Fixed- vs. variable-angle humeral neck cut in anatomic total shoulder arthroplasty: a randomized controlled trial.

Abstract

Variable neck-shaft angle (NSA) stemmed humeral components have been incorporated into certain implant designs to better re-create normal anatomy in total shoulder arthroplasty (TSA). The purpose of this study was to determine if premorbid glenohumeral joint anatomy is better restored with a fixed- vs. variable-NSA prosthesis. A randomized controlled trial was performed including 50 patients with osteoarthritis indicated for primary anatomic TSA. Patients were randomized preoperatively to receive either a variable- (n=26) or fixed-NSA (n=24) prosthesis. Humeral neck cut in the variable-NSA group matched the patient's anatomic neck, with prosthetic NSA of 127.5°, 132.5°, and 137.5° available. Fixed-NSA cuts were made with an intramedullary guide of 132.5°. Preoperative and postoperative radiographs were evaluated for specific radiographic anatomic variables: NSA, head thickness, tuberosity-to-head height, head offset, articular arc, greater tuberosity offset, and center of rotation (COR). Postoperative radiographic criteria were compared between groups. No differences were found between groups in demographics or preoperative radiographic measures. When comparing average difference in preoperative and postoperative measurements in the fixed-NSA group, the humeral head offset from the humeral shaft axis significantly decreased by 1.4 mm (P=.046), and the COR moved superiorly (3.0 mm, P=.002) without significant medialization or lateralization. In the variable angle group, humeral head offset decreased but did not reach significance (1.2 mm, P=.091), and the COR also moved superiorly (2.9 mm, P<.001) without significant medialization or lateralization. All remaining radiographic parameters did not significantly change from pre- to postoperative imaging. In comparing the fixed- and variable-NSA groups' net change from the premorbid measurements, no significant differences were found in tuberosity-to-head height, head offset, or COR position in both the horizontal and vertical planes. Both fixed- and variable-NSA anatomic TSA humeral components demonstrate adequate restoration of premorbid anatomy radiographically.