The critical shoulder angle after total shoulder, what does it reflect?

Glenoid component loosening associated with increased critical shoulder angle at midterm follow-up

These authors define the 'critical shoulder angle' ( CSA ) at followup after a total shoulder as the angle between a line drawn from the inferior to superior glenoid rim and a line drawn from the inferior glenoid component edge to the inferolateral aspect of the acromion on a true anteroposterior (AP) radiograph.

They suggest that a larger critical shoulder angle (CSA) may cause superior glenoid component loading and more rapid component loosening.

They studied the relationship between the CSA and glenoid component loosening in midterm follow-up in 61 primary total shoulders for osteoarthritis with an average followup of 5.0 years without surgical revision. Standard true anteroposterior radiographs postoperatively and at longest follow-up were graded. An “at-risk” glenoid was defined as grade 3 or higher lucency.

The average CSA was 32° ± 5°, median midterm lucency grade was 2 (range, 0-5), and median progression of lucency grade was 1 (range, −1 to 4).

At midterm follow-up, 20% of TSAs were grade 3 or higher mean glenoid lucency, with an average CSA of 36°.

There was a statistically significant correlation between CSA and both glenoid lucency grade (odds ratio, 1.20 per degree CSA) and progression of lucency grade (odds ratio, 1.24).

An increase in CSA of 10° was associated with a 6.2-fold increased odds of having an at-risk glenoid.

Comment: This paper brings up some interesting points.

First 20% of the total shoulders had glenoid components that were 'at risk' for failure.

Second, the study did not include the preoperative or the immediate postoperative CSA measurements, so the prognostic value of CSA in these patients is unknown.

Third, the authors suggest that CSA is a 'modifiable' risk factor for glenoid loosening, but it is not clear if they are suggesting that the CSA be modified by shortening the acromion or by placing the glenoid component in a more inferiorly directed position.

Fourth, it is not clear if the post total shoulder CSA should be determined by a line drawing from the inferior to the superior rim of the glenoid component or a line drawn from the inferior to the superior rim of the residual bony glenoid.

Finally, it is not clear whether the association of CSA and glenoid loosening is because the increase in CSA is causing the loosening or because the loosening of the glenoid (typified by superior tilting) is increasing the CSA as suggested by the two radiographs below from this article.

The one on the left shows an initial CSA of 28 and grade 1 lucency, while the one on the right shows a same shoulder followup CSA of 32 or 40 (depending on the glenoid reference) and grade 4 lucency.

This may, in fact, be an example of rocking horse loosening with rather massive osteolysis.