M.Ing. (Mechanical Engineering) / A stand-alone anterior lumbar interbody fusion device is used to stabilise the spine and restore the disc space height without any other instrumentation. The stand-alone anterior lumbar interbody device is fixed to the adjacent vertebrae using titanium screws. In this research an experimental and numerical investigation on the structural strength of the SASCATM stand-alone anterior lumbar interbody fusion device are presented. The outcome of the investigation will be used as part of the device validation documentation necessary for market approval. The SASCATM device is manufactured from PEEK (a high strength polymer). Tensile and compressive testing was conducted to determine the appropriate mechanical properties of PEEK. The structural integrity of the SASCA device was evaluated by conducting full scale compression testing on a limited number of different design revisions. Comparisons as regards to their loaddisplacement behaviour were made. All specimens were visually inspected. The Finite Element Analysis (FEA) method was used in the numerical investigation of the SASCATM stand-alone anterior lumbar interbody device. Three studies were conducted. The first study aimed at comparing the full scale experimental compressive testing results with the FEA simulation. Although the desired results weren’t achieved, the model gave a fair representation of the initial region of the experimental setup in the sense that it had a similar slope. It was concluded that the nominal stress (4.1 MPa) fell within the proportional limit (35 MPa) as measured during the materials testing. The second study was aimed at determining the displacement at a worst-case load determined from the literature (2.7 kN). The study showed that the maximum Von Mises stress does not exceed the yield strength of the material. The third and final (parametric) study aimed at geometric optimisation of the cages. The motivation for the changes was based on the literature and customer suggestions for improvement. The geometric optimisation intended to show whether a desired increase in graft hole size would have an effect on the structural integrity of the device. The suggestion to move the screw holes of the threehole version closer to the center of the cage was also assessed. It was shown that enlarging the two graft holes does have an effect on the compressive strength. Higher stresses were presented in all but one case. Combining the holes also had an effect on the compressive strength. Movement of the screw holes more medially did have an impact on the compressive strength of the cages. The effect was significant. The closer the holes were to the center of the cage, the higher the Von Mises stress was. This change should therefore be considered before implementation. The results showed that different shapes and sizes of the graft holes do have an impact on the stress of this particular cage. None of the models exceeded the compressive yield strength of the material. The proposed graft hole opening design changes are therefore not warranted for the current SASCATM stand-alone anterior lumbar interbody device.

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Chronic or degenerative diseases affecting the lumbar spine, commonly referred to as low back pain (LPB), are a major cause of dysfunction, pain, and disability worldwide. According to the Global Burden of Disease (GBD) report of 2019, LPB affects over half a billion people, severely limiting their well-being and lifestyle. Unfortunately, these numbers have been steadily increasing over the last decade, with a rise of more than 15%, mainly due to demographic aging of the population, making it a significant socioeconomic global issue. When conservative treatments such as medications, drugs, and injections fail to alleviate the symptoms, surgical interventions become necessary. Spinal surgeries have become increasingly common and account for 40% of the top ten surgical procedures in the United States alone. As a result, the global market for spinal implants and medical orthopedic devices has been growing at a compound annual growth rate (CAGR) of 5.0% in the United States. Degenerative disc diseases, herniated intervertebral discs, and spondylolisthesis are among the most common problems requiring implant surgery, with lumbar interbody fusion cages or total disc replacements being the most common options. These surgical techniques often utilize a metal endplate or hollow cage as a load-bearing structure to ensure correct load transmission and biomechanical spinal functionality. Currently, endplates for total disc replacement are produced using subtractive manufacturing techniques from bulk biomedical-graded metal alloys like Ti-6Al-4V. The endplates are inserted between two adjacent vertebral bodies, where bone ingrowth and implant fusion are necessary. However, the elastic properties of bulk metals and bone tissue do not match, resulting in stress-shielding phenomena, implant loosening, or implant subsidence. These complications frequently necessitate surgical revision of the implant, which not only impacts the daily activities of the patients but also has a relevant economic impact. Therefore, researchers are exploring alternative design and manufacturing strategies to develop next-generation prosthetic devices that overcome these challenges. Metal additive manufacturing (MAM), particularly Laser-Powder Bed Fusion (L-PBF), has revolutionized the fabrication of specialized components with complex shapes, including architected cellular materials - a novel class of engineered materials with tunable mechanical properties. The biomedical field is a prime example of where lattice application has proved beneficial. MAM provides numerous advantages, including patient-specific customization, a vast design space, and reduced stress shielding. However, issues with structural integrity, lack of AM-specific norms, and the need for fine-tuning process optimizations are still hindering MAM's widespread adoption on the international market. An essential issue that requires resolution is the impact of process-induced flaws on the fatigue behavior of components made of L-PBF lattices. Despite a growing body of scientific literature on the fatigue behavior of lattice unit cells, little attention has been given to the function of fatigue at a millimetric scale, specifically the role of sub-unital lattice elements such as struts and junctions. As fatigue is highly localized, understanding primary fatigue behavior and fracture mechanisms at a strut scale may be critical to addressing the aforementioned problems. Moreover, designing proper prosthetic devices requires fulfilling both biomechanical and biological requirements, leading to a bottleneck in component quality. Proper tuning of osteointegration often requires large porosity and small strut dimensions, approaching the limits of industrial 3D printers. This increases the likelihood of manufacturing lattices with unconnected struts, drosses, parasitic masses, and severe deviations from the nominal as-designed geometries, leading to highly susceptible components under fatigue. To address these challenges, combined approaches with bone tissue engineering may be advantageous. Biopolymers from natural sources, such as silk fibroin and collagen derivatives (i.e., gelatin), are widely used for bone-filler applications due to their exceptional biological properties. These polymers can create highly interconnected biodegradable porous 3D scaffolds suitable for cell differentiation towards an osteogenic phenotype, such as in the form of foams. These foams can be embedded into metal lattice structures, resulting in a hybrid composite device that simultaneously fulfills the load-bearing, fatigue, and osteointegrative requirements that a spinal prosthetic device necessitates. This thesis work covers a range of topics mentioned above. Firstly, an introductory theoretical background is presented in Chapter I, followed by experimental findings which are presented in three different chapters. Chapter II is dedicated to the fatigue behavior of L-PBF Ti-6Al-4V sub-unital lattice elements in the form of miniaturized dog-bone specimens that mimic struts and nodes. This chapter is divided into four sections. The first section investigates the fatigue strength of strut-like specimens based on their building orientations at four different angles with respect to the printing job plate. Morphological features of the miniaturized specimens such as average and minimum cross-section, eccentricity, waviness, and surface texture are correlated with fatigue strength. The role of inner and surface defects, such as lack-of-fusion (LoF) and gas holes, is also considered to explain the main failure mechanisms. The impact of building orientation on the printing quality of the specimens is highlighted, with an increase in surface roughness and defectiveness as the printing angle decreases, resulting in a shorter fatigue life for miniaturized struts. In the second section, the fatigue effect is studied across different fatigue regimes. The role of the mean stress effect is assessed using the Haigh diagram, which reveals an increase in fatigue life moving towards compressive loading regimes. The effect of the printing angle is also investigated, showing different trends according to the different stress ratios, suggesting different fatigue failing mechanisms. The third section introduces strut-junction miniaturized specimens and evaluates their fatigue behavior according to building orientations. Horizontal specimens show an increased fatigue life compared to their thin strut counterparts, and different morphological outcomes are highlighted, including improved surface quality even at lower angles, possibly related to the node acting as an additional supporting structure. The fourth section presents a design-led compensation strategy for sub-unital lattice specimens, aimed at reducing as-designed/as-built deviations. This systematic decrease in geometrical mismatch suggests potential new design strategies for fatigue enhancement. In Chapter III, bone tissue engineering strategies are explored for the design of foam scaffolds as bio-fillers for lattice-based design. The feasibility of the polymer-metal composite is assessed, using an N2O-based gas foaming technique to fabricate silk fibroin and silk fibroin/gelatin porous scaffolds infilled into a cubic L-PBF Ti-6Al-4V lattice structure. The adhesion at the polymer/metal interface is assessed, with simultaneous electrowetting, showing promise for better and more intimate contact on the outermost surface of the lattice struts. A statistical-based analysis of the foam porosity is then carried out, aimed at optimization towards osteointegration improvement. Selected foams are biologically evaluated, revealing good cell adhesion and differentiation towards an osteogenic phenotype. Chapter IV reports on two different strategies for the design of a Ti-6AL-4V L-PBF lattice-based endplate for total disc replacement. The first strategy focuses on homogenized-based topology optimization, designing an octet-truss prosthetic device with a graded structure and a cell size suitable for polymeric infilling. The second strategy aims at optimizing octet-truss lattice components for fatigue, evaluating the optimal building orientation for the specimens. Experimental results reveal an improvement in the fatigue life of three-point bending test specimens, suggesting the potential of the proposed model. In Chapter V, the major takeaways of this thesis work are discussed, highlighting important advancements in understanding the fatigue behavior of lattice structures and the development of novel hybrid strategies for the design of biomedical devices, with a particular focus on spinal orthopedics. Future possible directions for research are also explored.

Novel surgical strategies for spinal disc repair are currently being developed that require materials that (1) possess the appropriate mechanical properties to mimic the tissue the material is replacing or repairing and (2) maintain their mechanical function for long durations without negatively affecting the tissue response of adjacent tissue (i.e. bone). Polymers formed through photopolymerization have emerged as candidate biomaterials for many biomedical applications, but these materials possess limited toughness in vivo due to the presence of water inherent in most tissues. Therefore, the overall objective of this research was to develop photopolymerizable (meth)acrylate networks that are both mechanically and biologically compatible under physiological conditions to be implemented in spinal repair procedures. The fundamental approach was to determine structure-property relationships between toughness and network structure in the presence of phosphate buffered saline (PBS) using several model copolymer networks in order to facilitate the design of photopolymerizable networks that are tough in physiological solution. It was demonstrated that networks toughness could be optimized in PBS by tailoring the Tg of the copolymer network close to body temperature and incorporating the appropriate "tough" chemical structures. The ability to maintain toughness up to 9 months in PBS was dependent upon the viscoelastic state and overall hydrophobicity of the network. In tandem, the effect of network chemistry and stiffness on the response of MG63 pre-osteoblast cells was assessed in vitro. The ability of MG63 cells to differentiate on (meth)acrylate network surfaces was found to be primarily dependent on surface chemistry with PEG-based materials promoting a more mature osteoblast phenotype than 2HEMA surfaces. Amongst each copolymer group, copolymer stiffness was found to regulate osteoblast differentiation in a manner dependent upon the surface chemistry. In general, photopolymerizable (meth)acrylate networks that were deemed "tough" were able to promote osteoblast differentiation in a manner comparable if not exceeding that on tissue culture polystyrene (TCPS). This research will impact the field of biomaterials by elucidating the interrelationships between materials science, mechanics, and biology.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Neck pain is a common phenomenon that occurs in a large percentage of the population every day. While many occurrences are not deemed critical such as those from muscle strain which can be treated with rest and pain medication, others due to sports injuries, whiplash from car accidents, bad posture or degeneration of the intervertebral disc can be quite severe. In extreme cases failure of the vertebra(e) or the intervertebral disc requires surgery and possibly the use of cervical implants. Where intervertebral discs fail due to herniation or Degenerative Disc Disease (DDD), Anterior Cervical Discectomy and Fusion (ACDF) is a common surgical method used to remove the a ected disc and replace it with a cervical cage implant. These implants are designed to restore the height between the vertebrae, allowing bone from both vertebrae to grow through them and mineralise. Additive Manufacturing (AM) technologies can produce parts with complex geometries not possible using conventional manufacturing methods. This design freedom, coupled with CT scans of a patient, allow for tailoring an implant to the speci c anatomy of the a ected vertebrae using CAD software. Such an approach must be regulated and shown to be technically and commercially feasible before it can be implemented in industry. This study sought to develop a framework for manufacturing customized cervical cage implants using additive manufacturing. The e cacy of customization to reduce the risk of subsidence was investigated by means of non-destructive and destructive mechanical testing on six cadaver specimens, using readily available PEEK cage implants as a benchmark. The results showed that the customized implant was comparable to the PEEK, with no statistically signi cant di erence between the two. In extreme cases, where PEEK implants cannot be used, customized implants could be a suitable alternative to reduce the risk of subsidence. A manufacturing cost analysis was conducted to determine economic feasibility. The estimated cost and selling price of the customized implants under various utilization scenarios and mark-ups was compared to readily available PEEK implants. The estimated selling prices of the customized implants compared favourably to the PEEK verifying the economic viability of using AM. / AFRIKAANSE OPSOMMING: Nek pyn is 'n algemene verskynsel wat daagliks na tevore kom in die bevolking. Baie gevalle word nie as krities geklasi seer nie soos byvoorbeeld spier pyn wat behandel kan word deur genoegsame rus en pyn medikasie. Pyn wat deur sportbeserings, sweepslag beserings 'whiplash' tydens motor ongelukke, verkeerde postuur, of deur slytasie van 'n intervertebrale skyf veroorsaak is, word dikwels as ernstig geklasi seer. In ekstreme gevalle waar die werwel(s) of die inervertebrale skyf(we) faal, sal chirurgie en servikale inplantate moontlik nodig wees. Waneer intervertebrale skywe faal weens herniatie of Degeneratiewe Skyf Siekte (DDD) kan 'n algemene chirurgiese metode, Anterieure Servikale Discectomie en Fusie (ACDF), gebruik word om die gea ekteerde skyf te verwyder en dit te vervang met 'n servikale samesmelting implantaat. Hierdie implantate herstel die hoogte tussen rugwerwels en is ontwerp sodat die been deur dit kan groei en mineraliseer. Komplekse geometrieë kan vervaardig word deur toevoegingsvervaardiging (AM) tegnologieë. Die ontwerp vryheid, gepaard met CT-skanderings en CAD-sagteware stel mens in staat om die geometrie van die implantaat aan te pas tot die spese eke anatomie van die gea ekteerde vertebra. So 'n benadering moet gereguleer word en eers tegnies en kommersieel uitvoerbaar bewys word voordat dit in die bedryf geïmplementeer kan word. Hierdie studie poog verder om 'n raamwerk vir die vervaardiging van persoonlike servikale implantate deur middel van toevoegingsvervaardiging te ontwikkel. Die doeltre endheid van persoonlike implantate om te verhoed dat die chirurg die eind-plaat beskadig, en sodoende die risiko van insakking te verminder, is ondersoek deur middel van meganiese toetse op ses kadawer monsters. Hierdie toetse is gedoen met behulp van geredelik beskikbaar PEEK servikale implantate as 'n maatstaf. Die resultate het getoon dat die persoonlike- en PEEK implantate vergelykbaar is. In moontlike gevalle waar PEEK implantate nie geskik sou wees nie, kan persoonlike implantate 'n alternatiewe opsie wees om die risiko van insakking te verminder.

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Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / This thesis is concerned with the simulation of the in vivo biomechanical performance of a cervical disc replacement. A representative (averaged) maximum range of motion (ROM), determined by measurement of 10 student participants (5 male, 5 female), was used as head motion input to a simulation model of the cervical spine containing a disc implant at the C5/C6 intervertebral level. Intradiscal pressure, relative applied force on the C5 vertebrae, bending moments and vertebral rotations were recorded. The force and motion components of the results obtained were critically evaluated against the ISO and ASTM experimental protocol standards, probing the representativeness of these standards to the actual in vivo behaviour of the cervical functional spinal unit. Further, the wear resulting from a lifetime (10 million cycles) of the ISO prescribed -and simulation determined input cycles was simulated using a linear wear model with a triangulation technique for volume lost due to wear, and compared to in vitro results in the literature. The inputs used for the wear model were determined from a validated non-linear static contact finite element method (FEM) model. The simulation “chain” shows great potential as a comparative tool for the preexperimental testing of spinal implant designs and may be used with relative success as an alternative to expensive prototype testing.