The role of patient and implant factors, and surgical techniques in the development of acromial and scapular stress fractures after reverse shoulder arthroplasty

Reverse shoulder arthroplasty has revolutionized the treatment of complex shoulder pathologies, particularly in patients with severe rotator cuff tears and arthritis. however, the increasing prevalence of this procedure has brought to light several previously under-recognized complications, amongst which acromial and scapular spine stress fractures. while studies, both biomechanical and clinical, have been performed trying to evaluate risk factors associated with acromial and scapular spine stress fractures, most have involved small sample sizes, and several have had conflicting results. our aim was to identify patient and implant factors, as well surgical technique aspects that are associated with these fractures associated, that could be generalizable to all patient populations and implant designs. we did this by performing a large, multicenter retrospective cohort study involving 6320 reverse shoulder arthroplasty patients done by 24 different surgeons. the delphi method was used to identify study parameters. we analyzed various patient factors, including age, gender, comorbidities like osteoporosis and inflammatory arthritis, and indications for surgery, like glenohumeral osteoarthritis and rotator cuff arthropathy. for implant factors we analyzed parameters such as total glenoid lateral offset and neck-shaft angle. additionally, radiographic measurements for the final implant position were performed, which included lateralization shoulder angle, distalization shoulder angle and lateral humeral offset. these radiographic measurements provide an objective assessment of the final implant position, which takes into consideration implant design and surgical technique. we found the incidence of acromial and scapular stress fractures to be 3.8%. increased risk for acromial stress fractures was significantly associated with inflammatory arthritis (or 2.29, p<0.001), massive rotator cuff tear (or 2.05, p=0.010), osteoporosis (or 2.00, p<0.001), prior shoulder surgery (or 1.82, p<0.001), cuff tear arthropathy (or 1.76, p=0.002), female sex (or 1.74, p=0.003), older age (or 1.02, p=0.018), and greater total glenoid lateral offset (or 1.06, p=0.025). conversely, revision surgery showed a protective effect (or 0.38, p=0.019) compared to primary surgery. scapular spine stress fracture risk factors included female sex (or 2.45, p=0.009), rotator cuff disease (or 2.36, p=0.003), osteoporosis (or 2.18, p=0.009), and inflammatory arthritis (or 2.04, p=0.024). radiographically, we found that a greater increase in the lateralization shoulder angle from before to after surgery (or 1.42, p=0.005) and a higher postoperative lsa (or 1.76, p=0.009) increased fracture risk of fractures; conversely, increased lateral humeral offset appeared to be protective of this complication (or 0.74, p=0.031). these findings emphasize the complex interplay between patient-specific factors and implant/surgical technique in the development of these complications. this study can help surgeons identify which of their reverse shoulder arthroplasty patients are at a higher risk for developing these complications. additionally, it can provide guidelines on implant and surgical technique choices when performing these procedures in these high-risk patients. for example, positing the arthroplasty with increased humeral sided lateralization but limited global lateral offset could reduce the risk of stress fractures in a patient with rotator cuff arthropathy.