2.3 MPLANT DESIGN CRITERIA: EFFECTS OF THE BODY ON THE IMPLANT (DEGRADATION A. Chemical 1. Corrosion of metals: metal ion release due to an anodic(reduction)reaction a. Pitting and crevice corrosion and"concentration cell"effect at sites of depleted oxygen b. Galvanic corrosion due to contact of dissimilar metal. The more reactive metal (in the galvanic series) becomes the anode c. Stress corrosion due to accelerated metal ion release at a crack tip where the strain is high d. The oxide"passivation"layer reduces potential for corrosion e. Corrosion facilitates cracks initiation and thereby weakens the device f. Ranked according to their potential for corrosion Stainless steel >Co-Cr alloy >Ti alloy 2. Oxidation of polymers Oxidation of polyethylene results in chain scission and a reduction in the average molecular weight. This causes increases in the density, modulus of elasticity, and percent crystallinity. Oxidation can be determined by detecting the carbonyl groups that are formed 3. Hydrolysis of poly Ester linkages(e. g, polylactic and polyglycolic acid) are attacked by water leadi to chain scission 4. Water absorption Water absorption can lead to an alteration in the mechanical properties of certain hydrophilic thermoplastic polymers(e. g, polysulfone) id absorptio bsorption of lipid by certain hydrophobic polymers(e. g, polydimethyl siloxane) 6. Dissolution The effect of water and pH in dissolving certain substances(e.g, calcium 7. Precipitation Deposition of calcium salts(calcification2.3 IMPLANT DESIGN CRITERIA: EFFECTS OF THE BODY ON THE IMPLANT (DEGRADATION) A. Chemical 1. Corrosion of metals: metal ion release due to an anodic (reduction) reaction a. Pitting and crevice corrosion and "concentration cell" effect at sites of depleted oxygen b. Galvanic corrosion due to contact of dissimilar metal. The more reactive metal (in the Galvanic series) becomes the anode. c. Stress corrosion due to accelerated metal ion release at a crack tip where the strain is high. d. The oxide "passivation" layer reduces potential for corrosion. e. Corrosion facilitates cracks initiation and thereby weakens the device. f. Ranked according to their potential for corrosion: Stainless steel > Co-Cr alloy > Ti alloy. 2. Oxidation of polymers Oxidation of polyethylene results in chain scission and a reduction in the average molecular weight. This causes increases in the density, modulus of elasticity, and percent crystallinity. Oxidation can be determined by detecting the carbonyl groups that are formed. 3. Hydrolysis of polymers Ester linkages (e.g., polylactic and polyglycolic acid) are attacked by water leading to chain scission. 4. Water absorption Water absorption can lead to an alteration in the mechanical properties of certain hydrophilic thermoplastic polymers (e.g., polysulfone). 5. Lipid absorption Absorption of lipid by certain hydrophobic polymers (e.g., polydimethyl siloxane). 6. Dissolution The effect of water and pH in dissolving certain substances (e.g., calcium phosphates) 7. Precipitation Deposition of calcium salts (calcification)