Assessing intraocular lens calcification in an animal model.

PURPOSE: To describe an animal model used to evaluate the propensity of various biomaterials to calcify intraocularly. SETTING: Research Department, Allergan Inc., Irvine, California, USA. METHODS: Intraocular lens (IOL) optic materials were implanted intramuscularly and/or subcutaneously in rabbits for up to 90 days. The materials included silicone, poly(methyl methacrylate) (PMMA), hydroxyethyl methacrylate hydrogel, and several hydrophobic acrylic materials. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) were used to detect calcification demonstrated by characteristic discrete nodules containing both calcium and phosphate. Histological methods were used to evaluate tissue reactivity. Disc lenses fabricated from the experimental material were also bilaterally implanted in rabbit eyes that were monitored by slitlamp biomicroscopy. The lenses were explanted at 1, 2, 5.5, 10, and 20 months for SEM/EDS analysis. RESULTS: No calcification was noted in the intramuscularly or subcutaneously implanted silicone, PMMA, and acrylic optic materials. Calcification was noted on the intramuscularly, subcutaneously, and intraocularly implanted experimental acrylic and the intramuscularly implanted hydrogel material; the calcification was more extensive on the hydrogel. Signs that suggested intraocular calcification were first noted on the experimental IOLs at 4 months, but calcification was not confirmed until 10 months. CONCLUSIONS: Material calcification occurred more quickly in an intramuscular or subcutaneous environment than in an intraocular environment. Intramuscular and subcutaneous implantation appears to be an excellent model for screening materials for calcification potential. However, calcification is both host environment and material dependent. Using intramuscular or subcutaneous implantation in animal models to predict intraocular calcification in humans must be done with caution.

Biochemical properties of heat-treated valvular bioprostheses.

BACKGROUND: Preliminary studies showed that heat treatment of glutaraldehyde preserved valvular bioprostheses mitigates calcification. This study was carried out to define the physicochemical characteristics of the heat-treated tissues to elucidate the mechanism involved in the mitigation. METHODS: Glut bovine pericardium or porcine valve samples were treated at 50 degrees C in a 0.625% glutaraldehyde solution for 2 months. Some samples underwent assay for shrinkage temperature, moisture content, ninhydrin test, and acid hydrolysis, and other samples were incubated in human serum for 3 days and then analyzed by electrophoresis to study protein adsorption. RESULTS: Heat treatment mitigated calcification without adversely affecting shrinkage temperature (84.81 degrees C versus 83.95 degrees C) and moisture content (78.68% versus 78.71%). A significant reduction in free amino groups (0.15 versus 0.37 mol NH2/mol collagen) and a significant increase in resistance to acid hydrolysis were observed. Total protein content was similar, but significant differences were found for four proteins adsorbed in the tissues (167, 45, 11.6, and 10 kDa). CONCLUSIONS: The anticalcification effect of heat treatment may be attributed to structural changes, lipid extraction, increased resistance, and modifications of the type and concentration of the proteins adsorbed in the tissue.

Tissue characterization and calcification potential of commercial bioprosthetic heart valves.

BACKGROUND: Tissue properties may contribute to intrinsic calcification of bioprosthetic heart valves. Phospholipids have been proposed as potential nucleation sites for calcification. Other tissue properties might also be important in calcification. METHODS: Commercial and control bioprosthetic valve tissues were characterized by shrinkage temperature, moisture content, free amine content, phospholipid content, and calcification level after 90-day rat subcutaneous implantation as described. RESULTS: Shrinkage temperature, moisture content, and free amine content were typical for glutaraldehyde-cross-linked tissues. Phospholipid and calcium levels varied considerably among valve types. There was a significant correlation between phospholipid levels and calcification (r = 0.63, p = 0.04). Sulzer Carbomedics Mitroflow and Toronto SPV valve tissues had significantly more calcification than other commercial bioprostheses in this study (p < 0.01). Carpentier-Edwards Duraflex, CE SAV, and CE PERIMOUNT valve tissues had significantly less calcification than Medtronic Mosaic in this animal model (p < 0.02). CONCLUSIONS: Processes that reduce phospholipid levels are associated with reduced calcification in the rat subcutaneous model. Significant differences in calcification level were found among commercially available valves. The clinical significance of these results is unknown.

Contact-angle analysis of intraocular lenses.

PURPOSE: To present contact-angle measurements of commercially available intraocular lenses (IOLs) in air and in water to facilitate the understanding of how various IOLs might interact in different environments. SETTING: Laboratory. METHODS: Five commercially available IOLs were studied: AMO DuraLens PS-59NB. AMO PhacoFlex SI-26NB, AMO PhacoFlex II SI-30NB, Chiron ChiroFlex C10UB, and Alcon AcrySof MA60BM. The AMO soft acrylic model AR40, currently under clinical study, was also evaluated. Contact-angle measurements were made in air and in water using sessile drop and captive bubble methods. RESULTS: The sessile drop method indicated that all materials were hydrophobic in air. The captive bubble method differentiated materials based on their polar and dispersive forces. CONCLUSION: Contact-angle measurements differed depending on the test conditions. Proper choice of contact-angle measurement method can generate useful information about a material surface and its potential biomaterial interactions.

Surface properties of intraocular lens materials and their influence on in vitro cell adhesion.

An in vitro model to assess lens epithelial cell adhesion to a variety of intraocular lens materials was developed. Rabbit anterior lens capsules were isolated and cultured in serum-containing medium. Test surfaces included poly(methyl methacrylate), two new silicones (SLM-1/UV, SLM-2/UV), two hydrogels (HEMA, Lidofilcon A), and polytetrafluoroethylene (PTFE). Following the application and culturing of cells on the test surfaces, adherent cells were removed by trypsinization and counted at eight and 24 hours. The material surfaces were characterized by electron spectroscopy for chemical analysis and scanning electron microscopy. The captive bubble technique was also used to assess interfacial free energy. More cells adhered to PMMA than to the other materials tested (P less than .01). The two silicones, HEMA, and PTFE did not differ significantly from each other; Lidofilcon A had the lowest cell adhesion of all materials tested. Cell adhesion results were related to the interfacial free energy of each material. Materials of low (less than 5 ergs/cm2) or high (greater than 40 ergs/cm2) interfacial free energies had lower cell adhesion than materials of intermediate free energies (5 to 40 ergs/cm2) which exhibited the highest cell adhesion.