Corneal Transplant: Methods and Post-Operative Care

What is a corneal transplant?

The Importance of the Cornea in Vision

The cornea is a lens located in the anterior part of the eyeball, the most powerful in the entire visual system with a total power of approximately 43 diopters, precisely +48 in its convex anterior surface and -5 in its concave surface. The cornea has two main functions, which make it a fundamental element of our sight: it allows the passage of light from the outside to the inside of the eye, grouping the rays towards the fovea, and it also serves to protect the eyeball from external agents or foreign bodies.

Its two functions are due to the particularity of this tissue, which is composed of several layers of different compositions: the key to understanding the importance of the cornea certainly lies in its particular anatomy.

Each layer of the cornea has its own composition and function:

• Epithelium: This is the outermost layer of the cornea, whose function is to protect the eye from abrasions and aggression from external agents or bodies;
• Bowman's Layer: Also known as the anterior elastic membrane, it is a cell-free layer that functions as an elastic layer between the epithelium and the stroma;
• Stroma: This is the largest part of the cornea, composed mainly of collagen fibers, whose thickness increases from the central zone to the outer zone, as well as naturally with age. The stroma constitutes, depending on the case, 75 to 90% of the cornea;
• Descemet's Membrane: This membrane, the fourth of the layers that make up the cornea, is very similar to Bowman's lamella in its composition, also composed of collagen fibers that give it its elasticity;
• Endothelium: This is the fifth and final layer of the cornea, made up of cells closely linked to each other, whose function is that of a posterior filter and hydration of all the other layers of the lens;

In addition to these five layers, there is a sixth layer, discovered only in recent years, located in the innermost part of the cornea: it is so thin (15 microns) that it can only be seen with an electron microscope.

Indications and Objective of Corneal Transplant

A corneal transplant involves replacing the portion of the ocular surface that has become opaque with an equal-sized portion taken from a donor.

A corneal transplant may be used when drug treatments no longer allow the cornea to heal or when it has become too weak and therefore risks rupturing spontaneously.

A corneal transplant is therefore necessary when the microscopic structure of the cornea or its metabolism is altered. This results in a localized or diffuse loss of its transparency due to the formation of scar tissue or the abnormal formation of blood vessels in an area where they are normally absent. The indications for corneal transplantation are either optical (improvement of vision due to opacification of the cornea), reconstructive (for example, replacement of a perforated cornea), or therapeutic.

Different Methods for a Corneal Transplant

Surgical techniques have evolved in recent years, and it is now possible to selectively transplant the affected corneal layers according to the pathology of each patient.

There are different corneal transplant techniques depending on the layers to be replaced:

• SALK (superficial anterior lamellar keratoplasty): the most superficial layers are replaced;
• DALK (deep anterior lamellar keratoplasty): the superficial and deep layers are replaced, except for the last layer called the endothelium;
• DMEK (Descemet's membrane endothelial keratoplasty): is performed when the lesion is in the endothelium;
• DSAEK (Descemet's stripping automated endothelial keratoplasty): replaces only the deepest layers of the cornea, the endothelium and Descemet's membrane;

How is a corneal transplant performed?

Procedures Before the Operation

After collection, the harvested corneas are analyzed in the laboratory. The tissue is prepared by treating it according to the type of transplant it is intended for. The tissue is then stored at +4°C (short-term cold storage, up to approximately 10 days) or at +31°C (in culture, up to 30 days).

To be transplanted, corneas must be analyzed by assessing the viability of their cells and certain biological parameters.

To ensure maximum safety for the cornea recipient, the donor's health status is also carefully reassessed.

After being analyzed, prepared and stored, the corneas deemed suitable are sent to the healthcare facilities that request them for a corneal transplant.

Procedure of the Operation

Corneal transplant surgery involves replacing a diseased cornea with a healthy cornea from a donor. There is a particular situation, autografting, in which the patient's cornea is turned to avoid opacity, or the cornea of the other eye is used.

The operation lasts approximately 45 to 90 minutes, depending on the complexity of the case. It is performed in the operating room under local or general anesthesia. The surgeon removes the cornea, or the layer of the cornea that is affected, from the patient, then proceeds to the sutures.

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Post-Operative Care

Due to the invasive nature of the operation and the delicacy of the areas undergoing keratoplasty, the postoperative period can be long and problematic. Corneal rejection is fortunately a rare possibility, which can often be resolved with drug treatment, but which can occur even after years in the most unfortunate cases. After the operation, extreme exertion and water sports should also be avoided for several weeks, bandages should be worn, and a very strict cleaning regimen should be observed in order not to risk infecting the operated areas.

In general, the results of transplants are good. The cornea is an ocular tissue that is not vascularized, no veins or arteries reach it, so the occurrence of an immune reaction or rejection is very low.

In general, the ophthalmologist informs the patient of the prognosis of their particular case during the consultation. Some patients require the use of immunosuppressants to reduce their defense reaction and decrease the risk of rejection.

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