Comparison of the mechanical behaviors of locked and nonlocked plate/screw fixation applied to experimentally induced rotational osteotomies in canine ilia.

Objective: To compare the mechanical behaviors of 2 locked (parallel and diverging screws) and 1 nonlocked (NL) version of triple pelvic osteotomy (TPO) plate/screw fixation.

Study Design: In vitro biomechanical evaluation.

Animals: Cadaveric canine hemipelves.

Methods: Comparison 1-NL screws 20° TPO (NL-20) construct versus locked parallel (LP) screws 20° TPO (LP-20) construct (n = 7). Comparison 2-LP-20 construct versus locked diverging (LD) screws 20° TPO (LD-20) construct (n = 6). Condition 1-Nondestructive loading to determine stiffness. Condition 2-Cyclic loading to determine stiffness, screw loosening, and osteotomy gap displacement. Condition 3-Load to failure (yield load, yield displacement, maximum load, load to failure, failure mode).

Results: Stiffness was not significantly different for NL-20 versus LP-20 constructs (P = .48) or for LP-20 versus LD-20 constructs (P = .83). Screw loosening was significantly more frequent for NL-20 versus LP-20 (P = .01) and for LD-20 versus LP-20 constructs (P = .02). The relative risk for screw loosening with NL-20 constructs versus LP-20 constructs was 1.4 (95% CI: 1.1-1.8). The relative risk for screw loosening with LD-20 versus LP-20 was 1.6 (95% CI: 1.1-2.2). Yield load was significantly greater for LP-20 versus NL-20 and LD-20 constructs (P = .04, P = .03), respectively.

Conclusions: No TPO constructs tested maintained complete integrity after cyclic loading; however, screw loosening was significantly reduced and yield loads were significantly larger for LP-20 plate/screw constructs.