MECHANICAL STABILITY OF THE IN-VIVO LUMBAR SPINE - IMPLICATIONS FOR INJURY AND CHRONIC LOW-BACK-PAIN

One important mechanical function of the lumbar spine is to support th e upper body by transmitting compressive and shearing forces to the lo wer body during the performance of everyday activities. To enable the successful transmission of these forces, mechanical stability of the s pinal system must be assured. The purpose of this study was to develop a method and to quantify the mechanical stability of the lumbar spine in vivo during various three-dimensional dynamic tasks. A lumbar spin e model, one that is sensitive to the various ways that individuals ut ilize their muscles and ligaments, was used to estimate the lumbar spi ne stability index th ree times per second throughout the duration of each trial. Anatomically, this model included a rigid pelvis, ribcage, five vertebrae, 90 muscle fascicles and lumped parameter discs, ligam ents and facets. The method consisted of three sub-models: a cross-bri dge bond distribution-moment muscle model for estimating muscle force and stiffness from the electromyogram, a rigid link segment body model for estimating external forces and moments acting on the lumbar verte brae, and an 18 degrees of freedom lumbar spine model for estimating m oments produced by muscle forces and their 90 muscle fascicles and lum ped passive tissues. Individual associated stiffness estimated from th e EMG-assisted optimization algorithm, along with external forces were used for calculating the relative stability index of the lumbar spine for three subjects. It appears that there is an ample stability safet y margin during tasks that demand a high muscular effort. However, lig hter tasks present a potential hazard of spine buckling, especially if some reduction in passive joint stiffness is present. Several hypothe ses on the mechanism of injury associated with low loads and aetiology of chronic back pain are presented in the context of lumbar spine sta bility.