Not All Polyaxial Locking Screw Technologies Are Created Equal: A Systematic Review of the Literature

Locking plate fixation strength relies on axial alignment of the screw axis and plate hole, with small deviations in alignment substantially decreasing the load to failure. In an effort to overcome this technical deficiency, polyaxial locking plates were designed to provide increased flexibility of screw positioning with the intent of not sacrificing fixation strength. The purpose of this article is to review the variety of polyaxial locking mechanisms currently available, to compare the biomechanical performance of these designs, and to highlight their differences, which may have clinical implications.

A systematic review using the search terms "polyaxial locking," "variable angle locking," "polyaxial screws," and "variable angle screws" was conducted to identify all English-language articles assessing variable-angle locking screw technology. All articles directly comparing the biomechanical performance of polyaxial locking technologies were included.

Polyaxial locking is achieved by 5 described mechanisms: point-loading thread-in, cut-in, locking cap, expansion bushing, and screw-head expansion. With increasing insertion angulation, point-loading thread-in and cut-in designs demonstrate reduced failure strength. However, locking-cap fixation maintains consistent failure strength with increasing off-axis insertion angles.

Reports comparing polyaxial locking technologies are limited. The current biomechanical literature raises concerns that these mechanisms have various strengths and performance characteristics. Based on the results of the few studies that exist, it appears that locking-cap fixation provides superior biomechanical strength when compared with point-loading and cut-in designs. Additional studies are needed to assess variable-angle locking mechanisms more completely.