Kinematic study of the relation between the instantaneous center of rotation and degenerative changes in the cervical intervertebral disc.

PURPOSE: We located the instantaneous center of rotation (ICR) for the cervical spine at various ages and investigated age-related changes. We evaluated the impact of cervical disc degeneration on the ICR using a scoring system based on plain radiographs.

METHODS: Flexion, extension, and neutral lateral radiographs were obtained from 680 asymptomatic subjects (363 men, 317 women; ages 20-79 years) divided into six 10-year-age groups. The ICRs from C3/C4 to C6/C7 were determined from the radiographs using MIMICS software. A scoring system determined from lateral radiographs quantitatively assessed degeneration of cervical intervertebral discs. ICRs were compared among groups to analyze age-related changes and the relation between degenerative changes and ICR location.

RESULTS: In asymptomatic subjects, the ICR was located approximately at the superior half of the lower vertebral body height and the posterior half of its width. The ICR at the C5/C6 level was located more anterior and higher in patients >50 years than in younger subgroups (P < 0.05). Degenerative changes produced more anterosuperior translation of the ICR, which was significantly correlated with height loss (P < 0.05). In moderately or severely degenerated segments, the ICR location change reached statistical significance (P < 0.05).

CONCLUSIONS: Baseline data for Chinese cervical spine ICRs were established for the third through eighth decade of life, including age-related changes and the kinematic effects of degenerative change on the ICR in the functional spine unit. These findings should be considered in clinical practice and when designing disc prostheses.