Stand-alone lumbar cage subsidence: A biomechanical sensitivity study of cage design and placement.
Introduction
Spinal degeneration and instability are commonly treated with interbody fusion cages either alone or supplemented with posterior instrumentation. The aim of this surgery is to stabilise the segment and restore intervertebral height. Lumbar cages are widely employed in combination with additional screw instrumentation to ensure segment immobilisation and avoid the risk of non-union. However, although widely accepted as a successful treatment, this additional fixation, apart from being more invasive, has been reported to present some complications such as screw loosening or implant failure. Besides, some biomechanical studies have suggested that lumbar intervertebral disc (IVD) cages are sufficiently stable to be used as stand-alone devices [1], provided that they are introduced using a minimally invasive technique that ensures preservation of important stabilising structures [2], [3]. Large scale clinical studies have also demonstrated no differences in clinical outcomes between patients with stand-alone cages versus those with additional posterior fixation [4]. Although instability was initially defined as a loss of stiffness, and later as a reduction of the neutral zone [5], it is generally accepted that instability is associated with an abnormal load pattern which not necessarily would imply an increase in segmental movement as occurs during disc degeneration [6]. For that reason, and given that the fusion surgery aims to reduce the movement, throughout this paper the outcomes would be discuss in terms of segmental stiffness, directly related with the relative movement of the segment.
The use of stand-alone cages has shown to be very controversial presenting several problems chief of which is the risk of subsidence of the device into the bone owing to the high contact pressures on the bony endplates. For this reason, this study was focused on the investigation of possible factors which may contribute to reduce this risk in cages placed as a stand-alone construct. Although subsidence in early postoperative stage may increase the contact area, avoid the peak pressures caused by irregularities and prevent the progression of subsidence, high-grade subsidence can lead to a reduction in the intervertebral space height [7]. Thus, the use of an appropriate constitutive material of the vertebral bone incorporating a plasticity formulation would lead to a better prediction of the risk of subsidence in a stand-alone fashion. Previous studies have used Von Mises equivalent stress as the criterion for bone yielding [8], [9]. However, considering that bone should be treated as a brittle material, the Von Mises criteria would not be suitable [10]. In this study, we used the modified Drucker–Prager Cap model, which takes the contribution of hydrostatic stress into consideration, as the yield criterion to model the inelastic behaviour at a continuum level.
On the other hand, cage characteristics such as shape, material and positioning are also expected to have a significant influence on surgery success. Previous studies have used finite element (FE) models to compare among commercial cages, but only some of them have discussed the influence of cage material [11] or shape [8], [12] using parametric or optimisation methods. In their study, Hsu et al. [12] used a genetic algorithm to find the cage shape with an optimal subsidence resistance. However, they assumed flat endplates instead of real geometry which may lead to a more uniform pressure distribution and an underestimation of subsidence risk. Later on, a study comparing a standard cage with a custom-fit one showed that patient-specific cage geometry could reduce the stress concentration on the endplates [8]. However, these studies provided a limited prediction of subsidence as they used elastic material models.
In this study we aimed to prove a model which will serve as a tool for the preclinical evaluation of surgery outcomes that any interbody device or supplementary fixation may have in each specific patient. Particularly, in this work, the influence of different design parameters and positioning of a bean-shaped cage on segment stiffness and subsidence risk has been studied. The selection of a stand-alone device responds to the higher risk of subsidence reported for this technique but in no case it is aimed to show a comparison with additional instrumentation or to demonstrate a superiority of one technique over the other. To this end, we conducted a parametric FE analysis of a cage design of varying size, cross-sectional area and position, and evaluated it in a patient-specific functional spinal unit (FSU). The main contribution of this study is the evaluation of cage subsidence with an elasto-plastic material formulation with different behaviour for traction and compression for the bone. In addition, this formulation accounted not only for the stresses over the yield stress limit, but also considered the hardening of the material.
Section snippets
Materials and methods
A 3D FE model of a FSU was developed to evaluate the influence of cage design and positioning in the surgical outcome. Firstly, the intact segment was simulated and validated to set a control scenario. Then, the elements corresponding to the nucleus and inner annulus were removed to place a stand-alone cage. A total of 50 models were built (intact FSU + cage with neutral parameters + 8 variations of each parameter) as explained below.
Intact FSU validation
The ROM of the intact FSU was validated by comparison with in-vitro [27] and computational data [28] from literature. In Fig. 2, the rotation of the intact FSU for every movement is compared with data in the literature, and a good result was achieved for every case. Notwithstanding the fact, that the variability obtained by Heuer and collaborators [27] is very high, our results also fall in the range of the computational analysis made by Zander and collaborators [28].
Segmental stiffness
As it has been mentioned in
Discussion
In this work, the effect of varying interbody cage design and placement on segmental stiffness and risk of subsidence has been studied using FE models with a post-yield characterisation of the vertebral bone. Because the main goal of this work was to provide a useful tool for preoperative evaluation: range of motion, facet forces and bone integrity have been studied as key factors to achieve a successful fusion surgery.
Historically, supplementary fixation has proved to provide greater
Acknowledgements
This work was supported by the Spanish Ministry of Economy and Competitiveness through project DPI2016-79302-R and by the Spanish Ministry of Education, Culture and Sports (grant FPU13/01070).
Conflict of interest statement
The authors have not conflict of interest to declare.
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