The shape of the acromion had been initially divided into three types (which was known as the Bigliani classification) ³, to which a fourth has been added ². They are used as a standardized way of describing the acromion, as well as predicting to a degree the incidence of impingement.

Classification

This classification was initially proposed by Bigliani et al. in 1986 on outlet view radiographs and later modified by Kitay et al. ⁷ and Vanarthos et al. in 1995, which remains the most widely accepted classification at the time of writing (2022).

Acromial morphology is variable on sagittal oblique MRI depending on the slice position. A slice position just lateral to the acromioclavicular joint (no joint capsule or acromioclavicular ligament visible) is considered the single best slice ⁸.

Acromion morphology (types) are based on sagittal oblique MRI:

flat inferiorly (12%) ⁶
curved (56%)
- parallel to the humeral head with a concave undersurface
- down-sloping in the middle-third of the acromion ⁸
- considered most common type ³
hooked (29%)
- most anterior portion of the acromion has a hooked shape
- down-sloping in the anterior-third of the acromion ⁸
- associated with increased incidence of shoulder impingement
convex (upturned) (3%)
- most recent classification of acromion process shape
- the undersurface of the acromion is convex near the distal end ⁴
- no convincing correlation between a type 4 acromion and impingement syndrome exists ^4,5