MECH 4013 Lab 1 Stress Shielding

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Carleton University *

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4013

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Mechanical Engineering

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Jan 9, 2024

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MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 1 MECH 4013 – BIOMEDICAL DEVICE DESIGN EXPERIMENT 1: Stress Shielding after Fracture Fixation ( Location: 7103 Canal building ) 1. Introduction The objectives of this laboratory exercise are to give students practical experience of how to treat bone fractures, evaluate a simple fracture fixation device and explore the effect of such devices on the load transfer in the bone. Fixation of fractured bone fragments involves a trade-off between rigid fixation and stress shielding. Fixation must be sufficiently rigid to allow the fracture to heal otherwise fibrous tissue forms, resulting in non-union or mal-union. However, the excessive rigidity of the fixation device causes shielding of the bone from physiological load after fracture healing has occurred, resulting in lower stresses in the bone and a localized decrease in bone density. Stress shielding can adversely affect long-term outcome and may result in implant loosening and re-fracture of the bone. In this experiment, the stress shielding effect of a fracture fixation plate is evaluated on a porcine femur. The porcine femur is loaded in a servo-hydraulic testing machine (MTS) with and without a fracture fixation plate. A transverse fracture is simulated by cutting the porcine femur with an oscillating saw. Three strain gauges glued to the femur will measure the longitudinal deformation close to the fracture and the plate. 2. Safety Every student is required to bring a lab coat and safety goggles. While handling the biological tissue, the students are required to wear surgical gloves. Please advise the instructor beforehand if you are allergic to latex. You may use a scalpel to remove soft tissues from the cadaveric porcine bone. Use caution when using the scalpel. A special container is provided for disposal of scalpel blades after use. The scalpel blade is removed from the handle using pliers. While operating the drill press and the oscillating saw the students are required to wear the provided face shields. The servo-hydraulic testing machine will be operated by the teaching assistant (TA). Ref [1] contains information pertinent to the safe operation of this machine. 3. Materials Servo-hydraulic testing machine MTS (Canal Building Room 7103)
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 2 Cadaveric porcine femur A custom designed fracture fixation plate appropriate for the porcine femurs. #8 wood screws will be used to fix the bone plate to the femur. The screw holes will be predrilled with a #29 drill bit Drill press Manual screwdriver to drive the screws into the bone. A permanent marker to mark the screw holes and the line that will be cut. An oscillating saw will be used to create the simulated a fracture. Three strain gauges, which will be glued to the femur; these will be provided with wires attached. Cyanoacrylate (crazy glue) Ruler Clear tape P-3500 Vishay bridge amplifier. Calipers Laser level Femur clamp fixture 4. Experimental Procedure 1. Decide where on the femur the fixation device should be placed. Make sure you put the plate onto the side where you expect the greatest tensile stresses. Using the permanent marker, mark the position of the four screw holes and trace the edges of the fracture fixation plate. 2. Using the permanent marker, mark the location of the simulated transverse fracture located midway between the inner two screw holes screws (i.e. middle of the plate). Mark the entire circumference of the femur. 3. Check the distal end of the femur and the clamp fixture; some bone may need to be removed. Cut the distal end of the femur so that a flat surface will sit on the bottom of the clamp fixture and the femur will remain vertical. Do not remove too much bone from the distal end, otherwise the clamp fixture may interfere with the fixation plate. 4. Using the drill press drill the screw holes through the full diameter of the diaphysis. Wear the face shield for your protection. 5. Glue the strain gages just above the intended fracture line. One should be adjacent to the fixation plate, one opposite the plate, and a third midway between the other two strain gauges (i.e. the approximate neutral axis of the bone). Be sure to account for the possibility that screws may pierce the bone opposite the fixation device when placing your farthest strain gauge. Figure 1 shows a reasonable placement pattern. The gauges should be placed so that the wires are away from the fracture line. 5. Thoroughly clean the areas where the strain gauges will go. Pick up a strain gauge with the tape, put cyanoacrylate on the part of the bone where the gauge is to be placed, and press the
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 3 strain gauge down. Hold it until the cyanoacrylate dries, then carefully remove the tape. Repeat this procedure for the other two strain gauges. See Appendix A for more details. 6. Place the femur into the clamp fixture and tighten the eight securing screws. Ensure that the metal buffer plates stay between the femur and the securing screws. Make sure you mark the alignment in case you have to remove the femur. 7. Attach the strain gauges to the amplifier. 8. Ensure that the cooling valve for the MTS is open, that the machinery is on and the emergency stop buttons are pulled out. Note all operation of the MTS machine will be performed by the TA but are noted here to document the procedure. 9. At the computer terminal, open ft40 and use the manual command button to enable exclusive control. 10. Zero the amplifier connected to the strain gauges by pressing the “balance” button. 11. Lower the actuator of the MTS machine so that it just touches the femoral head. Using the ruler measure and record the perpendicular distance from the point of contact to the neutral axis of the femoral diaphysis. 12. Using the MTS program, load the porcine femur in increments of 0.2 kN up to a maximum of 1.2 kN. Record the applied force and the strain at each gauge for each load step. Repeat these measurements 3 times. Note the exact load applied can vary, but should be the same (within 10-20 N) for each test. 13. Unload the femur and fix the fracture fixation plate to the femur without removing it from the clamp fixture. The screws should project through the opposite cortex. Be very careful not to disturb the strain gauges. This configuration simulates a healed fracture. 14. Using the MTS program, load the porcine femur with the plate in increments of 0.2 kN up to a maximum of 1.2 kN. Record the applied force and the strain at each gauge for each load step. Repeat these measurements 3 times. 15. Unload the femur and without removing the femur, using the oscillating saw cut the femur along the line you marked in step 2 all the way to the fixation device. This cut should be level with the centre of the fracture fixation device and slightly below the strain gauges. This configuration simulates the initial fracture fixation situation. 16. Using the MTS program, load the porcine femur with the plate and simulated fracture in increments of 0.2 kN up to a maximum of 1.2 kN. Record the applied load and the strain at each gauge for each load. Repeat these measurements 3 times. Try to determine at which load the gap is closing. 17. Unload the femur, ensure the MTS is immobile and remove your femur from the clamp fixture. Disconnect the wires from the amplifier. 18. Remove the plate and measure the average outer and inner diameter of the bone at the fracture site and the width and height of the fracture fixation plate 19. Clean all the instruments and fixation devices. Wipe down the MTS and the tables with soap.
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MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 4 5. Theory Bone plates serve two primary functions. They are intended to hold bone fragments in place while the bone heals and they carry some of the load the bone would otherwise have to carry alone. Although it is necessary for a fixation device to carry a large part of the load in the short term to allow healing, the bone plate will continue to carry a large part of the load after the bone has healed. From solid mechanics, we know that for two parallel elements subject to a load, the stiffer element will carry a greater portion of the load, and will thus be subjected to higher stress. In this case, bone is the less stiff element, and in the region of fixation the stresses are reduced compared to an intact bone with no fixation. Another possible cause of concern can occur soon after the bone plate is installed. A bone fracture can leave a gap between bone fragments. This gap opens and closes depending on the kind of load applied. This further complicates the shared-loading dynamic between the bone and the fixation device. Its effects will be exaggerated in this experiment by creating a large gap in the bone. Observing the effects of this change will help students understand how a gap between bone fragments would affect the load dynamics experienced by the plate and bone system. It is possible to make predictions about the stress experienced by a bone and plate system. Numerous simplifying assumptions must be made about the system to allow this. It is assumed Figure 1: Example of placement pattern of strain gauges. The clamp fixture is partially visible at the bottom.
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 5 that the bone and plate act as a composite beam, and that both components exhibit elastic behaviour. The bone is modelled as an intact hollow cylinder rather than a fractured asymmetric body for simplicity. The plate can be assumed to have a square cross-section and is rigidly attached to the femur (Figure 2). Using composite beam theory, the stresses in the bone can be estimated using: for axial load for bending With the neutral axis located at: With:
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 6 6. Analysis Calculate the average measured strain at the three locations and report the average and standard deviations for each load step and each condition (intact, healed, fracture). Using the averaged strain values calculate the stresses at these positions for all three scenarios, intact femur, femur with plate (healed condition) and simulated fracture (initial fixation condition). Using composite beam theory calculate the maximum strains and stresses at the measured locations for the intact femur and the femur with the plate (healed condition). Address the following questions in your discussion: 1. What are the strains in the bone for the fracture condition? If this differs from the expected values, explain the possible cause(s). 2. What is the percentage difference of your measured and calculated stresses? (Use the average values at the maximum load step). 3. What are the sources of error? 4. Determine the Young’s modulus that best fits your model. 5. How does the closing of the gap affect your measurements? 6. Where do the max. and min. stresses occur in the 3 different scenarios? 7. What is the amount of stress shielding of the bone due to the plate? 8. What is the stress in the plate? 9. Estimate the max number of load cycles the plate could sustain without fracture. 10. Based on your analysis where would you expect changes in the bone to occur? Figure 2: Simplified bone fracture plate model
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MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 7 7 References [1] Authors unknown, (2008) Series 370 Load Unit Product Information Manual Part Number 100-184-093 A, 100-184-093 B, MTS systems corporation, Minnesota [2] D.L. Bartel, D.T. Davy and T.M. Keaveny (2006) Orthopaedic Biomechanics: Mechanics and Design in Musculoskeletal Systems. New Jersey: Pearson Prentice Hall.
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 8 Appendix Placing strain gauges 1. Remove all soft tissues and clean the bone thoroughly, especially around the fracture site and gauge locations. Any tissues or fluids will prevent adhesion of the gauges. 2. Place tape on top side of gauge.
MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 9 3. Place Cyanoacrylate on the bone where gauge will go. 4. Press gauge down firmly and hold thumb on gauge until cyanoacrylate has set.
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MECH 4013 – BIOMEDICAL DEVICE DESIGN Experiment 1 10 5. Carefully peel tape off the gauge at an acute angle. 6. Secure the connections with more cyanoacrylate. Wires may be glued to the bone surface to limit any accidental forces that could pull gauges off bone.