How do spinal segments move?

Abstract Purpose To study and clarify the kinematics of spinal segments following cyclic torques causing axial rotation ( Tz ( t )), lateral-flexion ( Tx ( t )), flexion/extension ( Ty ( t )). Methods A 6D--Measurement of location, alignment, and migration of the instantaneous helical axis (IHA) as a function of rotational angle in cervical, thoracic, and lumbar segments subjected to axially directed preloads. Results IHA retained an almost constant alignment, but migrated along distinct centrodes. Thoracic segments IHA was almost parallel to Tz ( t ), Tx ( t ), or Ty ( t ), stationary for Tx ( t ) or Ty ( t ), and migrating for Tz ( t ) along dorsally opened bows. IHA locations hardly depended on the position or size of axial preload. Lumbar segments IHA was also almost parallel to Tz ( t ), Tx ( t ), or Ty ( t ). In axial rotation IHA-migration along wide, ventrally or dorsally bent bows depending on segmental flexional/extensional status. Distances covered: 20–60 mm. In lateral-flexion: IHA-migration to the left/right joint and vice versa. In flexion/extension IHA-migration from the facets to the centre of the disc. Cervical segments In flexion/flexion IHA was almost stationary for and parallel to Ty ( t ). In axial rotation or lateral-flexion IHA intersected Tz ( t )/ Tx ( t ) under approximately −30°/+30°. Conclusions Generally joints alternate in guidance. Lumbar segments: in axial rotation and lateral-flexion parametrical control of IHA-position and IHA-migration by axial preload position. Cervical segments: kinematical coupling between axial rotation and lateral-flexion. The IHA-migration guided by the joints should be taken into account in the design of non-fusion implants. FE-calculations of spinal mechanics and kinematics should be based on detailed data of curvature morphology of the articulating surfaces of the joint facets.