Circular keratotomy technique uses guided-trephine system

Issue: November 1, 1999

ByBob Kronemyer

---Coupling of astigmatic cornea to spherical obturator in the guided trephine system.

BOCHUM, Germany — Circular keratotomy using the guided-trephine system ([GTS] Rhein Medical, Tampa, Fla.) is a popular astigmatic procedure in Germany that will soon be available in the United States.

“I believe that circular keratotomy is the only refractive procedure based on mathematics that allows us to calculate patient outcomes,” said Jorg H. Krumeich, MD, a refractive surgeon in private practice here. In addition, the GTS is needed “to be able to cut to the same level in the cornea. To me it seems state of the art.”

Dr. Krumeich noted that there are various approaches to making a sphere out of two different radii. “Basically we are dealing with the Law of Gauss,” he said. The Law of Gauss reflects the observation that “all volumes contained in a perfectly deformable membrane vessel will assume a spherical surface, such as you see in soap bubbles in daily life.” Modifications of one radius results in an opposite modification of the other radius; however, the multiplication of both radii is a constant. “In the hyperbaric system, such as the eye, every cornea should actually be spherical,” he said. But the cornea is prevented from being spherical by the firmness of the Bowman’s and Descemet’s membranes.

Circular cut

---The guided trephine system on the suction ring (left), and the suction ring (right) separate from the system. The ring allows for high adhesion of the globe without elevating the intraocular pressure.

“Cutting Bowman’s membrane circularly allows both radii to follow the Law of Gauss. This is the mathematical coupling factor,” said Dr. Krumeich, who shared the steps of a circular keratotomy procedure at the International Society of Refractive Surgery Symposium in Miami. First, “we execute a 7-mm cut through Bowman’s and allow the cornea to assume the spherical shape. And within this rail, you can deepen the steeper radius to help the spherification.”

Furthermore, the GTS “allows you to descend the blade to the exact setting you want,” Dr. Krumeich said. In his example, the blade descended to 250 µm. “Separately from cutting, you can mark the descent and verify that the cut has been performed,” he said. Under the keratoscope a tune-in deepening is performed in the keratotomy track with the RK knife or better the arcuate knife. The keratoscopic image is used to verify sphericity All control is with the keratoscope and no tables must be used.

To verify the efficacy of the procedure, Dr. Krumeich believes a vector analysis must be conducted. Else absolute results in cylinder are not meaningful. For example, “let’s say you start with a +2 D –4 D 3 0 on the blue line and it comes out +2 D –2 D 3 90,” he said. “You have an absolute value of correction of 2 D, but in reality you have corrected 6 D considering the change of the axis.”

Special suction ring

---Arcuate knife to perform redeepenings within the keratotomy cut.

In essence, the surgical principle of circular keratotomy “is to bring the aspherical cornea against a spherical body inside the trephining system,” Dr. Krumeich said. “So while you are rounding the corneal surface, you cut a 7-mm island with the trephine, while the cornea remains rounded during the cut.”

However, a special suction ring is required that does not increase intraocular pressure. With perforating keratoplasties in which the GTS is primarily used, “this allows you to do a 360° cut without the use of scissors.”

The arcuate knife used in circular keratotomy is a radial keratotomy knife whose inside “turns around like in a shell,” Dr. Krumeich said. “You have different inlays to run the knife along.”

Another feature of the circular keratotomy “is that you can avoid corneal graffiti. You have only a very faint cut that is hardly visible on the slit lamp,” Dr. Krumeich said.

Photos of results from the surgery on the cornea show incisions that are hardly noticeable.

For Your Information:

Jorg H. Krumeich, MD, can be reached at Propst-Helmich-Promenade 28, 44866 Bochum, Germany; (49) 232-782002; fax: (49) 232-788171; e-mail: jk@krumeich.ruhr.de. Dr. Krumeich has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

The Guided-Trephine System is distributed by Rhein Medical, which is located at 5460 Beaumont Center Blvd., Ste. 500, Tampa, FL 33634; (813) 885-5050; fax: (813) 885-9346.