Because spherical IOLs are more curved at the periphery than at the center, light rays are refracted to a greater degree passing through peripherally than those passing paraxially (Figure 1A). This can lead to a reduction in the quality of the image formed. Camera lenses have for some time incorporated aspheric optic elements to improve on image quality. In recent years, attempts have been made to improve patients' image quality from intraocular implants by using aspheric IOLs (Figure 1B).

In the young human eye, positive corneal spherical aberration is cancelled out by the negative spherical aberration of the natural crystalline lens. As the lens thickens throughout life, however, positive spherical aberration develops and adds to that of the cornea. This may lead to a reduction in image quality, even before a cataract forms. A cataract operation replaces the crystalline lens with a spherical artificial lens that retains the additive positive aberrations of the cornea and new lens.

Development of aspheric implants began with the Tecnis lens (originally manufactured by Pharmacia, Uppsala, Sweden; now manufactured by Advanced Medical Optics, Inc., Santa Ana, California). The Tecnis, derived on the basis of analyzing 70 corneas, was configured with an aspheric front surface of -0.27 µm. This was calculated to negate the positive spherical aberration found on the average cornea and leave the patient with no spherical aberration (Table 1) so that patients can experience better contrast sensitivity. In return, this may have implications for overall visual quality.

Subsequent aspheric lenses have incorporated different amounts of negative spherical aberration. The AcrySof IQ (Alcon Laboratories, Inc., Fort Worth, Texas) is based on asphericity data from 700 corneas; it aims to neutralize 2 µm of positive spherical aberration. The SofPort Aspheric Optic (AO; Bausch & Lomb, Rochester, New York) purposely neutralizes a smaller amount of aberration, intentionally leaving patients with positive spherical aberration that is said to provide a greater depth of field.

A recent pilot study carried out at Stoke Mandeville Hospital in the United Kingdom compared these three aspheric lenses with their spherical counterparts. Contrast sensitivity was improved in patients implanted with an aspheric IOL, as was the amount of residual spherical aberration. A patient-based visual outcome questionnaire failed to show meaningful differences in perceived visual quality, however. This was a small study with 30 patients; a study with larger numbers may be needed to show this sort of subjective change.

Subjective improvement in vision has been shown in patients with severe dysphotopsia after spherical lens implantation. Typically, these visual disturbances are hard to describe for the patient. They are also difficult to detect without a wavefront analyser and therefore often go un- or misdiagnosed for many months or occasionally years. Patients will complain of (1) difficulty seeing after dark, (2) sheets of light, or (3) an overall poor quality of vision. When tested with Snellen acuity charts, they will often reach the bottom of the chart unaided (20/20 visual acuity), however, they will still complain of poor vision.

Because there is nothing of note to see in the eye or implant, the first reaction is often a Nd:YAG laser capsulotomy to clear the visual axis and improve symptoms. Only when the patient is examined on a wavefront analyser will the type and amount of aberration become obvious, however. Figures 2 through 4 show wavefront maps from such a patient. By switching on/off the different higher-order aberrations, it can be seen that in this case, the symptoms were mainly caused by spherical aberration. In these patients, who tend to be in the under-60 age group, aspheric lens exchange may relieve symptoms and reduce nighttime myopic shift seen in the predicted phoropter refraction (Figure 5).

Lens exchange can be undertaken more easily when a capsulotomy has not been previously performed. If it has, then an anterior vitrectomy should be anticipated. Explanting the primary implant may be tricky, depending on how fibrosed into the capsular bag the lens has become. Many patients do not have a lens exchange for a number of months after the initial cataract extraction, because the diagnosis of spherical aberration can be difficult to make. AcrySof lenses are always easy to explant, even years after the initial surgery. Others such as the CeeOn Edge (Advanced Medical Optics, Inc.) silicone lens, however, are almost impossible to remove without damaging the capsular bag. In these cases, haptic amputation may be considered.

The primary implant may be refolded or cut and removed through a 3-mm wound or alternately with a larger scleral tunnel, allowing for explantation of the whole IOL. Once completed, an assessment of the capsular bag is made. If possible, the new aspheric implant is positioned into the capsular bag.

If a capsulotomy had been performed preoperatively, sulcus fixation should be considered. This may guide the choice of aspheric implant; the AcrySof IQ must not be placed into the sulcus, as its haptics are too thick and cause iris pigment dispersion. Appropriate allowance is made for positioning of the replacement lens with regard to its biometric strength.

An alternative to lens exchange would be to carry out a wavefront-guided corneal ablation with the excimer laser. This is possible because the wavefront analysis is of the eye's total aberrations, and the correction can be applied to the cornea.

Clearly, this is time consuming and potentially risky for the patient, so how can we avoid it in the first place?

CUSTOMIZING ASPHERIC IOL IMPLANTATION
Separating the corneal from the lenticular aberration will provide the amount of spherical aberration resulting from each component. Knowing the corneal asphericity allows a choice of implant to be made to negate—either fully or partly—this amount. It is probably sensible to analyze younger patients to start with, as they seem to be the most sensitive to residual spherical aberration. This may be because (1) the initial drop in quality of vision is partly due to the aberration itself rather than the cataract, as the crystalline lens thickens and changes its asphericity and (2) younger patients tend to have larger pupils, and as can be seen from the predicted phoropter refraction chart in Figure 5, aberrations become more pronounced as a pupil enlarges.

Corneal asphericity can be established using a Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany) scanner. There is also a software method of separating corneal from lenticular aberration available; it is the VOL-CT commercially from Sarver and Associates, Inc. (Carbondale, Illinois). In the future, this sort of customization may become more important in trying to predict which patients may suffer from aberrations postcataract extraction, and thereby implanting the appropriate aspheric implant to prevent aberrations.

Other forms of aberration may also account for alteration in visual quality. It is likely that in the future, new implants with correction for chromatic aberration, for example, will appear as the quest for ultimate visual correction occurs.

Larry Benjamin, DO, FRCS, FRCOphth, is in the Department of Ophthalmology at Stoke Mandeville Hospital, in Aylesbury, UK. He states that he has no financial interest in the products or compaines mentioned. Dr. Benjamin is a member of the CRST Europe Editorial Board. He may be reached at larry.benjamin@btopenworld.com.