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).

Rotator cuff retears: is the critical shoulder angle clinically significant?

Combination of risk factors affecting retear after arthroscopic rotator cuff repair: a decision tree analysis

These authors investigated the combination of factors associated with postoperative retear after repair of a rotator cuff tear in 286 patients having magnetic resonance (MR) imaging at 6 months after surgery.

254 of the repairs were intact and 32 were retorn.

 

The retear group had increased tear size as well as greater prevalence of hyperlipidemia, global fatty degeneration index, supraspinatus fatty degeneration and critical shoulder angle (CSA) equal to or greater than 37.

The healed group had an average CSA  33.5±3.54 while the retear group and an average of 35.7 ± 3.72. The difference was statistically significant (p .0015). However the difference in the means was only 2.2 degrees, a value much less than the standard deviations. So the question is, "is this difference clinically significant?" Can we say that a two degree difference increases the risk of retear? If an increased CSA increases the risk of retear, is it because of a difference in line AB or line BC, i.e. because the acromion is slightly more lateral or the superior glenoid is slightly more medial? Food for thought.

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

We have a new set of shoulder youtubes about the shoulder, check them out at this link.

Be sure to visit "Ream and Run - the state of the art" regarding this radically conservative approach to shoulder arthritis at this link and this link

Use the "Search" box to the right to find other topics of interest to you.

You may be interested in some of our most visited web pages  arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

The critical shoulder angle: what does it mean?

A biomechanical confirmation of the relationship between critical shoulder angle (CSA) and articular joint loading

These authors state that the angle between a line connecting the upper and lower lips of the bony glenoid and a line connecting the inferior glenoid lip and the lateral acromion (the so called, "critical shoulder angle" or CSA) is often altered in different conditions of the shoulder.

Shoulders with cuff disease tend to have greater angles while those with arthritis tend to have smaller angles. Such alterations could result from changes in the radiographic anatomy of any of the three points that determine this angle: the inferior glenoid lip, the superior glenoid lip or the lateral acromion.  In that these points can be altered by degenerative joint disease or cuff failure, it is easy to see how the CSA might be changed in these conditions. 

This article uses a sophisticated inverse dynamics 3-dimensional musculoskeletal model of the shoulder

to estimate the impact on glenohumeral biomechanics of modifying the deltoid attachment to the acromion.

The CSA was changed by altering the attachment point of the middle deltoid into (1) a normal CSA (33 degrees), (2) reducing the angle by 5 degrees and (3) increasing the angle by 5 degrees.

Subject-specific kinematics of slow and fast speed abduction in the scapular plane and slow and fast forward flexion measured by a 3-dimensional motion capture system were used to quantify joint reaction shear and compressive forces.

This model suggested that that a more lateral deltoid attachment resulted in increased superior-inferior forces (shearing forces; integrated over the range of motion and a more medial deltoid  the attachment resulted in increased lateromedial (compressive) forces for both the maximum and integrated sum of the forces over the whole motion. While some of these differences were statistically significant, the variability in the results was large compared to the effect sizes

While this model suggests that changing deltoid attachment to the acromion can alter glenohumeral joint biomechanics to a degree, it does not establish whether the CSA differences observed in different groups of patients are a result of their pathology or whether the differences in CSA contribute to the development of the pathology.   

Note in the two examples below how the shape of the glenoid determines the CSA in (a) osteoarthritis with medial wear of the inferior glenoid 

and (b) chronic cuff disease with medial wear of the superior glenoid.

The paper does not support the conclusion that "surgical restoration to a ‘‘normal’’ CSA is recommended when treating patients with such pathologies, for example, lateral acromioplasty after rotator cuff repair or ensuring control of glenoid inclination when conducting arthroplasty surgery."

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To see our new series of youtube videos on important shoulder surgeries and how they are done, click here.

Use the "Search" box to the right to find other topics of interest to you.

You may be interested in some of our most visited web pages  arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

"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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 To see our new series of youtube videos on important shoulder surgeries and how they are done, click here.

Use the "Search" box to the right to find other topics of interest to you.

How you can support progress in shoulder surgery

You may be interested in some of our most visited web pages  arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

Critical shoulder angle and cuff tears - are the differences clinically significant?

Critical shoulder angle in an East Asian population: correlation to the incidence of rotator cuff tear and glenohumeral osteoarthritis

These authors examined the correlation between the critical shoulder angle (CSA) measured on anteroposterior radiographs 

and the presence or absence of rotator cuff tears or OA changes was assessed in 295 patients. Rotator cuff tears were diagnosed with magnetic resonance imaging or ultrasonography.

The mean CSA with rotator cuff tear (33.9° ± 4.1°) was significantly greater than that without a rotator cuff tear (32.3° ± 4.5°; P = .002). Multivariable analysis also showed that a greater CSA had a significantly increased risk of rotator cuff tears, with the odds ratio of 1.08 per degree. 

In contrast to prior studies, osteoarthritis showed no significant correlation to the CSA.

Comment: These authors suggest that the CSA "may be an independent risk factor for the incidence of rotator cuff tears in the Japanese population. "

While the difference in CSA between patients with and without cuff tears appears statistically significant, one must wonder whether a difference of 1.6° can contribute to the risk of rotator cuff tear. Might instead the difference in CSA be a result rather than the cause of cuff tears (see this link)?

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We have a new set of shoulder youtubes about the shoulder, check them out at this link.

Be sure to visit "Ream and Run - the state of the art" regarding this radically conservative approach to shoulder arthritis at this link and this link

Use the "Search" box to the right to find other topics of interest to you.

How you can support progress in shoulder surgery

You may be interested in some of our most visited web pages arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

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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We have a new set of shoulder youtubes about the shoulder, check them out at this link.

Use the "Search" box to the right to find other topics of interest to you.

How you can support progress in shoulder surgery

You may be interested in some of our most visited web pages   arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

Rotator cuff tears, the critical shoulder angle and a bunch of other angles - do they matter?

Shoulder surgeons have been busy measuring angles on images of patients with and without rotator cuff disease. Here are eight examples where the authors have found higher rates of cuff disease when the measured angle is above or below a certain value:

Critical shoulder angle >35 degrees (see this link)

Greater tuberosity angle >70 degrees (see this link)

Acromial angle > 25 degrees (see this link)

Lateral acromial angle < 70 degrees (see this link)

Superior glenoid inclination angle > 9 degrees (see this link)

Acromial arch angle > 120 degrees (see this link)

Acromiohumeral centre edge angle >20 (see this link)

Decreased coracoid inclination angle (see this link)

A few questions arise about these measurements (see this link):

(1) What is their clinical utility, i.e. in this era of high quality MRI and ultrasound do these measurements affect clinical decision making?
(2) Do differences in these angles between shoulders with and without cuff disease suggest that the morphology reflected by the angle caused the cuff problem or is it likely that the cuff problem caused the morphology reflected by the angle?
(3) While it can be accomplished (see this link) is there evidence that surgically modifying these angles will change either the likelihood of subsequent cuff disease or the outcome of cuff repair surgery?
(4) Do these measurements matter? Contrast "Large Critical Shoulder Angle Has Higher Risk of Tendon Retear After Arthroscopic Rotator Cuff Repair" with "Critical Shoulder Angle and Acromial Index Do Not Influence 24-Month Functional Outcome After Arthroscopic Rotator Cuff Repair"

In our practice, we do not find that these measurements are of value in the treatment of our patients. The facts remain that
(1) Older patients with atraumatic cuff tears are less likely to benefit from rotator cuff repair surgery than their younger counterparts with traumatic cuff tears.
(2) Shoulders with inadequate cuff tendon quantity and quality have less chance of being durably reparable.
(3) The literature does not provide evidence that modifying the acromion is an important aspect of the treatment of cuff disease (see this link).
(4) Irreparable cuff tears in shoulders with retained active elevation can be well treated with a smooth and move procedure (see this link).
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Use the "Search" box to the right to find other topics of interest to you.

How you can support progress in shoulder surgery

You may be interested in some of our most visited web pages   arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'