The role of zygapophysial joint orientation and uncinate processes in controlling motion in the cervical spine.

Five linear and 2 angular measurements on each of C3 to T1 in a sample of 67 human skeletons were used to examine 3 hypotheses about the function of uncinate processes and zygapophysial joints in the cervical vertebral column. The material was sexed and each vertebra was rated for pathological changes. The effects of gender and pathology on the measures was assessed. The upper 4 vertebrae studied had the largest disc-facet angles, supporting the view that the articular facet orientation is responsible for the greater intervertebral disc translation occurring during sagittal motion in the neck. These upper 4 vertebrae also have the largest uncinate processes, and this observation supports the hypothesis that uncinate processes function to guide and control the anteroposterior translation which occurs during sagittal motion. The 3rd hypothesis that uncinate processes function to facilitate axial rotation is not supported by the recorded interfacet angles which appear to promote axial rotation only in the lower 4 vertebrae. Two further hypothesis are suggested. First, that the interfacet angle is responsible for controlling how strictly lateral flexion and axial rotation are coupled in cervical motion segments. Second, the suggestion is made that the high frequency of pathological change seen at the disc margins of the middle cervical vertebrae may be a result of the unusual combination of disc-facet and interfacet angles permitting more degrees of freedom than the cervical intervertebral discs can withstand.