These authors retrospectively evaluated tear progression in 48 non-operatively treated rotator cuff tears using magnetic resonance imaging (MRI) with the goal of identifying risk factors for tear progression >5 mm over 22 months (range, 12-65 months).
26 of these tears (54%) showed medial-lateral (M-L) progression while 20 (41%) showed anterior-posterior (A-P) tear progression on MRI follow-up.
Multivariate analysis revealed that MRI follow-up duration, diabetes mellitus, and infraspinatus muscle atrophy were associated with progression in the A-P plane. A high critical shoulder angle and supraspinatus and infraspinatus muscle atrophy were risk were associated with with M-L tear progression.
Comment: This is not a study of "risk factors" a phrase that implies causation; rather it is a study of associations. Following shoulders for longer periods of time does not increase the risk of cuff tear progression, for example.
The assertion that an increased "critical shoulder angle" is a risk factor for cuff tear progression, suggests that a five degree change in this angle from 33 to 38 degrees would be causative.
It seems at least equally likely that the change in CSA is not a cause, but rather the result of cuff disease. Note in the example below that the CSA measurement is increased by drawing a line from the inferior glenoid to the edge of a thin calcification on the lateral acromion, a change that may well arise from increased loading of the coracoacromial arch in shoulders with failing rotator cuffs.
There is a temptation in some corners to measure angles on the AP x-ray and attribute great clinical significance to small differences.
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