Implantable End Pad for Lower Limb ProsthesisMike DiCicco, Stephen Osterhoff, Trevor Taormina – Projects 1, Dr Nasir

Biomedical Engineering Program, Lawrence Technological University, MI 48075

Abstract: Roughly 185,000 lower limb amputations occur each year in the United States. Amputations are performed relatively easy and come with little complications. However, complications arise during the socket fitting process. Due to the lack of load transfer, often time there is pain and discomfort leading to a decrease in quality of life. A new product known as the implantable end pad transfers weight bearing loads back onto the skeletal structure. The research plan along with the methods is reviewed as well as anticipated challenges, future research, impact, responsibilities, and the costs associated are discussed. Over the course of this study, material characterization and mechanical testing will form a basis for the improvement of transtibial amputations

Keywords: Amputation, Implantable End Pad, Transtibial, Characterization, Mechanical Testing

In America alone, there are roughly 185,000 lower limb amputations that occur each year

according to the Amputee Coalition of America. More specifically, 131,000 amputations occur

that are below the knee joint [1]. The majority of patients that are subjected to these amputations

fall under two main categories: disease or trauma. In fact, about 70% of patients that receive an

amputation are for diseases like vascular disease or diabetes while about 22% are seen from

trauma accidents [2].

The amputation process is a relatively simple, defined, and cost efficient process.

During the procedure, the surgeon often sections the tibia near the midline of the bone; that is,

roughly 8 to 10 inches from the knee joint. This is because when a prosthetic is formed for the

patient, enough bone length must be kept to provide proper stability and momentum during the

walking gait. In addition, enough bone length must be removed so that an efficient prosthetic is

able to implemented along with the necessary hardware. Once the bone is cut, the end of the

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section is smoothed out to eliminate any jagged edges that could damage the surrounding tissue.

Existing muscle tissue is wrapped over the bone tightly and is sutured closed using skin flaps.

In the current market, the most notable technique for prosthetic limbs is attaching to the

patient via a socket. The socket is customized for each patient after recovery. During the

molding process, the prosthetist “shapes” the limb accordingly for optimized fit and to keep the

tibia from swinging within the socket. However as time passes, the socket can become loose due

to the change in the limb’s geometry. The changes can arise from the type of loading that is

exerted on the socket as well as fluid retention within the body. For example, if a patient has a

high intake of sodium throughout the day, more water will be retained in the body which will

change the limb ever so slightly causing a bad fitting with the socket. Bone bridging between the

tibia and fibula is another technique which has been used to assist with load distribution.

However, this technique does not have enough research to become a standard practice for

amputations. It can be seen through literature that building a bridge between the tibia and fibula

does not appear to have better outcomes than a standard transtibial amputation surgery [3].

Another newer process that is up and coming is attaching a titanium rod directly to the tibia and

connecting the prosthetic to the rod. The limitation with this process is whenever a medical

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Figure 1: (left) Prosthetic socket fitting (middle) Titanium Rod fixation (right) Tibia-Fibula bone bridging

device protrudes from inside the body through the skin barrier, it is much more likely to cause an

infection at the site. However, the method of using a titanium rod has a clear advantage over the

prosthetic socket because of the load capabilities. In the prosthetic socket, there is no load

distribution that occurs between the tissue and the skeletal structure. The entire load in this case

is distributed over the soft tissue which often leads to bone spurs, skin sores, and other

complications that will reduce the overall quality of life for an estimated 80% of patients [1].

Implantable End Pad

To solve the patient’s complications and discomfort, the proposed solution is to develop a

product that is able to dampen or transfer weight bearing loads back to the skeletal structure.

This product is known as the implantable end pad (IEP) by Advanced Amputee Solutions, LLC

(AAS). This product will absorb the excessive forces that are

placed on the tissue and eliminate the complications that normally

come with prosthetic sockets. This is important because then the

prosthetic that fits over the limb will be able to work more

efficiently and cause less fit-up issues from limb changes.

A good deal of the benefits comes from its unique shape. The

unique tear drop shape, shown in Figure 3, is based on how

prosthetic sockets are fitted once the prosthetist shapes limb

accordingly. The half spherical bottom portion provides a comfortable fit inside the limb while

the cone shaped top provides stability. With the incorporation of an offset bone insertion site, it

not only provides stability on the top end but it also increases the amount of material on the

anterior side of the tibia. This is important because with the natural walking gait, the tibia is

subjected to higher loads on the anterior side.

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Figure 3: Implantable End Pad

Research Plan

Literature Review

The first step in our research plan is to do a comprehensive literature review. In this

review, we aim to look at applications which the material has been used. Another area of

research is ISO (International Organization of Standardization) and ASTM (American Society

for Testing and Materials) standards required by the Food and Drug Administration (FDA) to

bring a product to market.

The material chosen was Bionate®, a polycarbonate-polyurethane composite

thermoplastic. From this, research and data about the polymer material was done about

biocompatibility and material characteristics. In literature provided by the polymer supplier

DSM, Bionate® has passed biocompatibility tests and maintains a FDA master file verifying

material biocompatibility with the FDA [4]. We also researched applications which the

Bionate® polymer was used. We have found that it has been used in load bearing prosthetic

products manufactured by Active Implants®. Active Implants® utilizes this material for hip and

knee prosthetics [5]. Material characterization studies were also researched and have found to

list various mechanical properties of the material [6]. From this research we determined

Bionate® to be a suitable polymer to use due to its biocompatibility and prior use in load bearing

prosthetics.

In our ISO and ASTM standards research, we have determined picked out key standards

to look at. ISO 10993 defines biocompatibility requirements needed prior to a clinical study.

ISO 14871 defines risk management assessments needed to determine the safety of the medical

device. ISO 13485 defines the quality management requirements needed for producing the

device. These ISO standards largely define requirements needed for later stages of producing the

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implantable end pad and are less relevant during the design phase. One ASTM standard we have

found is ASTM D695-02. This ASTM standard defines test methods for compression properties

of rigid plastics. This standard outlines the basic procedure we will follow for our mechanical

testing.

Computational Modeling

The second step of research is to do computation modeling via finite element analysis

(FEA). To achieve this, a variety of software programs and packages will be used that are freely

available to LTU students. The first program is MIMICS®, which takes computed tomography

(CT) scans of the body and generates 3d models of the selected tissue or body part. MIMICS®

will be used to generate a 3D model of the tibia to be used to generate the bone interface portion

of the IEP. From here various computer aided design (CAD) software can be used to create and

edit the IEP model. The created IEP model can then be imported into COMSOL®, where finite

element analysis can take place. The FEA process will allow various designs of the IEP to be

tested under simulated loading cycles. This will allow for areas of stress to be visualized, from

here the model can be altered to reduce areas of high stress where failure can occur. This

process will allow us to create an optimal design without having to spend the time testing a real

world models.

Compression Testing

The final part of our research plan is to conduct mechanical testing. Using the ASTM

standard D695-02, we intend to conduct fatigue testing with material samples provided by DSM.

In addition to this, implant fixation methods are to be tested as well. We plan to test suturing and

bone cement fixation, to characterize how well each fixation method works. This is important

for the product because we believe that the next point of failure after material is at the point of

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fixation. During the fixation test, cyclical stress mimicking the natural loading parameters of the

tibia will be applied to the bone and IEP device. After the compression test, analysis of the

fixation interface will be observed for degradation. In addition to this strain gages will be

utilized to determine the stress experienced by the material to help determine points of failure.

Methods and Tasks

In this project there are two main parts that will take place. The first part of the project

will model the IEP within the COMSOL® software package and simulate loading to find areas

of high stress and strain. This will be used to quickly design an IEP device that evenly

distributes loading and minimizes stress and strain of the material. In the second part the

experiment, test samples of material will undergo fatigue compression to determine which two

durometer of material that holds up the best to the stress. In addition to this, an injection molded

IEP device will be created and then attached to a bone model with various fixation methods. The

IEP and bone would then experience cyclical compression to see how well the fixation method

withstands the loading.

Computational Modeling

In the computational model portion of the project, finite element analysis software

COMSOL® will simulate the experiment. In the beginning stages of this, simple models created

directly in COMSOL® will be utilized to quickly set up loading parameters (Figure 4).

Once material properties and loading parameters have been correctly set, more complex and

accurate models can be imported into the software to produce better results (Figure 5).

In addition to using and modifying this scanned IEP geometry, several other software

packages will be used to generate the

geometry of the bone and IEP hole

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shape. MIMICS® will be used to produce 3D models of sectioned tibias and generate the bone

interface portion (hole) of the IEP. From here, various CAD software import models from

MIMICS® and combine it with the IEP model. From here, the IEP and bone models are

imported into the COMSOL® software and the simulation will be run. Based on results of the

simulation the IEP model can also be modified in the CAD software to change shape and size.

This will allow us to rapidly design and test different IEP designs to find the best one to use in

the compression testing.

Specific testing that will be conducted in the FEA software will be cyclical compression

of the bone and device. The device will be loaded to approximately 3.3 times the body weight of

a 95th percentile based male weight. The 3.3 times body weight parameter was the max load

placed on the tibia in a normal adult male as reported by one scientific study [7]. This

compression will load to the peak value and unloaded in cycles and timeframes consistent with

human gait cycles. In addition to using these loading parameters for the computational

modeling, the same loading parameters will be used for the real would cyclical compression

study as well.

Compression Testing

In the compression testing study, mechanical testing equipment will be used to test both

material samples and injection molded IEP models. For material sample testing, sample disks

(see figure 6) will be tested to natural testing parameters as established in the previous section.

The sample disks will be tested before degradation of the material will be characterized. To

characterize the material degradation, optical microscopy and SEM imaging will observe the

material for deformation a fracturing. From this

process, two appropriate durometers of the

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material will be chosen from the select four. When the appropriate Bionate® durometers have

been chosen, the IEP models will be created using injection molding techniques. Protocol for

injection molding the IEP has not been established, although outside contracting and in house

fabrication processes are being explored. Once the IEP models have been created, they will be

attached to either sawbone or pig bone samples using one of three fixation techniques: no

fixation (control), PMMA bone cement and surgical suturing. Each of these attachment methods

will be tested a minimum of two times, for a total of at least six trials for each durameter – 12

devices in total. Each trial will be placed in an electromechanical compression testing machine

located in CIMR laboratory at LTU. In addition to this each fixture will have strain gages placed

in high strain areas determined by the FEA process. This will allow for us to see if FEA

modeling was accurate in describing the stresses present. The fixtures will then be tested

according the natural testing parameters as established in the previous section. After cycling the

fixtures, optical microscopy and SEM imaging will be used to characterize the damage at the

fixation site to the cement, sutures and Bionate® material.

From this data, an analysis and report will conclude our findings about the material,

design and fixation method viability. The report will detail the key results, limitations and areas

of interest for further research.

Deliverables

The deliverables of this project will be broken down into 3 sections, computational

modeling, mechanical testing, and prototype development.

Computation Modeling

The computational modeling of this project will include the use of modeling programs

and a simulation program. The modeling programs are MIMICS®, SOLIDWORKS®, and

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CATIA®. In MIMICS®, three dimensional models can be made from CT scans. This allows an

accurate representation of a human tibia for this project. Within SOLIDWORKS®/CATIA®, a

three dimensional model of the implantable end pad can be created. This model can then be

manipulated to optimize the performance of the end pad. This is one deliverable from modeling,

this will allow AAS to have a base model that they can change for future applications. Once the

model has been developed it can be imported into COMSOL®. COMSOL® is a program that is

used for finite element analysis. A test can be set, in this case a compression test, and initial

parameters of the material and test cycle can be inputted. This will then show profiles of where

stress occurs on the end pad and can be used to determine where strain gauges should be placed

to give relevant data.

Mechanical Testing

The first part of mechanical testing will consist of a material fatigue test that will help

determine what durometers of material should be used. This in combination with the

computational modeling will help us to verify that the correct material has been chosen. The

second part of mechanical testing will be to test fixation techniques between the end pad and the

bone. From this it can be determined that what is the best way to attach the end pad to the tibia

and though the use of strain gauges valuable data about the deformation of the material.

Prototype Development

Prototypes of the implantable end pad will be made from the durometers of material that

is chosen.

Anticipated Challenges

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Some of the anticipated challenges we feel we might come up against along the way

while working on this projects starts with literature dealing with the FDA. The FDA has strict

regulations on medical devices before they are able to be marketed. Interpreting these

regulations in order to produce a quality product from FDA approval testing standards could

pose an issue. Another challenge deals with finite element modeling. The combination of

different programs, such as CATIA® or MIMICS®, and inputting data into COMSOL®, can

produce complications between files accurately. In addition, this modeling needs to ensure

accurate data amongst computational modeling and real mechanical testing. Mechanical testing

with the different methods of fixation – suturing and PMMA bone cement – that properly

simulates real applications could be difficult as well as understanding the machine interface of

the Instron instrument. And finally, during these mechanical testing, time constraints are bound

to occur due to the complexity and duration of fatigue and cyclical tests. For example, for a

typical 10 year study, to simulate the longevity of a medical device, it is placed under 10 million

cycles. This amount of cycles can last up to a month and half of continuous testing, leading to

clear time constraints.

Impact

The implantable end pad aims to help patients that underwent lower limb amputation,

specifically below the knee. Currently the only options a prosthetist and an amputee have is to

load all of the amputee’s weight onto the soft tissue at the site of amputation. This is a painful

experience for the amputee, because the body naturally works by loading the skeletal structure.

By implanting the end pad onto the bone, the load can then be transferred back onto the skeletal

structure. This will revolutionize the world of prosthetics. This will allow the prosthetist to

develop a better fitting socket that puts the load back where it should be. The stages that this

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group is focusing on will help to develop the product for the company AAS. The data provided

will help show that this product will be a success.

Future Directions

There are many items that can be addressed for future plans. The things that can be

addressed are environmental testing, dual durometer material, customized bone interface, method

for mass production, and new socket design.

Environmental Testing

An aspect that still needs to be looked at in terms of the implantable end pad is how the

material will act when it is placed within the body. A simulation of this can be done with

environmental testing. This will have to be done in a few stages. The first is by creating an

environmental chamber that will simulate the human body. After a simple environment test,

mechanical loads can be placed on the end pad while it is in the environmental chamber.

Dual Durometer Material

One potential factor that could be looked at is creating an end pad that has a dual

durometer. This means that the end pad would be made out of a material that has two different

stiffness. This was suggested by the material supplier DSM. This could be accomplished by

molding an internal portion then adding another layer on top of that. This could possibly give a

better load for the bone.

Customized Bone Interface

Another focus that the CEO of AAS would like to see is a customized bone interface.

This means making the insertion site on the end pad fit to the particular person that it is being

used by. This can be used by some of the techniques that were used in this project. Computed

tomography scans can be taken of the bone before it is to be amputated. These scans can then be

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loaded into the program MIMICS® and a 3D model of the bone can be created. This can then be

imported into CAD and an end pad model could be made to fit that bone. The end pad could

then be made through a process like 3D printing.

Method for Mass Production

Mass production is an area that needs to be investigated. Once the prototype models have

been made a decision needs to be made of what will be the most efficient way to produce the end

pad.

New Socket Design

A potential focus of this project it designing a new type of socket for the amputee once

the end pad has been implanted. Since the load transfer will be placed back onto the bone, a new

socket can be designed and patented.

Roles and Responsibilities

This team consists of Michael DiCicco, Trevor Taormina, and Stephen Osterhoff. For

our technical expertise, we have a well-rounded group consisting of our advisors Gordon

Maniere CEO of AAS, Dr. Nasir, and Dr. Meyer. Along with our vast contributors of Tech

Hwy, WEC Group, and a surgeon consultant Dr. Jon Ilhas of DMC and St. Mary’s Mercy

Hospital, we have a very dedicated team. The roles each of the engineering team members

overlap so we are all involved in the process of completing our goals. However, each of us are

leads in areas that allow us to accomplish these goals. Michael is most responsible for collecting

the data from the various tests, designing and refining the implantable end pad design, and

developing the protocol for our tests. Trevor is responsible for documenting our progress, the

lead on the mechanical testing, and the interpretation of the FDA documentation. Stephen is

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responsible for our finite element analysis, fabricating the prototype device, and also FDA

interpretation as well. Each of these tasks highlights sections of our individual skillsets so we

are able to work independently, but also lead in group work.

Cost Analysis

These costs are based on what is currently available on the market and are subject to

change. The test equipment that is planning on being used is an Instron. Currently within

Lawrence Technological University there are two of these machines that are available for use.

The material that is being used for the IEP has been provided by the company DSM and they are

continuing to work in conjunction with this project to make sure all material needs are met. Test

fixtures will be made out of metal and any other parts that are needed (i.e. screws, nuts, bolts).

For fabrication of the test fixture Lawrence Tech has a fabrication lab that is available for use.

Strain gages will be bought prewired to save time and ensure quality. The electronic setup will

consist of a strain gauge monitoring system and any extra wires that may be needed. Proposals

will be made to LESA and NCIIA for the funding of this project.

Table 1: Projected cost analysis for project objectives

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Materials Costs

Testing Equipment Provided by Lawrence Tech

IEP Material Provided by DSM

Test Fixtures $300-$500

Strain Gages $30-40 ea.

Electronic Setup (Strain Gages)

$100

Total $430-$640

**Planning to apply for funding

LESA, NCIIA Program

References

[1].."Understanding Limb Loss in the United States." Understanding Limb Loss in the United

States. Amputee Coalition, 27 Nov. 2012. Web. 23 Oct. 2013.

[2].. "Causes of Amputation." MossRehab.com. Web. 14 Dec. 2013.

[3].. Pinzur, M. S., J. Beck, R. Himes, and J. Callaci. "Distal Tibiofibular Bone-Bridging in

Transtibial Amputation." The Journal of Bone and Joint Surgery 90.12 (2008): 2682-687.

Print.

[4].."DSM in Medical." DSM. N.p., n.d. Web. 14 Dec. 2013.

[5].."Implant Materials." Implant Materials. N.p., n.d. Web. 14 Dec. 2013.

[6] Geary, C., C. Birkinshaw, and E. Jones. "Characterisation of Bionate polycarbonate

polyurethanes for orthopaedic applications." Journal of Materials Science: Materials in

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Medicine 19.11 (2008): 3355-3363.

[7]..Wehner, Tim, Lutz Claes, and Ulrich Simon. "Internal Loads in the Human Tibia during

Gait."Clinical Biomechanics 24.3 (2009): 299-302

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