The Effect of Lag Screw Position on Rotational Stability and Stress Concentration in Unstable Basicervical Intertrochanteric Fractures: A Finite Element Analysis

Background/Objectives: Due to the inherent rotational instability of the proximal fragment in unstable basicervical intertrochanteric (IT) fractures, the biomechanical effect of lag screw position may differ from that observed in typical unstable IT fractures. This study aimed to evaluate the influence of lag screw positioning on proximal fragment displacement and stress distribution after cephalomedullary nailing (CMN) in unstable basicervical IT fractures using finite element analysis. Methods: Twelve finite element models of unstable basicervical IT fractures fixed with a CM nail were constructed with lag screws placed in four anteroposterior (AP) positions (superior 5 mm, center, inferior 5 mm, and inferior 10 mm) and three axial positions (anterior, center, and posterior). The positional change of the proximal fragment and stress concentration on the nail construct were measured. Results: In this computational model, proximal fragment displacement and stress concentration, including peak von Mises stress and mean stress over a region of interest, increased as the lag screw was positioned more inferiorly on the AP view and more posteriorly on the axial view. Conversely, a relatively superior-anterior lag screw position was associated with the lowest proximal fragment displacement and reduced stress concentration on the nail construct and around the lag screw tip. Conclusions: Within the limitations of this finite element analysis using a single femoral model and axial loading condition, a relatively superior-anterior lag screw position was associated with more favorable biomechanical behavior compared with more inferior or posterior positions. These findings should be interpreted as hypothesis-generating biomechanical observations rather than direct clinical guidance.