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Femtosecond laser produces minimal localized effects
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The newest entry in the burgeoning microkeratome field is the IntraLase Pulsion FS (femtosecond) laser. This is a solid-state, infrared scanning laser that cuts tissue via a laser-tissue interaction referred to as photodisruption (note: the Nd:YAG laser is also a photodisruptor, while the excimer cuts tissue via photoablation). It is also classified as an “intrastromal” laser due to its ability to be precisely focused within the corneal stroma with no absorbence of the laser wavelength by the overlying corneal tissue.
The Pulsion FS is reported to produce minimal localized effects due to its very low energy pulse (a femtosecond is 10-15 second), thereby allowing for contiguous cutting. While its current Food and Drug Administration (FDA) clearance is as a laser microkeratome, it is also being investigated as a stand-alone refractive surgery instrument in European clinical studies.
Increased accuracy, precision
After a low vacuum suction ring is applied to the eye, a disposable contact lens assembly is used to applanate the cornea. The suction ring and applanating contact lens of the IntraLase Pulsion FS system creates about 30 to 35 mm Hg of intraocular pressure to the eye compared with standard mechanical microkeratomes of 65 to 100 mm Hg. A computer-controlled scanning optical delivery system then delivers an interconnecting series of laser pulses in a spiral pattern to create a dissection plane (i.e., the keratectomy bed).
The manufacturer maintains that the IntraLase system produces far greater accuracy and precision than traditional microkeratomes or other steel-bladed surgical instruments with a depth accuracy of 5 µm for its system vs. 30 µm for traditional microkeratomes. The laser completes the flap by making an arc-shaped side cut from the plane to the corneal surface. Aligning the patient, placing the ring and applanating the cornea can be performed in 1 to 2 minutes. The laser takes approximately another 45 seconds to create the flap.
The surgeon can vary flap parameters such as depth of cut, diameter of flap, hinge position and side architecture. The side architecture is the angle from the bed to the corneal surface. The IntraLase laser can create a very steep angle so that the flap can be positioned in a fashion analogous to a manhole cover. This should afford more flap stability than mechanical microkeratomes, which have a more gradual entrance cut. It is also postulated that the incidence of epithelial ingrowth would be decreased with the IntraLase secondary to the steep flap angle.
After the flap is created, the patient’s refractive error is corrected via an excimer laser. IntraLase refers to this new flap procedure as IntraLASIK. Researchers have seen no epithelial defects or ingrowth, or buttonhole, partial or free flaps while using this system.
Clinical trial results
The IntraLase system is being used clinically as a microkeratome at two sites in the United States, and a total of 209 eyes underwent the IntraLASIK procedure between May and September 2000. Average spherical error was –2.8 D (range: –10.25 to + 6 D), with an average of –1.28 D of cylinder (range: 0 to –6 D). Age ranged from 20 to 64, with an average of 38 years.
No complications or adverse events occurred that prevented completion of the IntraLASIK procedure on the same day. Postoperatively, none of the complications and adverse events commonly associated with LASIK were observed. Specifically, no flap dislocations, wrinkles, striae or peripheral epithelial defects associated with blade-created resections were observed. In addition, there were no cases of diffuse lamellar keratitis or epithelial ingrowth. At 3 months, 64% of eyes demonstrated uncorrected visual acuity of 20/20 or better, while 97% were 20/40 or better. No eyes experienced a loss of more than one line of best-corrected visual acuity.
No collateral tissue damage has occurred in the patients we have treated. The edges of the flaps are more easily seen, however; they look more like the edge of a graft due to the entrance angle. If the angle is too vertical, we have seen some epithelial “plugs” similar to radial keratotomy incisions that have been re-treated. There is virtually no chance for flap override.
Future applications
According to the company, the ultimate goal is a stand-alone laser to perform vision correction procedures. Intralase has begun research in Europe for using the laser for refractive procedures and the company hopes to begin similar trials in the United States very soon. This laser is also being investigated for therapeutic keratoplasty, both full and partial thickness.
For Your Information:
- Richard N. Baker, OD, FAAO, is in private practice in Houston and is clinic director for The Laser Center of Houston and adjunct faculty for the University of Houston. He can be reached at 3900 Essex #101, Houston, TX 77027; (713) 626-5544; fax: (713) 626-7744. Dr. Baker has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Intralase Corp. can be contacted at 3 Morgan, Irvine, CA 92618; (877) 393-2020; fax: (949) 461-3323; Web site: www.intralase.com.