March 2014

 

COVER FEATURE

 

Femtosecond phaco techniques

Operating on the laser's edge


by Maxine Lipner EyeWorld Senior Contributing Writer

 
   

The ASSORT femto LRI calculator allows planning of LRI procedures with well-known nomograms, together with postoperative astigmatism analysis using the Alpins Method.

Source: Noel Alpins, MD

Corneal topographic astigmatism (CorT) better matches the manifest refractive cylinder (R) than simulated keratometry (Sim K), according to Dr. Alpins.

Source: Noel Alpins, MD

Femtosecond cataract outcomes

Blade-free cataract surgery is on a lot of practitioners' minds these days as practices begin to incorporate the technology, akin to their refractive brethren a few years back with femtosecond LASIK. EyeWorld wondered about the effect on outcomes with femtosecond cataract surgery. From astigmatism correction to effective lens position and beyond, here's what you can expect with femtosecond technology.

Jack T. Holladay, MD, MSEE, FACS, professor of ophthalmology, Baylor College of Medicine, Houston, said that the introduction of bladeless cataract surgery and that of femtosecond refractive surgery is very similar. "We always jump in and think, 'What is the actual scientific evidence for improvement?' and that's going to be related to the adoption by the doctors," Dr. Holladay said. "When the IntraLase [Abbott Medical Optics, Santa Ana, Calif.] first came out, the same questions were raised in terms of comparing the microkeratome to the femtosecond keratome." People wondered whether use of the femtosecond made a difference. Dr. Holladay said while there were some differences in femtosecond LASIK outcomes in terms of visual acuity and contrast sensitivity, you would be hard-pressed to find a study showing this. "Yet in a period of two to three years, the femtosecond completely took over 90% of the market in terms of femtosecond LASIK," he observed. It was the difference perceived by patients in the chance of having a catastrophic complication with mechanical microkeratomes that was the driving force with femtosecond LASIK, he explained. "People are afraid of blades. They don't like the idea of cutting," he said. "The laser is perceived as safer."

The laser is safer and more precise, Dr. Holladay said. "With cataract surgery when the laser makes the primary cataract incision, the secondary incision, the AK cut, everything that it does, the precision with which it does that is an order of magnitude 10 times better than a surgeon can do," he said. Creating and placing incisions with the laser in every case is always the same, he said. "It's never the same from one patient to the next with the doctor." The pilot study with the Verion Image Guided System (Alcon, Fort Worth, Texas) did show a significant improvement in prediction error over manual cataract surgery and the Optiwave Refractive Analysis (WaveTec, Aliso Viejo, Calif.) intraoperative aberrometry. The study also showed that results with AK incisions were competitive with toric IOLs.

Eric Donnenfeld, MD, clinical professor of ophthalmology, New York University Medical Center, New York, thinks that for astigmatism correction, the femtosecond lasers add a level of predictability, reliability, and safety that can never be achieved with a manual limbal relaxing incision. "The fact that the majority of ophthalmologists never perform a manual limbal relaxing incision speaks to the point that many ophthalmologists are not comfortable with using a diamond knife to incise a cornea," Dr. Donnenfeld said. He said recent surveys show only about 25% of ophthalmologists currently do limbal relaxing incisions. "The arcuate incisions with the laser add a level of safety that can't be achieved manually," he said. Dr. Donnenfeld said that several of the lasers have OCT visualization that show how deep the incisions go and significantly help to avoid perforations. "The incisions can be set at a specific depth and that's achieved," he said. George Stamatelatos, OD, senior clinical optometrist, New Vision Clinics, Melbourne, Australia, likewise pointed to this as a strength. "With the OCT that's available on the femtosecond lasers, you can get the precise depth that you want," Dr. Stamatelatos said. When you take an average thickness of the cornea in the peripheral zone, it is just an average, whereas with the femtosecond you know that you're going to go to 85% of the actual depth. Another advantage that Dr. Donnenfeld believes will take the femtosecond cataract approach into the mainstream is that it is controlled by the computer and doesn't rely on a certain skill level. "The novice surgeon can do just as well as the experienced surgeon," Dr. Donnenfeld said. The laser incisions are adjustable after surgery, allowing outcomes to be titrated, Dr. Donnenfeld said. Akin to the intact serrations in a postage stamp, the incision can be put in place by the laser. The stamp isn't separated until these serrations are pulled apart. "I will make the incisions with a laser and then open these one incision at a time," he said. This almost eliminates the possibility of overcorrection and allows him to titrate the results to the desired effect. In addition, he said that the femtosecond allows for the possibility of creating intrastromal relaxing incisions that could never be done manually. Noel Alpins, MD, medical director, New Vision Clinics, also believes femtosecond lasers can be beneficial in correcting astigmatism. He cited a small series of LRIs that he did with the laser after first gaining access. "In about three out of 10 patients we did LRIs straight off the bat," Dr. Alpins said. "All of the patients had a good reduction of corneal astigmatism, as well as refractive cylinder."

In his view, the real benefit of using the femtosecond laser is that practitioners can precisely determine the depth of the incision. "You get exactly what you dial into the machine," he said.

Measurement obstacles

He stressed, however, one variable people are not thinking about that needs to be considered is the white-to-white measurement. "If the white-to-white is 10 versus 12, then that incision can be anywhere between 1 and 3 mm from the limbus," he explained. "You can have a lot of variability in the distance between that incision and the distance to the limbus." When this is done manually, practitioners tend to put the incision just inside the limbus. "I think a lot of the refinement yet to come in femtosecond LRIs is to hone down the white-to-white and to know where the incision is in relationship to the physical surgical limbus," Dr. Alpins said.

Another obstacle in attaining best correction of corneal astigmatism with femtosecond lasers is determining the true power of such astigmatism. Dr. Alpins said that simulated keratometry, used to help ascertain corneal astigmatism, relies on only one ring of topography readingsthe seventh ring, located roughly over the 3 mm zone. "The thing about using only one ring when the topographers have up to 32 rings is that it's a bit of a lucky shot," Dr. Alpins said, adding that in a fat cornea the ring will sit over the 4 mm zone and in a steep one over the 2 mm zone. "It doesn't measure the same point in the cornea every time if you use only one ring," he said.

To try to obtain better results, Dr. Alpins has been using the parameter corneal topographic astigmatism (CorT). For this, he considered all of the rings and all of the data captured on corneal topography and took the vectorial average of them all. "What we showed was the CorT value of the anterior cornea was so much more accurate than the SimK, manual keratometry, paraxial curvature matching and corneal wavefront," Dr. Alpins said. "It was the best of them all."

Dr. Alpins is now using three machines to ascertain total corneal powerthe Sirius Corneal Topographer (CSO, Florence, Italy), Pentacam (Oculus, Arlington, Wash.), and Ziemer (Port, Switzerland). Together these offer a CorT number that is more accurate not just for the anterior cornea but for the whole cornea, Dr. Alpins said, adding that this will give practitioners the number needed for LRI and toric IOL placement.

Douglas D. Koch, MD, professor and the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, contends there's a variability introduced into femtosecond measurements by two things that don't currently get considered. "One of them was pointed out by Dr. Holladay in terms of toric lenses, and that is the fact that there is less effective toricity as the anterior chamber deepens and as the IOL power diminishes," Dr. Koch said. With astigmatism measurement in general, there is a lot of variability in anterior corneal measurement. "You do measurement with three or four devices and it's astonishing the amount of variability that occurs." The second issue is the posterior cornea, which introduces an element of astigmatism that can be 0.5 D or more. "We are struggling to get devices that accurately predict it on a patient-by-patient basis," he said. Once that occurs, Dr. Koch thinks, there will be a distinct, incremental improvement in outcomes with the femtosecond lasers. Dr. Holladay pointed out that tilt and decentration can be factors because the human eye is not along the optical axis. The fovea, which has the finest vision, is not at the posterior pole, it's five degrees temporal, allowing people to avoid having to look through all the wires of the optic nerve. As a result we also end up with a lens that's tilted about five degrees. "Both that decentration and tilt result in about 0.5 D of against-the-rule astigmatism," Dr. Holladay said, adding that this also includes the against-the-rule astigmatism from the posterior cornea. "That has always been there, but we've never gotten down to where we've been worried about 0.5 diopter of cylinder," he said. "It's now getting down to the level where we're beginning to realize that if we want to get it perfect, we've got to compensate for those, too."

Effective lens position

Can the precision of femtosecond technology translate into better results? Whether or not IOLs can be better positioned as a result is the key consideration here, Dr. Donnenfeld said. He views the effective lens position as a missing link to achieving optimal results with cataract surgery. "Knowing where the lens is going to sit after cataract surgery is the key to achieving good refractive results," he said. He added that with biometry now so good, the advantage of the arc of the femtosecond capsulotomy is that it makes the same size capsulotomy every time. As a result, the thinking is that the lens is less likely to vault forward because the capsulotomy is too large or that the lens will be trapped and pulled backward. "There have been several studies showing this improves effective lens position and one study I know of showing it did not," Dr. Donnenfeld said. "In my estimation, I don't think there's any way the femtosecond laser can degrade effective lens positionit can only help." He thinks he is getting tighter results with femtosecond laser capsulotomies. However, he stressed, studies need to be done on where such capsulotomies need to be placed for best outcomes. "We're still worrying about where the best place to put these capsulotomies is," Dr. Donnenfeld said.

Dr. Holladay pointed to a study he presented at the 2013 European Society of Cataract & Refractive Surgeons Congress that examined if the location and diameter of the capsulorhexis that are made absolutely consistent because of the precision of the laser shows up in prediction error. He reported that for + or 0.25 D of prediction error with the LenSx laser (Alcon, Fort Worth, Texas), that was about 20% better than the Optiwave Refractive Analysis, which was 10% better than standard cataract surgery at one month.

Dr. Koch thinks the jury is still out on whether the femtosecond laser helps with effective lens position. "I think the next step where femtosecond lasers will be more beneficial will be when we have lens implants that are designed to take advantage of that very predictable size and location and [we can] attach lenses in a different way," Dr. Koch said. "I think then we will see a benefit from the effective lens position."

Dr. Alpins concurs. "It would be nice to be able to say that it's more predictable and more accurate because then you're going to get better outcomes," he said. "But in the hands of experienced surgeons that hasn't been shown to be the case." He stressed that the accuracy refractively is no better than that attained by an excellent cataract surgeon. "[For a] less than excellent cataract surgeon, femtosecond technology may significantly help by giving consistency to their surgery that their own manual techniques are not giving them," Dr. Alpins said.

Additional tools

Pairing the femtosecond laser with other tools can also potentially help to boost outcomes. Dr. Donnenfeld noted that the Verion (Alcon) allows surgeons to account for cyclorotation of the eye at the time of surgery that could improve outcomes. "That allows you to take preoperative pictures of the eye and then overlay them at the time of surgery," he said. However, he thinks using the Optiwave Refractive Analysis in conjunction with surgery is the best way to get a true reading of the actual cylinder. "The advantage of the Optiwave Refractive Analysis is that not only does it measure the anterior cylinder, it also measures the posterior corneal cylinder as well," Dr. Donnenfeld said, adding that getting such a true reading allows the practitioner to adjust cylinder very precisely. He finds this helps for toric IOLs as well, pointing out that the Optiwave Refractive Analysis tells the practitioner where to rotate the axis and whether or not to open an incision at the time of surgery to achieve an optimal result.

"I think that intraoperative readings are the only way we're going to always achieve the next level of accuracy with cylinder control because the variables of the visual axis, posterior corneal astigmatism, and the cylinder induced by the cataract incision are all considered with intraoperative aberrometry," Dr. Donnenfeld said.

Overall, Dr. Donnenfeld views astigmatism management as the number one limiting step for most ophthalmologists in achieving optimal refractive outcomeswhich he thinks the femtosecond can help to overcome. "With new technology and new interest I believe that we can improve outcomes in a significant way to achieve emmetropia in patients undergoing cataract surgery," he concluded.

Editors' note: Drs. Alpins and Stamatelatos have financial interests with the Assort.com web calculators and the Assort.com surgical management systems. Dr. Stamatelatos also has financial interests with ASSORT Surgical Management Systems (Victoria, Australia). Dr. Donnenfeld has financial interests with Abbott Medical Optics, Alcon, Bausch + Lomb (Rochester, N.Y.), and WaveTec. Dr. Holladay has financial interests with AcuFocus (Irvine, Calif.), Alcon, Abbott Medical Optics, Oculus, Visiometrics (Terrassa, Spain), WaveTec, and Carl Zeiss Meditec (Jena, Germany). Dr. Koch has financial interests with Alcon and Abbott Medical Optics.

Contact information

Alpins: alpins@newvisionclinics.com.au
Donnenfeld: ericdonnenfeld@gmail.com
Holladay: holladay@docholladay.com
Koch: dkoch@bcm.tmc.edu
Stamatelatos: george@newvisionclinics.com.au

Operating on the laser's edge Operating on the laser's edge
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