[A planar nonlinear model of the human spine]

In this study the derivation of a two-dimensional mathematical model of the human lumbar spine and its approximate solution using the method of finite elements is described. The computer model LUSP (Lumbar Spine) serves as a basis for studying the kinematic and load-bearing behaviour of the lumbar spine. The underlying working hypothesis is that the smallest spinal unit, the so called functional spinal unit (Junghanns, reflects the basic characteristic behaviour of the musclefree spine. On the basis of Lagrange's virtual work principle the nonlinear static and dynamic equations of motion for a sagitally symmetrical spine model of comprising rigid bodies, springs, beams and dampers are derived. The finite element method is used as an appropriate approximation scheme. Intensive research was conducted to provide the necessary geometrical and material input data. Special attention was paid to achieving a realistic description of the nonlinear stress-strain relationships for the soft tissue involved. A database-type preprocessor and a graphics-oriented postprocessor are made for convenient handling of the input and output data. The efficiency of the present computer model is demonstrated by means of an orthopaedic-biomechanical study on degenerative phenomena in so-called juxta-fused lumbosacral motion segments.