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The hardware and software used in refractive surgery has undergone a steady evolution that has resulted in more precise refractive outcomes.
London-The evolution in refractive surgery techniques and technology has significantly improved the accuracy of the refractive outcomes, and future advances are expected to lead to even greater benefits, including with respect to correction of higher-order aberrations (HOAs), according to Julian D. Stevens, MD, consultant ophthalmologist and director, refractive surgery, Moorfields Eye Hospital, London.
"A year-by-year review of outcomes in my personal series of patients treated for myopia up to –4 D shows a progressive improvement in predictability that is a tribute to the current precision of modern refractive surgery," he said. The standard deviation of the mean manifest refraction spherical equivalent for his first PRK treatments performed with a proprietary laser (Summit) in 1990 was about 1 D, Dr. Stevens said, and it dropped to 0.47 D after introduction of another laser (20/20 VISX) and use of a 6-mm optical zone. "Following the introduction of LASIK, wavefront-guided treatment, optical smoothing, and Fourier reconstruction, the refractive predictability has improved further to a present value of about 0.30 D, which surpasses the standard deviation of my optometrists in masked refraction," he said.
This shortfall led to closer scrutiny of all of the parameters that possibly could influence outcomes and the development of hardware and software solutions for improving treatment results. Initial attention focused on the wavefront sensors and has led to better capture algorithms and wavefront reconstruction algorithms using Fourier or zonal reconstruction. These innovations have led to better fidelity with the wavefront shapes obtained and improved results thanks to reduced loss of wavefront data, said Dr. Stevens.
To demonstrate the differences, he compared HOA maps from the same patient created with Zernike reconstruction, Fourier calculation, or zonal reconstruction techniques.
"There are marked differences in the appearance of these maps, and the reason is that there is tip and tilt in the cornea optics that is not reflected in the Zernike reconstruction," he said.
Replacement of mechanical microkeratomes with the femtosecond laser for flap creation also is contributing to improved outcomes because the microkeratome-created flaps usually are meniscus-shaped and vary in thickness depending on preoperative corneal curvature and thickness. In contrast, the femtosecond laser affords greater control for more precise creation of close-to-true lamellar flaps of a defined thickness and position within the cornea, Dr. Stevens said.
Treatment delivery also is more precise, thanks to improvements in positioning technologies.
"We currently have better tracking systems than ever before, and they will continue to improve. Trackers have gotten faster, and we now have iris registration technology for X-Y offset control, which is crucial because even small X-Y errors can lead to big problems in terms of the accurate delivery of high-fidelity wavefront treatment," Dr. Stevens said.
Better laser cavities are also in development and feature better energy sources that provide more stable output and better optics for improved smoothing, he said, predicting that the future also will bring autocalibration technology in which the wavefront sensor is integrated into the laser to offer even better control of treatment delivery.
For the future, researchers also are considering the potential importance of thermal loading from laser treatment applied to the cornea and the potential of thermal imaging and thermal delivery control as well as methods for incorporating corneal biomechanics effects into treatment planning.
"Examination and refinement of each and every component of the diagnostic and treatment process is an ongoing effort. Therefore, I anticipate that over the coming years, we will see increased precision in our treatments and a continuation of the existing trend of decreasing standard deviations," Dr. Stevens said.