Experimental Studies on Non-Metallic Composite Bone Implants With a Special Reference to Staphylococcal Biofilm Infection

Non-metallic implants made of bioresorbable or biostable synthetic polymers are attractive options in many surgical procedures, ranging from bioresorbable suture anchors of arthroscopic surgery to reconstructive skull implants made of biostable fiber-reinforced composites. Among other benefits, non-metallic implants produce less interference in imaging. Bioresorbable polymer implants may be true multifunctional, serving as osteoconductive scaffolds and as matrices for simultaneous delivery of bone enhancement agents. As a major advantage for loading conditions, mechanical properties of biostable fiber-reinforced composites can be matched with those of the bone. Unsolved problems of these biomaterials are related to the risk of staphylococcal biofilm infections and to the low osteoconductivity of contemporary bioresorbable composite implants. This thesis was focused on the research and development of a multifunctional implant model with enhanced osteoconductivity and low susceptibility to infection. In addition, the experimental models for assessment, diagnostics and prophylaxis of biomaterial-related infections were established. The first experiment (Study I) established an in vitro method for simultaneous evaluation of calcium phosphate and biofilm formation on bisphenol-Aglycidyldimethacrylate and triethylenglycoldimethacrylate (BisGMA-TEGDMA) thermosets with different content of bioactive glass 45S5. The second experiment (Study II) showed no significant difference in osteointegration of nanostructured and microsized polylactide-co-glycolide/β-tricalcium phosphate (PLGA /β-TCP) composites in a minipig model. The third experiment (Study III) demonstrated that positron emission tomography (PET) imaging with the novel 68Ga labelled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) CD33 related sialic-acid immunoglobulin like lectins (Siglec-9) tracer was able to detect inflammatory response to S. epidermidis and S. aureus peri-implant infections in an intraosseous polytetrafluoroethylene catheter model. In the fourth experiment (Study IV), BisGMATEGDMA thermosets coated with lactose-modified chitosan (Chitlac) and silver nanoparticles exhibited antibacterial activity against S. aureus and P. aeruginosa strains in an in vitro biofilm model and showed in vivo biocompatibility in a minipig model. In the last experiment (Study V), a selective androgen modulator (SARM) released from a poly(lactide)-co-ε-caprolactone (PLCL) polymer matrix failed to produce a dose-dependent enhancement of peri-implant osteogenesis in a bone marrow ablation model.

PET Imaging of Osteomyelitis. Feasibility of 18F-FDG, 68Ga-Chloride and 68Ga-DOTAVAP-P1 Tracers in Staphylococcal Bone Infections

Due to technical restrictions of the database system the title of the thesis does not show corretly on this page. Numbers in the title are in superscript. Please see the PDF-file for correct title. ---- Osteomyelitis is a progressive inflammatory disease of bone and bone marrow that results in bone destruction due to an infective microorganism, most frequently Staphylococcus aureus. Orthopaedic concern relates to the need for reconstructive and trauma-related surgical procedures in the fast grow¬ing population of fragile, aged patients, who have an increased susceptibility to surgical site infections. Depending on the type of osteomyelitis, infection may be acute or a slowly progressing, low-grade infection. Peri-implant infections lead to implant loosening. The emerging antibiotic resistance of com¬mon pathogens further complicates the situation. With current imaging methods, significant limitations exist in the diagnosing of osteomyelitis and implant-related infections. Positron emission tomography (PET) with a glucose analogue, 18F-fluoro¬deoxyglucose (18F-FDG), seems to facilitate a more accurate diagnosis of chronic osteomyelitis. The method is based on the increased glucose consumption of activated inflammatory cells. Unfortunately, 18F-FDG accumulates also in sterile inflammation regions and causes false-positive findings, for exam¬ple, due to post-operative healing processes. Therefore, there is a clinical need for new, more infection-specific tracers. In addition, it is still unknown why 18F-FDG PET imaging is less accurate in the detec¬tion of periprosthetic joint infections, most frequently due to Staphylococcus epidermidis. This doctoral thesis focused on testing novel PET tracers (68Ga-chloride and 68Ga-DOTAVAP-P1) for early detections of bone infections and evaluated the role of pathogen-related factors in the appli¬cations of 18F-FDG PET in the diagnostics of bone infections. For preclinical models of S. epidermidis and S. aureus bone/implant infections, the significance of the causative pathogen was studied with respect to 18F-FDG uptake. In a retrospective analysis of patients with confirmed bone infections, the significance of the presence or absence of positive bacterial cultures on 18F-FDG uptake was evalu¬ated. 18F-FDG and 68Ga-chloride resulted in a similar uptake in S. aureus osteomyelitic bones. However, 68Ga-chloride did not show uptake in healing bones, and therefore it may be a more-specific tracer in the early post-operative or post-traumatic phase. 68Ga-DOTAVAP-P1, a novel synthetic peptide bind¬ing to vascular adhesion protein 1 (VAP-1), was able to detect the phase of inflammation in healing bones, but the uptake of the tracer was elevated also in osteomyelitis. Low-grade peri-implant infec¬tions due to S. epidermidis were characterized by a low uptake of 18F-FDG, which reflects the virulence of the causative pathogen and the degree of leukocyte infiltration. In the clinical study, no relationship was found between the level of 18F-FDG uptake and the presence of positive or negative bacterial cul¬tures. Thus 18F-FDG PET may help to confirm metabolically active infection process in patients with culture-negative, histologically confirmed, low-grade osteomyelitis.

Reconstruction of cranial bone defects with fiber-reinforced composite–bioactive glass implants

A cranial bone defect may result after an operative treatment of trauma, infection, vascular insult, or tumor. New biomaterials for cranial bone defect reconstructions are needed for example to mimic the biomechanical properties and structure of cranial bone. A novel glass fiber-reinforced composite implant with bioactive glass particulates (FRC–BG, fiber-reinforced composite–bioactive glass) has osteointegrative potential in a preclinical setting. The aim of the first and second study was to investigate the functionality of a FRC–BG implant in the reconstruction of cranial bone defects. During the years 2007–2014, a prospective clinical trial was conducted in two tertiary level academic institutions (Turku University Hospital and Oulu University Hospital) to evaluate the treatment outcome in 35 patients that underwent a FRC–BG cranioplasty. The treatment outcome was good both in adult and pediatric patients. A number of conventional complications related to cranioplasty were observed. In the third study, a retrospective outcome evaluation of 100 cranioplasty procedures performed in Turku University Hospital between years 2002–2012 was conducted. The experimental fourth study was conducted to test the load-bearing capacity and fracture behavior of FRC–BG implants under static loading. The interconnective bars in the implant structure markedly increased the load-bearing capacity of the implant. A loading test did not demonstrate any protrusions of glass fibers or fiber cut. The fracture type was buckling and delamination. In this study, a postoperative complication requiring a reoperation or removal of the cranioplasty material was observed in one out of five cranioplasty patients. The treatment outcomes of cranioplasty performed with different synthetic materials did not show significant difference when compared with autograft. The FRC–BG implant was demonstrated to be safe and biocompatible biomaterial for large cranial bone defect reconstructions in adult and pediatric patients.