Corneal Transplant Techniques

Until a few years ago, the only common method of corneal transplantation was full-thickness corneal transplantation, and all corneal diseases were treated in this way. However, many corneal diseases affect only a portion of the corneal thickness, so replacing one or more layers of the cornea is sufficient and there is no need to replace the entire cornea with donor tissue. This transplant is called lamellar keratoplasty, which is divided into two parts, anterior and posterior. Common corneal transplantation methods can be divided as follows:A- Penetrating Keratoplasty: In this method, a circular disc is removed from the donated cornea in its entire thickness with its five layers using the Trephine device. The diameter of the disc is usually 8-5/7 mm and is replaced with a disc larger than it by 25/0-5/0 mm of full-thickness donated tissue and is sutured to the recipient's cornea with 0-10 nylon thread. B- Anterior Lamellar Keratoplasty: This method is used in corneal diseases that are determined by the stromal layer and the endothelial layer is intact. Anterior lamellar keratoplasty is performed in two ways. If the disease involves the superficial stromal layer, it is superficial, and if it involves the deep stromal layer, it is deep. If the extent of the injury is less than 50% of the corneal thickness, superficial lamellar keratoplasty is performed, which is performed in two ways: microkeratome incision (ALTK) and manual incision (Manual LK). In manual anterior lamellar keratoplasty (MLDK), a circular disc is removed with a Trephine device at a thickness of 50% of the corneal thickness and then the anterior stroma is removed with a sharp blade. The entire lesion should be removed to leave a uniform surface. The donor tissue, which is 0.25-0.5 mm larger than the damaged disc, is sutured with an equal thickness to the recipient cornea with 0-10 nylon suture. In deep anterior lamellar plasty, the recipient's stroma should be completely removed, leaving the endothelial layer and Descemet's membrane with a thickness of about 20 microns. For this, 80% of the recipient's cornea thickness is incised using a trephine device, and then the stroma layer is separated from the endothelial layer using different methods such as air injection (Big Bubble), fluid injection, viscoelastic injection, or using a slow-edged blade (Melles technique). Air injection and using a slow blade are common techniques for deep anterior lamellar plasty in Iran. On the other hand, the Descemet's membrane is separated from the donor's corneal endothelium and incised with a diameter of 0.25 mm greater than the recipient's cornea. Finally, this tissue is sutured with a 0-10 nylon thread to the recipient's cornea. 'Benefits of Lamellar Keratoplasty' Since the use of the endothelial layer is the most common reason for corneal transplant rejection, if the recipient's endothelial layer is preserved, a great success in corneal transplantation can be expected, especially in patients with keratoconus, where the patients are young and need a transplant in which the cornea remains transparent for a long period. The other benefit is avoiding getting close to the inside of the eye, which reduces the risk of bleeding during the operation or infection after it. Sometimes when the remaining thickness of the cornea is more than 30 microns.Stromal and subepithelial rejection of grafts are also seen in anterior lamellar plasty and although they are less important than endothelial rejection, their occurrence and untreated occurrence may lead to decreased vision due to corneal opacity and corneal neovascularization. C- Posterior Lamellar Keratoplasty One of the reasons for corneal transplantation is corneal stromal edema caused by a dysfunction of the endothelial cells either due to genetic reasons (such as Fuchs' endothelial dystrophy) or due to injuries resulting from intraocular surgeries, especially cataracts, which is the most common cause in European countries and the United States. If the disease is limited to the endothelial layer and does not leave any problem for the corneal stromal without edema and swelling, there is no need to replace the entire recipient's cornea. In this case, it is sufficient to replace the endothelial layer with a layer from the donor's cornea containing the natural endothelial layer. This procedure is called DSAEK, in which the Descemet membrane and endothelium are separated from the back of the recipient's cornea and replaced with a layer from the back of the donor's cornea, approximately 100 microns thick, containing the endothelium, Descemet membrane, and part of the posterior stroma, and is fixed to the back of the recipient's cornea using air, without the stitches used in penetrating keratoplasty and anterior lamellar keratoplasty. Therefore, vision recovery with this method is faster and refractive errors and astigmatism are much less than with penetrating keratoplasty. Since the majority of patients are elderly, penetrating keratoplasty is usually accompanied by a superficial defect of the eye including a delay in repair of the epithelium, which reduces the eye's ability to withstand trauma, which is why the DSK procedure has become widespread in the last ten years. Endothelial transplant rejection occurs in DSK as in penetrating keratoplasty.