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Central Placement of the Screw in Simulated Fractures of the Scaphoid Waist

A Biomechanical Study

Investigation performed at the Department of Orthopaedics and Sports Medicine, University of Washington School of Medicine, Seattle, Washington
Wren V. McCallister, MD
Thomas E. Trumble, MD
Department of Orthopaedics and Sports Medicine, University of Washington School of Medicine, 1959 N.E. Pacific Street, Box 356500, Seattle, WA 98195. E-mail address for T.E. Trumble: trumble{at}u.washington.edu

Jeff Knight, MS
Albert Einstein College of Medicine of Yeshiva University, Jack and Pearl Resnick Campus, 1300 Morris Park Avenue, Bronx, NY 10461

Robert Kaliappan
Northwestern University, Feinberg School of Medicine, Honors Program in Medical Education, P.O. Box 3060, Evanston, IL 60204-3060

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from an AO North American Resident Trauma Research Grant. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer, Warsaw, Indiana, loaned the Herbert-Whipple screw set and donated the screws used in this study). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: Recent reports on internal fixation of acute fractures of the scaphoid waist have demonstrated higher rates of central placement of the screw when cannulated screws were used than when noncannulated screws were used. This cadaveric study was designed to determine whether central placement in the proximal fragment of the scaphoid offers a biomechanical advantage.

Methods: Eleven matched pairs of scaphoids were removed from fresh cadaveric wrists. Each scaphoid was placed in a custom manufactured jig that was used to create reproducible central and eccentric positioning of the guidewire. Then a linear osteotomy was made followed by placement of a Herbert-Whipple cannulated screw to fix the osteotomy. The specimen was then potted in a holder with use of polymethylmethacrylate with a Kirschner wire passed through the proximal end of the scaphoid and placed into a fixture with a pneumatically driven plunger resting on the surface of the distal pole. The load acting through the plunger was measured with use of a load-cell, and its excursion was measured with use of a linear variable differential transformer. Stiffness, load at 2 mm of displacement, load at failure, and mechanism of failure were measured, and the two groups were compared with regard to stiffness and strength.

Results: Central placement of the screw in the proximal fragment of the scaphoid had superior results compared with those after eccentric positioning of the screw. Fixation with central placement of the screw demonstrated 43% greater stiffness (12.7 N per mm compared with 8.9 N per mm; p < 0.01), 113% greater load at 2 mm of displacement (126 N compared with 59.1 N; p < 0.01), and 39% greater load at failure (712 N compared with 513 N; p > 0.05).

Conclusions and Clinical Relevance: Central placement of the screw in the proximal fragment of the scaphoid offers a biomechanical advantage in the internal fixation of an osteotomy of the scaphoid waist. Clinical efforts and techniques that facilitate central placement of the screw in the fixation of fractures of the scaphoid waist should be encouraged.

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