SPECIAL REPORT

Corneal Applications for Femtosecond Lasers

By Roger F. Steinert, MD

Some roles for lasers in corneal surgery, such as using an excimer laser to shape the cornea, are long established. In recent years, new roles have emerged through advances in femtosecond laser technology, which shows an enormous potential to perform corneal incisions in ways that we just cannot achieve with the diamond blade. Although the technology isn't present in every facility, its use is expanding. The evolution of its applications has occurred rather rapidly as we've found new surgical advantages of the device and followed the surgical outcomes.

Femtosecond Evolution

The evolution of this technology started with the IntraLase FS laser (AMO), which was first used in a clinical setting to make LASIK flaps. The technology advanced quite a bit, becoming more sophisticated in the laser's speed, pulses and cutting ability. Flaps became more and more precise and predictable. We learned to perform reverse-angle side cuts that settle better and are more secure. We also gained the potential to create flaps with oval or other shapes.

Other companies began to work on their own femtosecond lasers. The Intralase laser is now joined by the Ziemer Femto LDV (Ziemer), the TECHNOLAS 520F (Technolas Perfect Vision), and the VisuMax (Carl Zeiss Meditec). Today, the majority of LASIK flaps are created using femtosecond lasers — a clear success story.

The second application of this technology in a major clinical setting was the modification of the laser for corneal transplant incisions. The femtosecond laser aids in penetrating keratoplasty because it can recreate patterns of incisions that can't be made with a straight blade, such as the now-common zigzag, top hat, and mushroom incisions. Although the laser is still being used by in the minority of cases, it's gaining favor with more and more corneal surgeons.

At the same time that we started performing penetrating keratoplasty with the femtosecond laser, we started applying it to another corneal procedure: creating tunnels for intracorneal ring segments. A large number of the rings are now being placed with a corneal laser instead of a surgical steel device.

Figure 1. FS Laser PKP with laser alignment marks. Ink marks have highlighted the 8 marks in the recipient rim. The donor marks are easily seen in the upper left.

From intracorneal ring segments, it was a relatively short step for us to begin using the femtosecond laser for astigmatic keratotomies. This highly controlled alternative does the same thing as a diamond blade, but with a programmable way to achieve the exact length and depth desired. Again, although the majority of procedures are still performed with a diamond blade, the femtosecond laser has a growing following.

Advantages Over Diamond Blades

There are several key advantages to making corneal incisions with a femtosecond laser instead of a diamond blade. Because the laser makes very fast pulses, it creates a perforated incision connected by many microscopic tissue bridges. As a result, we have to open the incision with an instrument like a Sinskey hook, just as we do with the LASIK flap. The key is that we don't always have to open the whole incision. A number of surgeons are looking into the efficacy of opening only part of the incision, and then expanding patients stepwise over time to titrate the effect.

Figure 2. Anterior segment OCT of zig-zagged interlocked incisions. Note that with the suture at half depth, the posterior incision of donor and host remain apposed due to tissue biomechanics.

In addition, unlike the diamond blade, the femtosecond laser has the ability to create nonpenetrating incisions. This is particularly helpful in cases of astigmatism, which starts deep and stops about 100 microns below the ocular surface. A nonpenetrating incision, created with a less powerful application of the laser, can achieve 1 or 1.5 diopters of effect. Many people only have that low level of astigmatism.

In addition, because the incision doesn't penetrate the anterior corneal surface, there's no healing involved, no plugs, and no infection. We'll find out more about the outcomes of this procedure over time, but it may be a more pre-dictable option.

Luis Ruiz studied a similar approach called the IntraCor procedure, an intrastromal correction for presbyopia in which the surgeon makes circular patterns in the cornea, creating central plus power in the center of one eye.¹ He and his colleagues described the results as “promising,” in that the procedure “preserves the corneal epithelium and anterior stromal fibers.”

Refractive Cataract Surgery

I think that the femtosecond laser's use will continue to expand quickly because it also has the ability to create very precise incisions for refractive cataract surgery. This part of the cataract procedure isn't covered by insurance, so the patient is responsible for the cost of refractive use of the laser.

For refractive cataract surgery, the femtosecond laser impacts all key steps in the procedure: the main and sideport incisions, astigmatic keratotomy, the anterior capsulotomy and nuclear softening.

From a reimbursement standpoint, in the United States, the only allowable patient shared billing is for refractive components, as that is not a covered service by third parties like Medicare. Thus, astigmatic incisions are the only unquestioned refractive component. However, a second refractive advantage may occur by making the capsulotomy precise in size and location for more accurate lens placement and centration. Many of the early adopters using femtosecond lasers are studying the clinical benefits. The final and perhaps most obvious application of the femtosecond laser for cataracts is in the safer disassembly of the nucleus. We can use the laser to prepare the nucleus, reducing or eliminating the amount of ultrasound phacoemulsification power needed, which may increase safety and efficacy. I believe this area will see ongoing technical advances over the next few years.

Looking Toward the Future

In a matter of a decade, femtosecond lasers have expanded laser applications in corneal procedures to include incisions for intracorneal rings, lamellar keratoplasty and keratotomy for correction of astigmatism. Although the size of the investment required to acquire a femtosecond laser means it's still not widely adopted for applications beyond LASIK flaps, the continued study of outcomes created with the device, particularly for billable procedures, will likely change that picture in the future. ■

Reference

1. Ruiz LA, Cepeda LM, Fuentes VC. Intrastromal correction of presbyopia using a femtosecond laser system. J Refract Surg. 2009;25:847-854.