BACKGROUND:
Little is known about the role of disorders of the acromioclavicular joint (ACJ) and how they relate to complications after reverse shoulder arthroplasty (RSA). The purpose of this study is to compare the severity of ACJ osteoarthritis in patients undergoing RSA with and without postoperative acromial and scapular spine fractures.

METHODS:
A retrospective review was performed to identify all patients who underwent primary RSA between 1/1/2007 and 10/31/2019 with a postoperative acromial or scapular spine stress fracture from a single institution. Patients who underwent RSA with a fracture were compared with an age-, sex-, and preoperative diagnosis-matched control group (1:4 controls) with a minimum 2-year follow-up. We compared demographics, medical comorbidities, and ACJ osteoarthritis between the 2 groups. Preoperative radiographs and 3-dimensional computed tomography scans were evaluated for ACJ osteoarthritis in all patients. The Petersson classification, a modified Petersson classification, location of the osteophytes, subchondral cysts, ACJ space, and size of the largest osteophyte were recorded and compared between the 2 groups.

RESULTS:
The study included 11 patients who underwent primary RSA (8 women and 3 men) with acromial (6) and scapular spine (5) fractures confirmed radiographically and 44 matched controls (average follow-up 3.1 vs. 4.3 years, P = .17). Average age at surgery was similar between study and control groups (69.6 vs. 70.0 years, P = .86). ACJ osteoarthritis with osteophytes larger than 2 mm was common and similar between the 2 groups (91% of patients with acromial fracture and 66% of controls, P = .15). There was no significant difference in the size or location of the ACJ osteophytes. The Petersson classification was similar between groups. However, the percentage of patients with subchondral ACJ cysts was higher in the fracture group (91% vs. 50%, P = .02), and the percentage of patients with large spanning or fused osteophytes was significantly higher in the fracture group (55% vs. 14%, P = .008).

CONCLUSION:
Radiographic ACJ osteoarthritis is common in patients undergoing RSA. Severe ACJ osteoarthritis with completely spanning or fused osteophytes may predispose patients to acromial or scapular spine fractures after RSA.





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