The effects of porosity in selective laser melted titanium interbody cages and bone mineral density on subsidence

A Biomechanical Study

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Degenerative disc disorders are among the most commonly diagnosed conditions and the leading cause for back and neck pain, affecting more than 266 million individuals annually. Interbody fusion is the proposed surgical treatment whenever lighter approaches such as physiotherapy and painkillers fail to improve the patient’s condition. It consists of the total or partial removal of the damaged disc, followed by the insertion of a spinal implant, referred to as interbody fusion cage or spacer, to block the vertebral segment and restore disc height. Unfortunately, in some cases this treatment can fail due to a loss in disc height as a result of subsidence, that is the sinking of the cage in the adjacent vertebrae. The investigation conducted in this thesis work focused on the effects of porosity of selective laser melted (SLM) implants and bone mineral density on subsidence, by means of a biomechanical study and a finite element model. In previous works, it was demonstrated that the most influential factor for the occurrence of subsidence is the condition of bone mineral density. Therefore, it was hypothesized that by using an implant with high porosity, the overall stiffness of the bone-implant system would decrease, as well as the stresses at the bone-implant interface, reducing the risk of subsidence and bone damage. At the same time, osseointegration would benefit due to the increased porosity, which is an important factor in the design of orthopaedic implants. The biomechanical study consisted of compression tests according to the standard ASTM-F2267 for subsidence evaluation in interbody fusion implants, according to which polyurethane foams were used to simulate the mechanical behaviour of trabecular bone. Cages designated for the cervical and lumbar regions of the spine were tested in two different porosities, in terms of apparent density of the cage, in combination with three foam densities, low-, medium- and high-density, corresponding respectively to low, average and above-average quality of bone. The results confirmed the findings in the literature and revealed significantly different outcomes when the foam density was changed, while the change in porosity did not affect the stiffness of the foam-implant system. The finite element model of a simplified cervical cage was used to explore the influence of the thickness of the solid frame in porous implants, revealing changes in stiffness of the simulations with the cages alone, while almost no stiffness variations were detected in the simulations for the implant-foam systems. Although the reduction of the stiffness of the system could not be achieved by tuning porosity, it was demonstrated that the mechanical performance of the system was not affected by an implant with higher porosity. These findings opened new research opportunities in the study of osseointegration, since it could be concluded that the use
of implants with higher porosity would not affect the overall stiffness, while allowing more bone ingrowth to avoid cage migration.