Laser channels improve accuracy in corneal inserts placement

March 1, 2005

Los Angeles—Implantation of micro-thin prescription inserts (Intacs, Addition Technology, Sunnyvale, CA) using the IntraLase femtosecond laser (IntraLase Corp., Irvine, CA) to create the channels seems to be a less traumatic and a more accurate means of placing the prescription inserts in the eye, compared with the mechanical procedure for implanting them in a group of patients with keratoconus who were contact lens intolerant, reported Yaron Rabinowitz, MD.

Dr. Rabinowitz and colleagues conducted a prospective study in which they compared results obtained in 20 patients who underwent implantation of the prescription inserts using the femtosecond laser to create the channels and those in whom a mechanical keratome was used according to the technique proposed by Joseph Colin, MD, from Bordeaux, France.

Dr. Rabinowitz explained that as the channels in which the prescription inserts are implanted got narrower, more flattening effect was achieved, that is, up to 3 D with a small channel of 6.6 by 7.4 mm compared with 1 D of effect with a larger channel that was 6.6 by 8.6 mm.

"The site of the implantation of the prescription inserts was determined by topography. We inserted the arc of the prescription inserts to bisect the thinnest part of the cornea," he explained.

In oval cones, the investigators used asymmetric prescription inserts, 0.30 mm superiorly and 0.35 mm inferiorly. In nipple cones, they implanted the 0.35-mm prescription inserts symmetrically, and in mild cones they implanted a single prescription insert depending on the thinnest part of the cornea. In the United States, surgeons are limited to implanting 0.25-, 0.30-, and 0.35-mm prescription inserts.

Results in 20 patients The results from the first 20 patients with the prescription inserts implanted using the IntraLase laser are promising. Six patients have been followed for 6 months and 14 patients for 3 months.

"The average reduction in the K value was 2.2 D (range, 0.4 to 4.6 D), and the reduction in the spherical equivalent (SE) was 3.56 D (range, -1.7 to 7 D). The uncorrected visual acuity (UCVA) was 3.42 lines. The permanent best-corrected visual acuity (BCVA) was 3.35 lines," Dr. Rabinowitz reported.

In addition, the surface regularity index improved by 0.3 and the surface asymmetry by 0.75.

After they obtained these results, the investigators then analyzed the data in comparison with two studies performed by Drs. Colin from France and Siganos from Greece and one by themselves in which prescription inserts were implanted using a mechanical spreader.

"The procedure in which the mechanical device was used resulted in slightly more flattening. With the IntraLase procedure, there was a greater decrease in SE, better UCVA levels, and better BCVA levels," Dr. Rabinowitz said.

The goal of the study, he noted, was to make patients contact lens or spectacle tolerant and to avoid the need for a penetrating keratoplasty in those who cannot tolerate contact lenses.

"Six months after the procedure, the six patients (100%) achieved a BCVA of 20/20 to 20/40. At 3 months after the procedure, seven of 13 patients (53%) were contact lens tolerant, five (38%) did not attempt contact lens wear, and one patient (9%) was not able to tolerate contact lenses. No prescription inserts had to be removed. Importantly, the channels were not too shallow in any of the 20 patients," Dr. Rabinowitz said.

When he compared these results with those in patients in which he and his colleagues used a mechanical device to implant the prescription inserts, there was a 70% success rate; the prescription inserts were removed from one eye because the channels were too shallow and in two eyes that underwent corneal transplantations.