Tunnel creation examined as another femtosecond laser application

Baltimore-Tunnel creation for placement of corneal implants (Intacs, Addition Technology) in eyes with corneal ectasia is a promising new use for the femtosecond laser (IntraLase FS, IntraLase), according to Alisa Kim, MD.

There are several potential advantages for using the femtosecond laser instead of performing the dissection manually, said Dr. Kim, instructor of ophthalmology, Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore. However, there are also some possible drawbacks to consider, and there is a lack of prospectively collected outcome data for patients who have undergone the femtosecond laser-assisted procedure. To address the latter issue, a prospective cohort study has been launched at the Wilmer Eye Institute enrolling patients with keratoconus and other forms of cornea ectasia who are candidates for treatment with the corneal implants.

"The possible safety advantage also has potentially positive marketing implications," Dr. Kim said.

"That concern is theoretical and remains to be proven," Dr. Kim said.

In addition, the laser can currently only cut to a depth of 400 μm, although the manufacturer is developing technology to allow deeper cuts.

The laser dissection

To perform the laser dissection, the first step is to preset the parameters to create the tunnels with a 1.2-mm incision at a 400-μm depth. The energy level varies depending on the software version of the femtosecond laser. Initially, the inner diameter is 6.6 mm and the outer diameter is 7.4 mm. As the surgeon gains more experience, the outer diameter can be reduced to make the tunnel narrower to achieve a greater refractive effect.

Dr. Kim observed there is variability among surgeons with respect to incision location. At the Wilmer Eye Institute, the incision site is placed at the topographic steep axis, not the refractive axis of cylinder.

After the laser settings are programmed, the geometric center of the cornea is marked and the femtosecond laser applanation plate is centered over that marking. Once the incision and tunnels are created, the procedure can be continued despite the presence of interface bubbles, which will disappear over time.