Artificial Cornea Transplantation

Excerpt

There are approximately 4.9 million people with bilateral blindness secondary to corneal disease worldwide, accounting for 12% of total global blindness. Common causes are anterior corneal pathologies such as trachoma, infectious keratitis, ocular trauma, chemical injuries, and high prevalence in developing countries. Primary corneal transplantation (lamellar or full-thickness) has high graft survival of 87% and 93% at one year and 72% to 73% at 5 years in non-complex eyes. Graft survival decreases with repeat transplantation and in complex eyes, despite advances in keratoplasty techniques and more selective tissue transplantation.

High-risk factors include recurrent and chronic inflammation of the ocular surface, glaucoma, and eyes with corneal vascularisation. Globally, the availability of donor cornea material can be limited due to donor supply and the need for eye banking facilities. An artificial cornea transplant can be considered for end-stage corneal diseases such as multiple graft failures or inflammatory ocular surface disease.

There have been many proposed artificial cornea transplant devices (keratoprosthesis; KPro). Pellier de Quengsy first described the initial concept in 1789. They generally have a central clear optic with either hard skirt plates which sandwich donor cornea tissue in between or a soft optic and skirt in a one-piece integrated design. The importance of a suitable skirt material with good tissue incorporation was made clear from earlier models made from rubber, milk protein, Dacron, crystal, glass, and celluloid, which resulted in device extrusion after implantation. The successful introduction of cadaveric corneal transplantation resulted in decreased interest in artificial corneal transplantation. However, the discovery of polymethylmethacrylate (PMMA) enabled a biocompatible device to be implanted, and earlier devices have been described by Choyce and Stone.

More recently, soft polymers have been used to simulate the natural cornea. Poly-2-hydroxyethyl methacrylate was used for the AlphaCor, which gained FDA approval in 2003. After one and two years, retention rates were 80% and 62%, and stromal melt occurred in 27% of the total cases, many requiring explantation. A similar design using polytetrafluoroethylene (PTFE; Legeais BioKPro-III) had worse outcomes, with 86% of devices failing after implantation. The focus of this review is to describe indications and management for the most commonly used artificial cornea transplants currently: Boston KPro type 1 and the Osteo-odonto-Keratoprosthesis (OOKP).

Current Keratoprostheses

Boston KPro Type I

The most widely implanted artificial cornea transplant is the Boston KPro type 1, which was first introduced by Dohlman in 1965 and gained FDA approval in 1992. Its popularity started increasing from the beginning of the 21st century, and to date, over 19,000 have been implanted. The design consists of a front plate with a central optical stem, a backplate, and a corneal donor button sandwiched in between. The front plate and optic are made from PMMA, and the optical power is determined by the radius curvature. The original design involved screwing the backplate into position. This was improved with a titanium locking ring in 2003 and a threadless stem in 2007. The backplate is available in PMMA and titanium, which are both biologically well tolerated. There is no difference reported in the frequency of retroprosthetic membrane formation (RPM) between the two materials at 12 months.

Osteo-odonto-Keratoprosthesis (OOKP)

The OOKP utilizes an autologous tooth root-alveolar bone complex as the keratoprosthesis skirt material for better tissue integration. Invented by Strampelli and modified by Falcinelli et al., the principle of OOKP surgery is the bypass of the diseased ocular surface by a buccal mucous membrane patch and replacement of the anterior segment structures with the OOKP. The mucous membrane patch can tolerate dry environmental conditions and some level of inflammation. The good tissue integration ensures that OOKP can retain for a number of decades. Long-term anatomical retention is good, with 81% retention over 5 years reported in a cohort of 36 eyes, 98% retention in 85 patients over a 20-year follow-up, and 80% in 224 eyes over 18 years.

Keratoprostheses in Development

There are numerous alternative keratoprostheses in development. Studies have found that 5-year survival both for anatomical retention and functional recovery were higher for the Boston type 1 KPro compared with the Aurolab Keratoprosthesis. However, these were not statistically significant. Therefore the Aurolab Keratoprosthesis can be an alternative to the Boston type 1 KPro if there are affordability or availability limitations. The Lucia keratoprosthesis is a modification of the Boston type 1 KPro to improve affordability. Machinist time was reduced by changing the locking interface between the front and backplates. Photoetching was used instead of using a lathe, and the round holes in the backplate were replaced with petaloid radial slits. Anodised titanium allowed changes in the color of the backplate for improved cosmesis.

Several different keratoprostheses for eyes with defective blinking and dry eyes or cicatrization are being evaluated. The Lux keratoprosthesis consists of a cone-shaped PMMA cylinder, titanium sleeve, and a 7.8mm titanium backplate. A donor cornea is double trephined centrally at 3 mm and 7.5 mm peripherally. The PMMA cylinder is secured in the titanium sleeve and placed through the central 3mm opening in the donor cornea. The backplate is secured and sutured into place in the host after removal of the patient's cornea with interrupted nylon sutures. A mucous membrane graft is sutured over with an opening for the PMMA cylinder optic. Short-term results with good retention and functional outcomes have been reported.

Improvements in skirt materials could further enhance keratoprosthesis development. OOKP is at risk of bone resorption, and a synthetic substitute with a hydrogel composite of nano-crystalline hydroxyapatite (nHAp) coated poly lactic-co-glycolic acid (PLGA) microspheres have been evaluated in the lab. A graphene oxide titania-based biomaterial has been implanted in vivo in rabbit corneas without causing an immune or inflammatory reaction and can be a potential new skirt material for keratoprosthesis.