New methods to study lumbar spine biomechanics: Delineation of in vitro load-displacement characteristics by using a robotic/UFS testing system with hybrid control

Delineation of the load-displacement characteristics of osteoligamentous spinal specimens has become fundamental to the investigation of the biomechanics of the spine. In vitro biomechanical testing of the spine requires not only highly specialized testing machines and devices for measuring loads and displacements, but also implementation of a control method to govern the application of loads and displacements to a specimen. Traditionally, in vitro load-displacement characteristics of the spine have been obtained through biomechanical tests that are performed in either a load control or a displacement control mode, with relative advantages and disadvantages. These control methods are in fact complementary, in that one method or the other is viewed as being more applicable in certain regions of the nonlinear load-displacement curve. A combination of load control and displacement control methods, termed hybrid control, offers advantages over either load control or displacement control methods alone for in vitro biomechanical testing of the spine. This article describes the application of a robotic/UFS (universal force-moment sensor) testing system with hybrid control to the delineation of the in vitro load-displacement characteristics of human lumbar functional spinal units (FSUs). Preliminary results obtained using this system demonstrate successful delineation of FSU load-displacement response in quarter-degree increments throughout specimen flexion/extension range of motion, with minimal coupled loads in secondary degrees-of-freedom. Characteristic S-shaped curves of the moment-rotation response were obtained, consistent with previous reports of the FSU having a region of low stiffness (neutral zone) bounded by regions of increasing stiffness (elastic zones). Further developments of hybrid control theory as applied to the spine could help to elucidate the interactions that occur among the passive, active, and neural control subsystems of the spine.