Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Senior Design 2012: Critical Design Review Client Ha Van Vo, MD, Ph.D., DPM Department of Biomedical Engineering Team Members _____________________________ KaLia Burnette _____________________________ Kenny Tang _____________________________ Griffin Pafford Manager Hodge E. Jenkins, Ph.D., P.E. Department of Mechanical Engineering Technical Advisors Ha Van Vo, MD, Ph.D., DPM Department of Biomedical Engineering Dr. Richard Kunz, P.E. Department of Mechanical Engineering Submitted on April 9, 2012 Mercer University School of Engineering School of Engineering Mercer University Macon, GA 31207 2 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Acknowledgements Thank you to Quang Luong for running the MTS machine for our test and also for the idea of the dashed gluing method used to attach the Neoprene to our socket. Thank you to Trung Le, Kevin Ngo, and Mr. John Mullis for machining some of the parts needed for the prosthetic. 3 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Executive Summary An amputation includes any removal of a body extremity by trauma or surgery. Our client Dr. Ha Vo had designed, built, and tested prosthetics for the people of Vietnam (developing countries). He recently designed a universal prosthetic for above knee (trans-femoral) amputees. While the device was proven successful it was not the most efficient for fitting all types of trans-femoral amputations. A prosthetic that is not properly adjusted can lead to skin breakdown and an unnatural weight distribution, which can be hard on the patient’s body. It would be desirable to have an improved design that is more universal for trans-femoral amputees. This would further support the mission program in Vietnam, thus enabling future teams to reach out to more amputees faster and more efficiently. Dr. Ha Vo has specified that the redesigned prosthetic be able to fit a wider range of amputees with fewer socket size variations than the current design. The prosthetic should remain inexpensive and should increase no more than 5% of the original design cost. A protective shell should also be added to the ‘knee’ j oint. This Critical Design Review (CDR) represents all the testing and finalized results of an improved universal trans-femoral socket design and the implementation of a casing for the knee joint. 4 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Table of Contents 1. Introduction ....................................................................................................................................... 7 1.1 Problem Statement ..................................................................................................................... 7 1.2. Objectives ................................................................................................................................... 8 2. Background ........................................................................................................................................ 9 3. Materials & Methods......................................................................................................................... 9 3.1 Materials ...................................................................................................................................... 9 3.2 Assembly .................................................................................................................................... 10 3.2.1 Rigid Knee Casing .....................................................................................................................................10 3.2.2 Partially Elastic Socket ..........................................................................................................................13 3.2.3 Snap Ring ....................................................................................................................................................15 4 Testing ............................................................................................................................................... 17 4.1 Preliminary Testing .................................................................................................................... 17 4.1.1 Glue Testing.................................................................................................................................................18 4.1.2 Neoprene Life Testing............................................................................................................................18 4.1.3 Finite Element Model Simulation.....................................................................................................18 4.2 Post Testing ................................................................................................................................ 21 4.2.1 Patient Testing ...........................................................................................................................................21 4.2.2 Case Testing ................................................................................................................................................21 5. Results & Discussion ........................................................................................................................ 22 5.1 Preliminary Testing .................................................................................................................... 22 5.1.1 Glue Testing.................................................................................................................................................22 5.1.2 Neoprene Life Testing............................................................................................................................24 5.1.3 Finite Element Model Simulation.....................................................................................................26 5.2 Post Testing ................................................................................................................................ 28 5.2.1 Patient Testing ...........................................................................................................................................28 5.2.2 Case Testing ................................................................................................................................................29 5.3 Updates from PDR ..................................................................................................................... 29 5.3.1 Knee Case......................................................................................................................................................29 5.4 Budget ........................................................................................................................................ 29 6. Summary and Conclusion ................................................................................................................ 30 7. Recommendations ........................................................................................................................... 30 5 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 7.1 Implementation improvement ................................................................................................. 31 7.2 Testing Improvement ................................................................................................................ 31 6. References........................................................................................................................................ 31 7. Appendixes ....................................................................................................................................... 31 I. Materials and Manufacturer .................................................................................................. 31 II. Drawings of Prosthetic ............................................................................................................ 32 IV. Project Schedule ....................................................................... Error! Bookmark not defined. Resumes ............................................................................................................................................... 53 Table of Figures Figure 1. Final Leg Design .................................................................................................................... 10 Figure 2. Making of the plaster mold ................................................................................................. 11 Figure 3. BT-1 Infrared Oven ............................................................................................................... 11 Figure 4. Making of polyethylene shell. ............................................................................................. 12 Figure 5. Finished Case ........................................................................................................................ 13 Figure 6. Making the Socket ................................................................................................................ 14 Figure 7. Snap Ring .............................................................................................................................. 15 Figure 8. Sketch for snap ring.............................................................................................................. 15 Figure 9. Fully Completed Prosthetic Leg ........................................................................................... 16 Figure 10. MTS Machine with Jig ........................................................................................................ 17 Figure 11. DAP Weldwood Marine Contact Cement Glue ................................................................ 18 Figure 12. Boundary Conditions and Loads ....................................................................................... 20 Figure 13. Meshing of the Model ....................................................................................................... 21 Figure 14. Neoprene attachment ....................................................................................................... 22 Figure 15. Testing sample setup ......................................................................................................... 23 Figure 16. Gluing Method: .................................................................................................................. 24 Figure 17. Sample one: ........................................................................................................................ 24 Figure 18. Sample two: ........................................................................................................................ 25 Figure 19. Max Principal Stress Analysis ............................................................................................ 26 Figure 20. Equivalent Stress Analysis ................................................................................................. 26 6 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Figure 21. Equivalent Elastic Strain..................................................................................................... 27 Figure 22. Total Deformation .............................................................................................................. 27 List of Tables Table 1. Problem and its corresponding prosthetic component ........................................................ 9 Table 2. Glue test results ..................................................................................................................... 23 Table 3. Max Stress Values .................................................................................................................. 27 Table 4. HDPE Yield Stresses ............................................................................................................... 28 Table 5. Neoprene Rubber Yield Stress .............................................................................................. 28 Table 6. Nylon Strap Yield Stress ........................................................................................................ 28 Table 7. Material Cost.......................................................................................................................... 30 Symbol and Abbreviations Glossary Ambulation: To walk from place to place; move about Distal: Situated away from the center of the body or from the point of attachment Fibula: The outer and usually smaller of the two bones between the knee and the ankle in humans Gait: HDPE: Hyperelastic : Lateral : A particular way or manner of moving on foot High Density Polyethylene - thermoplastic made from petroleum Materials which can experience la rge elastic strain that is recoverable. the side of the body or a body part that is farther from the middle or center of the body MTS: Medial: Material Testing Systems – Comprehensive system for testing materials properties Situated in the middle, in part icular Plantarfelxion: movement of the foot that flexes the foot or toes downward toward the sole Posterior: Pertaining to or toward the back plane of the body Proximal : Situated nearer to the center of the body or the point of attachment Sagittal: A vertical plane passing through the standing body from front to back Stump: The distal end of a limb after amputation Trans - femoral: Across or through the femur. Trans - tibial: Across or through the tibia. Valgus: outward angulation of the distal segment of a bone or joint Varus: inward angulation of the distal segment of a bone or joint 7 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 1. Introduction 1.1 Problem Statement According to the MUSE BME: “More than 2,000 Vietnamese are injured each year by land mines and unexploded bombs left during the Vietnam War. An estimated 100,000 amputees live in Vietnam today, and there are more than 18 million amputees around the world, with more than 80 percent of those living in developing countries.” Dr. Ha Vo has designed and tested a low cost, universal prosthetic for the amputees of the developing third-world country of Vietnam. Although Vietnam has its own prosthetic program, it is not nearly as advanced as the prosthetic program in the United States. The prosthetics themselves are very expensive and not very efficient due to their weight and crude design when compared to the prosthetics in the United Sates. Over the past three missions in Vietnam (2009-2011), 179 trans-tibial amputees were fitted and 23 trans-femoral amputees have been fitted. Each year, the team spends about four weeks treating amputees and opening temporary clinics throughout Vietnam. During each mission the Vietnamese government and its people express their support and gratitude towards the program and look forward to improved progress every year. As a result, the Mercer on Mission Vietnam Team strives to reach more and more patients, with a more optimized design each year. The universal prosthetic devices designed by Dr. Ha Vo has proven successful in fitting below and above knee amputees in Vietnam. While the above knee prosthetic is successful, it is not optimal for fitting all types of residual limbs. It would be desirable to have an improved design that is more universal for above knee amputees. This would further support the mission of the program in Vietnam, thus enabling future teams to reach out to more amputees. In an attempt to improve upon the current design Dr. Vo outlined several specific requirements needed to be addressed but encouraged creativity among the team. The following Critical Design Report (CDR) provides a detailed report for the design of a trans-femoral prosthetic which will lower the overall cost of fitting trans-femoral amputees by providing higher adjustability than previous designs as outlined by Dr. Vo. It will also extend the life of the prosthetic by providing better protection of the knee joint via a knee cover. The team was able to develop an improved design that effectively addresses and improved the function of the previous design. 8 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 1.2. Objectives The main objective was to develop an affordable improved Universal Trans-Femoral Prosthetic for above-knee amputees that can be utilized effectively in 3 rd world countries such as Vietnam. The inital objective in this project was to make sure that the chosen design adheres to the feasibility criteria as set by the team and Dr. Ha Vo. This criterion explicitly conveys the minimum requirements for the proper performance of the chosen design. Later, testing was conducted to ensure that the merit criteria were met with satisfactory. Designs proposed for the hip socket were to meet the following feasibility criteria: Adjustable: One of the defining goals for the socket is for it to be adjustable. The higher the degree of adjustability and the easier it is to adjust the better. This will allow more efficient fitting of patients with less equipment. Comfort: This socket will be worn by the patient throughout the day and will need to be able to support the wearer without causing discomfort or irritation to the stump. Inexpensive: The socket being made is for use in Vietnam and developing countries and should be inexpensive to make. Therefore it is important that the material and labor cost required to make this part remains low. Durable: Since the prosthetic will be worn throughout the day it will need to be able to withstand the stresses applied to it from impact, weather, etc. Easy attachment: The socket should be easy to attach to the patients stump, so that they can strap on or remove the leg at any time. Also all designs should be easily attached with the res to the current prosthetic. Easy maintenance: The design should be relatively easy to clean Designs proposed for the knee casing had to meet the following feasibility criteria: Attachment: The casing should securely fasten to the prosthetic. Protection: The casing should provide protection to the knee joint, especially the rubber tubing used for knee extension. Durable: The casing should be able to withstand small impacts they may occur throughout the day. Maintenance: the cover should be easy to clean. All feasibility criterion was presented in the Preliminary Design Review (PDR). In the PDR, two designs were proposed for the socket (Double Socket and Partially Elastic Socket) and two designs were proposed for the knee casing (Rigid Casing and Elastic Casing). The Partially Elastic Socket with the Rigid Knee Casing was chosen based on Merit Curve analysis. 9 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees The final objective, after the completion of testing, was to analyze the test results and identify improvements and/or changes that can be implemented in future projects to advance the performance of the Universal Trans-Femoral Prosthetic. 2. Background The teams’ research was focus on gathering information about the amputations of the lower limbs and how prosthetics are used to treat these types of amputees. An amputation includes any removal of a body extremity by trauma or surgery. Higher levels of amputation require a larger consumption of energy and oxygen by the patient, especially during ambulation [1] In addition, many factors must be considered when fitting a patient with the prosthetics which must be thought about when building a prosthetics. This includes: the size and shape of the stump, comfort of the patient, total length of the leg, and adjustment for valgus and varus knee and ankles. Each component of the prosthetic will address each issue as shown in the table below: Table 1. Problem and its corresponding prosthetic component Problem Prosthetic Component Size and shape of stump Socket Comfort of patient Prosthetic sock and stump wrapping Total length of leg Socket size and p y l on length Valgus and varus knee Washers to counteract valgus or varus Valgus and varus ankle Medial and lateral adjustment of ankle Since most of the team will be attending the next Mercer on Mission Vietnam trip, the team also focused on gaining familiarity on the old prosthetic design and its manufacturing process. Besides being familiar with the process, it is important that the team understands the mechanics behind ambulation and how it transfers to the performance of a prosthetic leg. 3. Materials & Methods The following section is a detailed description of the assembling procedures for the improved trans-femoral prosthetic which includes the Partially Elastic Socket and the Rigid Knee Casing. This section will include materials, testing, and assembly of the design 3.1 Materials The support of the Partially Elastic Socket was constructed from high density polyethylene (HDPE). The medial and lateral sides were replaced with neoprene. The Neoprene was attached to the plastic portion of the socket using DAP Weldwood Marine Contact Cement Glue. The pylons and knee joint, which remained unaltered, were made from Aluminum 10 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 316L. Surgical tubing was used to induce knee flection. The foot and ankle portion also remained unaltered as was constructed from HDPE for support and creep for impact absorption. The Rigid Knee casing was made from HDPE and steel snap rings were used to hold the knee casing in place. 3.2 Assembly Figure 1. Final Leg Design 3.2.1 Rigid Knee Casing Making the Mold The Rigid Knee Casing was made using a casting method. The negative mold was made using plaster, with the inside dimensions depicted in the Final Knee Covering Design in the Appendix. The mold was created by taping together two cans of soup which was then filled with plaster to create a base for the plaster mold. A metal rod was inserted into the mold before the plaster fully dried. When the plaster dried the soup can was cut off the plaster mold and then additional layers of plaster were applied by hand until the mold reached the desired diameter of 4 inches. Finally, the end of the mold was shaved until it represented the domed shape of the Rigid Knee Casing. 11 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Figure 2. Making of the plaster mold Making the Shell To form the shell, a 12in X 12in X 0.5in sheet of HDPE was heated to 390 o C in the BT-1 Infrared Oven. At this temperature the polyethylene can be easily molded for a short period of time. After reaching the desired temperature, the polyethylene was pulled down over the mold and placed under a vacuum to prevent wrinkles from forming. Figure 3. BT-1 Infrared Oven 12 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Figure 4. Making of polyethylene shell. The polyethylene was then allowed to cool. Then a handheld power saw was used to divide the case in half and the mold was removed. Originally, it was planned that the mold itself would create a satisfactory top for the knee case but unfortunately that was not the case as the vacuum was not powerful enough to pull it tightly to the top of the mold. Therefore the top of the case was not flat enough or consistent enough for the snap ring to keep the case up against the bottom socket plate. It was then determined that the best thing to do would be to cut off the upper end section and replace it with a flat piece of the same material. A sheet of HDPE was cut out into a rough circle on the band saw to the outer diameter of the case. A lathe was used to make it a perfect circle. A one inch hole was bored out in the middle on the milling machine. The plan was to then join the plate to the top of the knee case through the use of the same rubber cement glue that was used. Several different attempts were made to create samples to test the strength of the attachment between the two glued pieces of plastic however, all the samples created a weak bond even after being allowed to dry for an extended period of time. It was eventually determined after doing some research that there was no real glue that could bond together HDPE outside of some extremely expensive glue made by 3M ™ Research suggested that the only two ways to bond together two pieces was to either mechanically join them or one of two types of welding (Friction or Hot Air welding).The 13 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Welding Shop does have a hot air welder but neither John Mullis or the team could figure out how to make a satisfactory weld with it. However, we do know from the design of the prosthetic foot and our testing with the neoprene material that soft porous materials bond readily to the HDPE using the rubber cement. Therefore it was decided that to get around the bonding problem by bonding the plastic pieces to a creep foam intermediary. The bond that it created was sufficient for our purposes and was chosen as the method of attachment. Figure 5. Finished Case To attach the two pieces of the knee cover it was originally decided that they would be joined by lacing them up with shoestring. However, the client expressed reservations about that plan, as a result, Velcro was considered and purchased. When the original method of shoe strings was tested it was found that it was more than sufficient for our purposes. Because of this, the Velcro option was not pursued. Five holes were drilled in the back and six holes were drilled in the front. A shoelace was laced up in an alternating pattern between the holes as shown in Figure 5. 3.2.2 Partially Elastic Socket The Partially Elastic Socket was created from a mold resembling a person’s thigh. Once the socket was obtained ‘V’ cuts were made in the medial a nd lateral sides of the socket. The width of the top of the ‘V’ was 1.5in and extended 9in dow n the side of the socket. A 0.25in 14 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees diameter hole was drilled at the base of the cut. To decreas e the ‘V’ cut gap size to 1in, Velcro straps were used to compress the socket. Neoprene was glued over the gap on the inside of the socket using contact cement using the striped glue method. which will be outlined in the testing porting of the CDR. This was allowed to dry over 24hrs. Four holes were drilled in the bottom of the socket so that flat head screws could be used to attach the bottom socket plate to the socket, securing the Partially Elastic Socket to the rest of the leg. Figure 6. Making the Socket 15 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 3.2.3 Snap Ring Figure 7. Snap Ring To keep the Rigid Knee Case on the prosthetic leg a 1 inch snap ring was employed. A groove was cut into the upper leg pylon with the dimensions specified in figure below using a lathe. The groove was cut approximately 3.41in above the axis of knee rotation. The snap ring was seated by using a pair of TEKTON 3578 8-in-1 Universal Snap Ring Pliers. Figure 8. Sketch for snap ring 16 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Figure 9. Fully Completed Prosthetic Leg a. Partially Elastic Socket b. Ridgid Knee Casing a b 17 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 4 Testing Tests were done to ensure ultimate performance of the product. 4.1 Preliminary Testing To perform the material testing that our group needed to accomplish, a jig had to be created for the upper part of the MTS machine in the prosthetic lab. Figure 10. MTS Machine with Jig JIG 18 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees 4.1.1 Glue Testing The goal for this test is: 1) To see if the Neoprene and the DAP Weldwood Marine Contact Cement Glue can withstand enough forces to be used in the socket. 2) To determine the amount of DAP Weldwood Marine Contact Cement Glue needed to effectively hold the Neoprene to the high density polyethylene under various loads. 3) To observe the Neoprene under various loads. The amount DAP Weldwood Marine Contact Cement Glue used to attach the Neoprene to the plastic socket was determined by tension testing on the MTS machine. Neoprene- polyethylene surface overlaps of 0.25in, 0.5in, and 1in were pulled until failure occurred. Figure 11. DAP Weldwood Marine Contact Cement Glue 4.1.2 Neoprene Life Testing The neoprene life testing was done by gluing the material between 2 pieces of high density polyethylene. Using the MTS machine, a force was applied to stretch the neoprene 0.75in at a 1Hz frequency until it reached 2920 cycles or failure. 2920 cycles was chosen to represent the socket being worn for 2 years (being put on or taken off four times per day). It was assumed that in order to put on or take off the socket the circumference would change by 1.5 inch. Because this displacement would be shared between the medial and lateral portions of neoprene each section would only need to stretch 0.75 inches. It was also assumed that stretching of the socket during walking was negligible. 4.1.3 Finite Element Model Simulation Our client, requested that a second finite element model be constructed of our groups new socket design to model how the socket would respond to the interior pressure produced by the stump on the walls of the socket with the Velcro straps on it and with the holes drilled into the bottom of the Socket. Finite element analysis is a computational technique used to obtain the approximate solutions of boundary value problems in engineering. 19 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees The Partially Elastic Socket was modeled in Pro Engineer and was imported into ANSYS where a static structural analysis was performed. These results are shown in the figures below. The finite element analysis however is complicated by the fact that neoprene is a hyperelastic material which makes the material’s stress strain relationship non -linear. This makes the stiffness matrix of the element in the model no longer constant which complicates the solution of the finite element model in that it must now be solved by an iterative approach. ANSYS deals with most of the problems with hyperelastic materials in the background. However to be able to solve the model large deflections had to be turned on in the analysis settings because hyperelastic materials have a relatively huge potential to deflect when compared to something rigid like polyethylene. Most materials will, even under huge loads deflect very little thus there are hard coded restrictions against big deflections as this is usually a sign that the analysis has not converged. 20 | Improved Universal Trans-Femoral Prosthetic for Above-Knee Amputees Figure 12. Boundary Conditions and Loads There are several boundary conditions that were imposed on this model. The four holes in the bottom of the Socket are constrained with cylindrical supports, which mean that they cannot move in any direction. Near the top of the socket there are Nylon Velcro straps which keeps the socket se cure around the patient’s stump . A force of 300lbs was applied on the sides of the prosthetic. A force of 300lbs was used in order to simulate the maximum amount of force a 150lb person exerts on the socket when ambulating [13]