Refractive keratoplasty is a generic term which includes surgical procedures performed to reshape the cornea of the eye to correct vision problems. Vision occurs when light rays are bent or refracted by the cornea and lens and received by the retina, (the nerve layer at the back of the eye), in the form of an image, which is sent through the optic nerve to the brain. Refractive errors occur when the eye cannot properly focus light and images appear out of focus. The main types of refractive errors are myopia (nearsightedness), hyperopia (farsightedness) and astigmatism (distortion). Presbyopia (aging eye) is a problem of the lens and is characterized by the inability to bring close objects into focus. Refractive errors are generally corrected with glasses or contact lenses. Refractive eye surgery (or refractive keratoplasty) involves procedures that permanently change the shape of the corneal surface.
Refractive keratoplasties include but are not limited to the following surgeries:
- Radial keratotomy (RK) is a surgical correction for myopia (nearsightedness). Using a high-powered microscope, the physician places microincisions (usually eight or fewer) on the surface of the cornea in a pattern much like the spokes of a wheel. The incisions are very precise in terms of depth, length, and arrangement. The microincisions allow the central cornea to flatten, thus reducing the convexity of the cornea, which produces an improvement in vision.
- Minimally Invasive Radial Keratotomy (mini-RK) is intended in cases of myopia, to alter the cornea’s shape and consequently the refraction by reducing the millimeters of cornea that are incised.
- Photorefractive Keratectomy (PRK) uses a computerized laser to correct mild to moderate myopia. The laser delivers bursts of ultraviolet light that vaporize precisely targeted corneal tissue, thus altering the corneal curvature, and improving the focus of light on the retina in the back of the eye.
- Photoastigmatic Keratectomy (PARK or PRK-A) is a refractive surgical procedure to correct myopia with mild to moderate degrees of astigmatism.
- Conductive Keratoplasty involves the application of radiofrequency thermal energy to increase the curvature of the cornea and reduce hyperopia.
- Astigmatic keratotomy (AK) (arcuate incision, corneal wedge resection) is a refractive surgical procedure similar to RK that is used to reduce astigmatism. Instead of radial incisions, a curvilinear pattern is used to smooth the areas of the cornea that are too steeply curved.
- Lamellar keratoplasty (LKP) partial thickness corneal grafting.
- Automated Lamellar Keratoplasty (ALK) can correct hyperopia. For the treatment of moderate farsightedness, the cornea is opened across the top to form a type of “cap,” using an automated instrument. When the “cap” is positioned back into its original location on top of the eye, microscopic scar tissue is formed, causing the “cap” to bulge out, thus correcting the overly flattened cornea that is associated with hyperopia. Almost like Velcro, the cornea and “cap” adhere to each other, eliminating the need for sutures. Normally, one eye is treated at a time, with about three to four weeks allowed between each eye surgery. To ease any discomfort, the eye is anesthetized with special drops, and the patient is given a mild sedative to remain relaxed and aware throughout the procedure.
- Laser In-Situ Keratomileusis (LASIK) is a procedure to correct or reduce moderate to high levels of myopia. In LASIK, the surgeon creates a flap in the cornea using a microkeratome. An excimer laser is used to remove a micro-thin layer of tissue from the exposed corneal surface. The flap is replaced without the need for sutures. This procedure is very similar to ALK for myopia.
- Keratomileusis involves removing, freezing, and lathing the patient’s cornea, followed by its replacement onto the corneal bed. This surgery has been proposed for myopia and aphakic hyperopia (aphakia is the absence of the lens of the eye).
- Refractive Lensectomy is a refractive surgical procedure designed for use in patients 40 years of age and older whose natural lens has become more rigid and less flexible. This procedure involves replacement of the natural lens of the eye with an accommodative intraocular lens using the same technique as modern cataract surgery. This lens allows the restoration of the eye’s natural focusing ability.
- Refractive Epikeratophakia is a surgical procedure which involves the removal of the corneal epithelium from the recipient eye and the suturing of a prelathed donor corneal graft onto the surface of the recipient cornea.
Therapeutic keratoplasties include but are not limited to the following surgical procedures:
- Penetrating keratoplasty (PK) or keratoplasty (PKP) (corneal transplantation) is a therapeutic surgical procedure indicated for a number of serious corneal conditions, e.g., scarring, edema, thinning, and distortion.
- Lamellar keratoplasty (LKP) is a partial thickness corneal grafting.
- Penetrating keratoplasty (PK) is full thickness corneal grafting.
Lamellar or non-penetrating keratoplasty is a corneal transplant procedure in which a partial thickness of the cornea is removed. The diseased tissue is replaced with a partial-thickness donor cornea. Lamellar keratoplasty may be indicated for a number of corneal diseases, including scarring, edema, thinning, distortion, dystrophy, degenerations and keratoconus.
Penetrating Keratoplasty (PK) involves the replacement of the full thickness of the cornea with donor cornea, while retaining the periphereal cornea. Most penetrating keratoplasties are performed to improve poor visual acuity caused by an opaque cornea. PK is also used to restore altered corneal structure; to prevent loss of the globe that has been punctured; and to remove active corneal disease, such as persistent severe bacterial, fungal, or amebic inflammation.
Epikeratophakia is a surgical procedure which involves the removal of the corneal epithelium from the recipient eye and the suturing of a prelathed donor corneal graft onto the surface of the recipient cornea. This surgery has been proposed as a means of correcting adult and pediatric aphakia, keratoconus (a conical protrusion of the cornea, caused by thinning of the stroma, and resulting in major changes in the refractive power of the eye), and myopia.
Endothelial keratoplasty (EK), also referred to as posterior lamellar keratoplasty (PLK), is a form of corneal transplantation in which the diseased inner layer of the cornea, the endothelium, is replaced with healthy donor tissue. Specific techniques include Descemet’s stripping endothelial keratoplasty (DSEK); Descemet’s stripping automated endothelial keratoplasty (DSEAK), or Descemet’s membrane endothelial keratoplasty (DMEK) or Descemet’s membrane automated endothelial keratoplasty (DMAEK).
Descemet stripping with endothelial keratoplasty (DSEK) involves the scraping of the Descemet membrane and endothelium from the recipient cornea instead of the lamellar dissection and excision procedures performed in DLKP and DLEK. DSEK is also less technically challenging than DLEK. The primary complications of endothelial replacement procedures are disc dislocation and endothelial cell loss.
Deep anterior lamellar keratoplasty (DALK) is used when the pathology is confined to front layers of the cornea. In this procedure, most of the anterior layers of the cornea (i.e., epithelium, Bowman membrane, and stroma) are removed.
Deep lamellar keratoplasty (DLKP) is a surgical method that completely removes pathological corneal stromal tissue down to the Descemet membrane, followed by transplantation of donor tissue. This technique was modified with redesigned instrumentation, and renamed deep lamellar endothelial keratoplasty (DLEK).
Descemet’s stripping endothelial keratoplasty (DSLEK) is a variation of the DLEK. In this procedure the surgeon removes the endothelium and some (or all) of the Descemet’s membrane and transplants the endothelium, Descemet’s membrane, and a thin layer or stroma.
Keratoprosthetic devices are designed to be implanted in patients with severe bilateral corneal conditions, such as Stevens-Johnson syndrome, chemical burns, and repeated failure of PK. In general, keratoprostheses consist of a transparent cylinder-shaped optical portion and a haptical portion. The optical cylinder is inserted into a central circular opening of the opacified cornea, focusing images on a functioning retina. The haptical section is fixed to and buried under neighboring tissue. The different designs of keratoprostheses vary primarily in the haptical portion of the devices. For example, the osteo-odonto-keratoprosthesis (OOKP) is a method of corneal substitution which uses a prosthesis composed of an acrylic optical cylinder mounted in a section of one of the patient’s own teeth. This type of implant is proposed for use in patients who are at high risk of graft rejection, as autologous tissue is utilized for the procedure. The biocolonisable microporous fluorocarbon haptic keratoprosthesis (BIOKOP) procedure utilizes a synthetic hydrogel core surrounded by a porous skirt that allows biointegration and prevents epithelial down growth.
The established surgical treatment for corneal disease is penetrating keratoplasty (PK), which involves the creation of a large central opening through the cornea and then filling the opening with full-thickness donor cornea that is sutured in place. Visual recovery after PK may take one year or more due to slow wound healing of the avascular full-thickness incision, and the procedure frequently results in irregular astigmatism due to the sutures and the full-thickness vertical corneal wound. PK is associated with an increased risk of wound dehiscence, endophthalmitis, and total visual loss after relatively minor trauma for years after the index procedure. There is also risk of severe, sight-threatening complications such as expulsive suprachoroidal hemorrhage, in which the ocular contents are expelled during the operative procedure, as well as postoperative catastrophic wound failure.
A number of related techniques have been, or are being, developed to selectively replace the diseased endothelial layer. One of the first endothelial keratoplasty (EK) techniques was termed deep lamellar endothelial keratoplasty (DLEK), which used a smaller incision than PK, allowed more rapid visual rehabilitation, and reduced postoperative irregular astigmatism and suture complications. Modified EK techniques include endothelial lamellar keratoplasty, endokeratoplasty, posterior corneal grafting, and microkeratome-assisted posterior keratoplasty. Most frequently used at this time are Descemet’s stripping endothelial keratoplasty (DSEK), which uses hand-dissected donor tissue, and Descemet’s stripping automated endothelial keratoplasty (DSAEK), which uses an automated microkeratome to assist in donor tissue dissection. A laser may also be utilized for stripping in a procedure called femtosecond laser-assisted corneal endothelial keratoplasty (FLEK). These techniques include some donor stroma along with the endothelium and Descemet’s membrane, which results in a thickened stromal layer after transplantation. If the donor tissue comprises Descemet’s membrane and endothelium alone, the technique is known as Descemet’s membrane endothelial keratoplasty (DMEK). By eliminating the stroma on the donor tissue and possibly reducing stromal interface haze, DMEK is considered a potential improvement over DSEK/DSAEK. A variation of DMEK is Descemet’s membrane automated EK (DMAEK). DMAEK contains a stromal rim of tissue at the periphery of the DMEK graft to improve adherence and increase ease of handling of the donor tissue.
EK involves removal of the diseased host endothelium and Descemet’s membrane with special instruments through a small peripheral incision. A donor tissue button is prepared from corneoscleral tissue after removing the anterior donor corneal stroma by hand (e.g., DSEK) or with the assistance of an automated microkeratome (e.g., DSAEK) or laser (FLEK). Several microkeratomes have received clearance for marketing through the U.S. Food and Drug Administration (FDA) 510(k) process. Donor tissue preparation may be performed by the surgeon in the operating room or by the eye bank and then transported to the operating room for final punch out of the donor tissue button. To minimize endothelial damage, the donor tissue must be carefully positioned in the anterior chamber. An air bubble is frequently used to center the donor tissue and facilitate adhesion between the stromal side of the donor lenticule and the host posterior corneal stroma. Repositioning of the donor tissue with application of another air bubble may be required in the first week if the donor tissue dislocates. The small corneal incision is closed with one or more sutures, and steroids or immunosuppressants may be provided either topically or orally to reduce the potential for graft rejection. Visual recovery following EK is typically achieved in 4-8 weeks, in comparison with the year or more that may be needed following PK.
Eye Bank Association of America (EBAA) statistics show the number of EK cases in the United States increased from 1,398 in 2005 to 14,159 in 2007. Approximately one third of corneal transplants performed in the United States were EK procedures, and EK was performed for more than 85% of patients with endothelial disease. As with any new surgical technique, questions have been posed about long-term efficacy and the risk of complications. EK-specific complications include graft dislocations, endothelial cell loss, and rate of failed grafts. Long-term complications include increased intraocular pressure, graft rejection, and late endothelial failure. Also of interest is the impact of the surgeon’s learning curve on the risk of complications.