Overview of Artificial Cornea Transplantation
There are about 4.9 million people around the world who are completely blind because of corneal diseases, making up 12% of all blindness. Common causes include diseases that affect the front of the cornea like trachoma, infections, eye injuries, chemical damage, and these are particularly common in developing countries. One way to treat these conditions is through corneal transplantation (where part or all of the cornea is replaced), which is often quite successful with most grafts (the transplanted corneas) still working well after one and even five years. However, this success rate goes down if multiple transplants are needed or if the operation is particularly complicated.
There are factors, like recurring or long-term inflammation of the eye’s surface, glaucoma (a group of eye conditions that damage the optic nerve), or the development of blood vessels in the cornea, which can make a cornea transplant risky. Also, there may be limitations in the availability of donor corneas due to a lack of donors and the need for specialized eye banks. In these cases, an artificial cornea transplant might be considered.
There have been many suggestions for artificial cornea transplants, also known as keratoprosthesis. The first idea was proposed back in 1789 by Pellier de Quengsy. The designs often include a clear center with hard skirt plates which hold the donor cornea tissue or a single-piece design with a soft optic and skirt. The early designs used materials like rubber, milk protein, and Dacron, but these didn’t work out well because they didn’t integrate well with the tissue and resulted in the device being pushed out after being implanted. The successful use of corneas from cadavers reduced the interest in artificial corneas. However, the discovery of a material called polymethylmethacrylate allowed biocompatible (safe to use in the body) artificial corneas to be implanted.
More recently, soft polymers have been used which simulate the natural cornea. One such artificial cornea, AlphaCor, was approved by the FDA in 2003. But after one or two years, up to 27% of these cases experienced a ‘stromal melt’, where the cornea weakens and thins, resulting in many of them needing to remove the artificial cornea. Another similar design that used a different material had even worse results, with 86% of the implants failing.
The two most common types of artificial cornea transplants today are the Boston KPro type 1 and the Osteo-odonto-Keratoprosthesis (OOKP). The Boston KPro type 1, introduced in 1965 and FDA-approved in 1992, is now the most common type of artificial cornea transplant. It’s made up of a front plate that has a central optical stem, a backplate, and a donor cornea in between. The OOKP uses a patient’s own tooth root-alveolar bone complex to better integrate with the tissue. This procedure replaces the front part of the eye with the OOKP and a patch of mucous membrane, which can handle dry conditions and some inflammation.
Currently, several other artificial cornea designs are being studied and developed. However, the results so far indicate that the Boston type 1 KPro still performs better overall. That said, affordability and availability could make some of the newer designs a valid alternative.
Improvements in ‘skirt’ materials may lead to further developments in artificial corneas. Various alternative materials are being explored in labs and animal experiments. For example, OOKP may encounter issues with bone resorption (loss of bone tissue), so scientists are testing a synthetic replacement in the lab. Other materials, like a graphene oxide titania-based biomaterial, have been implanted into rabbit corneas without causing an immune or inflammatory reaction, showing promise for future keratoprosthesis.
Anatomy and Physiology of Artificial Cornea Transplantation
There are different ways to place a keratoprosthesis – these are medical devices that are implanted into the eye to try and restore vision. Here are three examples:
1. The Boston KPro or Auralab KPro involves two plate-like structures, with a piece of donor cornea tissue sandwiched between these plates.
2. Legeais, AlphaCor, KeraKlear and others use soft materials for the optic (the part you see through) and a sort of soft ring that’s held in place inside the layers of the cornea.
3. The osteo-odonto-KPro (OOKP) has a clear optic in the center, and it’s the device is held in place on the outside of the cornea and sclera (the white part of the eye).
The Boston KPro used to have a solid backplate, but this caused the cornea to break down over time. This happened because the solid plate didn’t allow enough fluid to reach the donor cornea tissue, causing it to die and fall off. That could lead to infections. To fix this issue, the Boston KPro’s backplate was changed to have tiny holes, and a bandage contact lens was added. This design allows more fluid to reach the donor tissue and helps it bond better with the keratoprosthesis, reducing the risk of infections.
The Osteo-odonto-KPro (OOKP), on the other hand, uses part of a tooth and its surrounding bone to replace the ring structure of the keratoprosthesis. Older keratoprostheses tried to use a variety of materials for this, like Teflon or ceramic, but they had problems lasting a long time and integrating with the eye tissue. This tooth-and-bone solution works better, and sometimes a piece of shinbone is used if the patient doesn’t have any teeth. A graft (a piece of tissue moved from one part of the body to another) from the inner part of the cheek is used, and the device replaces certain parts of the eye’s structure. This graft can handle dryness and some inflammation, and the tooth-bone piece can bond with the eye tissue and last for quite a long time. The surgery is performed by both oral and eye surgeons in several stages.
Why do People Need Artificial Cornea Transplantation
Patients with severely cloudy corneas who cannot see out of either eye, and for whom traditional cornea and stem cell transplants would not work, can be considered for a procedure to implant an artificial cornea in one eye, provided that the back of the eye is in good health. Doctors would need to evaluate the amount of scar tissue or hardening and inflammation on the eye’s surface, any shortening of the eye “pockets,” basic tear production, ability to blink, and any eyelid abnormalities. These factors will influence the appropriate surgical approach and choice of the artificial cornea (known as a keratoprosthesis).
In the case of the Boston-type artificial cornea (known as KPro Type I), patients need to have a good blink reflex and tear production, as measured by a standard test. Any type of eye condition affecting the front part of the eye can be acceptable. However, severe conditions or diseases affecting the back of the eye can influence the success of the procedure. These conditions may include corneas that have become cloudy and grown excessive blood vessels due to repeated failed transplant attempts or diseases such as aniridia, corneal dystrophies, herpes-related eye inflammation, or previous corneal infections. The patient will also need to be able to apply and keep in a soft contact lens if the Type I KPro is being considered. For patients who don’t meet these criteria, and who also can’t have a different procedure (OOKP – or Osteo Odonto Keratoprosthesis) due to poor or missing teeth, the Boston Type II KPro may be appropriate.
Patients with dry eyes, difficulties blinking and eyelid abnormalities, hardening and inflammation on the surface of the eye such as in conditions like Sjögren’s syndrome, graft versus host disease, Stephen Johnson Syndrome, mucous membrane pemphigoid, chemical burns, Lyell syndrome, and an advanced form of an infectious eye disease known as trachoma, might be suitable for the OOKP surgery if they have at least light perception vision.
When a Person Should Avoid Artificial Cornea Transplantation
In simpler terms, the Boston KPro Type I, a type of artificial cornea transplant, might not be the right choice for some people due to various reasons. Firstly, if you cannot blink, make tears, or if you have underlying eye conditions, like retinal detachment which is a severe condition where the retina separates from the back of the eye, this treatment may not be for you. If your vision is extremely poor, i.e. you can only perceive light, or if your eyelids are abnormally structured, it can complicate the process. There are other factors like certain conditions which shorten the eyelid’s length and also cause difficulty in keeping a soft contact lens in place.
Patients who cannot see light or have a shrunken and non-functional eyeball, or are unrealistically expecting results might not be suitable for this procedure. If a patient has an incomplete closing of the eyelids, they need to consult with a specialist to reduce possible exposure risks.
Previously, the Boston KPro implantation was done in children due to several benefits over traditional cornea transplantation. These include lesser chances of body rejection, faster recovery of vision as the procedure is less disruptive and fewer anesthetic follow-up exams. However, recent studies showed higher complication rates and progression to a shrunken and non-functional eyeball condition. Hence, it is no longer advised for the pediatric age group.
For OOKP, another type of artificial cornea transplant, advanced glaucoma (a group of eye conditions that damage the nerve that transmits visual images to your brain) is a borderline reason to avoid the procedure. It’s strongly advised against using this procedure in patients below 18 years due to their high metabolic activity in their bones. It’s also not suitable for patients who have severe vision conditions like inability to perceive light or having a shrunken and non-functional eyeball. Patients with the detached retina or other serious eye conditions, making vision recovery unlikely, are also advised against OOKP. Other factors where OOKP might not be suitable include patients who will not be able to attend the follow-up procedures regularly or who have poor mental health.
Equipment used for Artificial Cornea Transplantation
When doctors perform a Boston KPro Type I procedure, they use a variety of tools and equipment. This includes the Boston KPro type 1 device itself, which is a special type of artificial cornea. Other key items include a donor cornea tissue, a tool known as a trephine used for cutting circular sections, and very fine sutures or stitches made from nylon. They may also use a soft bandage contact lens, an intraocular lens if they’re also removing your natural lens, and specially-equipped surgical trays designed for this type of surgery. If they’re performing other eye surgeries at the same time, additional tools like a vitrectomy tray or a glaucoma drainage device could be required.
In an OOKP procedure, which is another type of corneal transplant, the doctors will again employ a corneal graft surgical tray. They’ll also use a cautery tool for controlling bleeding, a bone saw, retractors (tools that hold the eye open), and a diamond-coated flywheel which is a device for cutting bone. A drill, an optic cylinder for vision improvement, a fliering ring to secure the new cornea, a trephine, silk traction sutures, and fine nylon stitches are also part of the tools required for this procedure.
Who is needed to perform Artificial Cornea Transplantation?
A team of eye doctors, specially trained in different areas, such as the front part of the eye, glaucoma, retinal diseases, and cosmetic eye surgery work together along with dental surgeons, anesthesiologists (doctors who make you sleep during surgeries), regular doctors, nurses, and psychologists to provide the best possible care. This team not only provides medical support but also emotional and social support. They also ensure that there is a plan in place for you to get to the hospital for both scheduled and emergency care.
As part of their care process, they also have, attached to them, patient support groups for you to share and learn from other’s experiences. They provide easy-to-understand written guides or digital resources that explain what your surgery involves and how to care for yourself afterwards. This way, you always have a clear idea of what to expect before, during, and after the procedure.
Preparing for Artificial Cornea Transplantation
The Boston KPro Type 1 is a procedure performed when other medical and surgical treatments have not been successful in preserving or improving a patient’s vision. Before the surgery, doctors will assess your visual function, as well as look at any past treatments you’ve had, such as surgeries for glaucoma or previous corneal transplants. They’ll also note any reaction you may have had to steroids, and existing conditions in the back part of your eye. If your doctor is planning to remove your cataract during the procedure, they may measure the dimensions of your eye. They will also need to address any existing issues like herpes infections or eye inflammation before the procedure.
OOKP refers to a specific type of eye surgery. Before undergoing OOKP, your vision should allow you to at least perceive light. To anticipate the outcome of the surgery, doctors use multiple tests including ultrasonography, which uses sound waves to capture images, and electrodiagnostics to measure the electrical activity in the eye. Due to the nature of the condition, measuring the pressure inside your eye might be difficult, and sometimes the doctor might have to estimate it by touch. It’s also important for doctors to evaluate your eye surface and dryness, as well as your oral health. Sometimes, doctors even use X-ray imaging of the teeth to pick a suitable tooth for donation.
Before the first stage of the OOKP surgery, you’ll need to start using an antiseptic and antifungal mouthwash a day before. During the first phase, a tissue, called BMM, is taken from you and attached to the eye surface bed, and a specially prepared piece of bone, tooth, and acrylic is implanted into a soft pouch in your opposite lower eyelid. Between the two stages of the surgery, it’s essential to carefully look out for any complications such as thinning or infection, both in the mucus layer of the eye and in the implanted piece.
How is Artificial Cornea Transplantation performed
There are two main types of surgeries to treat severe corneal damage: Boston KPro Type I and OOKP.
Boston KPro Type I is performed as follows:
First, any thickened cornea is removed to get a better view. The center of the cornea is marked using a dye. If additional work on the eye’s internal structure (vitreous) is needed, this is preplanned. In cases where the natural lens isn’t functioning properly, it’s removed. An artificial lens might be placed inside the eye.
Before taking out the patient’s cornea, a replacement cornea (a “donor”) is prepared. This donor cornea is cut to a specific size and has a hole made in the center. The donor cornea is secured between two plates and is then ready to be implanted into the patient.
A tiny cut is made in the eye, after which a special gel is injected to maintain the shape of the eye while the patient’s cornea is removed. The donor cornea is then stitched into place. Additional procedures can be performed at this stage due to the clear view inside the eye. Once everything is in place, the surgical area is cleared of any debris, and a special contact lens is placed in the eye.
As for the OOKP procedure, it is a more complicated two-stage process.
In the first stage, a layer of mucous membrane (a kind of skin tissue) is removed from the inside of the mouth. This is prepped for grafting onto the eye. Any damaged areas on the eye are removed, and the membrane is stitched onto the outer layer of the eye.
Interestingly, a tooth from the patient is used in this procedure. The tooth and part of the surrounding bone are taken out. From this material, a small support structure is created that will hold a small lens. This construct is then implanted into a pouch beneath the skin to allow it to integrate with the body’s tissues.
In the second stage, the support structure is removed from under the skin. It’s then fitted onto the eye after lifting part of the grafted membrane. Further work is done inside the eye to reduce risks of complications. The support structure is finally secured onto the eye, and sterile air is injected to seal everything in place. The operation is completed by cutting a hole in the membrane to expose the small lens. After the operation, lifelong topical antibiotics and careful cleaning of the eye are advised.
Possible Complications of Artificial Cornea Transplantation
Complications may happen if an artificial cornea replacement, known as a kPro, doesn’t properly integrate with the body. This can cause issues like ongoing inflammation in the eye’s front chamber (anterior chamber), the formation of tissue around the artificial cornea (retroprosthetic membranes), or even melting of the surrounding natural cornea tissue. This melting could result in leaks and severe inflammation in the eye (endophthalmitis) which is typically fixed by replacing the cornea tissue and using cartilage from the patient.
Patients with autoimmune diseases- conditions where the body’s immune system attacks its own cells- are more at risk. In the U.S, up to 12.5% of patients with a Boston brand artificial cornea have reported inflammation, compared to 17% internationally, and this can lead to total blindness.
Glaucoma, a condition that damages the optic nerve, is a common complication that can occur after any artificial cornea transplantation. Conditions like high eye pressure before surgery, mucous membrane pemphigoid and Steven Johnson syndrome increase the risk of developing and worsening glaucoma after surgery. Recent studies have found that almost a quarter of patients who’ve had a Boston brand artificial cornea transplanted have developed glaucoma after the operation.
Using eye imaging technology and regular visual field testing, doctors can detect glaucoma. Doctors have found it beneficial to implant devices to control eye pressure either before or during the artificial cornea surgery. These devices can help prevent the progression of glaucoma.
The artificial cornea combined with a surgery to remove the eye’s visual jelly (vitreous) can reduce the formation of tissue around the artificial cornea and stabilize long-term vision. Risks of cornea melt, inflammation, and glaucoma that need further surgery increase over time. For the Boston brand artificial cornea, having a full replacement seems to lower the risk of recurrent cornea melts, with localized repairs only being a temporary option.
The OOKP version of artificial cornea, which uses the patient’s tooth, does have the advantage of being fully compatible with the body, but complications can still arise. Issues can include the risk of developing glaucoma, retinal detachment, inflammation, hemorrhage in the jelly-like part of the eye (vitreous), or even a severe eye inflammation. Sometimes, patients undergo a surgery to remove the visual jelly in the eye at the time of the artificial cornea surgery to reduce these risks.
Mucosal melt and ulceration can expose the laminar (the tougher layer of connective tissues in the eye), which would need to be repaired with a mucosal graft. Poor oral health and scarring increase the risks for mucosal ulcers and thinning. Laminar resorption, where your body breaks down and absorbs part of the laminar, can occur and is detected with an eye examination or a CT scan. This risk is higher in patients with Steven-Johnson Syndrome.
What Else Should I Know About Artificial Cornea Transplantation?
Corneal opacities, also known as clouding of the eye’s cornea, are one of the top causes of blindness across the world, alongside conditions like cataracts, glaucoma and macular degeneration. Unfortunately, not everyone has access to corneal transplants, a commonly used treatment. This is because the necessary infrastructure and donor tissue supply is lacking in some parts of the world.
However, there have been improvements in the area of the artificial cornea, also known as keratoprostheses. These devices, which are compatible with the body, have become more stable and widely available. Currently, there is no equivalent replacement for a natural corneal transplant, but these artificial corneas offer a possible option for those who cannot undergo a regular corneal transplant due to pre-existing issues such as scarring and the formation of new blood vessels in the eye.
There are many new types of artificial cornea devices being developed, and it’s important that we study, test, and evaluate these properly. Recent guidelines have been put in place to ensure all groups researching these devices follow the same reporting standards. This allows for better data collection and more accurate comparisons between devices.
