniaxial Surgical Mesh Experiments ...................................... 90
J. ABAQUS INP File: Lap Shear Model ................................................................................... 92
K. FEA Lap Shear Simulation vs. Lap Shear Experiments ....................................................... 92
L. Artist Permission ................................................................................................................... 92
vii
List of Tables
Table Page
Table 1: Literature Review On Interfacial Strength ..................................................................... 12
Table 2: Mechanical Strength Of Mesh And Tissue ..................................................................... 20
Table 3: FEA Model Part Characteristics ..................................................................................... 35
Table 4: Uniaxial Results With Averaged Normalized Forces ..................................................... 41
Table 5: Strain Energy Function Coefficients For Abdominal Wall Tissue ................................. 43
Table 6: Strain Energy Function Coefficients For Mesh .............................................................. 44
Table 7: Averaged Normalized Forces For Glued Vs. Sutured Specimens .................................. 49
Table 8: Statistical Analysis Using Mann-Whitney-Wilcoxon U-Test ........................................ 49
Table 9: 2-Way Anova Test .......................................................................................................... 49
Table 10: Lap Shear Test Failure Mode Occurrences .................................................................. 53
Table 11: Mesh Convergence Study For Tissue ........................................................................... 55
Table 12: Mesh Convergence Study For Surgical Mesh .............................................................. 56
Table 13: Goodness Of Fit Of Experimental Data With Uniaxial Simulation ............................. 57
Table 14: Goodness Of Fit Of FEA Simulation To Experimental Data ....................................... 59
Table 15: Mesh Convergence Study For Lap Shear Models ........................................................ 60
viii
List of Figures
Figure Page
Figure 1: Schematic Of A Typical Ventral Hernia With Intestinal Protrusion ............................................. 1
Figure 2: Hernioplasty Repair For An Onlay Ventral Hernia Surgery ......................................................... 6
Figure 3: Bard Soft Knitted Polypropylene Surgical Mesh .......................................................................... 7
Figure 4: Mesh Placement. Top) Onlay, Middle) Inlay, Bottom) Sublay ..................................................... 8
Figure 5: A) Peel Test Schematic, B) Indention Test Schematic ................................................................ 15
Figure 6: Lap Shear Test Schematic ........................................................................................................... 16
Figure 7: Mesh To Tissue Ratio For Mechanical Testing. ......................................................................... 17
Figure 8: Uniaxial Tension Test Schematic ............................................................................................... 19
Figure 9: Description Of The Orientation ................................................................................................... 25
Figure 10: Implanted Surgical Mesh With Different Fixation Techniques ................................................ 26
Figure 11: Lap Shear Test Specimens. Left) Glued Fixation, Right) Sutured Fixation .............................. 27
Figure 12: Typical Sample Division ........................................................................................................... 27
Figure 13: A) Instron 3380 B) Bluehill Readings ...................................................................................... 28
Figure 14: Uniaxial Mesh Tests. Left) Direction 1, Right) Direction 2 ...................................................... 29
Figure 15: Lap Shear Test Sample .............................................................................................................. 31
Figure 16: Zoomed In Image Of Surgical Mesh. Left) Threads; Right) Pores ........................................... 35
Figure 17: Partitioned Part For Uniaxial Simulation .................................................................................. 36
Figure 18: FEA Model Of Lap Shear Experiments .................................................................................... 38
Figure 19: Representative Uniaxial Test Data ............................................................................................ 40
Figure 20: Representative Tissue Experimental Response And The Material Model ................................ 44
Figure 21: Representative Mesh Experimental Response And The Material Model .................................. 45
Figure 22: Representative Lap Shear .......................................................................................................... 46
Figure 23: Summary Of Results From Lap Shear Experiments ................................................................. 48
ix
Figure 24: Typical Mesh-Tissue Response At 2 Weeks Vs. Excised Tissue Response ............................. 50
Figure 25: Failure Modes ............................................................................................................................ 51
Figure 26: Stress Contour Plot A) Surgical Mesh B) Abdominal Tissue ................................................... 54
Figure 27: Mesh Convergence Study For Tissue Model ............................................................................ 55
Figure 28: Representative Tissue Experimental Response And The FEA Model ...................................... 55
Figure 29: Mesh Convergence Study For Surgical Mesh Models .............................................................. 56
Figure 30: Representative Mesh Experimental Response And The FEA Model Results. .......................... 56
Figure 31: FEA Model Reaction Force Contour Plot ................................................................................. 58
Figure 32: Comparison Of Experimental And FEA Results Of Lap Shear Experiments ........................... 59
Figure 33: Mesh Convergence Study For Lap Shear Models ..................................................................... 60
x
Table of Abbreviations
Abbreviation Unabridged
HMR Hernia Mesh Repair
VHMR Ventral Hernia Mesh Repair
SEF Strain Energy Function
FEA Finite Element Analysis
FE Finite Element
ν Poisson’s Ratio
RSS Sum of the Residuals
TSS Sum of the Total Squares
R2 Residual Squared
F-D Force vs. Displacement
BC Boundary conditions
3D Three- Dimensions
3D Two- Dimensions
SEF 2nd
Order R.P. Second Order Reduced Polynomial Strain Energy Function
SEF 1st Order Ogden First Order Ogden Strain Energy Function
MPa Mega-Pascal
cm centimeters
σ Stress
ε Strain
A Cross-sectional Area
PP Polypropylene
N/cm Peak Force Per Unit Width
σ-ɛ Stress vs. Strain
UTS Ultimate Tensile Strength
N/cm2 Peak Force Per Unit Surface Area
Rs Sum of the Ranks
N Sample Size
S.C. Specimen bound in Stationary Clamp
D.C. Specimen bound in Dynamic Clamp
C.S. Center of Specimen
Sp. Specimen
IACUC Institutional Animal Care Unit Committee
1
Chapter 1: Introduction
Hernia is a defect in the tissue wall that surrounds various organs of the body. It
eventually leads to protrusion of organs through the weak spot in the muscle, or
connective tissue, called fascia, as illustrated in Figure 1. The two important factors
involved in the formation of hernia are i) local weakness of tissue and ii) increased intra-
abdominal pressure. Some hernias are asymptomatic, whereas some can cause significant
symptoms such as pain, nausea vomiting and change in bowel habits in the affected
individual. These symptoms can progress to surgical emergencies in the event of
incarceration, obstruction, or strangulation. This can instigate further complications as the
affected tissue begins to break down (Canziani et al., 2009; Carriquiry, 1996; Shell IV, de
la Torre, Andrades, & Vasconez, 2008).
Hernias are usually repaired surgically and require that the protruding organ be
pushed back into the human body and the defect zone repaired. This is followed by
closing of the defect in the soft tissue via suturing. Last, if the surgeon chooses to do so,
it is possible to reinforce the sealed hole by fixating a piece of mesh over it.
Complications that can occur after surgical repair of the hernia include reoccurrence of
the hernia, post-operative pain, and post-operative infection (Shell Iv et al., 2008).
A B
Hernia
Figure 1. Schematic of a Typical Ventral Hernia with Intestinal Protrusion.
A) Cross-Sectional View, B) Gross Appearance.
2
While there are different types of hernias that occur in the body, this specific
study applies to ventral hernias, which appear in the abdominal wall at a location of a
previous incision. In 2003, it was estimated that nearly one million abdominal wall hernia
repair operations were performed in the United States, with a predicted 1% annual growth
rate (Rutkow, 2003). Rutkow’s work inspired a more recent study by Poulose et al. which
determined that in 2006 there were 365,400 ventral hernia operations in the United
States, with an annual increase of 3% (Poulose et al., 2012; Rutkow, 2003). This number
is thought to have grown with the increase of obesity in the population, a well-
acknowledged factor in the development and recurrence of ventral hernias. Together,
these factors elevate the necessity of developing better ventral hernia repair techniques.
Traditional ventral hernia mesh repair (VHMR) surgery, using sutures as a mean
of mesh fixation, has proven very effective in practice in reducing recurrence rates
compared to non-mesh repair operations. The recurrence rate of non-mesh repairs is 23%
whereas the recurrence rate of mesh-repairs is 46% (Luijendijk Rw Fau - Hop et al., 2000
). However, post-operative pain associated with sutures still remains a major concern,
which has been postulated to be due to stress associated with sutures and tissue
penetration (Stoikes et al., 2013).
Recently, modern adhesives, such as biological glue, have been utilized as an
alternative fixation technique that does not require suturing to hold the mesh in place
(Canonico et al., 2005; Kaul et al., 2012). In Canziani’s (2009) study, a sutureless
incisional hernia repair technique was examined and found to result in a low occurrence
of chronic post-operative pain. Only 1 out of 40 patients reported recurring post-operative
pain, indicating that the use of staples, or sutures, for securing the mesh, was in fact
3
invasive and unnecessary (Canziani et al., 2009). This was previously indicated in a 2006
study by Petter-Puchner et al., where perforation of the bowel was noted in techniques
involving staples to fixate hernia meshes in surgeries performed on rats. This indicated
that fixation techniques involving tissue penetration raised the risk of damage to other
organs and also increased the risk of possible infections, or further post-op injury ( Petter-
Puchner et al., 2006).
One of the major determining factors of the strength and durability of the overall
repair is the mesh-tissue interface, which must remain intact during recovery such that
adequate tissue growth occurs to seal the hernia defect and embed the implanted mesh.
Grevious et al. stated that on average a typical adult requires a tensile strength of 16
N/cm to prevent a sealed and repaired abdominal wall from reopening (Cobb, Kercher, &
Heniford, 2005; Grevious, Cohen, Shah, & Rodriguez, 2006). On the other hand, it was
reported by Cobbs and Kercher that on average polypropylene mesh could withstand a
force of 32 N/cm (W. S. Cobb et al., 2005; Grevious et al., 2006). Ultimately, these prior
studies suggest that the use of fibrin glue fixation is both viable and generally preferred.
However, lack of data on fixation response of the sutureless methods, especially in
ventral HMR surgeries, is preventing most surgeons from adopting this technique
(Deeken, Faucher, & Matthews, 2012; Katkhouda et al., 2001).
The main goal of this master’s thesis project is to measure and compare the
strength of the mesh-tissue interface a) with sutures and b) with adhesive in a porcine
model. This study looks at the effect of biological glue, specifically human fibrin glue,
and 2-0 polypropylene sutures on the interface strength between the mesh and abdominal
wall. The goal being that with such information available there will be less controversy in
4
accepting the biological glue as an acceptable replacement in practice compared to
current traditional methods. Moreover, this experimental study provided data to validate a
preliminary computational model of the mesh-tissue interface.
5
Chapter 2: Background
Ventral hernia is classified such that the herniated tissue region occurs anywhere
on the abdominal wall where a previous incision has created a weakness in the tissue
layers. Some factors that contribute and increase the risk of ventral hernia manifestation
include abnormal collagen formation, malnutrition, vitamin deficiency, ageing, obesity,
pregnancy, previous invasive surgery, mechanical strains (i.e., chronic coughing or
constipation), and physical trauma (Norton et al., 2008). While environmental conditions
can result in hernia, most reported cases are generally due to uncontrollable congenital
factors (Shell IV et al., 2008). In the case where a ventral hernia is left untreated, patients
risk the possibility of the protruding organs to be subjected to incarceration and
strangulation leading to bowel obstruction and possibly death (Norton et al., 2008).
2.1 Surgical Repair
Hernias are usually repaired surgically and require that the protruding organ be
reduced into the human body before the defect is repaired. There are a handful of surgical
techniques that can be used when treating a hernia on an abdominal wall. The more
traditional method of repair is the tension repair technique (also known as
herniorrhaphy), which involves suturing together the edges of the tissue defect zone
(Norton et al., 2008). Tension repair has mostly been replaced in modern practices with
the tension-free repair technique (also known as hernioplasty), which involves fixating a
piece of mesh on top of the hernia to cover up and seal the defect while allowing a brace
to exist, on top of which tissue integration will occur (Figure 2) (Lau, Patil, & Yuen,
2006). A general consensus of tension-free repair being superior to traditional tension
repair is due to the significant reduction in post-operative morbidity and complications
6
(Luijendijk Rw Fau - Hop et al., 2000; Prieto-Diaz-Chavez et al., 2005).
Figure 2. Hernioplasty Repair for an Onlay Ventral Hernia Surgery
2.1. A. Surgical Mesh
The success of a surgical mesh in terms of preventing hernia-related
complications depends on several factors which include, among others, biocompatibility,
pore size, weight, and mechanical strength (Norton et al., 2008; Patel et al., 2012). While
surgeons are opt to select their preferred mesh product, it has been noted in literature that
polypropylene (PP) mesh (Figure 3), characterized as macro-porous, light-weight (LW),
with monofilament threads, is more commonly used in hernia related surgeries then other
mesh types (Canonico et al., 2005). A tensile test study performed on eight different
types of surgical meshes showed that not all the meshes behaved as effectively in vivo
when compared to a PP mesh (Patel, Ostergard, & Sternschuss, 2012).
7
Figure 3. Bard Soft Knitted Polypropylene Surgical Mesh. Left) Scaled, Right) Close Up
Mesh fixation with the host tissue can occur using any combination of sutures,
staples, clips, tacks, and biological glues; while mesh placement can be done at either
ends, or within, the site of the herniated tissue (i.e. onlay, sublay, inlay mesh placement)
(Clarke et al., 2011) (see Figure 4). There is no specific surgical approach that has been
acknowledged as the gold standard for a hernia repair, and as such it is the operating
surgeon’s preference on mesh type, placement, and fixation technique used for repair
(Clarke et al., 2011; Israelsson, Smedberg, Montgomery, Nordin, & Spangen, 2006;
Kingsnorth, Shahid, Valliattu, Hadden, & Porter, 2008).
8
Figure 4. Mesh Placement. Top) Onlay, Middle) Inlay, Bottom) Sublay
2.1. B. Adhesive Fixation Technique
The use of biological glue for fixating mesh onto tissue is still relatively new to
VHMR operations in comparison to the standard suturing techniques. One common
product applied for adhesive fixation is the off label use of Tisseel (Baxter, Deerfield, IL,
USA), fibrin-based glue. This adhesive is biodegradable and formed by combining
human-derived fibrinogen, calcium chloride, and thrombin to create a matrix of
polymerized fibrin fibers. The biological properties of Tisseel allow homeostasis, wound
healing, and fibroblast proliferation to occur at the fixation site, all of which are added
benefits over the sutured or stapled mesh-fixation techniques (Campanelli et al., 2012). It
is important to note that there are currently no FDA approved adhesive products in the
Rectus Abdominis Peritoneum
Onlay Mesh Placement
Inlay Mesh Placement
Sublay Mesh Placement
Rectus Sheath
9
market for this surgical application.
Katkhouda et al. has demonstrated the comparable efficacy of fibrin glue to staple
fixation in a 2001 study using porcine tissue for inguinal hernia repair. Samples prepared
after a 12-day recovery showed the two methods to be generally comparable in tensile
strength when subjected to a make-shift pull-off test (Katkhouda et al., 2001). Regarding
data for ventral hernia repair, Chevrel and Rath reported in a trial including 389 patients
that there was a recurrence rate of 18.4% when no mesh was used to reinforce the repair,
5.5% with mesh onlay reinforcement, and 0.97% when fibrin glue was used to fixate the
mesh in place (Chevrel & Rath, 1997). The use of fibrin glue was further supported by
Canonico, (2005), who presented an argument for the sutureless repair technique (n =
80), showing drastic reduction in surgical time and greater surgeon satisfaction,
especially the perceived ease of the operation and reduced reports of post-operative pain
(Canonico et al., 2005).
2.2. Hernia Repair Complications
Some general complications associated with hernia repair operations include
recurrence of the hernia, nerve entrapment (chronic pain), bowel obstruction, seroma
build-up, fistula formation, post-operative pain, and wound infection (Norton et al.,
2008). Failure modes of the mesh upon being implanted into the host include mesh
mechanical failure, mesh contraction, mesh migration, mesh curling/buckling, mesh
infection, undesired adhesion of mesh with other organs or local tissue, fistula formation,
erosion of mesh, and seroma formation (Chevrel & Rath, 1997; Robinson, Clarke,
Schoen, & Walsh, 2005). Variations in fixation techniques for tension free repair will
additionally alter the likelihood of particular complications from occurring.
10
Traditionally, the tension-free technique was performed with fixating mesh onto
the abdomen using any combination of sutures, staples, clips, or tacks; the mesh suturing
fixation method being the most popular in traditional tension-free repair operations. The
one thing all of these fixation methods have in common is that they require tissue
penetration for mesh anchoring. Consequently, any location where such an incision is
made into the abdominal tissue wall will become susceptible to the reoccurrence of future
incisional ventral hernias. A brief summary of the fixation strength obtained from
different fixation techniques for VHMR is shown in Table 1. In comparison to the
tension repair procedure any combination of these fixation techniques are superior in
reducing complications.
As stated previously in this document, the main concerns associated with sutured
mesh-fixation is the post-operative, acute and chronic, pain speculated to be caused by
the excess tissue penetration necessary for mesh-anchoring (Canonico et al., 2013). A
detailed investigation comparing post-operative pain associated with fixation techniques
was done in a 2009 study by Canziani’s. In this study, a sutureless incisional hernia repair
technique was examined and found to result in an extremely low occurrence of chronic
post-operative pain (only one out of 40 reported recurring post-operative pain), indicating
that the use of staples or sutures for securing the mesh was in fact invasive and
unnecessary (Canziani et al., 2009). Further support toward suture alternatives was
provided by a 2012 meta-analysis of PubMed data for inguinal hernias which
demonstrated comparable recovery times and recurrence rates of sutureless mesh (e.g.
fibrin glue) fixation to sutured mesh fixation, as well as reduced occurrences of post-
operative pain in the sutureless reinforcement (4% sutureless compared to 12% sutured)
11
which is a clear advantage over the traditional sutured technique (Kaul et al., 2012). In
the end, these prior studies suggest that the use of fibrin glue fixation has an upper hand
in i) reducing acute and chronic post-operative pain ii) recurrence rate iii) surgical time
iv) seroma formation v) better patient acceptability, and is both viable and generally
preferred. Even so, the amount of data available in literature has proven to be insufficient
in convincing many surgeons from adopting this sutureless technique in VHMR
operations and therefore additional experiments, such as this study, are being performed
to provide greater support.
12
Table 1. Literature Review on Interfacial Strength
Literature Animal Model/
Mesh
Fixation
Technique
Healing
Time
Mechanical
Testing
Procedure
Interfacial
Strength
(as reported)
Translated values for
comparison
(N/cm)
(Katkhouda et al.,
2001) Pigs / PPm
Tiseel
Nothing
Staples
12 Days Pull-off
0.955 kg
1.03 kg
0.46 kg
1.87 N/cm
2.02 N/cm
0.90 N/cm
(Clarke et al., 2011) Pigs / PPm
Tiseel
Tiseel & Tacks
Tacks
Suture & Tack
4 Weeks Pull-off
5.2 x 104 N/m
2
5.0 x 104 N/m
2
6.4 x 104 N/m
2
6.8 x 104 N/m
2
31.20 N/cm
30.00 N/cm
38.40 N/cm
40.80 N/cm
(A. Petter-Puchner,
Fortelny, Mittermayr,
Öhlinger, & Redl,
2005)
Rats / Ti mesh
Rats / VYPROII
Tiseel
Staples 17 Days
Burst
(80 mmHg)
Pull-off
(300 g pull
force)
No failure in any
interface No failure in any interface
(Eriksen, Bech,
Linnemann, &
Rosenberg, 2008)
Pigs/ Motif
Pigs / Proceed
Tiseel
Ti Tacks
Tiseel
Ti Tacks
30 Days Peel Test
3.0 ± 1.5 N/cm
3.87 ± 1.2 N/cm
3.44 ± 0.7 N/cm
2.69 ± 1.3 N/cm
3.00 ± 1.50 N/cm
3.87 ± 1.20 N/cm
3.44 ± 0.70 N/cm
2.69 ± 1.30 N/cm
(McGinty, Hogle,
McCarthy, & Fowler,
2005)
Pigs/ PPm
Pigs / ePTFE
Pigs / PCO
Suture & Tack 28 Days Peel Test
2.1 N/cm
1.3 N/cm
2.8 N/cm
2.10 N/cm
1.30 N/cm
2.80 N/cm
(Majercik, Tsikitis,
& Iannitti, 2006) Pigs/ ePTFE Tacks
2 weeks
4 weeks 6
weeks 12
week
Lap Shear
0.83 ± 0.06 lbs
1.06 ± 0.07 lbs
0.88 ± 0.08 lbs
1.13 ± 0.07 lbs
1.85 ± 0.13 N/cm
2.36 ± 0.16 N/cm
1.96 ± 0.18 N/cm
2.51 ± 0.16 N/cm
(d'Acampora,
Kestering, Soldi, &
Rossi, 2007)
Rats/ PPm
Rats / Vypro Nothing 28 Days Lap Shear
48.05 ± 9.05 N
45.32 ± 16.8 N
24.03 ± 4.53 N/cm
22.66 ± 8.40 N/cm
(Schug-Pass, Lippert,
& Köckerling, 2010)
Pig/ PPm
(Bard soft)
Nothing
Tiseel 0 hour
Indention
3.17 ± 0.50 N
73.6 ± 13.4 N
0.32 ± 0.05 N/cm
7.36 ± 1.34 N/cm
(Gonzalez et al.,
2005)
Pig / PPm
Pig / PEm Sutures 3 Months Pull-off
159 N
194 N
23.73 N/cm
22.30 N/cm
Polyester mesh (PEm); Polypropylene mesh (PPm); Expanded polytetrafluoroethylene mesh (ePTFE) ; Polyester mesh with anti-adhesive collagen layer (PCO)
13
2.3. The Mesh-Tissue Interfacial Strength
Before the hernia is fully healed, stress due to intra-abdominal pressure will
concentrate and build up at the hernia repair site. Respectively, the mesh-tissue repair is
responsible in resisting the internal loads until a time arrives that the wound has fully
healed. In most cases it is not as much necessary for the repair site to be fully healed to
resist the internal loads as much as for complete tissue ingrowth to occur. While waiting
for adequate tissue in-growth, the mesh-tissue interface becomes an important
determining factor in preventing recurrence (Majercik, Tsikitis, & Iannitti, 2006). In
light of this, it was determined in a previous study by Majercik et al., that nearly 70% of
the full tissue ingrowth occurred within 2 weeks of wound healing (Majercik et al.,
2006). It was therefore suggested that prior to this time the mesh-tissue interface was a
significant contributor when predicting the possibility of particular failure modes
occurring (i.e., mesh migration, curling/buckling, and reoccurrence) (Eriksen, Bech,
Linnemann, & Rosenberg, 2008).
Since the fixation strength of the mesh-tissue interface is reliant on the fixation
technique used for binding mesh and tissue together. There becomes a relative need to
better understand the fixation techniques in order to understand and improve the mesh-
tissue interface and respectively the mesh-tissue repair. More specifically, in any system
subjected to loading there is a buildup of stresses. How the system distributed these
stresses provides insight on which part of the system is most responsible toward resisting
failure. In regards to mesh-tissue fixation there becomes a fundamental difference in
stress distribution, which is specific to the fixation technique.
14
In the case of sutured mesh fixation the stresses will concentrate at the site where
the sutures are embedded within the tissue. On the other hand, when adhesive fixation is
used, the stress will distribute uniformly over the entire mesh-adhesive surface area. As
such, suture fixation prior to adequate tissue ingrowth will be reliant on the tensile
strength of the sutured material and the surrounding fascia it is embedded within;
whereas glued fixation will be dependent on the adhesive strength of the product and the
surface area of the mesh-adhesive interface. Because of this, one of the added benefits of
using sutures over adhesives is that sutures have a relatively stronger raw tensile strength
than biological glue and will be able to withstand a greater load before failing (Klinge et
al., 1996; Klinge, Klosterhalfen, Muller, Ottinger, & Schumpelick, 1998).
2.4 Interfacial Strength Measurements
Due to the importance of the fixation strength of a mesh-tissue repair it is
necessary to perform mechanical tests, which can provide quantitative data of the
interfacial strength. This information would make it possible to compare different
fixation techniques and better evaluate the added benefits of fixation strength to other
related complications. One mechanical approach that is often utilized for determining
interfacial strength is called adhesion tests. A few of the most common types of adhesion
tests performed, among others, include indention, lap shear, peel-off, pull-off, and burst.
For meaningful results, the mechanical test chosen must be designed to subject the mesh-
tissue specimen to the stresses it would encounter in vivo within the abdominal wall.
Mechanical failure of the interface due to stress build-up initially may occur by
developing a defect within the interface at a reentrant corner of the bonded stiffener (i.e.
the mesh). A bonded stiffener is the substrate material in the system that increases the
15
global stiffness of the composite structure (Lacombe, 2005). Consequently, once this
initial weakness occurs within the interface the failure site will propagate and increase in
size over the distance of the implanted mesh piece. For mechanical tests where a tensile
load is applied laterally through the gauge length the failure will occur along the samples
width. Therefore, it is common practice within literature for report values, from such
mechanical tests, to be normalized in the form of peak force per unit width (N/cm).
In vivo, when internal pressure of the abdominal wall is applied to the mesh
interface, failure propagation can occur along any direction. For adhesive tests such as
peel and lap shear the load is applied on the specimen in one particular direction, which is
labeled as the length (Figure 5A). For mechanical tests such as indention or burst were
the load is applied over the entire surface area of the specimen the resulting max force
cannot be normalized using the width since failure could have occurred along any
direction (Figure 5B). For those mechanical tests the data is generally normalized using
the surface area of the mesh and reported as UTS (N/cm2).
Figure 5. A) Peel Test Schematic, B) Indention Test Schematic
A B
16
2.4. A. Lap Shear Test
For this study, the mesh-tissue interface was evaluated using a lap shear test
procedure. Similar to peel, the lap shear test is performed by having a tension load
applied on the specimen such that stress concentration occurs at the interfacial region. In
lap shear tests the loading occurs differently in both ends of the sample, such that a
different substrate is loaded on either end (see Figure 6). Specifically, one substrate is
displaced at a particular velocity while the second substrate is fixed in place, preventing
all movement and rotation. This will cause the stress propagation to occur within the
interface; in an ideal case, failure would occur due to shearing of the two substrates.
Figure 6. Lap Shear Test Schematic
In respect to the previous testing methods mentioned in this text, the main
advantage of using a lap shear test over an indention test is that it does not require a
17
larger tissue to mesh ratio of surface area for individual test specimens (Figure 7).
However, the loading can only be applied in one specific direction versus all directions as
would have occurred in an indention test. On the other hand, compared to the peel test the
advantage of the lap shear experiment is that the axial loading is applied in the same
direction as shear stresses that occur in the interfacial region in vivo; one failure mode
directly related to shearing is mesh migration.
Figure 7. Mesh to Tissue Ratio for Mechanical Testing.
Green) Surgical Mesh, Red) Tissue Specimen
2.5. Material Properties
Biocompatibility of an implant is not limited to the chemical reaction of the host
body but also its biomechanical compatibility to the unique environmental loads of the
biological system. The presence of embedded mesh within a tissue will ultimately make
the repaired tissue stiffer than its healthy counterpart. Consequently, a stiffer tissue
response in the abdominal wall is associated with a reduction in the physiological
compliance of the tissue (Hernández et al., 2011).
The compliance of the abdominal wall is very important as it enables the
18
abdominal wall to withstand and adjust to increased stresses and minor pressure changes
due to simple and/or strenuous activities. Since the abdominal wall is anisotropic, it has
become essential to consider the following two factors to achieve a superior repair: 1) the
finest material and 2) accurate mesh orientation for each hernia repair. This would ensure
that the mesh has characteristics that match closely to the natural characteristics of the
abdominal wall (Hernández-Gascón et al., 2011). The mechanical anisotropic behavior of
the surgical mesh is determined by the filament composition, mesh weave, and spatial
arrangement of the filaments, while in the abdominal wall, it is the collagen fibers rather
than muscle fibers responsible for determining passive tissue tensile strength (Hernández
et al., 2011). It is therefore important, to determine the material properties of the
implanted prosthetics and the associated tissues as to determine if the presence of a
particular prosthetic will cause any biomechanical change in the system. In this study,
material properties were obtained by performing uniaxial mechanical tests on samples of
excised abdominal tissue and surgical mesh.
2.5. A. Linear Material Properties
Uniaxial tension experiments are a type of tensile test used for obtaining
mechanical properties of materials (Lacombe, 2005). These tests cannot account for the
anisotropic nature of a material and instead obtain isotropic material properties, which are
characterized by a specific orientation. The test is done by loading the ends of a uniform
material specimen such that the sample is stretched until failure along a specific axial
direction. Figure 8 shows a schematic of a uniaxial tensile test, when done using a testing
apparatus raw data will be in the form of force vs. displacement (F-D).
19
Figure 8. Uniaxial Tension Test Schematic
Material properties cannot be directly derived from the F-D data and must first be
translated to stress vs. strain (σ-ɛ), which is done using Equation 1. In this study the stress
(σ) was translated from this data by dividing every force (F) data point by the specimen’s
initial cross-sectional area (A) (i.e. Sample width x thickness). Similarly the strain (ε)
was translated from the data by dividing every displacement (Δl) data point by the initial
gauge length (lo) (i.e. specimen height subtracted by specimen height bounded within
clamps). Comparably, other studies have also obtained material properties of abdominal
tissue, PP surgical mesh, and PP sutures. Table 2 provides a brief list of some of the
material properties reported in literature regarding similar studies to this thesis.
(1)
20
Table 2. Mechanical Strength of Mesh and Tissue
Literature Material Test Type Average Strength
± STD
(Klinge et al., 1996) Mesh Indention Test 40-100 N/cm
(W. S. Cobb et al., 2005) PP Mesh Indention Test 43.2 N/cm
(Schug-Pass et al., 2010) PP Mesh
(BARD soft) Indention Test 29 N/cm
Manufacturer’s Information PP Mesh
(BARD soft) Indention Test 36 N/cm
(Cobb, Harris, Lokey, McGill, & Klove,
2003) PP Mesh Indention Test 32 N/cm
(Hernández-Gascón et al., 2011) PP Mesh
(Optilene)
Uniaxial Tests:
Direction 1
Direction 2
7.57 ± 0.74 N/mm
10.79 ± 1.05 N/mm
(Klinge et al., 1996) PP sutures Uniaxial 30 N/cm
(Klinge et al., 1996) Human
Abdomen
Uniaxial Test:
Horizontal Direction
Vertical Direction
Theoretical Value:
(rupture strength)
60-80 N/cm
20-30 N/cm
4-16 N/cm
(William S. Cobb et al., 2005) Human
Abdomen
Theoretical strength:
(at max intra-
abdominal pressure)
11 – 27 N/cm
2.6 Finite Element Method in Biomechanics
Computational modeling refers to an engineering application at which material,
loading, and environmental conditions are simulated virtually giving a visual
representation of how mechanical loads would propagate within a composite structure
(Reddy, 2004). One common method for computational modeling is finite element
analysis (FEA), which is a numerical technique used to compute approximate solutions to
boundary-value problems (i.e., a differential equation with specific boundary conditions)
(Venkatesh, 2011).
In this approach for theoretical computations, the stress distribution is evaluated
by creating a virtual simulation of the system with particular characteristics, such as
loading, geometry, material properties, boundary constraints, and material interface
conditions, translated into the form of mathematical equations (Reddy, 2004). The
21
mathematical equations used are generally determined from known material values or
experimental data which is translated into an approximate numerical solution using
various principles of solid mechanics. The goal is to find values for unknown stresses at
points on or within the system (Holzapfel, 2000; Seshu, 2004). These stresses are known
as field variables which are infinite in number due to the continuous nature of the body
(Holzapfel, 2000).
2.6. A. Non-Linear Mechanical Properties
In FEA it is required to give material properties for each simulated part. In
regards to surgical mesh and abdominal wall tissue these properties cannot be described
as linearly elastic materials and require a relatively more complex application of
continuum mechanics. In brief, continuum mechanics is an area of mathematical physics
which describes the fundamental laws governing motion and deformation of a structure
as a continuum mass rather than as discrete particles (Reddy, 2004). Within the field of
continuum mechanics, nonlinear materials such as hyperelastic materials are often
described using the constitutive laws with corresponding constitutive equations
(Holzapfel, 2000). Respectively, the constitutive equations are further described in the
field of solid mechanics (Holzapfel, 2000).
For constitutive theories, it is important to note that the resulting mathematical
model created to represent a real-life material behavior is based off of actual
experimental data obtained from mechanical testing (e.g. excised tissue and surgical
mesh). Constitutive models or material models are very important and necessary when
creating a computational model. These models are generally quite simple for elastic
behavior but become relatively more complex and less likely to predict realistic behavior
22
when used toward describing nonlinear continuum mechanics (Holzapfel, 2000). One
approach to modeling the behavior of nonlinear hyperelastic material is using a strain
energy function (SEF) to describe a mechanical response. Some of the more well-known
strain energy functions that are readily available to be used in engineering software
packages include Hookean, Mooney-Rivlin, Polynomial, Reduced Polynomial, and
Ogden descriptions (Holzapfel, 2000).
Modern software’s used for computational studies such as finite element analysis
(FEA) have built in curve fitting generators that can take experimental stress-strain values
and relate potential fits with various strain energy functions (ABAQUS/CAE user's
manual : version 6.4, 2003). Two of the more relevant strain energy functions used in this
study were the 2nd
order reduced polynomial and the 1st order Ogden SEF’s. The
corresponding equations and characteristics of these functions are described in the
Equation 2 and Equation 3 (ABAQUS/CAE user's manual : version 6.4, 2003). In the
equations below “W” is work, “U” is energy, “J” is Jacobian and is used to describe the
total volume change at any given point, is stretch ratio, “μ” and “α”, and “N” are
material constants, “D” is the constant used to describe rate of deformation and is
dependent on the Poisson’s ratio, the different “I” values are strain invariants, and “Ci” is
a temperature-dependent material parameters.
23
∑
∑
∑
(First Order Ogden SEF)
∑
∑
∑
(Second Order Reduced Polynomial SEF)
(2)
(3)
24
Chapter 3: Methods and Materials
The aim of this thesis was to obtain a more in-depth understanding of the load
carrying capacity of the mesh-tissue interface resulting from a VHMR operation. The
major interest in this research is in whether there is a difference in the resulting interfacial
strength if fixation of the prosthesis is performed using fibrin glue instead of sutures. The
end goal is to shed light towards the application of adhesive fixation techniques in
VHMR surgeries so that surgeons can better weigh the benefits of either technique when
making an informed decision regarding their surgical approach.
Animal Model
Dr. Stoikes and his team at the University of Tennessee Health Science Center
(UTHSC) obtained porcine abdominal specimens through an animal study. All
procedures were approved by the Institutional Animal Care Unit Committee, IACUC ID
# 12-103.0-A. Female mongrel pigs that weighted 25-30 kg were used as the animal
models. Polypropylene soft knitted (BARDTM
) surgical mesh was implanted onto the
abdominal wall using the onlay mesh placement. Specific tissue layer onto which mesh
was fixated was generally the rectus sheet and rectus abdominis. Two prosthetic mesh
pieces with an area of 6 by 4 cm were implanted in each animal model with 2-3 cm of
space between them. Both soft tissue and surgical mesh are anisotropic materials (see
Figure 9). In this particular study the mesh was implanted by the surgeon without
consideration of mesh/tissue alignment.
25
Of the two implanted mesh pieces, one was fixated using 4 ml of the Tisseel fibrin
glue (Baxter Healthcare), spread uniformly on and throughout the mesh surface by the
surgeon’s finger (see Figure 10). The second mesh piece was fixated onto the abdominal
wall using 4, 2-0 Prolene PP sutures (Ethicon); which were sewn near the edges of the
mesh as shown in Figure 10. After the mesh was implanted the animal specimens were
allowed to recover for a given amount of time so that tissue ingrowth into the mesh could
occur. The amount of time allowed for recovery was regulated at 24 hours, 1 week, and 2
weeks for 8 animal models per group. For these experiments, animal care and operations
were carried out at the UTHSC and only excised samples provided by Dr. Stoikes and his
team was brought over to the University of Memphis for testing. Samples of mesh-tissue
specimens are shown in Figure 11.
Figure 9. Description of the Orientation
Direction 1
Directio
n 2
Direction 1
Directio
n 2
26
Figure 10. Implanted Surgical Mesh with Different Fixation Techniques
The excised abdominal wall for each animal specimen was used to create two
specimens for mechanical testing, as shown in Figure 11. The individual samples
included the 6 by 4 cm2 mesh piece fixated onto tissue that on average had a surface area
of 8 by 5 cm, the extra 2 cm of tissue height extending from one end of the sample;
thickness of samples were on average 0.75 cm. An area of approximately 1 by 4 cm of
mesh-tissue was cut away from the sample for a separate study regarding histological,
mesh contraction, and tissue ingrowth evaluation at the Department of Pathology of the
UTHSC (see Figure 12A) (Stoikes et al., 2013). The removal of part of the mesh-tissue
sample damaged the end of the mesh and tissue, which was cut away, creating defects
where stress propagation under loading could occur. This defect zone was accounted for
in the mechanical tests by binding and directly loading that specific region (see Figure
Sutured
Fixation
Mesh
Placement
Prior to
Fixation
Glued
Fixation
27
12B). Biomechanical testing of the interfacial strength was done using a lap shear testing
procedure and uniaxial testing procedure for determining material properties of tissue and
mesh.
Figure 11. Lap Shear Test Specimens. Left) Glued Fixation, Right) Sutured Fixation
Figure 12. Typical Sample Division
Zone A:
grips only
mesh
Zone B:
grips only
tissue
28
3.1 Mechanical Testing Instrument and Software
Mechanical testing was done using an Instron 3380 (Canton, MA) testing device
(see Figure 13A). The device functioned by displacing the position of a single clamp.
This moving clamp was capable of exerting either a tension or compressive load on the
sample, depending on the direction of the clamps movement. The load developed by
stretching the tissue is measured using a load cell and the displacement of the clamp
measured with a built in extensometer. The proposed test procedure used a 5 kN load cell
and a software protocol programmed to provide a displacement velocity of 0.42 mm/s.
The specific velocity was used to apply a relatively slow loading so that a near quasi-
static material response could be recorded. Raw force and displacement data was
recorded by a digital output reader running the Bluehill 2 software (see Figure 13B), and
extracted as an excel comma separated spread sheath, .csv file type. The multiple
peaks/spikes in the F-D data for sutured fixation, in Figure 13B, marks the point of the
lap shear test when a single suture pulls out of the mesh-tissue specimen.
Figure 13. A) Instron 3380 Mechanical Testing Apparatus with Pneumatic Clamps
B) Bluehill 2 Force vs. Extension Readings
A B
29
3.2 Biomechanical Evaluation
3.2. A. Uniaxial Test Method and Procedure
The mechanical response of the mesh and abdominal tissue under loading was
measured through uniaxial extension experiments. The tissue was cut so that each tested
sample had dimensions approximately 4 cm in width, 5 cm in height, and on average 0.75
cm in thickness. The thickness of individual samples varied between 0.5 and 1 cm
depending on the topological location of the specimen and was never uniform. The tested
mesh pieces were 3 cm in width, 6 cm in height, and 0.044 cm in thickness. Figure 14
shows a sample uniaxial test on surgical mesh in both directions; in these tests the fiber
orientation strongly influenced the load bearing capacity of the specimen.
Figure 14. Uniaxial Mesh Tests. Left) Direction 1, Right) Direction 2
3.2. B. Nonlinear Material Properties
The non-linear hyperelastic material model was determined in ABAQUS by curve
fitting various SEF’s with the experimental data to obtain a best fit. In this step, the curve
fitting was done to the experimental data up to a strain of 1, ϵ = 1, such that the
corresponding SEF modeled the initial mechanical response of the material. Additionally,
SEF’s in ABAQUS require the input of a Poisson’s ratio for the material. These values
30
were not determined in the scope of this study and were instead approximated from
known information. For surgical mesh a Poisson’s ratio of 0.45 was applied, ν = 0.45,
this value corresponds to the poisons ratio of a solid block of polypropylene. In regards to
soft tissue, previous literature has modeled this material with a nearly incompressible
Poisson’s ratio (0.4 < ν < 0.5). For the sake of this study, different Poisson’s ratios
between 0.46 and 0.498 were tested and a best fit was selected.
The best fit was decided by evaluating the goodness of fit as determined by the R2
value between the experimental and theoretical data sets. This was done by first plotting
the material response of the SEF in Excel (Microsoft, Redmond, WA) and curve fitting it
with a tread-line so that an equation which matched the data set could be obtained. This
equation was then used to determine the stress from the material model for all strains in
the uniaxial experimental data sets, such that the size of the array would be identical for
both data sets. The array of values for experimental stress and theoretical stress were
compared for a goodness of fit using a coefficient of determinates method, 0 < R2 ≤ 1
where a value of 1 portrays identical data sets. The outputted R2 value was determined by
first solving the sum of the squares (TSS) using Equation 4A, followed by the residual
sum of the squares (RSS) using Equation 4B, and finally using TSS and RSS values to
solve for R2 as denoted in Equation 4C.
∑ ∑
B C A (4)
31
3.2. C. Lap Shear Test
The interfacial strength between mesh and tissue were measured using a lap shear
test procedure. Individual specimens were prepared for testing by shaving off excess
tissue from the edges and the tissue thickness. The sample thicknesses were reduced at
times so that the specimen would be able to fit within the clamps. Approximately, 1x4
cm2 surface area of mesh was released from the tissue so that it could be held by the
upper clamp. The excess tissue, which remained untouched by the prosthetic mesh, was
held by the lower clamp. Eventually after enough imposed strain, the ultimate strength of
the mesh-tissue interface was observed. For each sample the initial gauge height between
the two clamps before testing was recorded as well as the initial width of the mesh, the
height of the overall sample, and the location at which failure occurred (i.e. interface,
mesh, and tissue). Figure 15 shows a sample of a mesh-tissue lap shear test.
Figure 15. Lap Shear Test Sample
3.2. D. Data Analysis
Interfacial strength and the relevant material strength were reported in the form of
peak force per unit width (N/cm). These values as well as other computations were done
32
in Matlab (MathWorks Inc., Natick, MA) and Microsoft Excel. In particular, a Matlab
code was written that graphed the force to displacement data and outputted all the
maximum forces for individual samples. These values were than stored and sorted in
Excel, where they were normalized with their respective initial mesh width. Statistical
analysis to determine significant difference between fixation techniques at evaluation
time was done through a 2 sample t-test for normally distributed variables and Mann-
Whitney-Wilcoxon statistical U-test, also known as a Wilcoxon’s Rank Sum test, for
ordinal variables.
Student t-tests were done within excel spreadsheet while Mann-U tests were done
using an online statistical package called VarrarStats (VarrarStats, New York). Samples
were evaluated at a confidence level of 95%; such that there was a significant difference
reported from the t-test if P < 0.05. On the other hand for the Mann-U test significant
difference was evaluated using the UA value as the determinate factor (Equation 5).
Which was done by comparing the UA value with the lower and upper limit of the U-test;
if the value falls outside the range provided, lower-upper limit, the two data sets are
significantly different. An additional statistical test was done using a 2-way analysis of
variance (Anova), performed in SigmaPlot (Systat Inc., Illinois), which was able to
compare the influence of 2 different independent variables on one dependent variable. In
this case the two independent variables were the evaluated time points (24 hours, 1 week,
and 2 weeks) and the fixation method (glued or sutured), where the dependent variable
was the fixation strength reported. Other analysis included identifying outliers in Matlab
and removing them from calculations regarding average normalized loads.
33
3.3 Finite Element Analyses of the Uniaxial Extension and Lap Shear Experiment
The long-term goal of the computational study is to generate a simplified, robust
model of hernias with mesh repair, which would help researchers better understand the
biomechanics of a repaired abdomen. Wound healing after mesh fixation includes a
biological response of hemostasis, inflammation, proliferation, granulation, remodeling,
and maturation. At two weeks of recovery it has been noted in literature that nearly 70%
of tissue ingrowth has already occurred (Majercik et al., 2006). The tissue ingrowth
corresponds to the proliferation step where collagen formation and new tissue fibers are
created. Therefore, a computational model mimicking interfacial properties at two weeks
would provide insight on a repaired abdominal wall where the mesh has been almost
completely embedded within the tissue. The first step in creating this model is attempting
to create a method to model the mechanical responses of both surgical mesh and the
excised abdominal tissue. After which a coupling method was sought for that could
accurately mimic the interfacial strength of a repaired abdomen at two- week recovery
post operation.
Individual tissue and mesh material models were made on the assumption of the
continuum theory of finite strain. It was also assumed that the materials were best
represented by nonlinear hyper-elastic strain energy functions. The anisotropic response
was defined by identifying a preferred material direction (same orientation as recorded
during experiments). Lastly, the tissue response was modeled to mimic a passive
abdominal wall.
(5)
34
3.3. A. Simulation of Uniaxial Extension Using FEA
A commercially available FEA software ABAQUS (Providence, Rhode Island)
was used for this work. Parts for the mesh and tissue specimens were drawn to the
corresponding dimensions recorded for each of the uniaxial experiments. The shapes for
both parts were simplified when modeling; for tissue it was assumed that the shape of the
specimens were uniform and the material homogenous such that the part could be drawn
simply as 3D solid rectangular pieces. In real life surgical mesh is formed of woven
threads and contains many pores, see Figure 16, such that the surface area is
exceptionally smaller than that of a uniform rectangular block with the same dimensions
of length, width, and thickness. Solid parts in FEA, such as those used to model tissue,
cannot be in contact with wired elements which would be required to model surgical
mesh formed of woven threads (ABAQUS/CAE user's manual : version 6.4, 2003).
Therefore, a compromise was made and the surgical mesh was simplified as a
homogenous 3D shell rectangular piece. Parts varied in dimension from sample to sample
but remained constant in element and design characteristics as listed in Table 3. A sample
of the ABAQUS standard input file for these simulations is available in Appendix D and
Appendix G.
35
Table 3. FEA Model Part Characteristics
Abdominal Tissue Bard Soft PP Mesh
Part Type 3D Solid Homogenous 3D Shell/Continuum Homogenous
Element Shape Hex Dominated Quad Dominated
Element Type
C3D8H Elements: quadratic
elements, 8 node linear
bricks, hybrid formulation,
with constant pressure
S4R Elements: Quadratic Elements, 4
node doubly curved think or thick shell
structure, reduced integration, hourglass
control, finite membrane strains
Material SEF Reduced Polynomial 2
nd
Order Ogden 1
st Order
Poisson’s Ratio Nearly Incompressible:
0.46 ≤ v ≤ 0.498
Polypropylene:
v = 0.45
Dimensions: 5 x 4 x 0.75 cm 6 x 3 x 0.044 cm
Figure 16. Zoomed In Image of Surgical Mesh. Left) Pores; Right) Threads
The modeled parts were partitioned into three regions; the first region
representing the area bound in the upper clamp, the second being the gauge area, and the
third being the area bound by the bottom clamp (see Figure 17). The first region was
given a displacement boundary condition. The magnitude of this displacement was
equivalent to the value obtained in the experimental data at which this specific simulation
supposedly failed. The third partitioned region was given a boundary condition to be
pinned in all directions such that there was no movement.
36
Figure 17. Partitioned Part for Uniaxial Simulation.
Arrows) Upward Displacement, X’s) Fixed Region
The computational test was run and the sum of the reaction forces in the y-
direction for each node within the fixed region was collected. The reaction forces were
divided by the initial cross-sectional area which allowed for the FEA results to output in
the form of stress and strain. To compare the computational predictions from FEA with
the experimental results, both data sets were graphed simultaneously and a goodness of
fit determined by calculating the R2 value. Mesh convergence studies were conducted to
determine the number of elements necessary for optimal results with respect to
computational time and cost.
3.3. B. Lap Shear Simulation Using FEA
This FEA simulation used customized dimensions for each specimen to match
recorded dimensions during testing. Each part remained consistent with the part and
element characteristics determined by the uniaxial FEA simulations (see Table 3). These
parts were then assigned the material SEF coefficients for the orientation where the
37
original test load had been applied. The mesh and tissue were adhered together using an
interaction constraint, surface-surface contact. This constraint was further customized to
adjust over closures and to tie adjusted surfaces, i.e., surface area of both mesh and tissue
part depicted by yellow squares in Figure 18. Contact properties comprised of “normal
behavior”, which included a pressure over closure, “Hard Contact”. The full FEA model
with associated constraints can be seen in Figure 18. The loading in ABAQUS was done
by applying a displacement boundary condition onto the mesh shell edge and having it
moved upward equivalent to the displacement of the first peak for that specimen from the
lap shear experiments. The ABAQUS standard input file for this simulation is available
in Appendix J.
One other boundary condition was applied; fixing in place the bottom partitioned
region of the tissue. The sum of the reaction force at the fixed tissue region was recorded
at various extensions and compared to the load vs. extension data of the lap shear
experiments. The two data sets were graphed simultaneously and compared to each other
by analyzing the goodness of fit. Last, a mesh convergence study was undertaken to
determine the number of elements necessary for optimal results with respect to
computational time and cost.
38
Figure 18. FEA Model of Lap Shear Experiments. Blue) Mesh, White) Tissue, Arrows)
Upward Displacement, X’s) Fixed Region, Squares) Tied Surfaces
39
Chapter 4: Results and Discussion
The aim of this thesis is to determine whether the use of fibrin glue as an
alternative fixation method for mesh placement on the abdominal wall results in adequate
interfacial strength between mesh and tissue. This was accomplished by measuring and
comparing the fixation strength of mesh-tissue interface with fibrin glue or classical
sutures. Three major activities were undertaken: (1) uniaxial extension experiments with
only the abdominal tissue and only mesh (BARD, New Jersey) to identify the material
properties, (2) lap shear test to determine interface strength between abdominal wall and
mesh without hernia defect, and (3) FE modeling of the lap shear experiment to initiate a
computational modeling effort to model hernia.
4.1 Uniaxial Extension Experiments
Uniaxial experiments were conducted to measure the mechanical response of the
mesh and tissue. Both Bard soft mesh and abdominal tissue are known to display
anisotropy. Due to this characteristic, two different orientations were tested and used as a
means of comparison for this study. Representative raw data from the uniaxial
experiments from tissue and mesh are shown in Figure 19.
40
Figure 19. Representative Uniaxial Test Data.
Tissue Specimen (Direction 1), Mesh Specimen (Direction 2)
4.1. A. Average Normalized Force
Uniaxial experiments were performed and F-D response of surgical mesh and
excised abdominal tissue collected. This data was used to obtain the peak force per unit
width used to describe the material strengths (Table 4). The average normalized force for
direction 2, the weaker of the two orientations for tissue, was 16 N/cm +/- 3.0 STD (Table
4). A similar value was reported by Grevious et al. as a minimum strength required in
mesh repair for a successful operation of an average adult (Grevious et al., 2006).
Direction 1 of tissue is characterized as following the fibers along the transverse
abdominis and transverse to the muscle fibers in the external oblique. This orientation
was previously reported to have a stiffer response than tissue tested in direction 2
(Hernández et al., 2011; Song, Alijani, Frank, Hanna, & Cuschieri, 2006). While the
uniaxial results for tissue indicate an average value relatively stronger in Direction 1 then
0
10
20
30
40
50
60
0 10 20 30 40 50
Load
(N
)
Extension (mm)
Uniaxial Test Samples
Tissue Uniaxial Test
Mesh Uniaxial Test
41
Direction 2, it was revealed in a 2 sample statistical T-test that there was actually no
significant difference between the two orientations. On the other hand, a similar statistical
test confirmed that there is a significant difference in the mechanical response of surgical
mesh with respect to the orientation tested.
As stated previously, a relatively stiffer F-D response of the prosthetic mesh in
comparison to the surrounding tissue will result in a stiffer regional response in the
abdomen. However, the fittingly stronger mechanical response is necessary in providing
reinforcement to resist internal loads at the hernia repair site. Specifically, the axial and
radial stresses relevant to a hernia will cause stretching perpendicular to the direction that
the hernia defect is propagating. Consequently, in a hernia repair operations, optimal
results will be best achieved if anisotropy of the prosthetic implant is orientated such that
the weaker direction is aligned to the direction the hernia is propagation. For this study
however, where there is no hernia defect, optimal results will be best achieved if
anisotropy of the prosthetic implant closely resembles the fixated tissue (i.e. direction 2
of mesh is aligned to direction 2 of tissue).
Table 4. Uniaxial Results with Averaged Normalized Forces
Material Average +/- STD
Direction 1
Average +/- STD
Direction 2 P-Value
Abdominal Wall 22.0 +/- 7.4
n = 6
16.0 +/- 3.0
n = 6
P = 0.21
No Significant
Difference
Bard Soft Mesh 48.0 +/- 1.3
n = 5
17.0 +/- 1.6
n = 5
P < 0.01
Significant
Difference
42
4.1. B. Nonlinear Mechanical Properties for Mesh and Tissue
When curve fitting a strain energy function with experimental data the ABAQUS
software will provide coefficient values for the specific SEF’s such that the equation will
be able to simulate the non-linear material response. Each strain energy functions used a
total of 3 coefficients. For the 1st order Ogden SEF the coefficients were µ, α, and D1 and
for 2nd
order reduced polynomial the coefficients were C10, C20, and D1. These
coefficients represented specific material properties. For example µ, C10, and C20 were
coefficient related to shear modulus and were expressed in the units of MPa. Alpha is a
dimensionless and unitless constant which plays a role in the theory of finite elasticity.
Last the D1 constant called “Dashpot” is the time derivative of strain, resisting changes in
length; the units are in MPa-1
. All coefficients for the material models have been
summarized in Table 5 and Table 6. The goodness of fit between the material model and
experimental data is shown in Figure 20 for tissue and in Figure 21 for surgical mesh. All
curve fitting results for both mesh and tissues are included in Appendix E and H.
The Poisson’s ratio of the abdominal wall soft tissue was not solved for in our
experimental tests. However, other studies indicate that soft tissue can be modeled using
a nearly incompressible Poisson’s ratio (0.45 < ν ≤ 0.499). Poisson’s ratios were tested
and a best fit was selected for each individual model, such that the uniaxial simulation in
ABAQUS gave similar results to the experimental data. Best fit at this stage was
determined through visual inspections, but quantitative values were determined for the
final fit using the coefficient of determinate method. It is worthwhile to note, that
Poisson’s ratio for best fit varied between 0.46 and 0.499 between models of individual
specimens. It is very unlikely that the compressibility of soft tissue is ever at a value of
43
0.499, which suggest that further investigation should be done on the significance of the
Poisson’s ratio for abdominal soft tissue with respect to our current experimental set-up.
In using this method a few assumptions were made to simplify the problem. In
reality the abdominal tissue samples included many different types of tissue including
superficial fascia, rectus sheath, and rectus abdominis muscle tissue as well as possibly
fascia from the linea alba and tendinous intersections. However, for the simplification of
determining a function, which could represent the material properties of the tissue sample
it, was assumed that the tissue was a homogenous structure. Similarly, surgical mesh
which is a woven structure with many pores was modeled in this study as a continuous
homogenous shell part. Additionally, the surgical mesh model used a 1st order Ogden
SEF, with a Poisson’s ratio of 0.45, to model mechanical response. This Poisson’s ratio is
the material property value for a block of polypropylene and would be significantly lower
for a woven material. Consequently, because of these simplifications there is a chance the
material model will be unable to fully account for the mechanical response of different
fiber orientations in the structure.
Table 5. Strain Energy Function Coefficients for Abdominal Wall Tissue
SEF: Reduced Polynomial n=2
Tissue Direction 1 Tissue Direction 2
Coefficients C10 C20 D1 R2 C10 C20 D1 R
2
Units MPa MPa Mpa-1
MPa MPa Mpa-1
Specimen 1 4.40E-03 2.62E-02 1.14 0.985 1.02E-02 1.28E-02 0.99 0.972
Specimen 2 2.00E-04 4.50E-03 3.34 0.993 1.00E-02 1.24E-02 1.57 0.989
Specimen 3 7.82E-03 1.06E-02 2.58 0.999 1.50E-03 3.90E-03 3.66 0.954
Specimen 4 8.60E-04 2.00E-02 2.33 0.994 3.04E-02 4.32E-02 1.60 0.998
Specimen 5 1.80E-02 1.10E-02 1.12 0.934 1.76E-03 3.24E-02 1.14 0.995
Specimen 6 1.0E-09* 4.25E-03 1.54 0.913 1.30E-02 1.50E-02 1.40 0.997
Average 5.21E-03 1.27E-02 1.15 1.12E-02 1.99E-02 1.34
STD 0.01 0.01 0.01 0.01
* Unusual output value from ABAQUS software
44
Figure 20. Representative Tissue Experimental Response and the Material Model Fit.
A) Direction 1 B) Direction 2
Table 6. Strain Energy Function Coefficients for Polypropylene Soft Knit Mesh
SEF: Ogden n=1
Soft Mesh Direction 1 Soft Mesh Direction 2
Coefficients μ α D1 R2
μ α D1 R2
Units MPa MPa-1
MPa MPa-1
Specimen 1 2.80 7.20 0.134 0.976 0.817 4.74 0.253 0.992
Specimen 2 7.00 6.70 0.031 0.972 0.780 5.20 0.339 0.986
Specimen 3 4.80 7.20 0.050 0.867 0.840 5.00 0.266 0.953
Specimen 4 5.20 7.00 0.043 0.891 0.730 4.40 0.325 0.984
Specimen 5 1.100 4.40 0.234 0.968
Average 4.95 7.03 0.065 0.792 4.835 0.296
STD 1.72 0.24 0.047 0.05 0.35 0.04
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1
Str
ess
(MP
a)
Strain (mm/mm)
Specimen 3
Tissue Direction 1
Uniaxial Data
SEF R.P. n=2
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 6
Tissue Direction 2
Uniaxial Data
SEF R.P. n=2
A B
45
Figure 21. Representative Mesh Experimental Response and the Material Model Fit.
A) Direction 1 B) Direction 2
4.2 Lap Shear Tests for Obtaining Interfacial Strength
Biomechanical analyses conducted by lap shear tests measured the mechanical
strength of the mesh-tissue interface of the specimens. Maximum force required to cause
failure in the specimens, Figure 22, was normalized by the width of the mesh-tissue
specimen. Results of these experiments are displayed as a boxplot in
Figure 23 numerically summarized in Table 7. Comparisons of normalized force of
fixation technique at each individual time point was evaluated using a statistical Mann-
Whitney-Wilcoxon test with 95%, Table 8. An additional statistical test was performed
evaluating all fixation techniques and evaluation time points against each other using a 2-
way Anova test with 95% confidence, Table 9.
0
2
4
6
8
10
12
0 0.1 0.2 0.3 0.4
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 2
Mesh Direction 1
Uniaxial Data
SEF Ogden n=1
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 2
Mesh Direction 2
Uniaxial Data
SEF Ogden n=1
A B
46
Figure 22. Representative Lap Shear
This box plot in Figure 23 shows the spread of fixation strength resulting from the
lap shear experiments. At each later time point there is a clear increase in the average
fixation strength corresponding to the amount of tissue ingrowth which has occurred.
However, the sutured boxplots are clearly stronger then the glued counterparts. The solid
orange line marks the 16 N/cm load suggested by Grevious et al., to be the minimal
strength required for a successful abdominal repair (Grevious et al., 2006). Both fixation
methods pass this benchmark at 2 weeks while at 1 week only sutured specimens do.
Supporting this benchmark value, our uniaxial results also indicated that on average the
mesh and tissue could withstand a minimum strength around 16 N/cm before failing.
Therefore, it can be predicted that once a specimens normalized forces exceed the
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80
Load
(N
)
Extension (mm)
Lap Shear Test
24 Hours
1 Week
2 Weeks
47
benchmark value there is a higher likelihood that failure in the mesh-tissue specimen will
occur in either the mesh or tissue rather than in the interface.
Only at 24 hours did tested specimens fail most frequently in the mesh-tissue
interface. The sutured interface had a mean strength relatively higher than the glued
interface [9.4 +/- 2.4
STD and 5.3 +/- 3.6
STD]. The mean of the normalized
strength remained stronger in the sutured interface for each higher time point measured.
More notably, the load vs. extension graphs revealed a stiffer response in the sutured
specimens than to the glued specimens. This outcome was most prominent in the two-
week specimens, where the mesh was fully incorporated into the abdominal wall. Table 7
shows the average normalized force values from the lap shear tests. A complete table of
results from all specimens can be found in Appendix A.
The lap shear experiments failed the test for normal distribution performed in
Sigmaplot. Therefor the Mann-U test which compares ordinal sample distributions of two
populations was utilized for evaluating these tests. The analysis revealed a significant
difference between glued and sutured fixation at all-time points were sutured specimens
were relatively stronger. This difference in average strength was between 44-47 % higher
during early recovery, t ≤ 1 week; suggesting, that the comparably weaker fixation
strength reported in literature for fibrin glue fixation is nearly half of that associated with
sutured fixation.
An additional statistical test was done using a 2-way analysis of variance
(Anova), with 95% confidence, which compared fixation strength with both evaluated
time points and fixation techniques. The summary of this statistical tests are as follows.
There was a statistical difference between both fixation techniques and all evaluation
48
times. In addition there was a statistical difference between glued and sutured fixation, at
both, 1 and 2 weeks but none at 24 hours. Next, there was a statistical difference between
glued fixations at all time points except for 1 week comparison to 24 hours, where no
difference was reported. Last, there was a statistical difference between sutured fixations
between all-time points. A complete tabulated result from this Anova test is shown in
Table 9. Matlab code for determining peak force is shown in Appendix B and Matlab
code for obtaining the boxplot in Figure 23 is shown in Appendix C.
Figure 23. Summary of Results from Lap Shear Experiments
49
Table 7. Averaged Normalized Forces for Glued vs. Sutured Specimens
Healing Time Glue
Average +/- STD
Suture
Average +/- STD
24 Hours 5.7 ± 3.6
; (n = 8) 9.4 ± 2.4
; (n = 8)
1 Week 12.2 ± 4.0
; (n = 7) 23.1 ± 12.2
; (n = 8)
2 Weeks 22 ± 7.4
; (n = 9) 30.9 ± 7.4
; (n = 8)
Table 8. Statistical Analysis using Mann-Whitney-Wilcoxon U-test
Glue Suture
Z-
value P-Value UA UB 95 % Confidence Level
24
Hours n = 8 n = 8 -2.73 0.003 < P < 0.006 109 19 36 < U < 92 S.D.
1
Week n = 7 n = 8 2.03 0.021 < P < 0.042 10 46 13 < U < 43 S.D.
2
Weeks n = 9 n = 8 -1.97 0.024 < P < 0.049 57 15 18 < U < 54 S.D.
S.D. = Significant Difference; N.S.D. = No Significant Difference
Table 9. 2-Way Anova Test
Two Way Analysis of Variance
All Pairwise Multiple Comparison Procedures (POC with Holm-Sidak method) Comparisons
for factor: Comparison
Diff of Means
t P Critical Level
Significant
Time points
2 week vs. 24 hours 18.965 7.847 <0.001 0.017 Yes
1 week vs. 24 hours 10.292 4.126 <0.001 0.025 Yes
2 week vs. 1 week 8.673 3.525 0.001 0.05 Yes
Fixation T. Suture vs. Glue 7.86 3.917 <0.001 0.05 Yes
24 hours Suture vs. Glue 3.578 1.032 0.308 0.05 No
1 week Suture vs. Glue 11.129 3.102 0.003 0.05 Yes
2 week Suture vs. Glue 8.875 2.635 0.012 0.05 Yes
Glued
Fixation
2 week vs. 24 hours 16.316 4.844 <0.001 0.017 Yes
2 week vs. 1 week 9.8 2.805 0.008 0.025 Yes
1 week vs. 24 hours 6.516 1.816 0.076 0.05 No
Suture
Fixation
2 week vs. 24 hours 21.614 6.235 <0.001 0.017 Yes
1 week vs. 24 hours 14.068 4.058 <0.001 0.025 Yes
2 week vs. 1 week 7.546 2.177 0.035 0.05 Yes
50
One additional evaluation was performed comparing the lap shear and uniaxial
results. The mean strength for failure in the sutured specimens was 31.0 +/- 7.4
STD,
while the glued specimens failed on average at 22.0 +/- 7.4
STD. These averages
indicate that the tissue with glued mesh had the same mechanical response as excised
abdominal tissue, whereas the sutured specimens had relatively stiffer F-D mechanical
response (See Figure 24). Such differences in relative material strength of mesh-tissue
specimens and abdominal wall tissue indicate a regionally stiffer tissue response when
using sutured fixation (with Prolene sutures).
Figure 24. Typical Mesh-Tissue Response at 2 weeks vs. Excised Tissue Response
In the lap shear experiments the reported normalized max force indicates the
sample has begun to fail. Failures however does not necessary have to occur in the
interface and can just as easily occur in the materials, i.e., the bound tissue or mesh, as
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6 0.8 1
Norm
ali
zed
Load
(N/c
m)
Extension (cm)
2 Week Sutured
2 Week Glued
Direction 1 Tissue Uniaxial
Direction 2 Tissue Uniaxial
51
seen in Figure 25. These locations vary during different stages of wound healing and play
a role in identifying how well the mesh was incorporated into the tissue structure. In the
case when a majority of failure is no longer occurring in the interface the normalized
loads can no longer be reported as the mesh-tissue interfacial strength. The locations at
which the tested specimens failed are summarized in Table 10.
.
Figure 25. Failure modes. Left: Mesh failure, Center: Interface failure, Right: Tissue
failure
At 24 hours, the interface failed for glued samples 87.5% of the time, as was
previously predicted for this level of tissue ingrowth. For the sutured samples the
interface failed 60% of the time, while the other 40% was failure in the tissue region.
Two reasons exist that could cause failure at the tissue region. The first reason for failure
would be that enough tissue ingrowth had occurred that the stress was no longer
concentrated in the interface, and the second reason would be that there was some level
52
of tissue damage caused by the suturing which resulted in stress build up at the damaged
tissue region and ultimately led to failure in the tissue. On the other hand, at 24 hours the
glued specimens still had an observable glossy layer of glue surrounding the mesh pieces
and had not fully dissolved, this protective layer would limit the amount of tissue
ingrowth possible between the mesh and tissue. The sutured specimens had no such layer
and tissue ingrowth with mesh was able to occur immediately.
At 1 week, both types of specimens had enough time to form an adequately strong
interface. However, the level of tissue ingrowth varied from specimen to specimen and
depended on the animal model and that individual animal’s rate of healing. It would be
predicted at this stage for results to be in a binomial distribution where the average
failures occurred within the tissue. This was indeed observed in the glued specimens;
nearly 55% of the time the tissue failed, 30% of these failures were at the interface and
15% of the time they were in the mesh. Results were far more variable in the sutured
samples; where 38% of the time the mesh failed, 24% the tissue did, and 38% the
interface failed. Standard deviation of the normalized force at failure was also high for
sutured specimens at 1 week, with the samples ranged from mesh almost fully embedded
to samples with barely any tissue ingrowth. One major concern of sutured fixation, which
could cause a delay in tissue ingrowth, was mesh curling; all samples with failed
interfaces at 1 week time points had nearly no tissue ingrowth and the sutured specimens
had observable mesh curling. Regardless, our failure mode analyses suggest that as early
as 1 week there was an adequate amount of tissue ingrowth such that the interfacial
strength was stronger than the bound material substrates.
53
At 2 weeks mesh pieces were well embedded into the tissue and presented great
difficulty in separating mesh from tissue when preparing samples for lap shear tests.
Nearly full tissue ingrowth would be characterized at this stage and stress propagation
would be expected to occur along the mesh in the lap shear test. In the glued specimens,
there was a 100% failure at the mesh. For the sutured samples there was a 75% failure in
the mesh and 25% failure in the tissue.
Table 10. Lap Shear Test Failure Mode Occurrences
Mesh Interface Tissue
# Failed Percentage # Failed Percentage # Failed Percentage
24 Hours
Glue 0 0% 7 87.5% 1 12.5%
Suture 0 0% 5 62.5% 3 37.5%
1 Week
Glue 1 14% 2 29% 4 57%
Suture 3 38% 2 24% 3 38%
2 Weeks
Glue 9 100% 0 0% 0 0%
Suture 6 75% 2 25% 0 0%
4.3 Computational Study using FEA to Simulate Mechanical Experiments
4.3. A. Uniaxial Simulation with FEA
Finite element analysis was used to create a uniaxial simulation of the mesh and
tissue specimens (Figure 26). The force-displacement data obtained from the uniaxial
tests were compared to the sum of the reaction forces at the pinned tissue region of an
FEA model created in ABAQUS (Figure 28). The uniaxial model showed a reasonable
fit to the experimental results, which validated the individual models for mesh and tissue.
The goodness of fit was determined using the coefficient of determinate value R2, Table
13. All results are included in Appendix F and Appendix I. FE mesh size was optimized
54
such that computational time was reduced without drastically influencing the output
results. This was done by performing a mesh convergence study on surgical mesh and
abdominal tissue uniaxial simulations, Figure 27/Table 11 and Figure 29/Table 12. It was
decided from these convergence studies that 1500 elements for tissue models and 160
elements for the surgical mesh was sufficient for the simulations.
Figure 26. Stress contour plot A) Surgical Mesh B) Abdominal Tissue
A B
55
Figure 28. Representative Tissue Experimental Response and the FEA Model Results.
A) Direction 1 B) Direction 2
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1
Str
ess
(MP
a)
Strain (mm/mm)
Specimen 3
Tissue Direction 1
Uniaxial Data
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 6
Tissue Direction 2
Uniaxial Data
FEA Simulation
Table 11. Mesh Convergence Study for Tissue
ELEMENTS Sum(σ) Run Time
24486 3.11E+00 < 10 min
10000 3.11E+00 < 5 min
1914 3.13E+00 < 30 sec
1500 3.14E+00 < 30 sec
864 3.15E+00 < 30 sec
640 3.16E+00 < 30 sec
540 3.17E+00 < 30 sec
416 3.18E+00 < 30 sec
165 3.19E+00 < 30 sec
140 3.21E+00 < 30 sec
80 3.25E+00 < 30 sec
A B
3.1
3.1
3.2
3.2
3.3
3.3
0 10000 20000
Su
m o
f N
om
inal
Str
ess
(N/c
m2)
# of Elements
Mesh Convergence for
Tissue Model
Figure 27. Mesh Convergence Study for Tissue
Model
56
Figure 29. Mesh Convergence Study for Surgical Mesh Models
Figure 30. Representative Mesh Experimental Response and the FEA Model Results.
13.0940
13.0945
13.0950
13.0955
13.0960
13.0965
13.0970
13.0975
13.0980
13.0985
0 1000 2000 3000Su
m o
f N
om
inal
Str
ess
(N/c
m2)
# of Elements
Mesh Convergence for Surgical
Mesh
0
2
4
6
8
10
12
0 0.1 0.2 0.3 0.4
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 2
Mesh Direction 1
Uniaxial Data
FEA Simulation
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 2
Mesh Direction 2
Uniaxial Data
FEA Simulation
Table 12. Mesh Convergence Study
for Surgical Mesh
ELEMENTS Sum(stress) Time
3240 1.31E+01 < 5 min
1800 1.31E+01 < 1 min
561 1.31E+01 < 30 sec
450 1.31E+01 < 30 sec
378 1.31E+01 < 30 sec
288 1.31E+01 < 30 sec
210 1.31E+01 < 30 sec
162 1.31E+01 < 30 sec
120 1.31E+01 < 30 sec
84 1.31E+01 < 30 sec
45 1.31E+01 < 30 sec
18 1.31E+01 < 30 sec
6 1.31E+01 < 30 sec
A B
57
4.3. B. Lap Shear Simulation with FEA
Finite element analysis (FEA) was used to create a lap shear simulation of the 2
week glued test specimens, Figure 31. The load to extension data obtained from the lap
shear tests were compared to the FEA simulation. In Figure 32A, there is a noticeable
offset between the simulation and the actual test data. The initial toe in region of the lap
shear results where rate of load to extension is exceptionally low can be associated with
both material (i.e. some uncontrolled property of healing tissue) and experimental
conditions (i.e. slack in the sample, fluid leakage from specimen, etc.). Whatever the
condition may be that causes this response, it did not occur in the uniaxial experiments
(mesh and tissue) and so was not captured in the SEF’s mechanical response.
For the most part, it was clear the FEA simulation had a stiffer response in
comparison to the actual response. Representative results are shown in Figure 32. All
results are included in Appendix K. Goodness of fit, R2 value, was determined using the
coefficient of determinate method as described in the Equation 4. There was a significant
variation between the R2 values reported for all models. Most frequently the simulation
Table 13. Goodness of Fit of Experimental Data with Uniaxial Simulation
Specimen
Tissue
Direction 1
R2
Tissue
Direction 2
R2
Mesh
Direction 1
R2
Mesh
Direction 2
R2
1 0.992 0.983 0.584 0.826
2 0.976 0.979 0.978 0.843
3 0.990 0.910 0.974 0.932
4 0.942 0.995 0.984 0.862
5 0.977 0.983 0.942
6 0.987 0.998
58
and experimental results were within 6% difference of one another; however R2 varied
significantly between models ranging from worst case of 64.3% difference to best case at
1.6% difference. The full list of R2 values for lap shear simulations are given in Table 14.
The number of elements used in these models was approximately 1500 for the
tissue part and 460 for the surgical mesh part; these values were determined by a mesh
convergence study performed for each lap shear models created. Since there was a
separate model with customized parts created for every single specimen all the individual
mesh convergence studies are not shown in this thesis. However, Figure 33 and Table 15
provide a sample of one of the mesh convergence studies, specimen 2_4.
Figure 31. FEA Model Reaction Force Contour Plot
59
Figure 32. Comparison of Experimental and FEA Results of Lap Shear Experiments.
Lap Shear Model A) Sub-Optimal Fit and B) A Good Fit by Visual Inspection.
Table 14. Goodness of Fit of FEA Simulation to Experimental Data
Specimen # R2 % Difference
Specimen 1_1 0.357 (64.3%)
Specimen 1_3 0.836 16.4%
Specimen 1_8 0.938 6.2%
Specimen 2_3 0.946 5.4%
Specimen 2_4 0.469 53.1%
Specimen 2_7 0.944 5.6%
Specimen 2_8 0.546 45.4%
Specimen 9_4 0.984 (1.6%)
Specimen 9_5 0.831 16.9%
0
5
10
15
20
25
30
35
40
0 10 20
Load
, N
Displacement, mm
Specimen 2_8
Lap Shear…FEA…
0
10
20
30
40
50
60
70
0 5 10
Load
, N
Displacement, mm
Specimen 9_4
Lap Shear Data
A B
60
Figure 33. Mesh Convergence Study for Lap Shear Models
112
113
114
115
116
117
118
0 2000 4000 6000Su
m o
f R
eact
ion
Forc
e (N
)
# of Elements
Mesh Convergence for Lap Shear
Model
Specimen 2_4
Table 15. Mesh Convergence Study for Lap Shear Models
Elements
in tissue
Elements
in mesh
Total # of
elements
Sum(RF) Run time
4070 700 4770 113.023 < 7 min 3400 460 3860 113.074 < 5 min 1500 460 1960 113.816 < 2 min 858 195 1053 114.896 < 1 min 627 154 781 115.033 < 30 sec 540 130 670 115.132 < 30 sec 288 108 396 117.29 < 30 sec 225 80 305 117.335 < 30 sec 168 63 231 117.707 < 30 sec
61
Chapter 5: Conclusions
5.1. Conclusions
The average normalized load with sutures is stronger than glued at all-time points.
At or greater than 1 week time point, the fixation strength became independent of the
technique and failure occurred in the materials rather than the interface.
After 2 weeks of recovery, the glued specimens show nearly identical load
measurement readings to the material strength of healthy tissue.
At 2 week recovery, the sutured samples exhibited interface strength globally stiffer
than what is characterized with healthy tissue while glued specimens showed a nearly
identical F-D response.
The strain energy model used to mimic the material response under loading was
comparably similar to experimental data such that FEA uniaxial simulations
frequently reported σ-ɛ values less than 9% difference of the experimental data.
Modeling approach used for mimicking lap shear experiments was not always
comparable to the experimental data and would require further investigation to
develop a consistent model.
5.2. Clinical Significance
Lower interfacial strength at earlier time points, 24 hours, has a significant
relevance for susceptibility of reoccurrence due to mesh migration. However both
techniques (sutures or adhesive) lead to a mesh-tissue interfacial strength lower than the
benchmark value of 16 N/cm, at this early stage. At two weeks, every sample tested had a
mesh-tissue interface exceedingly stronger than the mesh or tissue materials. Samples
fixated using sutures had a far stiffer global F-D response and higher strength value than
62
associated with healthy tissue, such that the strength values more closely resembled
relative material strength of polypropylene sutures.
63
Chapter 6 Future Work
6.1 Mechanical Experiments and Sample Preparation
6.1. A. Future Work
It would be clinically significant to repeat experiments using earlier time points
(t ≤ 1 week), where fixation technique is still relevant toward the interfacial
strength.
Additionally, it would be clinically significant to perform experiments where the
advantages of using adhesive fixation are compared to the disadvantages of the
relatively weaker fixation strength at these earlier time points.
For future studies, it may be beneficial to perform tests comparing the efficiency
of biodegradable suture fixation with fibrin glue fixation post 1 week recovery, in
order to determine if the higher strength values obtained with sutured specimens
was due to the use of non-degradable sutures.
Other potential studies that could be informative and helpful for medical
practitioners include: A) Testing different coating techniques for glue applications
in order to determine if there is any relation with coating technique and fixation
strength; B) Testing different biological glues other than Tisseel fibrin glue to see
if another product is more efficient for mesh repair operations; C) Testing
alternative surgical meshes besides BARD soft knit polypropylene mesh to see if
another product reacts better with adhesive glue for fixation on soft tissue.
Isotropic mesh should be utilized in any future studies where mesh is implanted
onto healthy tissue. The significance of anisotropic mesh is in resisting the higher
stress concentration along the tissue direction that is perpendicular to the direction
64
the hernia is propagating. In the case where a study is not done on a herniated
tissue region the benefit of having the anisotropic mesh becomes irrelevant.
6.1. B. Limitations that need to be addressed in Future Studies
Surgical procedures used for suture fixation have four evenly-spread surgical
knots tying the corners of the mesh piece to the abdominal tissue. In common
surgical practice for VHMR operations, sutures are generally knitted into the
mesh and abdominal wall such that the sutures follow through the entire perimeter
of the mesh. This technique would uniformly holds the mesh flat onto the
abdominal wall and could potentially influence better fixation.
Future work should have control samples, which evaluate mesh-tissue interface
with no fixation. This would determine the level of tissue ingrowth that would
have occurred naturally under the absence of any anchoring or adhesive materials
used for fixation.
Greater precautions can be taken in future studies to prevent infection, blood
clotting, or other such biological responses that artificially reduce the natural
strength of the implant.
For the lap shear clamping tests, a scalpel was used to free the mesh from tissue,
possibly creating many artificial microscopic tears and nicks in the tissue
interface before testing. Additionally, the level of tissue ingrowth after 1 week
made it difficult to clear all the tissue from the mesh before clamping. It is
unlikely that the mesh cleared for clamping was completely unbound from all
tissue fibers.
65
Inconsistency on amount of tissue shaved off samples to allow fitting in clamps
also added potential error to the tests. Too much tissue shaved off specimens
would create an area of weakness where failure could occur while too little tissue
would cause samples to slip out of the clamps during clamp tightening. Future
studies should consider a more consistent method of preparing specimens for
testing.
The most common source of loading on the mesh-tissue repair site of the
abdominal wall is due to intra-abdominal pressure. Therefore, future studies may
consider using burst tests for biomechanical evaluation to accompany shear
experiments.
Lastly, one limitation during sample preparation was the difficulty in visually
determining the mesh piece location of 2 week specimens. Use of color coated
mesh or other identification method would counter this limitation. Chances of
accidently cutting and damaging mesh as well as accidently clamping portions of
tissue or mesh into the wrong clamp is significantly higher in the 2 week samples.
6.2 Finite Element Models for Computational Simulations
6.2. A. Next Step toward Developing Full Robust Model of a VHMR
A future step is using the material models and interfacial fit to developing a more
robust model of a VHMR abdomen. A full model of an abdomen with a hernia
defect zone and mesh sealant would help provide a macro scale visualization of
how stress distribution or potential failure would occur. The clinical significance
of this will be that such a model would be able to provide a general idea on how
variation in internal pressure and other loading mechanisms due to trunk
66
movement will influence the stress concentration at the repair site. It can also
provide practitioners with a general idea of whether the patient is at high risk of
suffering from a hernia reoccurrence due to mesh failure.
6.2. B. Limitations in Current Models that should be addressed in Future Work
Further investigation should be done to determine the true Poisson’s ratio of the
surgical mesh and abdominal tissue.
Surgical mesh was modeled as a 3D continuous homogenous shell part instead of
as a knitted fiber with large pores. Similarly, abdominal tissue was modeled as an
isotropic homogenous material rather than a heterogeneous one. Future models
should better account for the heterogeneity of tissue and monofilament fiber
structure of mesh.
The interfacial fit was performed by tying the surface of the mesh and tissue
together instead of embedding the mesh in tissue as it occurs in vivo. Interfacial fit
will have a different mechanical response than that of an embedded implant.
Goodness of fit between FEA simulation and lap shear experiments were
inconsistent. FEA models should be further improved so that computational
simulations better resemble the experimental data.
Future models could also be improved by taking into account the material
properties associated with muscle-active tension
67
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71
Appendix A
Experiment Results
The following is tables include the results and specimen dimensions recorded for all the
mechanical tests performed.
2 Weeks Lap Shear Experiments
Date Sampl
e Fixation
Techniq
ue
Heigh
t Wid
th Gauge
Length Max
Force Normalized Max
Force Slop
e Failure
Mode
Remove
d
cm cm cm N
5/1/12 73-1 Glue 2.8 1.5 1.5 27.83 18.55 3.86 Mesh
5/1/12 74-1 Glue 3 3 2.5 50.31 16.77 1.87 Mesh
5/2/12 75-1a Glue 3.5 2 2.3 27.06 13.53 9.36 Mesh
5/2/12 75-1b Glue 3.5 2 2.4 43.27 21.63 4.35 Mesh
5/2/12 76-1a Glue
1.5 2.5 26.62 17.75 5.45 Mesh
5/2/12 76-1b Glue
1.5 2.3 39.41 26.27 5.29 Mesh
5/9/12 80-1 Glue 2.5 2.5 1.8 48.21 19.28 6.21 Mesh
5/9/12 79-1a Glue 3.3 2 3 50.88 25.44 8.86 Mesh
5/9/12 79-1b Glue 3.3 1.5 2.8 57.91 38.60 10.7
9 Mesh
5/1/12 73-2 Suture 3 1.5 2.5 55.56 37.04 3.00 Tissue
5/1/12 74-2 Suture
3 3.7 53.37 17.79 8.00 Mesh
5/2/12 75-2a Suture 2.5 1.5 1.5 41.30 27.53 4.64 Mesh
5/2/12 75-2b Suture 2.5 1.7 1.5 40.17 23.63 20.3 Mesh
5/2/12 76-2a Suture
1.5 3.6 57.66 38.44 0.99 Tissue
5/2/12 76-2b Suture
1.5 4.3 75.04 50.03 5.82 Tissue Outlier
5/8/12 78-2 Suture 5 3 3 102.0 34.01 7.61 Tissue
5/9/12 80-2 Suture 4 3 2.5 92.45 30.82 10.6 Mesh
5/9/12 79-2 Suture 3 2 2.6 75.17 37.58 9.16 Mesh
5/8/12 77-2 Suture 5 3.8 3 61.76 16.25 2.78 Mesh Damag
ed
72
1 Week Lap Shear Experiments
Date Sam
ple
Fixation
Techniq
ue
Hei
ght
Wi
dth
Gauge
Length
Max
Force
Normalized
Max Force Slope
Failure
Mode
Remo
ved
cm cm cm N
6/6/12 53-1 Glue 4 3 3.3 31.10 10.37 2.18 Tissue
6/12/12 55-1 Glue 5 4 4 50.82 12.70 4.49 Interface
6/12/12 56-1 Glue
3.9 5 51.05 13.09 3.03 Mesh
6/13/12 58-1 Glue 5 4 4.2 58.18 14.54 1.48 Tissue
6/27/12 59-1 Glue 5.5 4.5 4.5 35.86 7.97 3.01 Tissue
6/27/12 60-1 Glue 4.5 5 3.5 76.53 15.31 6.75 Interface
6/27/12 61-1 Glue 5 4 4.5 45.12 11.28 3.28 Tissue
6/13/12 57-1 Glue 4.5 4.2 4.1 90.10 21.45 8.32 Tissue Outlie
r
6/12/12 56-2 Suture
3 5 53.39 17.80 2.41 Interface
6/13/12 57-2 Suture
3 4.1 58.08 19.36 6.05 Mesh
6/27/12 59-2 Suture 5 4 4.5 47.39 11.85 4.35 Tissue
6/27/12 60-2 Suture 4.5 3.2 4 61.52 19.23 5.23 Interface
6/27/12 61-2 Suture
3.5 4 34.71 9.92 3.13 Tissue
6/6/12 53-2 Suture 3 2 2.2 78.18 39.09 4.91 Mesh
6/12/12 55-2 Suture 4.5 2.5 4.8 108.6 43.45 7.47 Interface
6/13/12 58-2 Suture 3.7 4 3.3 103.1 25.77 8.56 Mesh
6/5/12 51-2 Suture 4.5 2.6 2.5 55.24 21.25 2.76 Interface Infect
ed
73
24 Hours Lap Shear Experiments
Date Sampl
e
Fixatio
n
Techni
que
Hei
ght
Wi
dth
Gauge
Length
Max
Force
Normalized
Max Force
Slo
pe
Failure
Mode
Remo
ved
cm cm cm N
5/8/12 82-1 Glue 5.2 3.5 4.3 12.02 3.43 0.67 Interface
5/8/12 81-1 Glue 5.1 3.4 4 9.49 2.79 1.11 Interface
5/11/12 83-1 Glue 5.1 3.6 4.1 18.45 5.13 1.48 Interface
5/11/12 84-1 Glue 5.1 3.6 4 7.74 2.15 0.58 Interface
5/15/12 97-1 Glue 5 3.8 3.5 32.51 8.56 2.65 Interface
5/15/12 98-1 Glue 4.5 4 4 30.23 7.56 1.31 Interface
5/17/12 99-1 Glue
3.7 4.5 12.30 3.32 1.67 Tissue
5/17/12 100-1 Glue 5.2 3.8 4.2 47.01 12.37 3.49 Interface
5/8/12 81-2 Suture 5 3 3 29.20 9.73 2.00 Interface
5/8/12 82-2 Suture 5 3.4 3 18.97 5.58 1.21 Interface
5/11/12 83-2 Suture 5.2 3.4 4 39.87 11.73 1.69 Interface
5/11/12 84-2 Suture 5.1 3.6 3.6 27.92 7.75 1.21 Tissue
5/15/12 98-2 Suture 4.5 3.8 4 37.80 9.95 1.21 Tissue
5/17/12 100-2 Suture 5 3.7 3.2 43.72 11.82 1.60 Interface
5/17/12 99-2 Suture 4.8 3.6 3.5 33.86 9.41 1.32 Tissue
5/17/12 97-2 Suture 5 3.5 4.5 81.64 23.32 3.28 Interface Outlier
Uniaxial Experiments for Abdominal Tissue Material Strength Sample
Type
Orie
nt.
S
p.
Heig
ht
Wid
th
Thickn
ess
Gauge
Length
Max
Force
Normalized Max
Force
Slo
pe Failu
re
Site # (cm) (cm) (cm) (cm) (N)
Tissue D1 1 5 4 0.5 < t
< 1 3.5 122.2 30.55
5.2
0 S.C.
Tissue D1 2 5 4 0.5 < t
< 1 2.5 48.2 12.04
3.8
1 S.C.
Tissue D1 3 5 4 0.5 < t
< 1 2.5 110.3 27.57
3.3
6 S.C.
Tissue D1 4 5 4 0.5 < t
< 1 3 100.2 25.04
4.7
7 C.S.
Tissue D1 5 5 4 0.5 < t
< 1 2.5 89.7 22.43
3.1
2 D.C.
Tissue D1 6 5 4 0.5 < t
< 1 2.5 56.0 14.00
4.0
5 D.C.
Average Normalized Force +/- Standard Deviation
21.94 +/- 7.44 (N/cm)
74
Uniaxial Experiments for Abdominal Tissue Material Strength Sample
Type
Orie
nt.
S
p.
Heig
ht
Wid
th
Thickn
ess
Gauge
Length
Max
Force
Normalized Max
Force
Slo
pe
Failu
re
# (cm) (cm) (cm) (cm) (N)
Site
Tissue D2 1 5 4 0.5 < t
< 1 3 55.09 13.77
2.41
S.C.
Tissue D2 2 5 4 0.5 < t
< 1 3 72.33 18.08
5.27
C.S.
Tissue D2 3 5 4 0.5 < t
< 1 3.5 57.48 14.37
3.02
D.C.
Tissue D2 4 5 4 0.5 < t
< 1 2 78.28 19.57
3.40
S.C.
Tissue D2 5 5 4 0.5 < t
< 1 3.5 55.62 13.90
4.34
S.C.
Tissue D2 6 5 4 0.5 < t
< 1 2.5 75.41 18.85
4.15
S.C.
Average Normalized Force +/- Standard Deviation
16.43 +/- 2.69 (N/cm)
Orient. = Orientation; SP. = specimen; S.C. = Specimen region at Stationary Clamp; D.C. = Specimen Region at Dynamic Clamp; C.S. = Center of specimen
Uniaxial Experiments for BARD Soft Knitted Polypropylene Surgical
Mesh Material Strength Sample
Type
Ori
ent.
Sp
. Height Width
Thick
ness
Gauge
Length
Max
Force
Normalized Max
Force Slope
Failu
re
# (cm) (cm) (cm) (cm) (N)
Site
PP Mesh D1 1 6 3 0.044 2 142.9 47.6 16.4 S.C.
PP Mesh D1 2 6 3 0.044 1.9 145.7 48.6 21.7 S.C.
PP Mesh D1 3 5.9 3 0.044 1.8 138.6 46.2 19.9 S.C.
PP Mesh D1 4 6 3 0.044 2 136.3 45.4 14.6 D.C.
PP Mesh D1 5 6 3 0.044 2 153.9 51.3 16.8 C.S.
Average Normalized Force +/- STDev 47.83 +/- 2. 29 (N/cm)
PP Mesh D
2 1 6 3 0.044 2 53.0 17.7 3.8 S.C.
PP Mesh D
2 2 6 3 0.044 2 57.0 19.0 2.8 S.C.
PP Mesh D
2 3 6 3 0.044 2 45.7 15.2 2.8 C.S.
PP Mesh D
2 4 6 3 0.044 2 46.4 15.5 2.5 S.C.
PP Mesh D
2 5 6 3 0.044 2 53.4 17.8 3.1 D.C.
Average Normalized Force +/- STDev 17.03 +/- 1.62 (N/cm)
Orient. = Orientation; SP. = specimen; S.C. = Specimen region at Stationary Clamp; D.C. = Specimen Region at Dynamic Clamp; C.S. = Center of specimen
75
Appendix B
Matlab Code: F-D Data Evaluation
The following is the Matlab code used for evaluating the raw data files retrieved
from the Bluhill2 software for the mechanical tests. This graph evaluated the force and
displacement data points to determine the maximum force at failure, the slope of the F-D
graph to the first peak, and the area under the graph up to the first peak.
% This file opens the .csv files (Raw data files from Bluehill2 software) and obtains F-D results
clear all
clc
close all
dirName = ('03082013');
filesInDir = dir(dirName);
numFiles = length(filesInDir);
%Bard mesh max load:
maxLoad = 25*9.81 / 2.2;
cc=0;
for ii=1:14;
if length(filesInDir(ii).name) > 2 & filesInDir(ii).name(1:2)=='Sp'
cc=cc+1;
fileName = [dirName '/' filesInDir(ii).name];
[num txt raw] = xlsread(fileName);
data = num(:,:);
data(:,1) = num(:,2);
data(:,2) = num(:,3);
sizeN(cc,:) = size(data);
figure(1)
plot(data(:,1),data(:,2),'b')
[maxMag(cc) maxInd(cc)] = max(data(:,2));
[maxX(cc) maxY(cc)] = ginput(1);
[minX(cc) minY(cc)] = ginput(1);
indMax(cc) = min(find(((maxX(cc)-0.01) < data(:,1)) & (data(:,1) < (maxX(cc)+0.01 ))));
indMin(cc) = min(find( minX(cc)-0.01 < data(:,1) & data(:,1) < minX(cc)+0.01 ));
dataNew = data(indMin(cc):indMax(cc),:);
76
% The slope:
P(cc,:) = polyfit(dataNew(:,1),dataNew(:,2),1);
Y = polyval(P(cc,:),dataNew(:,1));
figure(3)
plot(dataNew(:,1),dataNew(:,2),'r')
hold on
title(num2str(P(cc,1)))
plot(dataNew(:,1),Y,'k')
hold off
figure(10)
plot(data(:,1),data(:,2),'b')
axis([ 0 100 0 60])
grid on
figure(2)
subplot(7,2,cc)
plot(data(:,1),data(:,2),'b')
axis([ 0 100 0 60])
grid on
% Finding the first maxima that is within 40mm.
[yMax(cc) yInd] = max(data(:,2) .* (data(:,1) < 40))
% Area under the curve:
z(cc) = trapz(data(1:yInd,1),data(1:yInd,2))
hold on
plot(dataNew(:,1),Y,'r')
title([fileName(10:17) ' Slope =' num2str(P(cc,1)) ' N/mm'])
hold off
figure(5)
plot(data(:,1),data(:,2))
hold on
plot(data(yInd,1),data(yInd,2),'ro')
pause(1)
else
continue
end
end
77
Appendix C
Matlab Code: Box Plots
The following is the Matlab code used for evaluating the normalized mas force
results from the lap shear experiments. This code performed an anova test which was
used as an initial statistical approach to determine difference between data sets. This test
was also used to identify and remove outliers from the data set before final averages were
determined.
close all
%
24 Hour Glue Vs Suture
glue24 = ...
[ 3.43 2.79 5.13 2.15 8.56 7.56 3.32 12.37 4.16 9.79 1.33 6.02 6.89 1.77 4.72 4.55]; %Normalized Max
Force 24 hours Glue
suture24 = ...
[ 9.73 5.58 11.73 7.75 9.95 11.82 9.41 7.96 NaN NaN NaN NaN NaN NaN NaN NaN];%Normalized Max
Force 24 hours Suture
%%
close all
glue1w = ...
[ 10.37 12.70 13.09 14.54 7.97 15.31 11.28 NaN NaN NaN NaN NaN NaN NaN NaN NaN];%Normalized
Max Force 1 weeks Glue
suture1w = ...
[ 17.80 19.36 11.85 19.23 9.92 39.09 43.45 25.77 NaN NaN NaN NaN NaN NaN NaN
NaN];%Normalized Max Force 1 weeks Suture
%%
close all
glue2w = ...
[ 18.55 16.77 13.53 21.63 17.75 26.27 19.28 25.44 28.60 38.60 NaN NaN NaN NaN NaN
NaN];%Normalized Max Force 2 weeks Glue
suture2w = ...
[ 37.04 17.79 27.53 23.63 38.44 50.03 34.01 30.82 37.58 NaN NaN NaN NaN NaN NaN
NaN];%Normalized Max Force 2 weeks Suture
%%
figure
[h pi ci] = anova1([glue24(1,:); suture24(1,:); glue1w(1,:); suture1w(1,:); glue2w(1,:); suture2w(1,:)]'); %
Anova, T-test
title('Normalized Force')
axis([0 7 -5 65])% 0-7 x-axis (catagories), -5-65 Y-axis (normalized force magnitude)
78
Appendix D
ABAQUS INP File: Tissue Uniaxial Model
The following is an ABAQUS Standard input file for the computational uniaxial
simulation of the excised abdominal tissue. The complete list of nodes and elements has
been removed in order to shorten the document.
*End Part
**
**
** ASSEMBLY
**
*Assembly, name=Assembly
**
*Instance, name=TissueUniaxial-1, part=TissueUniaxial
*Node
[REMOVED]
*Element, type=C3D8H
[REMOVED]
*Nset, nset=Set-1, generate
1, 2184, 1
*Elset, elset=Set-1, generate
1, 1500, 1
** Section: Section-1
*Solid Section, elset=Set-1, material=Material-1
,
*End Instance
**
*Nset, nset=Set-1, instance=TissueUniaxial-1
[REMOVED]
*Elset, elset=Set-1, instance=TissueUniaxial-1, generate
901, 1200, 1
*Nset, nset=Set-2, instance=TissueUniaxial-1
[REMOVED]
*Elset, elset=Set-2, instance=TissueUniaxial-1, generate
1201, 1500, 1
79
*End Assembly
**
** MATERIALS
**
*Material, name=Material-1
*Hyperelastic, n=2, reduced polynomial
0.0102, 0.0128, 0.99, 0.
** ----------------------------------------------------------------
**
** STEP: Step-1
**
*Step, name=Step-1, nlgeom=YES
*Static, direct
0.1, 1.,
**
** BOUNDARY CONDITIONS
**
** Name: BC-1 Type: Displacement/Rotation
*Boundary
Set-1, 2, 2, 3.
** Name: BC-2 Type: Symmetry/Antisymmetry/Encastre
*Boundary
Set-2, ENCASTRE
**
** OUTPUT REQUESTS
**
*Restart, write, frequency=0
**
** FIELD OUTPUT: F-Output-1
**
*Output, field, variable=PRESELECT
**
** HISTORY OUTPUT: H-Output-1
**
*Output, history, variable=PRESELECT
*End Step
80
Appendix E
Strain Energy Model vs. Uniaxial Tissue Experiments
The following content include all the graphs for each individual strain energy
material model created for each individual abdominal tissue specimen.
81
82
83
Appendix F
FEA Uniaxial Simulation vs. Uniaxial Tissue Experiments
The following content include all the graphs for each individual FEA abdominal
tissue uniaxial simulation and relates it to the experimental data.
84
85
86
Appendix G
ABAQUS INP File: Surgical Mesh Uniaxial Model
The following is an ABAQUS Standard input file for the computational uniaxial
simulation of the surgical mesh. The complete list of nodes and elements has been
removed in order to shorten the document.
*Heading
** Job name: Surgical Mesh Model name: Model-1
** Generated by: Abaqus/CAE 6.12-1
*Preprint, echo=NO, model=NO, history=NO, contact=NO
**
** PARTS
**
*Part, name=Mesh
*End Part
**
**
** ASSEMBLY
**
*Assembly, name=Assembly
**
*Instance, name=Mesh-1, part=Mesh
*Node
[REMOVED]
*Element, type=S4R
[REMOVED]
*Nset, nset=Set-2, generate
1, 496, 1
*Elset, elset=Set-2, generate
1, 450, 1
** Section: Mesh
*Shell Section, elset=Set-2, material=Mesh
0.044, 5
*End Instance
**
*Nset, nset=Set-1, instance=Mesh-1
[REMOVED]
*Elset, elset=Set-1, instance=Mesh-1, generate
87
151, 300, 1
*Nset, nset=Set-2, instance=Mesh-1
[REMOVED]
*Elset, elset=Set-2, instance=Mesh-1, generate
1, 150, 1
*End Assembly
**
** MATERIALS
**
*Material, name=Mesh
*Hyperelastic, ogden
2.7, 7.4, 0.19
** ----------------------------------------------------------------
**
** STEP: Step-1
**
*Step, name=Step-1, nlgeom=YES
*Static, direct
0.1, 1.,
**
** BOUNDARY CONDITIONS
**
** Name: BC-1 Type: Displacement/Rotation
*Boundary
Set-1, 2, 2, 2.
** Name: BC-2 Type: Symmetry/Antisymmetry/Encastre
*Boundary
Set-2, ENCASTRE
**
** OUTPUT REQUESTS
**
*Restart, write, frequency=0
**
** FIELD OUTPUT: F-Output-1
**
*Output, field, variable=PRESELECT
**
** HISTORY OUTPUT: H-Output-1
**
*Output, history, variable=PRESELECT
*End Step
88
Appendix H
Strain Energy Model vs. Uniaxial Surgical Mesh Experiments
The following content include all the graphs for each individual strain energy
material model created for each individual surgical mesh specimen.
89
)
90
Appendix I
FEA Uniaxial Simulation vs. Uniaxial Surgical Mesh Experiments
The following content include all the graphs for each individual FEA surgical mesh
uniaxial simulation and relates it to the experimental data.
91
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1
Str
ess,
(M
Pa)
Strain, (mm/mm)
Specimen 5
Mesh Direction 2
(R2 = 0.942)
Uniaxial Data
92
Appendix J
ABAQUS INP File: Lap Shear Model
The following is a sample ABAQUS Standard input file for the computational lap
shear simulation (specimen 2_7). The complete list of nodes and elements has been
removed in order to shorten the document.
*Heading
** Job name: sp27 Model name: Model-1
** Generated by: Abaqus/CAE 6.12-1
*Preprint, echo=NO, model=NO, history=NO, contact=NO
** PARTS
*Part, name=Mesh
*End Part
*Part, name=Tissue
*End Part
** ASSEMBLY
*Assembly, name=Assembly
*Instance, name=Mesh-1, part=Mesh
*Node
[REMOVED]
*Element, type=S4R
[REMOVED]
*Nset, nset=Mesh, generate
1, 468, 1
*Elset, elset=Mesh, generate
1, 425, 1
** Section: Mesh
*Shell Section, elset=Mesh, material=Mesh
0.044, 5
*End Instance
*Instance, name=Tissue-1, part=Tissue
*Node
[REMOVED]
*Element, type=C3D8H
93
[REMOVED]
*Nset, nset=Tissue, generate
1, 1890, 1
*Elset, elset=Tissue, generate
1, 1352, 1
** Section: Tissue
*Solid Section, elset=Tissue, material=Tissue,
*End Instance
*Nset, nset="Tissue Tied Region", instance=Tissue-1
[REMOVED]
*Elset, elset="Tissue Tied Region", instance=Tissue-1, generate
1093, 1352, 1
*Nset, nset="Mesh Edge", instance=Mesh-1
5, 6, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98
99, 100
*Elset, elset="Mesh Edge", instance=Mesh-1, generate
409, 425, 1
*Nset, nset=Set-7, instance=Mesh-1
1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18
19, 20, 21, 22, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96
97, 98, 99, 100
*Elset, elset=Set-7, instance=Mesh-1, generate
409, 425, 1
*Nset, nset=Set-9, instance=Tissue-1
9, 11, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182
*Elset, elset=Set-9, instance=Tissue-1, generate
132, 1092, 80
*Elset, elset="_MeshTied Face_SPOS", internal, instance=Mesh-1, generate
1, 408, 1
*Surface, type=ELEMENT, name="MeshTied Face"
"_MeshTied Face_SPOS", SPOS
*Elset, elset="_TissueTied Face_S4", internal, instance=Tissue-1, generate
56, 1092, 4
*Surface, type=ELEMENT, name="TissueTied Face"
"_TissueTied Face_S4", S4
*End Assembly
** MATERIALS
*Material, name=Mesh
*Hyperelastic, ogden, test data input, poisson=0.45
*Uniaxial Test Data
[REMOVED]
*Material, name=Tissue
*Hyperelastic, n=2, reduced polynomial
0.521, 1.275, 0.15, 0.
94
** INTERACTION PROPERTIES
*Surface Interaction, name=IntProp-1
1.,
*Surface Behavior, pressure-overclosure=HARD
** BOUNDARY CONDITIONS
** Name: Tied Region Type: Symmetry/Antisymmetry/Encastre
*Boundary
"Tissue Tied Region", ENCASTRE
**
** INTERACTIONS
** Interaction: Int-2
*Contact Pair, interaction=IntProp-1, small sliding, type=SURFACE TO SURFACE, adjust=0.0,
tied
"MeshTied Face", "TissueTied Face"
** ----------------------------------------------------------------
** STEP: Step-1
*Step, name=Step-1, nlgeom=YES
*Static
0.05, 1., 1e-05, 0.1
** BOUNDARY CONDITIONS
** Name: Displacement Type: Displacement/Rotation
*Boundary
"Mesh Edge", 1, 1
"Mesh Edge", 2, 2, 2.3
"Mesh Edge", 3, 3
** OUTPUT REQUESTS
*Restart, write, frequency=0
** FIELD OUTPUT: F-Output-1
*Output, field, variable=PRESELECT
** HISTORY OUTPUT: H-Output-1
*Output, history, variable=PRESELECT
*End Step
95
Appendix K
FEA Lap Shear Simulation vs. Lap Shear Experiments
The following content include all the graphs for each individual FEA lap shear
simulation and relates it to the experimental data.
96
0
10
20
30
40
50
0 5 10
Load
, N
Displacement, mm
Specimen 9_4
(R2 = 0.984)
Lap Shear Data
FEA Simulation
97
Appendix L
Artist Permission
Artist permission to use specific figures in this thesis
12/20/2013
Dear Ms. Coalinn Golden
I am writing to request some images from your collection entitled:
Schematic, cross-sectional view, of a typical ventral hernia with intestinal protrusion.
Schematic, cosmetic defect, of a typical ventral hernia with intestinal protrusion.
Hernioplasty repair for an onlay ventral hernia surgery
Mesh placement (onlay, inlay, sublay)
Abdominal wall description of orientation This image will appear in a book by Hummad Tasneem currently entitled “Dependence of the
Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia Repair” to
be published by the University of Memphis Press in the Spring of 2014. This is a scholarly
undertaking that will reach a limited and specialized academic audience.
I am requesting permission to use the image as both an interior illustration and other forms of
illustration connected with this volume, including but not limited to advertising, publicity, and
direct mail, or other similar uses, but excluding use as a cover illustration. I ask that you grant
nonexclusive world rights for the reproduction, as part of this thesis only, in all languages and
for all editions (including ebook).
Please sign and return this letter to me along with the image in question. Please contact me if
you have any questions regarding this request.
Sincerely yours,
Hummad Tasneem
Approved: _________________________________ Date: ____________
(signature)
98
12/20/2013
Dear Ms. Kathryn Hicks
I am writing to request some images from your collection entitled:
Surgical mesh close up view of mesh pores
This image will appear in a book by Hummad Tasneem currently entitled “Dependence of the
Abdominal Wall-Mesh Interfacial Strength on the Fixation Method for Ventral Hernia Repair” to
be published by the University of Memphis Press in the Spring of 2014. This is a scholarly
undertaking that will reach a limited and specialized academic audience.
I am requesting permission to use the image as both an interior illustration and other forms of
illustration connected with this volume, including but not limited to advertising, publicity, and
direct mail, or other similar uses, but excluding use as a cover illustration. I ask that you grant
nonexclusive world rights for the reproduction, as part of this thesis only, in all languages and
for all editions (including ebook).
Please sign and return this letter to me along with the image in question. Please contact me if
you have any questions regarding this request.
Sincerely yours,
Hummad Tasneem
Approved: _________________________________ Date: ____________
(signature)