G. Piskoty

A NONLINEAR, DYNAMIC, MULTI-BODY MODEL OF THE LUMBAR SPINE

This paper addresses the current development of a rigid-body model of the lumbar spine and those parameters affecting its calibration process in the passive state. Many rigidbody models have been developed and integrated with active muscles to obtain load and moment profiles of lumbar spine, however, the effects of the passive elements were not individually validated. The present model includes a 6-DOF non-linear constraint representation of the intervertebral disc, individual ligaments and non-linear facet contacts. The model is evaluated by applying typical flexibility inputs similar to in vitro testing protocols in all planes (i.e. flexion/extension, lateral bending and axial rotation). The values of facet forces as well as range of motion are measured. The main range of motion showed satisfactory results, contrary to the facet forces and coupled motions. The facet forces in extension were at least two times larger than those measured in in vitro studies. It is concluded that such a model lacks the flexibility of the pedicle bone and therefore improvements of this aspect are required to more closely approximate facet joint contact forces.