Constrained Testing Conditions Affect the Axial Rotation Response of Lumbar Functional Spinal Units

Study Design. Human cadaveric spine specimens were tested in axial rotation using constrained and unconstrained methods. Objectives. To determine the degree to which constrained methods affect the response of the functional spinal unit in axial rotation at lumbar and lumbosacral levels. Summary of Background Data. A substantial controversy exists in the literature regarding the appropriateness of different testing methods. No study has been found in which the effect of constraint on axial rotation behavior was objectively examined. Methods. Ten human cadaveric spine specimens (five L3‐L4, five L5‐S1) were tested in axial rotation, using both constrained and unconstrained methods. In the unconstrained test, pure moments were applied to the upper vertebra, and its complete three‐dimensional motion was measured using an optoelectronic camera system. In the constrained test, the specimens were loaded in a fixed‐axis servohydraulic test machine individually around five rotational axis positions within the vertebral body, and the rotational motion was measured. Results. The rotational angles in the constrained tests were not different among the five rotational axis positions. However, the maximum rotation from the five axis positions was approximately 40% greater than the minimum rotation, a significant difference. The axial rotational motion of the unconstrained tests was always less than the maximum rotation measured in the constrained test. However, the total rotational angle using the helical axis of motion was not significantly different from the constrained angles. Conclusions. The large differences between maximum and minimum rotation angles demonstrate that the behavior of the functional spinal unit in axial rotation is 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.