Type IV acromion according to Bigliani et al. classification: which statement is true?

• A. It is a defined type in Bigliani classification
• B. It does not exist in the Bigliani system; some references describe it as convex
• C. It is identical to Type III
• D. It is always associated with rotator cuff tears

Answer: (B)

Bigliani’s acromion classification includes three shapes seen on imaging: flat, curved, and hooked. There is no Type IV in this system. Some references describe a convex variant and label it as Type IV, but that labeling is not part of the original Bigliani scheme. So the true statement is that Type IV does not exist in the Bigliani system; if you see a mention of convex as Type IV, it’s an nonstandard extension rather than an official category. The other options don’t fit because the classification itself is limited to three types, it’s not identical to Type III, and there isn’t an absolute rule that a particular shape always accompanies rotator cuff tears.