Bioengineering
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Method and Instrumented Fixture for Femoral Fracture Testing in a Sideways Fall-on-the-Hip Position
Chapters
Summary August 17th, 2017
In this manuscript, we present a protocol to fracture test cadaveric proximal femora in a sideways fall on the hip configuration using instrumented fixtures mounted on a standard servo hydraulic frame. Nine digitized signals comprising forces, moments, and displacement along with two high speed video streams are acquired during testing.
Transcript
The overall goal of this experimental cadaveric femoral fracture testing protocol is to obtain consistent reproducible fracture strength measurements. This method can help answer key questions in osteoporosis research, by measuring the differences between osteoporotic and normal femoral strength. The main advantage of this technique is that, the testing apparatus mimics a physiological site based fall on the hip.
For attachment of the large fixture, first, securely fasten the aluminum block onto the machine. Next, use bolts to secure the main fixture structure onto the aluminum block and support the fixture with the placement of a four ton jack, under the part of fixture that does not rest on the aluminum block. Then use more bolts to secure a six channel load cell fixture onto the main fixture.
For attachment of the crosshead fixture, attach the first base plate with the machine crosshead set to absolute zero and the curved edges of the base plate, facing the front of the testing machine. Use one pivoting screw to attach the second base plate. Then, attach the assembly of the two slide bearings, to the second base plate.
Rotate the second base plate in a such a way, that the second set of screws can be accessed from the top of the first base plate. Then set the machine crosshead to a relative position of 65 degrees, to allow manual rotation of the slides, orthogonal to the six channel load cell. Once the instrumented fixture has been assembled on the standard server hydraulic machine, attach a high speed camera and lighting equipment to tripods.
Position the camera facing the testing machine and connect camera to data acquisition unit. After connecting the data acquisition system, manually push the load cell and observe the data signal traces in the view panel of the DAQ software. To confirm the proper connection and synchronization of the trigger signal, tricanteric load cell, head load cell, linear potentiometer and six channel load cell.
Calibrate the linear potentiometer, secure the linear potentiometer fixture to the crosshead and attach the potentiometer to the fixture. Tighten the screws to lock the potentiometer body and plug the connector into the DAQ unit. Next, manually move the actuator 25 millimeters on a load frame, so that the potentiometer position translate from the maximum compression to the maximum extension and record the displacements and the corresponding voltage for at least three data points.
Plot the displacement versus the voltage and fit a linear function to the data. Then input the slope of the linear equation as the calibration factor into the scanning parameter box of the DAQ software. To prepare the bone for testing, place a clean 24 hour thawed bone in the acrylic scanning fixture and mix 60 grams of poly or PMMA powder, with 30 grams of liquid resin in a disposable cup under a fume hood, until the powder has dissolved.
When the PMMA is pourable, align an aluminum cup below the tricanter and decant the PMMA cement to half the height of the cup. Raise the fixture platform to fit the bone into the cup and allow the PMMA to polymerise for 10 to 15 minutes. When the PMMA is clear and hard, move the bone to the test fixture and center the aluminum cup on the plate, attached to the tricanteric load cell.
Adjust the six channel load cell linear stages, so that the aluminum cup lightly touches the load cell and remove the pin from the fixture to a lateral rotation of the fixture. Then center and lower the crosshead for contact with the femoral head and image the femur from two sides. After verifying that the servo mechanical load frame is appropriately programmed, click start to initiate the test sequence for fracturing the test femur.
After the test is complete, manually retract the actuator. Remove the bone from the fixture and tape the proximal broken end of the bone to the shaft. Then wrap the bone Wet towels and plastic bags and return the bone to 20 degree celsius storage.
After fracture, the force recorded at the greater tricanter and the displacement recorded at the femoral head, using the six channel load cell can be plotted. The broken parts of the bone can then be taped together for further fracture type imaging and classification. While attempting this procedure, it's important to remember to verify that all of the data acquisition systems, function properly with fiberglass femurs before testing the cadaveric specimens.
After its development, this technique paved the way for researchers in the field of computational biomechanics, to validate quantitative computer tomography based finite element predictions of strength in osteoporotic femurs. After watching this video, you should have a good understanding on how to test cadaveric femurs in a fall on a hip configuration.
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