The critical shoulder angle (CSA) is the angle between a line connecting the superior and inferior aspects of the glenoid fossa (the glenoid inclination) and another line connecting the inferior aspect of the glenoid with the most inferolateral point on the acromion as seen on a plain anteroposterior radiograph.

A CSA >35 degrees has been associated with rotator cuff tears while a CSA <30 degrees has been is associated with primary glenohumeral arthritis. It has been suggested that a larger (more obtuse) CSA increases the elevating vector of the deltoid, leading to fatigue of the posterior-superior rotator cuff while smaller (more acute) CSA angles lead to increased compressive forces across the glenohumeral joint leading to wear of the glenoid joint surface.

A few issues come to the fore:

1. Does it seem reasonable that a change in the CSA by as little as 5 degrees would change the pathology of a shoulder from cuff disease to arthritis?

2. Is the CSA influenced primarily by glenoid inclination or by the position of the acromion?

3. What does CSA tell us about the acromion? The author of A prospective observational case control study investigating the coronal plane scapular morphological differences in full-thickness posterosuperior cuff tears and primary glenohumeral osteoarthritis, points out that the position of most inferolateral point on the acromion is affected both by the length and the height of the acromion: a longer acromion will increase the CSA

and a higher acromiom will decrease it.

4. Should preoperative CSA affect the type of arthroplasty? The authors of The association between critical shoulder angle and revision following anatomic total shoulder arthroplasty: a matched case-control study suggest that surgeons consider using a reverse total shoulder arthroplasty rather than an anatomic total shoulder (aTSA) in cases of primary shoulder arthritis with a preoperative CSA of 35 degrees or greater. This recommendation is based on their finding that aTSAs revised for glenoid loosening or cuff failure had a higher likelihood of having a CSA >35 (although the difference in mean CSA between revised and unrevised shoulders was only two degrees).

This article did not report glenoid inclination, so it is not known if the observed difference in CSA between revised and unrevised shoulderS is related to glenoid inclination or to acromial anatomy; it would be of interest to know whether shoulders revised for glenoid loosening and cuff failure had more superiorly inclined glenoids (see Anatomic total shoulder glenoid component inclination affects glenohumeral kinetics during abduction: a cadaveric study). Shoulders with increased CSA have been reported to have a higher prevalence of rotator cuff tears; the preoperative status of the rotator cuff is not presented for the patients in this series. The authors of The implications of the glenoid angles and rotator cuff status in patients with osteoarthritis undergoing shoulder arthroplasty found that in osteoarthritic patients, the CSA was higher in those with secondary osteoarthritis with torn rotator cuffs than in those with intact rotator cuffs and that the CSA was positively correlated with glenoid inclination.

The effectiveness of decreasing CSA by increasing the inferior tilt of the anatomic glenoid is not known.

5. Is there evidence that "correcting" a high CSA by lateral acromioplasty improves the outcome of rotator cuff surgery?

The authors of The Effects of Arthroscopic Lateral Acromioplasty on the Critical Shoulder Angle and the Anterolateral Deltoid Origin: An Anatomic Cadaveric Study found that in cadavers a 5-mm lateral acromion resection combined with an acromioplasty reduced the CSA from a mean of 34.3 degrees to a mean of 31.5 degrees. The authors of Arthroscopic Correction of the Critical Shoulder Angle Through Lateral Acromioplasty: A Safe Adjunct to Rotator Cuff Repair used lateral acromioplasty to reduce the mean CSA was reduced from 37.5° preoperatively to 33.9° postoperatively, but no clinical outcomes are presented. The authors of Lateral acromioplasty for correction of the critical shoulder angle used lateral acromioplasty to reduce the mean preoperative CSA from 39.7 ± 1.0°, to an average value of 32.1 ± 1.2° in patients having surgery for cuff disease, but clinical results are not presented. A review of the literature revealed one non-randomized retrospective case series that showed a small and inconsistent benefit of lateral acromioplasty Lateral Acromioplasty has a Positive Impact on Rotator Cuff Repair in Patients with a Critical Shoulder Angle Greater than 35 Degrees

Two recent editorials provide a useful perspective on CSA. Editorial Commentary: It's Deja Vu All Over Again: Critical Shoulder Angle X-ray Measurements Do Correlate With Disease if the X-rays Are Carefully Taken:

"The critical shoulder angle and its correlation with rotator cuff tears and alternatively glenohumeral osteoarthritis has become a popular research topic in recent years. With carefully standardized x-rays, a correlation emerges. This development has generated interest in potential clinical usefulness for this measurement, as well as possible surgical interventions to modify the course of these shoulder problems. Lateral acromioplasty may have a role in rotator cuff surgery, but early study results are mixed. Prophylactic lateral acromioplasty has been proposed but would not be evidence based at this time, and there could be unintended negative consequences. For now, accurate x-ray films and awareness of the critical shoulder angle by the clinician could impact the index of suspicion and also may prove useful in patient counseling for these 2 shoulder diseases."

Editorial Commentary: Critical Shoulder Angle: Perhaps Not So "Critical" for Clinical Outcomes Following Rotator Cuff Repair:

"The critical shoulder angle (CSA) has been associated with the development of rotator cuff pathology. More recent studies have also identified a correlation between an increased CSA and recurrent tears after arthroscopic rotator cuff repair. Although this observation is significant, several studies have failed to identify a correlation between CSA and clinical outcomes after rotator cuff repair. As a result, the usefulness of this measurement, and the need to address it with lateral acromioplasty, remains ill defined. Further research is required to demonstrate an association between CSA and clinical outcomes before treatment algorithms should be altered."

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