The Future of Dental Implants

The German chemist M. H. Klaproth discovered zirconium dioxide in 1789 although this "miracle material" with its outstanding properties has only been re-discovered in the last few decades. For instance, various types of zirconium dioxide have been introduced to dentistry as a substitute for metal. This material is attractive because of its extraordinary properties such as high flexural strength (in excess of 1,000 MPa), hardness (1,200 – 1,400 Vickers) and Weibull modulus (10-12). Yttrium partially stabilises zirconium oxide to provide these positive properties. Adding aluminium oxide boosts the flexural strength of the zirconium dioxide alloy once again. Zirconium dioxide is used for manufacturing kitchen knives, industrial cutting tools and components under great thermomechanic stress in the automobile and aircraft industry. However, it is not only very strong, it is also biocompatible so that zirconium dioxide is also used in medicine (hearing devices and artificial fingers and hips) and dentistry (pins, crowns, bridges and implants). The fact that zirconium dioxide has the same colour as teeth along with its biotechnical characteristics mean it is used for manufacturing biocompatible, high-quality and aesthetic tooth and implant reconstructions. There have only been animal experiments and laboratory examinations on applying dental zirconium oxide implants to date, meaning no long-term data exists on the clinical application of these implants.

Manufacturing Zirconium Dioxide

The mineral zirconium (ZrSiO4) is the main raw material for zirconium dioxide while melting it with coke and lime (reducing the SiO2) produces ZrO2 for industrial uses. Since extremely pure constituents have to be used for producing high-performance ceramics, special ways to synthesise it have been developed for high-purity ZrO2. This includes production with reactions in molten salts, reactions in the gaseous phase, hydrothermal powder synthesis and the sol-gel process. Gaseous phase and sol-gel process production provides powder at very small particle sizes ranging from 0.01 to 0.10 µm. This powder is then mixed with additives to create what are known as green bodies with film casting, slip casting or drying pressing. We distinguish additives such as sintering additives (that have a specific effect on the sintering behaviour and the properties of finished ceramics) and auxiliary materials that facilitate shaping. While the sintering additives stay in the ceramics, all residues of the auxiliary materials (mostly slightly volatile organic compounds along with water) are removed from the moulded component before the sintering process. The green body is passed into the raw product by sintering and ground or polished depending upon use. The sintering process can be carried out at atmospheric pressure and under high pressure and it is only with the sintering process that the moulded components receive their actual properties. The ceramic powder particles are compressed by lowering the specific surface with temperature-dependant diffusion processes with alternating components of surface, particle size grading and volume diffusion. If solid body diffusion is too slow, sintering can also be carried out with a liquid phase or under pressure, the latter being called hot pressing or hot isostatic pressing (the HIP process). The velocity of solid body diffusion can be boosted with the right selection of sintering additives. A great deal of research needs to be done here since the high sintering temperatures (in excess of 1,200° C) and manufacturing under pressure causes production costs for ceramic components to shoot up. Along with providing systematic clarification of the impact that additives have on the sintering process, there are also attempts to enhance power transmission onto ceramic components by coupling in microwaves for lowering sintering temperatures.

ZrO2 Ceramics

The properties of ZrO2 ceramics substantially pivot on the chemical composition of the material and the manufacturing process. We distinguish fully stabilised ZrO2 (FSZ „fully stabilized zirconia“) and partially stabilised ZrO2 (PSZ „partially stabilized zirconia“). It can be partially stabilised by adding 3-6% CaO, MgO or Y2O3 and depending upon the conditions of manufacturing this stabilises the cubic, tetragonal or monocline modification. Partially stabilised ZrO2 demonstrates high thermal fatigue resistance, meaning it fills the bill for use as high-temperature mechanoceramics. Adding 10-15% CaO, MgO or Y2O3 also allows cubic modification of the zirconium dioxide from absolute zero to the solidus (FSZ) and the ceramic material is thermally and mechanically stable to a temperature of 2,600°. However, its low caloric conductivity and higher thermal expansion factor as compared with partially stabilised ZrO2 mean that the thermal fatigue resistance of the fully stabilised zirconium dioxide is lower. The zirconium dioxide that is suited to use as an implant has the following composition: 95% ZrO2 + 5% Y2O3.