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Effect of a Variable Prosthetic Neck-Shaft Angle and the Surgical Technique on Replication of Normal Humeral Anatomy

¹ Department of Orthopaedic Surgery, The Cleveland Clinic, 9500 Euclid Avenue, A-41, Cleveland, OH 44195. E-mail address for J. Jeong: osjeong{at}hotmail.com. E-mail address for J.P. Iannotti: iannotj{at}ccf.org
² Department of Biomedical Engineering, The Cleveland Clinic, 9500 Euclid Avenue, ND-20, Cleveland, OH 44195. E-mail address: bryanj2{at}ccf.org

Investigation performed at The Cleveland Clinic, Cleveland, Ohio

Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from commercial entities (DePuy, a Johnson and Johnson Company, and Tornier). Also, a commercial entity (DePuy, a Johnson and Johnson Company) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

A video supplement to this article will be available from the Video Journal of Orthopaedics. A video clip will be available at the JBJS web site, www.jbjs.org. The Video Journal of Orthopaedics can be contacted at (805) 962-3410, web site: www.vjortho.com.

Methods: A total of 2058 cadaveric humeri were measured to define the normal distribution of neck-shaft angles. Thirty-six humeri were selected to represent a wide variation in neck-shaft angles, and computed tomographic scans with three-dimensional reconstruction were made of these specimens. With use of a three-dimensional computer surgical simulator, the humeral head was then cut at the anatomic neck to replicate a normal neck-shaft angle and version or it was cut at a fixed 135° angle with anatomic version. The anatomy of an adjustable-angle prosthesis and that of a fixed-angle prosthesis of the same design were both compared with native humeral anatomy in three dimensions.

Results: The average neck-shaft angle of the 2058 humeri was 134.7° (range, 115° to 148°), and the angle was between 130° and 140° in 77.84% of the humeri. In the setting of a high varus or valgus neck-shaft angle, an adjustable-angle prosthesis allowed optimal reconstruction when the humeral head was cut along the anatomic neck and allowed a standard and consistent surgical technique with use of anatomic landmarks. A fixed-angle prosthesis also replicated the anatomic center of rotation, tuberosity-head height, and head volume if the surgical procedure was altered to adapt to variations in humeral anatomy. There was no significant difference in anatomic parameters between the two types of prostheses, except that in all cases the head thickness was decreased when a fixed-135°-angle prosthesis was used in a humerus with a high valgus or high varus neck-shaft angle, resulting in a smaller articular arc and percent articular surface match.

Conclusions: This study demonstrates the ability of both an adjustable and a fixed neck-shaft angle prosthesis to replicate humeral anatomy. However, the fixed-angle device requires specific modifications of the surgical technique to accommodate the specific prosthetic design and optimize the surgeon's ability to replicate normal anatomic parameters in humeri with an extreme neck-shaft angle.

Clinical Relevance: This information can be used to optimize humeral component placement intraoperatively.

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