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Corneal transplantation



 

  Corneal transplantation, also known as corneal grafting or penetrating keratoplasty, is a surgical procedure where a damaged or diseased cornea is replaced by donated corneal tissue which has been removed from a recently deceased individual having no known diseases which might affect the viability of the donated tissue. The cornea is the clear part of eye in front of the iris and pupil. The surgical procedure is performed by ophthalmologists, medical doctors who specialize in eyes, and are often done on an outpatient basis (the patient goes home following surgery).

Additional recommended knowledge

Contents

Indications

Indications for corneal transplantation include the following:

  • Optical: To improve visual acuity by replacing the opaque host tissue by clear healthy donor tissue. The most common indication in this category is pseudophakic bullous keratopathy, followed by keratoconus, corneal degeneration, keratoglobus and dystrophy, as well as scarring due to keratitis and trauma.
  • Tectonic/reconstructive: To preserve corneal anatomy and integrity in patients with stromal thinning and descemetoceles, or to reconstruct the anatomy of the eye, e.g. after corneal perforation.
  • Therapeutic: To remove inflamed corneal tissue unresponsive to treatment by antibiotics or anti-virals.
  • Cosmetic: To improve the appearance of patients with corneal scars that have given a whitish or opaque hue to the cornea. [1]

Pre-operative examination

In most instances, the patient will meet with their ophthalmologist for an examination in the weeks or months preceding the surgery. During the exam, the ophthalmologist will examine the eye and diagnose the condition. The doctor will then discuss the condition with the patient, including the different treatment options available. The doctor will also discuss the risks and benefits of the various options. If the patient elects to proceed with the surgery, the doctor will have the patient sign an informed consent form. The doctor might also perform a physical examination and order lab tests, such as blood work, X-rays, or an EKG.

The surgery date and time will also be set, and the patient will be told where the surgery will take place.

Procedure

On the day of the surgery, the patient arrives to either a hospital or an outpatient surgery center, where the procedure will be performed. The patient is given a brief physical examination by the surgical team and is taken to the operating room. In the OR, the patient lies down on an operating table and is either given general anesthesia, or local anesthesia and a sedative.

With anesthesia induced, the surgical team prepares the eye to be operated on and drapes the face around the eye. An eyelid speculum is placed to keep the lids open, and some lubrication is placed on the eye to prevent drying. A metal ring is then stitched to the sclera, which will provide a base for a trephine.

A trephine is then placed over the cornea and is used by the surgeon to cut the host cornea. The trephine is then removed and the surgeon cuts a circular graft (a "button") from the donor cornea. Once this is done, the surgeon returns to the patient's eye and removes the host cornea.

The donor cornea is then brought into the surgical field and maneuvered into place with forceps. Once in place, the surgeon will fasten the cornea to the eye with a running stitch (as used in the upper image above) or a multiple interrupted stiches (as in the lower image). The surgeon finishes up by reforming the anterior chamber with a sterile solution injected by a canula, then testing that it's watertight by placing a dye on the wound exterior.

With the metal ring removed and antibiotic eyedrops placed, the eye is patched, and the patient is taken to a recovery area while the effects of the anesthesia wear off. The patient typically goes home following this and sees the doctor the following day for the first post operative appointment.

Risks

While the cornea is avascular, there is still a potential for some blood loss, usually from suturing the metal ring to the sclera. Any blood loss is typically less than a teaspoon, or less than 2 cc.

There is also a risk of infection. Since the cornea has no blood vessels (it takes its nutrients from the aqueous humor) it heals much slower than a cut on the skin. While the wound is healing, it is possible that it might become infected by various microorganisms. This risk is minimized by antibiotic prophylaxis (using antibiotic eyedrops, even when no infection exists).

Graft failure can occur at any time after the cornea has been transplanted, even years or decades later. The causes can vary, though it is usually due to new injury or illness. Treatment can be either medical or surgical, depending on the individual case.

Prognosis

When the primary purpose of a cornea transplant is to improve visual acuity, the prognosis is dependent upon whether the rest of the eye is healthy. If it is, then it should be possible to recover normal vision.

History

The first cornea transplant was performed in 1905, by Eduard Zirm, making it one of the first types of transplant surgery successfully performed.

Advances in microscopes enabled surgeons to get a better view of the surgical field, while advances in materials science enabled them to use sutures finer than a human hair.

Instrumental in the success of cornea transplants were the establishment of eye banks. These are organizations located throughout the world to coordinate the distribution of donated corneas to surgeons, as well as providing eyes for research. Some eye banks also distribute other anatomical gifts.

Synthetic corneas

In cases where there have been several graft failures or the risk for keratoplasty is high, synthetic corneas can substitute successfully for donor corneas. Such a device contains a peripheral skirt and a transparent central region. These two parts are connected on a molecular level by an interpenetrating polymer network, made from poly-2-hydroxyethyl methacrylate (pHEMA). AlphaCor is an FDA approved type of synthetic cornea measuring 7.0 mm in diameter and 0.5 mm in thickness. The main advantages of synthetic corneas are that they are biocompatible, and the network between the parts and the device prevents complications that could arise at their interface. Although the probability of retention is less than that of donor corneas, it does not carry the risk of diseases that could be transmitted through donor tissue. Another difference is the price; synthetic corneas usually cost about $7,000, while processing fees for donor corneas run about $2,800.[citation needed]

Alternatives

Phototherapeutic keratectomy (PTK)

Main article: Phototherapeutic keratectomy

Diseases that only affect the surface of the cornea can be treated with an operation called phototherapeutic keratectomy. With the precision of an excimer laser and a modulating agent coating the eye, irregularities on the surface can be removed. However, in most of the cases where corneal transplantation is recommended, PTK would not be effective.

Intrastromal corneal ring segments

Main article: Intrastromal corneal ring segments

In this procedure, ring segments are placed into the stroma in order to push out against the curvature of the irregular cornea shape. The eye returns to its more natural curvature, allowing improved vision. There is a good chance of vision improvement with these rings, but it is not guaranteed.

Contact lenses

Main article: Contact lens

In the early stages of some diseases (such as keratoconus), contact lenses can be used to improve vision. Contact lenses improve vision by tear fluid filling the gap between the irregular cornea surface and the regular inner surface of the lens. However, this is only for the early stages and corneal transplant is necessary for vision correction if eyesight worsens.

New technology

High speed lasers

Blades are being replaced by high speed lasers in order to make surgical incisions more precise. These improved incisions allow the cornea to heal more quickly and the sutures to be removed sooner. The cornea heals more strongly, and the risk of transplant failure is smaller than with standard blade operations. Not only does this dramatically improve visual recovery and healing, it also allows the possibility for improvement in visual outcomes.

In 2007, Seattle-based SightLife™, one of the leading corneal tissue banks in the world, introduced a process for the preparation of donated corneal tissue using a Femtosecond Laser. This process is known as Custom Corneal Tissue™.

Riboflavin

A treatment that involves a one-time application of riboflavin eye drops and thirty minutes of UV-A light can arrest the progression of Keratoconus. The riboflavin, when activated by UV-A light, augments the collagen cross-links in the stroma. With this treatment, the cornea regains some of its mechanical strength. When this treatment is combined with intrastromal cornea ring segments, it has been shown to reverse Keratoconus.

DSEK/DSAEK

Descemet's Stripping Automated Endothelial Keratoplasty (DSAEK) is one of the newest techniques in corneal transplantation. In DSAEK, the diseased Descemet's membrane is removed and replaced with that of a healthy donor. DSAEK uses a only a small incision that is either self-sealing or may be closed with a few sutures. The small incision offers several benefits over traditional methods of corneal transplant such as Penetrating Keratoplasty. Because the procedure is less invasive, DSAEK leaves the eye much stronger and less prone to injury than full-thickness transplants. Additionally, DSAEK has a more rapid rate of visual recovery. Vision is typically restored in one to three months rather than one to two years.

Stem cells

There is a bioengineering technique that uses stem cells to create corneas or part of corneas that can be transplanted into the eyes. Corneal stem cells are removed from a healthy cornea. They are collected and, through laboratory procedures, made into five to ten layers of cells that can be stitched into a patient’s eye. The stem cells are placed into the area where the damaged cornea tissue has been removed. This is a good alternative for those that cannot gain vision through regular cornea transplants. A new development, announced by the University of Cincinnati Medical School in May of 2007, would use bone marrow stem cells to regrow the cornea and its cells. This technique, which proved successful in mouse trials, would be of use to those suffering from inherited genetic degenerative conditions of the cornea, especially if other means like a transplant aren't feasible. It works better than a transplant because these stem cells keep their ability to differentiate and replicate, and so keep the disease from recurring, longer and better.

Epidemiology and economics

Corneal transplant is one of the most common transplant procedures.[citation needed] Although approximately 100,000 procedures are performed worldwide each year, some estimates report that 10,000,000 people are affected by various disorders that would benefit from corneal transplantation.[2]

In Australia, approximately 1,500 grafts are performed each year. [3] According to the NHS Blood and Transplant, over 2,300 corneal transplant procedures are performed each year in the United Kingdom. [4] Between April 1, 2005 and March 31, 2006, 2,503 people received corneal transplants in the UK. [5]

In the United States, the cost is usually covered in part by Medicare and health insurers. Reimbursement depends on your personal healthcare provider. Usually 80% of the cost will be covered by your agency. Those on Medicare will be reimbursed up to $1,200 while the remainder is left up to the patient. The average cost of the procedure ranges from $7,500 to $11,000. However, there is a company that claims to be able to cover all costs for as little as $3,700. In 2005, there were about 32,840 corneal transplant recipients. The estimated first year billed charges per patient, including medications, was $19,100. There were a larger number of transplants for patients over 65 than under, 18,000 compared to 14,840. Milliman estimates that there will be 32,700 corneal transplant recipients in 2006 [6]. The predicted cost for the first year after transplantation in 2006 is $21,500[7].

In Sweden, corneal transplants are available free of cost. Every year about six hundred of these operations are performed.

References

    Sources

    • http://biomed.brown.edu/Courses/BI108/2006-108websites/group11corneal%20implants/index.html
    • http://www.crstoday.com/PDF%20Articles/1104/crst1104_pr.pdf
    • http://www.vision-institute.com/techniques/ptk.html
    • http://dro.hs.columbia.edu/icrs.htm
    • http://www.indystar.com/apps/pbcs.dll/article?AID=/20060419/CUSTOMPUB01/60413016/-1/ZONES04
    • http://www.news.com.au/story/0,10117,18829980-29277,00.html
    • http://www.cnyeyecare.com/newdetail.php?id=21
    • http://science.howstuffworks.com/news-item94.htm
    • http://www.milliman.com/expertise/healthcare/publications/rr/pdfs/US-Organ-Tissue-Transplant-2005-RR.pdf
    • http://www.milliman.com/expertise/healthcare/publications/rr/pdfs/US-Organ-Tissue-Transplant-2006-RR.pdf

     
    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Corneal_transplantation". A list of authors is available in Wikipedia.
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