Michael Yancey - Dr. Ting Cong - Week 1

For the summer immersion term, I am working with Dr. Ting Cong, who has partnered with the HSS Biomechanics department to explore the mechanical efficacy of allograft bone product in repairing femoral head impaction fractures. To do so, we have used the product on a cohort of cadaveric femurs. In short, each femur has a 2cm deep core removed from the femoral head and half of that core is morselized and then replaced to simulate the bone defect caused by an impaction fracture. We then perform force-controlled loading to 800N (approximating body mass) on the replaced core to obtain force-displacement characteristics of the defect area, especially the force-displacement slope and linear displacement at 800N. The femur is then repaired through a "metaphyseal-entry retrograde fluoroscopic approach" using a torchar cannula, which allows us to impact 2cc of chain-milled allograft bone into the defect, reducing fracture volume. The force-controlled loading is repeated on the repaired fracture for comparison. In addition, we performed 3D surface scans on the femur throughout the process for future volumetric analysis. It is that volumetric analysis that I primarily performed this week.

An important outcome of the femoral impaction fracture project is the volume of the defect. To do so, we perform the 3D surface scan at three stages: after we simulate the impaction fracture and load the femur to 800N, after we repair the fracture using allograft bone, and after loading the repaired bone to 800N again. I then convert those scans into STL files and analyze them in 3D modelling software. 

To describe that approach in brief, I estimate the diameter of the femoral head and create a sphere of the same diameter which I iteratively fit to the femoral head until the surfaces match as closely as possible. I then subtract the femur from the sphere, leaving just the cored defect area and other small portions of the sphere because the fit is never perfect. After some post-processing to remove the extraneous remainder of the sphere, I measure the volume of the defect area. This is repeated for the three stages mentioned before. Typically, the defect volume is approximately 2000 cubic mm after simulated failure and compression, near 0 after repair, and in the range of 200-800 cubic mm after compressing the repaired bone. This reduction in defect volume after repair is promising regarding the efficacy of the allograft bone product.

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