Reverse shoulder arthroplasty for patients with cuff tear arthropathy: do clinical outcomes differ by inlay vs. onlay design?

Background: The influence of position of the humeral tray (inlay or onlay) on clinical outcomes in reverse shoulder arthroplasty (RSA) is a topic of debate. The purpose of this study was to compare clinical and radiographic outcomes of patients with cuff tear arthropathy treated with RSA systems with inlay or onlay humeral tray design, similar neck-shaft angles, and lateralized glenospheres.

Methods: This was a retrospective study of prospectively obtained data from 1 tertiary care center. We identified all patients who underwent primary RSA between 2009 and 2017 (N = 511). We included 102 patients with diagnosed cuff tear arthropathy treated with RSA prostheses with a lateralized glenosphere and 135° neck-shaft angle (with either an inlay or onlay humeral tray design) who had a minimum of 2 years of follow-up (mean, 44 months; range, 24-125 months). Sixty-three patients (62%) had an inlay humeral tray (inlay group) and 39 (38%) had an onlay tray (onlay group). All patients underwent preoperative and postoperative evaluations, including measures of patient-reported outcomes (PROs), shoulder range of motion (ROM) testing, and radiographic imaging. Clinical relevance of changes in PROs and ROM was evaluated using published values for minimal clinically important differences.

Results: The 2 groups did not differ by demographic characteristics except for a higher proportion of women in the inlay group (75%) than in the onlay group (56%) (P = .04). Preoperative PROs and ROM were not significantly different between groups. At final follow-up, PROs and ROM were not different between groups in terms of statistical significance or clinical relevance. We found no significant differences in the rate of baseplate loosening (inlay, 3.2% vs. onlay, 5.1%, P = .63), revision surgery (inlay, 0% vs. onlay 5.1%, P = .07), acromial stress fracture (inlay, 3.2% vs. onlay, 5.1%, P = .63), prosthesis dislocation (inlay, 0% vs. onlay, 2.6%, P = .20), or scapular notching (inlay, 21% vs. onlay, 7.7%, P = .08).

Conclusion: At 2-year minimum follow-up, the position of the humeral tray in RSA prostheses (either inlay or onlay) for cuff tear arthropathy was not associated with PROs, shoulder ROM, or rates of complications, including baseplate loosening, acromial stress fracture, and scapular notching.