Objective To explore whether insertion of the lag screw into the head-neck fragment in left-sided intertrochanteric fractures can cause fragment rotation and loss of reduction.Methods A retrospectively analysis was performed in 32 patients with left-sided unstable intertrochanteric fracture,who underwent close reduction and Gamma-3 fixation from January to December 2013.The angle between the central axis of head-neck and the central axis of femur shaft in lateral X-ray film was measured,and compared among closed reduction,lag screw insertion,and final state before patient discharge.Result When lag screw was wrenched into the head-neck fragment,rotational displacement was observed in 9 of 32 patients (28%).All the 9 fractures were classified OTA31A2.3.The angle between the central axis of head-neck fragment and femur shaft was 16.78°±4.55° after closed reduction 27.78°±6.0° after lag screw insertion 17.67°±4.30° after back rotation,and 17.89°±3.89° in discharge,respectively.A statistical test revealed a significant difference between closed reduction,lag screw insertion,and back rotation of the lag screw (P<0.05).Conclusion Wrenching-in lag screw can potentially lead to head-neck fragment rotation and loss of fracture reduction in unstable left-sided intertrochanteric fractures.