Biomechanical evaluation of a new type of hip compression screw with retractable talons

Objectives: We investigated the biomechanical performance of a compression hip screw with four reversibly deployable talons. We hypothesized that the talons would increase purchase strength of the lag screw and prevent rotation of the femoral head by resisting torsional forces between the lag screw and the femoral head. Design: Reproducible, stable two-part intertrochanteric fractures were created on matched pairs of embalmed human femurs. Fractures were fixed using the compression hip screw with either the talons deployed (talon) or not deployed (screw) configuration. Pre- and post-implantation radiographs and dual energy x-ray absorptiometry scans were used to ensure geometric and balanced equivalency between contralateral limbs. Torsional tests were performed on femoral pairs at 1 Hz for 5000 cycles prior to torque failure. Compression tests were performed on femoral pairs with the screw in the inferior or central position. Results: Peak torque and torque at 15degrees of rotation were significantly greater for the talon versus screw device (6.5 +/- 1.7 Nm vs. 1.9 +/- 1.7 Nm for peak torque, P = 0.0002; and 4.2 +/- 2.0 Nm vs. 1.2 +/- 1.2 Nm for torque at 15degrees rotation, P = 0.003). The peak compressive forces generated by the talon device were significantly greater than for the screw device in the inferior position (P = 0.005), with compression values of 1398 +/- 467 N and 713 +/- 323 N, respectively. Peak compression forces did not differ significantly between talon and screw devices in the central position. Conclusions: Talon deployment significantly improved interfragment compression and torsional strength. Engagement or penetration into or through the cortical bone at the base of the femoral head-neck junction in the inferior lag screw position is the critical techincal step to maximize the talon purchase.