A new intraoperative measurement system for rotational motion properties of the spine

Abstract

Low back pain due to spinal segmental instability is common in lumbar degenerative diseases. Instability of the spinal segment is difficult to define and occur in any motion direction. In particular, rotational instability has been implicated in the pathogenesis of low back pain. Biomechanical measurements assess motion properties that are essential for estimating segmental instability. To detect spinal segmental instability, we developed a new system for measuring spinal mobility during lumbar spine surgery. The system consists of a motor driven mechanical apparatus with a computerized controller, load cell, optical displacement transducer and spinous process holders that allow handling of the spinal process without damaging the supraspinal and interspinal ligaments. In this study, the repeatability test was performed to demonstrate the potential of the measurement system. Displacement generated by the actuator at a speed of 1.0 mm/sec was applied to the tip of the holders with a maximum displacement of 4.0 mm from the neutral position for five cycles of axial rotation. After 5 min, the same specimen was retested for five more cycles of axial rotation, and this process was repeated ten times. In total, this test generated 50 load-displacement curves for each specimen. From the load-displacement curve, we determined three motion parameters: stiffness, neutral zone and absorption energy, and calculated the coefficients of variation (CVs) by dividing the standard deviation by the mean of each parameter and converting it to a percentage. The small CVs for each parameter measured reflected the high repeatability of the load-displacement curves. The measurement had sufficiently high repeatability for clinical use, as indicated by the low CV for each motion parameter.