Retrograde femoral nailing: an understanding of the intercondylar insertion site.

Background: The intercondylar starting site for retrograde femoral nailing has been selected to avoid damage to the articular weight-bearing surface. This starting point assumes that implants will adapt to the femoral radius of curvature. Implant-femur radius mismatch may result in postoperative fracture angulations, translation, or increase in hoop stress. Methods: Twenty cadaveric femurs were analyzed. The posterior cruciate ligament (PCL) was preserved. Two different femoral nails were analyzed. After creating an osteotomy at the superior level of the lesser trochanter, a cannulated nail was inserted to the level of the articular surface of the distal femur. A pointed guide wire was advanced through the nail and driven through the articular surface of the distal femur. The exact location of the guide wire exiting the articular surface was anatomically and radiographically determined. Results: The mean anterior distance of the wire to the PCL was 20.4 mm for the ACE nail and 13.9 mm for the Synthes Femoral Nail (SFN). A Student's t test showed a significant difference between the two implants (p = 0.0002). The mean medial distance of the guide wire exit compared with the PCL was 4.2 mm for the ACE and 4.1 mm for the SFN nail and showed no significant difference. Conclusions: The tested nails require a more anterior insertion site than what has been described to match the femur curvature. Of the two nail designs evaluated, the SFN was more compatible with the currently recommended starting point.