Morphological aspects and biomechanical properties of the vertebroaxial joint (C2-C3).
The C2-C3 intervertebral joint must be regarded as a transitional area situated between the upper cervical spine where most rotation of the neck and little flexion and extension occur and the lower cervical spaces where chiefly motion in the sagittal plane and also somewhat rotation take place. Under normal circumstances the range of flexion-extension reaches 11 degrees, slighter than below (19.5 degrees at C5-C6); on the opposite, the range of rotation attains 7 degrees; less than above but much more than below (0 degrees at C5-C6). The motion in the sagittal and coronal planes is relatively poor because of the location of Penning's motor-axis of C2 which runs far from the vertebral body and the lowness of the intervertebral disc. However, the rotation of C2 with respect to C3 is fair by the peculiar inclination of the articular facets which slope sagittally but also coronally and trace a sphere whereupon C2 may move around its motor-centre in any plane. When C2-C3 is surgically fixed by bone graft, the lack of motion is completed by a "compensatory movement" in the upper cervical spaces and especially at the atlantooccipital joint for flexion-extension. In the same way, C2-C3 may improve its mobility especially in the sagittal plane when the inferior partner is blocked by surgical or arthritic fusion.