Wavefront LASIK with iris registration best for mixed astigmatism, says study
April 8, 2009
Wavefront LASIK with iris registration provides better correction of mixed astigmatism than conventional or wavefront-guided LASIK with manual marking of the cornea to account for rotation of the eye during surgery, according to a recent study performed in Alexandria, Egypt.
What is mixed astigmatism?
Mixed corneal astigmatism is a special type of astigmatism whereby the unequal curvature of different meridians of the cornea causes the meridian of least curvature to be farsighted and the meridian of greatest curvature to be nearsighted.
This differs from the more common types of astigmatism, where both the flattest and steepest meridians of the eye are either nearsighted or farsighted, but in different degrees.
What is iris registration?
Iris registration is a automated method to ensure that the laser ablation delivered during wavefront LASIK is aligned perfectly with the eye and that no misalignment occurs due to rotation of the eye when the patient is lying down (in the prone position) during surgery.
Iris registration may require an software and hardware upgrade to the laser equipment used to perform conventional LASIK treatments.
Why iris registration matters
In custom (or "wavefront-assisted") LASIK, detailed wavefront measurements of the eye prior to surgery. The patient is sitting upright for these measurements. But during surgery, the patient is in the prone position. Studies show that most peoples eyes rotate slightly then they are lying down, compared to when they are upright. This rotation can cause errors in wavefront LASIK treatments, unless measures are taken to compensate for this rotation of the eyes.
The two methods of compensating for rotational errors are: 1) manually marking the eye with a surgical pen, and 2) using an instrument that "registers" the wavefront measurements with unique characteristics of the patient's iris.
The iris registration method also includes an upgrade to the treatment laser that continuously monitors the position of the patients eye during the ablation to compensate for any eye movements during surgery.
The goal of both methods is the same: To properly align the laser treatment to minimize errors caused by movements of the eye.
Results of iris registration study for mixed astigmatism
The patients with mixed astigmatism in the Egyptian study were divided equally into three treatment groups: 1) conventional LASIK and manual marking, 2) wavefront-guided LASIK and manual marking, and 3) wavefront-guided LASIK and iris registration (IR).
All patients were examined for at least three months following surgery.
The researchers found:
- Eyes treated with wavefront LASIK and IR had the best post-operative visual acuity, with 100 percent of eyes 20/30 or better, and 90 percent 20/20 or better.
- Unlike the other treatment groups, all eyes in the wavefront LASIK and IR group had best spectacle-corrected visual acuity (BSCVA) that was equal to or better than preoperative levels.
- The wavefront LASIK and IR group had the highest percentage of eyes that gained lines of BSCVA: 20 percent gained one line of acuity on the eye chart, 10 percent gained two lines.
- Eyes in the wavefront LASIK and IR group had the greatest improvement in contrast sensitivity for night vision.
Conclusion: Wavefront LASIK with iris registration is superior
The researchers concluded that wavefront LASIK with iris registration is safe, efficient and more predictable than conventional LASIK and wavefront-guided LASIK with manual marking in correcting mixed astigmatism.