Involvement of the scapulothoracic articulation after well-functioning reverse total shoulder arthroplasty

Shoulder movements result from the combined movements of the glenohumeral (GH) joint and the scapulothoracic (ST) joint, following a rhythm known as the scapulohumeral rhythm (SHR). The SHR represents the ratio of glenohumeral movement to scapulothoracic movement during arm elevation. Numerous studies have demonstrated that scapulothoracic kinematics and the SHR are significantly modified after reverse total shoulder arthroplasty (rTSA). This study aimed to analyze scapulothoracic motion post rTSA and compare it to that of an asymptomatic healthy shoulder.

Thirteen shoulders formed the rTSA group. All patients had undergone rTSA for primary osteoarthritis (9 shoulders) or cuff tear arthropathy (5 shoulders) between 2017 and 2022. The healthy group consisted of 25 adult volunteers with no previous medical or surgical history involving the shoulder or spine. Analyses were performed bilaterally. For each patient, clinical (shoulder range of motion), functional (Constant score, American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form, and Subjective Shoulder Value), and scapular kinematic assessments were conducted using miniature inertial measurement units.

Scapular upward rotation was significantly increased in rTSA patients compared with healthy controls starting from 60° of elevation (P < .01). Scapular retraction was also significantly higher in rTSA patients compared with healthy controls from the beginning of elevation (P < .01). No significant difference in scapular tilt was found between rTSA and healthy patients. The SHR of healthy controls varies during abduction, from 7 at 30° to 4 at the end of the movement, indicating increased involvement of the ST joint relative to the GH joint. In contrast, the SHR in the rTSA cohort remains stable throughout abduction, from 2.4 at the beginning to 2.8 at the end, suggesting a greater contribution of the ST joint to abduction in rTSA patients than in the healthy controls, especially at the movement's initiation.

This study demonstrates that the kinematics of the ST joint during overhead motion after a well-functioning rTSA are significantly altered compared with a healthy shoulder. Overhead motion was associated with a significant increase in scapular external rotation and retraction in rTSA patients, with no difference in scapular tilt. The ST contribution to overall shoulder movement is significantly increased in patients with an rTSA compared with a healthy shoulder. Better understanding of ST motion after rTSA could help improve rehabilitation protocols and preoperative planning for rTSA.