Biomechanical Evaluation of Variable-Angle Locking Systems. A Micro-CT Analysis

Formerly poor bone stock and periprosthetic fractures used to jeopardize monoaxial constructs. Polyaxial locking screws have substantially supported those particular fixation constructs. However, those systems rely on complex alignment between the screw head and the plate hole. This study aimed to investigate the mechanical properties of several polyaxial systems and to correlate these parameters with the screw head and the plate hole engagement.

Polyaxial locking systems were tested with screws inserted into the corresponding plates at various angles (0, 5, 10, and 15 degrees). A micro-computed tomography (micro-CT) scan of the plate hole and screw head interface with the quantification of average thread engagement (ATE) was performed before destructive tests. The screw-plate interface of each system was tested in a cantilever bending setup. Representative screws and plates were also examined by scanning electron microscope.

The standard insertion at 0 degrees sustained the greatest maximum bending strength in all analyzed systems. Point-loading thread-in and cut-in screws inserted off-axis showed a significant reduction in bending strength ( P < 0.001) ( P = 0.041) ( P < 0.001). By contrast, locking cap screws maintained similar bending strength with disregard to the angle of insertion ( P < 0.4849). A micro-CT analysis confirmed that the ATE of point-loading thread-in, cut-in, and locking cap screws was significantly reduced when placed off-axis ( P = 0.005) ( P < 0.001) ( P = 0.002) ( P < 0.001). The locking cap mechanisms maintained the highest ATE among all analyzed systems.

The mechanical performance of polyaxial locking plates usually comes at the price of reduced bending strength. Surgeons should limit polyaxial insertions depending on the particular system's characteristics.