The Ideal Location of the Lateral Hinge in Medial Closing Wedge Osteotomy of the Distal Femur: Analysis of Soft Tissue Coverage and Bone Density

Background: Although an appropriate hinge position to prevent unstable lateral hinge fractures is well established in medial opening wedge high tibial osteotomy, the position during medial closing wedge distal femoral osteotomy has not been elucidated. Purpose/Hypothesis: The purpose was to evaluate the ideal hinge position that would prevent an unstable lateral hinge fracture during biplanar medial closing wedge distal femoral osteotomy based on soft tissue coverage and bone density around the hinge area. The hypothesis was that the ideal hinge position could be clarified by analyzing soft tissue coverage and bone density around the lateral hinge area. Study Design: Controlled laboratory study. Methods: In 20 cadaveric knees (mean age, 70.3 +/- 19.2 years), the femoral attachment of the gastrocnemius lateral head was quantitatively analyzed as a soft tissue stabilizer using digital photography and fluoroscopy. Then, medial closing wedge distal femoral osteotomy was performed, locating the lateral hinge either inside (group 1) or outside (group 2) the femoral attachment of the gastrocnemius lateral head, and the incidence of unstable lateral hinge fractures was compared between the 2 groups. Cortical bone density around the lateral hinge was measured using Hounsfield units on 30 computed tomography scans and reconstructed as a 3-dimensional mapping model. The transitional zone with low bone density was regarded as the safe hinge position with an increased capacity for bone deformation. Results: The upper and lower margins of the femoral attachment of the gastrocnemius lateral head were 9.1 +/- 0.9 mm above and 8.0 +/- 1.4 mm below the upper border of the lateral femoral condyle, respectively, and the femoral attachment of the gastrocnemius lateral head was widest in the anteroposterior dimension 0.4 +/- 1.7 mm above the upper border of the lateral femoral condyle. The incidence of unstable lateral hinge fractures during osteotomy was significantly decreased in group 1 compared with group 2 (group 1: 0/10; group 2: 5/10; P = .01). An isolated transitional zone with low bone density was observed in all 30 knees and located 1.3 +/- 0.8 mm above the upper border of the lateral femoral condyle. Bone density of the transitional zone with low bone density was significantly lower than surrounding femoral cortices (P < .001). Conclusion: Only the upper border of the lateral femoral condyle can be recommended as an ideal hinge position to prevent unstable lateral hinge fractures during biplanar medial closing wedge distal femoral osteotomy based on soft tissue coverage and bone density.