Stand-alone lumbar cage subsidence: A biomechanical sensitivity study of cage design and placement.

Highlights

• A 3D lumbar functional spinal unit finite element model was created and a parametric model of an interbody cage was designed.

• A Drucker–Prager Cap plasticity formulation was used to predict plastic strains and bone failure in the vertebrae.

• The most influential parameters on the functional spinal unit biomechanics were size, curvature and cage placement.

• Wider cages and a more anteriorly placement would increase the volume of failed bone and, therefore, the risk of subsidence.

• This model provides a tool for the preoperative evaluation of the surgery outcome.

Abstract

Background and objective

Spinal degeneration and instability are commonly treated with interbody fusion cages either alone or supplemented with posterior instrumentation with the aim to immobilise the segment and restore intervertebral height. The purpose of this work is to establish a tool which may help to understand the effects of intervertebral cage design and placement on the biomechanical response of a patient-specific model to help reducing post-surgical complications such as subsidence and segment instability.

Methods

A 3D lumbar functional spinal unit (FSU) finite element model was created and a parametric model of an interbody cage was designed and introduced in the FSU. A Drucker–Prager Cap plasticity formulation was used to predict plastic strains and bone failure in the vertebrae. The effect of varying cage size, cross-sectional area, apparent stiffness and positioning was evaluated under 500 N preload followed by 7.5 Nm multidirectional rotation and the results were compared with the intact model.

Results

The most influential cage parameters on the FSU were size, curvature congruence with the endplates and cage placement. Segmental stiffness was higher when increasing the cross-sectional cage area in all loading directions and when the cage was anteriorly placed in all directions but extension. In general, the facet joint forces were reduced by increasing segmental stiffness. However, these forces were higher than in the intact model in most of the cases due to the displacement of the instantaneous centre of rotation. The highest plastic deformations took place at the caudal vertebra under flexion and increased for cages with greater stiffness. Thus, wider cages and a more anteriorly placement would increase the volume of failed bone and, therefore, the risk of subsidence.

Conclusions

Cage geometry plays a crucial role in the success of lumbar surgery. General considerations such as larger cages may be applied as a guideline, but parameters such as curvature or cage placement should be determined for each specific patient. This model provides a proof-of-concept of a tool for the preoperative evaluation of lumbar surgical outcomes.

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.