AXIAL ROTATION IN THE LUMBAR SPINE FOLLOWING AXIAL FORCE WRENCH

The report presents measurements of axial rotation of lumbar motion segments (L1/L2, L3/L4, L4/L5), particularly with small angles of rotation (in the range of +/- 1 degrees) following axial force wrenches. The investigation focussed on determining the influence of geametrically varying configurations in axial wrench (consisting of axial torque and axial force) applied on the kinematics (as defined by the migrating instantaneous helical axis, IHA) of lumbar motion segments under constant resulting axial force, and relating IHA-migration to anatomical structures. In all segments, IHA migrated over several centimetres (up to 6 cm). The main portion of IHA-migration was linked to the angle of rotation interval of +/- 1 degrees. 3. The shape of the IHA-migration was greatly dependent upon the position of the force line F(2). The-force-wrench-dependent wide IHA-migration found for the rotational angle interval of +/- 1 degrees suggests that joint guidance predominates in segment kinematics. The segment kinematics can be adjusted by means of the geometrical configuration of the force wrenches. The design of non-fusion spine implants and FE calculations have to take into consideration joint guidance and muscular force distributions with small intervals of axial rotation.