Finite element modeling of lag screw with plate fracture fixation: Effects of screw angle, countersinking, and plate configuration.

Abstract

A lag screw, or a lag screw combined with a neutralization plate, is commonly used for compressive fixation of simple fracture patterns. There are several surgical variables that potentially affect fixation stability including lag screw angle, fracture gap, screw countersinking, screw diameter, and neutralization plate configuration. This study involved finite element modeling of these biomechanics in a diaphyseal bicortical oblique fracture model of (1) lag screw application under controlled torque; and (2) postoperative stability with 4.5 mm diameter lag screw and corresponding neutralization plate combinations, subject to external compression, bending, and torsion. In models of lag screw application, predicted interfragmentary misalignment had good agreement with corresponding validation experiments. When the screw was applied perpendicular to the fracture, with moderate countersinking and no initial fracture gap, fracture compression was maximized and interfragmentary misalignment minimized. A 30° deviation from perpendicular screw placement reduced fracture compression by 19% and produced 0.7 mm of interfragmentary misalignment, while screw deviation at 15° had negligible effects. Not countersinking the screw head also decreased fracture compression and increased lag screw stress at the head-shaft junction, with the model predicting larger effects on compression than observed in the experiment. During postoperative loading, the addition of a neutralization plate limited shear interfragmentary motion to < 0.4 mm under all loadings, and reduced lag screw stress and bone strains, particularly under torsion. The study quantifies the decreases in fracture compression in lag screw fixation due to screw angular deviation and not countersinking the screw head, and characterizes the biomechanical benefits of combining lag screw fixation with a neutralization plate.