Do intact rotator cuff tendons and soft tissue tensioning affect stress shielding in reverse total shoulder arthroplasty? Findings from the Medacta shoulder system.

Abstract

Stress shielding has received increased attention in research, with higher filling ratios, longer stem length, and female gender identified as risk factors. The aim of this study was to investigate the effect of the remaining intact rotator cuff tendons (RCTs) and soft tissue tensioning on stress shielding in uncemented reverse total shoulder arthroplasty (rTSA). A database of patients undergoing rTSA (Medacta Shoulder System) between 2017 and 2021 was reviewed. Excluded were proximal humeral fractures, tumors, incomplete clinical or radiographic data, or revision within the first 2 years. Demographic variables included age and gender. Radiographic variables included stress shielding, bone quality (deltoid tuberosity index), metaphyseal and distal filling ratios (dFR), lateralization angle, and distalization angle (DSA). The number of intact RCT was determined based on available imaging and intraoperative evaluation. The effect of the assessed variables on stress shielding was analyzed with regression analyses. Optimal thresholds were calculated using recursive partitioning. In 77 consecutive patients (mean age 72 years), rTSA resulted in a mean absolute Constant Score (aCS) of 71 points and a mean relative CS of 98% at 2-year follow-up. The presence of stress shielding, seen in 32 cases (aCS: 70 points), did not affect functional outcome compared to those without stress shielding (aCS: 72 points; P = .390). Linear regression analyses identified higher dFR (P < .001), <3 intact RCT (P = .006), lower DSA (P = .044), and female gender (P = .041) as independent risk factors for stress shielding. metaphyseal filling ratio (P = .769), age (P = .241), deltoid tuberosity index (P = .722), lateralization angle (P = .215), and stem length (P = .231) had no influence. Cut-off values to prevent stress shielding were >55° for DSA and <0.79 for dFR, reducing stress shielding by a factor of 16 when both are met. Less than three intact RCT and low DSA were found to be independent risk factors for stress shielding, in addition to known risk factors such as high filling ratios and female gender. Lower distal filling ratios, ideally <0.79, remains the most important factor. However, additional distalization with a DSA >55° substantially reduces stress shielding, with a 16-fold decrease when both thresholds are met.