The cervical spine consists of seven vertebrae that are divided into two distinct regions. The upper region consists of the occiput, C1, and C2 vertebrae and lower region includes the vertebrae of C3-C7. Precise movements of the cervical spine require optimal arthrokinematics and osteokinematics and depend on muscle length, strength, and recruitment patterns. Motions of the cervical spine are comprised of coupled motions.
Neutral mechanics, also known as Type1 mechanics, result in coupled movement of side bending and rotation to opposite sides. Neutral mechanics occur in the thoracic and lumbar spine. Non-neutral mechanical coupling, or Type2 mechanics occur when side bending and rotation of vertebrae occur to the same side.
C0 = occiput, C1 = atlas, C2 = axis, C3-C7 = typical
The primary movement of the occipitoatlantal articulation (C0-C1) is forward and backward bending. There is also a small amount of coupled side bending and rotation to opposite sides. Left rotation of the occiput on the atlas is associated with anterior displacement of the right occipital condyle on the right articular process of the atlas. As the occiput turns to the left, the occipital condyles are displaced to the left, resulting in side bending to the right.
The geometry and orientation of the C0-C1 and C1-C2 articular processes appear to dictate the type and amount of motion available at the atlantoaxial joint. The primary movement here is rotation. (There is no intervertebral disc between C1 and C2.)
Typical Cervical Vertebrae
Intervertebral discs are found between two typical vertebral bodies. At the posterolateral corner of each vertebral body is a small synovial joint called the uncovertebral joint of Luschka. These joints are found only in the cervical region and are subject to degenerative changes that occasionally encroach on the intervertebral canal posteriorly. Flexion, extension, side bending, and rotation are all permissible in the typical cervical vertebrae; lateral flexion and rotation are always facet controlled. Side bending in one direction will always be coupled with rotation in the opposite direction (Type 1 mechanics).