Similar scapular morphology in patients with dynamic and static posterior shoulder instability

Background
There is evidence that specific variants of scapular morphology are associated with dynamic and static posterior shoulder instability. To this date, observations regarding glenoid and/or acromial variants were analyzed independently, with two-dimensional imaging or without comparison with a healthy control group. Therefore, the purpose of this study was to analyze and describe the three-dimensional (3D) shape of the scapula in healthy and in shoulders with static or dynamic posterior instability using 3D surface models and 3D measurement methods.

Methods
In this study, 30 patients with unidirectional posterior instability and 20 patients with static posterior humeral head subluxation (static posterior instability, Walch B1) were analyzed. Both cohorts were compared with a control group of 40 patients with stable, centered shoulders and without any clinical symptoms. 3D surface models were obtained through segmentation of computed tomography images and 3D measurements were performed for glenoid (version and inclination) and acromion (tilt, coverage, height).

Results
Overall, the scapulae of patients with dynamic and static instability differed only marginally among themselves. Compared with the control group, the glenoid was 2.5° ( = .032), respectively, 5.7° ( = .001) more retroverted and 2.9° ( = .025), respectively, 3.7° ( = .014) more downward tilted in dynamic, respectively, static instability. The acromial roof of dynamic instability was significantly higher and on average 6.2° ( = .007) less posterior covering with an increased posterior acromial height of +4.8mm ( = .001). The acromial roof of static instability was on average 4.8° ( = .041) more externally rotated (axial tilt), 7.3° ( = .004) flatter (sagittal tilt), 8.3° ( = .001) less posterior covered with an increased posterior acromial height of +5.8 mm (0.001).