TECHNIQUES FOR FULL-THICKNESS KERATOPLASTY IN RABBITS USING FRESH AND FROZEN CORNEAL TISSUE

Corneal preservation

Significant advances in corneal preservation have been made over the past decade. The introduction of chondroitin sulfate-containing media for use at 4 degrees C allows storage of corneas for up to ten days prior to transplantation. Organ culture techniques have also been improved with the addition of chondroitin sulfate. There has been an increase in our understanding of preservation using McCarey-Kaufman medium. Studies have been published that compare these methods and help the clinician decide which method to use based on objective data. Also, our understanding of existing methods of evaluating endothelial viability has increased and new methods have been developed. Finally, the acquired immunodeficiency syndrome is having an increasing effect on eye-banking and the supply of donor corneas.

Cryopreservation of the rabbit cornea: freezing with dimethyl sulphoxide in air or in medium

There have been considerable difficulties in developing a satisfactory method for the cryopreservation of corneas. In this paper we describe the effect of two variables that appear to influence the effectiveness of preservation, the concentration of cryoprotectant and the medium that surrounds the cornea during freezing. Rabbit corneas were exposed to the cryoprotectant dimethyl sulphoxide (Me2SO) in concentrations of 1 or 2 mol/l in a high-potassium well-buffered solution, and then cooled to -196 degrees C either in air or surrounded by 5 ml of the Me2SO solution. After storage at -196 degrees C, thawing and removal of the Me2SO, survival was assessed by electron microscopy and measurement of stromal thickness during perfusion on the specular microscope. The least degree of damage was observed when corneas were equilibrated with 1M Me2SO and frozen in air. The evidence suggests that 2M may be an excessive concentration of Me2SO in this system and that damage to the stroma may be reduced by freezing the cornea in air rather than surrounded by the Me2SO solution.

Cryopreservation of rabbit corneas: assessment by microscopy and transplantation

Rabbit corneas were frozen and thawed by three methods and compared by full thickness transplantation as well as specular microscopy, histology, and transmission electron microscopy. Two of the methods used a recently described technique, in which the excised cornea was immersed in a potassium-rich buffered solution containing the cryoprotectant dimethyl sulphoxide (Me2SO, 2 mol/l). This solution was designed to restrict the loss of intracellular potassium and to prevent cell swelling at low temperatures. In one group the corneas were frozen and thawed surrounded by 5 ml of medium, while in the second group corneas were drained of excess fluid and frozen in air. The third group consisted of corneas cryopreserved by Capella and colleagues' method. All the cryopreserved corneas were damaged, but those that had been frozen in air after exposure to the new medium showed better structure and function than corneas frozen by either of the other two techniques.

Clinical cryobiology of tissues: preservation of corneas

It is well recognized that the clarity of the cornea is a function of its hydration, and that this hydration is controlled by a "pump-and-leak" mechanism operating across the posterior monolayer of cells called the endothelium. A breakdown of the endothelium through disease or injury causes a marked increase in corneal thickness as the stroma imbibes fluid from the aqueous humor in the anterior chamber of the eye. This thickened, edematous condition of the stroma results in a cloudy cornea with an associated marked decrease in visual acuity. Treatment for this condition is usually by full-thickness corneal transplantation (penetrating keratoplasty), the success of which is dependent upon the donor cornea having an intact and healthy endothelium. It is essential, therefore, that any method of corneal storage for penetrating keratoplasty should protect and preserve the endothelium in a viable state. Current clinical practice relies upon short-term methods of preservation by two principal methods. Moist Chamber Storage is the time-honored corneal preservation method; it consists of keeping enucleated eyes at 0-4 degrees C in a sealed jar containing a pad of cotton gauze soaked in saline to provide a humid environment. The time limit placed upon this method of storage is 24-48 hr after which the viability of the endothelium deteriorates rapidly. Storage in M-K (McCarey-Kaufman) Medium involves excision of the corneoscleral segment from the donor eye and immersing it, endothelial side uppermost, in a medium consisting of tissue culture medium, 5% Dextran 40, and antibiotics. Laboratory and clinical studies indicate that storage in M-K medium at 4 degrees C preserves human endothelial cells for up to 4 days when the eye has been removed from the cadaver in less than 10 hr postmortem. Long-term preservation of corneas by freezing has long been a major goal in eye banking because indefinite storage by cryopreservation offers significant advantages for the quality and the quantity of material for use in keratoplasty, as well as for its distribution. However, procedures that have been developed for the cryopreservation of corneas have not been widely used, and a number of studies have shown that these procedures are inadequate for maintaining the integrity of the corneal endothelium. Not surprisingly, clinicians are now reluctant to accept corneas that have been frozen by these methods, though the clinical need is now greater than ever.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo survival of the endothelium of freeze-stored corneal homografts in the rabbit

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