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  • Title: New Subtype of Atlantoaxial Rotatory Fixation (Type IIIa).
    Author: Guo Q, Zhang M, Wang H, Chen F, Lu X, Ni B.
    Journal: World Neurosurg; 2023 Feb; 170():e622-e628. PubMed ID: 36410702.
    Abstract:
    OBJECTIVE: Some atlantoaxial rotatory fixations (AARFs) cannot be classified according to the Fielding and Hawkins classification. This study aimed to introduce a new subtype of AARF (type IIIa AARF) with a C1 anterior displacement >5 mm, but with one lateral mass being displaced anteriorly and another posteriorly. METHODS: Data from 10 cases of AARF with anterior C1 displacement of >5 mm were retrospectively reviewed. The exclusion criteria were as follows: 1) type I, II, or IV AARF according to the Fielding and Hawkins classification; 2) cases caused by trauma, tumor, or infection; 3) AARF with os odontoideum or odontoid fracture; and 4）age ≥18 years. Imaging features were analyzed. The atlanto-dental interval was measured to evaluate C1 anterior displacement. RESULTS: Three cases that did not match type III AARF were classified under type IIIa AARF. They had the following common imaging features: 1) atlanto-dental interval of >5 mm, being similar to type III AARF; 2) one lateral mass of C1 displaced anteriorly and the other posteriorly (the most important feature distinguishing the type from type III AARF in which both C1 lateral masses displaced anteriorly); and 3) C1-C2 separation angle (mean 44.2 ± 2.9°) being larger than that in type III AARF. CONCLUSIONS: AARF with anterior C1 displacement of >5 mm, but with one lateral mass displaced anteriorly and the other posteriorly, was defined as type IIIa AARF. It should not be confused with type III AARF because these 2 types differ in biomechanics and imaging parameters.
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