Why do you need to learn how to perform limbal-relaxing incisions? Limbal-relaxing incisions (LRIs) are used alone and in combination with implantation of toric and multifocal/presbyopia IOLs.
As a resident, or if you are new to this technology, your best candidates are patients with astigmatism levels of 1.00 to 2.00 diopters of cylinder. You can quickly determine the degree of astigmatism by looking at your manual keratometry readings, then perform a corneal topography to confirm the axis and degree of astigmatism and to rule out any abnormal pathology, such as early keratoconus or irregular astigmatism.
For beginning surgeons, it is also best to become comfortable with astigmatism that is steeper near the 90 degree meridian (with-the-rule) before attempting to correct astigmatism that is against-the-rule. The reason for this is that many of the nomograms require paired-incisions, which means that, for a surgeon operating through a temporal wound, the LRI may overlap the wound. This can cause wound leak if you are not comfortable with the equipment, thus it is best to avoid incisions placed near your main wound while learning the technique.
The next step is to become familiar with one of the many LRI nomograms that are available to you. One option we use is the Nichamin nomogram. One company has also created a free, online LRI calculator. We recommend you create a reminder note for the nurses to inform them that the patient will need an LRI on the day of the surgery.
The Day of Surgery
The patient should first be marked in the sitting position, prior to allowing the patient in the operating room. Noting reference marks at the three and nine o’clock positions is critical because cyclotorsion can occur when the patient is supine, which can affect the axis of reference. Some also place a reference mark at the six o’clock position.
The reference-marking technique is one that you will have to master as you progress to using toric and multifocal/presbyopia-correcting IOLs. The other elements to consider in the surgical plan include:
- The number of paired incisions, which you get from the nomogram;
- The location of the main incision;
- The axis;
- The size of arc; and
- The depth of the relaxing incisions.
Accurate corneal pachymetry is imperative. The normal central corneal thickness is approximately 550 microns; the normal peripheral corneal thickness is approximately 700 microns. If central corneal thickness is less than 515 microns, you should consider using a shorter LRI blade, such as a 500 micron blade. (The standard LRI blade depth for most nomograms is 600 microns.) Alternatively, you can use an adjustable diamond blade.
In the Operating Room
Once you are in the operating room, I highly recommend that you place your surgical plan in an accessible place, such as the arm of your microscope, for quick reference. In my experience, it is always best to start performing the LRI incisions prior to making any perforating incisions to keep that globe as firm as possible.
Place viscoelastic material on the cornea in the areas where you intend to create the LRIs. Once you have confirmed the axis and the central pachymetry, perform the LRI incisions by pressing firmly against the globe, holding the blade parallel to the cornea. Immediately, check for micro perforations and depth of your incisions.
At this point, tell yourself that the rest of the procedure is just your standard “phaco.” At the one-week, post-op visit, repeat the corneal topography and a quick refraction to assess your results.
After three to four cases, you will begin to feel comfortable with the technique, which can be easily combined with a toric IOL to correct high degrees of astigmatism. These will be your happiest patients!!
Tests Needed Prior to Performing an LRI
There are several tests you’ll need to perform prior to using an LRI. They include:
- Keratometry to identify the magnitude and meridian of astigmatism. This may be done with either a manual keratometer or an automated keratometer (such as the one built into the IOL master). Manual keratometry has the advantage of enabling you to subjectively identify the quality of the mires; however, the disadvantage is it that you are prone to operator error. Either method is acceptable, but it’s important to be consistent with your approach.
- Corneal Topography to identify the amount and location of astigmatism; additionally used to identify any evidence of corneal abnormalities, including keratoconus and irregular astigmatism that could represent dry eyes and/or anterior basement membrane dystrophy. Also, evaluate for pellucid marginal degeneration; do not perform an LRI if this is suspected based on topography.
- Corneal Pachymetry to measure cornea thickness. If the cornea is less than 515 µm, then a different blade is used (500 or 550 µm). If the corneal thickness is greater than 515 µm, then use standard blade (600 µm depth blade). If corneal pachymetry is less than 490 µm centrally, avoid LRI due to increased risk of corneal perforation.
Conditions to Be Aware Of
Use caution when performing LRIs on patients with dry eye syndrome. Their astigmatism can be variable due to irregular surface changes and you can easily over or under correct astigmatism with your LRI. Also, these cuts in the peripheral cornea can exacerbate dry eye symptoms.
Carefully check the cornea on slit lamp exam for signs of anterior basement membrane dystrophy. These corneas may look relatively normal on topography, but this astigmatism can be difficult to correct with either LRIs or toric IOLs.
Pellucid marginal degeneration is typically noticeable on corneal topography. Pellucid corneas will be thin peripherally, and LRIs should be avoided.
Avoid LRIs for patients with keratoconus or forme frust keratoconus. If the cornea is thicker than normal, LRIs will not have as much effect as predicted on nomogram. Consider making the arc slightly longer.
Younger patients tend to heal quickly following LRIs, which can lessen the astigmatism-correcting effect. Increasing topical steroid use postoperatively may slow the healing process enough to retain the astigmatism correction.
Also, there is a slight increased risk when performing LRIs, as they are designed to penetrate to Descemet’s membrane in order to have the astigmatism-altering effect. Descemet’s membrane is not impervious to bacteria, so there is a chance of developing endophthalmitis from an LRI incision.
For this reason, we do not perform LRIs on monocular patients, and all patients who are receiving an LRI are provided with an informed consent so they can know the risks, benefits and alternatives to this refractive procedure.
Eyes with prior refractive surgery, such as LASIK or PRK, may have LRIs at the time of cataract surgery, but approach with caution, as adding LRIs is one more way of reducing the predictability of the complicated post-refractive surgery IOL calculation. It is advised that you reassess the amount of astigmatism postoperatively and then correct as needed.
Finally, we have found that OCB Ks tend to over predict with-the-rule corneal astigmatism. We rely on an average of the OCB Ks and the topography Ks to get an accurate assessment of the amount of astigmatism. If the axis differs by more than 15 degrees between the two readings, we repeat both measurements.
Common LRI Mistakes
There are a few pearls that are key to performing LRI successfully. The most common mistake novice surgeons encounter is to operate on the wrong axis. Always perform the LRIs on the steep axis.
The second most common mistake is to perforate the cornea. The ideal depth of the LRI incisions is 90 percent of the thinnest pachymetry reading from the periphery of the cornea. The incisions are placed on the clear cornea just inside the surgical limbus.
Therefore, when you obtain the pachymetry readings, place the tip of the pachymeter where you expect to make the incision. The pachymetry reading is one step that Dr. Montoya will not delegate. Another important factor in preventing corneal perforations is using an adjustable diamond blade instead of preset blades.
A final but important pearl pertains to marking the cornea and what type of marker to use. We started with a simple double-axis marker used in conjunction with a Mendez marker to mark the desired axis and arc. Currently, we use the Chu LRI marker, which has preset marks at 30/45/60 degrees of arc. This saves some time and increases the accuracy of your markings.
Also, there are cases where it is not obvious that an LRI is needed to reduce the astigmatism. Consider a 56-year-old patient with keratometry readings of 44.00 x 44.75 @ 90. Assuming that your main clear corneal incision is temporal and your surgically induced astigmatism is 0.50 diopters, your final induced astigmatism would 1.25 diopters.
In this case, an LRI would be required, even though the keratometry readings indicate 0.75 diopters of cylinder. However, if you take this same patient and decide to make your main incision on the steep axis, your final astigmatism would be 0.25 diopters, which obviates the need for an LRI.
LRIs are a simple, effective and cost-efficient method of treating astigmatism intraoperatively. LRIs can be used on standard and premium IOL cataract patients. Furthermore, they can be combined with toric IOLs to reduce larger amounts of astigmatism successfully.
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About the author: Carlos F. Montoya Jr., MD, attended the University of California, Irvine Medical School, and completed his training in ophthalmology at Nassau County Medical Center/Stony Brook University Hospital in New York. He is currently in private practice in Los Angeles, Calif.
Robert F. Melendez, MD, MBA, is a partner at Eye Associates of New Mexico and assistant clinical professor at the University of New Mexico. Additionally, Dr. Melendez is author of Ophthalmology Buzzwords™, co-founder of the Juliette RP Vision Foundation and editor and chair of YO Info.
Anne Dwyer is the anterior segment services coordinator at Eye Associates of New Mexico. She has a bachelor's degree in business management and is a certified ophthalmic medical technologist, ophthalmic surgical assistant and ophthalmic coding specialist.
Drs. Montoya and Melendez do not have any financial interests in any of the products described in this article.