Is tip apex distance as important as we think? A biomechanical study examining optimal lag screw placement.

Abstract

Intertrochanteric hip fractures pose a significant challenge for the orthopaedic community as optimal surgical treatment continues to be debated. Currently, varus collapse with lag screw cutout is the most common mode of failure. Multiple factors contribute to cutout. From a surgical technique perspective, a tip apex distance less than 25 mm has been suggested to decrease the risk of cutout. We hypothesized that a low-center lag screw position in the femoral head, with a tip apex distance greater than 25 mm will provide equal, if not superior, biomechanical stability compared with a center-center position with a tip apex distance less than 25 mm in an unstable intertrochanteric hip fracture stabilized with a long cephalomedullary nail. We attempted to examine the biomechanical characteristics of intertrochanteric fractures instrumented with long cephalomedullary nails with two separate lag screw positions, center-center and low-center. Our first research purpose was to examine if there was a difference between the center-center and low-center groups in cycles to failure and failure load. Second, we analyzed if there was a difference in fracture translation between the study groups during loading. Nine matched pairs of femurs were assigned to one of two treatment groups: low-center lag screw position and center-center lag screw position. Cephalomedullary nails were placed and tip apex distance was measured. A standard unstable four-part intertrochanteric fracture was created in all samples. The femurs were loaded dynamically until failure. Cycles to failure and load and displacement data were recorded, and three-dimensional (3-D) motion was recorded using an Optotrak(®) motion tracking system. There were no significant differences between the low-center and center-center treatment groups regarding the mean number of cycles to failure and mean failure load. The 3-D kinematic data showed significantly increased motion in the center-center group compared with the low-center group. At the time of failure, the magnitude of fracture translation was statistically significantly greater in the center-center group (20 ± 2.8 mm) compared with the low-center group (15 ± 3.4 mm; p = 0.004). Additionally, there was statistically significantly increased fracture gap distraction (center-center group, 13 ± 2.8 versus low-center group, 7 ± 4; p < 0.001) and shear fracture gap translation (center-center group, 12 ± 2.3 mm; low-center group, 6 ± 2.7 mm; p < 0.001). Positioning of the lag screw inferior in the head and neck was found to be at least as biomechanically stable as the center-center group although the tip apex distance was greater than 25 mm. Our findings challenge previously accepted principles of optimal lag screw placement.