Tuesday, August 9, 2016

The 'critical' shoulder angle- cause or effect of shoulder pathology?

Predominance of the critical shoulder angle in the pathogenesis of degenerative diseases of the shoulder.

In 200 patients these authors measured the angle between a line connecting the inferior margin to the superior margin of the glenoid fossa and a second line connecting the inferior margin of the glenoid fossa to the lateral margin of the acromion.

The average angle was 34° ± 3° in the control group, 36° ± 3° with supraspinatus tears, 40° ± 3.5° with supraspinatus and infraspinatus tears, and 28° ± 2° with concentric osteoarthritis. Patients with large cuff tears had significantly greater angles compared with those with isolated supraspinatus tears (P = .03). The angle was associated with cuff tears (odds, 1.7; confidence interval [CI], 1.4-2.0). The Spearman coefficient between the angle and grade of eccentric osteoarthritis was 0.4 (P = .01). 

Larger angles were associated with increased rate of symptomatic cuff tears, larger cuff tears, and the severity of eccentric osteoarthritis. Smaller angles increased the risk and severity of concentric symptomatic osteoarthritis. These associations remained significant even after removal of some of the potentially confounding variables.

Comment: While it has been suggested that this angle is "responsible" for the occurrence of cuff tears and concentric osteoarthritis when the angle is significantly different from the corresponding angle in normal shoulders, we observe that this study demonstrates association but not causation. It seems more likely that changes in the angle are the result of the shoulder pathology rather than its cause. In that regard, it would be of interest to compare the shoulder angles between the pathological shoulder and the contralateral shoulder in the same subject to see if the angle was developmental or acquired.

For those readers interested in proving causation, this link will be of interest.