The role of secondary variables in the measurement of the mechanical properties of the lumbar intervertebral joint.

A general method is proposed for the testing of the structural properties of lumbar intervertebral joints. The apparatus designed includes a loading jig capable of applying any wrench resultant load while allowing unconstrained motion of the specimen and a three-dimensional displacement transducer for measurement of principal and coupled displacements. Four secondary independent variables, load axis position, nucleus pressure, the number of initial cycles and the load duration are shown to significantly affect the dependent variable, displacement. When performing this type of experiment, these secondary variables must be standardized and controlled.