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.
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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