Fixation of uncemented implants

My thesis postulates that bone ingrowth and direct bone apposition combined with implants engineered to produce interfacial strains lead to beneficial bone remodelling which may result in fixation of joints that will last for the patients life-time. The concept of extra-cortical plate fixation was investigated by assessing the bony response to plates of different design and surface coating. The study found that only one geometric design (holes) significantly increased bone ingrowth into the plate when compared with the control (p=0.01). A crystalline HA coating encouraged significantly greater interfacial contact when compared with a roughened titanium surface (p=0.01), a HA coating of lower crystallinity (p=0.004) and a solution precipitated HA coating (p=0.02). No significant differences were found when bone ingrowth into the plates were compared, except significantly more bone had grown into plates coated with a HA coating of lower crystallinity (p=0.036). Differences in bony reaction induced by the plates of different design were evident and therefore a combination of the correct design and surface coating are required for optimal bone attachment and ingrowth to extra-cortical plates. An experimental goat model was developed to investigate hydroxyapatite coated extra-cortical plate fixation in massive segmental bone tumour replacements. On retrieval, all of the plates were securely fixed by new bone. Bone apposition had occurred through a combination of periosteal bone production, invasion of bone through slots and bone growth over the ends of the plate. It was concluded that due to both mechanical and biological effects, extra-cortical plate fixation generated new bone growth that enhanced fixation and encouraged plate integration into cortical bone. The importance of the implant surface was demonstrated in a series of human autopsy retrieved hip implants. The proximal region of each implant was coated with either a plasma sprayed porous ingrowth surface (plain porous), a HA coated porous surface (porous HA) or a grit blasted surface. Significantly more bone ingrowth (p=0.012) and bone attachment (p<0.05) was measured to the porous HA surface when compared with the plain porous surface. There was no significant difference in bone attachment between the plain porous and grit blasted surfaces. A combination of a HA surface combined with extra-cortical plate fixation has been used to treat a number of bone tumour patients

Mechanisms of Bone Loss in Space

The unique conditions of microgravity and radiation exposure in space have a substantial impact on human tissue function, resulting in extreme bone loss. The goal of this project is to synthesize the current knowledge on bone loss in space for use in future research. There is clear evidence, based on past research, that microgravity, cosmic radiation, fluid flow, and mechanotransduction influence bone function during space travel and result in significant loss. However, there is a need for future research on how these factors relate to each other and their collective influence on mechanisms of bone regeneration and repair. In addition, there is a need for more research on potential solutions to prevent the bone loss seen during space travel

The effect of strontium and silicon substituted hydroxyapatite electrochemical coatings on bone ingrowth and osseointegration of selective laser sintered porous metal implants

Additive manufactured, porous bone implants have the potential to improve osseointegration and reduce failure rates of orthopaedic devices. Substantially porous implants are increasingly used in a number of orthopaedic applications. HA plasma spraying-a line of sight process-cannot coat the inner surfaces of substantially porous structures, whereas electrochemical deposition of calcium phosphate can fully coat the inner surfaces of porous implants for improved bioactivity, but the osseous response of different types of hydroxyapatite (HA) coatings with ionic substitutions has not been evaluated for implants in the same in vivo model. In this study, laser sintered Ti6Al4V implants with pore sizes of Ø 700 μm and Ø 1500 μm were electrochemically coated with HA, silicon-substituted HA (SiHA), and strontium-substituted HA (SrHA), and implanted in ovine femoral condylar defects. Implants were retrieved after 6 weeks and histological and histomorphometric evaluation were compared to electrochemically coated implants with uncoated and HA plasma sprayed controls. The HA, SiHA and SrHA coatings had Ca:P, Ca:(P+Si) and (Ca+Sr):P ratios of 1.53, 1.14 and 1.32 respectively. Electrochemically coated implants significantly promoted bone attachment to the implant surfaces of the inner pores and displayed improved osseointegration compared to uncoated scaffolds for both pore sizes (p<0.001), whereas bone ingrowth was restricted to the surface for HA plasma coated or uncoated implants. Electrochemically coated HA implants achieved the highest osseointegration, followed by SrHA coated implants, and both coatings exhibited significantly more bone growth than plasma sprayed groups (p≤0.01 for all 4 cases). SiHA had significantly more osseointegration when compared against the uncoated control, but no significant difference compared with other coatings. There was no significant difference in ingrowth or osseointegration between pore sizes, and the bone-implant-contact was significantly higher in the electrochemical HA than in SiHA or SrHA. These results suggest that osseointegration is insensitive to pore size, whereas surface modification through the presence of an osteoconductive coating plays an important role in improving osseointegration, which may be critically important for extensively porous implants

Mesenchymal stem cells with increased stromal cell-derived factor 1 expression enhanced fracture healing

Treatment of critical size bone defects pose a challenge in orthopedics. Stem cell therapy together with cytokines has the potential to improve bone repair as they cause the migration and homing of stem cells to the defect site. However, the engraftment, participation, and recruitment of other cells within the regenerating tissue are important. To enhance stem cell involvement, this study investigated overexpression of stem cells with stromal cell-derived factor 1 (SDF-1) using an adenovirus. We hypothesized that these engineered cells would effectively increase the migration of native cells to the site of fracture, enhancing bone repair. Before implantation, we showed that SDF-1 secreted by transfected cells increased the migration of nontransfected cells. In a rat defect bone model, bone marrow mesenchymal stem cells overexpressing SDF-1 showed significantly (p=0.003) more new bone formation within the gap and less bone mineral loss at the area adjacent to the defect site during the early bone healing stage. In conclusion, SDF-1 was shown to play an important role in accelerating fracture repair and contributing to bone repair in rat models, by recruiting more host stem cells to the defect site and encouraging osteogenic differentiation and production of bone

The effect of bone growth onto massive prostheses collars in protecting the implant from fracture

Limb-sparing distal femoral endoprotheses used in cancer patients have a high risk of aseptic loosening. It had been reported that young adolescent patients have a higher rate of loosening and fatigue fracture of intramedullary stems because the implant becomes undersized as patients grow. Extracortical bone growth into the grooved hydroxyapatite-coated collar had been shown to reduce failure rates. The stresses in the implant and femur have been calculated from Finite Element models for different stages of bone growth onto the collar. For a small diameter stem without any bone growth, a large stress concentration at the implant shoulder was found, leading to a significant fracture risk under normal walking loads. Bone growth and osseointergration onto the implant collar reduced the stress level in the implant to safe levels. For small bone bridges a risk of bone fracture was observed