The Neutral Hinge for Medial Closing Wedge Distal Femur Osteotomy. A 3-dimensional Surgical Simulation Study.

Background: Because a valgus deformity is frequently associated with patellar instability, medial closed wedge distal femoral osteotomy (MCWDFO) has gained popularity. While the influence of MCWDFO on the leg axis in the coronal plane is widely understood, information is relatively scarce on possible concurrent effects on the posterior distal femur angle (PDFA) and femoral torsion (FT) caused by different hinge axes. Hypothesis: The authors hypothesized that a neutral hinge axis changes only the alignment in the frontal plane and that rotation of the hinge axis significantly changes sagittal and axial anatomic parameters. Study Design: Descriptive laboratory study. Methods: The study used a 3-dimensional surgical simulation to perform MCWDFO with a stepwise increase in varization by 1° (0°-15°). Surface models were created from computed tomography scans of 12 cases of femoral valgus deformity and 23 physiologically aligned lower limbs, resulting in 6370 simulations. Anatomic landmarks were determined for each simulation to measure the hip-knee-ankle angle, mechanical lateral distal femoral angle, PDFA, and FT. Axial and sagittal rotations of the hinge axis were tested for their effects on the PDFA and FT. Results: Axial rotation of the hinge axis significantly affected the PDFA, with changes of 1.9°± 0.1° (mean ± SD) per 10° of rotation at a hinge closure angle of 10° ( Conclusion: With this 3-dimensional simulation study, a definition for a hinge axis in the MCWDFO could be found in which only coronal changes occur. By rotating the hinge axis in the axial plane, the PDFA can be significantly influenced; conversely, the FT can be changed by rotation in the sagittal plane. Clinical Relevance: Understanding the effect of hinge axis orientation in MCWDFO is crucial for enhancing surgical accuracy, thereby improving patient outcomes in treatments involving knee deformities and avoiding unintended changes.