Bioceramics
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s review describes bioceramics used as implants to repair parts of the body, usually hard tissues such as bones or teeth, but also to replace heart valves. Dozens of ceramic compositions hâve been tested,1"3 but few hâve achieved human clinical application. Clinical success requires the simultaneous achievement of a stable interface with tissue and a match of the mechanical behavior of the implant with the tissue to be replaced. Types of Bioceramics-Tissue Attachaient The mechanism of tissue attachment dépends on the type of tissue response at the implant interface (Table I).1 Figure 1 compares the relative chemical activity of différent types of bioceramics. The relative reactivity shown in Figure la correlates closely with the rate of formation of an interfacial bond of implants with bone (Figure lb).4 Figure lb will be discussed in more détail in the section on bioactive ceramics.
Table I: Types of Bloceramics-TIssue Attachment and Bloceramlc Classification. Type of Bloceramlc (1)
(2)
(3)
(4)
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Type of Attachment Dense, nonporous nearly inert ceramics attach by bone growth into surface irregularities, by cementing the device into the tissues, or by press-fitting into a defect (termed morphological fixation). For porous inert implants, bone ingrowth occurs, which mechanically attaches the bone to the material (termed biological fixation). Dense, nonporous surface-réactive ceramics, glasses, and glass-ceramics attach directly by chemical bonding with the bone (termed bioactive fixation). Dense, nonporous (or porous) resorbable ceramics are designed to be slowly replaced by bone.
Example Al 2 0 3 (single crystal and polycrystalline)
AI2O3 (porous polycrystalline) Hydroxylapatite-coated porous metals Bioactive glasses Bioactive glass-ceramics Hydroxylapatite Calcium sulfate (plaster of Paris) Tricalcium phosphate Calcium-phosphate salts
The relative level of reactivity of an implant influences the thickness of the interfacial zone or layer between the material and tissue. Analysis of failure of implant materials for 20 years generally shows failure starting at the biomaterial-tissue interface.3"8 When biomaterials are nearly inert (Type 1 in Table I and Figure 1) and the interface is not chemically or biologically bonded, movement occurs and a nonadherent fibrous capsule in both soft and hard tissues develops. Movement at the biomaterial-tissue interface eventually leads to détérioration in function of the implant, or the tissue at the interface, or both. The thickness of the non-adherent capsule varies greatly depending upon both material and extent of relative motion. The fibrous tissue at the interface with dense, medical-grade alumina implants can be very thick, about several micrometers.3"6 If alumina implants fit very tightly mechanically and are loaded primarily in compression, they are successful clinically.6 In contrast, if a nearly inert implant is loaded so that interfacial movement can occur, the fibrous capsule can become several hundred micrometers thick; the implant inevitahly loosens, leading
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