Tutorial
Introduction
Relaxing Incisions
Toric Intraocular Lenses
Summary
References
Slides
Correcting Astigmatism at the Time of Cataract Surgery
James P. Gills, MD · Michelle Van Der Karr, BA · Myra Cherchio, COMT
Whereas most patients who receive state-of-the-art sutureless cataract surgery have excellent unaided vision postoperatively, patients with significant preexisting astigmatism cannot achieve the same results without correction of their cylinder. Studies of astigmatism in cataract cases report a prevalence of 18% to 23% in patients with 1.5 D or more of keratometric cylinder and 9% to 12% of patients with 2 D or more of keratometric cylinder.1 Many of these cases benefit from reduction or elimination of their cylinder.
Although high astigmatism (more than 3 D) is rare, it can also be substantially reduced at the time of cataract surgery. While the cylinder may not be completely eliminated in very high cases, reducing the cylinder can improve not only unaided visual acuity, but also image quality.
Corneal Relaxing Incisions Corneal relaxing incisions (CRIs) have been used since the 1970s to reduce high preexisting astigmatism in cataract patients.1,2 Single or paired arcuate incisions should be placed concentric to the visual axis, at 99% of the peripheral pachymetry measurements, taking care not to perforate the cornea. Over the years, many surgeons have developed nomograms to improve the predictability of this technique.
Although CRIs remain a powerful tool for correcting high astigmatism, in our experience, CRIs have limited predictability and often result in overcorrections, especially in patients with lower-level astigmatism. We no longer employ CRIs as a "first-line" correcting technique; we reserve CRIs for patients with high astigmatism.
Limbal Relaxing Incisions Moving the relaxing incisions off the cornea to the limbus creates limbal relaxing incisions (LRIs).3,4 LRIs can be used with any type of cataract incision. LRIs have definite advantages compared with CRIs. Using LRIs results in more regular corneal topographies with less corneal distortion or irregularity and is effective in patients with low or moderate astigmatism (3 D or less). LRIs are easier to perform and more comfortable for the patient. LRIs are more "forgiving" than CRIs due to the placement and length of the incision. Precise placement "on-axis" is not as critical because the length of an LRI ranges from 4 mm to 10 mm. They are also more forgiving of variation in depth than CRIs. Postoperative refractions are less variable, and overcorrections are rare.4
We have found that extending the length of LRIs to 10 mm can add significant effect for cases with astigmatism of more than 3 D. However, customization of the surgical plan based on surgical keratometry, manual keratometry, and topography is essential. LRIs combined with CRIs placed near the limbus can correct even higher levels of astigmatism (up to 8 D).4
Incision Technique The amount, axis, and symmetry of the corneal cylinder are determined through keratometry and corneal topography. The refractive cylinder is not considered in phakic patients, because any lenticular astigmatism would be removed by the cataract surgery and thus cannot be included in the surgical plan. Topography is important to customize surgery. LRIs are created using an L320 micrometer knife (Lab Instruments) and are placed in the steep axis at the limbus just anterior to the palisades of Vogt. The number and length are determined according to the nomogram described below. The surgical keratometer is used to confirm the result.
The Gills nomogram for correcting astigmatism with LRIs is shown in the Table. This is a “starting point” nomogram that titrates surgery by length and number of LRIs. However, the length and placement can vary based on topography and other factors. The goal is to reduce cylinder power and absolutely avoid overcorrecting with-the-rule astigmatism, because against-the-rule astigmatism must be minimized. In cases with 0.5 D or less of cylinder, only an astigmatically neutral cataract incision is used.
Patients with low (less than 1.5 D) against-the-rule astigmatism (180°) receive only a single LRI in the steep meridian, placed opposite to the cataract incision. However, if astigmatism is higher than 1.5 D, a pair of LRIs must be used. In against-the-rule astigmatism cases, one pair of LRIs may be incorporated into the cataract incision. The length of the LRI is not affected by the presence of the cataract incision.
In low with-the-rule astigmatism cases, a single 6-mm LRI (0.6 mm in depth) is made at 90°. The LRI can be independent of the cataract incision in with-the-rule astigmatism cases (if the cataract incision is temporal and the LRI is superior).
A 6-mm relaxing incision generally corrects about 1 D of astigmatism for a 73-year-old patient. For more astigmatism, paired limbal relaxing incisions are used. If the patient has 2 D to 3 D vision, the pair of LRIs can be extended up to 8 mm. Four diopters can often be corrected or substantially reduced with a pair of 10-mm LRIs.
Limbal relaxing incisions can be effective in substantially reducing astigmatism even in high astigmatism cases. Two LRIs can be placed and lengthened to 10 mm to correct the first 4 D. Corneal relaxing incisions may then be added to provide additional correction. When used, CRIs are at 99% depth at the 8-mm or 9-mm optical zone, 2 mm in length for every diopter over 4 D. Added corneal relaxing incisions is reserved only for cases with higher levels of preexisting astigmatism, being careful not to overcorrect with-the-rule astigmatism cases.
We do take a conservative approach to astigmatism. It is better to make a long incision at the limbus than to use a small relaxing incision on the cornea. In cases of high astigmatism, there is often contributing pathology, such as corneal scarring or keratoconus. In these cases, the surgical effect is less predictable, so we are often conservative in planning surgery, preferring undercorrection. We can improve the quality of vision substantially in these high astigmatism cases, even if there is residual cylinder.
Results
We analyzed a series of mild to moderate astigmatic cases (less than 3 D) to evaluate the efficacy of the surgical
approach discussed above. Slide 1 shows the surgically induced cylinder versus
the preoperative keratometric cylinder. Most cases are within 1 D of desired
correction. Slide 2 shows the distribution of keratometric cylinder both
preoperatively and postoperatively. There is a substantial reduction in
cylinder for the cohort.
Twenty-one cases with preoperative keratometric CRIs. Surgery was planned conservatively in most cases. One case received an enhancement. Most cases had clear corneal incisions, but a few had scleral incisions.
The mean preoperative keratometric cylinder of 3.82 D was reduced to a mean of 1.64 D postoperatively. Mean percent reduction of keratometric cylinder was 58%. All cases were reasonably improved, most without the use of corneal relaxing incisions. Seventy-one percent of the cases had 20/40 or better uncorrected vision.
Implanting a lens with a toric correction provides an opportunity for reducing preexisting astigmatism without additional surgery. Many patients can have their astigmatism substantially improved without the added risk of complications from incisional surgery. Also, toric IOL implantation is not subject to regression of effect, as may happen with astigmatic keratotomy.
There are two disadvantages of a toric intraocular lens. First, the lens is currently available in only two cylinder add powers: 2 D with an effective correction of 1.4 D, and 3.5 D with an effective correction of 2.3 D, making it difficult to titrate the level of correction. Second, the lens occasionally rotates in the early postoperative period, which reduces the effective correction. A 10° shift reduces the effective correction of the lens by one third; a 30° shift negates the effect of the toric correction.5 The problem of rotation renders the lens unsuitable for patients with large capsular bags, as the lens can then freely rotate.
Rotation can be managed by adjusting the position of the lens as necessary during the postoperative period until the lens fibroses in the capsular bag; however, surgical rotation of the toric lens should be avoided for approximately 14 days to avoid the need for repeated procedures. To correct residual astigmatism, the lens is rotated toward the plus cylinder axis.
We have implanted a single toric IOL in
approximately 2,000 patients. We collected a consecutive series of 284
patients, with a mean follow-up of 41 days.
Slide 3 shows the preoperative
keratometric cylinder versus the cylinder induced by the cataract surgery with
toric implantation. Most cases are within 1 D of a complete correction of cylinder.
Slide 4 demonstrates the mean preoperative and postoperative cylinders.
Postoperatively, the keratometric cylinder is unchanged, because the toric lens
does not affect the cornea. The refractive cylinder is dramatically reduced.
Toric IOLs Combined with Relaxing Incisions The techniques of relaxing incisions and implanting toric IOLs can be combined to correct larger amounts of astigmatism. An advantage of using a toric lens is a reduction in the amount of incisional surgery required. For many cases of high astigmatism, adding a toric IOL allows the use of limbal incision rather than corneal relaxing incisions.
We have performed combined surgery on 11 eyes. Four eyes received a single LRI,
four received paired LRIs, two received a single CRI, and one received both an
LRI and a CRI. Preoperatively, the keratometric astigmatisms ranged from 2.62 D
to 10 D, with a mean of 5.04 D. Follow-up was 3 weeks to 5 months, with a mean
follow-up of 2 months. Slide 5 demonstrates the preoperative
keratometric cylinder versus the total amount of surgically induced cylinder.
All cases were within 1 D of desired correction of cylinder.
Slide 6 shows the
mean preoperative versus postoperative cylinder. The relaxing incisions
corrected approximately one half of the cylinder keratometrically, while the
toric lens corrected the remaining cylinder, resulting in a very low mean
postoperative refractive cylinder.
Slide 7 shows the corneal topography of the case
that received an LRI paired with a CRI combined with a toric lens. This patient
had 7.75 D of keratometric astigmatism. Using only astigmatic keratotomy, we
would have placed a pair of LRIs as well as CRIs to correct the astigmatism.
The single, 7-mm LRI and 3-mm CRI corrected approximately 6 D of keratometric
astigmatism. Using a toric lens allowed more correction of the astigmatism,
leaving the patient with a refraction of –0.25 – 0.75 x 95.
Piggyback Toric IOLs Recently, we have developed a new procedure that may eliminate the need for astigmatic keratotomy in most cases of high astigmatism. The procedure involves implanting two or more toric IOLs within the eye, in piggyback fashion. Although our results are preliminary, they are promising.
We have implanted piggyback toric IOLs in nine eyes, with follow-up of 1 to 75 days; mean follow-up was 1 month. Preoperatively, the mean keratometric cylinder ranged from 3.62 D to 5.88 D, with a mean keratometric cylinder of 4.48 D.
Slide 8 demonstrates the preoperative
keratometric cylinder versus the surgically induced cylinder. All cases were
within 1 D of desired correction.
Slide 9 shows the mean preoperative and
postoperative cylinders. Postoperatively, the mean keratometric cylinder was
unchanged, while the mean refractive cylinder was reduced to 0.69 D.
Since the advent of "astigmatism neutral" cataract surgery, preexisting astigmatism has been the limiting factor in achieving adequate unaided vision. Although both LRIs and the toric IOL produce reductions in preexisting astigmatism in low to moderate astigmatism cases, the optical quality of combining the toric IOL with LRIs appears to produce the best optical results. Both the combined toric/astigmatic keratotomy and toric piggyback procedures are still in their infancy. However, the early results show good predictability.
Posted April 2001
- Martin RG. Management of preexisting astigmatism. In: Gills JP, Martin RG, Sanders DR, eds. Sutureless Cataract Surgery: An Evolution Toward Minimally Invasive Technique. Thorofare, NJ: SLACK Inc; 1992.
- Troutman RC. Management of preexisting corneal astigmatism. In: Emery JM, Paton D, eds. Current Concepts in Cataract Surgery. St. Louis, Mo: CV Mosby; 1976.
- Gayton JL, Rowan P, Van Der Karr M. Cataract incisions at the steep axis. In: Gills JP, Martin RG, Thornton SP, Sanders DR, eds. Surgical Treatment of Astigmatism. Thorofare, NJ: SLACK Inc; 1994.
- Gills JP, Gayton JL. Reducing pre-existing astigmatism. In: Gills JP, Fenzl R, Martin RG, eds. Cataract Surgery: The State of the Art. Thorofare, NJ: SLACK Inc; 1998.
- Sanders DR, Grabow HB, Shepherd J. The toric IOL. In: Gills JP, Martin RG, Sanders DR, eds. Sutureless Cataract Surgery: An Evolution Toward Minimally Invasive Technique. Thorofare, NJ: SLACK Inc; 1992.