Incorporating Aspheric and Toric IOLs in Your Practice: Why and How?

Cataract surgery is a refractive procedure, and modern advances in that field have resulted in higher expectations from patients. Approximately 35% of the total cataract population has moderate to severe astigmatism, as defined by more than 1 D of cylinder, and 85% of those patients have 1 D to 2 D of cylinder. Recently available effective toric intraocular lenses (IOLs) provide patients with benefits that were previously difficult to obtain, and therefore their expectations have increased dramatically. As a result, there is a potential for placement of approximately 1 million toric IOLs each year.

Astigmatism must be correctly diagnosed prior to review of potential treatment options. Both phoropters and autorefractors can be used to diagnose astigmatism by refraction, and other techniques that can be effective include corneal topography and manual keratometry readings. Corneal astigmatism results from an irregularity of the shape of the cornea, causing improper focus of images on the retina. However, astigmatism can appear to be lenticular when it is assessed via refraction. Therefore, when considering methods for correcting astigmatism, an ophthalmologist should not consider refraction as an option. Refraction detects a combination of lenticular and corneal astigmatism, but only the corneal cylinder needs to be corrected because the lens is removed during cataract surgery.

Two toric IOLs are available: Staar (Monrovia, CA) and AcrySof (Alcon, Inc, Ft. Worth, Texas). The Staar model does not compensate for surgically-induced astigmatism, and during a Food and Drug Administration (FDA) clinical trial, 24% of the participants had their lenses rotated >10°, resulting in a fairly high repositioning rate of approximately 25%.¹ The Staar toric IOL also has large steps of 1 D at the corneal plane. In contrast, the AcrySof toric lens has small steps of 0.5 D, has a calculator that compensates for surgically induced cylinder, and has been shown to provide good precision and predictability outcomes.

In addition to assessing which toric IOL is most appropriate for treating surgically-induced astigmatism, the correct axis for implantation of lenses must be identified. Incisions themselves can also contribute to astigmatism, and therefore the ideal positioning of lens implants cannot be based on preoperative keratometric measurements alone. The AcrySof Toric IOL Calculator is specifically designed to identify the correct axis for implantation. The calculator utilizes vector analysis and compensates for surgically-induced astigmatism, and the data processed include preoperative manual keratometry, IOL power, location of incisions, and the estimated surgically induced astigmatism. This information helps surgeons adjust the locations of incisions, the orientation of the IOL, and the amount of dioptric power required.

Toric IOL Procedural Pearls

During toric IOL implantation, the surgeon performs a standard cataract procedure from capsulorhexis through phacoemulsification. The only difference is in the implantation of the lens itself. This begins with the IOL calculation. Proper alignment of the toric lens is critical for achievement of optimal results. Once the final position and power of the toric lens implant have been determined using the toric calculator, the eye is marked preoperatively. To aaccomplish this, a lid speculum is placed in the patient’s eye once the patient has been administered a local anesthetic and the eye has been dilated for surgery in the preoperative area. A marker is used to denote the 3, 6, and 9 o’clock positions with the patient sitting upright. These three dots will serve as orientation marks which aid in placing the atoric IOL in the proper position as cyclorotation frequently occurs when the patient is supine. Following removal of the cataract and utilizing the previously placed orientation marks, the cornea is marked intraoperatively with an axis marker denoting the steep axis of the cornea, i.e. the axis along which the toric IOL marks will be aligned. As the IOL is placed into the eye under an ophthalmic viscoelastic device (OVD), it is advisable to leave the IOL marks approximately 30° counterclockwise of the desired final position denoted by the corneal axis marks.

Following complete removal of the OVD, including from behind the IOL, an irrigating canula (used to keep the eye inflated) is placed through the paracentesis and a Sinskey hook is placed through the incision site or through another paracentesis. The lens can then be rotated and the dots of the toric IOL are oriented with the marks previously made that denote the desired axis.

AcrySof Toric IOL Clinical Data

An FDA study published in 2005 was designed to determine the safety and efficacy of AcrySof Toric IOL models SA60T3, SA60T4, and SA60T5 when implanted into the capsular bag.² This was a randomized, open label, parallel group, multi-centered study conducted in the United States that included 494 subjects. The results of that trial showed good rotational stability, with 97% of patients within 10° of the implantation axis and approximately 80% within 5° after six months (Figure 1). In addition, virtually 100% of the patients that received these implants no longer required spectacles for distance vision once the lenses had been implanted binocularly.

Figure 1. Rotational Stability of Toric IOLs Mean rotation for Toric IOL models was less than 4º with no lenses > 15º rotation at 6 months after surgery, indicating that these toric IOL models display excellent rotational stability. Source: U.S. Food and Drug Administration Center for Devices and Radiological Health. Summary of Safety and Effectiveness Data. Available at: http://www.fda.gov/cdrh/pdf/p930014s015b.pdf. Accessed January 15, 2009.

Aspheric IOLs

As individuals age, their visual acuity often deteriorates. The eyes of young people have, on average, zero spherical aberration, and visual performance usually peaks at the age of 19 (Figure 2).^3-7 At this age individuals also have maximal contrast sensitivity and quality of vision, and their eyes enable light to be sharply focused, thereby producing sharp, high-quality images.

However, as individuals age their spherical aberration may increase, resulting in reductions of functional vision (Figure 2).^8,9 Such deterioration of visual acuity is caused by crystalline lenses losing their ability to compensate for the positive corneal spherical aberrations thereby causing diffusion of light that results in blurred vision.

The goals of cataract surgery among older patients include employing a lens that can counteract positive spherical aberrations, thereby offering youthful functional vision. Eyes implanted with spherical IOLs have significantly higher spherical aberration compared to age-matched phakic eyes (Figure 2). Thus, while Snellen acuity may be improved after cataract surgery, functional vision may still be less than optimal.

Figure 2. Spherical aberration The eyes of young people have zero spherical aberration. As individuals age, their spherical aberration may increase, resulting in reduced functional vision. Traditional spherical IOLs may also induce spherical aberration. Source: Steven S. Lane, MD

Implanting an aspheric IOL can help surgeons offer better postoperative vision in patients with spherical aberrations. Such lenses enable focusing of marginal rays of light on the retina in aorder to improve visual acuity, particularly at night. There are several types of aspheric IOLs currently available, including the AcrySof IQ, Tecnis Z9000/9003, and Bausch & Lomb L161AO, each of which provides some unique aspects. AcrySof IQ adds 0.20 µm of negative spherical aberration for a 6 mm pupil, provides reduced optic center thickness compared to other spherical and aspheric IOLs, and offers both UV and yellow filtration properties. Both the Tecnis Z9000/9003 and Bausch & Lomb L161AO offer a thicker profile, multi-piece design, and UV light filtration properties; however, the Tecnis adds 0.27 µm of negative spherical aberration for a 6 mm pupil while the Bausch & Lomb L161AO adds no negative spherical aberration. Each of these factors should be considered when selecting lenses for patients. One of the key elements to consider is whether laser corneal refractive surgery has previously been performed. Individuals treated with hyperopic LASIK or wavefront-guided LASIK may have no spherical aberration and therefore should receive a lens that produces zero negative spherical aberration, such as the L161AO lens. In contrast, an individual with myopia corrected via non-wavefront guided treatment may require an IQ or Tecnis lens.

There are a few potential shortcomings of negative design aspheric IOLs that must be considered. The lenses can induce aberrations, especially coma, if they become decentered.¹⁰ As a result, it is essential that accurate centration of the IOL is assured during surgery and there is no displacement afterwards.

In conclusion, cataract surgery should be considered a form of refractive surgery, and eventually IOL refractive surgery may well surpass corneal refractive surgery as the most commonly employed procedure. Those practices that adopt these advances in equipment and techniques and then educate and engage their staff will be in the best position to provide the improved quality of life that our patients demand.

References

1. Holland E, et al. Journal of Cataract and Refractive Surgery. Accepted for publication.
2. U.S. Food and Drug Administration Center for Devices and Radiological Health. Summary of Safety and Effectiveness Data. Available at: http://www.fda.gov/cdrh/pdf/p930014s015b.pdf. Accessed January 15, 2009.
3. Artal P, Tabernero J, Piers P. Customized modelling to predict the optical quality in eyes implanted with different types of IOLS. Presented at ESCRS 2006.
4. Glasser A, Campbell, MC. Presbyopia and the optical changes in the human crystalline lens with age. Vision Research. 1998 Jan;38(2):209-29.
5. Smith G, Cox MJ, Calver R, Garner LF. The spherical aberration of the crystalline lens of the human eye. Vision Research. 2001 Jan 15;41(2):235-43.
6. Guirao A, González C, Redondo M, Geraghty E, Norrby S, Artal P. Average optical performance of the human eye as a function of age in a normal population. Investigative Ophthalmology and Visual Science. 1999 Jan;40(1):203-13.
7. Wang L, Koch DD. Ocular higher-order aberrations in individuals screened for refractive surgery. Journal of Cataract and Refractive Surgery. 2003 Oct;29(10):1896-903.
8. Guirao A, Redondo M, Artal P. Optical aberrations of the human cornea as a function of age. Journal of the Optical Society of America. 2000 Oct;17(10):1697-702.
9. Oshika T, Klyce SD, Applegate RA, Howland HC. Changes in corneal wavefront aberrations with aging. Investigative Ophthalmology and Visual Science. 1999 Jun;40(7):1351-5.
10. Altmann GE, Nichamin LD, Lane SS, Pepose JS. Optical performance of 3 intraocular lens designs in the presence of decentration. Journal of Cataract and Refractive Surgery. 2005 Mar;31(3):574-85.