Three-dimensional Scapular Orientation: A Comparison of Glenohumeral Osteoarthritis and Cuff Tear Arthropathy to the Unaffected Shoulder

INTRODUCTION Scapular orientation may play an important role in preoperative planning for shoulder arthroplasty; however there is currently no defined method for producing standardized three-dimensional measurements. The purpose of this study is to develop a reproducible measurement system of scapular orientation and to evaluate differences between unaffected shoulders and those with diagnoses of primary glenohumeral arthritis (GHOA) and rotator cuff tear arthropathy (CTA). METHODS Patients receiving computed tomography (CT) scans for preoperative shoulder arthroplasty planning were enrolled prospectively. Thin sliced axial images that included bilateral shoulders and the thorax were reformatted. The pathology of the bilateral glenohumeral joints were defined as GHOA, CTA, or unaffected. Scapular orientation was defined relative to the thorax using three-dimensional models to define the protraction, rotation, and tilt angles in the supine position. Univariate analysis was performed to compare demographics and scapular measurements between patients with GHOA, CTA, and unaffected shoulder pathology. Multivariate linear regression analysis was performed to identify independent predictors of scapular orientation. RESULTS One-hundred fifty-eight shoulders, comprising 79 patients were included. Twenty-seven percent (n = 21/79) of patients had an unaffected contralateral shoulder. Shoulders with GHOA had significantly greater downward scapular rotation (-14 ± 7 degrees) than both unaffected shoulders (-8 ± 7 degrees; P < 0.001) and those with CTA (-7 ± 8 degrees; P < 0.001). Increased downward scapular rotation was predicted by a shoulder pathology of GHOA in multivariate analysis (beta -7.09; P < 0.001). The magnitude of side-to-side difference was greatest in scapular rotation for patients with GHOA compared to CTA, with no significant differences in tilt or protraction. CONCLUSION Scapular rotation in the supine position varies by shoulder pathology, with greater downward rotation seen in patients with primary GHOA. These findings underscore the need for awareness of scapular orientation in addition to glenoid morphology when evaluating and planning shoulder arthroplasty cases.