Functionally adapted surfaces on a silicone keratoprosthesis.

BACKGROUND: Silicone intraocular lenses as well as silicone sponges and encircling bands on the bulbar surface are widely used and are well tolerated. The aim of this project is a new one-piece silicone keratoprosthesis with enhanced cell adhesion in the haptic region to optimize the keratoprosthesis stability. These investigations show how enhanced profileration of conjunctival fibroblasts and, therefore, improved tissue compatibility can be achieved by hydrophilizing and by protein immobilisation on a hydrophobic silicone surface. This allows a combination of desired chemical and mechanical properties of the silicone bulk material with surfaces of improved tissue compatibility. METHODS: Silicone foils with surface modifications of different kinds were tested. Experiments were done using cell cultures with murine fibroblasts L-929 and human conjuctival fibroblasts. Cytotoxicity assays were carried out with cells grown on the material in direct contact, as well as in indirect contact, with extracts (EN 30993-5). Viability stains by means of fluoresceindiacetate and ethidiumbromide together with morphology analyses by hemalaun-staining were performed. RESULTS: For the unmodified and modified foils themselves and their extracts any negative influence on cell cultures of murine and human cells could be excluded. There was a gradual improvement of cell morphology, spreading and proliferation dependent on the degree of surface modification. Covalently immobilised fibronectin showed the best results in contrast to adsorptive binding. CONCLUSIONS: Silicone surfaces can be modified chemically with bioactive proteins. These modifications are cell compatible and do not result in toxic reactions. The degree and type of silicone hydrophilization results in improved development of cell morphology, spreading and proliferation. Even better results are obtained after covalent binding of bioactive proteins like fibronectin. Improved biocompatibility with enhanced cellular overgrowth has been demonstrated in vitro for the modified silicone of the haptic region. We believe that this type of modification will help in reducing extrusion problems observed with former keratoprostheses.

[Biocompatibility of flexible intraocular lenses in vivo]. / Zur Biokompatibilität von faltbaren Intraokularlinsen in vivo.

PURPOSE: One aspect of biocompatibility of IOLs can be tested using specular microscopy of the anterior and posterior surface of the lens in vivo. METHODS: We examined 45 patients after implantation of a "Memory lens" and 31 patients after the implantation of an "Acrysof" lens 3-6 months after surgery. RESULTS: Typical deposits (Lens epithelial cells, spindle-shaped macrophages, giant cells, pigment, Wolter membrane) were rarely seen on both types of lenses. There were fewer lens epithelial cells on the posterior capsule and less secondary cataract after implantation of the "Acrysof" lens. CONCLUSIONS: We did not find any indication of reduced biocompatibility in any of the lenses examined. The reduced rate of secondary cataract after implantation of the "memory lens" may be explained by the fact that this lens type sticks to the posterior capsule better.

The "Aachen" keratoprosthesis: a new approach towards successful keratoprosthesis-surgery.

BACKGROUND: None of the keratoprostheses available today is absolutely successful in the long term, neither the problems of extrusion, retroprosthetic membrane formation and intraocular pressure rise are yet solved. A new type of keratoprosthesis is required which can show improved ingrowth characteristics and allow intraocular pressure measurements. In order to possibly meet the above mentioned requirements we developed a flexible silicone keratoprosthesis with scleral fixation and chemical surface modification. METHODS: The one-piece keratoprosthesis is made of silicone rubber. Its optical zone has a diameter of 11 mm and is 0.3 mm thick. The surface-modified haptic consists of a scleral rim and eight branches for scleral fixation. A ridge at the back of the keratoprosthesis fitting into the trephination hole shall avoid leakage and retroprosthetic membrane formation. Optical and mechanical qualities are characterised by tensile tests, spectrophotometry and topography. RESULTS: A method for keratoprosthesis-production was established. The optical quality of the device was improved by submicron lathing of the mould. Spectrophotometry showed high visible and ultraviolet light transmission of the silicone. Mechanical tests with silicone samples revealed high tensile strength and elongation at break. The mechanical properties were not impaired by surface modification. CONCLUSIONS: The production of a flexible silicone keratoprosthesis with high optical and mechanical properties was established. Its use both for the treatment of permanently opacified corneas and as temporary keratoprosthesis seems to be possible.

Changes in mineral composition of rabbit corneas after alkali burn.

BACKGROUND: Mineral balance is essential for maintaining corneal transparency. In preliminary investigations, we found alterations of mineral concentrations in the human cornea after chemical burns. In this prospective study the mineral content of the rabbit cornea was quantified after experimental alkali burns. METHODS: Thirty-two eyes of rabbits were burnt for 30 s with 1 M NaOH. A short period of saline rinsing followed, but no further therapy was given. The eyes of 20 healthy rabbits served as a control. We measured the water content by weighing the cornea before and after drying. The mineral content of the cornea was determined in different stromal layers using energy-dispersive X-ray analysis in the scanning electron microscope. Groups of eight rabbits were examined on days 0, 1, 4 and 8 after the experimental burns. RESULTS: After alkali burns the mineral composition of the corneal stroma changed considerably. Sodium was present in high concentrations immediately after burning, up to 708 mmol/kg dry weight, and returned to normal on day eight. Chlorine increased significantly throughout the whole period. Sulphur concentrations stayed almost normal, showing only a minute loss of ground substance after alkali burn. A considerable and increasing edema could be concluded from the water content. CONCLUSIONS: Alkali burns resulted in a dramatic change of the corneal mineral content. Therapeutic intervention with appropriate solutions is required to restore and maintain the normal mineral composition of the denuded corneal stroma.

Development of a surface modified silicone-keratoprosthesis with scleral fixation.

BACKGROUND: Many attempts have been made to create artificial corneas. The keratoprostheses currently available do not allow measurements of the intraocular pressure (IOP) and restrict the visual field. The main problem is extrusion due to an insufficient connection between implant and surrounding tissue. It is our aim to create a flexible keratoprosthesis with a wide field optic allowing measurements of the IOP. Surface modification will improve cell adhesion and therefore stability between implant and tissue. METHODS: The keratoprosthesis is made of silicone rubber. The optical zone is 11 mm in diameter with a thickness of 0.3 mm. The surface modified haptic consists of a scleral rim and 8 branches for scleral fixation. Optical and mechanical qualities were tested by tensile tests, spectrophotometry and topography. RESULTS: A method to produce one-piece silicone keratoprostheses was established. Submicron lathing of the mould led to an excellent optical quality. Spectrophotometry showed high degree of visible and ultraviolet light transmission of the silicone. Mechanical tests revealed high tensile strength and elongation at break which were not impaired by surface modification. CONCLUSION: The production of a flexible silicone keratoprosthesis with high optical and mechanical properties was accomplished, with possible use as both permanent and temporary keratoprosthesis.

[Temperature changes of the cornea by applying an eye bandage]. / Temperaturveränderungen der Hornhaut durch Anlegen eines Augenverbandes.

BACKGROUND: The corneal temperature is not often measured, but it may be useful to evaluate the temperature-changing effect of the application of eye bandages. The reason for applying an eye bandage is to calm an inflamed eye and to provide mechanical protection. Everyone knows that some patients have more complaints after an eye bandage has been applied. This phenomenon might be caused by the inflamed eye being warmed up by bandage application. MATERIALS AND METHODS: In 40 apparently healthy subjects 24 +/- 1.93 years of age we examined the temperature changes caused by two different types of eye bandage. The measurements were done with a Jeol infrared camera. Twenty probands received a monocular bandage consisting of a perforated plastic cap and another 20 subjects a monocular mull bandage with a perforated plastic cap. RESULTS: The mean corneal apex temperature was 32.05 +/- 0.74 degrees C. Both bandages caused the corneal temperature to go up considerably. The perforated plastic cap increased the mean apex temperature by 0.58 +/- 0.48 degree K and the combined mull bandage by 1.15 +/- 0.57 degrees K (P < 0.05). CONCLUSIONS: Application of an eye bandage increases the corneal temperature significantly. This is known to change enzyme activities and to cause prostaglandin liberation and pain. A change in the bacterial spectrum may result. Cooling as a universal principle in antiphlogistic therapy might be a supplementary therapy in treating sterile, but inflamed eyes.

The microtrephine. A new diagnostic tool for obtaining corneal biopsies.

We have developed a new tool for obtaining biopsies of the cornea in a manner which is less traumatic than former biopsy techniques. The new instrument consists of a drill with an aperture of 160 microns which functions as a trephine and is powered at 30,000 rpm. The instrument contains a 120 microns mandrel to remove the samples. In order to monitor electrolyte contents in alkali burnt corneas, 32 rabbit eyes underwent an alkali burn of one cornea (1 n NaOH for 30 sec) followed by a sequence of microtrephinations at different times. A total of 81 microtrephinations were performed. Fifty samples were obtained. One corneal perforation occurred. Forty-four puncture sites were examined by means of histology. Forty-eight biopsies were examined in a scanning electron microscope and identified as punctures of corneal tissue by way of an energy dispersive X-ray analysis. Human corneas had been punctured with this device.