Design and material evaluation for a novel lumbar disc replacement implanted via unilateral transforaminal approach

Abstract

The degeneration of the intervertebral disc is one of the principal causes of low back pain. Total disc replacement is a surgical treatment that aims to replace the degenerated disc with a dynamic implant to restore spine biomechanics. This paper proposes the first design of an elastomeric lumbar disc replacement that is implanted as a pair of devices via unilateral transforaminal surgical approach. Furthermore, several biomaterials (Polyurethanes (PU) and Polycarbonate Urethanes (PCU)) are evaluated for the purpose of the implant to mimic the axial compliance of the spine. Bionate II 80A (a pure PCU), Elast Eon 82A E5–325 (a PU with polydimethylsiloxane and polyhexamethylene oxide), Chronosil (a PCU based silicone elastomer) 80A with 5% and 10% of silicone were obtained and injection moulded according to the shape of the implant core, which was defined after a stress distribution analysis with the finite element method. The dimensions for each specimen were: 14.6 × 5.6 × 6.1 mm (length, width and height). Quasistatic compression tests were performed at a displacement rate of 0.02 mm/s. The obtained stiffness for each material at 1 mm displacement was: Bionate II 80A, 448.48 N/mm; Elast Eon 82A E5–325, 216.55 N/mm; Chronosil 80A 5%, 127.73 N/mm; and Chronosil 80A 10%, 126.48 N/mm. Dimensional changes were quantified after two quasi-static compression tests. Plastic deformation was perceived in all cases with a total percentage of height loss of: 4.1 ± 0.5% for Elast Eon 82A E5–325; 3.2 ± 0.5% for Chronosil 80A 10%; 2.7 ± 0.3% for Chronosil 80A 5% and 1.1 ± 0.2% for Bionate II 80A. The mechanical behaviour of these biomaterials is discussed to assess their suitability for the novel disc replacement device proposed.

Introduction

Low back pain (LBP) is a worldwide burden experienced by 80% of the population at least once in their lifetime (Baliga et al., 2015). One of the principal causes of LBP is the degeneration of spinal structures like the Intervertebral Disc (IVD) (Salzmann et al., 2017). Spinal fusion is a surgical option for the treatment for Degenerative Disc Disease (DDD) and consists of eradicating the source of pain by eliminating motion of the damaged spinal segment with implanted instrumentation. Total Disc Replacement (TDR) is an alternative treatment that aims to preserve spinal mobility by removing the damaged IVD and replacing it with a dynamic device. The better performance of TDR in comparison with fusion remains unclear (Salzmann et al., 2017; Rao and Cao, 2014) due to many marketed devices having caused numerous complications. A common problem is the alteration of the biomechanics of the spinal segment treated leading to degeneration of other spinal structures (Abi-Hanna et al., 2018). This complication may be avoided by providing a more physiological range of motion that includes axial compliance with the incorporation of an elastomeric component (Vicars et al., 2017). Elastomeric devices are demonstrating a better physiological range of motion in comparison to ball and socket devices because they replicate the viscoelasticity of the natural disc (Vicars et al., 2017).

Another drawback hampering the use of TDR treatment is the anterior approach undertaken when surgeons implant the devices (Salzmann et al., 2017). This surgical procedure requires well-trained surgeons as it presents a risk with respect to vascular structures around the spine (Vital and Boissière, 2014). None of the existing TDR devices on the market are implanted via the transforaminal approach, a widely applied surgical technique for spinal fusion that has shown favourable outcomes and advantages (such as less time of postoperative recovery) in comparison with posterior and anterior approaches (Deng et al., 2016, Zhang et al., 2014).

This paper describes a novel design for a lumbar disc replacement inserted as a pair of implants via an unilateral transforaminal approach and investigates the mechanical feasibility and the material selection for the device. The implant includes an elastomeric core in between two metallic endplates to mimic the viscoelastic behaviour of a healthy disc. The finite element (FE) method was used to verify that the endplates provided enough strength to prevent any plastic deformation during simulated spinal loading. Then, the elastomeric core was dimensioned and further investigated in order to find a suitable material that most closely reproduced the stiffness of the lumbar spine.

Polyurethanes (PUs) and Polycarbonate Urethanes (PCUs) represent two important classes of polymers used in a broad range of biomedical applications. Motion preservation of the spine is one of their uses because of the elastomeric nature of the collagen and fluids that constitute the IVD (John, 2014). Therefore, four long-term implantable PU and PCUs with and without silicone additives were obtained and mechanically evaluated to assess their suitability for the device purposes.