The physical and optical properties of the eye are very similar to those of a camera.
The cornea (the clear front surface of the eye) and the lens combine into a compound lens system, to fulfil the role of the lens in a camera. They collect parallel or distant light rays that enter the eye into one bundle, focusing an image on the retina at the back of the eye. An electrical impulse is created by the retina from this image, and is then transmitted via the optic nerve to the brain, where it is interpreted and converted into the images we see.
The cornea and lens make up the focal power of the eye, of which the cornea is responsible for about two-thirds, and the lens for about one-third of the total power. So, as you may now realise, the cornea plays an extremely important role in providing good vision. Any defect of the cornea, or any distortion introduced by the cornea, affects vision in a negative way.
What Is a Cornea Transplant and Am I a Candidate?
A cornea transplant refers to the surgical procedure were all or part of a damaged cornea is replaced with healthy donor tissue. It is also known as a Keratoplasty or a Corneal Graft.
Due to its specific nature and function, any abnormal condition of the cornea has a huge impact on the quality of vision a person perceives. There are 3 broad categories of conditions that may lead to a corneal transplant:
- Corneal opacity - a cornea that was affected by inflammation or injury may develop scarring. Due to the subsequent opacity, images are perceived as dull and blurred.
- Corneal thinning - a cornea that becomes increasingly thinner, starts to bulge outward and develop an irregular shape. This leads to a steady increase in short-sightedness and/or astigmatism.
- Failure to maintain corneal clarity - a cornea that is becoming increasingly swollen develops cloudiness due to failure of the endothelial layer of cells. This causes images to become progressively dull and/or blurred, as well as the cornea becoming uncomfortable.
If it is not possible to improve a patient`s vision through less-invasive methods (namely spectacles, soft- or hard contact lenses or less-invasive procedures, such as corneal cross-linking o implantation of an ICRS or phakic lens) then only do they become suitable for a corneal transplant.
Corneal transplant surgery is believed to be one of the most successful types of transplant surgery and the techniques are improving constantly. It is routinely performed as an outpatient procedure.
Cornea Transplant Techniques
A cornea transplant is usually considered as a last resort to correct corneal abnormalities, due to the relatively high cost, long recovery period and associated risk factors. However, when all else fails and it is indicated, a cornea transplant can restore good vision very successfully. Corneal transplants are known to be the most effective of all kinds of transplant surgery.
The recovery period following a corneal transplant may last between 1 and 2 years, depending on the type of transplant. Over this period, frequent follow-up visits are necessary to ensure proper healing of the cornea. It is routine to remove the corneal sutures in a gradual manner, so that by the one year post-operative mark the final sutures are removed. Another period of 6 months is then allowed for the cornea to stabilise.
At this point, the residual refractive error can be addressed, if required, by means of the most suitable refractive surgery technique.
Lamellar corneal transplants are the latest development in the field of corneal transplant surgery. With these techniques, only the affected layer of the cornea is removed and replaced with donor tissue. The recipient's own tissue is preserved as far as possible to minimize associated risks. Several transplant options are available, depending on the initial abnormality.
- Superficial Anterior Lamellar Keratoplasty (SALK)
This entails that only the superficial layers of the corneal stroma (100 to 200 microns or 0.1 to 0.2 mm) are removed and replaced by a similar donor section.
- Deep Anterior Lamellar Keratoplasty (DALK)
This is very similar to the SALK procedure, except that most of the corneal stroma is replaced. A number of variations to this technique are available, namely:
- 'Big bubble' DALK
This implies that Descemet’s membrane is separated from the stroma, followed by excision of the entire central stroma to be replaced by a similar donor section.
- 'Mushroom' Lamellar Keratoplasty
This is the only procedure among the lamellar variations that comprise a full penetrating procedure. The transplanted area resembles the shape of a mushroom with a larger outer “cap” and a smaller inner “stem”.
An Endothelial Keratoplasty procedure is done for isolated endothelial corneal disease. Here, two techniques are available, namely:
- Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)
This replaces the corneal endothelium, Descemet’s membrane and a thin layer of the deep internal stroma, which acts as a ‘splint’ to the delicate Descemet’s membrane and endothelium.
- Descemet Membrane Endothelial Keratoplasty (DMEK)
This is an intricate and very delicate procedure in which only the corneal endothelium and Descemet's membrane are replaced.
The above procedures are significantly less invasive, in contrast to a penetrating transplant, as visual recovery usually occurs within a much shorter time span as opposed to a penetrating keratoplasty.
Other Corneal Treatments
Probably one of the most exciting developments in corneal surgery the past decade is the introduction of UV-light induced corneal crosslinking. This innovative procedure is the result of careful observation by innovator Theo Seiler from Switzerland during the early 1990s. He got the idea after visiting his dentist, where he observed that the properties of UV light were utilized to strengthen the filling in his tooth. He theorised that if it was possible to apply this technology in dentistry, it should be possible to do the same in keratoconus, with a few adjustments to the technique.
The corneal stroma consists of collagen fibres, among other components, arranged in a geometric pattern. These fibres form a lattice, and are maintained in their highly-organized structure by chemical bonds between themselves. With ageing, under the influence of UV light from natural sources, these bonds tend to increase in number, thereby progressively strengthening the corneal structure over time. It is said that a cross-linking procedure selectively ages the cornea by approximately 10 years. This may offer an explanation of why it is rare to find progressive keratoconus in elderly patients.
Corneal cross linking is a medical intevention aimed at increasing the cross-links between the collagen fibres of the cornea, thereby reinforcing the ectatic unstable cornea.
The procedure involves removal of the corneal epithelium, where after Riboflavin drops are applied every 3 minutes for 30 minutes until the cornea is saturated with the vitamin. The reasoning is to provide a UV blocking filter in the cornea itself, to prevent UV damage to the internal ocular structures. Riboflavin has a yellow colour, which captures the UV light used in the procedure. Calibrated UV radiation is then focussed onto the cornea for 5 to 30 minutes, depending on the light source in use, while continuing to apply balanced salt solution every 3 minutes. The Riboflavin also acts as a catalyst through which oxygen rich free radicals are created which stimulates the linking between the corneal molecules.
The procedure is done on one eye at a time. The full effect of the cross-linking can take 6 months to 2 years to complete after the procedure, since the whole cascade of chemical linking is only activated or triggered by the procedure. One may therefore only assess the outcome after a period of 2 years.
This procedure is believed to stop progression and, in some cases, even lead to regression of keratoconus.
Intra-Stromal Corneal Ring Segments
ICRS are tiny, clear, ultrathin and precision-engineered crescents, made from polymethylmethacrylate (PMMA). This is the same material used in the manufacturing of certain hard contact lenses. Two of these crescents usually combine to achieve the desired correction, although it is possible to only use one segment, depending on the corneal curvature.
Two versions of the technology are available in South Africa, namely the Keraring™ and INTACS™.
The Keraring has a long track record, and finds its application mainly in the management of keratoconus and other ectatic corneal conditions.
INTACS™ were popularized by the company Keravision for the treatment of low to moderate myopia and was approved by the American FDA during April 1999 for the treatment thereof.
The management of keratoconus and other ectatic corneal pathologies underwent significant change since 2000 with the introduction of corneal cross-linking. In order to avoid (or at least postpone) a corneal transplant, the ICRS technology was introduced to improve the distorted corneal shape in these eyes. It is possible to alter and stabilise the cornea's shape through the placement of these rings. The Kerarings are especially popular for achieving this result, since they are available in different thicknesses, arc lengths and radii of curvature.
The amount of change of the corneal profile is dependent on:
- The thickness of the segment.
- The arc length thereof.
- The radius of curvature.
- The stromal depth at which it is implanted.
These segments are inserted into a circular tunnel, created in the periphery of the cornea within the corneal substance by a special surgical instrument, or alternatively by a femtosecond laser which is more accurate.