Abstract: Background  
The critical shoulder angle (CSA) is associated with the risk for rotator cuff tears (RCT) and primary osteoarthritis (OA). Musculotendinous retraction is considered the most relevant pathophysiological consequence of chronic tendon tearing. This study analyzed the existence of this association and explored a possible relation between CSA, supraspinatus muscle (SSP) retraction and tear extension. 

Methods 
We included 50 patients submitted to shoulder surgery, in which 38 have RCT and 7 have OA. Data about number of RCT muscles affected, tear gravity and extension, SSP retraction, gender and age were collected. CSA value quality was ensured using Suter-Henninger classification and with two-observer measuring, followed by a Bland-Altman plot. 

Results 
The CSA mean value on RCT patients was 35,3° ± 4,9° and on OA patients 27,9° ± 5,1°. The complete SSP tear group had a mean CSA of 36,7° ± 5,2°, significantly higher (p=0.03) than the partial tear group (33,3° ± 3,88°). Spearman test showed significant positive correlation between extension and retraction (R=0,525; p<0,01) but no correlation between CSA and retraction (p = 0,1). The large tear group is significantly older than the isolated SSP tear group (63,9 ± 9,02 years vs. 58 ± 7,35 years; p<0,05). 

Conclusions 
Higher CSA values are associated with the risk of RCT and lower values with OA. Larger angles are associated with increasing SSP tear gravity. No significant association between CSA and SSP retraction was found, although bigger SSP tears have more tendon retraction. Older age increases the risk of larger tears.

Keywords: Critical shoulder angle; scapular morphology; rotator cuff tear; osteoarthritis; shoulder radiograph; supraspinatus muscle.

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