Evolving laser technologies drive advances in LASIK safety, efficacy

Kerry D. Solomon

New and emerging laser technologies enable customized LASIK flap creation, which some experts view as a major enhancement to safety and postoperative refractive outcomes.

Ongoing developments in femtosecond laser technology may ultimately enable surgeons to make precise corneal incisions and move inside the eye to treat cataract, glaucoma and other pathologies, OSN Chief Medical Editor Richard L. Lindstrom, MD, said.

“I think that the anterior segment surgeons of the future might well find themselves sitting down in front of a femtosecond laser routinely in all their cases,” Dr. Lindstrom said.

OSN Refractive Surgery Board Member Kerry D. Solomon, MD, said femtosecond laser technology will be used to perform corneal transplantation procedures.

“The ability to really help with corneal transplantation, whether it’s full-thickness, deep anterior lamellar or DSEK procedures, there’s no question that’s going to play a major role in corneal transplantation,” Dr. Solomon said.

iFS femtosecond laser

The iFS femtosecond laser (Abbott Medical Optics), the fifth-generation IntraLase platform, operates at 150 kHz, enabling flap creation in 10 to 20 seconds, depending on the desired raster pattern. The machine’s predecessor, the FS60, runs at 60 kHz.

“That’s a good thing because that reduces the suction time, which reduces discomfort, perhaps makes it safer, although we really haven’t had complications from the suction as far as vascular occlusion, etc.,” Dr. Lindstrom said. “Still, it is a little bit uncomfortable for some patients, so making it faster is good.”

The iFS laser allows the creation of oval and round flaps.

“Oval flaps are logical in some patients with astigmatism,” he said. “We’re looking at the possibility that oval flaps may also be advantageous in regard to dry eye by sparing the inferior limbus, where a lot of dry symptoms occur, and perhaps cutting fewer nerves. The other thing is that an oval flap allows you to get the hinge more out of the way, so you can get more of a full ablation pattern. That can also be a potential advantage.”

The iFS system allows flap creation with an inverted bevel edge, which enhances flap adhesion and corneal biomechanical stability.

“It appears that [inverted bevel flaps] also create a more stable flap with less of a chance for a flap subluxation and for micro- or macrostriae,” Dr. Lindstrom said. “They look very good.”

The inverted bevel edge provides better wound sealing, Dr. Solomon said. “It’s like a manhole cover. It just fits in.”

He said studies by Michael C. Knorz, MD, and others show stronger flap adhesion with the inverted bevel edge, which enhances visual outcomes and prevents endothelial ingrowth.

“Minimizing epithelial ingrowth opens up whole new enhancement options for patients 6 months, a year, 2 years out, where currently, people are doing PRK on top of flaps because of the risk of epithelial ingrowth,” Dr. Solomon said. “We believe that this new inverted bevel will permit people to lift flaps longer-term because the risk of epithelial ingrowth will be less.”

Optimizing laser energy and creating a tight spot pattern lead to better flaps and a smoother residual stromal bed, resulting in rapid healing and early visual recovery.

“These beds look like glass,” he said. “The [flaps] lift far easier than any femtosecond flap I’ve ever seen. More importantly, the beds are so smooth, when I look at these patients 15 minutes later, the flaps look like they’re a week out. One week out, the flaps look like they’re about a month out. It’s earlier visual recovery and better quality of vision almost right away. The ‘wow’ effect is taken to a whole new level.”

Laser-enabled keratoplasty

The iFS system also offers IntraLase-enabled keratoplasty with various incision designs that enhance wound sealing and corneal graft fixation, Dr. Lindstrom said.

“You can cut both the donor and the recipient, so you get a much better fit of donor and recipient when you do a corneal transplant,” he said. “You can also use this for lamellar corneal transplants, and some people are looking at the possibility of even using it for both anterior and potentially posterior lamellar transplants, as we do Descemet’s stripping endothelial keratoplasty.”

Current research is focusing on use of the iFS to make limbal relaxing incisions for the treatment of astigmatism and pockets for implantation of intracorneal lenses, Dr. Lindstrom said. These femtosecond cuts would be faster and more accurate than manual incisions.

Visx STAR S4

The Visx STAR S4 (AMO) features eye tracking and iris registration, which compensate for cyclotorsion and pupil centroid shift, Dr. Solomon said.

“Iris registration, to me, is the big plus,” he said. “That’s what separated this laser from every other laser out on the market. Without registration, we are truly not doing the patient a service at all.”

Dr. Solomon described how a shift in the visual axis during pupil dilation or constriction can skew wavefront measurement and spot placement.

“Most people, when the pupil constricts to normal size for day-to-day vision, most pupils tend to shift nasally and superiorly,” he said.

Pupils may shift up to 200 µm superiorly and up to 400 µm or 500 µm nasally.

“That becomes significant when we start talking about placing small spots, not only for a custom treatment but even an optimized treatment,” Dr. Solomon said. “If we’re aligning our treatments up to a patient’s mid-dilated or dilated pupil and their day-to-day vision is going to be centered on something totally different, that’s where the rub is.”

Compensating for pupil centroid shift is critical for customized treatment.

“If you’re doing a customized treatment, where you’re treating people with spot sizes of 1 mm or less and you want to put those spots where they need to be, then the pupil centroid shifts become a major issue,” he said. “To me, the real advantage of the Visx S4 is the ability to register and the ability to perform truly customized wavefront-guided ablations.”

Femtosecond lens surgery

LenSx, a startup company, is developing a femtosecond laser to perform anterior capsulectomy and lens surgery, Dr. Lindstrom said.

“There are some surgeons in Germany who are actually looking at doing what I call a radial lensotomy, where they’re making little femtosecond radial incisions, almost like RK in the cornea, but now they’re doing it in the lens to improve and increase the elasticity of the lens with the goal in mind to restore natural accommodation,” he said.

According to Dr. Lindstrom, LenSar is another U.S. company that is working in the same field as LenSx.

The LenSx laser would also be used to soften the lens nucleus. In cases with mild or moderate nuclear sclerosis, the lens would be aspirated without the application of ultrasound power. The laser would assist cracking or chopping hard nuclei with reduced ultrasound power.

Dr. Solomon described how a femtosecond laser may be used to create a precise, perfectly centered capsulorrhexis in cataract surgery. Such precision would greatly enhance IOL alignment and refractive outcomes, he said.

“The advantage is that if I could center the capsulorrhexis around the undilated pupil or mid-dilated pupil and I would be able to register that and know where the centroid shift is, then at the time of surgery, I can center my multifocal or accommodative lens relative to the rhexis,” Dr. Solomon said. “Now I’m truly aligning my optics up where they need to be, relative to the patient’s line of sight.” – by Matt Hasson

• Richard L. Lindstrom, MD, can be reached at Minnesota Eye Consultants, 9801 DuPont Ave. S, Suite 200, Bloomington, MN 55431; 952-888-5800; fax: 952-567-6182; e-mail: rllindstrom@mneye.com. Dr. Lindstrom is a consultant to AMO, LenSx and LenSar, an investor of LenSx and LenSar, and receives royalties from LenSx.
• Kerry D. Solomon, MD, can be reached at Medical University of South Carolina, Storm Eye Institute, 167 Ashley Ave., Room 221, PO Box 250676, Charleston, SC 29425; 843-792-8854; fax: 843-792-6347; e-mail: solomonk@musc.edu. Dr. Solomon is a consultant to AMO.