From Grammont to a New 135° Short-Stem Design: Intraoperative 2-Hand-Lever-Test & Early Superior-Lateral Dislocations Reveal Critical Role of Liner Stability Ratio and Stem Alignment

Background: In reverse shoulder arthroplasty (RSA), the neck-shaft angle (NSA) has trended downward from 155° to 135° to reduce scapular notching, but concerns about instability persist. To assess superior-lateral stability, we developed the intra-operative 2-hand-lever-test (2HLT). This study evaluates the 2HLT's effectiveness, the learning curve with a new implant, and compares liner characteristics of 155° and 135° systems. Methods: In a single-surgeon learning curve study, 81 RSA procedures with the new Perform stem (Stryker) were included. Outcomes included the 2HLT test applied in 65 cases, early dislocations, stem alignment, stem length, liner type/thickness, and complications. The early dislocation rate was compared to 167 prior Ascend Flex RSA procedures (Stryker). Liner characteristics of three 135° systems (Perform/Stryker; Univers/Arthrex; Altivate/Enovis) were compared to traditional 155° Grammont systems (Delta Xtend/DePuy; Affinis Metal/Mathys; SMR 150/Lima, Aequalis Reversed/Stryker), focusing on jump height (JH) and liner stability ratio (LSR). Results: In 75% (49/65) of cases, the 2HLT detected superior-lateral instability, influencing implant selection. The early dislocation rate in the Perform cohort was 4.9% (0% with retentive liners, 8% with standard liners) versus 0% in the Ascend Flex cohort. The mean effective NSA was 133° (127°-144°) for short Perform stems and 135° (129°-143°) for long stems. Long Perform stems significantly reduced varus outlier density below 132° and 130° (p=0.006, p=0.002). The 36mm Perform 135° standard liner has a JH of 8.1mm and LSR of 152%, markedly lower than the Altivate (10.0mm/202%) and Univers (9.7mm/193%) and similar to traditional 155° Grammont liners (8.1-8.9mm/ 147%-152%). Perform retentive liners have LSR values of 185%-219%, comparable to established 135° design standard liners (195%-202%). In the Perform cohort, early complications included 4 superior-lateral dislocations (all standard liners, LSR 147%-152%) requiring 4 revisions. Conclusions: The 2HLT effectively identified superior-lateral instability and guided implant selection. Perform standard liners have a lower LSR and JH than established 135° designs, contributing to superior-lateral instability, particularly with an effective NSA < 135°. Retentive Perform liners with an LSR > 184% have a similar LSR compared to standard liners of established 135° designs and effectively mitigated instability. We recommend discontinuing the use of non-retentive Perform RSA liners (LSR <158%).