Pellier de Quengsy¹ first suggested the concept of an artificial cornea in the treatment of corneal blindness in 1789. In 1853, Nussbaum² performed experimental animal work and also published human trials using a quartz crystal implant, which he implanted into the cornea. During the next 50 years, further efforts in keratoprosthesis (KPro) design and insertion were continued by Heusser,³ Dimmer,⁴ Salzer,⁵ and von Hippel.⁶ Unfortunately, these KPros were associated with extremely high incidences of complications and typically failed early because of tissue necrosis around the device with subsequent leak, infection, and finally extrusion of the KPro.

After 1906, when the first human-to-human corneal graft was performed, there was a loss of interest in KPro development and surgery. In 1920, Verhoeff⁷ reported on a single case of insertion of a quartz button into a patient’s cornea; however, it had to be removed shortly afterward.

Similarly, Filatov,⁸ in 1936, implanted a full penetrating glass device into a patient’s opacified cornea and covered it with a double conjunctival flap after surgery. He left the flap in place, and after 6 months it had thinned sufficiently to give the patient an ambulatory vision of 1/200. More details on the history of keratoprosthesis surgery around this period have been reported previously in reviews by Day,⁹ Cardona,¹⁰ and Lund.¹¹

During World War II, Wunsche,¹² Stone and Herbert,¹³ and others noticed that polymethylmethacrylate (PMMA) fragments from shattered airplane canopies embedded in the cornea of pilots were well tolerated. This led to their experiments showing that PMMA discs could be retained in the cornea of rabbits. Soon human applications followed, and many ophthalmologists attempted to refine their procedure using these new inert plastics, primarily PMMA. However, once again, many of these cases were associated with severe complications, and the procedure lost favor. Some surgeons did persevere in their techniques and refined them over the years. However, the combined experiences of their keratoprosthesis surgeries probably did not amount to more than 4,000 or 5,000 cases during the past half century, which is quite small compared with the number of PKs carried out on a worldwide basis.

Among the studied KPros to date, two, namely, the Boston (Dohlman-Doane) and the AlphaCor devices, have been approved by the U.S. Food and Drug Administration and are available for clinical use. KPro designs consist of two principal types. The first is a so-called collar button-shaped device made of PMMA, which consists of two plates joined by a central clear optical stem (e.g., the Boston KPro) and the second is made with a central stem of transparent material that is held in place in the cornea by a peripheral skirt, which is usually placed intralamellarly in the stroma and is sometimes covered by transplanted autologous tissue or lid skin (e.g., the AlphaCor? KPro). The most frequently used Boston KPro is the type I device, of which more than 6,000 have been implanted worldwide to date (Boston KPro News, Fall 2011). Implanted in patients who have retained relatively normal lid function and the ability to maintain a “wet” ocular surface, this KPro is inserted such that the plates sandwich the fresh donor cornea between them. The advantages of this design include a short optical stem, thus providing a good view at the slit lamp, a generous visual field, and good stability because the wide plates prevent tilting of the device off the visual axis. The design also facilitates repair should necrosis of tissue occur around the stem. The refractive power of the KPro is selected based on whether the patient is pseudophakic or aphakic and the axial length of the eye. The technique for implanting the Boston type I KPro is as follows. A donor button (usual size, 8.5 to 9.0 mm) is prepared and a central 3-mm hole is trephined. The donor button is then placed over the stem of the front plate, and the back plate is screwed into place on top of this. A titanium locking ring is then snapped into place to prevent loosening of the back plate. The recipient cornea is prepared as for a traditional PK (usual host trephine, 0.5 mm less than the donor diameter). If pseudophakic, the intraocular lens can be left in place; if aphakic, a core vitrectomy is generally performed. The donor graft with the KPro is then sutured in place with multiple interrupted 10-0 nylon sutures, as with a standard PK. At the completion of the procedure, 400 g of intracameral dexamethasone is given, and a soft Kontur contact lens (Kontur Contact Lens, Richmond, California) is placed.

The type II KPro, which is similar to type I except that it has a 2-mm long anterior nub designed to extend through a surgically closed lid, is less commonly used and is reserved for those with end-stage dry eye conditions.

The material most frequently used for the central optical portion of the KPro has been medical-grade PMMA. PMMA is completely transparent and biologically inert, and there is now extensive experience with its use in intraocular lenses. However, many different types of material have been used for the skirts or plates used to secure the optical stem in the cornea, such as perforated grids of PMMA, Teflon, nylon, and ceramics. More recently, attention has been directed to porous designs using materials such as Proplast,¹⁴ polytetrafluorethylene,¹⁵ hydrogels,^16,17 collagen,¹⁸ and various copolymers.^19,20 These materials have been manufactured and developed with the idea that colonization with natural tissue elements would subsequently hold and anchor the device more securely in place, thus decreasing the likelihood of future extrusion. In the 1960s, Strampelli²¹ introduced a novel idea of inserting the PMMA stem into a slice of dental bone from the patient’s jaw, with the hope that the bone would subsequently heal better into the patient’s cornea. This technique was later modified by Falcinelli et al.²² These authors reported results from 224 eyes over 26 years of osteoodonto-keratoprosthesis (OOKP) insertion and noted that 18 years after OOKP surgery, the probability of retaining an intact OOKP was 85%. They also found that this type of KPro provided a rehabilitating recovery in visual acuity. This is, however, a formidable multistage procedure and results in limited visual field and difficulty measuring intraocular pressure postoperatively. Cardona and DeVoe²³ made an important observation that in an extremely dry eye, if the prosthesis is allowed to protrude through the skin rather than between the lids, safety is enhanced and the extrusion rate is subsequently reduced. Considerable modifications to various KPro designs have occurred over the years, but whether an ideal KPro can ever be designed that will always integrate into the human cornea without the risk of extrusion or necrosis has yet to be determined.

Our practice is to use an all-PMMA device like that shown in Figure 44.1 (Dohlman-Doane KPro types I and II). The standard type I device is used in all but extremely dry eyes, where a “through-the-lid” approach with a type II device is preferred. In the latter case, the anterior nub is designed to protrude through an opening in the lid skin. The dimensions described for the two devices allow satisfactory visual fields. Thus, type I permits a peripheral field of approximately 60 degrees; type II, with a longer stem, can give a visual field of approximately 40 degrees. The manufacture of these devices has previously been described.²⁴

Patient outcomes after keratoprosthesis surgery by a single surgeon using either the Cardona device or the Dohlman-Doane type I device showed improved outcomes with the newer Dohlman-Doane device.²⁵ The improvements in device design, surgical technique, and postoperative care have resulted in the dramatic improvement in results over the last quarter of a century.

A more recently developed KPro, developed in Western Australia, the AlphaCor, previously known as the Chirila KPro, does not require the use of a human donor cornea. It is a synthetic corneal replacement material made from the hydrophilic polymer poly(2-hydroxyethyl methacrylate). There is a transparent central optic region fused with a surrounding porous sponge skirt that allows biointegration into the host cornea by stromal fibroblast ingrowth. The AlphaCor KPro is implanted in a corneal stromal lamellar pocket in a two-stage procedure.²⁶ The first stage involves a 360-degree conjunctival peritomy and debridement of the corneal epithelium. A 180-degree superior paralimbal incision is extended at 50% depth into the corneal stroma, forming an intralamellar pocket. A central corneal trephination with a radius of 3.5 mm is then performed at the posterior bed of the cornea. The device is centered within the corneal pocket, and the perilimbal incision is closed with the device sandwiched between lamellae. A conjunctival flap is needed to cover the entire corneal surface. Eight to 12 weeks later, an anterior trephination, 3 mm in diameter, through the conjunctival flap and the anterior layer of the cornea to expose the optic of the KPro is performed as the second stage of the procedure. Implantation of this type of KPro has shown promising results in early reports.²⁶ In a series of 322 eyes who had the AlphaCor implanted by 84 surgeons, the retention rate was 65.8%, with stromal melting the most frequent complication, causing 64.5% of explantations of the device. Importantly, quality and quantity of the tear film was noted as a strong prognostic factor.²⁷

This type of KPro seems to biointegrate well with host corneal tissue.²⁸

It has become clear that the outcome of keratoprosthesis surgery differs markedly in various corneal disease causes, so the indications for surgery will have to be categorized accordingly. However, in general terms, certain criteria first must be fulfilled before qualifying for the procedure. Obviously, known end-stage retinal or optic nerve disease usually constitutes a contraindication. Also, young age of the patient or poor general health should be taken into consideration. One other very important consideration is the interest, commitment, and time of the surgeon because frequent follow-ups and readiness to intervene early in threatening complications are mandatory. In addition, keratoprosthesis evaluation, surgery, and follow-up are technically difficult; therefore, it is best undertaken by corneal surgeons with a special interest in the subject.

Of the patients in whom keratoprosthesis could be considered, the patients with the poorest outlook are those with SJS. These patients have binocular disease and are sometimes monocular because they frequently have undergone numerous unsuccessful surgical procedures in the past. A further difficulty with this group is that they are young and require the KPro to remain complication-free for many years. As already mentioned, they often have ongoing ocular inflammation, which increases the postoperative complication rate. Accordingly, at this time we advise against keratoprosthesis surgery in young patients with SJS; we would prefer waiting until techniques have improved.

Patients with OCP tend to be older, and this is in their favor. They also appear to have a more favorable outcome in the long term. However, they are prone to postoperative skin retraction and glaucoma.

Chemical burn patients can have good results after keratoprosthesis surgery. A simultaneous glaucoma shunt (Ahmed valve) is nearly always placed in these inflammatory conditions: the incidence of postoperative long-term glaucoma without such a shunt is extremely high.

Perhaps patients in the most controversial category are those with previous graft failures after noninflammatory edema, previous dystrophies, or trauma. The frequency of postoperative uveitis and uncontrolled glaucoma in these patients is quite low, and vision is usually restored fairly rapidly, in most cases more rapidly than a successful regraft would permit. Many of these are elderly patients with graft failure after bullous keratopathy. In certain instances, these patients may have a better chance of seeing well during their remaining days with a KPro rather than with another corneal graft.²⁹