The design, development and characterisation of a novel intervertebral disc prosthesis

Abstract

Back pain is one of the leading causes of disability worldwide. Including its concomitant effects, back pain costs the UK economy approximately £10 billion per annum. The majority of back pain stems from the deterioration of the intervertebral disc (IVD), known as degenerative disc disease. Conservative treatments for this condition are often insufficient and severe cases of degenerative disc disease are treated surgically with fusion or artificial disc replacement (ADR). Fusion is well established but causes unnaturally high loads in adjacent discs, which can lead to long-term complications; in theory, ADRs address this issue. An ideal ADR would be capable of transferring load through the spine with minimal risk of damage to the vertebrae, restoring the original stiffness of the damaged functional spinal unit and minimising soft tissue damage. To the author’s knowledge, no such device exists. This PhD project sought to establish a design for an ADR which fulfilled these requirements. Progressive design iterations were manufactured and mechanically characterised. The final ADR design comprised two components: (1) a core of polydimethylsiloxane elastomer and (2) a limac¸on-shaped periphery injection-moulded from polycarbonate-urethane, reinforced with an embedded stainless steel mesh. The mesh emulates the anisotropy generated by collagen fibres in the natural IVD. Immersed within this structure were flexible titanium endplates, to which a pin array was affixed to anchor the ADR to the vertebrae for primary fixation. Primary fixation of the ADR to the bone is vital for the device to function correctly and withstand the shear forces present. These shear forces have been reported at 230 N during arm elevation whilst holding a weight and 130 N during upper body flexion. In existing ADRs, fixation features protrude from stiff metal endplates, which interface with grooves cut into the vertebrae, introducing an unnatural strain distribution at the implant-bone interface. Associated postoperative complications include subsidence of the implant and, occasionally, even fracture of the vertebrae. In this work, the problem was quantified by characterising the strain distribution in the adjoining vertebrae and comparing the proposed ADR with a commercial device, with the goal of achieving isostatic contact between the implant and bone. Loading the proposed ADR between artificial vertebrae at physiological loads, stereo digital image correlation revealed that the standard deviation in vertical strain and horizontal strain averaged across the vertebrae reduced by 56% and 76% respectively when compared with a commercial disc replacement. This shows the biomimetic ADR promotes a more uniform strain distribution in the adjoining bone and a more even spread of strain energy in the horizontal plane. The potential implications of these results relates not only to a reduction in subsidence and vertebral fracture but also to pain alleviation from stress reduction on nerves in the vertebral endplate. The pin array for implant fixation was designed based on the penetrative behaviour of individual pins in porcine vertebrae. Scanning electron microscopy revealed the local failure mechanism in the bone was dependent on pin geometry: a flat-ended pin caused shear failure of the bone and a pointed pin resulted in compressive failure. A hollow pin required 39% less penetrative force than a solid flat-ended pin of the same diameter, which would result in a reduction in the compressive force required in surgery to seat the ADR, whilst maintaining the resistance to shear forces. Novel characterisation and verification techniques were also developed to support and justify the ADR development process. For the first time, an embedded fibre optic strain sensor was employed for ADR characterisation, providing spatially distributed strain sensing within the implant. This technique shows the potential to aid not only implant development but also for monitoring its ‘health’ in-vivo. Thermography was additionally employed to help verify the implant design and an analytical model was proposed as a means of measuring hysteretic energy loss, providing a better understanding of the mechanical response of the implant compared with other designs. This work encompasses the proof-of-concept step towards the development of a novel ADR. The proposed implant is theorised to allow the use of minimally invasive surgical techniques to minimise tissue damage; however, the next steps should focus on demonstrating this in a clinical environment.

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Identifiers

ORCID for Michael Robert Godfrey: orcid.org/0000-0002-6655-6437

ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

ORCID for Alex Dickinson: orcid.org/0000-0002-9647-1944

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