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 C_1–2 (RAC_1–2) 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 RAC_1–2 between 26.7° and 38.9°. There is no significant difference in RFD, HRA or RAC_1–2 (P > 0.05) in between levorotatory and dextrogyrate orientation, and there is a positive correlation between RFD and RAC_1–2 (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 RAC_1–2 in rotary position, are the normal anatomic features of atlanto-axial joints.