Threaded Screw Head Inserts Improve Locking Plate Biomechanical Properties

OBJECTIVES:The purpose of this study was to determine if the stiffness and fatigue life of locking one third tubular plates are enhanced by placing a locking screw head to fill the empty hole of the plate. We hypothesize that both the stiffness and fatigue life of the plates will be improved at physiologically relevant loads by filling the empty center hole of each plate. METHODS:The mechanical stiffness and fatigue life of plates with an open versus filled center hole were assessed through finite element analysis and experimentally using a synthetic bone model under four-point bending. Two plate manufacturers were evaluated, Synthes (n) and Stryker (r). Five-hole one third tubular plates were mechanically cycled with and without filling the central screw hole while load, displacement, and number of cycles were collected. Stiffness was calculated and cycles to failure and mode of failure were monitored. Five plates were evaluated for the filled (F) and open (O) configurations for the n and r plates. RESULTS:Finite element analysis indicated that filling the hole resulted in reduction in maximum stress at the periphery of the center hole by a factor of 2.43 and 2.29 for the n and r plates, respectively. Experimentally, a fourfold improvement was observed in fatigue life of the Synthes plates when a screw head was used to fill the central screw hole (P < 0.005; nF = 45,450 cycles versus nO = 10,305 cycles). The Stryker plates reached the maximum number of cycles (1 million) without fatigue failure in both O and F configurations. Improved bending stiffness was noted for both the n and r plates when the central hole was filled compared with open. For the Stryker plate, this increase was statistically significant (P < 0.011). CONCLUSIONS:The methodology proposed in this study for extending fatigue life and increasing stiffness of locking plates can potentially be extended to any locking plate. Adding a screw head or screw heads to open holes in locking plates adds little additional time or expense and no morbidity to the procedure but can have substantial effects on the mechanical properties of the implant, particularly in lower-profile plates that are initially less rigid and robust.