R. Bruce Wallace III, MD and John A. Hovanesian, MD, FACS
Restoring multifocal vision with a predictable and safe procedure is now a reality. The steps to success include all of the topics discussed in this text, with astigmatism correction being an essential ingredient. All manufacturers of presbyopic intraocular lenses (IOL) have either developed or are in the process of designing toric models to correct corneal cylinder. As we have learned from our results with monofocal toric IOLs, toric presbyopic IOLs show great promise. Until the option is widely available, surgeons will continue to depend on corneal reshaping to reduce astigmatism with spherical IOLs. Even when toric versions of presbyopic IOLs become available in the United States, corneal procedures may still be used because of cost and inventory issues. This chapter will focus on the use of peripheral corneal relaxing incisions, commonly referred to as limbal relaxing incisions (LRI), which have been shown to safely and predictably provide permanent astigmatic correction.1
ORIGIN OF LIMBAL RELAXING INCISIONS
LRIs are probably the friendliest and most cost-effective refractive procedures we can offer our patients. There’s no expensive laser, no central corneal or intraocular trauma, and perforations are rare in healthy corneas. So why is it that many cataract surgeons are not yet using LRIs? Some of us are not convinced that they are reliable, especially if instruments were purchased and results were disappointing. For many, just the awkwardness of incisional corneal surgery along with an uncomfortable change in routine for surgeon and staff have placed LRIs in a negative light. Yet judging by the swell in attendance at teaching events like LRI wet labs at the last few American Academy of Ophthalmology and American Society of Cataract and Refractive Surgery meetings, LRI popularity continues to grow.
We owe a great deal of thanks to early pioneers who promoted the benefits of combining astigmatic keratotomy with cataract surgery many years ago. A partial list would include Drs. Gills, Hollis, Osher, Maloney, Shepherd, Koch, Thornton, Gayton, Davison, and Lindstrom. Dr. Robert Osher has advocated peripheral relaxing keratotomy at the time of cataract surgery since 1983, learning the principles of the technique from Dr. George Tate.2
I have had the pleasure to teach LRI techniques for over 20 years. During these training sessions, I have learned the steps necessary to convince cataract surgeons that LRIs can be an important part of refractive cataract surgery. Before a cataract surgeon transitions to the routine use of LRIs, he or she must understand the benefits, be confident in the system of treatment, and be confident with his or her technique.
ASTIGMATISM IN THREE DIMENSIONS
In order to reduce unwanted astigmatism, the surgeon must lead the way in his or her practice to develop a systematic approach to surgical correction. Reducing astigmatism begins with effective preoperative assessment. Most cataract surgeons depend on trained technicians to perform preoperative astigmatism measurements, which include refraction, keratometry and videokeratography, or corneal topography. Unfortunately, most technicians do not think about astigmatism in 3 dimensions because these measurements only generate numbers or 2-dimensional color maps. For technicians and surgeons to be effective in astigmatism control, it is helpful to understand and visualize astigmatism, especially corneal astigmatism, in 3 dimensions. Such terms as the flat axis, the steep axis, and coupling become easier to grasp when thinking of corneal shapes rather than numbers or colors.
To determine whether your office staff perceives astigmatism in 3 dimensions, try this experiment. Ask your best-trained technicians to imagine that the oblong curvatures of an American football represent the astigmatic corneal surfaces of a patient’s eye with the curvature in one axis steep, the other flat. Imagine that the football is lying flat on the ground horizontally. Would that resemble with-the-rule or against-the-rule astigmatism? If they answer with-the-rule, they are correct and are probably thinking about astigmatism in 3 dimensions. With this fundamental understanding of what the term regular astigmatism means, all members of the surgical team will find astigmatism correction easier to understand.
SURGICAL PLANNING
The goal for astigmatism control should be the creation of a resultant cylinder of 0.5 diopter (D) or less at any axis. Most patients enjoy good unaided visual acuity with this degree of astigmatism.3 Some studies suggest a benefit to leaving some amount of residual against-the-rule cylinder so that uncorrected near vision after cataract surgery is improved.4 However, surgical practices using multifocal IOLs and/or monovision will not find this to be an advantage, because of the compromise of the loss of distance visual acuity with amounts over 0.5 D of cylinder.5
One of the more challenging tasks that the surgeon faces is deciding which astigmatic preoperative measurements should be used when planning a surgical correction. Do we depend on the cylinder diopters and axis from the refraction, the keratometry, or do we always need to perform corneal topography? One study showed the frequency of poor correlation of all 3 methods of measurement, especially with less than 2.0 D of astigmatism.6 Fortunately, unlike correction of spherical refractive errors, astigmatism correction is more forgiving, especially when treating moderate to low levels.
One way to plan surgical correction of astigmatism is to initially assess the refraction and the keratometry simultaneously. If good correlation exists as to the amount of cylinder and axis, the surgical planning for astigmatism correction during cataract surgery is fairly straightforward. If, however, there is poor correlation (even though keratometry should be more reliable), surgical correction can be less predictable, even with corneal topography. This is where the art of astigmatism correction applies. The surgeon needs to also judge the relative reliability of the astigmatic information. If, after careful consideration, there is doubt as to a reasonable surgical plan, the astigmatism correction should be postponed until after cataract surgery and adequate time for incision healing.
The recent improvements in computerized corneal topography provide more dependable preoperative astigmatic parameters and also signal important warnings, such as irregular astigmatism and forme fruste keratoconus. More reliable keratometry readings from biometers such as IOLMaster Series 700 (Carl Zeiss Meditec) and Lenstar LS 900 (Haag-Streit) continue to replace traditional manual keratometry.
POSTERIOR ASTIGMATISM
As Douglas Koch first pointed out, 87% of eyes have posterior corneal astigmatism that is not measured by keratometers, optical biometry devices like the IOLMaster or Lenstar, or Placido-based topographers. This posterior astigmatism has an optical effect similar to adding 0.5 D of against-the-rule astigmatism (ie, a cornea that is steeper at 180 degrees than it appears on the above measurements). This posterior corneal astigmatism does not drift with age, so for surgical planning, we can add this astigmatism to that measured by the traditional techniques mentioned previously.7
Another effect we may want to consider when planning surgery is the predictable drift of astigmatism with age described by Hayashi.8 Among the aged cataract surgery population, most patients will over 10 years have an axis shift of their astigmatism toward the horizontal axis amounting to about 0.4 D.
So how do we incorporate these 2 effects into surgical astigmatism planning? Most experts advocate the following approach:
▲ When correcting with-the-rule astigmatism (corneas steep near 90 degrees), correct 0.5 to 0.7 D less than measured on the corneal surface.
▲ When correcting against-the-rule astigmatism (corneas steep near 180 degrees), correct 0.5 D more than measured on the corneal surface.
▲ When correcting oblique astigmatism, make no changes.
A perhaps oversimplified way of looking at these corrections is to simply ignore the surgically induced astigmatism (SIA) of a temporal (180 degrees) clear corneal astigmatism, assuming the surgeon’s average SIA is about 0.5 D.
Many current online astigmatism planning tools do not automatically make these adjustments for posterior corneal astigmatism, so surgeons must manually account for them in planning. However, intraoperative wavefront aberrometry measurements do not require adjustment, because these devices measure the optics of the entire eye, including the posterior cornea.
Here are 2 examples of compensating for posterior corneal astigmatism in an LRI procedure.
Example 1: A patient’s IOLMaster reading shows keratometry values of 44.00 at 90 degrees and 43.20 at 180 degrees or about 0.8 D steep at 90 degrees. The surgeon plans a temporal clear corneal incision that will typically induce 0.5 D of flattening at 180 degrees. In this case, the phaco incision’s SIA of 0.5 D will negate all but 0.1 D of the patient’s presumed posterior corneal astigmatism of 0.6 D steep at 180 degrees.
Keratometric astigmatism: 0.8 D at 90 degrees
+ Posterior cornea: -0.6 D at 90 degrees (same as 0.6 steep at 180 degrees)
+ Surgically induced correction: +0.5 D at 90 degrees (same as 0.5 D flattening at 180 degrees)
= Astigmatism to correct: -0.7 D at 90 degrees
In this case, the resultant astigmatism might be corrected by 2 paired 30-degree corneal arcs, centered at 90 degrees.
Example 2: A patient’s IOLMaster reading shows keratometry values of 43.00 at 180 degrees and 42.00 at 90 degrees or about 1.0 D steep at 180 degrees. The surgeon plans a temporal clear corneal incision that will typically induce 0.5 D of flattening at 180 degrees. In this case again, the phaco incision’s SIA of 0.5 D will negate all but 0.1 D of the patient’s presumed posterior corneal astigmatism of 0.6 D steep at 180 degrees.
Keratometric astigmatism: 1.0 D at 180 degrees
+ Posterior cornea: +0.6 D at 180 degrees
+ Surgically induced correction: -0.5 D at 180 degrees
= Astigmatism to correct: -1.1 D at 90 degrees
In this case, the resultant astigmatism might be corrected by 2 paired 35-degree corneal arcs, centered at 180 degrees.
LIMBAL RELAXING INCISION TREATMENT SYSTEMS
A systematic approach to LRI use improves results. A number of LRI nomograms have been developed by Gills, Lindstrom, Nichamin, and myself. I first used Dr. Nichamin’s excellent nomogram and then modified it to slant more toward one incision for lower levels of cylinder (Figures 14-1 and 14-2). Since we make our LRI incisions so far in the corneal periphery, paired incisions were not found to be as important for postoperative corneal regularity as traditional astigmatic keratotomy made at the 7-mm optical zone. An advantage of Nichamin nomograms and their modification is that treatment is planned in degrees of arc rather than cord length. With corneal diameters varying and the fact that we make arcs, not straight line incisions, degree measurements are universally more accurate.
Figure 14-1. The NAPA nomogram.
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