Load- and displacement-controlled finite element analyses on fusion and non-fusion spinal implants

Abstract

This study used finite element (FE) analysis with the load-controlled method (LCM) and the displacement-controlled method (DCM) to examine motion differences at the implant level and adjacent levels between fusion and non-fusion implants. A validated three-dimensional intact (INT) L1-L5 FE model was used. At the L3-L4 level, the INT model was modified to surgery models, including the artificial disc replacement (ADR) of ProDisc II, and the anterior lumbar interbody fusion (ALIF) cage with pedicle screw fixation. The LCM imposed 10 Nm moments of four physiological motions and a 150 N preload at the top of L1. The DCM process was in accordance with the hybrid testing protocol. The average percentage changes in the range of motion (ROM) for whole non-operated levels were used to predict adjacent level effects (ALE%). At the implant level, the ALIF model showed similar stability with both control methods. The ADR model using the LCM had a higher ROM than the model using the DCM, especially in extension and torsion. At the adjacent levels, the ALIF model increased ALE% (at least 17 per cent) using the DCM compared with the LCM. The ADR model had an ALE% close to that of the INT model, using the LCM (average within 6 per cent), while the ALE% decreased when using the DCM. The study suggests that both control methods can be adopted to predict the fusion model at the implant level, and similar stabilization characteristics can be found. The LCM will emphasize the effects of the non-fusion implants. The DCM was more clinically relevant in evaluating the fusion model at the adjacent levels. In conclusion, both the LCM and the DCM should be considered in numerical simulations to obtain more realistic data in spinal implant biomechanics.

Effect of the cord pretension of the Dynesys dynamic stabilisation system on the biomechanics of the lumbar spine: a finite element analysis

“…For the adjacent levels, the present study revealed that the ROM increased by at least 22.4% in flexion and 15.6% in lateral bending; these trends conflict with the findings of some in vitro tests under the load control method [9,21]. Our earlier study demonstrated that the effects on adjacent levels are more prominent under the displacement control method after insertion of a spinal implant when compared with the load control method [18]. This conflicting result can be explained by the use of different testing protocols in the present FE simulation and previous in vitro tests.…”

“…First, ROMs in five levels of the INT model under different moments of 3.75, 7.5, and 10 Nm were validated with the results of the previous study [18]. Good agreement of ROMs was achieved under most of the loading cases.…”

“…The loading condition of the FE model was only applied to the displacement control method. The previous study [18] compared the biomechanical effect between the load control method and displacement control method. If the load control method is applied to the evaluation of non-fusion spinal implants, the variation of ROM for the stabilised motion segment is more remarkable.…”

“…The stiffness of the spinal structure changes depending on the contact status, so the standard contact option in ANSYS was adopted to account for the changing-state nonlinear problem in this study. A more detailed description of this intact lumbar spine FE model has been presented in our earlier studies [18,19]. The FE model of the intact lumbar spine consisted of 112,174 elements and 94,162 nodes.…”

“…The loading condition of a constant ROM has been shown to be more clinically relevant in predicting the adjacent level effects following the insertion of a spinal implant [18,20]; for this reason, this study used the displacement control method. For all Dynesys cases, three load steps were included in the whole simulation process.…”