Direct measurement of LASIK flap possible with femtosecond laser bubbles

The new technique may bypass the usual subtraction method with comparable accuracy. Studies are under way.

Issue: October 1, 2005

ByJared Schultz

The bubbles created during femtosecond laser flap creation can be used as an interface for direct pachymetry measurement of the flap’s thickness, according to one ophthalmologist.

Rather than perform subtraction pachymetry to calculate flap thickness, ophthalmologists can take a direct pachymetry measurement after the IntraLase laser pass while the bubbles are still present, according to Richard Eisner, MD.

In a normal cornea, pachymetry sound waves reflect from the posterior surface of the cornea to give a full thickness cornea measurement. But after flap creation with the IntraLase FS laser, the bubbles form a second surface, representing the posterior surface of the LASIK flap, Dr. Eisner explained.

In subtraction pachymetry, the thickness of the stromal bed after LASIK flap creation is subtracted from the full preoperative thickness of the cornea to derive the flap thickness.

Dr. Eisner said preliminary data indicate that measurements taken with the direct measurement technique using the Sonogage Corneo-Gage Plus pachymeter are as accurate as subtraction pachymetry.

“It turns out they are incredibly close, that if you do subtraction and compare that to the direct flap measurement technique, they are very close,” Dr. Eisner told Ocular Surgery News. Whether the technique is more accurate than standard subtraction pachymetry remains to be seen, he said.

Intraoperative measurement of flap thickness using the Intralase laser. (a) 50 Hz probe (Sonogage CorneaScan II) directly measuring flap thickness in situ. (b) Clear interface in area of probe tip compression.

Images: Binder PS

One of the benefits of using the IntraLase for flap creation is the ability to recreate the flap if the depth is incorrect, he said.

“If you make your flap, you haven’t lifted it, and you measured it, and it turns out it’s thicker than you thought it was going to be, you can decide to abort the treatment and come back later and make a thinner flap,” he said.

Accidental finding

Dr. Eisner said that his discovery of the bubble layer’s reflectivity was an accidental finding that emerged from his standard measuring practices. He overlooked repeating a pachymetry reading on a patient before applying the IntraLase laser. When his assistant reminded him of the oversight, he took the measurement immediately post-IntraLase.

“I measured it … and I should’ve gotten something like 540, 550, 560,” he said. “Instead, I got a reading of 110.”

When he took the measurement again and came up with the same number, Dr. Eisner theorized that the reading reflected flap thickness rather than full corneal thickness. After getting similar results with the method in other patients, he notified colleagues and began studies of the accuracy of the direct measurement.

Method studied

Perry S. Binder, MD, co-medical director for IntraLase Inc., was one of the physicians Dr. Eisner informed of his finding.

“Dr. Eisner called me and he told me what he found, and I immediately figured out he had a great idea, and I jumped right into it,” Dr. Binder told Ocular Surgery News. “I did about 100 eyes and measured them, and we submitted that for publication.”

The direct technique was compared to subtraction pachymetry in a study that included five attempted thickness targets, Dr. Binder said. The mean differences between the direct and subtraction methods ranged from 3.3 µm to 7.6 µm, he said. All measurements were higher than the attempted thickness, and most of the post-lift measurements came closer to the attempted flap thickness.

In 34 eyes in which a flap thickness of 100 µm was attempted, the pre-lift measurement was 112 µm and the post-lift measurement was 105 µm, he said. In five eyes in which a thickness of 140 µm was attempted, both the pre-lift and post-lift measurements were 143 µm.

The study has been accepted for publication in the Journal of Cataract and Refractive Surgery, Dr. Binder said.

Better technique?

While the direct measurement technique eliminates the steps involved in subtraction pachymetry, clinical experience must still determine which technique is more accurate, Dr. Eisner said.

“I don’t want to speculate and conclude that this is more accurate, but we can compare this to the way we do it now to find out what is the best way to measure it,” he said. “Does this eliminate the need to do a bed pach and calculate subtraction pachs? The answer is maybe.”

The technique eliminates the need to insert the pachymeter tip underneath the flap in the stromal bed, which may be an advantage, Dr. Eisner said.

“You can’t put it in the sterilizer every time, you can’t autoclave it like your other instruments, so a lot of guys don’t like the idea of taking that tip and putting it in the bed underneath the flap,” he said.

He also noted that the IntraLase air bubbles dissipate after a number of applications of the pachymeter tip.

“We found that the first reading or two are the most accurate readings, while the bubbles are still there,” he explained. “If you wait too long or you go too many times in the same spot, you won’t get the same kind of reading.”

The concept is also being explored for use with standard blade-based microkeratomes, Dr. Eisner said.

For Your Information:

Richard Eisner, MD, can be reached at Eisner Laser Center, 125 Plantation Centre Drive S., Suite 250, Macon, GA 31210-2083; 478-474-0020; fax: 478-474-7823. Dr. Eisner has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

Perry S. Binder, MS, MD, can be reached at Gordon Binder & Weiss Vision Institute, 8910 University Center Lane, Suite 800, San Diego, CA 92122; 858-455-6800; fax: 858-455-0244. Dr. Binder is co-medical director at IntraLase.

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