The IOL has a modified prolate anterior surface that compensates for changes in the aging lens. Results also indicate that the IOL improves the detection and identification distances for virtually all targets during night driving.
"The term 'functional vision' describes the impact of sight on the quality of life," explained Dr. Packer, clinical assistant professor, Casey Eye Institute, Oregon Health & Science University, Eugene. "Recognizing faces and facial expressions, reading the newspaper, driving at night, performing vocational tasks, and participating in recreational pursuits all bear a relation to functional vision for ophthalmic patients.
"Functional vision not only implies the role of sight in safety and accident prevention but also suggests the importance of high-quality vision in vocations such as astronomy, aeronautics, and the visual arts. Visual acuity alone does not entirely reflect functional vision," he added.
An important component of functional vision is contrast sensitivity. Dr. Packer said that contrast sensitivity declines with age even without the presence of ocular pathologies such as cataract, glaucoma, or age-related macular degeneration.
Retinal image quality
"Advances in wavefront science have shown that this decline in visual capability involves decreased retinal image quality due to changes in the spherical aberration of the natural crystalline lens," Dr. Packer said.
"It has been shown that spherical aberration of the human lens increases with age, but the amount of spherical aberration in the human cornea remains constant or tends to increase only slightly with age," he continued. "A young lens counteracts much of this defect by inducing negative spherical aberration, whereas the aging lens fails to compensate for corneal aberrations."
One drawback with conventional spherical IOLs is that they have positive spherical aberration, he said. Because of this, the contrast sensitivity of pseudophakic patients is no better than that of their age-matched counterparts who do not have cataract, Dr. Packer added.
"Patients who have undergone a previous IOL surgery show statistically significant elevations of fourth-order spherical aberration and total wavefront variance for pupil sizes greater than 5 mm compared with normal subjects," he said.
Modified prolate IOL concept
He recounted recent investigations that evaluated a complementary IOL designed by AMO that would mimic a youthful crystalline lens by reducing the total optical spherical aberration and improving contrast sensitivity levels. The amount of negative spherical aberration incorporated into this new IOL was based on the average positive spherical aberration present in a population of 71 cataract patients (Z [4,0] = 0.27 ± 0.02 μm). This value was confirmed by Wang et al. in a study that included 228 eyes of 134 subjects (Z [4,0] = 0.28 ± 0.086 μm). The results were reported in the Journal of Cataract and Refractive Surgery (2003;29:1514-1521).
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