This imaging sign is named after the Tam o' Shanter, a Scottish hat, named after the character in Robert Burns's ¹ poem of the same name. The appearances of advanced Paget disease of the skull are similar in appearance to the hat. 

Paget involvement of the skull, with widening of the diploic space and an overall enlargement of the cranium, combined with platybasia, leads to the appearance of the skull falling over the facial bones, such as a Tam o' Shanter hat would fall over the head.

It is important to note that strictly the sign should not be used for Pagetoid involvement of the skull only but more accurately for the additional presence of platybasia, which has been said to be pathognomonic for Paget disease ².

History and etymology

The way in which "Tam o' Shanter" is represented in text shows a wide range of forms in the medical (and lay) literature. The form used in this article was that used by Robert Burns in the poem of the titular character of the same name and is also that in Chambers Dictionary, one of the authoritative references on the English language ^3. As it is a proper noun, it should be capitalised, although as per Burns, the 'o' remains in lower case. Some articles insert a space between the 'o' and 'Shanter', others do not. Moreover, some sources, including at least one established dictionary, insert multiple hyphens, resulting in the somewhat quirky Tam-o' -Shanter or Tam-o'-Shanter sign ^2,4.

See also

Other Paget disease-related signs:

• banana fracture
• blade of grass sign
• cotton wool appearance of bone
• jigsaw pattern bone or mosaic pattern bone
• picture frame vertebral body