A new combined optical and robotic testing system to evaluate multisegmental spinal kinematics

Abstract

A testing system was developed with the aim of evaluating in vitro kinematics of multisegmental spine specimens. This includes a robot (KUKA-KR125) and an optical tracking system (NDI, Optotrak-Certus) measuring the three-dimensional kinematics of each vertebral body simultaneously. The robot has the option of saving kinematic and force/torque (F/T) data for repeating a prerecorded motion path. It can apply unconstrained physiologic movement. In a first evaluation study the robots' F/T control unit was conducted using a linear rod with a second F/T load cell in-line. Kinematic accuracy of the tracked optical data and the calculation software was determined. The study was complemented by testing six four-segmental lumbar calf specimens (L2-6). Three cycles of pure unconstrained moments (7.25±0.05Nm) were applied to each specimen in flexion-extension. Robot data and optical system records were synchronized using a self-developed software algorithm. For each spinal segment load-displacement data were analyzed for range of motion (ROM), neutral zone (NZ) and elastic zone (EZ). In addition the finite helical axes (FHA) for spinal segment motions were calculated. For each level ROM was compared to literature values from monosegmental studies matching with 102±17.6% for L2-3 to 101±15.7% for L5-6 of used reference. Segmental stiffness ranged from 0.3±0.14 to 0.7±0.14Nm/° for NZ and from 2.7±0.55 to 4.9±1.79Nm/° for EZ. Position and orientation of the inter-segmental FHA varied significantly for each segment and specimen. In general, the precision of the robot system seems to be adequate and the results are consistent with intact monosegmental data from literature. Therefore the testing system can be used to investigate adjacent level effects including coupled motion in a multisegmental specimen in future experiments. © 2010 International Federation for Medical and Biological Engineering.