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

Abstract

Replicating the normal anatomy of the shoulder is an important principle in the design of prosthetic devices and the development of surgical techniques. In this study, we used a three-dimensional surgical simulation to compare the abilities of an adjustable neck-shaft angle prosthesis and a fixed neck-shaft angle prosthesis to restore the normal geometry of the proximal part of the humerus. 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 degrees 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. The average neck-shaft angle of the 2058 humeri was 134.7 degrees (range, 115 degrees to 148 degrees), and the angle was between 130 degrees and 140 degrees 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 degrees-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. 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.