Innovations in femtosecond lasers broaden the technology's potential

Clinicians will be setting higher expectations for visual outcomes of lens-based and corneal refractive procedures.

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The femtosecond laser is gaining popularity in the medical community due to its expanding applications, as it is able to create finer, accurate dissections and predictable, uniform incisions with minimal collateral damage.

How the laser works

The femtosecond infrared laser emits ultra short optical pulses that can carry various levels of energy. It does not depend on the inherent wavelength of the absorption of tissue, unlike the excimer laser, and it is able to work at the subsurface level.

Femto lasers produce a type of tissue interaction called photodisruption. The application of multiple photons of energy at the same time and location can lead to a nonlinear absorption of laser energy. The photodisruption process creates plasma, small amounts of thermal energy, a low impact acoustic shockwave and eventually a cavitation bubble.

Femtosecond laser flap creation. Images: Cover-Miller DN

The firing pattern of the laser is controlled by a computer-based system that allows precise, contiguous placement of the pulses and, thus, bubbles, at desired depths and positions, allowing for tissue resection. It is this accurate placement of the pulses within tissue that allows the femtosecond laser to be used as a surgical tool, according to Pepose and Lubatschowski, Young and Lee.

The most common applications of the femtosecond laser in eye care include LASIK flap creation, donor or recipient tissue preparation for penetrating keratoplasty (PK), endothelial keratoplasty, lamellar dissections for anterior lamellar keratoplasty, astigmatic keratectomy, limbal relaxing incisions and creation of corneal tunnels for Intacs (Addition Technology, Des Plaines, Ill.).

The most recent technology is now allowing for laser cataract surgery, femto-created intrastromal pockets for riboflavin during collagen cross-linking, femtosecond lenticule extraction (FLEx), small incision lenticule extraction (SMILE) and intrastromal correction for presbyopia (IntraCor, Technolas Perfect Vision). This article will touch on a few of these desirable applications.

Several companies have femto lasers on the market. Intralase, by Abbott Medical Optics, is the most widely used system and has been on the market the longest. Newer femtosecond units come from Zeimer (Femto LDV Crystal Line for Z Lasik), Zeiss (VisuMax and ReLEx), Technolas Perfect Vision (Technolas Workstation and IntraCor), Alcon (LenSx), LensAR (LensAR Laser Systems) and Optimedica (Catalys). The activity among manufacturers is underway, and multiple competitors in this laser category will form a robust market. Innovation will make these laser platforms increasingly functional and easy to use.

LASIK flap creation

Although the initial introduction of all-laser LASIK was met with skepticism, femto flap creation is now accepted worldwide, with more than 1 million procedures performed to date according to the AMO website.

Femto enables a controlled flap creation of various shapes, diameters and depths. Hinge placement and angle of side cut can also be customized. Surgeon experience with selection of spot size, spot separation and energy per pulse will determine the time it takes to create a flap and also the ease with which the flap is able to be elevated. Flap dimensions are based on patient factors such as corneal thickness, steepness, refractive error and pupil position.

The femto uses precise individual micro photodisruptions of tissue to create a resection plane. The laser pulses are delivered through a patient interface composed of a suction ring assembly and an applanation cone. The applanation cone holds an applanation glass that flattens the cornea and creates a reference surface for depth control, while the suction ring assembly fixes the eye with respect to the cone and beam delivery device.

Flaps are created in a two-part process: first, the horizontal resection plane is created at the user-defined depth in either a spiral or raster pattern and, second, a partial cylindrical arc or side cut that extends from the resection plane to the corneal surface is created. Only a small section of the side cut is left untreated, thereby leaving a flap hinge.

Possible advantages over mechanical microkeratome include thinner flaps to maximize residual stromal bed; reduced incidence of flap complications, abrasions and dislocations; greater predictability and uniformity; and planar (nonmeniscus) flap configuration.

Femtosecond laser Intacs tunnel creation.

Sub-Bowman’s keratomileusis (SBK) involves creating a thin (90 microns to 110 microns), smaller-diameter flap with a femtosecond laser just below Bowman’s membrane. This membrane is the thin, noncellular layer of the cornea that separates the outer epithelium from the underlying corneal tissue that is reshaped by an excimer laser in procedures such as LASIK, photorefractive keratectomy and now SBK. Some surgeons feel this thinner, smaller diameter flap is less likely to cause biomechanical weakening of the cornea and can help reduce the chance of ectasia.

In addition, femto treatments can provide faster visual recovery, lower incidence of epithelial ingrowth and lower incidence of postoperative dry eye. It is fair to say that improved patient satisfaction and procedural ease has made the postoperative management more routine with fewer emergency consultations. Studies completed by Slade, Durrie and Binder support the above findings, reported Thompson and Boxer Wachler.

Arcuate astigmatic keratotomy

Astigmatism can often be congenital, post-traumatic and postsurgical. Corneal incisions have long been used to correct refractive error, including astigmatism. It has been found that when incisions are made perpendicular to the steep axis of the corneal astigmatism, a flattening effect is seen along that meridian. Subsequently, there is also a steepening effect 90° away from the incision, creating a coupling effect that generally maintains the spherical equivalent of the patient’s refractive error.

Femtosecond lasers combined with sophisticated software are now able to generate these incisions. Newer femto laser software even includes programming for nonorthogonal incision placement, thus giving us the versatility to manage irregular astigmatism. By using optical coherence tomography images and ultrasound pachymetry, one can predetermine the depth, arc length, axis and size of the optic zone of the automated femto astigmatic keratotomy (AK) cuts to greatly reduce the risk of corneal perforations. Femto-assisted AK enables precise, predictable and safe corneal keratotomy while maintaining simplicity and effectiveness, according to Young, Hurmeric and colleagues and Hurmeric and Yoo.

Intrastromal lamellar ring resection for Intacs

Intacs are small, precision lathe-cut implantable/removable intrastromal PMMA ring segments originally designed for myopia (approved by the U.S. Food and Drug Administration in 1999) that are now being used for keratoconus (approved by the FDA in 2004) or post-laser ectasia patients.

These rings work by restoring the cornea to a more natural prolate shape by flattening the steep central cornea, with hopes to achieve more functional vision and possibly delay the need for a corneal transplant.

Intacs come in various sizes, with the thickest segments having the most flattening effect. They are placed in the peripheral cornea between stromal layers at approximately 70% full corneal depth.

Femtosecond lasers are now able to create such channels for these segments. Intrastromal tunnels are an annular cut (without a side cut) at a desired corneal depth and predetermined inner and outer radius. The annular cut proceeds outward in a spiral fashion, which is then followed by a small continuous entry cut along a radial direction that starts at the tunnel depth and progresses anteriorly to the corneal surface. The femtosecond laser has greatly reduced the need for a mechanical spreader, intraoperative complications, risk of corneal rupture and clinical difficulty with insertion/removal of Intacs for treating keratoconus or ectasia. The regularity and accuracy of the channels also makes it easier to manipulate the segments and make adjustments with their placements.

Role in corneal collagen cross-linking

Ectatic diseases of the cornea are among the most frustrating for clinicians and patients. Their progressive nature requires many visits with frequent updating of prescriptions to maintain useful vision. Even with specialty contact lenses and intracorneal ring segments to slow the progression, a large percentage of patients eventually need a corneal transplant. Of course, this is the last resort, as ectatic conditions are not limited to the central cornea but change the far periphery as well. Grafts must be large, which increases the potential for induced astigmatism, prolonged healing times and the activation of inflammatory cells near the periphery.

Schematic diagram of femtosecond laser keratoplasty.

With the current lack of a truly successful therapy for corneal ectatic conditions, the eye care community has embraced a treatment called corneal collagen cross-linking. The goal with cross-linking is to strengthen the structure and stabilize the curvature of the cornea in patients with pellucid marginal degeneration, keratoconus or postsurgical ectasia.

Traditionally, riboflavin is applied to a debrided cornea, followed by ultraviolet exposure for 30 minutes. The interaction of the light and the riboflavin stimulates the production of free radicals that cause the formation of chemical bonds between and within the collagen fibers. Because the riboflavin absorbs the UVA light, it also protects the endothelium and other intraocular structures during the procedure.

An innovative approach to this procedure using the femtosecond laser spares the epithelium, thus reducing post-treatment pain and providing a more rapid visual rehabilitation. Intrastromal pockets can be created with the femto laser, which then allows for the direct injection of riboflavin with subsequent ultraviolet exposure, according to Don and Zhou.

Femtosecond laser-enabled keratoplasty

Deep anterior lamellar keratoplasty (DALK) is a surgical procedure that removes the corneal stroma down to Descemet’s membrane. It is most useful for treating corneal disease in the setting of a normally functioning endothelium.

Common indications for DALK include keratoconus and corneal scars. Patients with keratoconus are good candidates for DALK because they are typically young and have a healthy endothelium. These patients stand to lose the most from the occurrence of post-penetrating keratoplasty immunological reactions that can compromise endothelial function.

Traditionally, PK, which involves a full-thickness corneal graft, has been the treatment of choice for corneal stromal diseases. But PK can be complicated by graft rejection, irregular astigmatism and corneal opacification, thus resulting in visual impairment. DALK offers an alternative procedure that may lessen those risks because the recipient Descemet’s membrane and endothelium are preserved. At the same time, DALK carries the potential danger of decreased visual acuity due to possible opacification at the interface layers.

Femtosecond lasers can now be used in selective transplant of the anterior cornea allowing an intact globe, thus preserving Descemet’s membrane and the endothelium and greatly reducing the risk of graft rejection and intraocular complications. Femtosecond lamellar keratoplasty allows for diseased tissue to be replaced with a perfectly matched piece of donor tissue according to OCT measurements and subsequent laser preparation. Femto procedures allow for faster visual rehabilitation by greatly reducing suture-induced astigmatism.

The femtosecond laser can also be used in posterior stromal grafts for endothelial keratoplasty. Descemet’s membrane endothelial keratoplasty (DMEK) is a type of partial-thickness corneal graft procedure in which only the innermost corneal layers are replaced. The operation is performed where poor function of the inner endothelial cell layer has led to corneal edema. Normally, corneal transparency depends on the endothelial cells pumping fluid out of the cornea. If the cornea becomes edematous, the vision deteriorates.

Corneal endothelial failure is seen in the genetically determined condition of Fuchs’ corneal endothelial dystrophy and may also occur following intraocular surgery such as cataract extraction. As corneal failure becomes more severe, the cornea becomes increasingly hazy. If the cornea is too hazy, Descemet stripping automated endothelial keratoplasty (DSAEK) may be recommended, as DMEK requires a reasonable view into the anterior chamber for the surgeon to see the injected Descemet’s membrane and unroll and position it, according to Rose and colleagues and Price and colleagues.

Example of multilevel trephinations in femtosecond laser keratoplasty. Image: Tullo W

Femtosecond lasers are also being used for creating stepped or multilevel trephinations for penetrating keratoplasty. Creating unique wound shapes allows for stronger tectonic tissue bonding, faster healing with increased surface area, reduced risk of corneal rupture and potentially less astigmatism and change in astigmatism over time. Studies by Szentmary suggest that lasers avoid mechanical distortion induced by radial and tangential forces during trephination, “resulting in smooth, almost perpendicular cut edges, which are congruent in the donor and patient, thus avoiding ‘vertical tilt,’” according to Young.

Decentration of donor-recipient trephination was also reduced in conjunction with using a laser. In addition, patients having conventional trephination at times need to undergo subsequent transverse keratectomies due to high astigmatism, in contrast to those from the laser trephination group, Szentmary reported. “Furthermore, ‘all-sutures-out’ final topographic surface regularity of the graft was significantly better in the respective eye after having laser trephination in a given individual. In turn, this results in a significantly better potential visual acuity of such a corneal graft after laser trephination.”

Femtosecond lenticule extraction

Unlike traditional LASIK, femtosecond lenticule extraction (FLEx) is an all-femto procedure that involves creating a lenticule and a flap or small incision (small incision femtosecond lenticule extraction or SMILE), then physically extracting the lenticule as opposed to using an excimer laser to ablate the tissue under the flap.

During FLEx, the laser creates a refractive and a nonrefractive cut in a single step. The flap is elevated, the lenticule is removed and the flap is replaced. A possible advantage over LASIK is that there is no initial over correction as is seen in excimer ablation, which possibly leads to a faster visual recovery. Additionally, energy is not lost in the periphery of the cornea, which yields more prolate refractive zones similar to wavefront-optimized excimer laser ablation results. Some feel as though this could be a safer, less costly approach to myopic correction. However, some clinicians feel the initial results achieved were not on parallel with conventional LASIK, according to Sekundo.

IntraCor presbyopic treatment

Treatment of presbyopia remains the biggest challenge in the refractive arena, with monovision still being the most common method of correction. Along with conductive keratoplasty, intracorneal inlays and multifocal implants, the femtosecond laser is now being used to treat presbyopic emmetropic patients.

For the IntraCor procedure, Technolas Perfect Vision’s flapless intrastromal correction method, a curved interface is used between the eye and the femtosecond laser, which ensures fixation with minimal biomechanical impact. This has the advantage of not modifying the corneal shape or elevating the patient’s IOP. The IntraCor concept is to deliver five concentric circular intrastromal laser pulses around the visual axis of the eye, thus changing the biomechanics of the eye and creating a hyperprolate, multifocal shape. The purely intrastromal, flapless technique can reshape the cornea while maintaining the anterior and posterior corneal surfaces, giving patients a fast, comfortable recovery and minimal risk of infection. As noted on the Technolas website, more than 90% of patients studied had a cumulative uncorrected near visual acuity of J2 or better following the IntraCor procedure.

Cataract surgery

One of the most exciting and promising advances in ophthalmic surgery is the use of femtosecond lasers for cataract extraction. Femto technology will have the ability to reduce complication rates, increase predictability and provide reproducibility.

The anterior segment of the eye will be analyzed with integrated OCT or Scheimpflug technology. This will allow the surgeon to visualize, measure and focus with three-dimensional accuracy to perform the capsulorrhexis, limbal relaxing incisions, corneal tunneling, nucleus softening and lens fragmentation.

IntraCor femtosecond laser rings. Image: Technolas Perfect Vision

In September 2009, the LenSx laser received 510(k) approval from the FDA for creating anterior capsulotomies prior to cataract surgery. Alcon quickly received two additional clearances allowing for the creation of corneal incisions (December 2009) and the fragmentation of the lens (April 2010). Alcon’s LenSx was the first to receive FDA approval, however three additional companies are at work developing femto laser cataract procedures as well: LensAR, Technolas Perfect Vision and Optimedica Corp. Technolas Perfect Vision plans to offer a femto laser platform with multiprocedure capability to allow for cataract, refractive and corneal procedures, according to Bethke and Slade.

The femtosecond laser is able to create precise, continuous curvilinear capsulorrhexis of any desired shape, position or size. Precise, automated anterior capsulotomy for premium IOLs can help improve visual outcomes and possibly eliminate decentration-related problems with toric, multifocal or accommodating implants. Even experienced surgeons find creating a perfect capsulorrhexis challenging with poor visibility, zonular weakness or shallow chambers. More predictable anterior-posterior placement of the IOL will also be more beneficial in IOL power calculations and selections.

Customized lens softening and fragmentation with the femto laser will allow for the reduction of energy levels used during phacoemulsification and possible reduction of the resultant damage on the endothelium from the ultrasound as well as the zonular stress from sculpting, according to Lee.

The procedure

The surgeon pre-programs the laser for corneal incisions, capsulotomy and then the lens fragmentation pattern. The eye is docked to the laser, and the integrated OCT begins to capture three-dimensional images of the anterior segment. These images are projected onto the video microscope screen along with overlays of the pre-programmed treatments. The surgeon is then able to make any adjustments via touch screen.

Because the femtosecond laser works from the inside out, the cataract procedure is reversed from what we know to be customary. First the lens is fragmented, followed by the anterior capsulotomy, then the incisions are formed. Once the incision is complete, the surgeon uses irrigation and aspiration to remove the sectioned lens, performs cortical clean-up and then implants the IOL.

The first femtosecond laser cataract procedures in the U.S. were performed in 2010 by Stephen Slade, MD, and Steven Dell, MD, using a prototype LenSx laser. To see a video of this procedure performed by Dr. Dell, go to www.youtube.com/watch?v=h84x9QlvoNU.

According to Nagy, the early results of femto in cataract surgery show promising data. Results showed that all anterior capsulotomies with LenSx achieved perfect centration and exact intended diameter, 5.02 +/- 0.04 mm with no radial tears or adverse events. In the same study, average achieved diameters were 5.88 +/- 0.73 mm with manual technique; only 10% of those manually created achieved a similar diameter accuracy of +/- 0.25mm.

Also of note, compared to control porcine eyes, femtosecond laser phacofragmentation resulted in a 43% reduction in phacoemulsification power and a 51% decrease in phaco time. Both porcine and human studies exhibited similarly high levels of accuracy with no operative complications.

In a larger scale study, Nagy also found that femto capsulotomies yield better IOL centration as well as improved anterior capsule overlap with the IOL edge. This could possibly prevent posterior capsule clouding, which can hinder the performance of accommodating IOLs.

Nagy also looked at how the IOL position could impact higher-order aberrations. It was determined that both tilt and coma aberrations could be affected by poor IOL positioning.

Finally, laser-created corneal incisions, including limbal relaxing incisions, have better intraoperative self-sealing properties and possibly do not require stromal hydration. Additionally, the incisions created with the laser are extremely precise and should reduce risk of endophthalmitis and hypotony, according to Slade.

Femtosecond lasers in practice

Sophisticated patients will demand the best technology. We have learned through consumer research that given the choice, patients overwhelmingly prefer the thought of a laser procedure over traditional methods. Additionally, surgeons feel that the precision and predictability offered through laser procedures will enhance outcomes and drive business growth in both standard and advanced technology.

Elective medicine has been developed on the premise that patients are consumers who will pay for their upgraded procedures. Over time, cataract surgery has become commoditized, with third-party payers seeking to reduce its value, while surgeons try to preserve it. Laser cataract surgery is possibly a way to increase the value of the procedure once again.

The Centers for Medicare and Medicaid Services has deemed cataract surgery to have an elective component and now allows patients to pay for the use of specific IOLs or specific devices that may improve refractive outcomes. This is a key part of a market transition to label cataract surgery more accurately as a lens-based refractive procedure. With cataract surgery, the demand is already in place with more than 3.3 million procedures performed in the U.S. in 2010, according to Mahdavi. This market is already three to four times as large as that for LASIK. The industry will need to focus on market conversion rather than expansion.

Patients are well aware that they can visit a surgeon and have an outpatient procedure that lets them see without glasses. Education will be key when it comes to having the patient understand that they will need to pay for premium laser services, measurements and refractive-like outcomes. That being said, the only disadvantage to the femtosecond laser is, of course, the price tag. It is likely to have a base cost greater than the current femto lasers along with carrying a per-click fee for usage.

LenSx OCT photo. Image: Alcon LenSx

The activity among manufacturers is underway. Companies are refining their plans for commercialization, marketing, securing regulatory approvals and constructing financial models that will allow practices to integrate this expensive capital device. The pace of technological innovation is getting faster and faster. This transformation will take time, but it will be measured in months and years, not decades. It has been suggested that in the near term, femtosecond laser procedures will complement rather than replace phacoemulsification, attracting the interest of manufacturers and surgeons alike. Eventually, this technology will make it into everyone’s community.

It is imperative that all practitioners become educated and involved. Femtosecond lasers are now making it possible to look at lens-based and corneal refractive procedures in a different light by setting higher standards and goals for visual outcomes. We are no longer just looking at clearing the visual axis.

References:

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For more information:

• Denise N. Cover-Miller, OD, is area director of optometry for LasikPlus. She can be reached at 216 Mall Blvd., Suite #100, King of Prussia, PA 19406; (610) 265-5228; fax: (610) 265-1560; dcovermiller@lca.com.
• Disclosure: Dr. Cover-Miller has no direct financial interest in the products mentioned in this article, nor is she a paid consultant for any companies mentioned.