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Iris registration system compensates for cyclotorsion during laser eye surgery
The need for iris registration technology is supported by the literature and by clinical experience, surgeon says.
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An iris registration system from Visx allows compensation for cyclotorsional errors that occur in wavefront-guided treatments, according to a British researcher.
Julian Stevens, FRCS, of Moorfields Eye Hospital in London, said that the literature and clinical experience with wavefront-guided treatment support the need for iris registration technology.
The technology, to be used with the Visx CustomVue system, meets the challenge of compensating for cyclotorsion that occurs when a patient goes from the upright position required for WaveScan exams to the supine position in preparation for laser ablation, he said.
"For optimum correction of astigmatism and radially asymmetric refractive error such as in wavefront-guided treatment, you should not induce an error from a misregistration," Mr. Stevens said.
Cyclotorsional error data
Mr. Stevens presented cyclotorsional error data collected at Moorfields Eye Hospital. It showed that in 69 eyes, mean movement of the eye was 2.8º ± 2.0º, with movement in 4% of eyes greater than 7º and as high as 8.8º.
Of the 69 eyes examined at treatment, 14% had an angle error of more than 5º, and 86% had an angle error within 5º. According to Mr. Stevens, a 5º angle error translates to 17% undercorrection in astigmatism, with the degree of angle error and the percentage of undercorrection positively correlated.
Researchers then collected data showing angle error during registration of WaveScan ablations and found that the mean angle error in the 69 eyes dropped to 1.5º ± 3.2º. The absolute mean angle error was 2.8º.
Along with data collected at Moorfields, Mr. Stevens said that studies examining reductions in symmetric and asymmetric aberrations showed that there was a greater reduction of symmetric errors, such as spherical aberration, than of asymmetric errors, such as cylinder and coma.
"The evidence that you have a greater reduction in spherical aberration than coma suggests that you had a missed registration in a rotational manner, so that's indirect evidence that you need to improve in terms of your rotational alignment," Mr. Stevens explained.
Technique and benefits
According to Mr. Stevens, the Visx iris registration technology calculates torsional angles from multiple measurements on the iris. The pupil area is mapped using wavefront boxes that are prominently shown on the screen along with a cyclotorsion rotation indicator and WaveScan reference marks. The map is then uplinked into the laser. This positional information under the laser acquires the iris features in order to match to the iris itself. Once the iris is rotationally matched, the position of the mapping boxes is computed.
"It maps the iris like a circular bar code, and you simply unwrap that circular bar code and match it up with the iris picture under the laser," Mr. Stevens said.
For Your Information:
- Julian Stevens, FRCS, can be reached at Moorfields Eye Hospital, London EC1V 2PD; +44-207-251-4835; fax: +44-207-431-8622; e-mail: JulianStevens@compuserve.com. Ocular Surgery News was unable to confirm whether Dr. Stevens has a direct financial interest in the products mentioned in this article or if he is a paid consultant for any companies mentioned.
- Visx Inc. can be reached at 3400 Central Expressway, Santa Clara, CA 95051-7122; +1-408-733-2020; fax: +1-408-773-7278; www.visx.com.
- Jared Schultz is an OSN Staff Writer who covers all aspects of ophthalmology. He focuses geographically on Europe and the Asia-Pacific region.