Mismatch of Current Intramedullary Nails With the Anterior Bow of the Femur

OBJECTIVES: The anterior curvature of the femur affects intramedullary nail insertion, revision prosthesis design, and the biomechanics of the proximal femur. Two previous studies, using small numbers of femurs, determined femoral curvature and showed that it was significantly greater than that of the several intramedullary nails they evaluated. In this study, the curvature of 948 femurs (474 matched pairs) was determined and compared with current intramedullary nails. The correlation of femoral curvature to age, gender, femoral size, and race was also evaluated. SETTING: Museum skeletal collections and a hospital biomechanics laboratory. METHODS: The curvature of 892 femurs (446) from the skeletal collections of 2 museums was measured by processing the digital images of the femurs with a computer curve-fitting program. Fifty-six additional, embalmed femurs (28 pairs) from our collection were also digitally imaged and then radiographed and their medullary curvatures similarly determined for comparison. Curvatures of 8 current antegrade intramedullary nails and 3 long-stemmed femoral hip prostheses were obtained from manufacturers and confirmed by measurements from their templates after digitization. RESULTS: We found the average femoral anterior radius of curvature was 120 cm (+/- 36 cm). There was no effect of age on femoral curvature nor was there a correlation between femoral width or femoral length to curvature. Black donor femurs had less curvature than white donor femurs (P < 0.001). There was close correlation (r = 0.967) between the femoral curvatures determined from the digital images and the radiographs. Radii of curvature of the intramedullary nails ranged from 186 to 300 cm (eg, straighter than the femurs). CONCLUSIONS: There was a large mismatch between the curvature of some current antegrade intramedullary nails and the average femur. Although this is only 1 factor affecting nail insertion, the mismatch warrants a reappraisal of these intramedullary nail designs. CLINICAL RELEVANCE: Because ease of intramedullary nail insertion and possibility of cortical comminution are determined by a number of factors including insertion point and fracture location, it appears that a decrease in radii of curvature (less straight) of current nail designs is warranted, particularly for those larger diameter nails designed for hip fracture stabilization that have greater rigidity due to design or material.