Underlying Principle
Although the Boston KPro is referred to as a keratoprosthesis—that is, an artificial cornea—this is only part of the treatment. The procedure still relies on a donor cornea.
The donor cornea is implanted in a manner similar to a corneal transplant. However, it serves only as an anchor for the actual prosthesis. The donor cornea has no optical function.
This prosthesis consists of two parts:
- Front part made of transparent plastic: a sort of pin with a front plate attached
- Rear part: A larger, circular plate (now made of transparent plastic, formerly of metal) with holes and a metal fixation ring
The front part takes over the light-transmitting function of the cornea. The back part provides stability to the entire structure. The back plate also takes over the function of the iris that was previously removed.
The donor cornea and the posterior part of the prosthesis have a circular hole in the center that is the size of the pupil. The shaft of the anterior prosthesis is inserted through this hole. A ring secures it behind the ring plate. The plastic head of the anterior prosthesis is held in place by the sutured donor cornea and the ring plate.
Goals and Results of the Boston KPro
The target group consists of patients who are classified as blind due to corneal disease.
In most cases, patients tolerate the prosthesis well. Complications may include, among other things,
- the development of elevated intraocular pressure (glaucoma),
- mucosal overgrowth on the optical element,
- macular edema, or
- a retro-prosthetic membrane
. In many cases, however, these problems can be resolved.
The Boston KPro cannot provide perfect vision. After the procedure, patients may regain functional vision.
57–83% of patients with a Boston prosthesis can recognize letters on an eye chart from a distance of 6 meters that healthy individuals can identify from 60 meters. 19–23% of patients can even identify letters at 6 meters that healthy individuals can recognize at 12 meters. The cost of a Boston KPro is approximately 3,000 €.
In this treatment, physicians draw on 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 today at some clinics. For this procedure, the patient’s optic nerve and nerve cell layer must be intact. It is necessary to remove the lens and the iris. A tooth from the patient serves as the transplant material. This tooth is surgically removed along with its root and the surrounding jawbone.
The tooth root is then prepared so that an optical cylinder made of Plexiglas can be bonded to it. The resulting artificial cornea is then placed over the patient’s diseased cornea. As a rule, this keratoprosthesis enables orientation vision. After 10 years, 20–40% of patients have visual acuity greater than 5%. From a cosmetic standpoint, however, the result of such a keratoprosthesis surgery is not very appealing. Skin-colored buccal mucosa grows over the eye, covering the white areas and the colored iris.
If a patient does not have teeth suitable for this purpose, bone material can also be harvested from the tibia. This form of the procedure is also known as a “tibia corticalis” keratoprosthesis. A piece of bone can then serve to secure the optical cylinder in place.
The procedure described below is still in the experimental stage.
Here, a material with a structure capable of absorbing water is used for the artificial cornea. Thanks to a special surface coating and a chemical modification of the edge, it can integrate more easily with the surrounding tissue.
The newly developed forms of the artificial cornea were tested in the laboratory for compatibility. To this end, they were implanted into several laboratory animals. The implanted variants integrated into the surrounding tissue with largely successful results. They proved to be sufficiently light-transmissive and firmly anchored.
Over a period of six months, no rejection of the keratoprosthesis occurred. Overall, the animals tolerated the keratoprosthesis well.
A keratoprosthesis made from aporous matrix of the protein collagen shows great promise. Collagen is also the main component of the natural cornea. Therefore, it is particularly well-suited for a keratoprosthesis.
The artificial cornea made from the collagen matrix has so far been used only in a pilot study. It is still in the experimental stage. Advantages of the artificial cornea made from collagen fibers would include:
- Improved vision similar to that achieved with a donor cornea transplant
- Better tolerance of this artificial cornea: Patients would need to take little or no medication to suppress a rejection reaction
- Pathogens cannot be transmitted with this type of artificial cornea
- In nine out of ten patients, the nerves severed during surgery grew into the new tissue. This ensured that the implant became sensitive to touch
Only two out of ten participants experienced a decline in vision following implantation. However, this impairment could be corrected with contact lenses.
Even better results could be achieved if sutures were not used during implantation. The sutures used to date delay the artificial cornea’s integration into the eye. This ultimately impairs healing and increases the risk of complications. Instead, a biological adhesive could be used to anchor the implant.
The procedure to implant the keratoprosthesis is usually performed on an inpatient basis under general anesthesia. Depending on how the surgery goes, the patient must remain in the hospital for between four and six days after the operation. The length of stay depends on whether complications arise.
Infections generally remain the biggest problem following the procedure. For this reason, follow-up treatment with antibiotics is very important. An untreated or undetected infection can, under certain circumstances, lead to blindness in the affected eye.
Furthermore, intensive follow-up examinations of patients are required after keratoprosthesis surgery. During these examinations,
- the fit of the artificial cornea,
- intraocular pressure, and
- visual acuity
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 level of experience.
In general, further improvements in the field of artificial corneas are expected. In particular, the material—and thus its biocompatibility—is the focus of research projects. If the costs of this complex procedure can be reduced, this method will see wider application and adoption.
Existing problems will also be resolved through future developments. These include, for example, the relatively low durability of the artificial cornea. Last but not least, this will also help address the shortage of available donor corneas.
