Dynamic analysis of in vivo segmental spinal motion: An instrumentation strategy

A transducer for measuring three-dimensional segmental spinal motion was de signed to directly measure dynamic rotations (Rx, Ry and Rz) about three or thogonal axes using an array of liquid metal strain gauges (LMSGs). The con figuration of the LMSG array results in differential length changes due to segmental spinal motion. In vitro calibration utilized transducer attachmen t to Steinmann pins implanted into the dorsal spinous processes of anatomic al spinal segments. The response of the LMSGs approximated linearity (R-2 g reater than or equal to 0.980) over the calibrated ranges of angular displa cement (i.e., +/- 5 degrees). On average, artifactual mechanical noise of t he LMSGs was <3% of the signal recorded during locomotion. The minimum reso lution of the transducer was 0.07 degrees of flexion-extension, 0.46 degree s of lateral bending, and 0.56 degrees of rotation. Average resistive force for all transducers was 0.31 +/- 0.05 Nm at the neutral articular position (0 degrees) and 0.51 +/- 0.03 Nm at 5 degrees of flexion. Clinically, the modest mechanical resistance of the transducers did not affect spinal mobil ity nor locomotion. In vivo application of the transducer was demonstrated at thoracolumbar and lumbosacral spinal segments in horses during treadmill locomotion. The transducer was designed and tested on an equine model, but may be adapted for other quadrupeds. The dynamic and continuous measure of three-dimensional in vivo segmental spinal motion will provide an importan t new perspective for evaluating normal and altered spinal motion.