Iris registration significantly improved bitoric ablation outcomes

Using iris registration in mixed astigmatism treatment reduced postop higher-order aberrations and improved night vision, in a study.

ByJared Schultz

Iris registration is essential when treating mixed astigmatism with wavefront-guided ablation; without it, conventional ablation should be used, according to one ophthalmologist.

Mounir A. Khalifa

In a retrospective study presented at the International Society of Refractive Surgery meeting in Istanbul, Mounir A. Khalifa, MD, compared three groups of 20 eyes treated for mixed astigmatism with conventional ablation, wavefront-guided ablation with iris registration or wavefront-guided ablation without iris registration and found that results were significantly better with iris registration than without.

“Wavefront-guided LASIK with iris registration is the most efficient, predictable, and safest method for treating mixed astigmatism,” Dr. Khalifa said.

Initial thinking was that the quality of the flap affected outcomes in cases of mixed astigmatism, but Dr. Khalifa explained to Ocular Surgery News that he and his colleagues began to attribute the induction of higher order aberrations in these cases to the ablation patterns.

“The wavefront pattern is more complicated than the conventional pattern, so if I don’t use accurate registration, it’s better to go for the conventional treatment,” Dr. Khalifa told Ocular Surgery News during a telephone interview.

Improved outcomes

The mean patient age was 23 in the conventional group, 24 in the non-iris-registration group and 25 in the iris-registration group, and the data were statistically analyzed preoperatively and at 3 months postoperatively, Dr. Khalifa said.

He used the Visx Star S4 laser in all groups, with manual marking in the conventional and non-iris-registration groups. He used the WaveScan aberrometer with Fourier analysis for the wavefront patients.

According to the results, 10% of patients in the iris-registration group gained two lines of best corrected visual acuity by 3 months postop compared to none in the other two groups. Predictability of both sphere and coma to within 0.5 D was better in the iris-registration group compared to conventional and non-iris-registration.

Scotopic contrast sensitivity improved significantly in the iris-registration group compared to the other two groups, Dr. Khalifa reported.

The most telling results showed that coma, trefoil and secondary astigmatism increased significantly in the conventional and non-iris-registration groups whereas in the iris-registration group, there was no significant change.

“I found that when you misalign very little in the non-iris-registration group for mixed astigmatism, you find a significant difference, especially in secondary astigmatism,” Dr. Khalifa said.

Axial and torsional registration

The two key aspects of accurate registration are torsional registration, which compensates for eye movements that occur when switching from the upright to supine position, and axial registration, which compensates for the difference between the scotopic and the photopic pupil, Dr. Khalifa said.

Torsional registration is achieved by compensating for the shifts in the position of the entire eye. Axial registration is achieved by locking in the scan of the cornea with a dilated pupil and applying that scan after the pupil constricts under the light of the microscope.

For more information:

Mounir A. Khalifa, MD, can be reached at Horus Vision Correction Center, 456 Al Horeya Ave., Roushd, Alexandria, Egypt 22311; +203-5448744; fax: +203-5444480; e-mail: mounirkhalifa@yahoo.com; Web site: www.mounirkhalifa.eyemd.org.

Jared Schultz is an OSN Staff Writer who covers all aspects of ophthalmology. He focuses geographically on Europe and the Asia-Pacific region.