Novel Materials and Implant Designs for the Treatment of Degenerative Disc Disease

Abstract: The overarching objective of this thesis was to develop intervertebral implants for degenerated discs through design, fabrication, and mechanical validation. The research had four primary aims, each addressing different facets of implant development.The first aim was to design a structurally optimized fusion intervertebral cage capable of accommodating weak bioactive materials. Topology optimization was employed to design cages using titanium and calcium phosphate. The cage’s integrity was verified using finite element simulations, fabricated using additive manufacturing, and validated using ASTM F2077. Imaging techniques were utilized to assess the quality of the produced cages. These evaluations confirmed the mechanical reliability of the produced cage, able to incorporate around 75% in volume of a bioactive calcium phosphate material, protected by the titanium.The second aim was to develop a clinically relevant degeneration model for the biomechanical evaluation of percutaneous cement discoplasty (PCD). To this end, voids were generated in ovine functional spinal units (FSUs) using papain. The results were compared with clinical data where parameters such as void volume percentage, disc height, and morphology of the induced injury were assessed. FSUs were treated with PCD using bone cement, and mechanically evaluated under healthy, injured, and treated conditions to determine if PCD could stabilize the spinal segment. The void induced showed similar parameters compared to the clinical data. Further, the stability of the spine was significantly reduced after degeneration and restored after treatment, highlighting the effectiveness of the degeneration method and PCD treatment.The third aim was to evaluate the suitability of novel bone cements for their use in PCD. This study examined the tensile and fatigue properties of a low-modulus cement (VSLA) primarily intended for vertebroplasty. The formulation was tested in tensile and fatigue. VSLA showed a significant decrease in tensile and fatigue properties when compared to commercial cements. This study set a baseline for future low-modulus cements that may be tested for use in PCD.The fourth aim was to evaluate an alternative cement due to the low viscosity of VSLA, which may not be suitable for discoplasty. This study assessed the fatigue and long-term properties of a high-viscosity low-modulus cement (hv-LA-PMMA). The hv-LA-PMMA showed a significant reduction of mechanical and fatigue properties when compared to its commercial base. Notably, the fatigue properties were similar to those of the annulus fibrosus in the disc. Additionally, its high viscosity renders it a promising alternative to the bone cements currently utilized in PCD.In conclusion, this thesis successfully addressed the design, fabrication, and mechanical validation of two types of intervertebral implants for degenerated discs. The research outcomes contribute with valuable insights into the design of fusion cages, a degeneration model to evaluate PCD, and the assessment of low-modulus cements for use in PCD.