Accuracy and repeatability of a new method for measuring facet loads in the lumbar spine

Abstract

We assessed the repeatability and accuracy of a relatively new, resistance-based sensor (Tekscan 6900) for measuring lumbar spine facet loads, pressures, and contact areas in cadaver specimens. Repeatability of measurements in the natural facet joint was determined for five trials of four specimens loaded in pure moment (±7.5 N m) flexibility tests in axial rotation and flexion-extension. Accuracy of load measurements in four joints was assessed by applying known compressive loads of 25, 50, and 100 N to the natural facet joint in a materials testing machine and comparing the known applied load to the measured load. Measurements of load were obtained using two different calibration approaches: linear and two-point calibrations. Repeatability for force, pressure, and area (average of standard deviation as a percentage of the mean for all trials over all specimens) was 4–6% for axial rotation and 7–10% for extension. Peak resultant force in axial rotation was 30% smaller when calculated using the linear calibration method. The Tekscan sensor overestimated the applied force by 18±9% (mean±standard deviation), 35±7% and 50±9% for compressive loads of 100, 50, and 25 N, respectively. The two-point method overestimated the loads by 35±16%, 45±7%, and 56±10% for the same three loads. Our results show that the Tekscan sensor is repeatable. However, the sensor measurement range is not optimal for the small loads transmitted by the facets and measurement accuracy is highly dependent on calibration protocol.

Introduction

Spinal facet joints play a role in load transmission through the lumbar spine (Adams and Hutton, 1980; el-Bohy et al., 1989; Lorenz et al., 1983; Yang and King, 1984). Implantable devices that allow spinal motion may change the loading across the facets, which could have deleterious long-term clinical consequences. These effects have not been studied extensively because current techniques for assessing facet joint loads are limited.

Earlier approaches for measuring lumbar spine facet loads include extra-articular strain gauges (Buttermann et al., 1991; Kahmann et al., 1990). Results are highly sensitive to the placement and number of strain gauges used (Buttermann et al., 1992; Luo et al., 1996). Direct measurements of in vitro facet loads using Fuji Prescale film have been performed, (Buttermann et al., 1991; Dunlop et al., 1984; Hedman, 1992; Lorenz et al., 1983) but this method is limited to measuring only the peak contact pressure over one cycle.

Thin, flexible resistive sensors (Tekscan Inc., South Boston, MA, USA) use patterns of electrical conductor to produce a grid of sensing elements over which force distribution is measured. It is not clear how accurately and repeatably these sensors will assess load in the facet joints because sensor performance is dictated by such factors as compliance of interface materials and sensor type used.

In this study, our objectives were to determine: (1) What is the repeatability and accuracy of measuring facet joint loads with a Tekscan sensor? and (2) What effect does calibration protocol have on measured loads in the facet joints?