Despite enthusiasm for the ability of the laser to create more precise and more consistent astigmatic incisions compared with a manual technique, there are no solid data showing use of the laser leads to better outcomes for reducing astigmatism.
Dr Stevens said he uses the Catalys laser to perform intrastromal astigmatic keratotomy (ISAK), and he explained that he favours this technique over penetrating incisions because it provides better rotational/angular alignment, is associated with better postoperative comfort for patients, and theoretically eliminates infection risk.
In addition, the incisions heal quickly, visual recovery is fast, and, according to his data, stability is achieved by 1 month and maintained in patients having follow-up to 2 years.
“Unlike penetrating incisions, ISAK does not seem to result in cylinder overcorrection unless there is anterior penetration of the arc because of vertical gas breakthrough,” said Dr Stevens.
With that in mind, he noted that anterior surface detection and tilt compensation are critical for ISAK, and Dr Stevens also called for all systems to have automated identification of the steep corneal meridian to guide AK placement in order to minimise angle error.
He noted that achieving the desired result with ISAK requires complete separation of the arc walls. To avoid residual bridging of corneal fibres, Dr Stevens said the treatment should be performed with high power, aiming to generate sufficient gas to assure separation of corneal lamellae.
He also mentioned an outcomes study undertaken at Moorfields Eye Hospital showing that while ISAK was effective in reducing astigmatism, there is room for improving its predictability.3
“We found corneal biomechanics accounts for about half of the scatter in outcomes, and this is a place where we think Brillouin scanning to measure corneal elasticity in individual eyes may be important in the future,” Dr Stevens said.
In a discussion on methods of placing reference marks for aligning the horizontal corneal meridian, Dr Thomas Laube, who works in private practice in Düsseldorf, Germany, described his use of a sterile disposable ink pen (Devon utility marker, Covidien) to mark the conjunctiva when performing ISAK with the Catalys laser. He said the markings are clearly visible in the OCT image and are easily removed with a sponge.
Consistent with the outcomes study mentioned by Dr Stevens, Dr Anil Pitalia, SpaMedica Eye Hospitals, Manchester, UK, presented a comparative analysis of outcomes achieved in 52 eyes that had toric IOL implantation and 36 eyes treated with FS laser ISAK for up to 2 D of astigmatism.
Dr Pitalia noted he prefers to use the laser because it is simpler, but his results showed toric IOL implantation was associated with greater predictability and less residual astigmatism, particularly in eyes with >1.5 D astigmatism preoperatively.
Dr Stevens suggested that some ISAK undercorrections that occurred in his series may be due to tissue bridging that can be easily addressed postoperatively at the slit lamp.