a review of biocompatibility in hernia repair; considerations in vitro and in vivo for
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R E V I E W
A review of biocompatibility in hernia repair; considerationsin vitro and in vivo for selecting the most appropriate repair
material
N. Bryan • C. Battersby •
N. Smart • J. Hunt
Received: 14 January 2014/ Accepted: 29 August 2014 / Published online: 13 September 2014
Springer-Verlag France 2014
Abstract
Purpose Repair of hernia typically makes use of a pros-thetic material; synthetic or biologic in nature. Any mate-
rial which enters the body is subject to interrogation by the
inflammation and immune system in addition to numerous
other cell families, the outcome of which ultimately
determines the success of the repair. In this review, we
discuss the fundamental biology which occurs in situ when
a biomaterial associates with a tissue, compare and contrast
the techniques available to predict this in vitro, and review
how features of hernia repair materials specifically may
manipulate tissue interrogation and integration. Finally, we
conclude our article by examining how biocompatibility
impacts surgical practise and how a better understanding of
the manner by which materials and tissues interact could
benefit hernia repair.
Materials and methods A review of the literature was
conducted using appropriate scientific search engines in
addition to inclusion of findings from the groups’ primary
research.
Results Using pre-clinical assays to anticipate the bio-
compatibility of a medical device is critical; however, to
maximise the scientific power of in vitro findings, we must
carefully consider the in vivo niche of the cells with which
we are working. Excessive in vitro culture or contact to
non-self materials can add compounding complexity to
studies involving leucocytes for instance; therefore, we
must ensure careful and stringent assay design whendeveloping techniques for assaying pre-clinical biocom-
patibility. Furthermore, many of the features associated
with hernia repair material design specifically, included to
enhance their mechanical or biodegradation characteristics,
are inadvertently instructive to cells, and therefore,
throughout the prototype stages of a materials develop-
ment, regular biocompatibility assessment must be
performed.
Conclusion The biocompatibility of a material is rate
limiting in its ability to function as a medical device. The
future of hernia repair materials will rely on close cohesion
between the surgical and scientific communities to ensure
the most robust biocompatibility assessment techniques,
and models are utilised to predict the efficacy of a given
material in a particular surgical application.
Keywords Biocompatibility Inflammation Leucocyte
Hernia In vitro In vivo
Introduction
Hernia repair often incorporates the use of an exogenous
material to provide mechanical support to compromised
tissue. Immediately after implanting this material, it is
subject to contact with tissue and bodily fluids causing the
physiologic processes which allow the body to decide the
fate of the material to begin. Ideally, the material integrates
with surrounding tissue acting as a scaffold for appropriate
cells to colonise and provide permanent mechanical aug-
mentation to damaged tissue. However, if this interrogation
process is inappropriately stimulated, the material may
become encapsulated, degraded and require removal with
N. Bryan (&) C. Battersby J. Hunt
Clinical Engineering (UKCTE), Institute of Ageing and Chronic
Disease, Duncan Building Ground Floor, Daulby Street,
Liverpool L69 3GA, UK
e-mail: [email protected]
N. Smart
Exeter Health Science Research Unit, Royal Devon and Exeter
Hospital, Barrak Road, Exeter, Devon EX2 5DW, UK
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DOI 10.1007/s10029-014-1307-8
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morbidity to previously healthy interfacial tissues. There-
fore, understanding the mechanisms at a cellular level with
which a material interacts with tissue is paramount in
predicting its efficacy and longevity in vivo. This review
provides a concise overview of cellular considerations of
biocompatibility in addition to considering in detail how
specific features of hernia materials may influence this
process and the impact that this may have on surgicalpractise.
Materials and methods
A review of the literature was performed across a range of
scientific citation databases considering surgical and sci-
entific research in the area of biocompatibility and hernia
repair from over the previous three decades. In addition to
this, findings from our research teams own work are also
included.
Biocompatibility
Tissue healing involves complementary cellular and path-
ophysiologic processes which return compromised tissue to
normal function [2, 7]. From the instant, a surgeon places a
material in contact with tissue the processes that determine
its fate, and ultimately the quality of the patient’s life
commence. Material surfaces are coated with proteins from
blood, interstitial fluids or exudates which commences
cellular interrogation. A positive outcome is integration
with surrounding anatomy in a mechanically stable manner
having returned the tissue to normal homoeostasis. How-
ever, should a material’s cellular interrogation decide it
presents a risk to the function of the organism as a whole it
will degraded via enzymes, small molecules and reactive
oxygen species [12]. If destruction is not possible, mate-
rials are isolated in a capsule of fibrous matrix where risk to
surrounding tissue is contained. Broadly, the ability of a
material to repress the negative and encourage the positive
components of cell interrogation is termed
biocompatibility.
The interaction between material and tissue is dynamic.
Throughout the proceeding hours to years (in the instance
of permanent prostheses), the boundaries of its effects on
the host are pushed as the body attempts to use it as a
conduit to tissue regeneration, which may require its
remodelling, cell infiltration or removal.
We must disregard definitions that portray inflammation
as intrinsically negative. A material that stimulates
inflammation is a statement which informs us of inevita-
bility due to lack of any true biologically inert material.
Inflammation is the underlying process which repairs tis-
sue. Without inflammation, wounds do not heal, materials
do not integrate and infection prevails. A material is
required to encourage constructive elements of inflamma-
tion to support tissue remodelling through with appropriate
debridement and disinfection as opposed to destruction of
material, and healthy tissue that occurs during exacerbated
inflammation.
The cells that decide material fate are a varied with an
array of roles unified by a drive to return tissue to normalhomoeostasis. Primarily this involves leucocytes, but as
time progresses, non-circulatory cells also play a role in
material compliance. Within the first hours, a material is
investigated by neutrophils; which secrete many destruc-
tive proteins and small molecules capable of damaging
both material and native tissue. Although historically
thought of a cell whose role was purely destruction and
decontamination, we now recognise neutrophils as media-
tors of inflammation by signalling to attack and recon-
naissance cells to orchestrate pro and anti-inflammatory
processes [4].
Over the proceeding days, neutrophils are accompaniedby macrophages and famed debridement through phago-
cytosis, and these cells also signal and are arguably the
most rate limiting cell throughout tissue healing [5].
Additionally, these cells also present fragments of digested
material using MHC molecules, which drives generation
and persistence of long-term and targeted inflammation by
lymphocytes.
A site of tissue repair will require re-vascularisation
meaning that a biomaterial must support migration of blood
vessels through its bulk. This provides colonising cells with
a mass transport network to prevent necrosis of neo-tissue.
As cells enter the material and proliferate their nutrient and
waste, import/export requirements require constant
remodelling of neo-microvasculature throughout the graft;
changing shape, size and direction to ensure that cells
within receive support to keep the tissue healing and
healthy.
The interaction of cells with the periphery of the
material must also be considered. Blood cells are rela-
tively short lived, and as such, their presence at a
material site is constantly turned over. Therefore, we
must give thought to cells that remain throughout its
existence and provide security and stability beyond the
degradable suture used to secure it. A materials edge
must permit cell infiltration such that matrix secretion
knits the material with surrounding ECM. This is the
role of fibroblasts; excessive fibroblast activation or
proliferation results in encapsulation; enshrouding fibrous
tissue preventing integration, colonisation and vasculari-
sation. Therefore, the fibroblast cells must be managed to
ensure the correct degree of matrix deposition to provide
graft security, without fibrous encapsulation and material
isolation.
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Predicting biocompatibility in vitro and in vivo
Biomaterial development should be guided by clinical
feedback to researchers and companies, to define the ideal
requirements for a particular style of repair.
Any new material will be subject to direct and indi-
rect contact cytotoxicity screening to assess cell vitality
in response to a material using biochemical tests. Typi-cally, these report quantitatively (colorimetric or fluori-
metric) in response to changes in cell metabolism or
membrane integrity. These techniques quantify the pro-
portion of live or dead cells after treatment with a
material and can be tailored into deduce whether cell
death was a consequence of apoptosis or necrosis.
Cytotoxicity assays are run over short periods (hours) to
deduce acute cytotoxicity or extended over days/weeks to
consider modifications to a materials cytotoxicity as it
degrades or is remodelled. The cells used for cytotoxicity
screening can be tailored to a materials end use. Com-
monly, an established cell line is used for initial cyto-toxicity screening which progresses to primary cells
appropriate to the anatomy in which the material will be
situated, as its development is refined.
In cell specific models, the goal may not solely be one of
confirming cytocompliance, in favour of deducing the
influence of a material on the phenotype or function of a
relevant cell population. To this end, we must ensure that
in vivo culture platforms are designed with appropriate
stringency to mirror the cells native 3D microenvironment
as much as possible to obtain translatable data [6].
A second stage in anticipating material biocompatibility
is to subject it to contact with inflammatory cells and using
molecular biology to deduce its influence on cell longevity,
phenotype and function. Most commonly, these experi-
ments utilise human peripheral blood mononuclear cells
(PBMCs), extracted from whole blood via density gradient
centrifugation. PBMCs contain the mononuclear fraction of
leucocytes; monocytes and lymphocytes; granulocytes are
removed during density gradient centrifugation.
The reason for utilising mononuclear leucocytes is that
granulocytes rapidly become activated by the physical and
chemical factors to which they are subjected outside of the
cardiovascular system. When these cells are activated, they
secrete cytokines and reactive oxygen species (ROS) which
cause phenotypic changes across a cell population, making
subtle, constructive leucocyte responses difficult to
observe. ROS are an incredibly diverse class of signalling
molecules capable of both pro- and anti-inflammatory
processes, a number of which are detailed in Fig. 1.
This highlights the limitations of considering inflam-
mation in vitro. Although assays are regularly performed
throughout biomaterials science which place leucocytes in
contact with a test substance and record their response, we
must treat this data as a baseline and avoid making direct
translation to in vivo inflammatory physiology. This is
because the ex vivo environment triggers phenotypic
switches similar to invasion of the body with non-self
material. One must remember that cells recognise and
activate in the presence of materials deemed foreign to
healthy tissue, therefore, it should not be simply underes-
timated that a tissue culture environment distorts leucocytefunction and influences data collection.
Factors worthy of consideration include the physical and
haemodynamic forces associated with venipuncture and
centrifugation, and contact with xenoproteins in foetal calf
serum, a component of tissue culture media [11]. Impor-
tantly, we must consider the influence of contact with tis-
sue culture plastic vessels remembering that leucocytes are
constantly suspended in blood. Therefore, a direct surface
contact mirrors activation pathways such as contact with
damaged tissues or activated endothelium in vascular
walls. The compounding activating variables should be
accepted as significant when extrapolating to wholeorganism physiology.
Solutions have been developed which address and alle-
viate components of this in vitro activating milleau,
including non-adherent suspension culture systems using
bioreactors, acoustic forces [13], chemically modified
surfaces and serum-free media.
Typically, the data output from in vitro studies of leu-
cocyte activation should include several levels of molec-
ular interrogation which support an ultimate assessment of
changes in cell function based on transcriptomic, proteomic
and secretomic levels. We must take care to understand the
implications and limitations of each stage of this workflow.
Although PCR and microarray strategies provide insight
into cellular behaviour, we must use these genomic/tran-
scriptomic level analyses to provide support and robustness
for protein and secretion data. Fundamentally, it is the
protein level which allows a cell to perform its function,
and therefore, whilst changes in gene expression provide a
platform level of understanding into cell behaviour, it is
pertinent that we do not cease our analyses at a nucleotide
level and evaluate changes in proteins responsible for cell
function in vivo.
A further pitfall to studying material/leucocyte interac-
tion in vivo is in using clean and pure populations of
PBMCs, we remove the synergy which happens as a result
of material, cell and blood interaction. Biomaterial surfaces
are rapidly coated with proteins from blood and tissue fluid
which include immunomodulatory molecules such as
immunoglobulin and compliment. Therefore, whenever
possible, we must consider the inclusion of whole blood or
human plasma in in vitro biocompatibility workflows.
The authors have previously used a whole blood based
assay to conclude a materials capacity to activate
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leucocytes using chemiluminescent reporting of ROS pro-
duction. Using whole blood adds granulocytes into theassay; therefore, we designed experiments to consider the
initial phases of material interaction, running assays for
90 min with appropriate controls to gather quantitative data
before granulocyte degeneration [9, 10].
After in vitro studies have concluded sufficiently that
candidate materials do not possess inherent cytotoxicity
nor are they intrinsically pro-inflammatory the next stage in
predicting their biocompatibility at a whole organism level
are in vivo animal models.
Initially, implantation is performed to deduce inflam-
matory characteristics of a material in vivo without the
compounding variable of a healing or damaged tissue. Thisis typically a subcutaneous implant with a material placed
adjacent to the dorsolumbar musculature in a rodent. This
allows free movement of cells through the material,
allowing inflammation to proceed and be monitored with-
out compromise of the animal should the material fail
mechanically. We must remember, however, particularly in
device-specific models that animal models often present a
material to different mechanical loading (quadrupedal vs.
bipedal for instance), biochemistry, and differentiation
degradation kinetics based on much shorter lifespan to
humans. These factors should all be considered whendesigning and interpreting data from small animal series.
After preliminary animal studies, materials are tested in
a more anatomically correct model which describes the
defect which it is designed to repair clinically, such as a
full thickness abdominal wall excision model in the
instance of ventral/incisional hernia. In tissue-specific
models, material mechanical properties are also tested to
comply with their target tissue. Furthermore, models
should subject materials to specific populations of cells
with which they would contact clinically, such as muscle
cells and tissue-specific populations of leucocytes such as
peritoneal macrophages in the instance of abdominal wallrepair.
Classical analyses are performed by excising tissues
after predetermined end points and performing histo-
pathology in which the fixed, micron thickness sections
are non-specifically stained to colour the cells and ECM
to visualise using light microscopy. This enables path-
ologically indexing using a numerical scale to semi-
quantitatively elucidate material interaction with host
physiology.
Fig. 1 Leucocyte signalling is a vastly intricate process. The important signalling aspects of one class of leucocyte produced molecules, ROS,
are detailed
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This approach, whilst providing valuable baseline data,
is suboptimal for elucidating finer points of cell/material
interaction. Fundamentally, the system is subjective, based
on the opinion and training of scoring pathologists, and
negates of the importance of considering the function of
particular populations of cells at an implant in favour of
recording their mere presence.
Although pathologists undoubtedly poses the skill todiscern between lymphocytes and neutrophils for instance
based on cell morphology, the presence of these cells
provides only basic insight into how a material interacts
with tissue. Although greater numbers of neutrophils
indicate stronger acute inflammation, we remain devoid of
detail concerning cell activation or signalling making true
prediction of material fate difficult.
Cells with microscopically similar morphology indicate
vastly differing tissue consequences which only become
apparent with more detailed molecular analyses such as
CD4/CD8 T lymphocytes and classically versus alterna-
tively activated macrophages.Therefore to quantify and critically, standardise bio-
compatibility predictions, we must enhance pathologic
interrogation beyond traditional histopathology. This
should use validated SOPs to remove variability induced
by human scored systems whilst adding value to informa-
tion gained purely by tinctorial staining, providing a layer
of molecular deduction of the roles and purpose of specific
cells at an implant.
The clearest alternative is immunohistochemistry, a
process which utilises antibodies against antigens associ-
ated with particular components of inflammation to visu-
alise specific cells and tissue areas by visible light or
fluorescent microscopy. Once cells or areas have been
specifically allocated a spectral signature, human evalua-
tion can be removed in favour of image analysis to count
cells or calculate boundaries absolutely [8]. Using absolute
quantification inter-sample standardisation is required
when cell counting; a material may present with greater
numbers of a particular cell simply because the sample is
physically larger. Therefore, cells of interest should be
normalised to a concurrent nuclear stain to deduce total cell
number such as haematoxylin or Hoechst. This technique
differentiates morphologically similar yet functionally
distinct cells in addition to targeting surface markers
associated with defined cell processes such as activation,
phagocytosis, migration and apoptosis.
Immunohistochemistry also targets secreted molecules
which present in intercellular and interstitial spaces
allowing signalling substances involved in inflammatory
progression and remission to be observed, to real-time
monitor inflammatory progression.
Immunohistochemistry is, however, a relatively com-
plex process made up of several sub-elements which also
have potential to induce user mediated variation in the
analytic procedure. There are numerous individual pro-
cesses which can induce variation between practitioners
including fixation, antigen retrieval, antibody concentra-
tions/incubation incubation parameters, visualisation pro-
tocol and even mounting and colorimetric/fluorimetric
preservation. Therefore, although the technique undoubt-
edly has the potential to add value beyond traditionalpathologic indexing, we must proceed with caution and
ensure operating protocols are generated and followed to
produce data which supports the rigour of inter-centre
standardisation.
A skilled histologist produces sections 4–6-lm thick,
ensuring cells are sliced across their axis, presenting their
internal cytoplasm. This allows observation of intracellular
substances such as precursors to secrete factors and probing
the phagolysosome. Depending on the tissue fixation, we
may also probe a section at a nucleic acid level using
nucleotide hybridisation in which a fluorescently conju-
gated nucleotide sequence complementary to an mRNAsequence targets sequences within. Such techniques con-
clude the gene expression in cells surrounding an implant,
providing a window into their objectives as they decide the
material fate.
Explants can be dissociated and subject to gene probing
using polymerase chain reaction (PCR) analyses. A par-
ticularly useful modification to this, reverse transcriptase
PCR utilises the enzyme reverse transcriptase to convert
mRNA into DNA which enables PCR amplification. This is
particularly interesting as elucidates genes which are
expressed at a particular moment in time. It is now possible
to characterise expression of many genes simultaneously in
a single PCR meaning large data can be assembled rapidly.
This is useful for deducing novel biomarkers of disease
based on high throughput and cost effectiveness of PCR.
However, we must ensure during any molecular biology
transcripts of clinical interest have their translation to
protein confirmed.
Histopathology reports as a terminal procedure which
does not help to dynamically predict biocompatibility.
In vivo, the material will be excised when the animal is
killed, clinically a material may only be available patho-
logically when it failed and been removed/replaced.
Therefore, we were not able to decipher molecular pro-
cesses which preceded its failure which would provide a
group of dynamic biomarkers to monitor material/tissue
interaction throughout its integration/degradation profile.
This insight would improve pharmaceutical management
or refine surgical intervention to augment the process
before clinical failure.
Throughout animal studies, we must collection of bio-
markers from experimental subjects in addition to histo-
pathology. Most obviously, from this blood collection, we
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can monitor populations of cells as they migrate to an
implant or quantify substances in serum. We may also
consider less invasive fluids specifically urine which can
easily be collected from experimental animals.
From biologic fluids, there are several techniques to
monitor systemic molecules and rapidly screen for sub-
stances of interest. Similarly to immunohistochemistry, this
employs antibodies specific to particular molecules to flagthem up for luminescence/fluorescence detection. These
may be ELISA systems in which antibodies tethered to a
substrate bind molecules of interest, which allows quanti-
fication with the addition of further antibodies conjugated
to a reporter substance. Recently, these technologies have
been advanced multiplex in which many molecules can be
observed in a single assay using spectrally distinct reporter
antibodies with detection apparatus capable of reading
many emission spectra simultaneously.
Material factors influencing biocompatibility
Many factors influence biocompatibility of implanted
materials. The physical properties of a material, either by
design or as by-products of fabrication, inadvertently
instruct cells and tissues. When considering biocompati-
bility, we must remember material integration is syner-
gistic between material and patient tissue; therefore, there
are also patient-specific physiological variables which
influence material interaction and modify biocompatibility
at a personal level. This is highlighted in the Williams
definition of biocompatibility ‘the ability of a material to
perform with an appropriate host response in a specific
application’ we must consider the specificity of an appli-
cation to be personal given the heterogeneity of inter-
individual tissue dynamics [1]. Therefore, generalisations
regarding ubiquitous biocompatibility should be used with
extreme caution until sufficient prospective data confirms
this throughout a large range of subjects.
The skill of the surgeon and appreciation for maintain-
ing tissue vitality also contributes to the biocompatibility of
an implant. Excessive tissue manipulation causes damage
and extends wound remission. Furthermore, surgical
application of the material also influences its biological
efficacy, such as material/tissue overlap or suture to
material ratio. For this reason, deducing subtleties in
material biocompatibility are difficult from retrospective
analyses due to the variables introduced by lack of stand-
ardised surgical application.
Materials modify biocompatibility at the level of
chemical and biological composition and physical and
mechanical properties. There is no such thing as an inert
biomaterial, one which may be applied to a defect purely as
a mechanical bridge without cell interaction, and it is
essential to disregard this ideology. Fundamentally, we
must consider the way in which biomacromolecules inter-
act with a material surface as this gives a cell its first
impression of an implant and begins the process of deter-
mining acceptance or destruction.
Cell surfaces are dynamic and continually adapt to the
3D space to which they are subjected. As cells contact
substrate membrane, fluidity allows it to flex and mould tothe surface, producing focal adhesions which feed back to
the cellular cytoskeleton to change in shape and rigidity.
Once intracellular cytoskeletal scaffolding is laid down, the
cell produces ECM creating a more favourable environ-
ment for colonisation as extracellular receptors recognise
this environment is familiar using integrins and other rec-
ognition molecules [2].
If a surface environment is unfavourable to cells, the
material is perpetually reliant on its sutures to retain its
structural capacity as cells will never colonise and produce
autogenous matrix to bind the device with surrounding
tissues. The converse of this is a surface which signals cellsto hypersecrete matrix which results in fibrous encapsula-
tion. We must be mindful of the interaction of cells with
material surfaces and engineer implants to incorporate
known topographic parameters with balanced adhesive and
proliferative properties.
The physical microenvironment of a biomaterial greatly
influences cell response and supports interaction of the
material with tissues as a whole. Topography at a micron
scale, which is easily overlooked during materials fabri-
cation as it may play a minor part in conferring mechanical
properties, modifies cytoskeleton remodelling, matrix pro-
duction and cells recruitment. Techniques such as atomic
force microscopy (AFM) characterise material surfaces to
quantify the roughness and randomness of their topography
enabling the provision of optimal surface parameters for
cell compliance.
When a material contacts tissue fluids its surface is
immediately coated with protein, so therefore when con-
sidering surface topography in vitro, we allow this inevi-
table fouling which may mask subtle topographic changes
by providing a homogenous protein coat. On this basis, we
should also consider a materials protein adsorption capac-
ity when performing material evaluation and more specif-
ically investigate materials possess preferential adsorption
of certain specific proteins.
Despite protein coating, it has been demonstrated that
nanoscale modifications to material surfaces modify cell
fate and function, particularly in the instance of adult stem
cells, a pertinent cell in any healing tissue [15].
The hydrophobicity/hydrophilicity surface must also be
considered, as this will strongly influence protein adsorp-
tion and subsequent cell interaction. Conversely, the field is
continually seeking materials in which an anti-adhesive
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face can be applied to the materials visceral side to inhibit
intra-abdominal adhesions. Although a material which is
intrinsically repulsive to tissue presents a tantalising sub-
stance in herniorrhaphy, we must proceed with caution
with anti-adhesive biomaterials and use our ability to repel
brings surgical issues caused by compromised integration
and poor mechanical stabilisation. There may indeed be
more subtle pharmaceutical methods by which to manageintra-abdominal adhesions by targeting and suppressing
migration and activation of inflammatory cells [3].
Material integration will also benefit from colonisation,
increasing and securing the bond between material and
peripheral tissue by providing a meshwork of interfacial
cells inside and outside the graft. Therefore, we must
ensure that a material has sufficient porosity to allow cell
penetration and migration. In the instance of large grafts,
this should also allow the penetration of macroscopic tissue
architecture such as microvasculature to provide the graft
with mass transport capacity and preventing necrosis.
Although understanding physical and chemical compo-nents of a surface is critical in anticipating its tissue
interaction, we must also consider internal structure and
chemistry.
As material erodes, degrades or remodel’s its internal
structure becomes visible to cells and tissues; therefore, in
a material whose bulk is not homogenously distributed
resulting in differing internal and external structures or
chemistry, we must consider that this continually changing
environment modifies tissue behaviour beyond a standard
acute inflammatory interaction time frame, as with every
new material parameters, as the device degrades comes a
fresh wave of inflammatory investigation.
In vitro, testing of material degradation is also the
subject of debate. Various methods exist which include
most basically incubating the material for extensive time
courses in simulated body fluids and assessing changes in
physical appearance and mechanical properties as time
progresses. A further degree of complexity is induced by
incubating with single enzymes such as collagenase or
enzyme cocktails to mimic the tissue remodelling envi-
ronment or the influence of microbial contamination
secreting enzymes into the material field. Much like many
in vitro tests, however, modelling degradation in vitro is
challenging to directly extrapolate to a true in vivo tissue
reaction as the actions of continuing waves of cells inter-
rogating and potentially digesting the material, in con-
junction with the huge and undefined array of
metabolically active proteins present in interstitial fluids, is
difficult to recreate in vivo. In vitro, it is also difficult to
create tests which appreciate the repeated and perhaps
unpredictable influence of mechanical loading on a mate-
rial and how this may synergise with the tissue environ-
ment to modify the materials degradation kinetics.
Therefore, although we can broadly predict a materials
degradation profile based on in vitro tests, this is only
absolutely quantified when a material progresses to an
animal model.
The rheometry of a device, its stiffness, rigidity and
elasticity also influences cell function. Cells reside in
specific niches in which they are accustomed to a set of
sheer parameters; therefore, subjection to physical forces towhich they are unaccustomed has the capacity to modify
their response. We must also consider how this component
of cell/material interaction may change over time if the
mechanics of a device changes due to wear or repeated
loading.
In the instance of hernia, there are many compounding
engineering features which influence the biocompatibility
of a prostheses, this is particularly challenging in the
instance of biologics [14].
Biologic meshes are subject to proprietary processing
steps which are partially or completely undisclosed, this
modifies the inflammatory and mechanical characteristicsof the tissue to render it suitable as a medical device. A
number of these processes result in material changes at the
level to instruct cell fate.
Starting material may influence biocompatibility by
utilising a range of source species and tissues which it
would be naive to assume contain identical structural and
mechanical features and induce the same cell responses.
Proprietary grafts exist which are xenogeneic (porcine or
bovine) or allogenic (cadaveric) derived from a range of
tissues including dermis, small intestinal submucosa and
pericardium. These materials vary in degradation profile,
continually unmasking epitopes to inflammatory cells as
degeneration modifies material.
When materials are decellularised, it is critical that cells
and cellular remnants are completely removed or cell pattern
recognition receptors will flag the graft as non-self resulting
in destruction. The goal therefore is to ensure the cellular
component is removed, meaning the remaining matrix con-
tains only proteins (largely collagen) that are conserved
across individuals and species and provides a 3D environ-
ment for cells to colonise, without epitopes for the immune
system to differentiate graft from autogenous ECM.
In some instances, this core protein structure is modified
using cross-linking to join the collagen molecules within
the decellularised tissue. Cross-linking increases resistance
to bacterial proteases, which increases its persistence in
infected fields. It would be reasonable to assume changes
induced by cross-linking modify inflammatory character-
istics by presenting cells with a protein conformed in a
manner that they are not accustomed. However, studies by
our own group both in vitro and in vivo have generally
demonstrated that cross-linking does not significantly
influence cell response in dermis derived biomaterials [30].
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A final variable is sterilisation, with many techniques
employed including gamma rays, electron beams and eth-
ylene oxide.
The surgical consequences of unsatisfactory
biocompatibility
In the light of progress in the scientific understanding of
material/tissue interaction, and advances surgical applica-
tion, the ideal characteristics of hernia repair materials are
perpetually refined. The qualities described by Cumberland
and Scales, and Hamer-Hodges and Scott, are as follows:
non-carcinogenic, chemically inert, resistant to mechanical
strain, sterile, unresponsive to body and tissue fluids, able
to limit foreign-body reaction, modifiable to defect size,
and non-allergenic [16]. Required characteristics applied
specifically in the instance of biologic meshes include
resistance to infection, anti-adhesive, and the capacity to
act like native tissue [17]. Additionally, an ideal materialshould be associated with very little surgical morbidity,
such as seroma, easy to handle in open and laparoscopic
instances, and cost effective. Due to significant advances in
synthetic and composite meshes, a vast array of products
for repair of uncomplicated hernias is available [18].
Despite this panel of ideal material properties and the
apparent ease at which a material can be engineered to
modify its mechanical characteristics, synthetic meshes are
not without risk, even in clean, uncontaminated operations,
which highlights the need for a more thorough under-
standing at a material science level of how tissues and
material are interacting. Often the consequences of syn-
thetic mesh, failure can be catastrophic. This includes
erosion, adhesion, fistulation or encapsulation, which are
incredibly debilitating requiring challenging surgeries to
correct with potential for life modifying long-term out-
comes such as stoma as a consequence of bowel resection
during adhesiolysis [31, 32].
Undoubtedly, synthetics improve the quality of life of
many people each day, however at present, we can say that
these materials are not ideal in every patient, and a more
thorough understanding of how the biological and cellular
origins of inter-patient differences in response is required
as a matter of urgency to take the field forward.
Identifying the ideal repair material for complicated
hernias remains a challenge. Although classification of her-
nia complexity is debated and has been the subject of a
number of consensus meetings [19, 20], it is accepted that
complex hernias include those in which there is contami-
nated or clean–contaminated tissue, exposed bowel or fis-
tula, and significant loss of domain such that low- or tension-
free repair is not possible. The use of synthetic meshes for
complex hernias has been associated with infection,
recurrence and significant adhesions to intra-abdominal
organs, leading to obstruction, erosion and fistula formation
[21]. Infected synthetic meshes are difficult to treat, fre-
quently require surgery to explant, and may leave the patient
with a colonised defect requiring prolonged treatment [17].
For synthetic mesh repair in the presence of contamination or
infection, several authorities recommend a two-stage pro-
cedure in which contamination is managed surgically in thefirst stage, with the hernia repaired 6–12 months later [22].
Biologic meshes are promoted for repair of complex
hernias, as these meshes become incorporated into the
wound, acting as a scaffold for tissue repair leading to a
strong, well-healed, vascularised wound [22]. Due to the
nature of biologics, the adhesions associated with synthetic
mesh should not occur and vascularisation allows delivery
of immune cells and antibiotics [17]. Retrospective con-
sideration of biologic mesh efficacy is compromised due to
the compounding variables induced throughout their
application: the variety of available products, the tech-
niques used to repair hernias (onlay, inlay, sublay, com-ponent separation) and the complex nature of hernias being
repaired [23, 24]. To date, no data from randomised con-
trolled trials specifically designed to investigate biologic
meshes is available [21].
Besides the previously mentioned issues surrounding
classification of, the classification of ventral hernia repair
and subsequent outcome reporting is also the subject of
debate [24]. Current literature is based on single centre
case series, (and reviews of those series), with several
series reporting a variety of hernia complexity, a range of
surgical techniques, a variety of mesh types, and use of
biologic mesh in both clean and contaminated wounds [21–
23, 25, 26]. The recently launched European hernia registry
is a development designed to bring clarity to reporting of
ventral hernia repairs and outcomes, and should provide
robust, prospective data [24, 27]. Although overt recur-
rence of the hernia might be considered a failure, one
author, reporting a series of ventral hernia repairs using a
variety of biologic meshes in contaminated or clean–con-
taminated tissue, reported a 31.3 % rate of hernia recur-
rence at 21.7 months, but described the majority of
recurrences as asymptomatic, with only 17.5 % requiring
further surgery. The same study showed a recurrence-free
survival of 92 % at 1 year, 77 % at 2 years and 51 % at
3 years—a trend that questions the durability of such
repairs [22]. Another study, describing repairs of ventral
hernias secondary to emergency or trauma laparotomy,
reported a 100 % recurrence with AlloDerm at 1 year, and
a 31 % recurrence rate with FlexHDAs, all recurrences
progressed to further surgery [26]. A recent review of 17
retrospective series showed that recurrence depended upon
wound class, with an overall recurrence rate of 13.8 %. The
recurrence rates were 2.9 % in clean or clean/contaminated
176 Hernia (2015) 19:169–178
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cases, 19.4 % in complicated hernias, and 23.1 % in con-
taminated or dirty wounds. The rate of recurrences
requiring further surgery is not clear from that review [23].
Associated surgical and wound morbidity is well docu-
mented in ventral hernia repairs, with a review of 25 series
showing a 46.3 % complication rate, wound infections being
most common in 15.9 % of all cases [23]. It is interesting to
note that only 4.9 % of infected implants required removal tomanage infection; the remainder were salvageable via to
non-operative management. Although not all series report
seroma formation, that problem is commonly documented
following biologic mesh repair. The review of 25 series
shows an overall seroma rate of 14.2 % [23]; however, it is
likely that the surgical technique, mesh, and wound class all
influence seroma risk. A study describing Strattice to repair
contaminated or infected ventral hernias documented ser-
oma in 28 % of cases [28].
The future of biocompatibility—a surgeon’s perspective
The current literature relating to biologic meshes is not
sufficient to support evidence-based decision-making in
ventral hernia repair, as it is composed of relatively small
series, often with limited follow-up period. Moreover, the
series describe significant variations in technique, using a
range of biologic meshes. Interpreting data from such
series is compounded by the fact that the nature of com-
plicated or contaminated ventral hernias is also a signifi-
cant source of variability. The only valid conclusion that
can be supported from current literature is that it seems safe
to use biologics in contaminated fields; however, the
recurrence and morbidity rates remain high, especially in
challenging cases. As Belyansky and Heniford [29] point
out in response to the work of Kissane and Itani [21], large
prospective trials would be challenging and expensive,
considering the number of variables that have to be con-
sidered; however, from a surgical point of view, estab-
lishing the role of biologic meshes in this complex field
must be driven by evidence from prospective trials, in
combination with development of biologic materials.
Conflict of interest NB declares conflict of interest not directly
related to the submitted work.CB declares no conflict of interest.
NJS declares conflict of interest not directly related to the sub-
mitted work.
JH declares conflict of interest not directly related to the submitted
work.
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