Three-dimensional CT study on normal anatomical features of atlanto-axial joints.

It has long been a research hotspot to diagnose atlanto-axial disorder by observing the shape and motions of atlanto-axial joints. The basis for correlative studies is to ascertain the normal anatomic features of atlanto-axial joints. In our study, 33 normal subjects were examined for atlanto-axial joints, by three-dimensional CT (3D-CT) in functional positions (neutral, left and right rotary position). The contraposition between superior and inferior articular facets of lateral atlanto-axial joints (AFLAJ), including its shape and type, the width of rotational facets displacement (RFD), head’s rotational angle (HRA) and rotational angle at C1–2 (RAC1–2) were observed and measured on 3D-CT images. In neutral position, a complete contraposition of AFLAJ was found in 25 subjects as well as a basic contraposition in 8. In rotary position, the width of RFD was between 6.16 and 8.68 mm, the angle of HRA was between 30.2° and 45.8°, and RAC1–2 between 26.7° and 38.9°. There is no significant difference in RFD, HRA or RAC1–2 (P > 0.05) in between levorotatory and dextrogyrate orientation, and there is a positive correlation between RFD and RAC1–2 (r = 0.5078, P < 0.05). Our study results show that the contraposition of AFLAJ can be clearly displayed by 3D-CT, and complete or basic contraposition in neutral position and symmetry RFD, HRA and RAC1–2 in rotary position, are the normal anatomic features of atlanto-axial joints.