A new transducer for facet force measurement in the lumbar spine: benchmark and in vitro test results.

A new transducer capable of direct measurement of time-dependent loads in human lumbar facet joints was developed and tested. The transducer was comprised of a force-sensitive resistor (FSR) in series with a pressure-sensitive film. A wide range of experiments revealed the performance attributes and limitations of the FSR. The output signal of the FSR is actually sensitive to both force and area of contact independently. Therefore, a pressure-sensitive film was used to quantify the contact area. At least two transformation equations were calculated for each FSR corresponding to known contact areas. Each equation was a linearization of the log of the FSR output vs the log of the applied ramp loads. Coefficients of determination (CD) were calculated for small (21 mm2) and large (32 mm2) contact areas, and were found to exceed 0.900 for all data. The average of nine cycles was nearly linear for some FSRs (CD of 0.999). FSR output signal and contact area were recorded in cadaveric lumbar facets under ramp load. The appropriate transformation equation was determined by a linear interpolation between benchmark equations based on the contact area measured in vitro. Facet force measurements compared well with those of other researchers. The transducer was found to be quite easy to use.