What is a corneal graft?
Corneal grafting or corneal transplantation is the replacement of defective host corneal tissue with donor corneal tissue.
Advanced keratoconus is the most common reason for a corneal graft. A corneal graft is required once the vision deteriorates significantly or a contact lens can no longer be fitted to the irregular cornea.
The majority of corneal grafts are full thickness; all tissue is replaced from the epithelium (top layer) to the endothelium (inside layer). The grafted tissue is held in place by sutures (stitches) which can either be individual or continuous.
Deep anterior lamellar keratoplasty (DALK) is a partial thickness graft that preserves the inner-most layers of the cornea. This procedure improves the structural integrity of the post-graft cornea and reduces risk of graft rejection. DALK operations are now being used more frequently.
It is impossible to predict the quality of vision when the sutures are still in place; however, often vision is moderately to substantially better than unaided vision pre-graft. The sutures are usually kept in place for 12 to 18 months and then gradually removed at the discretion of the corneal surgeon, depending on the shape of the graft. Once the sutures are removed the graft shape may change considerably due to balancing of stresses between the graft thickness and the host corneal thickness. If the patient has good vision with the sutures still in place, there is a reasonable chance that they will have good vision following the removal of the sutures. However the majority of corneal grafts have significant astigmatism on removal of the sutures.
While some patients can manage without any optical correction, others with corneal grafts require further correction. Some patients with low levels of astigmatism may be able to successfully wear soft contact lenses. Soft contact lenses may also be an option if there is a substantial difference in prescription between the grafted eye and the fellow eye. Usually rigid gas permeable lenses cause fewer complications than soft contact lenses. Your consulting optometrist will advise you if soft contact lenses are an option for you.
Corneal grafts with irregular astigmatism will require rigid gas permeable (RGP) contact lenses to provide good quality vision. Patients who have successfully worn RGP lenses previously will usually prefer to resume RGP wear as they are familiar with the handling and cleaning regime of RGP lenses.
Rigid Gas Permeable Lenses for Corneal Grafts
Rigid gas permeable (RGP) contact lenses improve vision in the same way as they do in keratoconus. Placing a smooth rigid surface over the cornea allows the tears to fill in the space between the rigid surface and the corneal irregularity. This provides a reduction in image distortion to give better definition and contrast, reduced ghosting and consequently, much clearer vision.
RGP contact lenses for corneal grafts are available in a significant variation of designs. Unfortunately no one design can be guaranteed to be successful in comfort, vision or stability.
A significant factor in determining the shape of an RGP lens for a corneal graft is the degree of elevation of the graft above the host cornea. Corneal grafts are designed to be slightly large for the aperture of the host cornea and so tend to be slightly elevated; however in some cases the elevation can be substantial. This can substantially complicate the design of the RGP lens as the RGP contact lens has to be specially curved to prevent it being dislodged.
Long-standing corneal grafts can sag and also develop significant elevation.There have now been significant improvements in the ability for contact lens manufacturers to produce RGP contact lens with very complex designs to accommodate irregular corneal grafts. Often the main difficulty will be to find a lens design that will be stable on the corneal graft. It may require a 1 or 2 visits to arrive at an initial contact lens design that can be ordered. Even with an experienced contact lens expert it can take numerous visits to alter the RGP lens fitting to an acceptable level.
Larger Diameter Rigid Gas Permeable Lenses for corneal grafts
Advanced computer programs and digital lathes have facilitated the development of complicated larger diameter RGP lens designs. These lenses have the advantage of improved comfort, better centration and a larger area of vision. These designs are at the forefront of RGP contact lens technology.
These lenses require the same maintenance and handling as standard RGP lenses, but occasionally can be more difficult to insert and remove due to the larger diameter (13.0 mm to 15.8 mm).
Miniscleral Lenses for corneal grafts
Miniscleral lenses are also manufactured in a rigid gas permeable material but fit the eye in quite a different way. These lenses are much larger (usually 16.5 mm diameter) and vault the whole cornea, resting on the white part of the eye. Miniscleral lenses rest under the upper and lower eyelids and as the lids are not hitting the edges of the lens, the lenses are very comfortable. The lenses totally seal on the white part of the eye and cannot be dislodged making them ideal for dusty environments and for active sports where conventional RGP lenses may be dislodged. Miniscleral lenses require the same maintenance and cleaning regime as RGP lenses. Miniscleral lenses can be more difficult to insert as they need to be inserted full of solution and more difficult to remove due to the large diameter.
Hybrid Lenses for corneal grafts
Hybrid lenses have a hard RGP centre with soft plastic on the periphery. Hybrid lenses have so far have been unsuccessful in fitting corneal grafts, however there are ongoing developments in materials and designs. Hopefully Hybrid contact lenses will become available in the near future.