Saturday, February 8, 2020

"Risk" factors for progression of cuff tears - association vs causation

The natural course of and risk factors for tear progression in conservatively treated full-thickness rotator cuff tears

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.

The distinction between "association" and "causation" is important in that a surgeon convinced that an increased lateral extension of the acromion causes cuff tear progression, might be tempted to cut off the lateral acromion as was done in the case below.

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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