Five lines have been described and used by most of the radiologists to evaluate basilar invagination on frontal and lateral skull plain radiographs as well as coronal and midsaggital reconstructed CT images.
This is based on the relative projection of odontoid process to these lines, which is as follows:
- digastric line: tip of odontoid process is normally located 11 to 21 mm below this line
- bimastoid line: tip of odontoid process projects normally not more than 10 mm above this line
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