A New Predictive Parameter for Rotator Cuff Tears: Acromial Incidence Angle

Abstract

Background: A significant statistical association of increased critical shoulder angle (CSA) with rotator cuff tear (RCT) has been demonstrated; however, the mean difference between RCT and control groups varies between approximately 1° and 3.5°, with a wide range of distribution of the CSA from 18° to 55°. It may be more predictive to evaluate the CSA in conjunction with parameters that evaluate the morphology of the acromion in the sagittal plane.

Hypothesis: It was hypothesized that the acromial incidence angle (AIA), which can evaluate the position and orientation of the acromion in the sagittal plane, may be associated with RCT and that AIA, when evaluated together with the CSA, may provide a highly predictive measure of risk for RCT.

Study design: Cross-sectional study; Level of evidence, 3.

Methods: The study included 117 patients who underwent arthroscopic repair for posterosuperior RCT and 117 patients as a control group. The CSA, AIA, glenoid inclination, glenoid version, and anterior acromial coverage were measured on magnetic resonance imaging. The groups were compared in terms of these parameters.

Results: The mean CSA was significantly higher in the RCT group (35.7°± 5.3°) than in the control group (33.9°± 4.5°), with a sensitivity of 57.1% and a specificity of 61.3% (P = .009). The mean AIA was also significantly higher in the RCT group (77.4°± 12.9°) compared with the control group (63.7°± 9.4°), with a sensitivity of 80.2% and a specificity of 83.9% (P < .001). The anterior acromial coverage showed significantly less anterior coverage in the RCT group (-16.5°± 14°) than in the control group (-9.7°± 10.5°) (P = .033). Logistic regression analysis showed that the CSA and the AIA were associated with RCT independently of other parameters (P < .001, P < 0.001, odds ratio [OR], 1.32, and OR, 1.34, respectively). In the patient group with a CSA of <35°, AIA values of >72° predicted RCT with a sensitivity of 85.6% and a specificity of 84.8%, while in the patient group with a CSA of ≥35°, values of >65° predicted RCT, with a sensitivity of 70.2% and a specificity of 76.7%.

Conclusion: The AIA predicted RCT with a sensitivity of 80.2% and a specificity of 83.9% for values >70°. In patients with a CSA of <35°, it predicted RCT, with a sensitivity of 85.6% and a specificity of 84.8% at values >72°. By evaluating the relationship between the acromion and the glenoid in the sagittal plane, the AIA, as a novel parameter, allows for the reevaluation of the risk in the patient group with a CSA of <35°, which is considered to be in the low-risk category in terms of RCT.

Keywords: acromial incidence angle; acromial morphology; anterior acromial coverage; critical shoulder angle; rotator cuff tear.

Figure 1.

Demonstration of the CSA measurement. (A) Detection of the most lateral point of the acromion on the T1 sequence coronal MRI and fixation on the screen (red asterisk). (B) Advancement of the coronal slice to a slice showing the superior and inferior bony margins of the glenoid, followed by measurement of the angle between the line joining the superior and inferior margins of the glenoid and the line between the projection of the lateral acromion (red asterisk) and the inferior glenoid. (C) T2 coronal MRI image of a 56-year-old female patient with CSA measurement and full-thickness RCT. CSA, critical shoulder angle; MRI, magnetic resonance imaging.

Figure 2.

Demonstration of GV and GI measurements. (A) On the axial section, after determining the cross section through the most medial point of the scapula and the center of the glenoid, the GV is obtained by measuring the angle between the line through the glenoid articular face and the tangent of the line from the most medial point of the scapula to the midpoint of the glenoid articular face. (B) The GI is obtained by measuring the angle between the tangent of the line between the superior and inferior glenoid and the tangent of the line along the base of the supraspinatus fossa on the coronal MRI section. GI, glenoid inclination; GV, glenoid version.

Figure 3.

Demonstration of AIA measurement. (A) The glenoid center and the reference line between the glenoid center and the inferior corner of the scapula are identified in the sagittal section and fixed on the screen. (B) The sagittal section is advanced to the section where the acromion is seen widest. The AAC is obtained by measuring the angle between the line drawn from the glenoid center to the anterior aspect of the acromion and the reference line. The AIA is obtained by measuring the angle between the line drawn from the glenoid center to the anterior of the acromion and the line extending from the anterior to the posterior of the acromion. AAC, anterior acromion coverage; AIA, acromial incidence angle.

Figure 4.

ROC curves of the CSA and the AIA for RCT. AIA, acromial incidence angle; CSA, critical shoulder angle; ROC, receiver operating characteristic.

Figure 5.

(A) ROC curve for RCT in patients with a CSA of <35°. (B) ROC curve for RCT in patients with a CSA of ≥35°. CSA, critical shoulder angle; RCT, rotator cuff tear; ROC, receiver operating characteristic.

Figure 6.

Demonstration of the relationship between the acromion and the circle whose center is the same as the center of the glenoid in the sagittal plane.