The shape of the acromion has been initially divided into 3 types (which was known as the Bigliani classification), to which a fourth has been added:
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flat inferiorly
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curved: parallel to the humeral head with convex undersurface - considered the commonest type
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hooked: the most anterior portion of the acromion has a hooked shape, associated with increase incidence of shoulder impingement
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convex (upturned)
- this is the most recent classification of acromion process
- in this type of acromion, the undersurface is convex near the distal end
- no convincing correlation between a type 4 acromion and impingement syndrome exists