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Success rates with full-thickness penetrating keratoplasty, defined as clear grafts, can reach 90 percent at five years.1,2 Yet despite technically excellent surgery, many patients do not fall into this success category, including those patients with neovascularized corneas, multiple failed corneal grafts, autoimmune diseases or a poor ocular surface.
The type 1 Boston keratoprosthesis has shown promise for select patients suffering from corneal blindness. The use of this device has recently gained popularity, though the search for the optimal keratoprosthesis that provides long-term clear vision has continued for over two centuries, with mostly guarded results. The implantation of early keratoprosthesis prototypes was complicated by the extrusion of the implant and subsequent infection. However, the number of poor outcomes has been decreased by having the patient follow a regimen of long-term antibiotics to reduce infection, and a regimen of chronic contact lens use to improve tear distribution.3-5
There are several types of keratoprostheses, but our discussion here will focus largely on the type 1 Boston keratoprosthesis, as it is the most commonly used keratoprosthesis in the United States and worldwide.
The Boston Keratoprosthesis
There are two types of Boston keratoprosthesis (developed by Dr. Dohlman and colleagues). Both consist of an anterior plate attached to a 3.4 mm-diameter PMMA stem that must be secured intraoperatively to a PMMA posterior plate with a titanium ring (Figs. 1A and 1B). Each type requires mounting the complex into a donor corneal button and transplanting the whole as one typically does for penetrating keratoplasty (Figs. 2 and 3).
The type 1 Boston keratoprosthesis has the shape of a collar button, with a front-plate diameter of 5.0 mm, a stem diameter of 3.4 mm and a back-plate diameter of 7.0 to 8.5 mm. The back plate has 16 holes, each 1.5 mm in diameter, to facilitate nutrition and hydration of the endothelium. The type 2 Boston keratoprosthesis, which is implanted through the closed eyelid in eyes with severe ocular surface disease, is similar in shape but has an additional 2.0 mm anterior nub for through-the-eyelid implantation (Fig. 4). Both types of Boston keratoprosthesis are intended for use in aphakic and pseudophakic eyes.
Keratoprosthesis implantation surgery is similar to penetrating keratoplasty. The donor cornea for this type of procedure may be tectonic-quality tissue, since healthy corneal endothelium is not needed for clear vision. The patient is anesthetized with a retrobulbar block or general anesthesia.
Preparing the prosthesis. The donor cornea is trephinated using an 8.5-mm trephine to ensure that the donor corneal button is larger than the recipient corneal rim and posterior plate. A 3.0-mm punch is used to remove the central 3.0 mm of cornea from the donor button. The donor button is advanced over the stem of the keratoprosthesis anteriorly, ensuring that the anterior plate is on the same side as the donor corneal epithelium. The back plate is then advanced onto the posterior portion of the PMMA cylindrical stem to sandwich the donor cornea in between the two plates. A titanium ring is used to secure the posterior plate onto the central cylinder by advancing the ring, using a plunger, until the ring snaps into place.
Surgery. The recipient cornea is then trephinated using an 8.0-mm trephine. If the eye is phakic, a lens extraction is required. If the posterior capsule is intact, it is generally left in place. Vitrectomy may be performed as needed. The donor button/keratoprosthesis complex is sewn into place using 9.0 nylon interrupted sutures. Suture-induced astigmatism is not an issue, as the rigid PMMA cylinder is not deformed by suture tension. Subconjunctival injections of an antibiotic and a corticosteroid are given at the end of the procedure.
In most cases, the placement of a Boston keratoprosthesis should be the last-resort option after multiple corneal transplants have failed. Despite the use of chronic topical antibiotic prophylaxis to lower the incidence of endophthalmitis, and a bandage contact lens (BCL) to improve surface hydration, the postoperative management of an eye with a Boston keratoprosthesis remains complex.
This procedure is best suited for patients with bilateral pathology limiting vision in both eyes. The extent of ocular scarring, particularly on the surface, is a good preoperative prognostic factor for long-term outcome.6 The prognosis for long-term success is most favorable in patients with a history of multiple failed grafts without ocular surface disease and with no autoimmune disorders, and with satisfactory blink and tear function. Patients with the least favorable prognosis are those with a history of autoimmunity and/or severe ocular surface scarring as associated with, for instance, Stevens-Johnson syndrome, cicatrizing chemical burns and ocular cicatricial pemphigoid.6
Ideally, candidates for this kind of surgery should have a history of failed grafts in at least one eye, should have no evidence of retinal detachment or severe glaucoma and should be prepared to undergo follow-up regularly every few months to prevent irreversible complications.
Prior to transplantation, the eye should be quiet, and the patient should be informed of potential complications, which include infection, glaucoma, loss of vision and loss of the eye.
If the patient has glaucoma, consideration should be given to implanting a glaucoma tube at the time of, or following, keratoprosthesis surgery. Currently, we prefer to implant the keratoprosthesis in a single eye and keep the second eye as a “spare” in the event a complication occurs. The reasoning for this preference is that glaucoma continues to be the leading cause for irreversible loss of vision in our keratoprosthesis patients.
As caregivers, we often want to give every patient the opportunity to see again. But the selection of a good candidate is crucial for this procedure’s success. We recommend against this procedure in eyes with severe ocular surface scarring secondary to Stevens-Johnson syndrome, ocular cicatricial pemphigoid or chemical burns, or in patients with uncontrolled autoimmune disorders, unless one is prepared to treat the recurrent corneal melting that may follow. Although the Boston keratoprosthesis has improved outcomes for a select patient population, the technology is still limited by potential complications.
1 Thompson, R. W. et al. Ophthalmology 2003;110(7):1396–1402.
2 Price, F. W. et al. Arch Ophthalmol 1993;111(6):799–805.
3 Durand, M. L. and C. H. Dohlman. Cornea 2009;28(8):896–901.
4 Dohlman, C. H. et al. CLAO J 2002;28(2):72–74.
5 Harissi-Dagher, M. et al. Int Ophthalmol Clin 2008;48(2):43–51.
6 Yaghouti, F. et al. Cornea 2001;20(1):19–23.
Dr. Afshari is associate professor of ophthalmology in cornea and refractive surgery, and Dr. Santaella is a Horizon Grant fellow in cornea and refractive surgery. Both are at Duke University Eye Center.
Part One of The Nuts and Bolts of Keratoprosthesis covers implantation and patient selection. Next month, Part Two will look at postoperative management ranging from corneal melts to endophthalmitis. The authors would like to thank Claes H. Dohlman, MD, for reviewing this article.