Three-dimensional computed tomography-based modeling of sagittal cadaveric femoral bowing and implications for intramedullary nailing

Objectives: To establish the nature and extent of femur sagittal bowing by determining outer and inner anterior cortex geometries and sizes using analytical 3-dimensional computed tomography modeling and relate the resultant femoral curvature with the curvatures of available intramedullary nails. Methods: Computed tomography scans were rendered in 3D using modeling software to reconstruct 2 geometries: (1) outer cortex and (2) medullary canal. Femoral bow in the sagittal plane and the anterior cortical thickness were measured. Three curvature points were selected on both the geometries. Exterior anterior cortex radius of curvature (AROC) and medullary canal radius of curvature (MROC) were compared within and between each femur pair. Results: Three groups of femurs exhibiting significantly different geometry patterns emerged: (1) AROC significantly greater than MROC, (2) AROC equivalent to MROC, and (3) AROC significantly smaller than MROC. Anterior cortical thickness ranged from 2.2 to 7.0 mm was significantly different for males versus females, and it varied inversely with age. Conclusions: The study confirms that the radius of curvature of most intramedullary nails exceeds the sagittal radius of curvature of most adult femurs (both AROC and MROC). An intramedullary nail selected based on the anterior curvature would impinge on the anterior cortices at the proximal and distal anterior aspects of the femur in specimens with AROC > MROC. Conversely, in specimens with AROC < MROC, an intramedullary nail selected based on the anterior curvature would impinge on the proximal and distal posterior cortices. That cortical thickness varied significantly in accordance with gender and age is also relevant to surgical planning. MROC, in addition to the AROC alone, should be one of several design parameters used to match specific intramedullary nail design to an individual patient.