Finite element evaluation of spondylolysis taking account of nonlinear mechanical properties of ligaments and annulus fibrosus

The present study is concerned with the elucidation of spondylolysis observed in the lower lumbar vertebrae. A three-dimensional finite element model is constructed by taking account of L3, L4 and L5 vertebrae, the sacrum, the ilium, intervertebral disks and spinal ligaments. The mechanical properties of the ligaments and the annulus fibrosus of intervertebral disks are modeled using nonlinear elastic materials. To evaluate the effects of material nonlinearity, corresponding linear elastic materials are also introduced. The motions of extension, flexion and axial rotation are simulated by using nonlinear and corresponding linear elastic materials. The results show that the extension motion is the most dangerous and that spondylolysis is most frequently observed in L5. A comparison of nonlinear and linear analyses clarifies that the material nonlinearity of soft tissues plays an important role in reducing the extension force, flexion load and axial rotation moment.