Kinematic evaluation of atlantoaxial joint instability: An in vivo cineradiographic investigation

Although range of motion has been considered the best parameter to quantify atlantoaxial instability, no other kinematic parameters have been determin ed for dynamic quantification. The objectives of this study were to investi gate the kinematics of the normal and pathologic atlantoaxial joints by cin eradiography and to determine the in-vivo kinematic parameters, if any, for the quantification of atlantoaxial instability. Sagittal plane motion of t he atlantoaxial joints was analyzed by cineradiography in 12 healthy volunt eers and 15 patients with atlantoaxial subluxation. In both flexion and ext ension, C1-C2 sagittal rotation and C1 translation in the sagittal plane we re measured continuously to determine the time-displacement curves for both parameters. All patients with atlantoaxial subluxation and seven of the vo lunteers had the sigmoid pattern in their time-displacement curves in sagit tal rotation. In these cases, atlantoaxial motion showed different points o f the onset of rapid increase in motion in their sigmoid curves between fle xion and extension. The discrepancy between these points was more significa nt in the patients than in the volunteers. In most of the patients who had atlantoaxial instability, subluxation occurred when the atlantoaxial joints were still in a more extended position and they were reduced when they wer e still in more flexed position. The discrepancy showed characteristics sim ilar to those of the neutral zone observed during in vitro investigations, suggesting that it becomes a good indicator of in vivo atlantoaxial instabi lity.