Artificial Cornea That Can Be

¶ … artificial cornea that can be used to correct severe corneal cloudiness. Many people who have corneal disease can be assisted with regular corneal transplantation. This involves tissue that is transplanted from human donors. This is the most ordinary treatment for severe corneal opacity. In some cases such transplantation is thought to be hopeless. The Boston Keratoprosthesis can be used when standard corneal transplant would be not likely to succeed. Thus Keratoprosthesis implantation is a procedure designed to help patients whose conditions are the most difficult to treat (Artificial Cornea - the Boston Keratoprosthesis, n.d.).

The Boston Keratoprosthesis has been under development since the 1960s and has gradually been made better. It received FDA clearance in 1992. It has become the most widely used Keratoprosthesis in the United States. The Keratoprosthesis is made up of clear plastic that has outstanding tissue tolerance and optical assets. The device is put into a corneal graft. The graft is then fixed onto the patient's cloudy cornea. If the natural lens is in place, it is also removed. The physician may advise that a soft contact lens may be applied to the surface. With the recent success of the Boston Keratoprosthesis newest design, surgeons in Boston, Chicago and other major centers are expanding the applicability of this technology to help patients who suffer from repeated rejections/failure of standard corneal transplants (Artificial Cornea - the Boston Keratoprosthesis, n.d.).

More than 40,000 corneal transplants are performed per year in the United States for diseases such as pseudophakic bullous keratopathy, aphakic bullous keratopathy, trauma, infections, and corneal dystrophies or ectasias. Corneal transplants are very successful in initial grafts. The success rate is thought to be greater than 95% remaining clear over a four to ten-year period. Other studies have reported failure rates of between 9% and 30% over a three to ten-year period. Yet, the outlook for subsequent graft failure is inferior. In analyzing data on all corneal transplants performed for any indication in patients with initial corneal transplant vs. repeat corneal transplants, it has been determined that survival rates for first time repeat grafts were 55% at 3.5 years, and 28% at 4.5 years. Second time re-grafts had survival rates of 45% at 3.5 years and 20% at 4.5 years. This does not mean that once a graft has failed once, a repeat graft has an estimated 50% chance of failure. Consideration has to be taken into account as to the reason for failure. High risk factors for re-graft failure include glaucoma, ocular surface disease and inflammation, limbal stem cell deficiency, and history of multiple ocular surgeries.

Because of the high risk of re-graft failure in some patients, Keratoprosthesis have been designed to provide a clear corneal window in patients with severe corneal opacification and corneal blindness (Birkholz and Goins, 2009).

Postoperative glaucoma is still a considerable problem in KPro-implanted eyes, especially in those after chemical burns. Of patients accepted for KPro surgery in Boston, more than two thirds had glaucoma diagnosed already preoperatively and some were aggravated by the subsequent surgery. What makes the problem more challenging is the difficulty of measuring the intraocular pressure accurately in an eye with a KPro. In the Boston KPro the stiff back plate prevents measurements based on indentation of the cornea, even in its periphery. Scleral tonometry or the use of any device acting through the lids has been highly unreliable due to often distorted or scarred tissue arrangement. Despite all of variances, finger palpation over the upper lid with the patient looking down has been shown to be the best approach to gross judgment of the IOP (Melki, Lopez and Dohlman, 2009).

The KPros are manufactured to match the refractive need of the individual patient and, therefore, a substantial inventory is required to rapidly supply a device of the desired dimensions. For the eye that is pseudophakic and approximately emmetropic, and where the IOL is left in place at surgery, a single standard power is chosen for the manufacture. For aphakic eyes of different axial lengths, devices with a varying degree of power are made in order to match a patient's needs as closely as possible. Therefore, the axial length of the individual aphakic patient is always ask for from the surgeon in order to allow the choice of a KPro that in theory best matches the dioptric requirement of the eye which is being operated on (Dohlman, Harissi-Dagher and Graney, 2007).