The Latest Corneal Transplant Techniques
The cornea is the clear window at the front of the eye and the eyes's main focusing element. Uniquely, it is the only transparent structure in the human body and can be thought of much like the crystal face of a watch. Behind the cornea lies the coloured iris and in the middle, the dark pupil of the eye.
There are many different reasons why the cornea may be damaged and lose its clarity and transparency leading to loss of vision. These can include infective causes such as the herpes simplex virus (cold sore virus) or contact lens related infection, traumatic injuries, inherited diseases such as Fuchs' dystrophy or lattice dystrophy, or even previous cataract or glaucoma surgery.
The optical function of the cornea may also be affected by conditions that lead to a change in it's shape including keratoconus or pellucid marginal degeneration, or due to prior infection or injury.
The cornea is a layered structured. The main layers, the epithelium at the surface, the stroma in the middle and the endothelium at the back of the cornea consists of different cell types. Corneal diseases can affect selective layers of the cornea or may affect all layers of the cornea, each of which can lead to lost vision which may require surgery.
Corneal transplantation has been performed for over a century and is the only mode of treatment for some patients that have lost transparency or function of their cornea leading to loss of vision.
Types of corneal transplant
During all types of corneal transplantation the diseased part of the of the patient's cornea is removed and replaced with healthy corneal tissue from a human donor. For many years the predominant type of corneal transplantation was penetrating keratoplasty (PK) which involved removing a circular full thickness area of cornea from the patient and replacing it with full thickness donor tissue. More recently, due to advances in technology and technique, it has been possible to remove a specific layer of corneal tissue and replace just that layer. This type of surgery is called lamellar (layered) surgery. PK surgery is still being performed in cases where all layers of the cornea are involved.
Lamellar surgery is technically demanding and requires that your surgeon have completed advanced specialisation in these techniques. Mr Angunawela is an expert in lamellar corneal transplantation and spent two and a half years in training to be able to perform the entire spectrum of lamellar corneal surgery. Lamellar surgery is broadly divided into anterior lamellar (the front layer) or posterior lamellar (the endothelial layer) surgery. The diagram below illustrates the various types of corneal transplant that are now possible.
Lamellar corneal transplants are associated with lower rejection rates and preserve the structural integrity (strength) of the whole eye.
Different types of keratoplasties (corneal transplant)
(A) The cornea consists of five main layers: the superficial multilayered epithelial cell layer, Bowman's membrane, which is a condensation of the anterior stromal layer; the corneal stromal layer (making up the major thickness of the cornea) consisting largely of collagen with relatively low numbers of stromal keratocytes; Descemet's membrane; and most posteriorly, the endothelial cell monolayer that is responsible for deturgescing the stroma to maintain corneal transparency. (B) Penetrating keratoplasty. (C) Anterior lamellar keratoplasty. (D) Deep lamellar endothelial keratoplasty. (E) Descemet's stripping automated endothelial keratoplasty. (F) Descemet's membrane endothelial keratoplasty. *Image adapted from a publication by Professor John Dart
Where do donor corneas come from?
Human corneas used for transplantation are obtained from human donors that have given their eyes for medical use after their death. In the United Kingdom the majority of donor organs including hearts, kidneys and eyes are processed by the UK Transplant Service and undergo stringent tests to exclude risk of transmissible diseases and infection, and to ensure sufficient cell numbers for transplantation. Moorfields Lions Eye Bank also provides donor tissue to the hospital and works under the same stringent national guidelines. In the UK just over 4000 human eyes are retrieved from donors for transplantation each year. These can be stored for up to 28 days before use. We also obtain tissue from the North Carolina Eye Bank, USA.