Surgeon devises formula to predict flap diameter with Moria keratome

Inverse relationship found between corneal diameter and flap diameter.

NEW YORK — The reliability of the disposable Moria One microkeratome is extremely high, according to a clinical evaluation of 50 eyes of 25 patients who were each treated with a different Moria single-use unit.

“I had been using the reusable stainless steel Moria One microkeratome from Moria (Antony, France) for several years. Like many surgeons, I wasn’t very confident about the reliability of a disposable microkeratome system,” said Barrie D. Soloway, MD, director of the Vision Correction Center at The New York Eye and Ear Infirmary. “But I became surprisingly impressed that a disposable microkeratome could have such a high degree of predictability from one unit to the next.”

Dr. Soloway and his colleagues evaluated parameters of the one-use microkeratome ranging from suction time to flap thickness and flap diameter. Using a –1 suction ring exclusively, Dr. Soloway performed all surgeries with the LadarVision excimer laser system.

Keratometry was measured preoperatively with a manual keratometer, corneal diameter with the LadarVision limbus ring and flap diameter with the LadarVision software ruler on a tracked image. Keratometry measurements ranged from 41 D to 45.75 D (mean 43.75 D); corneal diameter range was 10.3 mm to 12.2 mm (mean 11.4 mm).

“The right eye and left eye showed no real difference for either keratometry or corneal diameter,” Dr. Soloway said.

Various relationships studied

The relationship of flap diameter to keratometry and corneal diameter was calculated. “The flap sizes we measured ranged from 8.6 mm to 9.7 mm (mean 9.3 mm). The flap diameter became greater as keratometry became greater. For example, a 42 D cornea had a smaller flap than a 45 D cornea. Everything was measured vertically because horizontally it would include the hinge,” Dr. Soloway said. A scatter plot of the vertical keratometry readings versus the vertical flap diameter indicated that the lower the readings, the smaller the flap diameter.

There was an inverse relation between flap diameter and corneal diameter. “Flap diameter correlates to keratometry as inversely related to corneal diameter, but when you break down corneal diameter into three groups — large, regular and small — you are able to better predict what your flap diameter will be,” Dr. Soloway said.

From a multiple regression analysis of the data, Dr. Soloway devised a formula to predict flap size: for a corneal diameter less than 11.2 mm, divide the keratometry reading by 4.7; between 11.2 mm and 11.8 mm, divide the reading by 4.72; larger than 11.8 mm, divide by 4.75 mm. “By using this formula with the Moria one-use microkeratome, you can achieve a good approximation preoperatively of what flap size you can achieve,” he said.

Retrospectively, by using the formula, the actual diameter versus the predicted diameter was within 0.1 mm for 60% of study patients. Likewise, 26% of patients were between 0.1 mm and 0.2 mm, 4% were between 0.2 mm and 0.3 mm and 10% were between 0.3 mm and 0.4 mm.

“For hyperopic treatment and for large optical-zone myopic treatment, flap diameter is very important. Flap thickness is also a consideration, but certainly in the hyperopes the diameter is typically more important,” Dr. Soloway said

Flap thickness

The 50 eyes had a mean flap thickness of 158 µm, with a standard deviation of 11 µm. The minimum flap thickness was 138 µm and the maximum was 175 µm. “We measured flap thickness by the subtraction method. Again, we were surprised. We found that the flaps we achieved were very close to what was advertised by the manufacturer: 160 µm,” Dr. Soloway said. Visual outcomes were also comparable to other microkeratomes.

Overall, the study showed that a –1 suction ring is reliable for flap diameter prediction based on keratometry and corneal diameter. Two limitations of the study were that it consisted of only 50 eyes and only one size ring was used. Although the single-use microkeratome is available in 0 and –1 suction ring sizes only (compared to a full range of rings with the reusable Moria One microkeratome), “most of the users at my center are happy with these two ring sizes,” Dr. Soloway said. Dual suction ports also minimize risk of pseudosuction and of speculum interference.

Dr. Soloway said his technicians “love” the single-use microkeratome. “They open up a bag and that’s it,” he said. The preassembled head has the same design as the stainless steel reusable Moria One microkeratome, “with excellent flap visibility. The transparent suction ring gives visual indication of active 360° suction.” Moreover, “engaging the head in the track is extremely easy; in fact, it is much easier than with the metal version and certainly easier than most other microkeratomes on the market,” Dr. Soloway said. The transit of the keratome across the eye “is also quite smooth, and better than any other manual keratome I have used.”

In general, the Moria One disposable microkeratome is ideal for a low-volume center and for new surgeons, Dr. Soloway said.

For Your Information:

• Barrie D. Soloway, MD, can be reached at 160 E. 56th St., Ste. 900, New York, NY 10022; (212) 758-3838; fax: (212) 758-4175; e-mail: bsolowaymd@pol.net. Dr. Soloway receives contributions toward research and travel expenses from Moria.

• For product information on the Moria One disposable microkeratome, contact Moria USA Inc., 4030 Skyron Dr., Unit G, Doylestown, PA 18901; (800) 441-1314; fax: (215) 230-7670; e-mail: moriausa@moriausa.com.