Biomechanical evaluation of locking plate radial shaft fixation: unicortical locking fixation versus mixed bicortical and unicortical fixation in a sawbone model.

PURPOSE: Compression plating is a commonly accepted technique for treating diaphyseal forearm fractures. The purpose of this study was to evaluate the stabilizing effects of two hybrid fixations that replace the end screws of a locked unicortical fixation with bicortical (locked or unlocked) screws and to compare these hybrid fixations to an unlocked bicortical fixation.

METHODS: Sixteen composite radius sawbones were equally divided into 4 groups. We performed a midshaft osteotomy and plate fixation on the volar surface with 1 of 4 different constructs: 3 unlocked bicortical screws on each side (unlocked bicortical), 3 locked unicortical screws on each side (locked unicortical), or with 2 unicortical locked screws near the fracture and 1 bicortical unlocked (unlocked hybrid) or locked (locked hybrid) screw distant from the fracture on each end (LCP system, Synthes USA, Paoli, PA). Specimens were tested in nondestructive 4-point bending and torsion on a servo-hydraulic material testing system. The construct stiffness was obtained from the linear portion of the load-displacement curves after 3 cycles of preconditioning. The results from all groups were compared using analysis of variance and post hoc Bonferroni tests.

RESULTS: Under torsional loads, replacing the end screws of a locked unicortical configuration with bicortical screws significantly improved the construct stiffness: 57.6% increase for the locked screws and 51.6% increase for the unlocked. In anteroposterior (AP) bending, the highest improvement over the locked unicortical configuration came from the locked hybrid constructs (42.9% increase). When compared with the unlocked bicortical configuration, both hybrid constructs provide equivalent stability in torsion but superior stability in AP bending.

CONCLUSIONS: Replacing a single set of unicortical locked screws with locked or unlocked bicortical screws distant from the fracture site improves torsional stability of the construct by more than 50%, giving stability equal to standard unlocked plating. The hybrid fixation, however, with locked bicortical end screws has the best stability in AP bending.