The Impact of Critical Shoulder Angle during Reverse Total Shoulder Arthroplasty: Is It Helpful?

Backgroud: The critical shoulder angle (CSA) is an important radiologic measurement that is significantly associated with deltoid and rotator cuff biomechanics. It was hypothesized that the CSA affects the biomechanics of the shoulder joint before and after reverse total shoulder arthroplasty (RTSA).

Methods: One hundred and fifty-six patients (156 shoulders) who underwent RTSA with medial glenoid/lateral humerus design were divided into 3 groups according to preoperative CSA: low CSA group (< 30°), intermediate CSA group (30°-35°), and high CSA group (> 35°). The preoperative and postoperative lateral humeral offset (LHO) and acromiohumeral distance (AHD) and postoperative deltoid wrapping angle (DWA) and deltoid moment arm (DMA) were measured on x-ray and computed tomography and compared between the 3 groups. The improvement of the pain score, assessed using the visual analog scale at rest and motion, American Shoulder and Elbow Surgeons score, University of California, Los Angeles shoulder score, and Constant shoulder score were compared between the 3 groups.

Results: The postoperative LHO was the highest in the low CSA group (16.62 ± 4.76 mm) and the smallest in the high CSA group (13.68 ± 5.59 mm, p = 0.03). The postoperative AHD was the highest in the low CSA group (28.72 ± 5.42 mm) and the lowest in the high CSA group (25.21 ± 5.12 mm) (p = 0.004). The postoperative DWA, DMA, and improvement of clinical scores were not significantly different between the groups (p > 0.05). Five patients (3.2%) experienced acromion fractures in the postoperative period, including 1 in the intermediate CSA group and 4 in the high CSA group.

Conclusions: RTSA with lateralized humeral design in patients with a low CSA resulted in more lateralization and distalization than in patients with an intermediate or high CSA, although the difference was less than 5 mm. The imaging and postoperative improvement in clinical outcomes associated with deltoid action such as range of motion, DWA, and DMA were not significantly different between patients with different CSAs. Therefore, the CSA is not useful in the selection of humeral components during RTSA, since there is no significant difference in clinical or functional outcomes based on CSA.