Simulated gastrocnemius traction alters interfragmentary motion in Hoffa fracture fixation

Background: Hoffa fractures remain biomechanically challenging due to their intra-articular location and limited fixation surface. The influence of posterior muscle forces—particularly from the gastrocnemius—on interfragmentary motion has not been adequately addressed in previous experimental studies. This study aimed to assess the impact of simulated gastrocnemius traction on interfragmentary motion in Hoffa fracture fixation. Methods: Patient-specific synthetic femora with anatomically realistic type I Hoffa fractures were manufactured from CT data using validated polyurethane-based materials. High-strength-fiber loops were embedded at the anatomical gastrocnemius insertion sites to simulate posterior muscle traction. Eight specimens with and eight without simulated gastrocnemius force (300 N constant pull) were tested under progressively increasing cyclic axial loading. Interfragmentary motion was captured via 3D motion tracking and analyzed for displacement and rotation. Results: Specimens with simulated muscle force exhibited significantly altered motion patterns compared to controls. Muscle traction reversed the direction of gap opening, increased gap twisting at higher loads (up to − 3.0°, p ≤ 0.005), and modified shear displacement and localized gap expansion. Despite these differences in fragment kinematics, no significant differences in construct failure load were observed (p = 0.599). Conclusion: Simulated gastrocnemius traction substantially influences interfragmentary motion in Hoffa fractures under axial load, even in the absence of changes in failure load. This study presents a novel test setup combining patient-specific fracture morphology and anatomically integrated muscle simulation, providing a transferable and physiologically relevant platform for future biomechanical investigations of distal femur fractures.