CONSTRAINED TESTING CONDITIONS AFFECT THE AXIAL ROTATION RESPONSE OF LUMBAR FUNCTIONAL SPINAL UNITS

Study Design. Human cadaveric spine specimens were tested in axial rot ation using constrained and unconstrained methods. Objectives. To dete rmine the degree to which constrained methods affect the response of t he functional spinal unit in axial rotation at lumbar and lumbosacral levels. Summary of Background Data. A substantial controversy exists i n the literature regarding the appropriateness of different testing me thods. No study has been found in which the effect of constraint on ax ial rotation behavior was objectively examined. Methods. Ten human cad averic spine specimens (five L3-L4, five L5-S1) were tested in axial r otation, using both constrained and unconstrained methods. In the unco nstrained test, pure moments were applied to the upper vertebra, and i ts complete three-dimensional motion was measured using an optoelectro nic camera system. In the constrained test, the specimens were loaded in a fixed-axis servohydraulic test machine individually around five r otational axis positions within the vertebral body, and the rotational motion was measured. Results. The rotational angles in the constraine d tests were not different among the five rotational axis positions. H owever, the maximum rotation from the five axis positions was approxim ately 40% greater than the minimum rotation, a significant difference. The axial rotational motion of the unconstrained tests was always les s than the maximum rotation measured in the constrained test. However, the total rotational angle using the helical axis of motion was not s ignificantly different from the constrained angles. Conclusions. The l arge differences between maximum and minimum rotation angles demonstra te that the behavior of the functional spinal unit in axial rotation i s sensitive to the axis's position but the location of the axis is not repeatable. This supports the use of unconstrained methods in spinal testing.