Cellular responses to implant materials: biological, physical and chemical factors.

Adhesion of bone and epithelial cells to the dental implant are vital to its retention in alveolar bone and to the prevention of infection via its 'gingival' margin. Studies of cytotoxicity, tissue irritability and carcinogenicity of implantable polymers, metals and ceramics and of tissue adhesion to them have been carried out in tissue culture and in animal experiments. The more similar the polymeric materials are chemically to living tissue the more easily are they dissolved and digested in the host. Therefore, implant materials having a molecular structure similar to protein or polysaccharide, e.g. Nylon, cannot be expected to function. On the other hand, silicones, polyethylene and Teflon (polytetrafluroethylene), which have molecular structures completely different from living substances, are generally more stable in the tissues. However, these polymers are hydrophobic and have little adhesion to living cells in spite of their high stability. They are not, therefore, suitable materials for the construction of implants. Studies on antithrombotic polymers have demonstrated the possibility of creating implantable polymers which have high stability as well as strong adhesion to the surrounding tissues. These properties may be conferred by grafting a hydrophilic polymer on to the surface of a hydrophobic polymer. Of the metals, Ti, Zr and Ta are fairly stable in living tissue, and allow cells to adhere strongly. Alloys of Co-Cr-Mo, Fe-Ni-Cr-Mo, Ti-Al-V, Ti-Mo, Ti-Pd and Ti-Pt deserve to be better evaluated because they are low in density, have high mechanical strength, stability and corrosion resistance in living tissue, and there is direct adhesion to the surrounding tissues. Biodegradable or bioactive ceramics which induce bone formation around the implant do not have sufficient mechanical strength. Implant ceramics have to be stable, e.g. crystal alumina, vitreous carbon, synthetic hydroxypatite and silicon nitrate. These exhibit high biocompatibility and excellent adhesion to tissue. Single crystal sapphire ceramics with high mechanical strength permit the delicate designs required for implants. Success with dental implants may depend upon combining the rigid retention of porous stable alloys or ceramics of suitable Young's modulus with a stress absorbing superstructure. A further development to be expected is the appearance of composite and polyphase materials which have tissue adhesiveness, stability in living tissues and various degrees of Young's modulus.