The cornea is the clear window at the front of the eye. It is the part of the eye that performs most of the focusing of light for the eye. To have good vision, a transparent cornea is required. If a disease affects the transparency of the cornea, the quality of vision will be affected. To improve vision, a corneal transplant may be required.
The term corneal transplant (also known as a corneal graft) refers to a group of operations in which the cornea is removed from an organ donor and used to replace the diseased cornea of a living person. There are several types of corneal transplants including Penetrating keratoplasty (PK), Descemet Membrane Endothelial Keratoplasty (DMEK), Descemet Stripping Endothelial Keratoplasty (DSEK) and Deep anterior Lamellar Keratoplasty (DALK).
Penetrating keratoplasty is the traditional form of corneal transplantation. It is used in conditions where there is disease affecting all layers of the cornea. In a PK, the entire central portion of the corneal is removed and this is replaced with a similarly sized button of cornea from a donor. The graft is then stitched into place. The best vision achieved following PK does not occur for at least 12-18 months following surgery. The stitches are usually removed after 12 months, after which glasses, contact lenses, or further surgery may be required to achieve the best possible vision.
Penetrating keratoplasty is a corneal transplant in which a diseased circular, full thickness section of the cornea is removed and replaced with a similar sized transplant from a donor cornea. It is performed in cases where there is disease which involves the entire thickness of the cornea. The corneal transplant tissue is stitched to the eye using tiny sutures. The manner in which these sutures are inserted must be done precisely and with exact tension to ensure that the transplant is well secured and that it’s shape is regular (to ensure good future vision). The stitches remain in place for 12 to 18 months until the transplant has embedded sufficiently to the eye. After this the stiches are removed (this is usually possible in the clinic). Once the stiches are removed, further measures may be required to achieve the best possible vision. These include glasses, contact lenses, laser surgery and artificial lens implant surgery.
DMEK is a recently developed technique in corneal transplantation. In DMEK the innermost layer of the cornea is replaced. A 15 micrometer (0.015mm) thick layer of tissue (the graft) is carefully removed from a donor cornea (the cornea from a recently deceased person). The graft is then inserted inside the eye through a 2.75mm incision made in the cornea. It is then unfolded and floated into position with an air bubble. This is all done without touching the tissue and without any sutures.
Prior to the development of DMEK and DSAEK, treatment of corneal diseases affecting the endothelium required a full thickness corneal transplant (penetrating keratoplasty), after which best vision is not achieved for 12-18 months.
After DMEK surgery, the best vision is achieved within 6 weeks. The likelihood of the immune system rejection a DMEK graft is significantly less than in penetrating keratoplasty or DSAEK. After DMEK surgery, the patient must lie on their back for 2 hours and then for 45 minutes in every hour overnight. This ensures that the graft adheres well to the inner side of the cornea.
DALK is a type of corneal transplant used in diseases of the cornea where the inner layer of the cornea (endothelium) is healthy. In such surgery, the front 80-90% of the cornea is removed and replaced with corneal tissue from an organ donor. The donor cornea is sutured (stitched) into place.
During the operation, benefits of DALK compared to Penetrating Keratoplasty include a reduced risk of complications during surgery such as bleeding and infection which can threaten vision if they occur. Following surgery, there is a much lower risk of the body’s immune system rejecting the transplant as the main target of the immune system in corneal transplant rejection is the inner layer which has not been transplanted in cases of DALK. Finally, the long-term of successful DALK surgery is a significantly longer likely survival.