A new dynamic six degrees of freedom disc-loading simulator allows to provoke disc damage and herniation

Abstract

Purpose The cause of disc herniation is not well understood yet. It is assumed that heavy lifting and extreme postures can cause small injuries starting either in the inner anulus or from the outside close to the endplate. Such injuries are accumulated over years until its structure is weakened and finally a single loading event leads to a sudden failure of the last few intact lamellae. This paper describes a novel, custom-developed dynamic 6-DOF discloading simulator that allows complex loading to provoke such disc damage and herniations. Methods The machine's axes are driven by six independent servomotors providing high loads (10 kN axial compression, 2 kN shear, 100 Nm torque) up to 5 Hz. A positional accuracy test was conducted to validate the machine. Subsequently, initial experiments with lumbar ovine motion segments under complex loading were performed. After testing, the discs were examined in an ultrahigh field MRI (11.7 T). A three-dimensional reconstruction was performed to visualise the internal disc lesions. Results Validation tests demonstrated positioning with an accuracy of B0.08°/B0.026 mm at 0.5 Hz and B0.27°/ B0.048 mm at 3.0 Hz with amplitudes of ±17°/±2 mm. Typical failure patterns and herniations could be provoked with complex asymmetrical loading protocols. Loading with axial compression, flexion, lateral bending and torsion lead in 8 specimens to 4 herniated discs, two protrusions and two delaminations. All disc failures occurred in the posterior region of the disc. Conclusion This new dynamic disc-loading simulator has proven to be able to apply complex motion combinations and allows to create artificial lesions in the disc with complex loading protocols. The aim of further tests is to better understand the mechanisms by which disc failure occurs at the microstructural level under different loading conditions. Visualisation with ultra-high field MRI at different time points is a promising method to investigate the gradual development of such lesions, which may finally lead to disc failure. These kinds of experiments will help to better investigate the mechanical failure of discs to provide new insights into the initiation of intervertebral disc herniation. This device will also serve for many other applications in spine biomechanics research.

Percutaneous posterolateral approach for the simulation of a far-lateral disc herniation in an ovine model

“…The rongeur only grasps tissue and does not cut through it; therefore, the volume of tissue sequestered from each respective individual is only predictable within a range. While AF tissue was not specifically identified as part of this procedure, the NP:AF ratio of such a sequester is typically 1:1, based on previous findings of sequester tissue composition [25,30]. Six weeks post-injury, all investigated segments show a resealed dorsal AF with obvious tissue irregularities as shown in Fig.…”

“…AF failure can be seen as a hybrid of local tearing of individual fibers and slippage of overstretched fibers from the main loading zone [23]. From recent in vitro and clinical studies, two typical types of herniation have been defined: mid-span failures and endplate junction failures [23][24][25]. Awl penetration of the AF requires approximately 5 s, comparable to the mid-span failure time of disc herniation triggered by body motion [26,27].…”

Homeostasis of the intervertebral disc requires regulation of STAT3 signaling by the adhesion G-protein coupled receptor ADGRG6

“…Col1a1, Col3a1, Col4a1, and Col5a1) in the IVDs at a juvenile stage of development (P20). Increased expression of fibrillar collagens and induced stiffness of the IVD is expected to result in disturbed stress distribution of the IVD, concentrate loading at the cartilaginous endplate, which increase the risk of endplate fractures and the formation of disc herniations (Adams et al, 1996;Vergroesen et al, 2015;Wilke et al, 2016). The adult IVD is thought to be an avascular tissue, as such its major source of nutrient flux occurs via diffusion through endplate (Urban et al, 2004).…”

The equivalence of multi-axis spine systems: Recommended stiffness limits using a standardized testing protocol

“…The stiffness and phase results were consistent between the sine and triangle waveforms, suggesting that it may not be necessary to use both waveforms in future tests that adopt the standardized protocol. Sine waveforms are commonly used in spine testing to simplify test control, and approximate physiologic motion (Amin et al, 2015;Chamoli et al, 2015;Costi et al, 2008;Wilke et al, 2016), though triangle waves have also been adopted in position controlled tests to ensure a uniform test rate (Bennett and Kelly, 2013;Gardner-Morse and Stokes, 2004;Holsgrove et al, 2015a;Kotani et al, 2006). However, triangle waveforms may be less applicable in load control testing where the test rate will vary according to the stiffness of the specimen.…”

“…There are many six-axis testing systems that have been used for the biomechanical testing of the spine (Chung et al, 2002;Ding et al, 2014;Holsgrove et al, 2014;Ilharreborde et al, 2010;Kelly and Bennett, 2013;Martínez et al, 2013;Stokes et al, 2002;Wilke et al, 1994;Wilke et al, 2016), however, the designs and control capabilities of those testing systems vary considerably. Additionally, despite previous studies having demonstrated the large changes in the mechanical properties of spinal specimens due to a preload (Gardner-Morse and Holsgrove et al, 2015a;Panjabi et al, 2001;Tawackoli et al, 2004) and the method of preload application (Cripton et al, 2000), testing rate (Costi et al, 2008;Gay et al, 2008), and testing environment (Costi et al, 2002;Pflaster et al, 1997;Wilke et al, 1998a), the standardization of in-vitro methods is still lacking, despite previous recommendations (Goel et al, 2006;Wilke et al, 1998b), which often makes it difficult, if not impossible, to compare different biomechanical studies (Holsgrove et al, 2015b).…”