Investigation of the Combinations of Loading and Facet Joint Contributions to Lumbar Disc Herniation

Author: Bethany Kamitakahara

Kamitakahara, Bethany, 2018 Investigation of the Combinations of Loading and Facet Joint Contributions to Lumbar Disc Herniation , Flinders University, College of Science and Engineering

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Intervertebral disc herniation can compress nerve roots, cause a localised inflammatory response and consequently lead to sciatica or low back pain (LBP). These conditions significantly decrease quality of life, limit activity and impose a large socio-economic burden on the individual and community (Duthey 2013). Disc herniations occur most frequently at the L4-L5 or L5-S1 disc levels in the postero-lateral region (Rajasekaran et al. 2013; Marshall & McGill 2010). Certain risk factors such as age, disc degeneration and sudden or repetitive mechanical loading can predispose the disc to herniation.

Numerous in vitro mechanical tests have been conducted in an attempt to characterise and simulate intervertebral disc herniation. Functional spinal units (FSUs) have been loaded under physiological conditions of fatigue loading or sudden overloading. Previous findings have clearly demonstrated that disc herniation can be achieved by fatigue or sudden overload in healthy, or mildly degenerated, discs (Adams & Hutton 1982). However, several studies have removed the facet joints during testing to improve visibility of the failure event (Wilke et al. 2016; Veres et al. 2010). The facet joints are imperative in restricting axial rotation, forward sliding and extension. Consequently, removing facet joints may compromise the physiological conditions that would be experienced in vivo. These studies have primarily focussed on characterising the specimen’s mechanical properties at failure. To the author’s knowledge, no work has been conducted to compare the six degree of freedom (6DOF) behaviour of the disc before and after a herniation event.

This study primarily aimed to develop a protocol to herniate sheep intact FSUs and isolated discs under varying directions of sudden overload in the hexapod robot and to investigate modes of failure between test groups. Twenty-nine sheep L4-L5 FSUs were used to model the human lumbar spine. Each specimen was rotated in flexion, lateral bending and axial rotation and loaded in compression at 6.67 mm/s. A 6DOF loading protocol was conducted to compare the mechanical properties of the specimens before and after the failure event.

Herniation was observed in 61.5% of intact FSUs, while 68.8% of isolated discs failed by nuclear extrusion and endplate-vertebral fracture. Two-way univariate ANOVAs revealed a significant difference in stress, modulus and toughness between intact and isolated discs. Evidently, the facet joints contribute significantly to an FSUs compressive stiffness and mechanical behaviour during failure. The findings also suggest that intact FSUs in lateral bending are least susceptible to injury in sudden overload, while combined flexion and lateral bending is the most at risk.

Keywords: Intervertebral disc herniation, biomechanics, lumbar spine, facet joints

Subject: Engineering thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: Associate Professor John Costi