Incongruity of the scapulohumeral arch: an indirect sign of posterior subluxation of the humeral head in primary glenohumeral osteoarthritis.

Background: In symptomatic glenohumeral osteoarthritis, arthroplasty is the gold standard to restore a mobile and painless shoulder. The kind of arthroplasty chosen is mainly based on the evaluation of the rotator cuff and the type of glenoid. The aim of this study was to analyze the scapulohumeral arch in primary glenohumeral osteoarthritis (PGHOA) with an uninjured cuff and to determine whether posterior humeral subluxation influences the Moloney line, which corresponds to an intact scapulohumeral arch.

Materials And Methods: Between 2017 and 2020, 58 anatomic total shoulder arthroplasties were implanted in the same center. We included all patients with complete preoperative imaging (radiographs and magnetic resonance imaging or arthro-computed tomography scans) assessing an intact rotator cuff. A total of 55 shoulders that had undergone surgery with a total anatomic shoulder prosthesis were analyzed after determination of the type of glenoid in the frontal plane according to the Favard classification on an anteroposterior radiograph and in the axial plane according to the Walch classification on computed tomography scans. The grade of osteoarthritis was evaluated according to the Samilson classification. We analyzed whether there was a rupture of the Moloney line on the frontal radiograph and assessed the acromiohumeral distance.

Results: After preoperative analysis of 55 shoulders, 24 shoulders had type A glenoids and 31 had type B glenoids. Scapulohumeral arch rupture was observed in 22 shoulders, and posterior subluxation of the humeral head was found in 31 shoulders, of which 25 had type B1 glenoids and 6 had type B2 glenoids according to the Walch classification. Most glenoids were type E0 (n = 47.85%). Incongruity of the Moloney line was more frequent in shoulders with type B glenoids (20 of 31 [65%]) than those with type A glenoids (2 of 24 [8%], P < .001). No patient with a type A1 glenoid (0 of 15) had a rupture of the Moloney line, and only 2 patients with a type A2 glenoid (2 of 9) had incongruity of the scapulohumeral arch.

Conclusion: In PGHOA, rupture of the scapulohumeral arch seen on anteroposterior radiographs, also known as the Moloney line, may be an indirect sign of posterior humeral subluxation, corresponding to a type B glenoid per the Walch classification. Incongruity of the Moloney line may indicate a rotator cuff injury or posterior glenohumeral subluxation with an intact cuff in PGHOA.