Solid-state laser promising in early cases
in San Francisco
THE CustomVis™ solid-state laser appears to be an effective tool for treating irregular astigmatism and myopia, according to the results of preliminary studies presented at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery.
Marguerite McDonald MD described the experience of Australian ophthalmologist Ian Anderson MD in the first three cases to be treated with the laser. All of the patients had undergone previous surgeries, including PRK, PK and AK with compression sutures. However, they all had serious refractive deficiencies and extreme irregular astigmatism.Following treatment with the solid-state laser they gained up to seven lines of uncorrected vision and experienced decreases in cylinder of as much as 8.0 D, she reported. "They are getting encouraging results with these first three difficult irregular astigmatism cases," she said.
The first astigmatic patient in the series had 8.4 D of pre-op cylinder, best-corrected visual acuity of 20/400, and uncorrected visual acuity of 20/400. The treatment zone was 6.8 mm and the optical zone was 4.8 mm. The maximum ablation depth was only 154 microns, because there were only 300 microns of residual tissue. The patient had undergone two previous PRKs and two AKs with compression sutures. The patient achieved a best-corrected visual acuity of 20/20 at three months post-op, uncorrected visual acuity of 20/80, and a decrease in cylinder of 4.0 D. The patient gained five lines of vision. Contrast acuity with best-corrected vision improved at contrast levels ranging from 10% to 100%. Contrast acuity approved with uncorrected vision at all but the 10% level, using standard tests.
The second patient presented with 6.7 D of cylinder, best-corrected visual acuity of 20/30, and uncorrected visual acuity of 20/80. Both best corrected and uncorrected visual acuity improved to 20/20, and cylinder improved to 3.4D. Uncorrected visual acuity improved by seven lines, according to Dr McDonald. The third patient was an extremely difficult case. The patient had a cylinder of 11.1 D, best-corrected visual acuity of 20/40, and uncorrected visual acuity of 20/400 before surgery. Following treatment with the CustomVis, the cylinder error dropped to 3.2 D, and uncorrected visual acuity improved to 20/50, with a six line improvement in UCVA. The patient also demonstrated a 12.0 D refractive change in one meridian and a 5.25 D refractive change in the opposite meridian. There was no loss in best-corrected visual acuity, Dr McDonald noted.
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Clinical trials of the CustomVis laser are scheduled to begin soon in the US . The US investigative team is looking for patients with irregular astigmatism who might be suitable candidates for treatment with the system, said Dr McDonald, who is slated to be one of the investigators in the US study, although she is neither an investor in the laser system nor a paid consultant. The CustomVis solid state laser has a Gaussian 0.6 mm random spot, a 300 Hz pulse repetition rate and a wavelength of 213 nm. From animal and human trials, scientists now know that a wavelength of 213 nm has no harmful thermal effects on the residual stroma. It can also ablate very well through a hydrated cornea and achieve ablation similar to a wavelength of 193 nm. The CustomVis laser has dual (analogue and digital) eye trackers, and a 1 kHz total closed loop response—a rate that is 10 times faster than any other commercially available laser. The system uses limbal tracking and does not require pupil dilation.
The CustomVis' Z-Cad system software is crucial to its success in treating myopia and astigmatism, noted Ralph Chu MD, Adjunct Clinical Assistant Professor of Ophthalmology of the University of Minnesota , in a related presentation at ASCRS. The system combines refractive data, as well as topographic and wavefront data to create a customised treatment plan for each patient. Wavefront data is used to treat lower order aberrations and topographic data to treat higher-order aberrations. Dr Chu noted that it is possible to import wavefront data from any device, but he recommended Tracey for the more irregular eyes. "We have to know what's going on with the cornea so we can customise treatments for our patients," Dr Chu said. Tracking is done on the limbus because it is the most stable anatomic part of the eye. Thus the limbus edge is used to determine the treatment centre, Dr Chu said.
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"We can simulate the treatment before we actually deliver it to the eye, and see what it does to the eye before we perform surgery with the laser. Based on the simulation, physicians can identify treatment parameters for each patient. That gives you surgical control and a laser that is customisable to each patient," he said. With the CustomVis, the surgeon has control over the treatment zone size and optical zone size, and may even change refractive data, Dr Chu said. "With topographic and wavefront data, the CustomVis allows surgeons to customise surgical plans and retain control of key parameters," he said.
Marguerite McDonald MD
Clinical Professor of Ophthalmology
Tulane University School of Medicine
New Orleans , LA
Ralph Chu MD
University of Minnesota
Minneapolis, Minnesota, US
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