The CSA: the confusing shoulder angle.

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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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link).

How to x-ray the shoulder (see this link).

The ream and run procedure (see this link).

The total shoulder arthroplasty (see this link).

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

The smooth and move procedure for irreparable rotator cuff tears (see this link).

Shoulder rehabilitation exercises (see this link).

Revision of anatomic total shoulders - relationship to the critical shoulder angle (CSA)

 The association between critical shoulder angle and revision following anatomic total shoulder arthroplasty: A matched case-control study

The critical shoulder angle (CSA) is a two dimensional measurement of the angle between two lines based at the inferior glenoid - one passing through the lateral acromion and one passing through the superior glenoid.

It is of note in the 2013 description of the critical shoulder angle, Is there an association between the individual anatomy of the scapula and the development of rotator cuff tears or osteoarthritis of the glenohumeral joint?, the authors  hypothesised that a large acromial cover with an upwardly tilted glenoid fossa would be associated with degenerative rotator cuff tears (RCTs), and conversely, that a short acromion with an inferiorly inclined glenoid would be associated with glenohumeral osteoarthritis (OA). They found that if the CSA was < 30°: 80 shoulders (93%) were osteo-arthritic. If the CSA was between 30° and 35° 78 of these shoulders (70%) had neither cuff tears or osteoarthritis, 18 (16%) had OA and 14 (14%) had cuff tears.  When the CSA was > 35°: 84 (84%) were in the RCT group and only four (4%) in the OA group. 

The authors of the current article conducted a matched case-control study using a shoulder arthroplasty registry from the Kaiser-Permanente healthcare system of patients who underwent primary elective anatomic total shoulder arthroplasty (TSA) for osteoarthritis.

The cases included 78 patients that underwent revision due to glenoid component failure or rotator cuff tear. Of the 78 cases, 25 were revised due to glenoid failure, 47 were revised due to rotator cuff tear, and 6 had both indications.

Two non-revised controls were matched to each case by age, gender, body mass index, American Society of Anesthesiologists classification, surgeon who performed the index TSA, and post-TSA follow-up time. 

Revised cases had a higher likelihood of a critical shoulder angle (CSA) ≥35° (odds ratio [OR]=2.41, 95% confidence interval [CI]=1.27-4.59). 

A higher likelihood of CSA ≥35° was observed for those revised for glenoid loosening (OR=4.58, 95% CI=1.20-17.50) and revised for rotator cuff tear (OR= 2.41, 95% CI=1.18-4.92) compared to non-revised controls. 

Every 5° increase in CSA had higher odds of overall revision (OR=1.62, 95% CI=1.18-2.21), glenoid loosening (OR=2.50, 95% CI=1.27-4.92), and rotator cuff tear (OR=1.51, 95% CI=1.07-2.14).

These authors suggest that a reverse arthroplasty might be considered in cases of primary shoulder arthritis with a CSA of 35° or greater.

Comment: This is an interesting study that brings up some interesting questions.

(1) The patients in the two groups were matched for age, gender, body mass index, and American Society of Anesthesiologists classification, surgeon and followup time, should they not have been matched for glenoid type - a factor that reflects the three-dimensional anatomy of the arthritic shoulder, that may be related to the CSA, and that has been associated with revision rate?

(2) The authors' database has enrolled 5274 TSA patients of which 78 underwent revision for glenoid component or cuff failure. That is a low revision rate of 1.4%. Is there evidence that a lower revision rate would have been achieved if a reverse total shoulder had been performed in these 78 shoulders (see the comparison of revision rates on this link)?

(3) The data in the table above show that the difference in the average CSA between the revised and non-revised shoulders was only 2 degrees; is this small difference sufficient to change the forces that might contribute to TSA failure?

(4) Over 75% (60/78) of the revised shoulders had a CSA < 35 degrees; how did the characteristics and failure modes for these shoulders compare with the 18 revised shoulders with CSA≥35?

(5) The CSA can be increased by superior glenoid inclination and by an increased length of the acromion; would it not be interesting to consider the independent effect of each of these measurements? 

Follow on twitter: https://twitter.com/shoulderarth

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How you can support research in shoulder surgery Click on this link.

Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link)

The smooth and move for irreparable cuff tears (see this link)

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

Shoulder rehabilitation exercises (see this link).

Critical shoulder angle and shoulder disease - chicken vs egg

Relationship Between the Critical  Shoulder Angle and Shoulder Disease

This review implies a causative effect of the  Critical Shoulder Angle (CSA) in both osteoarthritis and rotator cuff disease. 

"An increased CSA (>35°) is thought to alter deltoid vectors, which results in increased superior shear forces on the rotator cuff muscles. This increased loading of the rotator cuff may be a risk factor for the development of rotator cuff tears." This assumed causative relationship has even led some to consider arthroscopic lateral acromial resection as a means for optimizing long-term surgical outcomes after rotator cuff repair.

 "A decreased CSA (<30°) is associated with glenohumeral arthritis due to the increased compressive forces across the glenohumeral joint. "

This article often refers to statements about "prediction": a higher CSA predicts a cuff tear, while a lower CSA predicts osteoarthritis. These are actually statements of association rather than prediction or causation.  Evidence to prove prediction or causation would require data that young shoulders with higher CASs would be more prone to develop cuff tears in later years and that young shoulders with lower CASs would be more prone to develop OA in later years. To our knowledge such data are not available. 

It seems at least equally likely that the observation that the CSA is higher in patients with cuff tears is due to the fact that when the cuff is deficient the humeral head rides up, eroding the superior lip of the glenoid and causing laterally extending spurs on the acromion, both of which would increase the CSA. So the presence of a cuff tear may cause the change in the CSA rather than the other way around. See these examples:

It also seems likely that the observation that the CSA is lower in patients with osteoarthritis is due to the fact that OA results in medial erosion of the inferior lip of the glenoid, which would reduce the CSA. So the OA may cause the change in CSA rather than the other way around. See these examples:

Those readers interested in what it takes to prove causation are invited to visit the post "Causation and Statistics - a lesson from Sir Austin Bradford Hill"
 (link) and to look especially at #4.

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