Background: The importance of the subscapularis tendon in reverse shoulder arthroplasty (RSA) has been increasingly emphasized lately. Recent studies have indicated that a repaired subscapularis tendon has better functional outcomes. This study is aimed at comparing the healing rate of repaired subscapularis tendons between onlay and inlay Bony Increased Offset-Reversed Shoulder Arthroplasty (BIO-RSA).

Methods: This retrospective comparative review covers all patients who underwent BIO-RSA at a single center, comprising 189 cases performed by on a single surgeon from January 2012 till December 2021. We included all patients who underwent subscapularis tenotomy repair and who had a tendon ultrasound (US) examination at six months postoperatively (as requested in this single surgeon’s usual protocol). These patients were divided into two comparable groups, an onlay group and an inlay group. Healing status was determined using the Sugaya classification, with healed subscapularis tendons defined as having Sugaya type I–III integrity and the unhealed tendons as having Sugaya type IV and V integrity.

Results: In total, 2 patients were excluded because ultrasound was not performed (they missed their appointment); 187 patients were evaluated; 98 patients underwent an onlay BIO-RSA; and 89 patients underwent an inlay BIO-RSA. The healing rate of the repaired subscapularis tendon was 73% in the onlay group and 56% in the inlay one (p = 0.020).

Conclusions: The onlay systems may enhance subscapularis tendon healing compared to the inlay one, possibly due to the preserved intramedullary vascularity and the near-normal tendon excursion that can be achieved by the onlay system. Choosing an onlay design can minimize bone cuts during RSA, while achieving good subscapularis tendon healing.