Permanent and temporary keratoprostheses save sight

Albany, NY-Severely traumatized eyes may be salvaged with use of either a permanent or temporary keratoprosthesis. Both allow combined procedures in situations where visualization otherwise would preclude attempted posterior repair, according to Michael W. Belin, MD, FACS, professor and director of cornea and refractive surgery, Department of Ophthalmology, Albany Medical College, Albany, NY.

A keratoprosthesis, whether permanent or temporary, is used when corneal opacity precludes adequate visualization of the posterior segment during surgery, usually prior to procedures such as retinal attachment or vitrectomy. Currently, two major styles of temporary keratoprosthesis are appropriate for trauma cases-a reusable polymethylmethacrylate (PMMA) keratoprosthesis (Landers-Foulks) and a semi-reusable, silicone keratoprosthesis (Eckardt)-and a permanent, surgically implanted keratoprosthesis (Boston Keratoprosthesis [K-Pro]), said Dr. Belin.

Temporary prostheses provide visualization to the posterior segment and typically are used when the case will be followed by a full-thickness penetrating keratoplasty, he said.

The reusable PMMA keratoprosthesis comes in two sizes, 7.2 and 8.2 mm; the latter is more widely used. An opening is made in the cornea as if for standard penetrating keratoplasty, with the opening cut to size. The device is screwed into place and secured with a number of interrupted sutures. It is durable, reusable, and easy to insert; however, it does not give a very good view of the retinal periphery, Dr. Belin said.

The semi-reusable, silicone keratoprosthesis allows better peripheral visualization than the PMMA model, he added. This device is 7.5 mm in diameter and used in a 7-mm bed. Multiple sutures are passed through the peripheral skirt and sewn into the cornea. The silicone keratoprosthesis can be used several times but eventually has to be replaced. Additionally, it is not as watertight as the PMMA model, Dr. Belin said.

The permanent keratoprosthesis traditionally is not regarded as a treatment in trauma cases but may be used when suitable tissue is unavailable, he said. It also can follow the use of a temporary keratoprosthesis if the prognosis for a full-thickness penetrating keratoplasty is poor.

"The advantage of a permanent keratoprosthesis is that it does not need viable endothelial cells. However, you need to have a keratoprosthesis on hand, which most people do not," he said. "At our center, we have a number of [permanent keratoprostheses] as backup for our standard keratoprosthesis surgery, and we can-if needed-use them in trauma cases."

Dr. Belin noted that this device is not specifically designed for severely traumatized eyes and offers a relatively limited view of the posterior pole.

The most commonly used permanent keratoprosthesis in the United States (Boston K-Pro Type 1, manufactured under the auspices of the Massachusetts Eye and Ear Infirmary) is made of medical-grade PMMA. It has a two-part anterior optical with a posterior back plate. The plates are clamped onto a donor button of 8.5 mm or more, which then is sutured into the patient's cornea. Finally, a therapeutic soft contact lens is applied to the surface for postoperative safety.

Surgical technique

The technique for implanting the permanent keratoprosthesis is similar to that of a transplant, with the exception of the creation of a 3-mm trephination in the middle of the cornea. In a one-step procedure, the prosthesis is assembled around the punch, with the donor cornea sandwiched between the anterior and posterior plates. A titanium locking mechanism prevents the back plate from becoming unscrewed. Visual improvement usually occurs quickly, often within a day of surgery, and no astigmatism results because of the PMMA material.

Complications that may occur with permanent keratoprostheses include excessive inflammation, particularly at the posterior plate holes, Dr. Belin said. Medications such as a fluoroquinolone or topical prednisolone acetate 1% may be given. Occasional intracameral or subTenon?s injections of 40 mg of triamcinolone may be necessary as well, he added.

Another permanent keratoprosthesis (AlphaCor, Addition Technology Inc.) is not appropriate for the management of acute trauma because it requires a two-stage implantation procedure and intracorneal lamellar dissection, according to Dr. Belin.