Alumina ceramic: A preferred bioceramic material for bone tissue repair and regeneration

　　Bioceramics, a type of ceramic biomaterial, are used to fill dental and bone defects, fix bone grafts, fractures, or prostheses to bones, and replace diseased tissues. Their widespread use in the medical field stems from their excellent properties, including high strength, wear resistance, superior compressive and flexural strength, and high biocompatibility. The earliest use of bioceramics dates back to the 19th century, when a resorbable ceramic—plaster of Paris—was used in experiments and clinical applications, greatly stimulating scholarly interest in the field. In the early to mid-20th century, American scholar Talbert successfully implanted particulate ceramic materials (alumina ceramics) in the form of prostheses into the femurs of adult dogs. Alumina ceramics subsequently attracted significant attention from researchers.
　　01 Alumina Ceramics
　　The concept of alumina ceramics encompasses a broad range. Besides pure alumina ceramics, any ceramic material with an alumina content exceeding 45% can be termed alumina ceramic. Alumina ceramics exist in numerous polymorphs, but currently, only α-Al2O3 and γ-Al2O3 are commonly used. Their differing crystal structures result in different properties. α-Al2O3, also known as corundum, is the main crystalline phase in alumina ceramics, exhibiting high mechanical strength, high-temperature resistance, and corrosion resistance.
　　Generally, products with more than 99.9% alumina content are considered high-purity alumina. High-purity alumina possesses excellent properties, including a high melting point, high hardness, high electrical resistance, superior catalytic performance, good mechanical properties, wear resistance, corrosion resistance, and insulation heat resistance. The use of high-purity alumina polycrystals as biofunctional materials in the human body began in 1969. High-purity alumina fine ceramics used in medical engineering include both single crystals and sintered polycrystals. Single-crystal alumina, with its high strength and excellent wear resistance, can be processed into fracture fixators and artificial roots. Polycrystalline alumina, characterized by high strength, is used in the fabrication of joints, artificial roots, artificial bones, and double-cup artificial hip joints.
　　02 Application of Alumina Ceramics in Artificial Joints
　　In 1972, Boutin reported on the fabrication of human hip joints from alumina ceramics and their clinical applications. In 1977, Shikata et al. developed a hip joint prosthesis consisting of an alumina ceramic femoral head and a high-molecular-weight polyethylene acetabulum. In 1982, the U.S. Food and Drug Administration (FDA) officially approved the clinical use in the United States of artificial hip joints composed of Al2O3 ceramic balls and sockets and CoCrMo alloy stems.
　　High-purity alumina ceramics, with their very low coefficient of friction, high hardness, and good wettability, are well-suited for use as joint friction surfaces. According to the U.S. Food and Drug Administration (FDA), only high-purity alumina can be used in the medical field. Impurities that can form glassy grain boundary phases (such as silicon dioxide, metal silicates, and alkali metal oxides) must be below 0.1 wt%, as their degradation can lead to stress concentration sites and crack formation. Research has shown that controlling alumina grain size and porosity by selecting appropriate sintering parameters (temperature, time, heating/cooling rate) and doping with additives (such as magnesium oxide, zirconium oxide, and chromium oxide) can effectively improve the toughness and fracture strength of alumina.
　　Composite materials formed using zirconia and alumina are known as zirconia-toughened alumina (ZTA) or alumina-toughened zirconia (ATZ), and they also hold significant importance in artificial joint materials. The specific properties of these two composite materials depend on the content of the main component. These composites combine the toughening ability of zirconia with the low sensitivity to degradation of alumina in low-temperature biofluids. Depending on the design requirements of the material, ATZ can be chosen when high fracture toughness is needed, while ZTA is preferred for hardness. Currently, there is insufficient clinical data to demonstrate a greater advantage of ZTA joint bearing surfaces in terms of wear resistance. Studies have shown that ZTA and zirconia-based toughened alumina (ZPTA) are used far more extensively in joint surgery than ATZ.
　　03 Application of Alumina Ceramics in Oral Restoration
　　Alumina ceramics possess translucency and color matching well with natural teeth, and exhibit minimal toxicity. Their low thermal conductivity significantly reduces the stimulation of the dental pulp by hot and cold foods. Zirconia ceramics are notable for their wear resistance, corrosion resistance, and high-temperature resistance, and their color is similar to natural teeth, making them suitable for tooth restoration, with higher strength. Based on the phase composition and manufacturing process of alumina ceramic materials, alumina ceramics used in all-ceramic restoration can be classified as follows:
　　(1) Glass-infiltrated Alumina Ceramics
　　Glass infiltration, also known as slurry coating glass infiltration. Alumina, as the matrix material, exhibits a porous structure. A lanthanum-boron-silicon glass containing colorants is infiltrated into it. After forming, the microstructure consists of alumina and glass phases interpenetrating each other. Glass-infiltrated alumina ceramics have high mechanical strength, with flexural strength reaching 250-600 MPa and fracture toughness of 3-4 MPa.m1/2. A representative product is the base crown of the In-Ceram Alumina system from Vita (Germany), which was also the first all-ceramic restoration system capable of producing three-unit bridges in the posterior region.
　　(2) High-Purity Densely Sintered Alumina Ceramics
　　Composed of alumina with a purity of 99.9%, these ceramics are formed by pressing alumina powder into a preform under high pressure (dry pressing) followed by sintering. The pressing method gives the alumina ceramics high density and low porosity. The flexural strength of this ceramic material can reach 500-700 MPa, and the fracture toughness can reach 5-6 MPa.m1/2, allowing for clinical use as bridge structures in the posterior region. A representative product is the base crown of the Procera ALLCream system from Nobel Biocare (Sweden).
　　(3) Glass-Infiltrated Zirconia-Toughened Alumina Ceramics
　　This type of ceramic is formed by adding 35% partially stabilized zirconia to alumina ceramic powder that has permeated glass. Uniformly distributed tetragonal zirconia can be observed inside the formed material. This is also the strongest ceramic material in the alumina ceramic series. Due to the poor translucency of zirconia-toughened alumina ceramics, they are generally used in posterior teeth restoration in clinical settings where aesthetic requirements are not high. A representative product is the In-Ceram Zirconia system base crown from Vita, Germany.
　　Summary:
　　In medical engineering, the application of high-purity alumina ceramics has made significant progress. With the advent of various alumina medical products, alumina ceramics have been applied to artificial bones, artificial tooth roots, artificial joints, bolts, and other areas. Currently, many countries, including the United States, Switzerland, and the Netherlands, are widely using polycrystalline high-purity nano-alumina materials to manufacture artificial teeth and bones. With the continuous development of technology, the application of alumina bioceramic materials in the medical field is becoming increasingly widespread, and research on them will continue to move towards emerging medical applications with higher added value and greater prospects.