The cornea is the clear, dome-shaped outer layer. It serves a dual purpose, acting as a protective barrier against the environment while providing approximately two-thirds of the eye’s total focusing power. The corneal tissue must remain perfectly transparent and maintain a precise curvature for clear vision. A corneal transplant, medically known as keratoplasty, becomes necessary when the cornea loses either its clarity or its proper structural shape, leading to a significant loss of vision that cannot be corrected with glasses or contact lenses.
Changes to Corneal Shape
A common reason for needing a corneal transplant involves conditions where the cornea’s structure begins to weaken, causing it to change shape. The primary example of this is Keratoconus, a progressive disorder where the central cornea thins and bulges outward into an irregular cone shape. This structural distortion creates a high degree of irregular astigmatism and high-order aberrations that severely scatter incoming light. The misshapen surface prevents light rays from converging at a single focal point, resulting in ghosting and blurred vision.
Specialized rigid contact lenses can sometimes mask the irregularity by creating a smooth new refracting surface over the cone. However, if the condition progresses to a severe stage, causing the tissue to become excessively thin or develop scarring from chronic stress, a surgical replacement of the damaged layers is required. This intervention is often needed for younger patients whose Keratoconus is still progressing aggressively.
Failure of the Cornea’s Fluid Pump System
Another major category of corneal disease requiring transplantation involves the failure of the tissue’s fluid management system. The innermost layer of the cornea, the endothelium, contains specialized cells that act as a microscopic fluid pump. These cells actively transport water out of the corneal stroma, maintaining its slightly dehydrated state, which is required for transparency.
Since these endothelial cells do not regenerate, their loss or malfunction leads to fluid retention and swelling, a condition called edema. Fuchs’ Endothelial Dystrophy is a progressive, genetic condition characterized by the premature death and dysfunction of these endothelial cells. As the cell density drops below a functional threshold, the pump fails, causing the cornea to swell and become cloudy.
A similar outcome, known as Pseudophakic Bullous Keratopathy (PBK), can occur following complicated cataract surgery, where surgical trauma causes a significant, acute loss of endothelial cells. In both cases, the resulting corneal edema leads to blurred vision, which is often worse in the morning due to fluid buildup overnight, ultimately necessitating replacement of the dysfunctional inner layer.
Damage from Infection and Physical Trauma
External factors, such as severe infections or physical injuries, can also cause irreversible damage to the cornea, leading to the formation of dense, opaque scar tissue. Microbial keratitis, caused by bacteria, fungi, or viruses like Herpes Simplex Virus (HSV-1), can rapidly penetrate the corneal surface. If the infection reaches the thick middle layer, the stroma, the body’s healing response involves the activation of resident cells. This process results in a permanent white or grayish scar, or opacity, that directly blocks light transmission and severely impairs vision.
Chemical burns, particularly those caused by alkali substances like lye, are especially destructive because they allow the chemical to rapidly penetrate and cause widespread damage to all corneal layers. A deep laceration or acid burn can also induce this same scarring response in the stroma, creating a visually significant barrier that only a transplant can remove.
Addressing Failed Previous Transplants
A final indication for keratoplasty is the failure of a previous corneal transplant. The most frequent cause of this late failure is immunological allograft rejection, where the recipient’s immune system recognizes the donated tissue as foreign. This immune response attacks and destroys the donor endothelial cells, which are the most susceptible layer of the graft.
A rejection episode results in sudden corneal swelling and clouding. While it is often reversible with intensive steroid treatment, each episode causes a permanent loss of donor cells. Even without rejection, a transplanted cornea is subject to an accelerated rate of endothelial cell loss due to the stress of the procedure. Over many years, the cell density of the donor tissue may fall below the number required to maintain clarity, a condition known as late-stage endothelial failure, which requires a new graft to restore transparency.