• Michael Bottlang, Josef Doornink, Gregory D Byrd, Daniel C Fitzpatrick, and Steven M Madey.
• Legacy Biomechanics Laboratory, 1225 N.E. 2nd Avenue, Portland, OR 97215, USA.
• J Bone Joint Surg Am. 2009 Mar 1; 91 (3): 620-7.

BackgroundLocking plates transmit load through fixed-angle locking screws instead of relying on plate-to-bone compression. Therefore, locking screws may induce higher stress at the screw-bone interface than that seen with conventional nonlocked plating. This study investigated whether locked plating in osteoporotic diaphyseal bone causes a greater periprosthetic fracture risk than conventional plating because of stress concentrations at the plate end. It further investigated the effect of replacing the locked end screw with a conventional screw on the strength of the fixation construct.MethodsThree different bridge-plate constructs were applied to a validated surrogate of the osteoporotic femoral diaphysis. Constructs were tested dynamically to failure in bending, torsion, and axial loading to determine failure loads and failure modes. A locked plating construct was compared with a nonlocked conventional plating construct. Subsequently, the outermost locking screw in locked plating constructs was replaced with a conventional screw to reduce stress concentrations at the plate end.ResultsCompared with the conventional plating construct, the locked plating construct was 22% weaker in bending (p = 0.013), comparably strong in torsion (p = 0.05), and 15% stronger in axial compression (p = 0.017). Substituting the locked end screw with a conventional screw increased the construct strength by 40% in bending (p = 0.001) but had no significant effect on construct strength under torsion (p = 0.22) and compressive loading (p = 0.53) compared with the locked plating construct. Under bending, all constructs failed by periprosthetic fracture.ConclusionsUnder bending loads, the focused load transfer of locking plates through fixed-angle screws can increase the periprosthetic fracture risk in the osteoporotic diaphysis compared with conventional plates. Replacing the outermost locking screw with a conventional screw reduced the stress concentration at the plate end and significantly increased the bending strength of the plating construct compared with an all-locked construct (p = 0.001).

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