The Journal of Bone and Joint Surgery (American). 2009;91:620-627.
doi:10.2106/JBJS.H.00408
© 2009 The Journal of Bone and Joint Surgery, Inc.
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A Nonlocking End Screw Can Decrease Fracture Risk Caused by Locked Plating in the Osteoporotic Diaphysis

Michael Bottlang, PhD1, Josef Doornink, MS1, Gregory D. Byrd, MD2, Daniel C. Fitzpatrick, MD3 and Steven M. Madey, MD1

1 Legacy Biomechanics Laboratory, 1225 N.E. 2nd Avenue, Portland, OR 97215
2 Department of Orthopaedics and Rehabilitation, Oregon Health and Science University, 3181 S.W. Sam Jackson Park Road, Portland, OR 97239
3 Slocum Center for Orthopedics and Sports Medicine, 55 Coburg Road, Eugene, OR 97408

Investigation performed at the Legacy Biomechanics Laboratory, Portland, Oregon

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the Legacy Research Foundation and less than $10,000 from the Orthopaedic Research and Education Foundation. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

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.


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