Christina R. Prescott and James Chodosh

Background

Pellier de Quengsy first proposed the idea of a synthetic or partly synthetic cornea in 1789.¹ Dr. Claes Dohlman began development of the Boston keratoprosthesis in 1965; since then, over 7000 have been implanted.² The Boston keratoprosthesis, when constructed, includes four parts (Fig. 50.1). As configured for implantation, the most anterior component is the optic, which is lathed from a single piece of polymethyl methacrylate (PMMA) to form the front plate and its stem. A corneal carrier button (allograft or autograft) permits suturing of the device to host cornea and sits between the front plate and the back plate, which is also made of PMMA. Finally, a C-shaped, titanium locking ring encircles the posterior optic stem and prevents disassembly once the keratoprosthesis is implanted in the eye. Incremental improvements in the device since its inception include a snap-in, rather than screw-in, assembly; the addition of back plate perforations to improve nutrition to the corneal carrier and reduce keratolysis; inclusion of the titanium locking ring (Fig. 50.2); and, currently awaiting approval from the U.S. Food and Drug Administration, a change in back plate material from PMMA to titanium.^3,4 Other advances include the long-term use of antibiotics and soft contact lenses.⁵ The Boston keratoprosthesis type I is likely the most commonly implanted artificial cornea worldwide. The Boston keratoprosthesis type II, a modified version of the type I designed to fit through closed eyelids, is used less often.

Once the decision is made to proceed with implantation (see Chapter 49: Keratoprosthesis Indications), a choice must be made whether to implant a type I or type II design. Underlying pathology, as well as completeness of eyelid closure, quality and quantity of preocular tear film, and depth of conjunctival fornices must be determined. The type II device is only utilized in cases of corneal blindness in which damage to the ocular surface and adnexa is extensive enough to make successful retention of the type I device unlikely.⁶ Candidate patients for a type II keratoprosthesis typically have ocular surface keratinization, severe aqueous tear deficiency, symblephara, and foreshortening of the fornices.⁷

A detailed preoperative discussion of the risks and benefits of keratoprosthesis implantation is extremely important. Patients must understand that they will need continuous treatment, including daily antibiotics, bandage contact lens wear (for the type I keratoprosthesis), and lifelong follow-up care with a qualified corneal specialist. Long-term follow-up is necessary to maintain patient compliance with medications, recognize and treat indolent infection and early keratolysis, and diagnose new-onset or worsening of pre-existing glaucoma, any of which may compromise the visual gains from the initial surgery and lead to loss of the keratoprosthesis and/or the eye. Patients should also be aware that the cosmetic appearance of the eye will change with keratoprosthesis surgery (Fig. 50.3A), quite notably with the keratoprosthesis type II (Fig. 50.3B). To improve cosmesis following type I keratoprosthesis, some patients choose to wear a painted contact lens. After type II surgery, the cosmetic options are limited to the use of tinted glasses, so patients must receive complete preoperative counseling and be willing to accept this limitation prior to undergoing surgery. For best outcomes in patients with underlying inflammatory and autoimmune conditions, such as Stevens–Johnson syndrome/toxic epidermal necrolysis or mucous membrane pemphigoid, attempts should be made to minimize ocular surface inflammation prior to surgery. This may require use of a systemic immunosuppressive agent, such as mycophenolate mofetil. For chronically immunosuppressed patients, collaboration with a rheumatologist, or other similarly qualified physician, is critical to the long-term preservation of the keratoprosthesis.