As we age, so do our eyes. One part of this process is an increase in the total spherical aberration found within the eye. Most—or in fact all—of this increase is because the crystalline lens gains positive spherical aberration over time. This is due to an increase in the size and volume of the lens, combined with some degradation of the lens proteins. Thus, it is the crystalline lens that deterorates with age. By the time that someone gets a cataract, the total spherical aberration within the eye is quite high.

If we replace the cataractous crystalline lens with what is currently perceived as a standard IOL, we are more or less just regressing to the same optical quality that the patient had preoperatively. We treat a lot of baby boomers, and their optical quality is weaker than it was even 20 years ago. An option to combat the loss of optical quality is to use an aspheric IOL. Such IOLs have a changing curvature from the center to the periphery, producing a crisper point focus. The concept is to restore vision to that of a 20- to 30-year-old.

Standard IOLs add positive spherical aberration to the spherical aberration already found on the cornea. In turn, the total spherical aberration worsens, and blurred vision, halos, and nighttime glare may result. Studies have shown that aspheric IOLs offer improved contrast sensitivity that becomes more pronounced in low-light conditions, and hazard detection distance is also improved compared with standard IOLs.^1,2 The major patient benefit with aspheric IOLs is this improvement in the functional quality of vision, especially under low levels of illumination including nighttime driving.

Optically and clinically, it makes sense to have asphericity on all IOLs because there is a universal benefit of better functional vision compared with a standard IOL. There is a trend toward developing IOLs with asphericity optimization, however, it will take a few years before it happens.

SPHERICAL ABERRATION AND IOL DESIGN
All standard IOLs add positive spherical aberration into the optical system, however, two types of lens designs—zero aberration and negative aberration—exist. The idea behind both models is to reduce the amount of total spherical aberration in the eye.

Zero aberration aspheric IOLs. With this lens design, no spherical aberration is introduced into the system. The spherical aberration found on the cornea is still present, however, the lens does not have any spherical aberration of its own. The SofPort Advanced Optics (AO) (Bausch & Lomb, Rochester, New York) lens is an example of a zero aberration design.

Negative aberration aspheric IOLs. This lens type is equipped with negative spherical aberration. It is designed to negate, reduce, or compensate for the positive spherical aberration on the cornea. In return, the total spherical aberration in the eye is minimized. The Tecnis (Advanced Medical Optics, Inc., Santa Ana, California) and AcrySof IQ (Alcon Laboratories, Inc., Fort Worth, Texas) are negative aberration lenses.

The Tecnis IOL has -0.27 µm of spherical aberration on the lens surface, compensating for the average corneal spherical aberration (0.25 µm) of the general population. The goal is to make the total spherical aberration near zero. The AcrySof IQ has -0.20 µm of spherical aberration on the lens surface, leaving a small amount of positive spherical aberration in the eye.

ONGOING DEBATE
The debate of which aspheric strategy is best for patients is ongoing, because we do not really know the optimal amount of spherical aberration one should leave in the eye. In reality, there may never be a standard answer, as all patients are different, and lifestyles help dictate the correct amount of spherical aberration for their situation.

Finding the overall target of spherical aberration is the underlying challenge in the use of aspheric IOL designs. Another issue, however, is that even if we know the amount of spherical aberration to correct, aspheric IOLs with customized amounts of spherical aberration are not yet available. Instead, the aspheric IOLs on today's market have a fixed standard amount of spherical aberration. One way to compensate for the lack of spherical aberration options is to customize the use of the available aspheric lenses. This is dependant upon the amount of spherical aberration as measured on corneal topography.

By measuring the corneal topography preoperatively, the corneal spherical aberration of the eye is calculated, and then the appropriate aspheric IOL is chosen. For example, if a patient has a corneal spherical aberration of 0.37 µm, the Tecnis aspheric IOL would be the appropriate choice, as it would minimize the spherical aberration in the eye. If, on the other hand, you have a cornea with 0.1 µm, then an AcrySof IQ or the AO lens would be the appropriate options, depending on what amount of total spherical aberration you want to leave in the eye.

We must also consider appropriate aspheric IOL choice in patients who have had corneal refractive surgery. If the treatment was to correct myopia, there is usually a very high amount of positive spherical aberration left behind on the cornea. It makes the most sense to implant a Tecnis to negate that very high spherical aberration. If, however, the refractive procedure was to correct for hyperopia, the corneal spherical aberration is reduced or even negative. These patients would most benefit from a zero aberration lens such as the SofPort AO or Akreos AO (Bausch & Lomb).

Centration and tilt are also important and become more relevant in the Tecnis and AcrySof IQ lenses, because spherical aberration is added into the system. We cannot control or predict centration or tilt in any IOL, and all models tend to center themselves within the capsular bag. If the Tecnis lens decenters from the entrance pupillary center (ie, the ideal centration point) by more than 0.4 mm, the benefit of the aspheric correction is entirely lost. Similarly, if there is a tilt of more than 6º, again they lose the benefit of the aspheric correction. If your patient has a very decentered pupil, or if you suspect that the patient may experience an unusual degree of decentration, using spherical aberration additive lenses like the Tecnis or the AcrySof IQ may be problematic. In these instances, use zero aberration lenses like the Akreos or SofPort AO because they are much more tolerant of decentration and tilt. As you get comfortable using aspheric lenses, you can then start to customize the procedure by deciding what aspheric IOL design is best on a case-by-case basis.

START WITH A ZERO ABERRATION LENS
Surgeons who are just starting to use aspheric IOLs or those who only currently use standard IOLs should consider starting with the zero aberration lens. Patients almost always end up with a better optical quality than that offered with a standard IOL, and they do not have to worry about decentration and tilt. The spherical aberration additive lenses can then be introduced into their practice in a customized case-by-case manner. Typically, patients cannot tell the difference between aspheric and standard IOLs. This may sound bizarre, however, the improvement in image quality is small and estimated to be equivalent to the improvement seen by correcting the defocus of sphere by 0.25 D. Therefore, patients have a hard time perceiving it. It is only when we test them for contrast sensitivity with their full spectacle prescription, especially in low levels of light, that one can tell the difference. As almost all patients are left with small residual refractive errors, the improvement produced by correcting spherical aberration is difficult to perceive for patients.

Objective tests show that patients have better functional quality of vision with aspheric IOLs, and just because patients may not consciously see the difference does not mean that there is no benefit. In fact, there is a universal benefit when compared with standard IOLs, and in the future, all IOLs will include some asphericity. In the meantime, understanding the optics of currently available aspheric IOLs is the first step to their successful use in improving patients' quality of vision.

Milind V. Pande, DO, FRCS, FRCOphth, is Head of the Vision Surgery & Research Centre, in East Yorkshire, UK. Dr. Pande states that he has no financial interest in the products or companies mentioned. He may be reached at +44 01482 339515.

1. Bellucci R, Scialdone A, Buratto L, et al. Visual acuity and contrast sensitivity comparison between Tecnis and AcrySof SA60AT intraocular lenses: A multicenter randomized study. J Cataract Refract Surg. 2005;31:712-717. Erratum in: J Cataract Refract Surg. 2005;31:1857.
2. Packer M, Ginsburg AP. Contrast sensitivity and aging. Ophthalmology. 2007;114:1589.