Degenerative rotator cuff tears (RCT) and osteoarthritis (OA) are associated with differences in coronal plane scapular morphology, with particular focus on the effect of the Critical Shoulder Angle (CSA) on shoulder biomechanics. The effect, if any, of axial plane scapular morphology is less well established. We have noticed wide disparity of axial coracoid tip position in relation to the face of the glenoid, and sought to investigate the significance of this through measurement of the Critical Coracoid Process Angle (CCPA), which incorporates coracoid tip position and glenoid version.
Methods: CCPA, CSA and glenoid retroversion were measured by three independent reviewers from the cross-sectional imaging of 160 patients in four equal and matched case-control groups: 1) a control group of patients with a radiologically normal shoulder and no history of shoulder symptoms who had a CT thorax for another reason; 2) patients with primary OA with Walch Type-A glenoid wear pattern on CT scan; 3) patients with Type-B glenoid primary OA; 4) patients with MRI proven atraumatic tears of the posterosuperior rotator cuff.
Results: Interobserver agreement was excellent for all measured parameters. Median CCPA was significantly lower in the Type-B OA group (9.3˚) when compared to controls (18.7˚), but not significantly different in the other study groups. There was a trend towards greater glenoid retroversion in the Type-B OA group, but Receiver Operating Characteristic curve analysis demonstrated CCPA to be by far the most powerful discriminator for Type-B OA. Optimal cut-off value was calculated for CCPA at 14.3˚ with a sensitivity of 93% and specificity of 90% for Type-B OA. Compared to controls, CSA was significantly higher than controls in the RCT group, and lower in both OA groups, but did not differentiate between Type-A and Type-B OA.
Conclusion: Combined with a lower CSA, a lower CCPA (<14.3˚) is strongly predictive of Type-B glenoid OA. The authors propose a simple model of Pectoralis Major biomechanics to explain the effect of this axial plane anatomical variation, which requires further investigation.