Acromial and Scapular Fractures after Reverse Shoulder Arthroplasty: Comparison of 3,018 Reverse Total Shoulders by Inlay and Onlay Humeral Component Design.

Introduction: Periscapular fractures specifically acromial and scapular spine fractures, have been identified as one of the leading complications of RSA. However, very little is known of the etiology of these postoperative fractures, or how variations in humeral designs correlates with risk of postoperative fracture development. Therefore, the purpose of this study was to analyze the prevalence, timing, and relationship of humeral component design to acromial or scapular spine fractures.

Methods: A retrospective study of primary reverse total shoulder arthroplasty (RSA) performed for elective and traumatic indications from two tertiary institutions. Exclusions consisted of primary oncologic reconstructions, diagnosis of osteogenesis imperfecta, and less than 1 year of clinical follow-up. A total of 3,018 primary RSAs were included with a cohort of 1,739 (57.6%) females, a mean age of 71 years (range, 20 - 94 years), a mean BMI of 30.6 ± 6.6 kg/m2, and a mean follow-up of 6.4 ± 3.8 years. The implants utilized varied based on surgeon preference and included 9 different types. The humeral component of the RSA were categorized as an inlay design (n = 762; 25.2%), defined as a humeral component where the tray is seated within the metaphysis, or an onlay design (n = 2256; 74.8%) defined as a humeral component where the humeral tray sits on the metaphysis at the level of the humeral neck cut.

Results: A fracture of the acromion or scapular spine was radiographically identified in 64 of 3,018 (2.1%) RSA at an average of 8.5 months ± 12.6 months after surgery. The majority of fractures included the acromion (n = 57; 89.1%) and scapular spine (n = 7; 10.9). Non-operative management (n = 60; 93.8%) was the predominant treatment strategy for fractures, while 4 (6.2%) RSA underwent open reduction and internal fixation. When compared by humeral component design (inlay versus onlay), there was no differences in rates of acromial or scapular spine fractures (2.6% vs. 2.0%; P = .264). Similarly, there were no treatment differences between non-operative (90% vs. 95.5%) or operative management (10% vs. 4.5%) of the fractures based on the type of humeral component design (P = .403).

Conclusions: Acromial and scapular spine fractures complicated the postoperative course of 2.1% of primary RSA when performed across two high volume shoulder arthroplasty centers with multiple surgeons including a wide range of implant types. Most of the fractures involve the acromion, with less frequent involvement of the spine of the scapula. When comparing by inlay versus onlay humeral component design, the rates of postoperative acromial or scapular spine fractures were statistically similar.