Biomechanics of humerus fracture fixation by locking, cortical, and hybrid plating systems in a cadaver model

The goal of this study was to discover how locking or cortical screws or a hybrid of both would perform in stabilizing a simulated humerus fracture. We simulated stripping of screw threads or poor bone quality by overdrilling the screw hole, and also studied a control group with no overdrilling. A total of 38 fresh frozen cadaver humeri were divided into 2 groups: 16 undergoing overdrilling with a drill bit 0.3 mm less than the diameter of the screw and 22 undergoing no overdrilling. A 4-point bending test followed torsional fatigue of 1000 cycles with an amplitude of +/-10 degrees. The post-fatigued samples were retested in the same way after tightening the loose screws if necessary. Finally, each fatigued specimen was tested for failure in torsion at 0.5 Hz by applying a maximum rotational displacement of 60 degrees .The bending stiffness values (Nm/mm) of cortical and locking screws, with the exception of the hybrid system, were significantly higher for the overdrilled group than the non-overdrilled (cortical, 6.9 vs 5.6; locking, 9.1 vs 6.3; hybrid, 8.4 vs 6.8). Fatigue had no effect on the bending stiffness of all the screw/plate systems (cortical, 6.9 vs 7.4; locking, 9.1 vs 8.8; hybrid, 8.5 vs 8.1). The overdrilling had no effect on the failure loads and displacements for all the screws except cortical screws, where the failure displacement was significantly higher for the overdrilling group. The torsional stiffness retentions after 1000 cycles were significantly different for overdrilled specimens in the cortical screws group only.Overdrilling had a minimal effect on bending and torsional properties. The results of the locking and hybrid were close, and the cortical screw had only slightly lower bending stiffness.