What causes Tam O’ Shanter sign in the skull on plain radiography?
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Answer:
Tam O’ Shanter sign represents advanced Paget disease of bone involving the skull, characterized by thickening of the skull vault with widening of the diploic space, cranial enlargement, and platybasia, resulting in the frontal calvarium appearing to fall over the facial bones.
Why is it called so?:
Named after the Tam O’ Shanter, a traditional Scottish hat that is broad, flat, and tends to fall over the head, resembling the appearance of the thickened, enlarged calvarium overhanging the facial bones in advanced skull Paget disease.
Pathophysiology:
Paget disease of bone causes excessive, disorganized bone resorption followed by hyperactive new bone formation, leading to calvarial thickening predominantly of the inner table, diploic space expansion, overall cranial enlargement, and platybasia from basilar invagination, creating the characteristic overhanging frontal skull appearance.
Alternative names: Tam o’ Shanter sign, Tam-o’-Shanter sign
Other associated named signs: banana fracture, blade of grass sign, cotton wool appearance, jigsaw pattern bone, mosaic pattern bone, picture frame vertebral body
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