Cam femoro-acetabular impingement (FAI) surgery restores hip range of motion by trimming the headneck junction, which weakens the proximal femur. Sheep femurs resemble cam deformity and might be suitable for evaluating the impact of the surgical correction. Accordingly, this study addressed two questions: How does increasing head-neck resection affect ovine femoral strength? Are quantitative computed tomography (QCT)-based FE models able to discriminate the observed changes?
With approval of the veterinary board (Kantonales Veterinäramt Zürich, application 123/2006), 18 femoral pairs were distributed in three groups (3, 6 and 9 mm resection depth) and one random bone of each pair underwent surgery. Then, specimen-specific models were generated for each pair from QCT scans performed with a calibration phantom. Finally, compression tests were conducted ex vivo and in silico under stance configuration and experimental and simulated failure loads were quantified. Safety factors (SF) were calculated for walking and running activities based on the BMI. The weakening of the resected femurs was evaluated relative to their intact contralateral side.
Our results showed that ex vivo and in silico failure loads correlated strongly (r2=0.83, P<0.001) and exceeded hip forces induced during daily activities, even after strong resection (SF>1). For each group, the resections had more influence on the FE failure loads (−18%, −21%, −33%) than measured experimentally (−5%, −10%, −19%). The differences between resection groups were significant or close to P=0.05.
Two conclusions can be drawn from these results. First, just as for human hip, strength of ovine femurs significantly decreases with deeper resections. Fracture risk induced by the procedure, however, remains low even after 9 mm correction. Second, this study yielded first evidence of QCT-based models predicting the weakening of femurs after headneck resection. Based on pre-operative CT, those models could provide patient-specific guidelines to prevent over-correction during cam FAI surgery.
14 May 2016 - 17 May 2016