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Oxide Ceramics for Biomedical Applications
Oxide Ceramics for Biomedical Applications The biomedical applications of oxide ceramics are based on several characteristics of these materials that can be summarized as follows: $ chemical stability (absence of corrosion and ion release); $ resistance to wear and to scratching owing to high hardness; $ dimensional stability in physiological environments; and $ biocompatibility of bulk materials and of particulate derivatives. The high stability (low corrosion, low ion release) that characterizes oxide ceramics in the most aggressive conditions is due to the high heat of formation of their molecules. In nonmedical applications, oxide ceramics are used in components operating at high temperature in corrosive environments (melt steel valves, exhaust ports, fuel cells, chemical reactor liners, etc.) and in applications that requires high resistance to wear (extrusion dyes, paper mills, wire guides, cutting tools, etc.). The oxide ceramics in use as biomaterials are aluminum oxide (alumina, Al O ) and zirconium # $ with the interdioxide (zirconia, ZrO ), complying # national reference standard ISO 6474 (alumina, αAl O ), or with the standard ISO 13356 (zirconia, # $ Y-TZP). There have been many attempts to use alumina and zirconia in implantable medical devices; many of these devices never leaving the experimental stage. In laboratory tests and in many years of clinical use, alumina and zirconia ceramics have proved their suitability for human implantation. Both materials are classified as ‘‘nearly inert’’ biomaterials, taking into account that no material can be considered as absolutely inert when placed into living tissues. Alumina and zirconia have their main field of application in orthopedics for the manufacture of components of the joints of total hip replacements (THRs) (Table 1). The use of ceramics minimizes the wear debris released from the joint (Do$ rre and Hubner 1984) that causes osteolysis and aseptic loosening of prostheses, and makes ceramic joints suitable for younger and more active patients (Black 1997). Alumina was introduced in clinical practice in 1970. Table 1 Applications of oxide ceramics as biomaterials. Alumina
Zirconia
THR ball-heads, THR sockets and inlays, TKR, shoulder joints, finger joints, spinal spacers, bone screws, dental implants, orthodontic brackets, dental crowns, dental inlays, middle ear implants, orbital floors, keratoprostheses THR ball-heads, dental implants, dental posts
More than 2.2 million alumina and more than 500,000 zirconia ball-heads have been manufactured, and ceramic components are used in about 35% of THRs implanted worldwide (Heros and Willmann 1998). The design of components made from alumina has to take into account the fracture mechanics of the material, its brittleness implying that tensile stresses must be kept low. In THRs, ceramic ball-heads are coupled to the metallic stem using a cone taper. This solution offers the advantage of allowing for the same stem the selection of neck length that best suits the patient anatomy. The drawback is that the self-locking conical taper transforms the compressive load into tensile stresses in the dome part and in the rim zone of the ball-heads. Thus, stress distribution and intensity in the ball-head depend on the cone angle, on the extent of the contact, and on the careful matching of surface roughness, roundness, and linearity of the taper. In the first years of clinical use, alumina ball-heads experienced an unacceptable rate of failure, a problem that is now almost resolved (see Fig. 1). Such failures were due to the poor quality of some materials and inadequacy in design (Walter 1992, Piconi et al. 1999). The introduction of zirconia was made to exploit the toughness of this material, called a ‘‘ceramic steel.’’ The use of zirconia has allowed the production of ballheads smaller in diameter (e.g., 22 mm) with good safety coefficient. The reduction in the diameter of ball-heads is a prerequisite to lowering the wear of ultrahigh molecular weight polyethylene (UHMWPE) in THR joints and the extension of the lifetime of the prosthesis. Moreover, the better mechanical properties of zirconia make this ceramic more forgiving of coupling tolerances at metal ceramic–interfaces. A follow-up (Oonishi et al. 1999) on 1633 zirconia ballheads (1484 with 22 mm diameters) showed no failure over an eight-year period. While the use of zirconia is limited to the production of ball-heads, alumina is used also for the manufacture of the acetabular component of THRs, with massive ceramic socket or ceramic inlays for metal-backed acetabula (Richter et al. 1998) forming alumina– alumina joints. Besides implantable medical devices, alumina and zirconia are used in the manufacture of components for medical devices that require machining to tolerances near to the micrometer level and high wear resistance and stability, such as pins, guides, insulators, nozzles, etc. 1. Alumina Among the seven forms of alumina known, only αalumina, or corundum, is suitable for use as biomaterial, due to its high stability. α-Alumina has a crystal structure similar to single-crystal ruby and sapphire, formed by h.c.p. layers of oxygen atoms, with two-thirds of octahedral sites occupied by 1
Zirconia
THR ball-heads, THR sockets and inlays, TKR, shoulder joints, finger joints, spinal spacers, bone screws, dental implants, orthodontic brackets, dental crowns, dental inlays, middle ear implants, orbital floors, keratoprostheses THR ball-heads, dental implants, dental posts
More than 2.2 million alumina and more than 500,000 zirconia ball-heads have been manufactured, and ceramic components are used in about 35% of THRs implanted worldwide (Heros and Willmann 1998). The design of components made from alumina has to take into account the fracture mechanics of the material, its brittleness implying that tensile stresses must be kept low. In THRs, ceramic ball-heads are coupled to the metallic stem using a cone taper. This solution offers the advantage of allowing for the same stem the selection of neck length that best suits the patient anatomy. The drawback is that the self-locking conical taper transforms the compressive load into tensile stresses in the dome part and in the rim zone of the ball-heads. Thus, stress distribution and intensity in the ball-head depend on the cone angle, on the extent of the contact, and on the careful matching of surface roughness, roundness, and linearity of the taper. In the first years of clinical use, alumina ball-heads experienced an unacceptable rate of failure, a problem that is now almost resolved (see Fig. 1). Such failures were due to the poor quality of some materials and inadequacy in design (Walter 1992, Piconi et al. 1999). The introduction of zirconia was made to exploit the toughness of this material, called a ‘‘ceramic steel.’’ The use of zirconia has allowed the production of ballheads smaller in diameter (e.g., 22 mm) with good safety coefficient. The reduction in the diameter of ball-heads is a prerequisite to lowering the wear of ultrahigh molecular weight polyethylene (UHMWPE) in THR joints and the extension of the lifetime of the prosthesis. Moreover, the better mechanical properties of zirconia make this ceramic more forgiving of coupling tolerances at metal ceramic–interfaces. A follow-up (Oonishi et al. 1999) on 1633 zirconia ballheads (1484 with 22 mm diameters) showed no failure over an eight-year period. While the use of zirconia is limited to the production of ball-heads, alumina is used also for the manufacture of the acetabular component of THRs, with massive ceramic socket or ceramic inlays for metal-backed acetabula (Richter et al. 1998) forming alumina– alumina joints. Besides implantable medical devices, alumina and zirconia are used in the manufacture of components for medical devices that require machining to tolerances near to the micrometer level and high wear resistance and stability, such as pins, guides, insulators, nozzles, etc. 1. Alumina Among the seven forms of alumina known, only αalumina, or corundum, is suitable for use as biomaterial, due to its high stability. α-Alumina has a crystal structure similar to single-crystal ruby and sapphire, formed by h.c.p. layers of oxygen atoms, with two-thirds of octahedral sites occupied by 1