The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem

The purpose of this study was to assess the effects of lateralization and distalization on scapular spine fracture (SSF) after reverse shoulder arthroplasty (RSA). The hypothesis was that postoperative distalization would increase the risk of SSF, whereas lateralization would not. A multicenter retrospective review was performed at a minimum of 1 year postoperatively on primary RSAs with 3 different implants, 2 with an inlay design (n = 342) and 1 with an onlay design (n = 84). Functional outcome, range of motion, stem design, and radiographic measurements, including acromiohumeral distance and lateralization, were compared between groups with and without fracture. The incidence of SSF in the onlay group (11.9%) was significantly higher compared with the inlay group (4.7%; P = .043). Postoperative acromiohumeral distance was approximately 4 mm higher in the SSF group (37.5 mm) compared with the control group (33.7 mm; P = .042), whereas lateralization was similar between the 2 groups (52.8 mm vs. 53.9 mm; P = .362). Higher return to activity (92.1% vs. 71.4%; P < .001) as well as postoperative forward flexion was observed in the group without fracture (135° vs. 120°; P = .009). Increased postoperative distalization is associated with an increased risk of SSF after RSA. An onlay stem resulted in a 10 mm increase in distalization compared with an inlay stem, and a 2.5 times increased risk of SSF. Lateralization, however, does not appear to increase the risk of SSF.