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

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 80 A (a pure PCU), Elast Eon 82 A E5–325 (a PU with polydimethylsiloxane and polyhexamethylene oxide), Chronosil (a PCU based silicone elastomer) 80 A 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 80 A, 448.48 N/mm; Elast Eon 82 A E5–325, 216.55 N/mm; Chronosil 80 A 5%, 127.73 N/mm; and Chronosil 80 A 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 82 A E5–325; 3.2 ± 0.5% for Chronosil 80 A 10%; 2.7 ± 0.3% for Chronosil 80 A 5% and 1.1 ± 0.2% for Bionate II 80 A. The mechanical behavior of these biomaterials is discussed to assess their suitability for the novel disc replacement device proposed.