Accidents and injuries can irreparably damage the cornea of the eye. If a corneal transplant is not possible, the use of an artificial cornea (keratoprosthesis) can help. There are several procedures, but they are very complex.
Here you will find further information as well as selected keratoprosthesis specialists and centers.
The cornea is the "windshield" of the eye, protecting the sensitive interior of the eye from external influences. It is transparent so that light can pass through it into the eye. Behind the cornea, the lens of the eye focuses the light and projects it onto the retina at the back of the eye.
There, the information received is converted into impulses that are transmitted to the brain via the optic nerve. A regularly structured, clear and transparent cornea is therefore of great importance for a sharp image.
There are many diseases and disorders that can impair the eye's ability to see. One of these is corneal opacity. This means that less light passes through the cornea and the light is also scattered. The result is reduced or blurred vision. Corneal clouding can be caused by
be caused.
A corneal transplant can help with corneal clouding. This involves removing the cloudy cornea and replacing it with a clear cornea. Unfortunately, the availability of human donor corneas is limited.
In most cases, the entire cornea (thickness: approx. 500 micrometers) is not replaced during a corneal transplant. A thin layer (so-called lamella) only 10-20 micrometers thick is sufficient in the majority of cases.
The cornea protects the iris and the sensitive interior of the eye © Ramona Heim | AdobeStock
However, there are also diseases or injuries in which a corneal transplant fails. The transplant recipient must form the top layer of the cornea themselves. It cannot be transplanted.
In rare cases, however, the eye is not capable of doing this, for example in the case of severe chemical burns. In such cases, an artificial cornea (keratoprosthesis) can help to improve the patient's vision.
Researchers have developed different types of artificial corneas. Many patients can significantly improve their vision after the implantation of an artificial cornea.
A keratoprosthesis of this kind is generally difficult due to the special nature of the cornea, but it is possible. Body tissue cannot usually bond firmly with artificial material.
The use of artificial materials in the body is therefore problematic. This leads to immune reactions and often to rejection of the foreign material. A material must therefore be inserted with which the body's own tissue can grow together. Only then will it be firmly anchored and the patient can benefit from improved vision.
The structure of the human eye © reineg #61485340 | AdobeStock
A thorough ophthalmological examination is necessary before keratoprosthesis surgery.
In order to expect a good result from the keratoprosthesis, a number of conditions must be met:
In this article we present some methods of keratoprosthesis.
By far the most common procedure is the Boston keratoprosthesis, also known as Boston KPro. It has been used in the USA since 1992 and has been continuously developed since then. In Germany, the Boston KPro has only been CE-marked since 2015. However, it is now a reliable alternative for affected patients.
The healthy iris controls the amount of light entering the eye. It enlarges and reduces the pupil. With a Boston KPro, the patient's iris is also removed. This task is taken over by part of the prosthesis.
A Boston keratoprosthesis is used if
The Boston KPro requires careful and intensive monitoring after the operation. Only then can long-term success be achieved.
Over 11,000 Boston KPro treatments have been performed worldwide in recent years. Nevertheless, the procedure is associated with a higher complication rate than a normal corneal transplant.
Boston corneal prosthesis: The rear metal plate, the sewn-in donor cornea and the transparent front plastic pin can be seen. Transparent plastic is now used instead of metal © Mariagessa | Wikimedia CC4.0
A Boston KPro is referred to as a keraprosthesis, i.e. an artificial cornea. However, this is only part of the therapy. The procedure is still dependent on a donor cornea.
The donor cornea is implanted in a similar way to a corneal transplant. However, it only serves as an attachment for the actual prosthesis. The donor cornea has no optical function.
This prosthesis consists of two parts:
The front part performs the light-transporting function of the cornea. The rear part gives the entire structure stability. The posterior plate also takes over the function of the previously removed iris.
The donor cornea and the rear part of the prosthesis have a circular hole the size of the pupil in the middle. The shaft of the front part of the prosthesis is pushed through this hole. A ring secures it behind the ring plate.
The plastic head of the anterior part of the prosthesis is held in position by the sutured donor skin and the ring plate.
The target group are patients who are defined as blind due to a diseased cornea.
In most cases, patients tolerate the prosthesis well. Complications can include
can occur. In many cases, however, these problems can be corrected.
The Boston KPro cannot provide perfect vision. Patients can regain orienting vision after the procedure.
57- 83 % of patients with a Boston prosthesis can recognize letters on a vision test chart at a distance of 6 meters that healthy people can identify at 60 meters. 19-23% of those affected can even identify letters at 6 meters that healthy people can identify at 12 meters.
The cost of a Boston KPro is around €3000.
In this therapy, doctors make use of findings from dentistry. Metal crowns and fillings can be permanently bonded to the mineral substance of the tooth.
The Italian ophthalmologist Benedetto Strampelli made use of this observation in 1963. He used a tooth root to attach an artificial cornea to the surface of the eye.
This osteo-odonto-keratoprosthesis is used in some clinics today. The patient's optic nerve and nerve cell layer must be intact for this.
It is necessary to remove the crystalline lens and the iris. A tooth from the patient is used as the transplant material. This is surgically removed together with the tooth root and surrounding jawbone.
The root of the tooth is then processed in such a way that an optical cylinder made of Plexiglas can be bonded to it. The artificial cornea created in this way is then placed on the patient's diseased cornea.
As a rule, this keratoprosthesis enables the patient to see with orientation. 20-40% of patients have more than 5% vision after 10 years.
Cosmetically, however, the result of such a keratoprosthesis operation is not very appealing. Skin-colored cheek mucosa grows over the eye and covers the white areas and the colored iris.
If a patient does not have teeth suitable for this purpose, bone material can also be taken from the splint bone. This type of procedure is also known as a "tibia corticalis" keratoprosthesis.
A piece of bone can then serve as a fixation for the optical cylinder.
The procedure presented below still has experimental status.
Here, a material is used for the artificial cornea whose structure can absorb water well. A special coating on the surface and a chemical modification of the edge make it easier for it to grow together with its surroundings.
The newly developed forms of artificial cornea were tested in the laboratory for their compatibility. For this purpose, they were implanted in several test animals. Most of the implanted variants grew successfully into the surrounding tissue. They proved to be sufficiently translucent and firmly anchored.
There was no rejection of the keratoprosthesis over a period of six months. Overall, the animals tolerated the keratoprosthesis well.
A keratoprosthesis made from a porous matrix of the protein collagen is very promising. Collagen is also the main component of the natural cornea. It is therefore particularly suitable for a keratoprosthesis.
The artificial cornea made from the collagen matrix has so far only been used in a pilot study. It is still in the experimental stage. The advantages of the artificial cornea made of collagen fibers are
Only two out of ten test subjects experienced a deterioration in vision following implantation. However, this deficit could be improved with contact lenses.
Even better results could be achieved if no sutures were used. The sutures used to date delay the ingrowth of the artificial cornea into the eye. This ultimately impairs healing and increases the susceptibility to complications.
Instead, a biological adhesive could be used to anchor the implant.
The procedure to implant the keratoprosthesis is usually performed as an inpatient under general anesthesia and the patient must stay in hospital for between four and six days after the operation, depending on the course of the procedure. The duration depends on whether complications occur.
Infections are generally still the biggest problem after the operation. For this reason, follow-up treatment with antibiotics is very important. An untreated or unrecognized infection can lead to blindness in the affected eye.
Furthermore, patients require intensive follow-up checks after keratoprosthesis surgery. These include
are checked.
The total number of artificial corneas transplanted is still low. Due to the complexity of keratoprosthesis surgery, the surgical center must have a certain amount of experience.
In general, further improvements can be expected in the field of artificial corneas. In particular, the material and thus the tolerability is the subject of research projects. If it is possible to reduce the costs of this complex procedure, this method will become more widely used.
Existing problems will also be solved by future developments. These include, for example, the relatively low resilience of the artificial cornea.
Last but not least, the lack of available donor corneas can then also be compensated for.
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