RESTORING SIGHT

Corneal transplants are the most widely performed and most successful type of transplant surgery, yet corneal blindness remains the fourth leading cause of blindness worldwide after cataract, glaucoma and age-related macular degeneration (AMD). The causes of corneal blindness include lack of donor tissue, repeated graft failure, and ocular trauma and diseases. However, over the past few decades keratoprostheses have become available which can address most of those problems and their implantation is becoming a mainstream component of modern ophthalmic surgery. Since Pellier de Quensqy first suggested the use of a piece of glass as a substitute cornea in 1789, many designs of keratoprostheses have come and gone, but only two have stood the test of time and are in wide use today. They are the Boston type 1 KPro, formerly known as the Dohlman-Doane keratoprosthesis and the osteo-odonto-keratoprosthesis (OOKP).

[gallery link='file']

The two devices differ in design and indication, each filling distinct niches. The Boston KPro type 1 uses a donor corneal button as a skirt for the optic and is primarily indicated in eyes which have a history of repeated graft failure and which have normal tear and blink function. The OOKP uses dental and alveolar bone overlaid with buccal mucous membrane as a carrier for the optic. Its primary indications are patients with bilateral corneal blindness resulting from severe end-stage corneal disease, chemical burns and dry keratinised eyes.

The Boston keratoprosthesis
The Boston KPro type 1 was originally developed by Claes Dohlman MD, Harvard Medical School, in the 1960s, but was not FDA-approved until 1992. The device has a collar button design with a PMMA front plate and optic, and a PMMA or titanium back plate which sandwich around a donor button of corneal tissue which is sutured into the recipient's eye. The device's 5.5mm diameter front plate also includes a 3.35mm diameter stem, which is the optical portion of the keratoprosthesis and fits through the 7.0 or 8.5mm diameter back plate. The back plate has fenestrations to allow nutrition of the cornea, and is held in place with a titanium lockingring.

In addition to repeated failed corneal grafts, other indications for the Boston KPro type 1 include vascularised corneas, aniridia and Peter's anomaly in paediatric eyes. The keratoprosthesis requires a wet blinking eye with good lid function and good tear film to preserve a healthy ocular surface and prevent degradation of the implant and infection. Implantation of the Boston KPro is carried out in essentially the same way as a conventional keratoplasty, the only important additional step is the assembly of the keratoprosthesis/donor button complex. Postoperative care includes lifetime treatment with topical antibiotics, usually a fluoroquinolone and vancomycin, to eliminate the possibility of endophthalmitis. In addition, all patients use a bandage contact lens over the keratoprosthesis to prevent the donor tissue from drying out and melting around the neck of the KPro.

In the major clinical trials reported to date, the average retention rate after a follow-up of one year has been 91 per cent, ranging from 82 per cent to 100 per cent. In eyes with at least one year's follow-up, two-thirds of patients have achieved a BCVA of 20/200 or better, and around 44 per cent have achieved a BCVA of 20/50 or better. The major postoperative complication reported in the studies has been retroprosthetic membrane, which has occurred in about 40 per cent of eyes but has generally proved amenable to treatment with a YAG laser. The most serious complication to occur has been infectious endophthalmitis which has occurred in three per cent of eyes, overall. However, there have been no cases of endophthalmitis in the studies conducted since the adoption of lifelong vancomycin. Another frequent complication is IOP elevation, the incidence of which can be reduced by preventive measures such as placement of a tube shunt prior to or during the implantation procedure. Less frequent complications have included stromal melt, retinal detachment and sterile vitritis.

Long in the tooth
The osteo-odonto-keratoprosthesis is one of the oldest and most successful of artificial corneas. Originating from a design by Strampelli in the 1960s, and later improved upon by Falcinelli in the 1980s and 90s, a high proportion of the original patients implanted with the modified design (MOOKP) have obtained useful vision from their keratoprostheses for decades. However, the OOKP procedure is very specialised and time-consuming, and must be performed in two stages. As a result, the use of the OOKP is generally restricted to patients with bilateral corneal blindness resulting from such conditions as severe end-stage Stevens-Johnson syndrome, ocular cicatricial pemphigoid, chemical burns, trachoma, and those with dry eyes. In the first stage of surgery, the tooth and periodontal bone, with the optic in place, is sutured into a sub-muscular pocket to allow soft tissue to grow into it. The patient also undergoes suturing of the mucous membrane flap onto the sclera at this point.

In the second stage of surgery, which takes place two-tothree months later, the surgeon removes the implant from the sub-muscular pocket, reflects mucous membrane graft, trephines the cornea, removes the iris, lens and anterior vitreous, and then sutures the implant into place. The mucous flap is then reattached with a trephined hole through which the optical cylinder protrudes. Falcinelli has reported a retention rate for the OOKP of 85 per cent at 18 years and BCVA ranging from 0.41 to 0.8. In our series of 32 patients, the retention rate at 3.9 years of follow-up has been 72 per cent, with resorption of the lamina the main cause for failure. The BCVA in our series was 6/60 or better in 78 per cent, and 6/12 or better in 53 per cent and 61 per cent of cases retained best-achieved vision throughout the follow-up period. Aftercare of the OOKP is less intensive, particularly in terms of antibiotic usage. That is because the mucous membrane seals in the bone parts very well. Therefore patients only require a nightly application of antibiotic ointments. On the other hand, eyes with the OOKP are equally prone to complications such as glaucoma, retinal detachment as are eyes with the Boston KPro. Future improvements in keratoprosthesis design and surgical technique are likely to lead to improvements in visual outcomes and retention of the devices. But already, modern keratoprostheses have revolutionised corneal surgery by enabling the restoration of vision in cases where it previously would have been impossible.