Influence of the site of acromioplasty on reduction of the critical shoulder angle (CSA) – an anatomical study
A large critical shoulder angle (CSA) >35° is associated with the development of rotator cuff tearing. Lateral acromioplasty (AP) has the theoretical potential to prevent rotator cuff tearing and/ or to reduce the risk of re-tears after repair. It is, however unclear which part of the lateral acromion has to be reduced to obtain the desired CSA. It was the purpose of this study to determine which part of the lateral acromion has to be resected to achieve a desired reduction of the CSA in a given individual.
First, the influence of the exact radiographic projection on the CSA was examined. Second, the influence of anterolateral versus strict lateral AP on the CSA was studied in eight scapulae with different anatomic characteristics. Differences in CSA reduction were investigated using paired t-test or Wilcoxon test.
Scapular rotation in the sagittal and axial plane had a marked influence on the radiologically measured CSA ranging from -6 to +16°. Overall, lateral AP of 5/10mm reduced the CSA significantly greater than anterolateral AP of 5mm/10mm [5mm: 2.3° (range: 0.7°-3.6°) SD±0.8° vs. 1.2° (range: 0°-3.3°) SD±1.1°, p=0.0002]/[10mm: 4.8° (range: 2.1°-7°) SD±1.3° vs. 2.7° (range: 0°-5.3°) SD±1.7°, p=0.0001]. Depending on scapular anatomy anterolateral AP did not alter CSA at all.
For comparison of pre- and postoperative CSA, the exact orientation of the X-ray and the spatial orientation of the scapula must be as identical as possible. Anterolateral AP may not sufficiently correct CSA in scapulae with great acromial slopes and smaller relative external rotation of the acromion as the critical acromial point (CAP) may be located too posteriorly and thus is not addressed by anterolateral acromioplasty. Consistent reduction of the CSA could be achieved by lateral AP in all eight scapulae.
KeywordsRotator cuff tear Acromioplasty Critical shoulder angle Rotator cuff retear Digitally reconstructed radiograph Computed tomography
Critical acromial point
Rotator cuff tears
The morphology of the scapula shows great differences between individuals. Its variable radiographic appearance has led Bigliani et al. to distinguish three different forms of the acromion as early as 1986 . Later reports focused on different acromial spurs, acromial slope (AS), acromial tilt (AT), lateral acromial angle (LAA), acromion index (AI) and critical shoulder angle (CSA) [2, 3, 4, 5, 6, 7]. The interest of these studies is to understand how the scapular anatomy is related to clinical shoulder pathologies and how it might be altered in order to possibly reduce the incidence of degenerative rotator cuff tears or their recurrence after repair . Other reports found a highly relevant impact of a presumably “faulty” body posture on rotator cuff tears, as differences in posture alter the position of the scapula in space .
While Moor et al. noted little variation of the radiographic appearance and the CSA in different scapular rotation , a newer study has shown a greater susceptibility of the CSA to malposition especially in ante- and retroversion .
At our institution all standard AP shoulder radiographs are obtained with the x-ray beam angled 15 degrees caudally in the sagittal plane and vertically in the axial plane, independent of the patient specific scapular morphology especially regarding the acromion, the position in space or the patient’s posture. Using the above-mentioned protocol, it has been shown that a CSA of <28 is highly predictive (odds-ratio >10) for the development of osteoarthritis and a CSA > 35° is highly predictive of rotator cuff tears (RCT) . Consequently, we assume that normalization of a very high (>35°) CSA may be helpful in preventing rotator cuff re-tears and we therefore seek to achieve this goal with acromioplasty in these patients.
Step 1- Variation of the CSA in three anatomically different scapulae
In a first step, we assessed which point forms the most inferolateral part of the acromion (CAP) on the radiograph. We selected three patients (p1-p3) with distinctly different scapular anatomy regarding acromial slope and relative external rotation of the acromion.
Step 2- Effect of lateral vs. anterolateral acromioplasty
To increase the validity of the study five additional scapulae (p4-p8) were included. Segmentation of the scapulae from MRI Dicom data, simulation of acromioplasty and CSA measurement were performed identically as described above.
Written informed consent was obtained from all eight patients and Ethic Committee Approval was obtained (KEK Nr.: ZH2016-000826).
Differences in reduction of CSA between anterolateral and lateral acromioplasty in different flexion angles were investigated using paired t-test (or Wilcoxon test, where applicable). P-values <0.05 were considered statistically significant. Results are reported with mean, standard deviation and associated p-values if not stated otherwise.
Step 1- Variation of the CSA in three anatomically different scapulae
Extreme scapular positions, especially ±30° and ±40° flexion/extension, were deliberately included, fully aware that these are unacceptable for clinical use. They were performed to help understand how extreme positions may become relevant for the CSA. In Diagram 1 these are highlighted gray while clinically more likely variations are highlighted white. In clinically likely variations, the CSA varied from 33° to 54° in the “p1” scapula, from 34° to 52° in the “p2” scapula and from 35° to 45° in the “p3” scapula. Internal rotation consistently increases the CSA, while external rotation decreases the CSA in clinically likely variations.
Step 2- Effect of anterolateral vs. lateral acromioplasty
Anatomical characteristics regarding preoperative CSA, posterior acromial slope and relative external rotation of the 8 examined scapulae (p1-8)
Preoperative CSA [°]
Posterior acromial slope [°]
Relative external rotation [°]
Overall reduction of the CSA was significantly greater by lateral than by anterolateral acromioplasty of 5mm [2.3° (range: 0.7°-3.6°) SD±0.8° vs. 1.2° (range: 0°-3.3°) SD±1.1°, p=0.0002] and significantly greater by lateral than by anterolateral acomioplasty of 10mm [4.8° (range: 2.1°-7°) SD±1.3° vs. 2.7° (range: 0°-5.3°) SD±1.7°, p=0.0001].
In neutral position reduction of the CSA did not significantly differ between lateral and anterolateral acromioplasty of 5mm [2.0° (range: 0.7°-2.9°) SD±0.9° vs. 1.1° (range: 0-2.5°) SD±1.1°; p=0.15] and between lateral and anterolateral acromioplasty of 10mm [4.4° (range: 2.1°-6.2°) SD±1.5° vs. 2.6° (range: 0-4.5°) SD±1.8°; p=0.06].
In 10° flexion reduction of the CSA was significantly greater by lateral than by anterolateral acromioplasty of 5mm [2.5° (range: 1.2°-3.3°) SD± 0.7° vs. 1.6° (range: 0°-3.3°) SD± 1.1°; p=0.02] and significantly greater by lateral than by anterolateral acromioplasty of 10mm [5.3° (range: 3.5°-7°) SD± 1.1° vs. 3.3° (range: 0-5.3°) SD± 1.5°; p=0.008].
In 10° extension reduction of the CSA was significantly greater by lateral than by anterolateral acromioplasty of 5mm [2.3° (range: 1°-3.6°) SD± 0.8° vs. 1° (range: 0-2.4°) SD± 1.1°; p=0.007] and significantly greater by lateral than by anterolateral acromioplasty of 10mm [4.7° (range: 2.1°-6.3°) SD± 1.2° vs. 2.1° (range: 0-4.6°) SD± 1.7°; p=0.006].
The average correction achieved by lateral acromioplasty of 5mm (10mm) was 2.3° (4.7°) in 10° extension, 1.96° (4.44°) in neutral and 2.4° (5.3°) in 10° flexion.
The average correction achieved by anterolateral acromioplasty of 5mm (10mm) increased with increasing flexion of the scapula from 1.2° (2.3°) in 10° extension to 1.3° (2.95°) in neutral and 1.7° (3.70°) in 10° flexion.
Multiple reports [3, 6, 7, 14, 15, 16] leave currently little doubt that the radiologically visible lateral extension of the acromion is a relevant predictor for either development of osteoarthritis (“small” acromion) or RCT (“large” acromion). The acromial extension may be measured either by the critical shoulder angle (CSA) or with the acromion index (AI) . Altering these values by acromioplasty during rotator cuff repair may contribute to a lower rate of re-tears, as recently reported by Garcia et al. and Hong et al. [17, 18] even though this has not been widely verified in long-term follow up yet. For rotator cuff repair, the goal at our institution has been arbitrarily set to reduce the CSA to less than 35° in a postoperative follow-up x-ray, as a CSA of greater than 35° has been associated with a higher risk of rotator cuff disease [6, 10].
In our surgical practice however, we made the surprising observation that similar surgical corrections show variable corrections of the CSA. We observed that in one patient (scapula p1) with a preoperative CSA of 39° a purely anterolateral acromioplasty led to a correction of the CSA by 4° to 35° (Fig. 1), while in other patients subjectively similar acromioplasties barely altered the CSA. It was therefore the purpose of this study to understand how lateral and anterolateral acromioplasties can have a profoundly different effect on the postoperatively measured CSA by understanding how the CAP behaves in scapulae with different anatomies and in different spatial position.
The analyses of our experimental measurements confirmed our clinical observation in two regards:
Direction of the x-ray beam
Corresponding to the recently published study on the dependency of the CSA of the radiographic viewing perspective of Suter et al.  the CSA of the scapulae p1-3 showed a certain positional susceptibility of this clinically valuable parameter even at low angular variations (Diagram 1).
In our second experimental array we did not alter scapular rotation as malrotated x- rays are easily identified at the glenoid and rejected in clinical practice.
Effect of anterolateral vs lateral acromioplasty
Overall lateral acromioplasty of 5mm and 10mm reduced the CSA significantly greater than anterolateral acromioplasty of 5mm and 10mm, respectively. We believe this is mainly due to the fact, that the CSA is reduced consistently by lateral acromioplasty in all of the included scapulae independent of acromial anatomy. The reduction of the CSA by lateral acromioplasty depends on the amount of bone which is resected (Diagrams 2, 3, 4, 5).
On the other hand anterolateral acromioplasties have a variable effect on the postoperative CSA ranging from no effect at all in scapula p2 (average reduction by lateral AP= 4.1° vs average reduction by anterolateral AP = 0°) to a comparable effect in scapula p1 (average reduction by lateral AP 4.6° vs average reduction by anterolateral AP 3.2°).
We noticed that the CAP seemed to move anteriorly in scapulae with greater external rotation and posteriorly with greater acromial slope. As the CSA is only reduced if the CAP is included in the osseous resection; a more anterior located CAP is more likely to be included by an anterolateral acromioplasty. We concluded that in acromia with greater external rotation and smaller acromial slope, an anterolateral acromioplasty is more likely to reduce CSA.
Further supporting this conclusion is the tendency of greater CSA reduction in increasing flexion of the scapula by anterolateral acromioplasty. Increasing flexion of the scapula relatively reduces the posterior slope, moving the CAP more anteriorly and as mentioned above making it more likely to be included in an anterolateral acromioplasty. Vice versa, increasing scapular extension moves the CAP more posteriorly potentially reducing the effect of anterolateral acromioplasty, which can be seen well in Diagram 4.
There are several limitations to this experimental study. Segmentation was performed by hand using a standardized MRI by the first author. Only eight different scapulae were used to test our hypothesis and simulate acromioplasty. A larger number of test scapulae may be useful to confirm the above mentioned findings, however will most likely not change the intuitively well understandable principal findings we have made. The greatest limitation appears, that we do not know what the real biomechanical effect is that we generate with each acromioplasty procedure.
In summary, a large lateral acromial extension with large CSA and AI may be the result of the projection of a possible anterolateral spur in the coracoacromial ligament especially in scapulae with a smaller posterior acromial slope and a larger relative external acromial rotation (Fig. 1). In such patients, the CAP is located anterior and pure anterolateral acromioplasty may relevantly alter and “correct” the radiologically visible CSA. However, with greater posterior acromial slope and less relative external acromial rotation, the CAP moves further posterior and may exit the area affected by anterolateral acromioplasty. Therefore in those patients, only a lateral acromioplasty will lead to the currently desired reduction of the CSA.
For comparison of pre- and postoperative CSA, the exact orientation of the X-ray and the spatial orientation of the scapula must be as identical as possible. Anterolateral AP may not sufficiently correct CSA in scapulae with great acromial slopes and smaller relative external rotation of the acromion as the critical acromial point (CAP) may be located too posteriorly and thus is not addressed by anterolateral acromioplasty. Consistent reduction of the CSA was achieved by lateral AP in all eight scapulae.
Availability of data and materials
The datasets used and analysed during the current study are available from the corresponding author on reasonable request.
DK and DM designed and carried out the study. EB participated in its design and the computer analysis. CG and DM contributed substantially to the manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
Approval Number of Ethics Committee:
Kantonale Ethikkomission Kanton Zürich
KEK Nr.: ZH2016-000826
Consent for publication
The authors declare that they have no competing interests.
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- 1.Bigliani LU, Morrison DS, April EW. The morphology of the acromion and its relationship to rotator cuff tears. Orthop Trans. 1986;10:228.Google Scholar
- 10.Moor BK, Bouaicha S, Rothenfluh DA, Sukthankar A, Gerber C. Is there an association between the individual anatomy of the scapula and the development of rotator cuff tears or osteoarthritis of the glenohumeral joint? A radiological study of the critical shoulder angle. Bone Joint J. 2013;95-B:935–41.CrossRefGoogle Scholar
- 16.Beeler S, Hasler A, Götschi T, Meyer DC, Gerber C. The Critical Shoulder Angle: Acromial Coverage is More Relevant than Glenoid Inclination. J Orthop Res. 2018. https://doi.org/10.1002/jor.24053.
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