A robotic testing facility for the measurement of the mechanics of spinal joints.

A robotic testing facility for the measurement of joint mechanics was used to determine the significance of tears in the intervertebral disc on the mechanics of the spinal joint. Ten lumbar joints of sheep were dynamically loaded and manipulated. Comparisons were made between the behaviour of the intervertebral disc in flexion and extension at two test speeds. The influence of the posterior elements and of rim lesions was assessed by testing the joint immediately before and after removal of the posterior elements and after the creation of a 4 mm x 10 mm rim lesion in the disc. Stiffness of the spinal joint dropped significantly upon removal of the posterior elements, from 0.81 to 0.23 N m/deg for flexion and from 0.65 to 0.40 N m/deg for extension. Maximum moments dropped 37 per cent for flexion and 63 per cent for extension. The rim lesion caused a further significant stiffness reduction to 0.21 and 0.31 N m/deg respectively. Maximum moments reduced a further 12 per cent and 16 per cent respectively. A higher test speed (2 deg/s instead of 0.5 deg/s) usually did not change these results significantly. The novel six-degrees-of-freedom robotic testing facility used in this study was demonstrated to be an effective system for studying the mechanics of complex biological joints.