Implantable End Pad for Lower Limb Prosthesis
Mike 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 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 isattaching 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 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.
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 implantable endpad 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 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 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 material will be chosen from the select four. When the appropriate Bionate® durometershave 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 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 endpad can be created. This model can then be manipulated to optimize the performance of the endpad. 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 endpad 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 endpad and the bone. From this it can be determined that what is the best way to attach the endpad to the tibia and though the use of strain gauges valuable data about the deformation of the material.
Prototype Development
Prototypes of the implantable endpad will be made from the durometers of material that is chosen.
Anticipated Challenges
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.