Umeå University's logo

Pathological processes in the alveolar and facial bones can lead to bone loss that may not heal with complete regeneration. Biomaterials can be used to facilitate the healing process and/or as a bone substitute, but the mechanisms are not fully understood. Persistent leakage of bacteria/bacterial toxins, after root canal treatment, may lead to a residual bone defect. The healing is dependent on a placed dental biomaterial providing a tight seal. The composition of the filling material may also influence the healing process.

The general aim of this study is to investigate surface properties and biological interactions of biomaterials used in dento-alveolar surgery. A dental biomaterial, a bonded compomer (DAP) containing a corroding glass filler, was used as a root end filling material, promoting a new operation technique. The healing (assessed according to Molven´s x-ray criteria) demonstrates a significant improvement in healing results for the compomer group, compared to a commonly used technique. The surface properties and biological interactions of DAP were analyzed. ICP-OES of DAP cell culture medium extract demonstrated a significant release of Sr, Si and F from the dental biomaterial. Human periodontal ligament (PDL) cells grew on and around DAP specimens without any sign of toxic reactions. DAP extract stimulated proliferation of PDL cells, but caused an inhibition of osteoblastic gene expression in mouse bone marrow cells. The surface properties of the glass containing compomer may contribute to improved healing of the periapical lesions.

A bovine inorganic bone graft substitute (BO) is commonly used as a treatment option in dento-alveolar surgery with new bone formation in immediate close contact with BO material. ICP-OES dissolution analysis of cell culture media, after incubation with BO particles, demonstrated a dosedependent release of Si and a decrease of Ca and P. An uptake of Ca from the medium to the BO particle was demonstrated with calcium-45 labeling. The Si dissolution varied between different batches, possibly reflecting a variation in food intake in the animals. Stimulated osteogenic response was seen in close contact to the BO particles in cell cultures. Furthermore, it was clearly demonstrated that the study design is a critical factor for correctly understanding biomaterials’ biological interactions.

The surface properties of three bone graft substitutes reported to have good results in dento-alveolar surgery were investigated, in order to establish whether or not dissolution-precipitation reactions could contribute to the bone healing. Dissolution-precipitation extracts of BO, bioactive glass 45S5 (BG) and a marine algae hydroxyl apatite (AP) in cell culture media were analyzed. Dissolution of Si at significant levels was detected for BO and 45S5 over time. Significant uptake levels of Ca and P from the culture were seen for both 45S5, BO and AP but at different times. Surface analysis of the biomaterials with SEM/EDAX, before and after immersion in cell culture media, revealed a smoothing of the surface morphology for 45S5 over time. No obvious alterations for BO and AP were detected. Ca/P ratio decreased significantly for 45S5, but no major changes were detected by XPS for BO or AP. XPS further demonstrated a surface charge for BO, changing from negatively to positively charged when exposed to serum. 45S5 and AP had positive surface charges, both in the absence and the presence of serum. These demonstrated surface changes in biomaterials could contribute to adherence of cells and subsequently affect bone healing.

Conclusion: Biomaterials used in dento-alveolar surgery interact with biological surroundings through surface and dissolution-precipitation reactions which may have implications for bone healing.

Bone graft substitute materials are becoming more common as alternative therapy strategies when bone defects in patients need to be treated. The interaction between bone cells and biomaterials occur at the surface of the materials. A great deal is known about the importance of surface topography and physic-chemical properties of biomaterials. It is also known that cells require proteins in order to interact with biomaterials. Less is known about how material properties and proteins interact forming the biological interface that cells will be exposed to, and that might determine if new bone is formed or not in the patient. The overall aim of the present thesis was to systematically investigate bone graft substitute material surface reactions and the interface in order to better understand how biomaterials may promote bone formation. Bio-Oss (BO) is a commonly used bone graft substitute material in reconstruction of periodontal and dentoalveolar bone defects. BO is mainly considered to be “osteoconductive”, but we could show that it does interact with a biological fluid (α-MEM cell culture medium) through dissolution/precipitation reactions. A significant reduction of calcium and phosphate levels in the medium was obtained even with low concentrations of BO. A release of silicon from the material was also demonstrated. An osteogenic response was seen in close contact to the BO particles when cultured with different types of pre-osteoblastic cells (Paper I). X-Ray Photoelectron Spectroscopy (XPS) with fast-frozen sample technique was used to further characterize the surface of BO, Frios Algipore (AP) and 45S5 Bioglass (BG). These three bone graft substitute materials are used as “model systems”, because they have all demonstrated newly formed bone on the surface after implantation in patients. From the XPS analysis it can be concluded that AP and BG acquired a positively charged surface while BO gained a negatively charged surface. Only AP and BG adsorb organic components (amino acids) from the medium (Paper II). Next we investigated initial surface reactions and the formation of a biological interface in the presence of proteins (serum) for the three biomaterials. The major findings were that in the presence of proteins BO underwent a surface charge reversal, all three biomaterials adsorbed proteins on the surface and all three biomaterials altered the chemical composition of the cell culture medium (Paper III-IV). Silicon (Si), which was released from BO as well as from BG, is interesting in relation to bone health. Positive effects of BG Si dissolution products on osteoblasts have been reported earlier. In the present study inhibitory interactions of Si on the RANK/RANKL/OPG signaling pathway as well as with gap junction intercellular communication in vitro are reported. These new findings implicate that Si could potentially be beneficial for patients with imbalance in bone remodeling (osteoporosis) and treatments of bone defects (Paper V). In conclusion, biomaterials of different origins interact with a solution resembling the extracellular tissue fluid. The dissolution-precipitation reactions are influenced by the material concentration used and should be taken into consideration when designing experiments and when biomaterials are used clinically. The presence of proteins will influence surface reactions, the formation of the biological interface and have implications on cellular responses. Possible dissolution products from the biomaterials should be investigated.  Si, a dissolution product, is shown to have an inhibitory effect on osteoclastogenesis and bone resorption in vitro. Potential clinical value of Si in treatment of patients with bone defects should be further investigated.