| 1. |
- Zhou, Yijun, 1993-
(författare)
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Mechanical optimization of orthopaedic bone screw constructs
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Orthopaedic screw implants are crucial in surgical procedures for bone structures, with a longstanding issue being screw loosening. Improving screw stability requires an enhanced understanding of the bone-implant interactions and associated failure mechanisms. This knowledge can be used to optimize screw designs.In this thesis, a first step towards this end was taken through the development of enhanced trabecular bone models. Due to the trabecular bone's complex structure and the difficulties in obtaining micro-structural information, we proposed a numerical model based on Voronoi tessellation to mimic the trabecular bone morphology across varying porosities. This model's mechanical properties aligned well with analytical formulas and finite element modelling of real bone specimens, showing strong agreement with experimental results.Further investigation into screw-trabecular bone interaction was carried out using two numerical models. The explicit finite element models were able to replicate experimental screw push-in results. While only one of the models accounted for the screw insertion step, both models showed strong congruence with key experimental results. The two-step simulation however led to a more physically plausible Young’s modulus for trabecular bone material.Utilizing a validated numerical model, a numerical optimization process was initiated, where novel screw designs were proposed. Several neural network surrogate models were introduced, reducing evaluation costs while maintaining prediction accuracy. We found that screw insertion position, trabecular bone porosity, and orientation were significant factors, explaining about 96% of the variance in predicted response. Furthermore, a neural network workflow was developed to generate super-resolution trabecular bone models from clinical CT data, improving accuracy by up to 700% both morphologically and mechanically, using micro-CT models as a benchmark.Lastly, the potential enhancement of screw's primary stability was explored, by injection of hyaluronic acid mixed with hydroxyapatite particles. The augmentation effect was influenced by the hydroxyapatite particles' size and shape. Considering that these particles can also promote bone growth, a particle-laden hydrogel injection could potentially enhance screw stability throughout its lifespan.In conclusion, this study proposed methods to elucidate bone-implant interaction and enhance screw stability in trabecular bone, encompassing trabecular bone modelling, bone-screw interaction investigations, screw geometric optimization, super-resolution trabecular bone model generation from clinical CT data, and hydrogel optimization for stability. The results provide an enhanced understanding as well as optimization of complex geometric interactions, and could lead to future enhancements of clinical practice in terms of screw stability.
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| 2. |
- Carlsson, Jenny, 1984-
(författare)
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On Crack Dynamics in Brittle Heterogeneous Materials
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Natural variation, sub-structural features, heterogeneity and porosity make fracture modelling of wood and many other heterogeneous and cellular materials challenging. In this thesis, fracture in such complicated materials is simulated using phase field methods for fracture. Phase field methods have shown promise in simulations of complex geometries as well as dynamics and require few additional parameters; only the material toughness and a length parameter, determining the width of a regularised crack, are needed.First, a dynamic phase field model is developed and validated against experiments performed on homogeneous brittle polymeric materials, wood fibre composites and polymeric materials with different hole patterns. Then, a high-resolution model of wood is developed and related to experiments, this time without considering fracture. Attention is finally focussed on high-resolution numerical analyses of fracture in wood and other cellular microstructures, considering both heterogeneity and relative density.The phase field model is found to reproduce crack paths, velocities and energy release rates well in homogeneous samples both with and without holes. In more complicated heterogeneous and porous materials, the model is also able to simulate crack paths, but the interpretation of the length scale is complicated by the inherent lengths of the micro-structural geometry. In sum, the thesis points to possibilities with the proposed method, as well as limitations in our current understanding of both quasistatic and dynamic fracture of heterogeneous and cellular materials. The findings of this thesis can contribute to an improved understanding of fracture in such materials.
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| 3. |
- Corrêa Filho, Luimar N., 1985-
(författare)
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Functional properties of silicon nitride based materials for joint applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Total hip and total knee replacements are generally successful procedures for patients suffering with pain due to bone fracture or diseases affecting the joints. However, the materials that are commonly used still have limitations. In particular, corrosion products and wear debris may give rise to negative body reactions.In this thesis, silicon nitride based materials were investigated for use in joint implants, namely as a coating for e.g. femoral heads and the metallic modular taper junction in hip implants, as well as a bulk bioceramic for joint applications. One of the main advantages of these materials is the potential to dissolve slowly in aqueous solutions, releasing only biocompatible ions.To understand the mechanical and wear properties of these materials, thin film coatings were deposited using magnetron-based techniques onto Si wafers and a CoCrMo alloy, the latter frequently used in biomedical implants. Coatings up to 8.8 µm thick were deposited on 2D flat discs as well as full 3D implant heads, following a CrN interlayer for improved adhesion. The chemical composition, microstructure, surface roughness, adhesion, wear resistance, and dissolution properties of the coatings were evaluated as a function of substrate rotation, bias voltage, target power as well as the addition of different elements.Results show that it is possible to produce coatings with mechanical properties and a wear performance similar to bulk ceramics and other ceramic coatings already evaluated in vivo. It was evident that a high coating density is needed, to avoid premature failure in an in vivo environment. The coating density, and stability over time in solution, was found to increase when a higher target power and process heating were used.New bulk silicon nitride materials containing only biocompatible additives, were evaluated for potential use in joint applications by wear tests for the first time, showing very low wear rates of the counter material.Silicon nitride coatings and bulk materials tested in this work showed promising results for further investigation and a basis for future application in joint bearings.
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| 4. |
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| 5. |
- Ghandour, Salim
(författare)
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Novel Materials and Implant Designs for the Treatment of Degenerative Disc Disease
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 6. |
- Lewin, Susanne
(författare)
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Functional Aspects of Cranial Implants : Mechanical and Regenerative Properties
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In several neurosurgical procedures, the skull must be temporarily opened. The resulting bone defect can subsequently be reconstructed with a cranial implant. However, the complication rate of this surgical procedure is high (~20%). The most common complication for cranial implants is infection. Currently, the most frequently used implant materials are titanium alloys, PMMA or PEEK. An improved clinical outcome – in terms of increased bone regeneration, vascularization and soft tissue compatibility – could possibly be obtained through the use of bioactive and osteoconductive materials such as calcium phosphates (CaP).This thesis focuses on CaP–titanium composite (CaP–Ti) implants. This recently developed implant type is increasingly used with a promising outcome. However, a thorough understanding of its functional properties is lacking, something that is of high importance for their clinical use, but also for future biomaterial development. The overall aim of this thesis is to increase the knowledge of the in vivo functional aspects of CaP–Ti composite implants, with a specific focus on the mechanical and regenerative properties.The mechanical properties of the implant were investigated experimentally and numerically at quasi-static and impact loading rates. An important finding was that the titanium structure makes the CaP–Ti implant capable of cerebral protection in impact situations comparable to the one that was tested. Moreover, the mechanical response of the CaP–Ti implants could be predicted by the developed numerical models at both quasi-static and impact loading rates. The developed numerical framework makes an important contribution to future evaluations of patient-specific CaP–Ti cranial implant designs in various loading scenarios. A comparison of two additive manufacturing (3D-printing) processes demonstrated that lower geometrical accuracy and higher surface roughness made electron beam produced implants inferior in terms of mechanical strength, as compared to laser melted implants.In order to assess the regenerative properties, the volumetric balance of the implant was investigated by CT in ten patients. After one year, the total volume of the implant had decreased – mainly at the outside of the implants in the direction of the scalp. However, all patients had a volumetric increase at the interface between the implant and the bone defect. In a histological analysis of a retrieval specimen from one of the patients, the volumetric increase could be confirmed as bone regeneration, and the decrease as CaP degradation. Remodeling of the CaP into bone was also observed, but was not detected in the clinical CT. In retrieval specimens from an animal study, it was found that correlation of some µCT cross-sections to histology can result in improved and more robust quantitative µCT evaluations.
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| 7. |
- Bang, Le T., et al.
(författare)
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Synthesis and assessment of metallic ion migration through a novel calcium carbonate coating for biomedical implants
- 2020
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Ingår i: Journal of Biomedical Materials Research. Part B - Applied biomaterials. - : Wiley. - 1552-4973 .- 1552-4981. ; 108:2, s. 429-438
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Tidskriftsartikel (refereegranskat)abstract
- Titanium (Ti) implants are commonly regarded as well accepted by the body. However, metal ion release is still a cause for concern. A small decrease in pH, which can be caused by inflammation, may produce a large increase in the corrosion rate of Ti implants. Coating the alloy with a buffer layer could have a significant protective effect. In this study, a calcium carbonate coating was developed on commercially pure Ti and a Ti-6Al-4V alloy through a hydrothermal treatment of previously NaOH-treated surfaces in calcium-citric acid chelate complexes. The results showed that a superstructured calcite coating layer formed on the Ti substrate after treatment at 170 degrees C for 3 hr. The coating was approx. 1 mu m thick and covered the substrate surface uniformly. When prolonging the hydrothermal treatment from 5 hr to 24 hr, the rhombohedral structure of calcite was observed in addition to the superstructure of calcite. Dissolution test results showed no significant differences in solution pH between the coated- and un-coated samples. However, the CaCO3 coating reduced by approx. 2-5 times the Ti and V ion release from the substrate as compared to the uncoated material, at pH 4. CaCO3 and hydroxyapatite (HA) coatings gave nonsignificant effects at neutral pH although the HA coating showed a trend for better results at the longer time points. The reduction in metal ion release from the substrate and the buffering ability of the CaCO3 coating encourage further studies on this coating for clinical applications.
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| 8. |
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| 9. |
- Katsaros, Ioannis, et al.
(författare)
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Bioactive Silicon Nitride Implant Surfaces with Maintained Antibacterial Properties
- 2022
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Ingår i: Journal of Functional Biomaterials. - : MDPI. - 2079-4983. ; 13:3
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Tidskriftsartikel (refereegranskat)abstract
- Silicon nitride (Si3N4) is a promising biomaterial, currently used in spinal fusion implants. Such implants should result in high vertebral union rates without major complications. However, pseudarthrosis remains an important complication that could lead to a need for implant replacement. Making silicon nitride implants more bioactive could lead to higher fusion rates, and reduce the incidence of pseudarthrosis. In this study, it was hypothesized that creating a highly negatively charged Si3N4 surface would enhance its bioactivity without affecting the antibacterial nature of the material. To this end, samples were thermally, chemically, and thermochemically treated. Apatite formation was examined for a 21-day immersion period as an in-vitro estimate of bioactivity. Staphylococcus aureus bacteria were inoculated on the surface of the samples, and their viability was investigated. It was found that the thermochemically and chemically treated samples exhibited enhanced bioactivity, as demonstrated by the increased spontaneous formation of apatite on their surface. All modified samples showed a reduction in the bacterial population; however, no statistically significant differences were noticed between groups. This study successfully demonstrated a simple method to improve the in vitro bioactivity of Si3N4 implants while maintaining the bacteriostatic properties.
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| 10. |
- Montgomery, Cecilia
(författare)
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Early identification of motor problems in very preterm infants : An evaluation of the Structured Observation of Motor Performance in Infants
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Infants born very preterm are at risk of adverse neurodevelopment. It is important to identify motor problems early to initiate interventions aiming at ameliorating outcomes. Evaluating motor development in high-risk infants is a complex task. There is a need for assessment methods for early identification of abnormal motor performance. The aim of this thesis was to evalute the Structured Observation of Motor Performance in Infants (SOMP-I) method for early identification of motor problems in very preterm children and to investigate early motor performance in relation to neonatal characteristics, cerebral imaging and later outcome. Level of motor development and quality of motor performance was assessed at 2, 4, 6, and 10 months’ of corrected age. Study I validated the revised SOMP-I, and compared early motor performance in 111 very preterm infants with 72 full-term infants. The preterm infants were more delayed and had more quality deficits than the term infants, and the groups had different motor trajectories. We concluded that convergent validity and discriminant validity of the SOMP-I was supported and facilitated early identification of infants with atypical motor development.Study II investigated SOMP-I results in relation to motor outcome (Bayley-III motor index at 2.5 years) in 98 very preterm children. The 28 children with delayed development had significantly poorer SOMP-I scores in infancy. We concluded that level and quality of motor performance were significant markers of later motor problems and quality became more significant with increasing age. Study III investigated early motor performance (SOMP-I), in relation to neurodevelopment and motor competence at 12 years (Movement ABC-2) in 78 very preterm children. At all assessment ages, there were significant associations between SOMP-I and MABC-2 scores. At 6 and 10 months, SOMP-I level and quality scores separately explained unique variance of the MABC-2 scores at 12 years. Study IV explored the relation between neonatal cerebral MRI (morphology, apparent diffusion coefficient, regional brain volumes) and 4-month motor performance (SOMP-I), in relation to 2-year motor outcome in 66 very preterm infants (11 with motor problems). SOMP-I results correlated with several MRI measures and with motor outcome. The level of motor performance had the highest predictive value for motor outcome. Overall conclusion: The two SOMP-I domains, level and quality, explain unique variances towards later motor outcomes, meaning that the two separate domains give added value to the motor assessment and are useful markers of motor outcome in very preterm infants.
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