In vivo evaluation of dynamic characteristics of the normal, fused, and disc replacement cervical spines

Study Design. In vivo human and biomechanical study. Objective. To quantify the in vivo kinematic and kinetic variations in the normal, fused, and disc replacement cervical spines. Summary of Background Data. Clinical and cadaveric studies have reported the motions and forces after an anterior cervical decompression and fusion (ACDF) or implantation of a cervical artificial disc replacement (CADR). However, there is no current study that describes the in vivo dynamic data of these 2 groups and compares them with that of a normal group. Methods. Ten normal subjects, 10 patients treated with an ACDF (C5-C6), and 10 patients having CADR (C5-C6) performed full flexion to extension motions under fluoroscopic surveillance. Kinematic data were obtained from the fluoroscopic images. Kinetic data were derived based on an inverse dynamic model of the entire cervical spine. Results. Even though the range of motion was larger for the normal group than for the ACDF group, the intersegmental rotations at the adjacent C6-C7 and C4-C5 levels in the ACDF group were 13.4 and 8.8 compared with 3.7 and 4.8 in the normal group, respectively, during the neck motion from 20 flexion to 15 extension. The difference at the C3-C4 level was 1 on average. Both the transverse contact forces and the soft tissue forces in the ACDF group were significantly larger than those in the normal group. The vertical forces in the ACDF group were smaller than those in the normal group, but there were no statistical differences. The CADR group exhibited kinematic and kinetic results similar to the normal group. Conclusion. In terms of restoring the normal dynamic motion of the cervical spine, CADR may be an alternative to the ACDF.