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

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering
• Medical Imaging

Background:

• Medial closed wedge distal femoral osteotomy (MCWDFO) is increasingly used for valgus deformities and patellar instability.
• While coronal plane effects of MCWDFO are understood, its impact on posterior distal femur angle (PDFA) and femoral torsion (FT) is less clear.
• Hinge axis orientation is a critical factor potentially influencing these sagittal and axial parameters.

Purpose of the Study:

• To investigate the effects of different hinge axis orientations during MCWDFO on the posterior distal femur angle (PDFA) and femoral torsion (FT).
• To test the hypothesis that a neutral hinge axis primarily affects coronal alignment, while rotated axes alter sagittal and axial parameters.

Main Methods:

• A 3D surgical simulation was employed using CT scans from 12 femoral valgus deformity cases and 23 controls.
• MCWDFO was simulated with stepwise varization (0°-15°), and hinge axis rotations in axial and sagittal planes were analyzed.
• Measurements included hip-knee-ankle angle, mechanical lateral distal femoral angle, PDFA, and FT.

Main Results:

• Axial hinge axis rotation significantly altered the PDFA (1.9°±0.1° per 10° rotation at 10° closure, P < .001).
• Sagittal hinge axis rotation demonstrated a strong negative correlation with FT changes (-0.82, P < .001).
• A neutral hinge axis resulted in minimal, non-significant changes in PDFA (P = .85) and FT (P = .98).

Conclusions:

• The study defines hinge axis parameters for MCWDFO, achieving purely coronal plane changes with a neutral axis.
• Axial hinge axis rotation can significantly influence PDFA, and sagittal rotation can alter FT.
• Precise control over hinge axis orientation is essential for optimizing MCWDFO outcomes and avoiding unintended anatomical alterations.