Reverse shoulder arthroplasty (RSA), a special shoulder prosthesis developed by Grammont in the 1990s [1], is the last treatment for irreparable rotator cuff tears (RCTs) or cuff tear arthropathy (CTA) that has had poor results with hemiarthroplasty or total shoulder arthroplasty (TSA) [2]. RSA provides good pain relief and elevation ability for patients with massive RCTs or CTAs, comminuted proximal humeral fractures, or severe primary osteoarthritis of the shoulder with massive glenoid bone loss [[2], [3], [4]]. As indications for this treatment have increased, more complications have been reported. Stress shielding and humeral osteolysis are complications that may be observed around the humeral stem after TSA or RSA [5]. This was first noted after total hip arthroplasty and has been attributed to changes in stress distribution due to the insertion of the stem [6]. Conversely, previous reports have described that humeral osteolysis was caused by wear debris due to scapular notching [7]. The resulting bone resorption poses the risk of periprosthetic fracture and reduced bone stock, thereby making revision difficult [8]. Therefore, preventing stress shielding or humeral osteolysis is necessary and determining their causes is important, especially for cortical thinning or osteopenia (CTO) (Fig. 1) and proximal humeral osteolysis. Several causes of stress shielding or osteolysis after RSA have been reported [7,9,10], while several of them remain poorly understood.

RSA was first introduced in China and South Korea; it was introduced in Japan in 2014 and is now widely used throughout Asia. Asians, particularly Asian women, have smaller shoulders than Westerners. Smaller components have recently become available for such cases; however, the size of RSAs for Westerners is too large for Asians, which has previously led to overstuffing [11]. Therefore, the frequency or cause of stress shielding when using Western-sized components for Japanese patients may differ from that of Westerners. This study aimed to analyze stress shielding after RSA in an Asian population from multiple centers and analyze the predictors using multivariate regression analyses.