Restoration of joint line and soft tissue balance: a 3D reference system to quantify shoulder pathologies

While two-dimensional methods quantify glenohumeral relationships, they are unable to capture three-dimensional anatomy and define the geometric relationship of the humeral head in the registry of the glenoid. We hypothesize that a geometric relationship of the humeral head and the glenoid exists in the three-dimensional space defined by the relationship of the best-fit sphere of the humeral head relative to the best-fit sphere of the glenoid such that relative measurements will define the normal shoulder and the pathologic shoulder. A retrospective cohort study was conducted using computed tomography scans from 90 shoulders including 30 normal, 30 with glenohumeral osteoarthritis (GHOA) with Walch type A1 glenoids, and 30 with cuff tear arthropathy (CTA). Scans were then analyzed using surgical planning software. Angular measurements were calculated including horizontal displacement angle (HDA) and vertical displacement angle (VDA) in the coronal plane, and subluxation angle and relative subluxation axial angle (RSAA) in the axial plane. In the coronal plane, the mean HDA was significantly higher in GHOA (73.6&#xb0;) compared to normal (66.9&#xb0;, P = .003) and CTA (62.7&#xb0;, P < .01). Mean VDA was significantly elevated in CTA (54.7&#xb0;) compared to normal (42.2&#xb0;, P < .01) and GHOA (41.0&#xb0;, P < .01). In the axial plane, mean subluxation angle did not significantly differ between normal (95.6&#xb0;), GHOA (94.1&#xb0;), and CTA (98.3&#xb0;) shoulders. Similarly, the mean RSAA did not differ significantly across normal (87.6&#xb0;), GHOA (90.0&#xb0;), and CTA (89.9&#xb0;) cohorts. However, normal shoulders demonstrated significantly less variance in both axial parameters compared to pathologic shoulders. The variance in the subluxation angle was significantly lower in normal shoulders compared to both GHOA (P = .034) and CTA (P = .032). Likewise, the variance in the RSAA was significantly lower in normal shoulders compared to both GHOA and CTA (P < .01). Defined geometric relationships exist between the humeral head in the reference of the glenoid sphere. This study establishes a reliable method of using best-fit spheres of the glenoid and humeral head. The HDA and VDA in the coronal plane differentiate the normal shoulder from those with GHOA and CTA. The subluxation angle and the RSAA in the axial plane describe the glenohumeral relationship in both subluxation and relative subluxation. This method quantifies the expected relationship of the humeral head in the glenoid registry in the normal shoulder and characterizes predictable disruptions in disease, supporting improved understanding of premorbid anatomy and potential restoration of optimal shoulder function.