Accurate FE modeling of human annulus tissue for refined spinal implant design

This research is based on experimental and numerical investigations of human lumbar spine. Human intervertebral discs usually exhibit highly anisotropic mechanical responses in a finite strain regime under physiological loading conditions. Human disc body units basically consist of a fluid filled cavity (nucleus pulposus) approximately at the disc center and surrounding tissue reinforced by collagen fibers (annulus fibrosus). For the latter component a newly developed constitutive model is applied which accounts for nonlinear anisotropic stress response. By assuming appropriate nonlinear material parameters, this material law ensures numerical accuracy in the finite strain regime. In general, however, it is very complicated to obtain nonlinear constitutive data, since this requires exceptional anatomical expertise. This presentation enlightens numerical aspects on specimens obtained from the annulus fibrosus. By means of finite element (FE) analyses, the requirement for a careful specimen preparation is outlined. It proves that the entire constitutive theory can be based on this new experimental procedure for the determination of local, nonlinear and anisotropic stress response in annulus tissue. A distinct feature of the present theory compared with previous publications is the determination of nonlinear material parameters for single annulus lamellae. Besides a physically unique determination of the constituents, this also allows to account for stiffness variability within annulus fibrosus in radial and hoop directions, respectively. Based on the new experimental results and on the constitutive model implemented in a multi-purpose FE code, enhanced large scale FE analyses of lumbar disc bodies are discussed. Since all constitutive parameters can be interpreted in an unique and physically consistent way, the FE analyses show excellent performance and accuracy within the physiological range of human lumbar motion segments. Hence, the current approach is very attractive for refined spinal implant design and optimization.