''The Global Glenoid Component Inclination: why scapulothoracic orientation should be considered when defining glenoid component inclination in reverse total shoulder arthroplasty''.

Abstract

In reverse total shoulder arthroplasty (rTSA), glenoid component inclination critically influences complications and implant performance. Current literature predominantly advocates for neutral or 0° inclination referenced to the supraspinatus fossa in order to avoid superior tilt which may lead to loosening. However, the influence of resting scapular rotation (RSR) on the effectively obtained inclination relative to the global reference system (gravity) remains poorly defined. This study aims to address this gap by evaluating the impact of RSR on global glenoid component inclination following rTSA. This retrospective cohort included 93 consecutive patients who underwent rTSA with 3D preoperative planning and patient-specific instrumentation (PSI) for baseplate placement targeting 0° inclination. Standardized pre- and postoperative true anteroposterior radiographs were obtained with a vertical plumb line aligned to gravity. Measured parameters included scapular rotation angle (SRA), humeral abduction resting angle (HARA), scapulohumeral abduction resting angle (S-HARA), glenoid inclination angle (GIA), global glenoid inclination (GGI), and arthroplasty scapulohumeral abduction resting angle (AS-HARA). Interrater reliability was assessed using intraclass correlation coefficients (ICC). The effect of preoperative RSR on postoperative GGI was evaluated. Correlations among radiographic parameters and associations with clinical outcomes at 9 and 26 weeks were analyzed using multiple regression. Interrater reliability for the measured parameters was excellent (ICC=0.953-0.989). SRA varied widely from 14° upward rotation to 42° downward rotation with a mean of -15±9°. Three reproducible SRA types were identified, which remained mostly constant postoperatively. Despite a targeted glenoid inclination angle of 0°, GGI differed significantly with a mean of -18±10° and a range from 1 to -53° (p<0.0001) and demonstrated a strong correlation with SRA (r=0.77). Preoperatively, S-HARA averaged 113±10° and HARA 7±8°; postoperatively, S-HARA 117±10°, HARA 7±8°, and AS-HARA 116±10°. At 9 weeks, higher postoperative AS-HARA and S-HARA were associated with better ASES Index Scores, whereas by 26 weeks patients had recovered regardless. Preoperative RSR varied substantially across the cohort. Consequently, the effective postoperative global glenoid component inclination ranged from downward to superior inclination depending on the scapula rotation angle, despite targeted glenoid inclination angle of 0° executed with PSI. Therefore, accounting for scapula rotation in terms of the global glenoid inclination when planning the glenoid component inclination seems warranted. The consistency of SRA before versus after rTSA allows its incorporation into personalized planning to avoid superior inclination in the global reference system, which may increase shear forces on the glenoid component and lead to a slower recovery.