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

Abstract

In symptomatic glenohumeral osteoarthritis, arthroplasty is the gold standard to restore a mobile and painless shoulder. The kind of arthroplasty chosen is based mainly 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 a non-injured cuff and to determine whether posterior humeral subluxation influences Moloney's line, which corresponds to an intact scapulohumeral arch. Between 2017 and 2020, 58 anatomic total shoulder arthroplasties were implanted in the same center. We included all patients with complete preoperative imaging (X-rays and MRI or arthro-CT scan) assessing an intact rotator cuff. A total of 55 shoulders that had undergone surgery for a total anatomical shoulder prosthesis were analyzed after determining the type of glenoid in the frontal plane according to the Favard classification on an anteroposterior X-ray and in the axial plane according to the Walch classification on CT scan. The grade of osteoarthritis was evaluated according to Samilson. We analyzed whether there was a rupture of Moloney's line on the frontal radiograph and assessed the acromiohumeral distance. After preoperative analysis of 55 shoulders, 24 shoulders had a type A glenoid and 31 a type B glenoid. Twenty-two shoulders had a scapulohumeral arch rupture and 31 had a posterior subluxation of the humeral head: 25 shoulders had a type B1 glenoid and 6 had a type B2 glenoid according to the Walch classification. Most glenoids were E0 (n=47, 85%). The incongruity of Moloney's line was more frequent with type B than A glenoids (20/31, 65%, vs 2/24, 8%, p<0.001). No patient (0/15) with a type A1 glenoid had rupture of Moloney's line, and only two patients (2/9) with a type A2 glenoid had incongruency of the scapulohumeral arch. In PGHOA, the rupture of the scapulohumeral arch seen in anteroposterior X-rays, also known as Moloney's line, may be an indirect sign of posterior humeral subluxation, corresponding to a type B glenoid in the Walch classification. Incongruity of Moloney's line may indicate a rotator cuff injury or posterior glenohumeral subluxation with an intact cuff in PGHOA.