Alumina ceramics: the preferred bioceramic material for bone tissue repair and regeneration
Ceramics used as biomaterials to fill defects in teeth and bones, fix bone grafts, fractures or prostheses to bones, and replace diseased tissues are called bioceramics. They are widely used in the medical field due to their excellent properties such as high strength, wear resistance, higher compressive and flexural strength, and high biocompatibility.Bioceramics first emerged in the 19th century. At that time, a kind of absorbable ceramic—plaster of Paris—was used in experiments and clinical practices, which greatly stimulated scholars' interest in the field of bioceramics. From the early to the middle of the 20th century, American scholar Talbert made granular ceramic materials (alumina ceramics) into prostheses and implanted them into the femurs of adult dogs, and finally achieved success. Alumina ceramics also attracted the attention of many scientific researchers.
①Alumina ceramic
The concept of alumina ceramics includes a wide range. In addition to pure alumina ceramics, any ceramic material with an alumina content of 45% or more can be called alumina ceramics. There are many homogeneous and heterogeneous crystals in alumina ceramics, but currently the most commonly used ones are only α - Al2O3 and γ - Al2O3. Due to their different crystal structures, they have different properties. Among them, α - Al2O3, also known as corundum, is the main crystalline phase of alumina ceramics, with high mechanical strength, high temperature resistance, and corrosion resistance.
It is generally believed that products with an alumina content greater than 99.9% are high-purity alumina. High-purity alumina has excellent properties such as high melting point, high hardness, high electrical resistance, excellent catalytic performance, good mechanical properties, wear resistance, corrosion resistance, insulation and heat resistance.The use of high-purity alumina polycrystals as biofunctional materials in the human body began in 1969. There are two types of high-purity alumina fine ceramics used in medical engineering: single crystals and sintered polycrystals. Single-crystal alumina has high strength and good wear resistance, and can be made into fracture fixators, artificial tooth roots, etc. after processing. Polycrystalline alumina, characterized by high strength, can be used in the production of joints, artificial tooth roots, artificial bones, double-cup artificial hip joints, etc.
②Application of alumina ceramics in artificial joints
In 1972, Boutin reported on the fabrication of human hip joints from alumina ceramics and their clinical applications, among other aspects. In 1977, Shikata et al. developed a hip joint prosthesis composed of an alumina ceramic femoral head combined with a high-molecular-weight polyethylene acetabulum. In 1982, the U.S. Food and Drug Administration (FDA) formally approved the clinical application in the United States of artificial hip joints consisting of Al₂O₃ ceramic balls, acetabula, and CoCrMo alloy stems.
High-purity alumina ceramics have a very low friction coefficient, high hardness, and good wettability, making them well-suited for use as joint friction surfaces. According to the regulations of the U.S. Food and Drug Administration (FDA), only high-purity alumina can be used in the medical field, and impurities that can form glassy grain boundary phases (such as silicon dioxide, metal silicates, and alkali metal oxides) must be less than 0.1wt%. This is because the degradation of such impurities can lead to the formation of stress concentration areas, where cracks will occur. Studies have found that by selecting appropriate sintering parameters (temperature, time, heating/cooling rate) and doping additives (such as magnesium oxide, zirconium oxide, and chromium oxide), the grain size and porosity of alumina can be controlled, which can effectively improve the toughness and fracture strength of alumina.
Composites formed by zirconia and alumina are called zirconia-toughened alumina (ZTA) or alumina-toughened zirconia (ATZ), and they also play an important role in artificial joint materials. These two composites specifically depend on the content of the main components. They combine the toughening ability of zirconia and the low sensitivity of alumina to degradation in low-temperature biological fluids. According to the design requirements of the material, ATZ can be selected when high fracture toughness needs to be emphasized, while ZTA can be used when hardness needs to be highlighted. Currently, there is insufficient clinical data to show that the load-bearing surface of ZTA joints has greater advantages in terms of wear resistance. Studies have shown that the application of ZTA and zirconia-based toughened alumina (ZPTA) in joint surgery is far greater than that of ATZ.
③Application of alumina ceramics in oral restoration
Alumina ceramics have translucency and color matching natural teeth, with weak toxicity. The low thermal conductivity of alumina ceramics is significant, which reduces the stimulation of cold and hot food on the dental pulp. Zirconia ceramics have remarkable wear resistance, corrosion resistance, and high-temperature resistance, with a color similar to natural teeth, making them suitable for dental restoration and having high strength. According to the differences in the phase composition and manufacturing process of alumina ceramic materials, the alumina ceramics used in the field of all-ceramic restoration can be divided into the following categories:
(1) Glass-infiltrated alumina ceramics
Glass infiltration, whose full name is the slip-casting glass infiltration method. Alumina, as the matrix material, presents a porous structure, and the lanthanum-boron-silicon glass containing colorants infiltrates into it. After forming, it has a microstructure where alumina crystal phases and glass crystal phases interpenetrate. Glass-infiltrated alumina ceramics have high mechanical strength, with a flexural strength of 250-600 MPa and a fracture toughness of 3-4 MPa·m¹/². A representative product is the base crown of the In-Ceram Alumina system from Vita (a German company), which is also the first all-ceramic restoration system capable of manufacturing three-unit bridges in the posterior tooth area.
(2) High-purity dense sintered fully alumina ceramics
Composed of alumina with a purity of up to 99.9%, alumina powder is pressed into a green body under extremely high pressure (dry pressing forming) and then sintered. The pressure forming method endows alumina ceramics with high density and low porosity. This ceramic material can achieve a flexural strength of 500-700 MPa and a fracture toughness of 5-6 MPa·m¹/², so it can be used clinically as a bridge structure in the posterior tooth area.
(3) Glass-infiltrated zirconia-toughened alumina ceramics
This type of ceramic is formed by adding 35% partially stabilized zirconia to glass-infiltrated alumina ceramic powder. After forming, uniformly distributed tetragonal phase zirconia can be observed inside the material. It is also the ceramic material with the highest strength in the alumina ceramic series. Due to the poor translucency of zirconia-toughened alumina ceramics, they are generally used clinically for posterior tooth restoration where aesthetic requirements are not high.