PAINLESS SURGERY

High-repetition femtosecond lasers can now be used to alter the refractive index of living corneas and crystalline lenses in an animal model, and in hydrogel ocular implant materials. If stable, the technology could enable non-invasive vision correction in both phakic and pseudophakic eyes, Scott M MacRae MD told the innovator’s session at the annual ASCRS symposium. Unlike femto laser lens or flap cuts, these refractive index changes are accomplished by working below the threshold of tissue damage, said Dr MacRae, who is testing the technology developed by Wayne Knox PhD and Krystel Huxlin PhD at the University of Rochester, New York, US. In this new modality, the femtosecond laser does not need to disrupt the epithelium and leaves no microscopic or immuno-histochemical evidence of cell damage in living cat corneas. As a result, refractive surgery could be performed without pain or the risk of dry eye, or the need for postoperative antibiotics or shields, Dr MacRae said. “Using this technique, we hope to achieve custom vision correction of -3.0 to +3.0 D sphere or astigmatism on corneas, lenses and hydrogel implants.â€

High-speed, low power

Called femtosecond Intra-tissue Refractive Index Shaping (IRIS), the system uses localised energy deposition from laser pulses to create a change in refraction by increasing the refractive index in patterns that can treat myopia, hyperopia or astigmatism. This can be done within hydrogel materials, corneas and crystalline lenses, Dr MacRae said. IRIS is docked to the eye using a suction ring and liquid interface similar to those on existing flap cutting and cataract systems. However, IRIS works at a much higher repetition rate of 82 MHz, compared with 5 kHz to 5 MHz for corneal flap cutters. IRIS pulses are also shorter, at 30 to 100 femtoseconds compared to 200 femtoseconds to 2 picoseconds, and lower power, at 1 to 12 nJ compared to 150 to 6,000 nJ. This yields a total power per pulse about 100 times less than with flap cutters, imparting much less heat energy per pulse. As a result, refractive index changes can be made in corneas and lenses without disrupting corneal epithelium, endothelium, stroma or lens tissue, Dr MacRae noted. In cat corneas and lenses, refractive index changes of up to +0.037 have been obtained to date, which when enacted over a large region and in multiple layers, can attain power changes of several dioptres. In hydrogel lenses, the effect is even greater, with refractive index increasing by up to +0.05 in each lasered layer. After treatment the material remains optically clear with virtually minimal light scatter, he added. Potential treatment options include writing corrections directly in the cornea and/or crystalline lens; implanting an IOL and then adjusting the lens power by writing directly in the IOL; or implanting a lens and writing corrections in the cornea.

Unresolved issues

However, several issues remain to be resolved, Dr MacRae said. Stability in vivo is one. So far, lines inscribed in cat eyes have remained stable for three months. In hydrogel, the refractive change appears to be permanent and lasts more than 2.5 years. Tissue response may also be an issue as total treatment power approaches that of flap cutters. Dr MacRae expects these issues will be successfully resolved. “The field is moving fast and it is very exciting.â€