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Numeric Analysis of the Load Capacity of the Human Spine Fitted with L-Rod Instrumentation

WEILER, PETER J., MASc, MD, PEng*; MEDLEY, JOHN B., PhD, PEng; McNEICE, GREGORY M., PhD, PEng

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L-rod procedures with sublaminar wiring have become widespread in orthopaedic practice, and serious mechanical problems involving fatigue failure, creep, and fixation have been reported. To address these problems, numeric analysis was developed to provide estimates for the maximum bending stresses, approximately equal to maximum tensile stresses, in L-rods subject to various axial loads and postoperative Cobb angles. The superior load capacity of L-rods made from MP35N multiphase alloy compared with 316L stainless steel was demonstrated. Bending stresses in excess of the fatigue limit were predicted for eight patients with broken rods, suggesting that such failures might be prevented by prior analysis. This analytic approach provides a foundation for future studies of custom-designed L-rods and perhaps the successful use of L-rods without arthrodesis.

* Division of Orthopaedic Surgery, Department of Surgery, University of Toronto, Ontario, Canada

‡Department of Civil Engineering University of Waterloo, Ontario, Canada.

© Lippincott-Raven Publishers.