Comparison of Commercially Available Femtosecond Lasers in Refractive Surgery

Glauco Reggiani-Mello; Ronald R Krueger

Disclosures

Expert Rev Ophthalmol. 2011;6(1):55-65. 

In This Article

Five-year View

Femtosecond laser technology has evolved considerably over the past 5 years, and further advances are expected. This technology will probably guide the major surgical changes in ophthalmologic care over the next 5 years. In addition to expected changes in refractive, corneal and cataract surgery, this technology can introduce and refine the treatment options for newer procedures such as keratolimbal grafts,[21] glaucoma[22] and retina[23] surgeries (to cut vitreous traction fibers – (Figure 9).

Figure 9.

Retinal traction fibers could theoretically be cut with the femtosecond laser technology.

Refractive Surgery

In refractive surgery, a promising treatment modality using only the femtosecond laser (no excimer laser involved) is called FLEx.[6] In this procedure, the femtosecond laser is used to cut a lenticule of corneal stroma, instead of an excimer laser ablating this same tissue (Figure 10). The refractive outcomes are still not as good as the excimer laser treatments, but there is room for improvement. The theoretical advantage over the standard procedure is that it is faster, has no need of two different lasers and less energy is applied. In addition, in excimer laser ablation we observe a reduction in laser efficiency in the periphery of the cornea, yielding results that differ from that expected with wavefront-guided and optimized treatments. This would not be a problem with the femtosecond laser-cut lenticules, with the first results showing a large prolate treatment zone, and with less induction of high-order aberrations.[24] The development of faster, more advanced femtosecond laser systems, such as the Zeiss Visumax, allows for a more accurate placement of laser pulses, making this modality a potentially competitive treatment to excimer laser ablation. This feature, present in the Visumax, is not yet available in the USA.

Figure 10.

Flex procedure: a lenticule of stroma is cut at the same time as the flap is created, then the flap is lifted and the lenticule removed.

Manipulating Corneal Biomechanics

One of the promising new applications using the femtosecond laser in refractive surgery is the IntraCor procedure,[5,25] which is based on a controlled biomechanical manipulation.

In this procedure, femtosecond laser pulses are used to perform corneal 'intrastromal-only' incisions in a cylindrical shape pattern (Figure 11). The only device to date that has this software available is the Technolas Femtec. The incisions heal fast, since there is no damage to the epithelium. The incisions biomechanically induce a hyperprolate, negatively aspheric corneal shape, and aberrated refractive profile with both negative spherical aberration and positive secondary spherical aberration.[5] First results demonstrate an increased depth of focus and refractive corneal stability over the first year with no patients showing loss of best-corrected visual acuity.

Figure 11.

Intracor procedure. (A) Intrastromal-only cuts. (B) It is possible to visualize the incisions in the slit-lamp.

Our knowledge regarding cornea biomechanics has been increasing in the last decade with the development of new diagnostic tools and crosslinking procedures. However, the fear of biomechanical disasters,[26,27] resulting in corneal instability and ectasia (that occurred in procedures such as hexagonal keratotomy, automated lamellar keratoplasty and radial keratotomy), are still fresh in our minds and long-term results are needed to prove safety and stability.

Cataract Surgery

The main objectives within the surgical management of cataracts has always been in the treatment of disease. Residual refractive errors, delayed recovery, and even complications were accepted and expected by patients and physicians. With the development of intraocular lenses and small-incision surgical technique, patients and doctors became less tolerant to an imperfect and unexpected result. Modern-day cataract surgery is becoming part of refractive surgery, and despite the excellent results obtained with the current technology, perfection is demanded. In the years ahead, patients' expectations with cataract surgery will probably be the same as among LASIK patients. Femtosecond laser technology is bringing these two procedures even closer.

Currently, four companies (LensAR, LenSx Lasers Inc. [CA, USA], OptiMedica Corporation [CA, USA] and Technolas) are researching the use of femtosecond technology to make primary incisions, paracentesis, capsulotomy, lens fragmentation (Figure 12) and limbal-relaxing incisions (Figure 13).

Figure 12.

The emulsification can be performed in different cutting geometries.

Figure 13.

All the steps of cataract surgery that can be performed with the femtosecond laser technology. LRI: Limbal-relaxing incision.

The femtosecond laser devices for cataract surgery are more complex than those for flap creation. They require very precise imaging systems and a docking system that preserves the anatomy of intraocular structures. The LenSx laser (LenSx Lasers Inc.) and the OptiMedica laser (OptiMedica Corporation) use a real-time high-resolution optical coherence tomography. The docking system specifications have not been disclosed by the companies. The LensAR laser system uses a high-resolution 3D confocal structured illumination, a form of infrared-based imaging used together with a no-touch, non-applanating suction fixation device (an automatically filled miniature water chamber). The device from Technolas is able to do cataract surgery and flaps for refractive surgery. It uses the same curved applanation plate used in the current femtosecond laser device for flap creation. More data regarding the differences among the devices are expected after they become commercially available.

The precision involved in these lasers promises a safer and more predictable cataract surgery, and could be responsible for the most important evolution since the transition to phacoemulsification. The first results show a more reproducible and stronger capsulotomy, excellent incision architecture (less leakage and less potential endophthalmitis) and less energy used (less potential endothelial damage).[28,29]

The enhanced safety and predictability can improve outcomes, especially with premium IOLs, which depend on a regular, well-centered capsulotomy and minimum residual corneal astigmatism. These can lead to an expansion of the indications of refractive lens exchange and limbal-relaxing incisions, bringing the revolution not only to cataract surgery but also to the refractive surgery area.

Astigmatic Correction

An area that has been undergoing research in refractive surgery and has been a hot topic in the past few years is astigmatism correction. It is more challenging to correct astigmatism than myopia and extra precision is required. Four main options currently in the treatment of astigmatism include: excimer laser photoablation, manual limbal relaxing incisions, astigmatic keratotomy and toric IOLs.

The results of excimer laser photoablation to correct astigmatism are usually accurate, but is not as good as with myopic treatments. In cataract surgery, excimer photoablation of residual astigmatism would require a separately performed procedure. In addition, nonorthogonal astigmatism cannot be treated with astigmatic excimer laser photoablation and topographic customization is not currently available.

Toric IOLs have been the subject of intense research in refractive cataract surgery, and the technology has evolved into being the first option for correcting astigmatism when facing a patient that is undergoing to cataract surgery. More accurate visual results are found when compared with manual limbal relaxing incisions.[30] However, the placement of a toric IOL is not possible when using a multifocal or accommodative implant and it cannot be used in the absence of cataract surgery.

Manual limbal-relaxing incisions have been used mainly to correct low-grade astigmatism (<3 diopters) during cataract surgery with good results.[31] However, when dealing with manual incisions we must be aware of the lack of length, depth and orientation precision during incision placement, as the reproducibility of outcomes would probably be enhanced with imaging-based laser incisions.

Astigmatic keratotomy has been widely performed during the radial keratotomy era, but because of irregular astigmatism, instability and imprecision, it was essentially abandoned, except for high postkeratoplasty astigmatism, although excimer laser vision correction may be needed to correct the residual astigmatism.[32]

Early studies of femtosecond laser-assisted astigmatism correction have begun with good results.[33,34] In higher degree astigmatism (postkeratoplasty and high naturally occurring astigmatism) laser astigmatic keratotomy is effective, while among lower astigmatic eyes, limbal-relaxing incisions seem to be most effective. The results for high postkeratoplasty astigmatism show greater accuracy and less complication compared with manual techniques.[35]

In addition, femtosecond lasers can create intrastromal-only astigmatic incisions. Although these are expected to be less effective, they would also be safer and more stable, but these facts need to be verified in further studies.

The precision of femtosecond laser technology in creating incisions still needs to be matched with better nomograms for an accurate correction. In addition, a precise imaging system and flexible geometric cutting profile are required for a better placement of the incisions.[36] This technology is still evolving and advanced refinements are currently being developed in the newer generation femtosecond laser devices.

Presbyopia Correction

Presbyopia is one of the last major challenges in ocular surgery. Many different surgical techniques have been studied to correct this huge problem, yet the simplest solution, reading glasses, are still the most utilized, because other solutions are still potentially compromising. The prevalence of presbyopia is increasing in the world owing to aging of the population, and is expected to be approximately 1.8 billion in 2020.[37] The main pathology of this disease is the increasing stiffness of the crystalline lens related to aging, for which there is currently no restorative solution Table 2.

Although the IntraCor procedure uses femtosecond lasers in the cornea to increase the depth of focus for spectacle independence in presbyopia, intralenticular femtosecond laser pulses could also be noninvasively applied for accommodation restoration at the level of the crystalline lens.

The concept of using low-energy femtosecond pulses inside the crystalline lens is currently being studied. This would allow increased flexibility and sliding of lens fibers that could partially restore the accommodative loss of the lens with aging.[38–40]

The main fear of this approach is the potential in causing cataracts and a loss of best-corrected visual acuity. The studies thus far show the development of pinpoint opacities at the site of laser interaction but no progressive cataract formation in preclinical[41] and clinical studies (Figure 14). When comparing this novel method with other presbyopia-correcting options and their complications, the minimal invasiveness of this technique makes it a potentially attractive potential remedy. The risk of infection is negligible since no exterior wound is created, and the possibility of presbyopia correction involving lens exchange could still be performed without concern if unsatisfactory results with femtosecond laser correction are experienced. An undesirable outcome within the crystalline lens would be much easier to treat than an undesirable outcome secondary to a corneal procedure. Studies in animals have shown promise,[41] and human trials are underway to show the real potential for efficacy and safety in performing this technique.

Figure 14.

Note in the slit-lamp image the appearance of the cuts in the lens performed to increase the sliding of lens fibers and partially restore the accommodation.

Nonrefractive Corneal Applications

In corneal procedures, the potential of femtosecond laser technology has not yet been fully achieved, although its initial uses for shaped penetrating corneal transplantation have been reported successfully in the last years (Figure 15). In addition, femtosecond laser-created keratolimbal autografts have been easily prepared with very positive results.[21]

Figure 15.

Shaped edges can be created for a better wound architecture.

The laser cutting of Descemet's stripping automated endothelial keratoplasty (DSAEK) grafts and anterior lamellar keratoplasty are hot topics with this technology.[42–44] The current standard procedure for cutting DSAEK grafts utilizes a microkeratome. Femtosecond lasers promise thinner and more reproductible DSAEK grafts.[45] However, a real benefit compared with microkeratome would depend on the use of a curved applanation system (to maintain the posterior stroma and endothelium in a physiologic, unfolded position) and on a cutting plane reference along the posterior surface, rather than the anterior as it is today (since cornea's posterior curvature is steeper than anterior, a cut based on an anterior curvature will leave the graft thicker in the periphery). How close to the endothelium we can go without inducing damage is still to be determined. In deeper cuts, there is an increase in laser scattering, resulting in an irregular cut. Very similar challenges are found when using the laser with deep anterior lamellar keratoplasty.

There are many more advances to be explored in this field, and future studies will determine whether or not there is any benefit to using a Femtosecond device in these situations.

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