Thesis--Wisconsin. / Vita. Includes bibliographical references (leaves 187-207).

published_or_final_version / Orthopaedics and Traumatology / Master / Master of Philosophy

Post-operative osteomyelitis attributing to the biofilm formation on implant surface and medical grade 316L stainless steel have been reported to gain a higher rate of infection among other clinically applied metals. It is believed suppressing bacterial adhesion on implant surface at early stages can help prevent biofilm formation. The major challenges of current antibacterial surface treatments include limited biocompatibility, potential development of antibiotic resistant bacteria, short life cycle and high fabrication cost. In this study, it is aimed to explore the feasibility of an inexpensive and simple surface modification technique to achieve a long-term antibacterial effect on medical grade 316L stainless steel while maintaining its biocompatibility. Thus, a novel 1D nanostructured ZnO nanowire coating that can provide different special topographies and can be easily fabricated by simple hydrothermal method is suggested to coat on stainless steel surfaces. Two kinds of ZnO nanowire coatings, ZnO_5hrs and ZnO_17hrs, are fabricated for further investigation. Relatively well-aligned ZnO nanowires with diameters of ~50 nm were found on ZnO_5hrs samples, while randomly-oriented ZnO nanowires with diameters of ~150 nm were found on ZnO_17hrs samples. In the antibacterial tests, both ZnO_5hrs and ZnO_17hrs samples exhibited excellent antibacterial effects, which represent over 90% of bacterial reduction among all of the tested bacterial strains including S. aureus, P. aeruginosa and E. coli, with exception to the case of ZnO_17hrs sample with S. aureus. It is confirmed that antibacterial Zn2+ ions are released from the coatings during the test and help against bacterial adhesion. On the other hand, it is suspected that the increase in hydrophilicity and special physical topography are also antibacterial factors of the ZnO nanowire coatings. The cytocompatibilities in both ZnO_5hrs and ZnO_17hrs samples were not satisfactory. In the cell adhesion test, the GFP-OB cells did not habitually spread and attach on the treated sample surfaces after 6 hours incubation. Cytotoxicity test results further confirm no viable MC3T3 cells were found on the treated sample surfaces. The cytocompatibility of the coating remains to be improved. In the in-vivo study, the group of rats with ZnO_5hrs rod samples displayed a reduced number of bacterial cells in the implantation site at day 0, as well as a shorter duration (within 8 days) for bacterial termination as compared to that with untreated stainless steel rod samples. The presence of ZnO nanowire coating on medical grade 316L stainless steel rod samples demonstrates the in vivo antibacterial effect. In short, the novel 1D antibacterial ZnO nanowire coating is successfully fabricated and coated on medical grade 316L stainless steel surfaces by a simple and inexpensive hydrothermal method. However, the biocompatibility of the ZnO nanowire coating remains to be improved. One of the critical issues is to engineer the coating in order to precisely control the Zn2+ ions release rate. For future study, the key is to find out how to manipulate the characteristics of special surface topography, together with a controllable release of Zn2+ ions on the ZnO nanowire coating to maximize the antibacterial effect while maintaining the original biocompatibility of medical grade 316L stainless steel. / published_or_final_version / Orthopaedics and Traumatology / Master / Master of Philosophy

In mammalian cells, cyclin dependent kinase (CDK) is subjected to negative regulation by a conserved kinase, Wee1. In budding yeast, S. cerevisiae, Swe1 kinase, the Wee1 homologue, phosphorylates and inactivates Cdk1 associated with mitotic cyclins to prevent entry into mitosis. Both transcription and proteolysis ensure the periodic appearance of Swe1 protein during the cell cycle. To facilitate Swe1 protein degradation, the functional Hsl1-Hsl7 protein complex recruits Swe1 to the bud-neck. As a kinase, Hsl1 phosphorylates Hsl7 directly and this phosphorylation is essential for Swe1 degradation. Hsl1 itself is also a phosphoprotein and Elm1 kinase has been shown to be responsible for Hsl1 phosphorylation. Protein phosphatase 2A (PP2A) is a critical phosphatase involved in multiple cellular events. Previous work indicates that the absence of the B-regulatory subunit Cdc55 results in accumulation of Swe1 protein, which leads to the abnormal bud morphology. We present evidence indicating that the failure of Swe1 degradation in cdc55 mutants is a consequence of defective Hsl1-Hsl7 pathway. We found that the phosphorylation of both Hsl7 and Hsl1 deceased obviously in cdc55 mutant, suggesting the compromised kinase activity of Elm1. Moreover, casein kinases are also required for the phosphorylation and activation of Hsl1. / A Thesis submitted to the Department of Biomedical Science in partial fulfillment of the requirements for the degree of Master of Science. / Degree Awarded: Summer Semester, 2010. / Date of Defense: July 15, 2010. / PP2A, Cdc55, Yck1, Yck2, Swe1 / Includes bibliographical references. / Yanchang Wang, Professor Directing Thesis; Wuming Deng, University Representative; Mary Hurt, Committee Member; Akash Gunjan, Committee Member; Hongguo Yu, Committee Member.

Mesenchymal stem cells (MSCs) have the ability to differentiate towards cartilage, bone, fat and muscle, and therefore have great therapeutic potential. Human MSCs reside under hypoxia (4-7% oxygen) in vivo and recent investigations have described an increase in population doublings and maintenance of differentiation capacity upon culture under these conditions. The reason for these differences may be related to their cellular metabolism. This thesis examines MSC metabolism during proliferation, chondrogenic and osteogenic differentiation. Furthermore, MSCs were cultured under uninterrupted and controlled hypoxia (5% or 2% oxygen) to observe its effect on proliferation and differentiation. The production of lactate and consumption of oxygen by MSCs indicated a mixed metabolism, with the cells utilising both oxidative phosphorylation and glycolysis under 20% oxygen or normoxic conditions. The majority of cellular ATP production was through glycolysis, whilst the full oxidative capacity of the mitochondria was not fully utilised. During chondrogenic differentiation, oxygen consumption was significantly reduced with time in culture and the cells became highly glycolytic, whilst osteogenic differentiation maintained the mixed metabolism of expanded MSCs. Fewer colonies were formed at 5% oxygen, compared to 2% and 20% oxygen. Hypoxic culture induced fewer cells per colony compared with normoxia. However, MSCs expanded under hypoxia had reduced cellular senescence after five passages, potentially due to the reduced utilisation of oxidative phosphorylation that has been shown to lead to the production of reactive oxygen species (ROS). The oxygen levels during expansion did not affect chondrogenic potential but expansion under hypoxia prevented ostoegenic differentiation. Osteogenesis was also inhibited for MSCs expanded at normoxia and differentiated under hypoxia. These results may be related to the initial colony formation under hypoxia and the changes in cellular metabolism during osteogenic differentiation.

Cartilage is a type of connective tissue which has very little potential for spontaneous natural repair when damaged. The use of tissue engineering for the treatment of articular cartilage has been investigated for a number of years. However, due to its unique biomechanical properties and functional requirements, the neo-tissues produced thus far have proved to be of limited long-term functionality. One avenue being investigated is the use of cell-seeded scaffolds as cartilage templates. Chitosan is a natural, abundant polysaccharide, biocompatible and biodegradable, with the ability to form open, porous and interconnected networks. It has the potential to fulfil many of the requirements of a cartilage tissue engineering scaffold material. A procedure was developed to form glutaraldehyde cross-linked chitosan hydrogels under relatively mild conditions and seed them with bovine articular chondrocytes. The cylindrical scaffolds formed were subjected to several investigations, including FTIR, microstructure analysis via pycnometry and porosimetry, static compression, biochemical analysis and a number of microscopic techniques, including confocal microscopy, optical microscopy and SEM. These investigations have shown that the chondrocytes migrate readily into the chitosan hydrogel structure within 1 hour of seeding, attach within 1 day, and maintain a well-defined, rounded morphology for up to 28 days in culture. The GAG and DNA data implies that the chitosan hydrogels were not cytotoxic towards the chondrocytes and the growth rate corresponds to previous studies suggesting chitosan partially inhibits the cell cycle in G1 phase. The microstructure analysis suggests that the main limitation of the hydrogels produced in this work, from a cartilage tissue engineering standpoint, is their relatively low porosity. The compression data shows that these hydrogels behave as visco-elastic polymeric materials. However, the variation in the results suggests that the quality of the hydrogel is of grave importance and specimen selection should be carried out with great care.

Current golden standard therapy for bone repair and regeneration involves the use of auto grafts. Nevertheless, there are many drawbacks associated with auto grafts including donor site morbidity, requirement for an invasive surgery, post-operative pain and infection. The use of injectable tissue engineered bone is an attractive alternative, providing a minimally invasive approach to regenerate bone. It offers faster healing, less pain and exact conformation to irregular defects. The present work is designed to achieve injectable formulations of tissue engineered bone that fulfil the requirements needed. It involves investigation of potential polymeric binders that are biocompatible, biodegradable and allow bone formation when combined with cells. Chitosan binders were tested for biocompatibility, biodegradability, gelation, angiogenic potential and osteogenic differentiation and bone formation when mixed with goat and human bone marrow derived mesenchymal stem cells (gMSCs, hMSCs). An in vivo bone formation study was performed to investigate the bone formation ability of gMSCs in contact with chitosan binder. Chick chorioallantoic membrane assay was carried out to examine the angiogenic potential of the chitosan binder combined with/without hMSCs. Furthermore, MC3T3-El cells were employed to assess the osteogenic potential of cells exposed to chitosan polymeric systems. Chitosan binder was proved to be an attractive polymer to carry cell-scaffold combination. hMSCs were able to survive and differentiate along the osteogenic lineage when encapsulated with 1.5% (w/v) chitosan-15% (w/v) glycerol phosphate (GP)-0.18% (w/v) hydroxyethyl cellulose (HEC) in a 14-day study. Furthermore, chitosan-GP-HEC solutions demonstrated fast gelation at 37°C. Chitosan was biodegradable following 42 days in the presence/absence of lysozyme. Moreover, gMSCs combined with chitosan binder produced 24.6 ± 13.7% bone comparable to the control group after a 6-week implantation in mice. Chitosan was shown to be nonangiogenic unlike hMSCs which showed angiogenic potential. Also, chitosan was found to be osteogenic at 2 and 0.05 mg/ml concentrations.

16 New Zealand White (NZW) rabbits were ovariectomised (OVX) and 20 left intact as age-matched controls. 4 controls were killed at time 0, then 4 from each group at 1,3,6 and 12 months post-OVX. Serum oestradiol and body weight were recorded. Post-mortem of the OVX animals confirmed that all ovarian tissue had been removed. Femoral and humeral diaphyses were tested in 4-point bending, metatarsals in torsion and cancellous bone from the femora and humeri in compression. Oestradiol levels were not altered by OVX and there were no significant reductions in strength, stiffness or density of cancellous or cortical bone 12 months post-OVX, except that OVX femoral strength was significantly lower than control strength (p < 0.05). In conclusion, the species was not a suitable model for post-menopausal osteoporosis. Due to the failure of the rabbit model, it was necessary to revert to the rat model of post-menopausal osteoporosis. Two mechanical tests were used to investigate the efficacy of an anti-resorptive therapy (Oestradiol-3-benzoate) and an anabolic therapy (Parathyroid Hormone). An indentation test examined the properties of cancellous bone material, while the femoral neck bending test examined the properties of the bone as an organ by reproducing the in vivo events leading to a shear fracture of the hip. Biomechanical variables were related to Bone Mineral Density, a measure of bone mass currently in use in clinical practice, by scanning the bones using peripheral quantitative computed tomography (pQCT) before mechanical testing took place. It was found that cancellous bone density and indentation strength decreased and cortical bone density increased 6 weeks post-OVX: high levels of oestradiol-3- benzoate prevented these changes . Total bone mineral density was strongly correlated with indentation strength (r=0.92). Parathyroid hormone did not restore cancellous bone density of OVX to sham levels but restored biomechanical strength.

Over the last three decades tribological studies of polyethylene total hip replacements have been undertaken using a simplified model of normal walking. As hip prostheses are being implanted in younger and more active patients, coupled with the increased wear resistance of crosslinked polyethylene, such in vitro approximations in patient activity are limiting. Therefore an alternative wear testing methodology for total hip replacements has been proposed, measuring the influences of fast walking, stumbling and simulated jogging sequences, all at varying cycle speeds with both smooth and roughened femoral components. This hip simulator study has shown that the influence of femoral roughness on the wear of crosslinked polyethylene becomes significantly greater under increased patient activity, demonstrating that roughness may be a more influential factor than previously ascribed. The combined effects of high roughness (Re of 0.38 μm), high joint forces (4.5 kN max) and high sliding speed (1.75 Hz) generated excessive crosslinked polyethylene wear and high joint torque, with wear rates exceeding 3000 mm3/106 cycles (k = 50 x10-6 mm3/N m). Thus for more active patients, implant survival can be greatly increased by using harder and more damage resistant femoral heads compared to CoCrMo. Under smooth conditions however, the overall influence of a significant increase in patient activity showed a much weaker effect. It was found that with smooth heads and non-constraining socket fixtures, the occurrence of excessive stumbling at 1 Hz (5 kN max) had a negligible effect on the wear of crosslinked polyethylene, whilst simulated jogging at 1.75 Hz (4.5 kN max) only showed a median increase in wear volume of 40 % compared to normal walking. Fast walking produced the largest wear rate (53 mm3/106 cycles), and was consistently greater than for simulated jogging. Ignoring fixation and other factors, these results suggest that whilst preserving polished surfaces, short periods of increased patient activity, for example, aerobics, tennis etc, will not greatly reduce the survival of crosslinked polyethylene/metal implants. Sliding speed and the degree of socket clamping were shown to be the most influential factors under smooth conditions, with the results showing no significant differences in wear rate when testing in varying quantities of bovine serum, or using an inverted or physiological specimen orientation.

Hydroxyapatite(HA) is a highly biocompatible calcium phosphate material which in porous form, promotes rapid bone ingrowth and revascularisation. As such it has potential for use as a synthetic bone graft substitute. However, due to poor mechanical strength, its use has been limited to non-major load bearing applications. In response, secondary phase additions such as calcium/phosphate-based glasses have been used to reinforce HA. However, the improved mechanical properties obtained by secondary-phase reinforcement are often associated with decomposition of the HA to tricalcium phosphate(TCP), which may be undesirable due to the increased solubility and controversial biocompatibility of the latter. The aim of this thesis was to produce a calcium/phosphate-based additive for reinforcing HA and to investigate the mechanical and chemical stability of this composite in a physiological environment. Furthermore, the possibility of transferring this technology to porous structures was investigated. Prior to investigating the effects of second phase addition on strength and phase stability, the HA used in this study was characterized as having a biaxial flexural strength (BFS) of 65+11MPa and being chemically stable up to sintering temperatures of 1350°C. Two calcium/phosphate-based additives were produced with Ca/P ratios of 0.5 (CAP I) and 0.8 (CAP2); CAP1 was found to be amorphous, whilst CAP2 was predominantly crystalline in nature where the crystalline phase was primarily Ca2P2O7. The maximal BFS value found for HA doped with 2.5 wt% CAP I (CAPIHA) was 27MPa with up to 73%TCP, whilst the CAP2-doped HA (with 2.5 wt% CAP2) achieved a maximal BFS of 102±21MPa with up to 13%TCP. Thus CAP2HA was judged to be a successful composite suitable for more comprehensive investigation. Studies were carried out to decipher the ideal wt% of CAP2 to promote mechanical reinforcement with a minimal presence of TCP, using 1,2.5 , 3.25 ,4 and 5 wt % CAP2. The results indicated that 2.5 wt% CAP2HA was optimal in terms of both the mechanical and chemical criteria. For investigating mechanical and chemical stability, the HA and CAP2HA samples were soaked in 50% strength Ringer's solution for periods of 1-30 days. HA retained 60% of its original strength, whilst CAP2HA retained 78% of its original strength. In view of the success in using CAP2 as a reinforcing additive, a preliminary investigation was carried out using HA and CAP2HA. This involved development of a technique for producing porous HA, using a reticulated foam template, with a highly interconnected structure and mean porosities and strengths of 66% and 2MPa. Porous samples of CAP2HA were also successfully produced however, only mean porosities and strengths of 75% and 0.6MPa were achieved. This was attributed to processing complications arising from the solubility of Ca2P2O7in water leading to inhibited sintering, which may be overcome by the use of a different binder system.