Mid-Term Outcomes of a Rectangular Stem Design with Metadiaphyseal Fixation and a 135° Neck–Shaft Angle in Reverse Total Shoulder Arthroplasty

<b>Background/Objectives</b>: Classical reverse shoulder arthroplasty (RSA) with a high neck-shaft angle (NSA) of 155° has shown satisfactory outcomes. However, newer RSA designs aim to improve results by modifying the stem design. This study evaluates the 5-year outcomes of a stem design featuring a rectangular metadiaphyseal fixation and a 135° NSA. <b>Methods</b>: This prospective bicentric case series included and longitudinally followed up patients that were treated for cuff arthropathy, massive irreparable rotator cuff tears, or eccentric osteoarthritis using a non-cemented rectangular metadiaphyseal fixation stem with a 135° NSA (Univers Revers, Arthrex, Naples, FL, USA). Subjective and objective functional outcome scores (Constant-Murley Score (CS), Shoulder Pain and Disability Index (SPADI), and Subjective Shoulder Value (SSV)), range of motion (ROM), radiographic outcome, adverse events, complications, and quality of life were investigated. <b>Results</b>: This study enrolled 132 patients (59% female, mean age 75 years, SD 6). At the 5-year follow-up, subjective and objective outcomes significantly improved compared to baseline: CS (32.9 to 71.7, <i>p</i> < 0.001), SPADI (38.7 to 86.2, <i>p</i> < 0.001), and SSV (43.0 to 84.1, <i>p</i> < 0.001). ROM improved in flexion (80° to 142.4°, <i>p</i> < 0.001), abduction (71.5° to 130.2°, <i>p</i> < 0.001), internal rotation (<i>p</i> < 0.001), internal rotation at 90° abduction (12.7° to 45.0°, <i>p</i> < 0.001), and abduction strength (0.8 kg to 5.2 kg, <i>p</i> < 0.001). External rotation remained unchanged (32.1° to 32.0°, <i>p</i> = 0.125), but external rotation at 90° abduction improved (20.9° to 52.7°, <i>p</i> < 0.001). No signs of implant migration, subsidence, shift, tilt, alignment loss, or wear were observed, but scapular bone spur formation (11%), scapular notching grade 1 (10%), bone resorption (10%), and partial humeral radiolucent lines (1%) were reported. <b>Conclusions</b>: Rectangular stems with metadiaphyseal fixation and a 135° neck-shaft angle in RSA consistently improve shoulder function, showing no aseptic loosening and minimal radiological changes at 5 years.