Three-dimensional orientation of the femoral curvature. How well does it match with the sagittal curvature of femoral implants?

BACKGROUND: The curvature of the femoral shaft is generally considered as residing in the sagittal plane. While many studies have measured the femur's radius of curvature, very few have studied the orientation of the plane in which it resides. The orientation of this plane may affect the rotation of intramedullary nails or revision stems with diaphyseal locking. This led us to conduct a three-dimensional (3D) anatomical study to: 1) analyze the 3D curvature of the femoral diaphysis by specifying the orientation of the plane in which it resides, 2) look for relationships between the curvature's orientation and anatomical parameters of the femur.

HYPOTHESIS: We hypothesized that the femoral curvature resides in the anterolateral plane, not the sagittal plane.

MATERIALS AND METHODS: A computed tomography scan was performed on 45 dry femur bones provided by a cadaver laboratory. The 3D reconstructions were analyzed to characterize the curvature of the diaphyseal shaft, radius of its various portions and to determine the plane in which it resides relative to the standard coronal reference plane defined by three points: posterior side of both condyles and the greater trochanter. The following parameters were measured: length, neck-shaft angle, femoral valgus (between anatomical and mechanical axis) and neck anteversion.

RESULTS: The largest curvature was in a plane oriented on average of 78.3°±14.9° (35.2° to 106.7°) anterolateral. The correlation between femoral curvature and neck-shaft angle (R=0.172), size (R=0.095), valgus (R=0.104) and overall curvature (R=0.60) was low.

DISCUSSION: Implants with diaphyseal fixation are designed such that the femoral curvature resides in a strict sagittal plane. Long revision stems with diaphyseal fixation may have a tendency to rotate laterally (externally) during insertion. Correcting this external rotation to prevent anteversion may result in a less than satisfactory press-fit. The femoral curvature must be taken into account when designing intramedullary implants.

LEVEL OF EVIDENCE: IV, Cadaver study without control group.