Determination of spinal facet joint loads from extra articular strains--a theoretical validation

Loads on the facet joints of the lumbar spine may play a role in low back pain. Abnormal loading of the facets, either primarily or as a consequence of disc degeneration, may accelerate their degeneration. Study of these phenomena is difficult, since here are few methods for a direct measure of facet forces occurring in vivo. The authors developed a method for the direct measure of facet forces in a canine animal model of spine disease using strain gages. The method used empirical calibration of the strain gages, which was used to reduce experimental strain output to facet force during function. In this Technical Note, a theoretical model is formulated, solved and validated which gives a theoretical basis for the data reduction method. The facet is modeled as a cantilevered plate and deformations, as function of applied normal load, solved for using a finite difference method. The model is validated by comparison with two experiments with strain-gaged facets. Results from the model show that a minimum of three strain gages is required to determine uniquely the location and magnitude of an applied load to the facet, and that at least two gages should be placed in the region where the cranial articular process joins the pedicle with the gage axis parallel to that of the process. Plate surface strains were found to be insensitive to changes in the area of the applied loads, for a given resultant force magnitude. The method may be useful in other applications in which resultant force magnitude and location need to be measured on plate-like structures.