Application of a fully-parametric spine model to simulate lumbar interbody fusion procedures

Lumbar interbody fusion (LIF) is the gold standard for the treatment of several lumbar pathologies and involves the insertion of an interbody cage within the intervertebral disk, supported typically by a posterior fixation system. The study of LIF surgery through numerical models typically involves the segmentation of clinical images to reconstruct the spine morphology. However, this approach is excessively case-specific and is not able to represent wider range of morphologies without a large clinical image database. Parametric models are a good alternative to overcome such limitation, allowing the representation of different morphologies by modifying single geometric parameters. The present work aims to exploit a previously validated numerical model to simulate complex surgical scenarios. In particular, this study focused on two LIF techniques (XLIF and PLIF) applied at the L4-L5 level, for 3 different lumbar morphologies to study the effect of cage insertion on bone mechanical resistance. Both XLIF and PLIF led to a significant reduction in the Range of Motion ensuring sufficient primary stability. The analysis of the strains at the bone-cage interface highlights an increase in the bone failed volume moving from the XLIF to the PLIF model. This result is due to the wider bone-cage contact surface produced by XLIF, compared to PLIF. Finally, by analysing the stresses in the cage and rods it was verified that both devices were working in safety conditions for each morphology and loading. This analysis allowed to shed light on the applicability of a parametric models in the analysis of complex systems as LIF surgery, paving the way for the application of such tools in the design of innovative devices for spinal surgery.