Analysis of the mechanical properties of locking plates with and without screw hole inserts

The purpose of this study was to determine if the use of screw hole inserts in empty locking screw holes improves the strength and failure characteristics of locking plates. Twenty 5-hole 1/3 tubular locking plates (Synthes, Paoli, Pennsylvania) were mounted on an oak dowel with a 1-cm gap simulating a fracture with comminution and bone loss. Ten of the 1/3 tubular plates had a screw hole insert placed in the center hole (centered over the simulated fracture), while 10 of the 1/3 tubular plates remained empty in the center hole. The plate-dowel constructs were placed in an Instron 8800 Material Testing Machine and subjected to a series of loading conditions, replicating physiologic loading. The torsional and axial stiffness of each plate-dowel construct was calculated. All plates were then loaded to failure. No significant differences were found in the mechanical properties of the 2 plate constructs. Both the filled screw-hole plate constructs and unfilled screw hole plate constructs demonstrated the same torsional and axial stiffness, before and after being subjected to a combined cyclic and axial torsional load. Additionally, there was no significant difference in ultimate compressive strength or load to failure. Locking plate technology is a relatively new innovation in orthopedic fracture fixation. The evolution of new and varied applications and implants continues. Persistent, fundamental questions exist concerning the basic locking plate design. This study demonstrates that the addition of screw hole inserts does not significantly change the stiffness, torsional strength, or axial loading strength of 1/3 tubular locking plates.