Einfluss zweier Bandscheibenprothesen auf die Kinematik des C5/C6-Segmentes

Abstract

Purpose: The kinematical properties of human C5/C6 segments in axial rotation, ventral/dorsal and lateral bending are evaluated before and after total disc arthroplasty (TDA) with PRESTIGE LP and BRYAN Cervical Disc Systems (Medtronic) under flexion/extension and differing amount of central axial preload as parameters. Furthermore the effect of the physiological inclination of the cervical spine is evaluated. The results are compared with those of C3/C4 segments. Methods: Eight human segments (five positioned vertically, three with an inclination of 15°) were stimulated by triangularly varying, axially directed torque (Tz(t)), transversally directed torque (Ty(t)) and sagittally directed torque (Tx(t)) under compressing static axial preloads. Using a 6D-measuring device with high resolution, the response of segmental motion was characterized by the instantaneous helical axis (IHA). The position, direction, and migration path length of the IHA were measured before and after TDA (parameter: position of the axially directed preload). In addition stiffness and range of motion (ROM) of each segment before and after TDR were detected. Results: In axial rotation the periodic torque Tz(t) generated IHA migrations whereupon the IHA direction was constantly rotated to the dorsal by ≈15.5°. After TDA, the IHA0 (neutral positions) were significantly shifted to the dorsal (PRESTIGE: 4.3 mm, BRYAN: 7.0 mm) just as the points of balance of the entire IHA migration paths. In ventral/dorsal bending after TDA a significant dorsal shift of the IHA was found for both Implants, while under the BRYAN Protheses an additional significant cranial shift occurred. Stiffness and ROM of the segments were altered after TDR with each of both implants. Conclusions: Due to the configuration of the vertebral joints and their interaction with the intervertebral disc, the IHA migrates during the axial rotation within a distinct domain of each C5/C6-segment. Implantation of the PRESTIGE and BRYAN prostheses significantly alters these kinematical properties by dorsal (in ventral/dorsal bending by dorsal (PRESTIGE) and dorso-cranial(BRYAN)) displacements of the domains. Statistically TDA of C3/C4 and of C5/C6 are not correlated. The (physiological) inclination of the segment has an effect on the segments kinematics. Under axial rotation as also under ventral/dorsal bending of the cervical spine, additional lateral and/or ventral/dorsal displacements are produced by TDA. Consequently, adjacent level disease (ALD) may be mechanically stimulated.