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October 2009
 

CATARACT/ IOL
 

Aspheric versus spheric




by Matt Young EyeWorld Contributing Editor
 

 

 



Research has shown that best-corrected visual acuity isn’t much different between aspheric and spherical IOLs after cataract surgery. So what’s the reason the latest aspheric IOLs could displace spherical IOLs in the cataract surgeon’s armamentarium? The answer can be found via contrast sensitivity (CS) testing. “Eyes implanted with the aspheric AcrySof IQ IOL [Alcon, Fort Worth, Texas] showed better photopic and mesopic CS at medium and high spatial frequencies than eyes implanted with the spherical AcrySof SN60AT IOL [Alcon],” according to Peter R. Trueb, M.D., Ambulante Augenchirurgie Zürich, Zurich, Switzerland, in a report published in the May 2009 issue of Ophthalmology. There are interesting theoretical reasons behind the IOL differences in visual results, but Dr. Trueb’s conclusion is clear: Important differences do exist.


A tale of two IOLs


AcrySof IQ IOL Source: Alcon

Dr. Trueb analyzed 524 eyes, among which 262 were implanted with the AcrySof IQ IOL (SN60WF model), and 262 were implanted with the AcrySof Natural IOL (SN60AT model). Dr. Trueb suggested that this large sample size should properly address questions of visual acuity and contrast sensitivity related to both lenses. “One hundred percent of patients achieved BCVA of 20/32 or better in the AcrySof IQ IOL group and 86.3% achieved BCVA of 20/32 or better in the AcrySof Natural IOL group,” Dr. Trueb reported. “The percentage of patients who achieved a BCVA of 20/20 or better was 42.7% for the AcrySof IQ IOL group and 31.3% for the AcrySof Natural IOL group.” Dr. Trueb concluded that these differences were not significant. “Considering these outcomes, it can be concluded either that there are no visual acuity differences between eyes implanted with both types of IOL or that the visual performance metric used (high-contrast photopic BCVA measurement) is not accurate enough to detect subtle visual changes because of SA [spherical aberration] reduction by asphericity,” Dr. Trueb noted. He looked to the CSF measurement tool to determine if there were further significant differences between the lenses, noting that this gauge “most usefully characterizes human spatial vision.”
“The results reported herein revealed significantly better CS for eyes implanted with the AcrySof IQ at medium and high spatial frequencies under both photopic and mesopic conditions,” Dr. Trueb reported. “Statistically significant differences were found at 6, 12, and 18 cpd under photopic conditions and at 3, 6, 12, and 18 cpd under mesopic conditions.”
While contrast sensitivity results were already better with the AcrySof IQ in photopic conditions, they became more pronounced in mesopic ones. “Differences between both IOLs become more evident when lighting conditions are reduced and pupil diameter is increased,” Dr. Trueb reported. “This is an expected result considering the reduction of ocular SA for large pupil diameters when an aspheric IOL is implanted, compared with that found in eyes with a spherical IOL.”
Dr. Trueb cautions that the observed benefits of aspheric IOLs can be cancelled out if surgery is suboptimal. “The advantages of aspheric IOLs may be limited, canceled, or even turned into disadvantages by decentration,” Dr. Trueb reported. Nonetheless he encourages further usage of aspheric IOLs. “Surgeons, then, should consider aspheric IOLs for their patients and try to customize the asphericity depending on the corneal SA to obtain the optimal visual performance,” he concluded. In theory, the lenses restore the spherical aberration equilibrium of the eye achieved during one’s youth. “The young crystalline lens has negative SA that compensates for the positive SA of the normal cornea,” Dr. Trueb reported. “The SA of the crystalline lens becomes positive with aging, disrupting the balance with corneal SA and increasing therefore the overall positive SA for the entire eye. The SA has a significant impact on the reduction of retinal image quality throughout life.”
Aspheric IOLs can provide needed negative spherical aberration to compensate for the aged cornea, characterized by positive spherical aberration, Dr. Trueb noted. Ian Anderson, F.R.A.C.O., Subiaco Eye Clinic, Perth, Australia, said he likes aspheric lenses and uses them often. However, he said he realizes there are drawbacks. “If someone needs perfect night vision, he or she has to have an aspheric lens,” Dr. Anderson said. For instance, taxi drivers are people who could benefit from this lens. The trade-off is that while aspheric lenses give better quality of vision at one distance, there is a decreased depth of focus, Dr. Anderson said. “If you have an aspheric lens, you may actually decrease the pseudoaccommodation you see with it,” Dr. Anderson said. “We regularly see patients with a monofocal [spherical] lens that have good distance and near vision.”
But with a lens that has a very precise focus like an aspheric lens, it can be more difficult to achieve this good range of vision, he said.



Editors’ note: Dr. Trueb has no financial interests related to this study. Dr. Anderson has no financial interests related to his comments.



Contact information

Anderson: +61 8 6380 1855, ian@ianseyesite.net
Trueb: ptrueb@mac.com







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