A corneal implant in a rabbit eye did not trigger rejection or bacteria growth. (Laboratoires Corneal)
Tried and tested optical materials and careful application of engineering can be used to create a viable artificial human cornea, so says Dr Franck Villain and a research team from Laboratoires Corneal (Pringy, France). "To realize the non-perforated human eye cornea replacement, we had to create a new optical arrangement but one based on innovation rather than invention," says Villain, who is one of the coordinators of a pan-European project called CRAFT. "For example, we are using regular acrylic-based materials to avoid complications in biocompatibility. Trials on a rabbit had yielded encouraging results so we now have to scale up the dimensions of the process to suit a human eye."
Developed as a cooperative research project under the Fourth Framework Programme, BIOMED 2 of the European Union, the CRAFT project to design and develop a synthetic cornea is a collaboration between Corneal, the University of Göteborg (Göteborg, Sweden), Medicarb AB (Bromma, Sweden), Domilens Vertrieb für Medizinische Produkte GmbH (Hamburg, Germany), and Compania Italiana Oftalmologica Scarl (Parma, Italy).
The new approach is less invasive than previous techniques and involves only the surface layers of the cornea. The cornea is composed of five layers: epithelium, Bowman membrane, stroma (80% of the thickness), Descemet membrane, and endothelium. "Our technique is designed not to perforate the Descemet membrane," says Villain. "The endothelium is a cell layer, very fragile as the endothelial cells will never be regenerated. Old types of corneal prostheses required a full trephination of the cornea. As soon as the eye is opened, the risk of complications is increased." The new cornea consists of a central optical element 400 to 500 µm thick with an annular periphery that complies with the cavity cut into the cornea. The material is specially treated so that the surrounding cells will not reject it. It is based on existing polymeric materials that inhibit possible bacterial growth in the implant.
The implantation of an artificial cornea is the best way to recover vision. More rigid implants have become less popular since larger incisions have been found to be associated with an increased incidence of postoperative complications such as induced astigmatism. With recent advances in small-incision cataract surgery, increased emphasis has been placed on developing soft, foldable materials suitable for use in artificial implants.
Acrylic-based corneal implant consists of a central optical element with an annular periphery that complies with the cavity cut into the cornea. (Laboratoires Corneal)
A major problem in the success of corneal prostheses is the proliferation of epithelial cells. They are designed to protect the cornea, and they stop proliferation of confluence. "The risk is the proliferation under the prosthesis on the Descemet membrane; this will lead to an expulsion of the prosthesis," says Villain. "Experiments in rabbits with almost one year of follow-up show that this did not happenprobably thanks to the design of the prosthesis."
"Surgical techniques make it possible to place the implant without penetrating the membrane endotheliale of the cornea," adds Jean-Marie Parel, of the University of Miami School of Medicine (Miami, FL). "Surgical tools are difficult because of the size and shape of the cornea. But the approach is inexpensive and easy to manufacture so it has the potential for helping more people than previous approaches."
He has been working on scanning laser corneal surgery for some years, including within the CRAFT program. This has included long-term studies of changes in the rabbit cornea after photoablation with pulsed scanning laser operating at 213 nm (frequency-quintupled neodymium-doped yttrium aluminum garnet). "In the latest work the implants have performed well with no sign of rejection. Now we have to develop models of implants adaptable to the larger human eyes. The first patient will be someone for whom the chance of sight restoration by other means is not possible."