Biomechanical effects of far-cortex over-penetration in locking plate fixation: A finite element study

This study quantified the biomechanical consequences of incremental far-cortex over-penetration at terminal locking screw positions in a locking compression plate construct using a validated three-dimensional finite element model of a femoral diaphyseal fracture (AO/OTA 32-A3). Four terminal screw engagement configurations were evaluated: unicortical fixation, fully contained bicortical fixation, 1-thread over-penetration (0.6 mm), and 3-thread over-penetration (1.8 mm). Axial compression (1000 N), torsional loading (±10 Nm), and cyclic dynamic loading (100-1000 N, 1 × 10⁶ cycles) simulating the 8-week early postoperative period were applied. Model validation against published experimental data demonstrated agreement within 4% for all key biomechanical parameters. Fully contained bicortical fixation was the only configuration to remain below the predicted fatigue threshold, demonstrating the lowest peak von Mises stress (74.8 MPa) and a Miner's cumulative damage index of D = 0.291. Unicortical fixation (D = 1.103), 1-thread over-penetration (D = 1.362), and 3-thread over-penetration (D = 2.968) all exceeded the failure threshold (D ≥ 1.0). Under the loading conditions and material assumptions of the present finite element model, unicortical terminal fixation alone did not sustain simulated cyclic physiological loading during the pre-callus phase without exceeding the predicted fatigue damage threshold. These findings provide preliminary, model-dependent computational evidence suggesting that fully contained bicortical fixation may offer greater fatigue resistance, and that the biomechanical basis of current terminal screw placement guidelines warrants further experimental investigation.