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A Nonlocking End Screw Can Decrease Fracture Risk Caused by Locked Plating in the Osteoporotic Diaphysis

Bottlang, Michael PhD; Doornink, Josef MS; Byrd, Gregory D. MD; Fitzpatrick, Daniel C. MD; Madey, Steven M. MD

Journal of Bone & Joint Surgery - American Volume: 01 March 2009 - Volume 91 - Issue 3 - p 620–627
doi: 10.2106/JBJS.H.00408
Scientific Articles

Background: Locking 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.

Methods: Three 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.

Results: Compared 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.

Conclusions: Under 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).

Clinical Relevance: For bridge-plating in the osteoporotic diaphysis, the addition of a conventional end screw to a locked plating construct can enhance the bending strength of the fixation construct without compromising construct strength in torsion or axial compression.

1Legacy Biomechanics Laboratory, 1225 N.E. 2nd Avenue, Portland, OR 97215

2Department of Orthopaedics and Rehabilitation, Oregon Health and Science University, 3181 S.W. Sam Jackson Park Road, Portland, OR 97239

3Slocum Center for Orthopedics and Sports Medicine, 55 Coburg Road, Eugene, OR 97408

Copyright 2009 by The Journal of Bone and Joint Surgery, Incorporated
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