During cataract surgery, you can correct corneal astigmatism by toric IOL implantation, limbal relaxing incisions (LRIs) or a combination of the two. A new toric IOL will soon be on the market, and clinical nomograms are making LRI results even more reliable. Here, two experts share guidelines for choosing the best approach.
Toric Intraocular Lenses
Currently, Staar Surgical manufactures the only toric lens available in the United States. This is a silicone plate-haptic IOL with two cylindrical powers: a 3.5-D optic that corrects 2.4 D in the corneal plane, and a 2.0-D optic that corrects 1.3 D in the corneal plane. (For low powers, corrections in the spectacle plane are almost the same as corrections in the corneal plane.)
To succeed with a toric IOL, you must align the lens with the appropriate steep meridian of the astigmatism. Unfortunately, plate-haptic IOLs have a tendency to rotate. “If the lens rotates 31 degrees, there is essentially no reduction in astigmatic power, and the axis of residual astigmatism shifts to a new meridian, often oblique,” said Kevin M. Miller, MD, professor of clinical ophthalmology at the University of California, Los Angeles. “If a lens were to rotate 90 degrees off axis, the patient’s astigmatism could actually double.”
A new option. The Alcon AcrySof toric lens is nearing the end of FDA clinical trials and will likely become available this year. This acrylic open-haptic lens will be available in three cylindrical powers: 1.5 D, 2.25 D and 3.0 D in the IOL plane.
“It is identical in appearance to the single-piece SA60 AcrySof lens, and implantation is the same,” said Louis D. Nichamin, MD, medical director of the Laurel Eye Clinic, Brookville, Pa., and an investigator in the clinical trials. “The data from the FDA trial indicate that the AcrySof toric lens rarely rotates more than 5 degrees,” Dr. Nichamin said.
While this is good for long-term results, aligning the IOL during surgery is more difficult. “With practice, one learns to nudge the implant into position using two manipulators through side-port incisions, or using the irrigation/aspiration] hand piece, with infusion on,” Dr. Nichamin said, noting that the active flow rate keeps the lens more mobile in the capsular bag.
In general, the AcrySof toric IOL tends to rotate more easily clockwise, he said. When inserting it, you try to leave the lens slightly counterclockwise from where you want it to rest. At the end of the surgery, you can dial it into place clockwise.
“You can document the alignment of your toric lenses by overlaying a stock photograph of the toric lens on the topography map,” added Dr. Miller.
Is centration enough? The toric IOL must also be aligned with the visual axis. Surgeons usually judge centration of an implant by its position relative to the pupil. “This begs a subtle but important question,” Dr. Nichamin said, “because the center of the pupil does not necessarily coincide with the visual axis.”
In many patients, Dr. Nichamin explained, the line of sight in relation to the center of the pupil (the angle kappa) is positive, which means that the line of sight is nasal to the pupillary center. “One might have a perfectly centered toric implant, but the patient’s line of sight may not correlate with the center of that implant,” he said. “Whether or not this is significant depends on the amount of angle kappa.”
“We will have to study toric lenses vs. limbal relaxing incisions using tools such as wavefront aberrometry and comparative studies to assess their respective effects on vision,” Dr. Nichamin said.
Limbal Relaxing Incisions
Another option for correcting astigmatism during cataract surgery is peripheral corneal arcuate incisions, or LRIs. Dr. Miller noted that peripheral incisions are preferred over central incisions because the same incision can be used for phacoemulsification, and there is less pain and less induced irregular astigmatism.
Nomograms. Dr. Nichamin has been performing LRIs for more than 10 years and has developed two nomograms, which specify the use of LRIs according to the type of astigmatism and the patient’s age. The standard Nichamin nomogram, a conservative approach, does not use pachymetry or adjustable blade-depth settings, but rather an empirical blade depth of 600 micrometers.
His more aggressive age-and-pachymetry-adjusted nomogram (see Table, on page 23) is designed for use in younger refractive lens exchange patients, or in conjunction with LASIK for patients with high astigmatism. “With these patients,” Dr. Nichamin explained, “we try more aggressively to eliminate every last bit of astigmatism. We perform ultrasound pachymetry over the intended incision site, and we set an adjustable diamond blade at 90 percent of the thinnest reading obtained.”
Dr. Miller uses a nomogram of his own that is based on diopters of corneal astigmatism and on whether the bowtie appearance on corneal topography is symmetrical or asymmetrical. Incision lengths are specified in clock hours rather than in millimeters or degrees.
Whichever nomogram a surgeon chooses, Dr. Nichamin suggests using a diamond blade dedicated for this technique. “With a steel blade or an older radial keratotomy blade, one might achieve less effect,” he said.
Where to start. Dr. Nichamin will correct as little as 0.75 D, if the refraction is well documented and the same on K-readings, topography and refraction measurement. “We need to get patients down to plus or minus half a diopter for both sphere and cylinder,” he said.
He added that he “might be a little less aggressive with an elderly nursing home patient. We don’t want to overcorrect, to flip the astigmatism or to have it shift away to a different axis. In the older population, we are doing the patient a service just by reducing the astigmatism, getting it close.”
Slow LRI adoption. Although the technique is well within the purview of cataract surgeons, Dr. Nichamin noted, they have been slow to adopt the use of LRIs. The most challenging aspect of astigmatism correction is measuring the astigmatism and devising a strategy, he said. “It can be frustrating that the refraction and standard keratometry and even topography may not correlate. We increasingly rely on corneal topography as our guide.”
There may be a bad aftertaste from the RK days, Dr. Nichamin said, but he noted that when LRIs are performed properly, the chance of having problems like those seen with radial incisions is low. “In my practice, I have not seen downside effects such as dry eye, corneal denervation or instability to the globe.”
Combining the Approaches
For low astigmatism, up to about 1.5 D, Dr. Miller prefers LRIs. For moderate astigmatism, 1.5 to 3 D, he prefers toric lenses. For astigmatism greater than 3 D, he combines LRIs with toric IOLs. “When higher-power toric lenses become available, they will be my preference,” he said.
Depending on the patient’s age, Dr. Nichamin can correct between about 2 and 4 D with LRIs. A young patient might have only 2 to 2.5 D of astigmatism; a cataract-age patient, 3 to 4 D. “My nomogram goes up to incisions of 90 degrees arc length,” he said. “I will not go beyond that. In cases of higher astigmatism, I combine the use of a toric lens and LRIs.”
In patients with very high astigmatism, Dr. Nichamin has used the stronger 3.5-D Staar toric lens in conjunction with LRIs and subsequent bioptic LASIK. “It is gratifying to take patients with 7, 8 or 9 diopters of astigmatism and bring them down to 1 diopter,” he said.
Dr. Miller is an investigator for Alcon and is participating in the toric IOL clinical trials. He also has a small stock interest in Staar Surgical. Dr. Nichamin is participating in the clinical trials, but he has no financial interest in Alcon or in any of the LRI instrument makers.
|Toric IOLs |
- The lenses take a phacoemulsification incision, so recovery is quick.
- The incision is unlikely to induce irregular astigmatism.
- If new lenses come out, there is potential for correcting high amounts of astigmatism, possibly up to 10 D.
- Toric lenses don’t correct astigmatism at the source (in the cornea), so distortion may be induced, even if the astigmatism is fully nullified.
- Residual astigmatism with toric lenses is usually oblique.
- Toric lenses are not useful for correcting asymmetric bowtie astigmatism.
- The technique is easy to learn and perform.
- LRIs take a minute to do, adding minimal time to cataract surgery.
- LRIs correct the problem at the source (in the cornea).
- Results are predictable for low dioptric corrections.
- Incisions can’t rotate like IOLs.
- LRIs can work well for asymmetric corneal astigmatism.
- The incisions are longer and possibly more irritating to the patient than a phaco incision would be.
- Large incisions are less predictable, occasionally gape, can be difficult to hydrate and sometimes require sutures.
- LRIs cannot be used in a patient with keratoconus.