ON THE SIMULATION OF LUMBAR SPINE DEGENERATION

M. I. Godinho (1), V. Carvalho (1), P. R. Fernandes (1), A. P. G. Castro (1)

1. IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal;

Introduction collagen cross-linking. In all cases, a pre-load of 100 N and a moment of 7.5 Nm were applied on the top of Low back pain (LBP) is an emerging problem in L3, with full restriction of L5 movements [5]. The society, as the lumbar region is a leaning site for spinal main outcome was the relative changes in the range of diseases to occur. The number of spinal fusion motion (ROM). IVD degeneration was implemented surgeries for treatment of diseases as herniation, only on the reduced height model, by gradually stenosis and spondylolisthesis is increasing. As not increasing the stiffness of different components, while only the (IVD) is affected/removed reducing the annulus fibres mechanical response. during spinal fusion, the aim of this work was firstly to understand how degeneration links with LBP by determining the role of each ligament per movement Results and secondly to evaluate the impact of IVD height The outcomes for the healthy IVD model, after reduction in degenerative changes. This framework ligaments removal, revealed that the superficial enhances the separation of cause-effect mechanisms in ligaments and FCL had no influence for extension degenerated spines. Thus, treatment decisions can movements, being ALL the most influential ligament benefit from these finite element (FE) simulations of in that case (ROM increase of 14% when removed). In different scenarios to support clinical practice [1]. flexion, FCL removal caused a decrease of ROM up to 19%. After ligament stiffness reduction, there was a Methods 4% ROM increase in flexion when FCL was degenerated, and 1% increase in extension with ALL Two different L3-L5 FE models were developed to degeneration. For the model with height reduction, simulate the abovementioned degeneration/intervention ALL (extension) and FCL (flexion) remained the most scenarios. The first originated from computed active ligaments in extension and flexion. However, tomography (CT) images of a healthy 40-year-old their influence was increased, which could be observed woman, while the second was based on a 78-year-old on the 11% ROM growth after FCL degeneration. woman with marked IVD height reduction on the L4-

L5 level (figure 1) [2]. The segmentation software was ITK-Snap [3], followed by Solidworks® for IVD Discussion inclusion and Abaqus® for FE simulation (both from It was interesting to observe that, with IVD and/or Dassault Systèmes SolidWorks Corp., USA). ligaments degeneration, ROM variation trends were very similar with both models. This confirms literature indications that ALL and FCL rule the movement, but also that such trends are not altered along the degenerative process [6]. It must also be highlighted that, when comparing the outcomes between models with different levels of IVD degeneration, it was verified that ROM relative changes with mild ligament degeneration were slightly higher in the model with the most advanced stage of IVD degeneration. This is a strong indication that ligament degeneration is Figure 1: Healthy and degenerated spine models. primarily linked with IVD degeneration. Further

studies will consider more complex material models The FE models included the seven major ligaments of and a wider patient database. the lumbar spine: anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), facet capsular ligament (FCL), ligamentum flavum, interspinous References ligament, intertransverse ligament and supraspinous 1. El Bojairami et al, Sci Rep, 10(1):1-15, 2020. ligament (superficial ligaments). Four phases for 2. Korez et al, IEEE T Med Imag, 34(8):1649–1662, 2015. ligament removal around L4-L5 were followed, using 3. Yushkevich et al, NeuroImage, 31(3), 1116–1128, 2006. the healthy model [4]: (i) removal of superficial 4. Ellingson et al, CMBBE, 19(9):1009–1018, 2016. 5. Park et al, Comput Biol Med, 43(9):1234–1240, 2013. ligaments, (ii) FCL removal, (iii) PLL removal, and 6. Heuer et al, J Biomech, 40(2):271–280, 2007. (iv) ALL removal. Ligament degeneration was then implemented in both models, being modelled as an Acknowledgements age-related phenomena in two phases: (i) stiffness decrease (i.e. relaxation) due to reduced mobility, and This research was funded by Fundação para a Ciência e Tecnologia (Portugal), through IDMEC, under LAETA (ii) increase in ligament stiffness due to growing project UIDB/50022/2020.

26th Congress of the European Society of Biomechanics, July 11-14, 2021, Milan, Italy