Optimisation of Intervertebral Disc Mechanical Properties and the Impact of Vertebral Alignment in Subject-Specific Finite Element Models

Abstract

Subject-specific finite element models could improve understanding of how spinal loading varies between people, based on differences in morphology and tissue properties. However, determining accurate subject-specific intervertebral disc (IVD) properties can be difficult due to the spine's complex behaviour, in six degrees of freedom. Previous studies optimising IVD properties have utilised axial compression alone or range of motion data in three axes. This study aimed to optimise IVD properties using 6-axis force-moment data, and compare the resultant model's accuracy against a model optimised using IVD pressure data. Additionally, model vertebral alignment was assessed to determine if differences between imaged specimen alignment and in vitro 6-axis test alignment affected the optimisation process. A finite element model of a porcine lumbar motion segment was developed, with generic IVD properties. The model loading and boundary conditions replicated in vitro 6-axis stiffness matrix testing of the same specimen. The model was then optimised twice, once using experimental IVD pressures and once using forces and moments. A second model with geometry based on the specimen's vertebral alignment from the 6-axis testing was also developed and optimised. The 6-axis force-moment optimised model had more accurate overall 6-axis load-displacement behaviour, but less accurate IVD pressures than the pressure optimised model. Neither optimised model fully captured spinal behaviours, due to model and optimisation process limitations. The 6-axis vertebral alignment model had lower error and different optimised IVD properties than the imaged vertebral alignment model. Thus, vertebral alignment affected segment stiffness, so should be considered when developing spine models.