Direct measurement of LASIK flap, bed thickness essential for safety

Not measuring flap thickness puts the patient at risk for postoperative corneal ectasia.

CHICAGO — Surgeons performing laser in situ keratomileusis (LASIK) may be “putting patients at serious risk by not directly measuring flap thickness,” said Richard B. Foulkes, MD, the medical director at the Icon Eye Centers in the Chicago area. “The direct measurement of the flap gives one a much more accurate prediction of stromal bed thickness.”

Until recently, “We haven’t had a device to accurately measure these flaps,” Dr. Foulkes said. “The two-standard-deviation variability of the flap thickness in all microkeratomes is in the neighborhood of 50 µm. But we now have a pachymeter that can quite accurately measure down to around 100 µm.”

For the past year, Dr. Foulkes has been using an ultrasound pachymeter called the Pachette, manufactured by DGH, based in Exton, Pa. “Ever since I purchased the device last October, I have been measuring every eye,” he said. “I have also measured every enhancement,” he said. In that time period, Dr. Foulkes said, he has measured approximately 3,000 eyes.

Using the Pachette, “on average, we are 20 to 30 µm (plus or minus) in error when measuring the base as a predictor for bed thickness,” Dr. Foulkes said. “When measuring the flap, the error ratio turns out to be only about 7 µm.”

Easy to use

The Pachette costs less than $5,000, and there is a minimal learning curve, said said Dr. Foulkes, an associate professor at the Illinois Eye and Ear Infirmary in Chicago. “The device sits on top of your laser,” he said.

Traditionally, when a microkeratome passes through the corneal tissue, “you pull some fluid with you,” Dr. Foulkes said. After the flap is turned over, a sponge is used to dry the surface. Then, the stromal bed is measured.

“I roll the flap over on a very wet sponge and tap the flap two or three times with the pachymeter. I measure both the flap and the base. This practice takes only 20 seconds,” he said. “By doing this, I discovered that I was ending up with a different prediction for the actual post-laser bed. The base measurement tended to predict that I had a thicker bed.”

Dr. Foulkes concluded that the best predictor of the actual bed thickness “turns out to be the direct flap measurement,” he said. In contrast, “I usually erred by 20 to 30 µm when using the base measurement. Very rarely did I actually achieve my true bed thickness when I measured the base.” However, he continued, “more often than not, I was within 5 to 10 µm of accuracy when I measured the flap.”

Dr. Foulkes noted that the stromal tissue surgeons cut through is an artificially dry tissue. “The corneal stroma is being cleared of fluid every second by the endothelial cells. But when the endothelial cells fail to pump water out of the cornea, the cornea swells to almost double its original thickness,” he said. “There is a thicker sponge in the base than in the flap.”

Manufacturer claims

Dr. Foulkes began performing high-volume LASIK surgery about 18 months ago, favoring the Carriazo-Barraquer microkeratome by Moria. At the time, “I really did not trust that my corneal flaps were what the manufacturers said they were,” he said.

“The Mathematics of LASIK,” an article by Machat and Probst that appeared in Ophthalmology 2 years ago, “personifies how most surgeons approach LASIK,” Dr. Foulkes said. “The article states that you first need to perform a pachymetry measurement on the surface. Then you sit down with your calculator to determine if it will be safe for that patient. Munnerlyn’s formula predicts how much tissue is required to perform an excimer ablation.”

He described a hypothetical case in which inaccurate calculation could cause trouble. Using Munnerlyn’s formula, the surgeon subtracts 100 µm from the total thickness of perhaps a slightly thin 500-µm cornea. In this example, “you need 240 µm to complete this treatment. This is based on the prediction that your flap maker makes a 140-µm flap,” Dr. Foulkes said.

In this particular case, though, the patient returns 3 to 6 months after LASIK because of regression. “So now you need to laser an additional 11 to 20 µm or so,” Dr. Foulkes said. “You say to yourself, ‘The patient has 260 µm in the bed. I can legally reduce that to 250 µm, and if I reduce to even 245 µm, that’s okay.’ But after correcting for the additional 1.5 D, the patient returns within the next month with a –5 D or a –6 D.” Topography reveals a “big, bulging area in the middle part of the eye,” Dr. Foulkes said. Slit-lamp examination also shows ectasia. “Often, these patients progress to penetrating keratoplasty,” he said.

“Instead of making an original 140-µm flap, you made a 180 or perhaps a 200-µm flap,” Dr. Foulkes said. “But you didn’t know it.”

Dr. Foulkes’s strong conviction about the need for accurate measurements is reflected in his belief that “we are literally in an era when there is likely to be serious legal repercussions if we don’t begin to document flap and bed thickness. You’ll find that, by measuring flap thickness, you are often cutting a thinner flap than you thought. Therefore, you actually have the ability to treat patients who you may not have been treating previously.” On the other hand, “unless you measure, you are going to find that you occasionally cut a much thicker flap than you intended,” Dr. Foulkes said.

For Your Information:

• Richard B. Foulkes, MD, can be reached at 40 S. Clay, Ste. 9W, Hinsdale, IL 60521; (630) 520-5880; fax: (630) 920-8533; e-mail: foulkes52@att.net; Web site: www.glassesfree.com.

• The Pachette is manufactured by DGH Technologies Inc., 110 Summit Dr., Exton PA 19341; (610) 594-9100; fax: (610) 594-0390.