Atlantoaxial rotatory fixation: Part 1--Biomechanics of normal rotation at the atlantoaxial joint in children.

Objective: Atlantoaxial rotatory fixation (AARF) remains a recondite entity loosely included under the panoply of cervical trauma. The difficulty in finding a precise definition and reliable diagnostic criteria for AARF has been chiefly because of a lack of normative biomechanical data for C1-C2 rotation. As Part 1 and foundation of a comprehensive undertaking to define the biomechanics, mechanism, diagnosis, classification, and management of AARF, the present study focuses on the dynamic behavior of C1 and C2 during normal voluntary head rotation in children. Methods: Twenty-one normal children 3 to 11.5 years old underwent computed tomographic examinations from the lower clivus to the base of C3 in various head positions during axial rotation. The angles made by C1, C2, and the occiput with the vertical 0 degrees were recorded, and from these, the separation angles between C1 and C2 (C1-C2 degrees) were calculated for each head position (represented by the C1 angle) studied. In 18 children, the range of rotation was between 90 and -90 degrees, i.e., with the head making a full 180-degree turn from one side to the other. In 3 children, the head was first turned from 0 to 90 degrees and then back from 90 to 0 degrees, making only a half turn. All separation angles (C1-C2 degrees) were then plotted against the corresponding C1 angle to create a motion curve, which, in essence, describes the interaction between C1 and C2 through the full range of head positions. In the 18 children with full turns, both individual motion curves and a composite motion curve comprising all data were constructed. Results: There is a high degree of concordance for rotational behavior of C1 and C2 in the 18 subjects undergoing full turn. C1 always crosses C2 at or near 0 degrees, the null point of full rotation. The predictable relationship between C1 and C2 is depicted by three distinct regions on the composite motion curve: when C1 rotates from 0 to 23 degrees, it moves alone, with C2 remaining stationary at approximately 0 degrees (the single-motion phase). When C1 rotates from 24 to 65 degrees, C1 and C2 move together, but C1 always moves at a faster rate (the double-motion phase), C2 being pulled by yoking ligaments. From 65 degrees onward, C1 and C2 move in exact unison (the unison-motion phase) with a fixed, maximum separation angle of approximately 43 degrees, head rotation being carried exclusively by the subaxial segments. In the 3 children with half turn, the forward rotation curve and the reverse rotation curve are almost superimposable, suggesting that the "yoking" between C1 and C2 is a result of more than just tensing and relaxing of ligaments but probably also to a mutual dragging by irregular bony surfaces between the two bones. Conclusion: C1 and C2 in children move in a predictable manner during axial head rotation, with a high degree of concordance among subjects and a relatively narrow variance from the mean. The composite motion curve can thus be used as a touchstone against which may be judged all manners of pathological interlock or "stickiness" between C1 and C2 in rotation that could be defined as AARF.