Biomechanical Comparison of a Novel 3-Screw Fixation vs Conventional 2-Screw Fixation of Calcaneal Tuberosity Avulsion Fractures

Calcaneal tuberosity avulsion fractures are challenging to treat because of the poor bone stock and high risk of fixation failure secondary to the strong Achilles tendon pull. The purpose of this study is to compare the tensile force to failure of 2 different types of screw fixation construct in a cadaveric model of calcaneal tuberosity avulsion fracture.An oblique osteotomy was created in the calcanei of 7 matched pairs of cadaveric specimens to simulate a tuberosity avulsion fracture and one specimen from each pair randomized into one of the 2 groups for comparison. Two cancellous screws were inserted perpendicular to the fracture line at the posteromedial and posterolateral corners of the avulsed fragment for the 2-screw construct. For the second group, an additional stab incision was made at the midline of the Achilles insertional region for a screw placed between the initial 2 screws with a trajectory toward the calcaneocuboid joint in the 3-screw construct. These specimens were then mounted and loaded to failure.The mean force to a predefined failure at 3.0-mm gap for the 3-screw construct was 468.7 ± 267.9 N vs 278.9 ± 164.0 N for the 2-screw construct (P < .001). The addition of a central nonparallel screw in the 3-screw construct significantly increased the force required for fracture gap displacement at all cut-off points (1.0-8.0 mm) when compared to the 2-screw construct (P < .001). Notably, the mean peak tensile force for the 3-screw construct was 499.4 ± 255.4 N occurred at the gap displacement of 4.1 mm for the 3-screw construct whereas the mean peak tensile force for the 2-screw construct was 315.9 ± 162.4 N displacing the gap at 4.3 mm.This study showed that an additional central nonparallel screw in the 3-screw construct provided significant mechanical superiority compared to a 2-screw construct.The present study supported the use of augmented fixation with an additional central off-axis screw to reduce risk of fixation failure in calcaneal tuberosity avulsion fractures.