Cornea refers to the clear glass like structure in front part of the eye. This structure allows light to enter the eye, and also helps focus image on the light sensitive retina inside the eye. The cornea is composed of five layers. The outer four layers maintain the shape of the cornea and the innermost fifth layer contains active cells that help maintain the clarity of the cornea. Damage to the anterior layers following infection, or trauma leads to scarring and reduces the clarity of cornea, thereby affecting vision.  In certain conditions like Keratoconus, there is progressive thinning of the cornea, due to which the shape of cornea becomes abnormal.  This abnormal corneal contour results in distortion of images,  thereby affecting vision. The innermost layer of the cornea can become dysfunctional with aging, or following trauma etc. This leads to accumulation of water within the corneal layers resulting in loss of corneal clarity and hazy vision.
 
Corneal transplantation is performed in this situation to restore corneal clarity and improve vision. Traditionally corneal transplantation refers to full thickness replacement surgery, wherein all the five layers are replaced with donor tissue. The results of this transplantation procedure are good, and most patients are able to maintain a clear graft for several years after surgery.
 
Over the past 2 -3 years, newer surgical techniques have evolved wherein selective replacement of the diseased layers of the cornea is performed to restore corneal clarity. These procedures have several advantages over conventional full thickness transplantation surgery.
 
In the presence of corneal scarring wherein the innermost layer is functioning normally, replacement of the anterior layers is performed using a technique called Deep Anterior Lamellar Keratoplasty (DALK). Here the surgeon surgically removes the affected corneal layers and replaces them with healthy donor tissue. This technique has been successfully used with good results in a variety of corneal disorders associated with corneal scarring or abnormal corneal shape. Keratoconus is a corneal disorder, wherein there is progressive thinning and ectasia of the cornea. 

This results in poor vision, and is initially corrected using hard contact lenses. In advanced keratoconus, surgical replacement of the cornea is required. In traditional full thickness corneal transplantation surgery, the healthy innermost layer is also replaced with donor tissue. Following this corneal transplantation procedure,  the body tends to recognize the innermost layer as non-self, and attempts to reject the donor tissue. To prevent such rejection episodes, long term therapy with steroids is necessary following surgery. Prolonged use of steroids can lead to formation of cataract or cause a rise of pressure within the eye. Both of which may require further surgical intervention and or medical therapy.

With the lamellar corneal surgical procedures such as DALK, the healthy innermost layer of the host is retained, and only the outer layers are replaced with donor tissue. In this scenario, the body is not able to detect the donor tissue, and therefore there is no risk of rejection episodes unlike full thickness corneal grafts. Hence steroid therapy is given only for a limited short term period following DALK procedure.

 
In conditions, wherein the innermost layer of the cornea is not functioning properly, full thickness corneal transplantation procedures provide healthy tissue to restore corneal function. The full thickness corneal graft, requires multiple stitches to keep the donor tissue in position. This results in abnormal corneal curvature and glass power following surgery. Moreover the stitches have the risk of becoming loose over a period of time, and this can lead to corneal infections, or invite blood vessels into the corneal graft, increasing the risk of damage to the corneal graft.

Newer methods of corneal transplantation procedure such as Deep Lamellar Endothelial Keratoplasty (DLEK), allow selective transplantation of the diseased inner layer without the need for surface incisions or stitches to keep the donor tissue in place. The results of this technique are encouraging and provide better quality of unaided vision following surgery, compared to the full thickness corneal transplantation procedure. (the author, was the first corneal surgeon in India to perform the DLEK procedure.)

 
Both DALK and DLEK allow better utilization of donor tissues in the eye bank. This ensures that more patients are able to undergo corneal transplantation surgery, as donor corneal tissues are always inadequate for the number of patients awaiting corneal transplantation surgery. Both the procedures are technically demanding, and therefore require special training. Experienced corneal surgeons, should perform both DALK, and DLEK procedures whenever possible to provide better long term benefit to the patients undergoing corneal transplantation procedures.