Tuesday, March 28, 2023
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Associate Professor; Residency Director, Department of Family Medicine, University of California, Irvine
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Associate Professor; Residency Director, Department of Family Medicine, University of California, Irvine
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Over the last decade, availability of donor tissue and repeated human graft failures has prompted the search for artificial corneal substitutes that would be compatible in human eyes. Several of these devices are available and in clinical use today. These range from completely synthetic devices such as the Boston Keratoprosthesis (Boston KPro; Massachusetts Eye and Ear Infirmary, MA, USA) to the completely biological tissue-engineered cornea.
The Boston KPro received FDA approval in 1992.[223] It is suitable for patients with repeated failed corneal grafts where further transplants are judged to be at high risk for failure. The optical component is made from PMMA. A corneal graft is sandwiched between this unit of front plate and stem, and a larger back plate (Figure 9). This device is then locked into place with a titanium c-ring and sutured to the recipient’s eye (Boston KPro type I). Postoperatively, a continuous-wear BCL is placed on the cornea (Kontur Contact Lens Co., CA, USA), which is replaced every 3–4 months, and the patient is given a tapering regimen of antibiotic and steroid drops for life. As there is no reliable device to measure intraocular pressure in these patients, digital palpation and regular follow-up with visual fields and optic nerve evaluation is necessary to monitor glaucoma progression.
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EnlargeBoston Keratoprosthesis. (A) Schematic of Boston Keratoprosthesis. (B) Boston Keratoprosthesiss in situ. Courtesy of Allan Slomovic (University of Toronto, Canada).
A pseudophakic eye with a well-positioned intraocular lens can be corrected with a KPro of a standard power. An aphakic eye requires a chosen power depending on the axial length of the eye. In patients with severe ocular surface disease, an anterior cylinder may be added to the KPro, which protrudes through a permanent tarsorrhaphy in patients with severe ocular surface disease (Boston KPro type II).
In 2005, Aquavella reported the result of their series of 25 patients with the Boston KPro type 1.[224] All prostheses were retained with no extrusion. Almost half of the patients achieved vision of 20/25–20/200 in an average of 60 days. The first and largest multicenter study on the Boston KPro type 1 was published in 2006.[225] A total of 141 procedures by 39 different surgeons were analyzed at 17 sites. Postoperative visual acuity improved to 20/200 in 57% of the cases. The graft-retention rate at 8.5 months was 95%. Other recent studies have demonstrated retention rates of 83–100%, with visual outcomes of at least 20/200 in 77–83% of patients.[226,227]
First described by Strampelli in 1963,[228] this device uses the patient’s own tooth root and surrounding alveolar bone to support an optical cylinder. Other biological support materials include cartilage[229] and tibial bone.[230] The first stage involves assembling the tooth–optic lamina and implanting this in a submuscular pouch of the lower lid in the fellow eye for 2–4 months. A full-thickness buccal mucous membrane graft is then harvested and sutured on the recipient eye. In the second stage, this device is retrieved and then implanted in the recipient eye that has had its lens and iris removed, and anterior vitrectomy following trephination. The buccal mucous membrane graft cover is then repositioned and sutured into place. Surgical management is usually carried in a joint fashion with an ophthalmologist and maxillo–facial surgeon.
In 2005, Falcinelli reported on 181 patients with a median follow-up of 12 years.[231] The probability of retaining an intact osteo–odonto keratoprosthesis (OOKP) was 85% and the probability of retaining best postoperative visual acuity was 55.5% after 18 years. In 2008, Liu et al. reported on 36 patients with a mean follow-up of 3.9 years and found that OOKP retention rates approached 72%, with 61% retaining best-achieved vision during the follow-up period.[232] The same year, Tan et al. reported on 15 Asian patients with OOKP and showed a retention rate of 100% over 19.1 months.[233]
The AlphaCor™ (Argus Biomedical, Perth, Australia) was developed in Australia and received FDA approval in 2003.[234] It is composed of a clear central optical core and an opaque skirt made of a biocompatible polymer (poly-2-hydroxyethyl methacrylate)[235] joined by a polymer. The outer skirt is designed to facilitate colonization by invading keratocytes for integration with surrounding tissues. This device is implanted intrastromally following a careful lamellar dissection to 50% thickness and trephination of the central posterior lamella. In the second stage, performed 2–3 months later (to allow for biointegration), trephination is performed of the tissues superficial to the optic and removed to expose the optical zone.[234,236] In 2003, Hicks et al. reported retention rates of 62% over 2 years.[236] However, poor visual outcomes and late anterior stromal melts have limited the use of this device.
In 1979, Pintucci et al. used Dacron® as a skirt for a PMMA optic.[237] In the first stage, a mucous membrane graft is used to cover the eye following corneal debridement of the epithelium. The device is buried in a submuscular pouch in the lower lid for biointegration over at least 2 months. The device is then retrieved from the lower lid before implantation similar to that with OOKP. Pintucci et al. reported on 20 eyes with a mean follow-up of 58 months.[237] All achieved some improvement of vision, with 65% retaining this improvement for more than 2 years. In 2006, Maskati reported on 31 eyes, with 77% achieving an improvement in vision to greater than counting fingers at 1.5 m. Loss in vision was seen in seven eyes.[238]
With the growing need for a corneal substitute, there are a number of devices being developed. These range from supradescemetic synthetic corneas[239,240] to natural corneal substitutes from recombinant human collagen.[241–244] Having a range of synthetic and artificial corneal substitutes within our armamentarium will allow visual rehabilitation of blindness due to corneal disease despite a shortage of donor corneas.
