Potential applications of femtosecond lasers in ophthalmology expand

San Francisco-Femtosecond laser technology, used as a minimally invasive tool in intrastromal refractive surgery, may also have potential as a useful instrument for cataract and glaucoma surgeons, according to Daniele S. Aron-Rosa, MD, who delivered a presentation, "The Future of Femtosecond Lasers," during the Innovator's Session at the American Society of Cataract and Refractive Surgery annual meeting.

In refractive surgery, the femtosecond laser has been employed to create a precise corneal flap utilizing very short pulses with a spot size of about 3 μm. At a 1,053-nm wavelength, the femtosecond laser beam passes through the cornea with no effect on the tissue until reaching the appropriate target.

"With the femtosecond pulses and the new way to mod-lock, there is no collateral (tissue) damage and the pulse is so fast that water has no time to realize that it should boil," said Dr. Aron-Rosa, honorary professor, University Park VII and chairwoman of ophthalmology, Hospital Robert Debre and Foundation Rothschild, Paris.

"The photons are so powerful that one single photon can disrupt a molecule without any vibration of the molecule," she said.

Glaucoma application

Besides its use as a refractive surgery tool, investigators have also been studying the use of femtosecond laser technology for glaucoma surgery. At the polytechnic school, Dr. Aron-Rosa has used a titanium sapphire femtosecond laser to create an iridotomy in a series of rabbits. The femtosecond laser had a pulse energy of 150 mJ and a repetition rate of 15 kHz, which was increased to 30, 40, and then 50 kHz.

"The rabbits were killed at 6 months after surgery," Dr. Aron-Rosa explained. "One can see the small iridotomy, the filtration bubble, and the regular opening. We can obtain this in the trabeculectomy."

Cataract surgeons may also be interested in the possibility of creating an anterior capsulotomy as well as cataract fragmentation with femtosecond laser technology. Dr. Aron-Rosa, inventor of the picosecond pulsed Nd:YAG laser, showed the possibilities of the picosecond laser 25 years ago. She demonstrated an anterior capsulotomy similar to a can opening and cataract fragmentation on a hard nucleus. At that time the researchers used animal and cadaver eyes and found that 50-kHz repetition rate at 100 to 200 mJ was more efficient than 20 kHz.

"To open the lens, we worked from the back to the front," she said. "Otherwise, you are disturbed by the gas bubbles and are less efficient."

Depending on the target, the titanium sapphire femtosecond laser settings must be adjusted to avoid damage to the eye. If the surgeon wants to work on the cornea, the laser should be set at either 150-or 200-femtosecond pulse duration at 15 to 50 kHz repetition rate and a pulse energy of 10 mJ. For treating the lens or the trabeculum, the pulse energy would be increased to 100 to 150 mJ, she said.

"Femtosecond lasers have tremendous potential," Dr. Aron-Rosa said. "When you work in the infrared range, which is the wavelength range suitable for working inside the cornea or inside the eye, the choice of wavelength has to be in the part of the spectrum where the absorption of water, pigment, and hemoglobin is the lowest. This means it is between 800 and 1,064 nm."

The femtosecond pulses offer real, precise subsurface effects, with the least amount of energy required, she said.