Advances in cataract surgery and lens replacement have continued to push toward spectacle independence after cataract surgery. As this trend continues, we are faced with addressing the obstacles to spectacle independence. There are an estimated 15-20% of individuals with greater than 1.5 diopters (D) of cylinder1 and a much higher percentage with less than 1.5 D of cylinder. For these individuals, achieving spectacle independence and improved quality of vision after cataract surgery requires addressing their astigmatism either at the time of surgery or shortly thereafter. There are a number of techniques available for addressing corneal astigmatism including placement of clear corneal incisions (CCI), limbal relaxing incisions (LRI), corneal relaxing incisions (CRI), and toric intraocular lenses, and photorefractive keratectomy as part of a bioptics approach.
Our experience with keratorefractive surgery has shown that a residual astigmatism of 0.75 D may leave a patient symptomatic with visual blur, ghosting, and halos.2 Therefore, effort should be made to reduce residual astigmatism to less than 0.75 D. In addressing astigmatism, several factors such as age, axis of astigmatism, and magnitude of astigmatism must be considered. Other considerations include the against-the-rule drift that occurs throughout one's lifetime. Hence, some advocate a less aggressive approach toward with-the-rule astigmatism. Some consideration should also be given to astigmatism and how it relates to uncorrected distance and near vision. There is some thought that with-the-rule astigmatism may favor uncorrected distance vision and against-the-rule astigmatism favors near. Given these considerations, one can design a specific treatment plan that would work in favor of each patient's need.
Incision Techniques
Modern phacoemulsification techniques with small incisions have reduced the amount of surgically induced astigmatism. Addressing astigmatism with placement of the CCI over the steep meridian can reduce astigmatism by 0.25 D at 6 months using a 3.5 mm incision.3 Therefore, adjusting the CCI alone can be awkward for the surgeon and have minimal effect. The most popular of the incisional techniques are LRIs, which can be performed either during the cataract surgery or postoperatively. There are several LRI nomograms that are available and treat up to 4 D of astigmatism.4 LRIs are typically performed using a guarded 600 µm blade. The groove is made over the steep meridian immediately anterior to the limbus in a circumferential fashion. The length is based on a nomogram, which usually takes into account the patient's age plus or minus other variables. In general, a 6-mm incision results in approximately 1 D of flattening.4 LRIs of less than 90 degrees do not require a change in the power of the intraocular lens.2
CRIs are placed concentric to the visual axis over the steep meridian. CRIs have nomograms similar to LRIs, with the added variables of distance from the optical center and depth of incision. Because CRIs are placed near the optical center, they decrease optical quality and can cause glare. Their central location also increases healing time, discomfort, and the risk of perforation. Given the disadvantages of CRIs and the fact that LRIs are easier to perform and more forgiving, LRIs should be the preferred method.
Toric Intraocular Lenses
Two toric intraocular lenses (IOL) are currently available in the United States: Staar Toric IOL from Staar Surgical and Acrysof Toric IOL from Alcon Laboratories. Both lenses correct for aphakia and corneal astigmatism. The Staar Toric IOL was approved by the Food and Drug Administration (FDA) in 1998. It is available in 2 cylinder powers, 2.00 D and 3.50 D at the IOL plane, which correspond to 1.40 D and 2.30 D at the corneal plane. It is a single-piece silicone plate haptic that is injected using a MSI-TR or MSI-PR injector through a 2.8-mm incision. In studies, the Staar Toric IOL had greater than 10% of implants undergo significant rotation of greater than 10 degrees.5
The Acrysof Toric IOL was approved by the FDA in 2005. It is available in 3 cylinder powers, 1.50 D, 2.25 D, and 3.00 D at the IOL plane, which correspond to 1.03 D, 1.55 D, and 2.06 D at the corneal plane. It is a single-piece acrylic polymer that is injected using a Monarch II injector through a 3.0-mm incision. A multicenter trial showed that the Acrysof Toric IOL is rotationally stable; the average lens rotation was less than 4 degrees over a 6-month period.6 In this study, 97% of Acrysof Toric patients achieved 20/25 or better uncorrected visual acuity (UCVA) compared with 77% of patients that received a control lens (SA60AT).
The placement of either toric IOL requires a few minor modifications from routine cataract surgery. It begins with lens selection, which takes into account the surgeon's induced astigmatism. There are online calculators that help in this process. With the patient sitting up in the preoperative area, the eye is marked at the 3 and 9 o'clock positions. Cataract surgery proceeds normally until the toric IOL is placed into the capsular bag. At that point, the IOL is grossly aligned 15-20 degrees counterclockwise from the desired axis. During viscoelastic removal, it is imperative to remove all the viscoelastic to minimize postoperative lens rotation. The final step is the rotation of the lens to its final position.
As mentioned earlier, postoperative IOL rotation can occur and significantly degrade the toric effects. Every degree off axis results in a loss of approximately 3.3% of the cylinder power.7 If significant postoperative rotation occurs, repositioning should occur within one week before a bioadhesive bond forms between the IOL and the posterior capsule. Once this occurs, the next step should be an unplanned bioptics approach.
Bioptics Approach
The original description of bioptics by Zaldivar8 was the combination of a phakic IOL followed by laser-assisted in situ keratomileusis (LASIK) for high myopia. Later, others would describe this approach for correction of residual refractive error after posterior chamber IOL (PCIOL) placement. Initially, concern regarding endothelial safety and IOL dislocation during the suction phase of flap creation lead to flap creation prior to cataract surgery. Subsequently, it has been demonstrated that no endothelial damage occurs, which has lead to flap creation and LASIK on the same day postoperatively. LASIK is typically performed once a stable refractive error is achieved. This may be as early as 6 weeks after cataract surgery,2 but usually takes 2 to 3 months. In studies, 82% to 100% of eyes treated in a bioptics approach with LASIK after PCIOL placement ended up with less than 1 D of residual error.9
Maloney proposed photorefractive keratectomy (PRK) for residual refractive error after PCIOL placement.10 Its success has been documented in several series. Some advocate PRK versus LASIK, reporting that with increasing age comes decreasing corneal sensitivity and therefore less postoperative discomfort. Secondly, dry eye is a common compliant among these individuals, which is exacerbated by LASIK.9
Bioptics can take the form of an unplanned fine-tuning of an imprecise primary surgery, an unplanned repair of surgically induced astigmatism or wrongly powered IOL, or a planned approach to address high refractive errors when the data supports that residual refractive error/astigmatism will occur. A bioptics approach can also be used in conjunction with toric IOLs and LRIs.
Conclusion
There are several techniques available for addressing corneal astigmatism in the face of cataracts. As outlined above, each has its own set of advantages and limitations. The decision of which one of these techniques to use will of course dependend on the surgeon's comfort with each technique and the patient's expectations. Keep in mind that as patient's expectations increase, there may come a time when these consumers ask for spectacle independence after cataract surgery.
References
1. Hoffer KJ. Biometry of 7,500 cataractous eyes. Am J Ophthalmol. 1980;90(3):360-368.
2. Nichamin LD. Astigmatism control. Ophthalmol Clinical N Am. 2006;19(4):485-493.
3. Kaufmann C, Peter J, Ooi K, et al. Limbal relaxing incisions versus on axis incisions to reduce corneal astigmatism at the time of cataract surgery. J Refract Surg. 2005;31(12):2261-2265.
4. Tehrani M, Dick HB. Incisional keratotomy to toric intraocular lenses: an overview of the correction of astigmatism in cataract and refractive surgery. Int Ophthalmol Clin. 2003;43(3):43-52.
5. De Silva DJ, Ramkissoon YD, Bloom PA. Evaluation of toric intraocular lens with a Z-haptic. J Cataract Refract Surg. 2006;32(9):1492-1498.
6. Lane SS. The Acrysof Toric IOL's FDA trial results: a look at the clinical data. Cataract & Refractive Surgery Today. May 2006;66-68.
7. Horn JD. Status of toric intraocular lenses. Curr Opin Ophthalmol 2007;18(1):58-61.
8. Zaldivar R, Davidof JM, Oscherow S, et al. Combined posterior chamber phakic intraocular lens and laser in situ keratomileusis: bioptics for extreme myopia. J Refract Surg. 1999;15(3):299-308.
9. Leccisotti A. Bioptics: where do things stand? Curr Opin Ophthalmol. 2006;17(4):399-405.
10. Maloney RK,Chan WK, Steinert R, et al. A multicenter trial of photorefractive keratectomy for residual myopia after previous ocular surgery. Summit Therapeutic Refractive Study Group. Ophthalmology. 1995;102(7):1042-1052.
11. Gills JP, Van Der Karr M, Cherchio M. Combined toric intraocular lens implantation and relaxing incisions to reduce high preexisting astigmatism. J Cataract Refract Surg. 2002;28(9):1585-1588.
Author Disclosure
The author states that he has no financial relationship with the manufacturer of any product discussed in this article or with the manufacturer of any competing product.