Comparison of the syndesmotic staple to the transsyndesmotic screw: a biomechanical study

Controversy still exists about treatment of syndesmotic injuries. This study compared the fixation strengths and biomechanical characteristics of two types of ankle fracture syndesmotic fixation devices: the barbed, round staple and the 4.5-mm cortical screw.

Cadaveric testing was done on 21 fresh-frozen knee disarticulation specimens in biaxial servohydraulic Instron testing equipment. Submaximal torsional loads were applied to specimens in intact and Weber C bimalleolar fracture states. The specimens were then fixed with one of two techniques and again subjected to submaximal torsion and torsion to failure. Biomechanical parameters measured included tibiofibular translation and rotation, maximal torque to failure, and degrees of rotation at failure.

Compared to the intact state before testing, the staple held the fibula in a more anatomic position than the screw for mediolateral and anterior displacements (p < 0.01). With submaximal torsional testing, the staple restored 85% of the tibiofibular external rotation and all of the posterior translation values as compared to the intact state. The screw resulted in 203% more tibiofibular medial translation and 115% more external rotation than the intact state. The degree of tibial rotation during submaximal torsional loading was restored to within 15% of intact values but was 21% less with the screw. There was no statistical difference between the screw and staple when tested in load to failure. Tibiotalar rotation at failure was statistically different with the staple construct, allowing more rotation as compared to the screw.

The staple restored a more physiologic position of the fibula compared to the syndesmotic screw. Both provided similar performance for the load to failure testing, while the screw reduced tibial rotation more after cyclic loading. There was more tibial rotation before failure for the staple, suggesting a more elastic construct. This study provides biomechanical data to support the clinical use of the syndesmotic staple.