Geometry of Proximal Humerus Locking Plates

Proximal humerus fractures are commonly treated with open reduction and internal fixation with periarticular locking plates. This study compared the geometry of proximal humerus locking plate's screw distribution, amount of bone-screw interface, and the volume the screws occupy within the humeral head, as well as how leaving the screws short of the articular surface affects these measures.

Locking plates from 7 manufacturers were applied to foam humerus models. The entry and exit hole of each screw trajectory was digitized using a 3-dimensional motion tracking system. A trajectory for each screw was modeled, as well as the volume enclosed by the screw trajectories. The following outcome metrics were calculated: the bone-screw interface, the volume enclosed by the screws, and the effect of leaving the screws short of the articular surface.

Biomet had the most bone-screw interface (7259 mm), whereas Zimmer had the least (3982 mm). The original Synthes plate had the largest screw volume, occupying 31.5% of the humeral head, whereas Smith & Nephew and Zimmer had the lowest volumes, occupying 21.2% and 12.6%, respectively. Leaving the screws 15 mm short of the articular surface resulted in the most reduction in volume for the Depuy plate (50%) and the least reduction for the new version of the Synthes plate (29%).

Many different manufactured periarticular locking plates exist to stabilize a proximal humeral fracture. Clinicians need to be familiar with the different plate configurations, screw lengths, and trajectories, which affect potential biomechanical performance and can optimize fracture site maintenance.