Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions

Abstract

The objective of this study was to develop an improved finite element (FE) model of a lumbar functional spinal unit (FSU) and to subsequently analyse the deviations in load transfer owing to implantation. The effects of loading and boundary conditions on load transfer in intact and implanted FSUs and its relationship with the potential risk of vertebral fracture were investigated. The FE models of L1-L5 and L3-L4 FSUs, intact and implanted, were developed using patient-specific CT-scan dataset and segmentation of cortical and cancellous bone regions. The effect of submodelling technique, as compared to artificial boundary conditions, on the elastic behaviour of lumbar spine was examined. Applied forces and moments, corresponding to physiologic movements, were used as loading conditions. Results indicated that the loading and boundary conditions considerably affect stress-strain distributions within a FSU. This study, based on an improved FE model of a vertebra, highlights the importance of using the submodelling technique to adequately evaluate the mechanical behaviour of a FSU. In the intact FSU, strains of 200–400 µε were observed in the cancellous bone of vertebral body and pedicles. High equivalent stresses of 10–25 MPa and 1–5 MPa were generated around the pars interarticularis for cortical and cancellous regions, respectively. Implantation caused reductions of 85%–92% in the range of motion for all movements. Insertion of the intervertebral cage resulted in major deviations in load transfer across a FSU for all movements. The cancellous bone around cage experienced pronounced increase in stresses of 10–15 MPa, which indicated potential risk of failure initiation in the vertebra.

A novel approach for tetrahedral-element-based finite element simulations of anisotropic hyperelastic intervertebral disc behavior

“…This multi-component complex structure in conjunction with the mechanical loads that it experiences during various physical activities ( Schultz and Andersson, 1981 ; Schultz et al, 1982 ) results in complex internal load transfer mechanisms, which are expected to influence spinal pathologies such as back disorders and pain ( Pearcy et al, 1984 ; Kumar, 1990 ; Dvořák et al, 1991 ; Manchikanti, 2000 ; Thiese et al, 2014 ) as well as secondary complications after surgical interventions such as adjacent segment disease ( Bertagnoli, 2011 ), pseudoarthrosis ( Steinmann and Herkowitz, 1992 ), and screw loosening ( Bredow et al, 2016 ). In this context, finite element (FE) based models encompassing various spinal components have gained greater attention in recent decades to study spine biomechanics ( Noailly et al, 2005 ; Schmidt et al, 2006 ; Campbell et al, 2016 ; Dreischarf et al, 2014 ; Zander et al, 2009 ; Schmidt et al, 2013 ; Jaramillo et al, 2015 ; Maquer et al, 2015 ; del Palomar et al, 2008 ; Ayturk et al, 2010 ; Zander et al, 2017 ) with increasing applications towards pre-clinical/surgical studies ( Baroud et al, 2003 ; Rohlmann et al, 2007 ; Boccaccio et al, 2008 ; Talukdar et al, 2021 ), evaluating the influence of intervertebral disc degeneration ( Schmidt et al, 2007b ; Ayturk et al, 2012 ; Cegoñino et al, 2014 ), and towards subject-specific investigations ( Widmer, 2020 ; Pickering et al, 2021 ). These computationally powerful tools are particularly effective in combining hierarchic intricacies of complex spinal systems with material and geometrical non-linearities and a wide range of loading scenarios ( Schmidt et al, 2013 ).…”

Finite element analysis of an intact lumbar spine model: Effects of loading under different coordinate systems

“…The endplates have a thickness of 0.5 mm, and the nucleus covers 43% of the disc volume approximately. 4,23 The full lumbar FE model was meshed with 10noded tetrahedral elements. The mesh convergence study provided in the following section (Verification and validation) was used to determine the appropriate mesh size.…”

“…A combination of 150 N preload and 10 Nm moment was used to simulate all the physiological movements (flexion, extension and lateral bending). 4…”

“…3 Experimental investigations and finite element (FE) analysis have been proven useful in studying the biomechanics of the lumbar spine. [4][5][6][7] In an experimental investigation of spinal stability, Patwardhan et al 8 developed a follower load technique for applying physiological spinal loads. It was intended to replace the compressing effect of local muscle forces with a follower load whose direction followed the curvature of the spine.…”

“…Numerous FE studies considered intact and implanted lumbar spine models to compare the variations in ranges of motion (ROM), stresses, and strains due to fusion surgery. 4,[11][12][13] The follower load technique was considered in various FE studies to apply the loading and boundary conditions. [14][15][16] The outcomes of such FE studies were comparable to the results of standard experiments performed under identical conditions.…”