Glenoid vault and humeral head alignment in relation to the scapular blade axis in young patients with pre-osteoarthritic static posterior subluxation of the humeral head.

BACKGROUND Static posterior subluxation of the humeral head is a pre-osteoarthritic deformity preceding posterior erosion in young patients. Its etiology remains unknown. The aim of this study was to analyze the differences between scapular morphology in young patients with a pre-osteoarthritic static posterior subluxation of the humeral head and healthy controls with a centered humeral head. METHODS A retrospective analysis of all patients with a pre-osteoarthritic static posterior subluxation of the humeral head, who were treated in our institution between January.2018 and November 2019, was performed. Fourteen shoulders in 12 patients were included in this study and then matched according their age, gender and affected side with controls. Computer tomography images of both groups were compared on standardized axial images for differences in scapula morphology. Following parameters were measured: Glenoid version relative to the Friedman line and scapular blade axis, scapulohumeral and glenohumeral subluxation index, neck angle, as well as glenoidal and humeral offset. RESULTS The patients in the subluxation group showed a significantly higher scapulohumeral and glenohumeral subluxation index compared to controls (0.76 vs. 0.55, p<0.0001 and 0.58 vs. 0.51, p=0.016). The mean glenoid version according to Friedman and relative to the scapular blade axis were significantly higher in the subluxation group compared to controls (19° vs. 4°, p<0.0001, 14° vs. 2°, p=0.0002). The glenoid vault was significantly more anteriorly positioned in respect to the scapular blade axis in the subluxation group compared to controls (neck angle 166° vs. 173°, p=0.0003, glenoid offset 9.2mm vs. 4.6mm, p=0.0005). The midpoint of the humeral head showed a posterior offset in respect to the scapular blade axis in the subluxation group, while controls had an anteriorly placed midpoint of the humeral head (-2mm vs. 3.1mm, p=0.01). A higher scapulohumeral subluxation index showed significant correlations with an increased anterior offset of the glenoid vault (increased glenoidal offset and decreased neck angle) (r=0.493, p=0.008 and -0.554, p=0.002), with a posterior humeral offset (r=-0.775, p<0.0001) and excessive glenoid retroversion measured by both methods (Friedman line: r=0.852, p<0.0001, Scapular blade axis: r=0.803, p<0.0001). A higher glenohumeral subluxation index also correlated significantly with an increased anterior offset of glenoid vault (r=0.403, p=0.034 and -0.406, p=0.032) and posterior humeral offset (r=-0.502, p=0.006). CONCLUSION Young patients with pre-osteoarthritic static posterior subluxation of the humeral head have significant constitutional differences of scapula morphology in terms of an increased anterior glenoid offset, excessive glenoid retroversion as well as increased posterior humeral offset in relation to the scapular blade compared to healthy matched controls. LEVEL OF EVIDENCE Anatomy Study; Imaging.