Type II Acromion (Os Acromiale Variant): Anatomy, Prevalence, and Clinical Significance

The classification described corresponds to a variant of os acromiale (unfused acromial apophyses) with a specific bony bridge to the coracoid, rather than the classic Bigliani acromion types (I flat, II curved, III hooked). This entity is rare and distinct.

Anatomy of This Variant

• Osseous bridge between the acromion (typically the meta-acromion) and the coracoid process.
• Broad base at acromion, narrowing distally where supraspinatus tendon passes.
• Coracoacromial ligament ossified into this bony arch.
• Creates a fixed, narrow tunnel for the supraspinatus tendon.

Prevalence & Associations

• Prevalence: 1–6% (of os acromiale cases, which are ~8% of population → overall ~0.1–0.5%).
• AC joint degenerative disease: 85% (mechanical stress across pseudoarthrosis).
• Rotator cuff pathology: 25–50% (impingement in the bony tunnel).
• May be asymptomatic or cause subacromial impingement symptoms.

Imaging Findings

• Axial (transaxial) CT/MRI: Bony bridge with broad base spanning acromion-coracoid interval; usually no surrounding degenerative changes.
• Coronal: "Hooked beak" appearance from the osseous bridge + superior acromial curvature.
• Differential: Fibrous coracoacromial ligament (Type I acromion mimic); true os acromiale (unfused apophyses without coracoid bridge).

Clinical Implications

• The distal tip of the bony bridge may be the sole passage for the supraspinatus tendon → high impingement risk.
• If symptomatic (pain, weakness, night pain) and failed conservative care (PT, NSAIDs, injection):
• Surgical options: Resection of the bony bridge (coracoacromial decompression) ± acromioplasty ± AC joint resection (if degenerative).

Distinction from Bigliani Classification

• Bigliani types describe acromial shape (flat, curved, hooked) influencing impingement risk.
• This variant is a developmental anomaly (os acromiale + coracoid fusion) with distinct mechanics.

Accurate identification on imaging guides appropriate surgical planning for refractory impingement.