Type I = Flat inferiorly (12%)

Type II = Curved (56%)

• parallel to the humeral head with concave undersurface 


• considered most common type


Type III = Hooked (29%)

• most anterior portion of the acromion has a hooked shape


• associated with increased incidence of shoulder impingement


Type IV = Convex (upturned) (3%)

• most recent classification of acromion process shape


• the undersurface of the acromion is convex near the distal end


• no convincing correlation between a type IV acromion and impingement syndrome exists

Overview of parameters of acromial morphology.

A.   Acromial slope (d) according to Bigliani et al. (1986) and Kitay et al. (1995).

B.   Acromial tilt (b) according to Kitay et al. (1995) and Aoki et al. (1986).

C.   Lateral acromial angle (a) according to Banas et al. (1995).

D.   Acromion index (AI) according to Nyffeler et al. (2006).

Results

The acromial type according to Bigliani was not associated with any particular cuff lesion.

A statistically significant difference between controls and impingement patients was found for AS.

AT of controls was significantly smaller than that of impingement patients and cuff-tear patients.

LAA of cuff-tear patients differed significantly from that of controls and impingement patients, but LAA of controls was not significantly different from that of impingement patients. Differences between impingement patients and cuff-tear patients were also significant.

AI of controls was significantly lower than of impingement patients and of cuff-tear patients. A good correlation was found between acromial type and AS.

Interpretation

A low lateral acromial angle and a large lateral extension of the acromion were associated with a higher prevalence of impingement and rotator cuff tears. An extremely hooked anterior acromion with a slope of more than 43° and an LAA of less than 70° only occurred in patients with rotator cuff tears.