Sagittal-plane mobility of the cat cervical spine.

The present study was conducted to evaluate the nature of sagittal-plane motion across cervical vertebral joints and to identify the centers of rotation for each joint in anaesthetized cats X-rayed in a range of head-neck postures. Relative positions of adjacent pairs of vertebrae were assessed by constructing transparent templates for each vertebra that could be overlaid onto different X-rays, and then by digitizing the locations of three markers attached at a distance from each template. The finite centers of rotation for each joint were estimated using a rigid-body method. The errors associated with the estimates were quantified further by using a method in which the positions of digitized markers were fitted to concentric circles using a least-squares approach. The center of rotation between the skull and C1 was located at the cranial articular facets between the two bones. The center of rotation between C1 and C2 was situated near the tip of the odontoid process close to the site of attachment of the transverse ligaments. Centers of rotation for the other cervical vertebral joints were located in the region of the intervertebral disc. A similar range of values was obtained for centers of rotation when extended or flexed postures were used for the calculations, suggesting that the centers of rotation may not move markedly throughout a range of sagittal-plane rotations of approximately 90 degrees at the skull-C1 joint and about 15-30 degrees at the other cervical joints. These results showed that all cervical joints rotated during sagittal-plane motion. A minimal representation of the musculoskeletal mechanics of the feline neck probably requires multiple segments, each corresponding to a single vertebral bone.