Background: This biomechanical study was designed to evaluate the predictive ability of dual-energy x-ray absorptiometry, cortical bone geometry as determined with computed tomography, and radiography in the assessment of torsional load to failure in femora with and without notching.
Methods: Thirteen matched pairs of cadaveric femora were randomized into two groups: a notched group, which consisted of femora with a 3-mm anterior cortical defect, and an unnotched group of controls. Each pair then underwent torsional load to failure. The ability of a number of measures to predict femoral torsional load to failure was assessed with use of regression analysis. These measures included dual-energy x-ray absorptiometry scans of the proximal and the distal part of the femur, geometric measures of both anterior and posterior cortical thickness as well as the polar moment of inertia of the distal part of the femur as calculated on computed tomography scans, and the Singh osteoporosis index as determined on radiographs.
Results: The torsional load to failure averaged 98.9 N-m for the notched femora and 143.9 N-m for the controls; the difference was significant (p < 0.01). Although several variables correlated with torsional load to failure, distal femoral bone-mineral density demonstrated the highest significant correlation (r = 0.85; p < 0.001). Moreover, multiple regression analysis showed that a combination of distal femoral bone-mineral density and polar moment of inertia calculated with the posterior cortical thickness (adjusted r
2 = 0.79; p < 0.001) had the strongest prediction of torsional load to failure in the notched group. The addition of other measures of cortical bone geometry, proximal femoral bone-mineral density, or radiographic evidence of osteopenia did not significantly increase the model's predictive ability.
Conclusions: Femoral notching significantly decreases distal femoral torsional load to failure and is best predicted by a combination of the measures of distal femoral bone-mineral density and polar moment of inertia. Together, these values account for the amount of bone mass present and the stability provided by the cortical shell architecture.
Clinical Relevance: Femoral notching during total knee arthroplasty decreases distal femoral torsional load to failure. By examination of femoral bone density and distal femoral geometry, the relative decrease in torsional load to failure can be predicted and appropriate precautions taken.