Aspheric Ablations Look Promising
A U.S. surgeon gains experience with a new technique.
BY MANOJ MOTWANI, M.D.

Currently, many excimer laser ablations for refractive correction involve inducing a large dioptric power change between the optical zone and the transition zone. That abrupt power difference changes the shape of the cornea from prolate to oblate (most noticeable on topographies as the "red ring"). The resulting spherical aberration has been linked to various undesirable effects for patients, such as night glare, halos, decreased contrast sensitivity, and poor subjective vision -- even when acuity is objectively measured as 20/20. Therefore, the idea of a prolate cornea after myopic laser ablation has been considered a desirable goal for several years now.

Working toward that goal, researchers using the Nidek EC-5000 excimer laser around the world, including Paolo Vinciguerra, M.D., of Italy, and Arturo Chayet, M.D., of Mexico, have been performing what they call aspheric ablations. They've discovered that using an optical zone of 5 mm and a transition zone of 9 mm pushes the "red ring" much farther out, increasing the effective optical zone, and maintaining a more prolate corneal shape. (The EC-5000 has adjustable optical and transition zones. The function between the zones is close to linear, so the depth of the transition zone changes depending on the depth of the ablation.)

Their results so far have been positive. Patients are experiencing excellent night vision with zero to minimal change in night glare and halos, and minimal to no change in contrast sensitivity. Dr. Chayet even presented a small study showing a slight increase in contrast sensitivity in some patients. It's arguable that this increase was an aberration; however, all of the physicians using aspheric ablation are reporting similar results.

Topography and wavefront 3 months after an aspheric ablation for a patient whose correction was -9.00, 1.75 x 15 OD and -9.50, 0.75 x 160 OS.

Testing the Technique in the U.S.

At the 2002 American Academy of Ophthalmology meeting in Orlando, Nidek users learned that of the eight possible profiles of the EC-5000 laser, Profile 4 provided the most linear relationship between the optical and transition zone. Profiles 1 to 3 were progressively more concave, and Profiles 5 to 8 were progressively more convex.

Profile 4 isn't available in the United States, but I took it upon myself to question the Nidek engineer responsible for creation of the profile, and discovered that the U.S. lasers do have Profile 3. It is slightly concave, but very close to the ideal Profile 4.

Over the past 7 months, I have performed aspheric ablation on approximately 250 eyes. (See "Aspheric Ablation Results, Binocular Vision," above.) The nomogram I'm using is: 0.76*(Sph-((Cyl)(0.4))). If desired, an accommodation can be made for corneas of patients above the age of 45 by decreasing the first modifier to 0.72. Factors specific to different operating centers (e.g., temperature, humidity) can be accommodated by increasing or decreasing the first modifier.

Uniformly, my patients have done very well with aspheric ablation. Although there was not a significantly high amount of complaints of halo, night glare, or difficulty seeing at night with the 6.5-mm optical zone/7.5-mm transition zone profile, the number of complaints has gone down significantly. For example, a -4D helicopter pilot with 7-mm pupils had no difficulty seeing at night or night glare by 1 week. A medical company's quality assurance officer had LASIK performed for -8D OU and noted by 1 week that his vision was already better than with contact lenses; by 1 month he had no significant night glare or halos.

The range of correction in my first series was quite large, -1D to -12D. At 1 week post-op, 63% of patients had 20/15 post-op acuity; at 1 month that number increased to 77%. More importantly, any complaints of difficulty seeing at night, night glare or halos was linked to dry eyes or incorrect correction/regression. Once those issues were addressed, patients achieved good subjective day and night vision. In my opinion, this was the most important result because patients consistently reported natural clear vision, often better than with their contacts and glasses, and reported no significant change in their night vision from baseline by as early as 1 week.

Although there has not been enough time to fully evaluate the enhancement rate, so far only 9 eyes have required enhancement, an approximately 3.6% rate. This is the case despite the number of high myopes. More detailed overall results will be reported in the future.

The secret to aspheric ablation appears to be the very wide ablation connected to a moderate-sized central optical zone, and the key appears to be the gradual transition zone to the 9-mm edge. The results reported for contrast sensitivity with this large ablation zone appear to support (and better) the improved contrast sensitivity that occurred going from the 5.5-mm optical zone/7.0-mm transition zone to the 6.5-mm optical zone/7.5-mm transition zone. Furthermore, the amount of tissue removed centrally is not large, as the deepest point of the ablation appears to only remove approximately 12.5 mm to 13 mm per diopter.

Evaluation of aspheric ablation with the ARK-10000 device that includes wavefront analysis showed that most patients had little to no increase in higher-order aberrations after the procedure. (Many patients did not have significant amounts of higher order-aberrations to begin with.)

Next Step: Combine It with Wavefront

The results of aspheric ablation have been gratifying. The 20/15 or better rate has been very high, and subjective quality of vision has been excellent.

The next obvious step is to take this profile and combine it with a wavefront-guided ablation that will mitigate higher-order aberrations. This could result in a significant further increase in better than 20/20 results, and may bring us closer to that Holy Grail of 20/10 vision without aberrations for everyone.

Dr. Motwani is the medical director and chief refractive surgeon of Alpha Laser Center in San Diego, Calif.