The scapuloacromial angle: a 3D analysis of acromial slope and its relationship with shoulder impingement

To quantify acromial slope (the scapuloacromial [SA] angle).

We used shoulder MR studies in 64 subjects who were stratified surgically into impingement (with and without tear) and nonimpingement (shoulder instability) groups. We measure, in three dimensions, the angle between unit normal vectors to planes of best fit to the undersurface of the acromion and the medial surface of the blade of the scapula. We examine the projection of this SA angle onto the transverse, coronal, and sagittal planes to gauge some measure of rotation, lateral tilt, and anterior tilt with respect to the patient. Orientation of the acromion and scapular blade in relation to the MR scanner and therefore to the patient was also calculated.

We found no significant differences between these two groups when examining age and side of pathology. A statistically significant difference was seen between males and females.

Our study does not support the idea that a flatter acromial slope causes impingement and is an etiological basis for rotator cuff tears. We believe that acromial slope, regardless of imaging plane, is not useful in classifying patients who present with shoulder pain and should not be considered as a source of pathologic changes.