Atlantoaxial rotatory fixation: part 2--new diagnostic paradigm and a new classification based on motion analysis using computed tomographic imaging.

OBJECTIVE: This is Part II of a study on atlantoaxial rotatory fixation (AARF) that aims to introduce a new diagnostic paradigm and a new classification of this condition based on motion analysis of C 1 C 2 rotation using computed tomographic (CT) imaging. This phase of the study is possible because Part I succeeded in defining physiological C 1 C 2 axial rotation with CT data from 21 normal children, displayed in a highly concordant composite motion curve, which is used as the normal template for the present study. AARF is defined as flagrant departure from normal motion dynamics as delineated by abnormal motion curves. The new classification is predicated on the graded amount of pathological stickiness in the restricted rotation. METHODS: Forty children age 1.5 to 14 years with painful cock-robin necks resulting from minor trauma or otolaryngological procedures were subjected to 3 CT examinations: 1) in the presenting (P) position; 2) with the nose pointing up (P 0 position); and 3) with the head forcefully turned to the opposite side as much as the patient could tolerate (P _ position). The angles made by C 1 and C 2 and the separation angle C 1 C 2 ° (C 1 minus C, degrees) were obtained as described in Part I. The test motion curve was generated by plotting C 1 against C 1 C 2 angles, and all motion curves were analyzed in the context of the normal template. RESULTS: Five distinct groups with highly characteristic motion curves could be identified. Group 1 (n = 5) patients showed essentially unaltered (locked) C 1 C 2 coupled configurations regardless of corrective counterrotation, with curves that are horizontal lines in the upper two quadrants of the template. Group 2 (n = 7) patients had reduction of the C 1 C 2 separation angle with forced correction, but C 1 could not be made to cross C 2 . Their curves slope downward from right to left in the upper quadrants but never traverse the x axis. Group 3 (n = 9) patients showed C 1 C 2 crossover, but only when the head was cranked far to the opposite side. Their motion curves traverse the x axis left of C, = -20 degrees. Groups 1, 2, and 3 motion dynamics are respectively classified as Types I, II, and III AARF in descending degree of pathological stickiness, which is in essence a resistance against closure of the C 1 C 2 angle to counterrotation. Group 4 (n = 14) patients had normal dynamics, and Group 5 (n = 5) patients showed motion curve features between normal and Type III AARF, designated as belonging to the diagnostic gray zone, an uncertain group that may or may not revert to normal dynamics with only comfort measures. CONCLUSION: AARF can be reliably diagnosed with a simple and practical CT protocol and construction of a three-point motion curve superimposed on a reusable normal template. The type of AARF, reflective of the severity of pathological stickiness of rotation, can be identified readily by the shape of the motion curve. This system of classification is useful in selecting the best regimen of management.