Results from the clinical trial of the Intralase laser

Results from the clinical trial of the Intralase laser

Stephen G. Slade

University of Texas, Houston, TX, USA

We have been using an Intralase femtosecond laser for almost two years. We believe it has great promise, and may allow us to reduce the lamellar complications traditionally associated with LASIK, to create new types of lamellar applications, and perhaps even to do away with the excimer laser altogether.

Femtosecond lasers are similar to Nd:YAG lasers, but the burst length is very short – about 100 femtoseconds. In order to illustrate how fast a femtosecond is, if you gave a ray of light an entire second, it would go around the world seven and half times. If you gave it 100 femtoseconds, it would barely get across a human hair. So it is an amazingly tiny bit of time. Our Intralase sits right next to the Technolase, which fortuitously has a rotating bed. You simply swing out the bed, do the flap, swing it back, and do the ablation. It does not think flaps; it simply thinks about where it is going to place the next spot. So, everything is up to the physician: the diameter of the flap, the angle of the side cut, the location of the hinge (which can be anywhere), the hinge angle, the hinge size, and other factors. You could even do a custom flap, if you so wish, because it is a software-driven device. It mates with the eye with a docking maneuver, using a cone. The laser fires down to the glass cone that is attached to the laser, and into a suction ring that is attached to the patient’s eye, clamps on, and applanates the cornea. From the reference plane of its quartz plate, the laser knows exactly where it is, and can do the cut in a spiral pattern or in any other pattern that it is instructed to. The cut can first be placed wherever desired within the cornea. It produces a very nice bed, with the same diameter and hinge placement every time. The beds are always perfectly centered over the pupil, because the flap itself is not dependent upon where the suction ring is placed. Instead, the flap is realigned directly over the visual axis.

We no longer use a spiral pattern. We now use a raster technique, starting from the bottom and coming up to create a surgeon’s-eye view. The results have improved because, at the initial stage of laser use on the eye, vapor and plasma are produced. The bottom-up approach keeps all that down inferiorly at the area of the hinge, so that the dissection is much cleaner, and the flaps rise up more easily. We do not start the cut at the most critical part, the visual axis. These days, we

Address for correspondence: Stephen G. Slade, MD, FACS, 3900 Essex, Suite 101, Houston, TX 77027, USA

Wavefront and Emerging Refractive Technologies, pp. 107–109

Proceedings of the 51st Annual Symposium of the New Orleans Academy of Ophthalmology, New Orleans, LA, USA, February 22-24, 2002

edited by Jill B. Koury

© 2003 Kugler Publications, The Hague, The Netherlands

often can just sweep beneath the flap and lay it back, not peel it off like Velcro as we used to do.

We have carried out studies showing that the accuracy and depth are better, and histological studies showing that they appear to be planar all the way across. A keratome flap is typically thinner in the center than at the periphery. We believe we are going to find that Intralase flaps will be more aberration-neutral, and even if they are not, will be more predictable. If we are adding aberrations and we know in advance what we are adding, we can treat for these before we do it. The flaps have the same diameter every time. If flap aberrations occur due to the biomechanical effect from the edges of the keratectomy, with a typical keratome, they vary in diameter because of the preoperative Ks. With the femtosecond laser, we do not take the preoperative Ks into account, and moreover, the laser is perfectly centered. We all know that we cannot center a radial keratome flap perfectly, and make it planar.

The Intralase seems to be very safe. It has some unique safety aspects. If you have a suction break, this is not critical. You let the bubbles clear and bring the patient right back to the laser a few minutes later in order to do the ablation.

Lee Nordan and I compared our early results with a VisX femtosecond and a Technolase femtosecond laser with results from excimer lasers. These were very similar to excimer patients at three and six months postoperatively. However, the one-day postoperative results are not quite as good with the femtosecond laser. When you bounce a light back off the patients, you get slightly more edema, so the recovery is somewhat slower and less comfortable.

What are the advantages? We started working on various procedural changes, and we are now putting more angle on the side cut. The important point is that the last part of the cornea to be lasered is Bowman’s membrane. With a regular keratome, this is the first part cut and you disrupt the surface. With the femtosecond laser, any suction break or any other type of problem is contained because Bowman’s membrane remains intact until the end of the ablation.

We do see the occasional iron line or late postoperative complication, but we have not encountered epithelial ingrowth. We think this is because of the preciseness of the flap. We also think we see less striae because we do not have thinness in the center of the flap. And we certainly have not had any aborted cases.

Where is this technology headed? There was an idea to do intrastromal ablation, which we have done. You can do these in myopes by simply stacking a series of plates, removing more from the center than from the periphery, and hoping that the cornea will then collapse down and flatten the corneal first surface.

We are concentrating on the suggestion that this could be done more safely in hyperopes because the visual axis is not being treated. We have tried to do femtosecond ALK in which we isolated a lens, reached in through a little channel, and pulled it out. But ideally, of course, you should be able to do this totally intrastromally at the slit lamp in a true no-touch technique.

We are still involved in deciphering the results, but we have not had a loss of best-corrected vision, although we have seen some shifts. As well as custom ablation, we are also talking about using the laser for homoplastic lamellar procedures. The problem with doing these with a keratome is to match the cuts of the diameter of the donor and treatment eyes. With the femtosecond laser, you get an exact diameter match and the K factor is obviated.

We have performed two cases of posterior lamellar keratoplasty. This procedure

is still in the developmental stage and is very difficult to perform. Other possibilities for the laser include posterior keratoplasty, intrastromal correction of aberrations, and perhaps relaxation of the lens.

In conclusion, the femtosecond laser does appear to avoid some of the traditional lamellar complications encountered with LASIK. There is also the promise of multiple lamellar applications.