Synthetic inlays in early study for refractive correction

Sydney, Australia-The first synthetic polymer corneal inlays implanted in humans appear to be biocompatible with corneal tissue and represent a safe, biologically acceptable alternative to other forms of refractive surgery, according to observations made over an 18-month period, said Deborah F. Sweeney, B Optom, PhD.

The inlays, also known as implantable contact lenses, are made from a highly porous biomaterial called perfluoropolyether (PFPE), which was developed by Vision Cooperative Research Centre researchers. The inlay is implanted under the surface of the cornea and changes the shape of the corneal surface to achieve the desired correction, explained Dr. Sweeney, professor and chief executive officer, Vision Cooperative Research Centre (CRC), University of New South Wales, Sydney, Australia.

In the phase I trial, inlays were placed under a microkeratome flap (180 µm thickness) without sutures in the unsighted eye of five patients. A microkeratome flap without implantation was performed on the unsighted eye of an additional patient who served as a control.

Patients were assessed at baseline, immediately after surgery, at 7 days, 1 month, and every month thereafter for 12 months. For the duration of the study they were observed at least every 3 months. Confocal microscopy also was performed every 3 months.

According to Dr. Sweeney, the surgeries were uneventful and no episodes of inflammation, significant vascular in-growth, or other adverse reactions were reported. The inlay was removed from one patient 5 months postoperatively because of epithelial and stromal breakdown that may have been consistent with a retrospective diagnosis of forme fruste keratoconus. The inlays were left in place in the other four patients, with the corneal tissue surrounding the inlay unaffected; mild haze surrounding the inlay was reported in two cases.

Confocal microscopy indicated normal posterior stroma and endothelium appearance. The changes that were seen in the epithelium, anterior stroma, and interface were consistent with those observed in animal models and were less marked from 12 months onward, Dr. Sweeney said.

The inlay position remained stable throughout the 18-month follow-up; however, clarity decreased slightly for the first 3 months before stabilizing.

The inlays are intended to be permanent alternatives to refractive surgery, Dr. Sweeney said.

"We think it is a better procedure than refractive surgery because it is additive as opposed to ablating and removing tissue," she said.

Dr. Sweeney explained that laser surgery alters the curvature of the corneal surface by irreversible laser ablation, correcting refractive error but permanently damaging the cornea.

Implantation of an inlay, however, is a reversible procedure, which would allow modification if vision changes were required, and does not cause permanent damage to the central optical zone because no tissue is ablated.

Other advantages include being a relatively simple, in-office procedure, which would be less invasive than current surgical and refractive techniques and require less surgical skill than other refractive procedures, Dr. Sweeney said. In addition, there would be less surgical down time for patients and improved long-term stability of refractive correction, because there is no significant wound-healing response typical of that accompanying removal of corneal tissue.

The inlays would also be alternatives to contact lenses or spectacles because they are permanent means of correcting the vision.

The PFPE material was developed through 15 years of research and is ideal for vision-correction applications, Dr. Sweeney said.

"The material has a number of characteristics that we think make it very good for use as a corneal inlay," she said. "Our results show that it is biocompatible across time, and we are continuing to work on that polymer and its optimization in order to bring it to the next phase of testing."

Follow-up continues

A phase II study could begin within the next 1 to 2 years. In the meantime, the patients in the phase I study are being followed for an additional 12 months beyond the 18 months for which data have been presented.

The corneal inlay is just one application of the polymer, however.