Surface Modification of Dental Implant Improves Implant–Tissue Interface
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The implant material must have optimum surface compatibility with the host epithelial tissue, connective tissue, and bone tissue. Because, dental implants, which are partially exposed to the oral cavity, must have firm contact with tissues to prevent the bacterial infection. Such materials can be created under well-controlled conditions by modifying the surfaces that contact these tissues. The rough and grooved surfaced implant contributes to a more rapid cell migration and make osseointegration during wound healing. A number of chemical and physical methods for titanium and/or zirconium surface modification have already been established. Recently, plasma treatment can control surface physiochemical properties and affect protein adsorption for bioengineering. Moreover, the “motif-programming” methodology to “biologically” modify titanium and zirconium surfaces has created interfacing artificial proteins that endowed those surfaces with cell-binding activity. These technique should improve firm contact between tissue and dental implant.
KeywordsDental implant Surface modification Tissue interface
3.1 Dental Implant–Tissue Interface
Fibroblast face to the implant surface differentiates into osteoblasts also during the process of wound healing. The osteoblast deposits bone matrix on the implant surface, becomes calcified, and completes osseointegration which is complicatedly associated with the implant materials. It is known that a direct bond between implant and surrounding bone has been demonstrated with implants made of bioactive materials, i.e. bio-glasses and calcium phosphate ceramics [5, 6].
In view of the direct bone-implant contact, it plays important roles of surface geometry and surface chemistry of the implant material, and cell behavior surrounding implant.
3.2 Effect of the Surface Geometry
3.3 Control of Surface Chemistry
Surface chemistry involves the adsorption of proteins and cells on biomaterials. This adsorption reflects the affinity between two substances, and the strength of adsorption follows the order: chemical adsorption including covalent bonds and ionic bonds > electrostatic force found in electrokinetic potential or zeta potential > hydrogen bonds involved in hydrophilic groups such as –OH, –COOH, and –NH2 > hydrophobic interaction (i.e., adsorption of hydrophobic substances in water) > van der Waals forces. Adsorption characteristics are primarily evaluated by hydrophobicity (wettability), which can be determined by measuring the surface energy (contact angle), and electrokinetic potential (zeta potential, isoelectric point), which reflects surface electric charges and these affect creation of firm integration between implant and cells.
3.4 Protein Application
As for the surface chemistry, methods of modifying the titanium surface using adhesive proteins such as osteonectin, fibronectin or laminin-5 compatible with the soft tissue/implant interface have been proposed. For the implant surface in contact with subepithelial connective tissues, tresyl chloride treatment is used to adhere the selected proteins such as fibronectin to the amino residues . Thus the gingival epithelium attached to dental implants through the formation of hemidesmosomes using laminin-5 . However, a stable coating and prevention of protein denaturation at the time of implantation are necessary using motif-programming or plasma treatment.
3.5 Application of Motif-Programming
3.6 Plasma Treatment of Implant Surface
3.7 Calcium Phosphate (Ca-P) Coating by Plasma Spraying
Ca-P implants, including hydroxyapatite (HA), are well known for good osteoconductivity (the early stage of osteogenesis) as well as for direct binding to bone tissue in vivo. Alkaline phosphatase expression and parathyroid hormone response were higher in cultures grown in HA than in cultures grown in titanium  and the in vitro formation of extracellular matrices was greater on Ca-P coatings than on titanium.
In spite of their rapid and strong bonds to living bone tissues and favorable osteogenic ability, Ca-P ceramics alone cannot be used for implants because of their lack of strength. Accordingly, Ca-P coatings on Ti implants produced by the plasma spraying have frequently been used . These Ca-P coated implants, however, often develop fractures in their coatings as well as at the titanium interface after implantation. The reason for this is thought to originate in the comparatively thick, porous, non-uniform (crystalline surrounded by an amorphous mass), and poorly adherent Ca-P layer produced by plasma spraying. These fragments of a certain size cause phagocytosis by macrophages, leading to inflammation. It is therefore desirable for the materials to be rapidly and completely absorbed in the host tissues and to be entirely replaced with bone tissue. When osteogenesis occurs at the site where old bones are absorbed (remodeling of bones), the Ca-P coatings should be no thicker than necessary.
3.8 Thin Ca-P Coatings
Attempts have recently been made to solve problems, the cold plasma, ion-plating  and the ion sputtering , which are a kind of physical vapor deposition (PVD), are used to produce implant materials consisting of a thin, homogeneous, and adherent Ca-P coating. Ion beam dynamic mixing (IBDM) was also introduced as a suitable technique for fabricating a thin and adherent ceramic layer . This method is a combination of ion implantation and PVD, and has the advantages of a high deposition rate, producing defect-free transparent thin films, and excellent adhesion compared to conventional thin-film deposition techniques.
3.9 Future of Dental Implant
The next generation should be the surface modification of any of the materials for bio-functionalization of dental implants. Such materials can be created under well-controlled conditions by modifying the surfaces of metals that contact those tissues. “Tissue-compatible implants,” which are compatible with all host tissues, must integrate with bone tissue, easily form hemidesmosomes, and prevent biofilm accumulation.
This study was supported by Oral Health Science Center Grant 5A10, 5A08 from Tokyo Dental College, by MEXT. HITEKU (2002–2006), and by a Grant-in-Aid for Scientific Research No. 07672123, 10671845, 10085839, 15592065 and 14207093 from The Ministry of Education, Culture, Sports, Science and Technology in Japan.
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