Effect of multilevel lumbar disc arthroplasty on spine kinematics and facet joint loads in flexion and extension: a finite element analysis

Abstract

Total disc arthroplasty (TDA) has been successfully used for monosegmental treatment in the last few years. However, multi-level TDA led to controversial clinical results. We hypothesise that: (1) the more artificial discs are implanted, the stronger the increases in spinal mobility and facet joint forces in flexion and extension; (2) deviations from the optimal implant position lead to strong instabilities. A three-dimensional finite element model of the intact L1-L5 human lumbar spine was created. Additionally, models of the L1-L5 region implanted with multiple Charité discs ranging from two to four levels were created. The models took into account the possible misalignments in the antero-posterior direction of the artificial discs. All these models were exposed to an axial compression preload of 500 N and pure moments of 7.5 Nm in flexion and extension. For central implant positions and the loading case extension, a motion increase of 51% for two implants up to 91% for four implants and a facet force increase of 24% for two implants up to 38% for four implants compared to the intact spine were calculated. In flexion, a motion decrease of 5% for two implants up to 8% for four implants was predicted. Posteriorly placed implants led to a better representation of the intact spine motion. However, lift-off phenomena between the core and the implant endplates were observed in some extension simulations in which the artificial discs were anteriorly or posteriorly implanted. The more artificial discs are implanted, the stronger the motion increase in flexion and extension was predicted with respect to the intact condition. Deviations from the optimal implant position lead to unfavourable kinematics, to high facet joint forces and even to lift-off phenomena. Therefore, multilevel TDA should, if at all, only be performed in appropriate patients with good muscular conditions and by surgeons who can ensure optimal implant positions.

Spinal Facet Joint Biomechanics and Mechanotransduction in Normal, Injury and Degenerative Conditions

“…Similarly, the L1-L5 FE model developed by Schmidt et al [273] predicted facet force to increase during extension by 24% on average with two disc implants at any two levels, and by 38% with four implants at each level [273]. In addition, the antero-posterior placement of the implant has been shown to modify the relative segmental motions because it separates or compresses the facet articular surfaces [314].…”

“…Segmental mobility increases after capsulotomy [105,106,195,206,209]. Disc arthroplasty modifies facet loading at index and adjacent spinal levels [132,134,200,263,269,270,273]. Geometric parameters of a disc implant influence facet joint contact during loading [269,314,330].…”

“…In the same way as elemental modification is used to model facetectomy, computational models have also been modified by replacing the intervertebral disc elements with those modeling the geometric and material properties of disc implants in order to evaluate the influence of disc arthroplasty on spinal kinematics and facet loads [200,264,[266][267][268]270,273,274,314,329,330]. Those FE models are useful because they can be altered to study the influence of parameters like implant size or its placement on the mobility of the motion segment and the contact pressure developed in the facet articular cartilage.…”

Lumbar interbody fusion: a parametric investigation of a novel cage design with and without posterior instrumentation

“…A FE model of the L4-L5 human spine segment derived from a previously developed and validated model of the lumbar spine [31] was employed (Fig. 1a).…”

“…The fiber angle varied from ±24°to the mid-plane of the disc ventrally to ±46°at the dorsal side, according to a previous study [15]. The non-linear elastic material properties and the crosssectional areas of the disc collagen fibers and ligaments were the same used in previous elastostatic studies [29,31]. Facet joints were modeled by using a frictionless surface-tosurface contact; a layer of solid elements having thickness 0.5 mm represented the facet cartilage.…”