The theta angle: A reliable and clinically significant method of radiographic assessment after reverse total shoulder arthroplasty.

Abstract

Reverse shoulder arthroplasty (RSA) is effective for cuff tear arthropathy, but complications from the Grammont design emphasized humeral lateralization and distalization. Conventional parameters such as the acromio-humeral interval (AHI) and critical shoulder angle (CSA) show inconsistent outcome associations. This study introduces new parameters using the greater tuberosity as a reference: the acromion-greater tuberosity distance (AGT), the greater tuberosity-inferior glenoid/glenosphere distance (GTIG), and the theta angle, formed at their intersection. We retrospectively reviewed 60 patients who underwent primary RSA between 2010 and 2019. Clinical outcomes (ROM, Constant-Murley, UCLA, Oxford scores) were assessed pre-operatively and at 2 years. Radiographs were analyzed for AHI, CSA, AGT, GTIG, and theta angle. GTIG, theta angle, and CSA showed good inter-observer reliability (ICC 0.75-0.88). AHI had no outcome correlation. Absolute GTIG and AGT correlated weakly with forward flexion strength (R = 0.30, <i>p</i> = 0.02 and R = 0.27, <i>p</i> = 0.04 respectively). Change in theta angle correlated with ROM and improved Oxford scores (R = 0.36, <i>p</i> < 0.01 and R = -0.28, <i>p</i> = 0.04 respectively, where lower Oxford score is better). Lastly, absolute CSA demonstrated a negative correlation with ROM, strength, and Constant scores (R = -0.31, <i>p</i> = 0.02; R = -0.28, <i>p</i> = 0.03; and R = -0.31, <i>p</i> = 0.02 respectively). GTIG and theta angle are reliable radiographic parameters with clinical utility in RSA assessment.