Optimization of a multibody system of the human lumbar spine

A novel 3D multibody system (MBS) model of the human lumbar spine is presented, allowing the dynamic study of the entire lumbar spine and/or of each one of its intervertebral discs (IVD) by itself, predicting the complex stress conditions due to any arrangement of the main movements: flexion-extension, lateral bending and axial rotation, compression or traction. The multibody model is composed by six vertebrae, taken as rigid bodies, interconnected through fifty non-linear Maxwell elements, emulating the five intervertebral discs and twenty main ligaments, plus the facets. The intervertebral discs (IVD) three orthogonal axes rotation and translation behavior was characterized via a dedicated high degree viscoelastic finite elements model, developed within the research group, whereas the ligaments (PLL, ALL, SSL-ISL, LF) were typified through published experimental data. In both cases, the resulting MBS non-linear spring joints characteristics were carefully checked against the original data sources, and similarity of response assured within the physiological range of motion. The model, thoroughly validated against in vitro and in vivo available experimental data, showed to be very stable and with remarkably light computational demands.