Achieving Best Visual Outcomes with a Monofocal Intraocular Lens
aberrations (HOA) level by minimizing the fourth-order HOA known as spherical aberration (SA).
Spherical aberration is induced in an optical system when peripheral rays have a different focus compared with central rays (see Figure 10). The major contributors to ocular SA are the cornea and the lens. The SA of the cornea is positive, which means that when central rays are focused by the cornea onto the retinal photoreceptors, peripheral rays are focused in front of them. Several large studies97–99
have
determined that the average SA induced by the cornea for a 6mm aperture is approximately +0.27µm, a value that remains relatively unchanged with age.98
However, the effects of age can increase positive asphericity to even higher values.
Fortunately, the magnitude of corneal SA error is progressively lower for smaller pupil diameters. Approximate magnitudes of corneal SA at decreasing aperture diameters are +0.13µm at 5mm, +0.051µm at 4mm and +0.016µm at 3mm.100
Therefore, the effect of this aberration is sensed most acutely under mesopic or scotopic conditions when pupils are dilated, and is negligible in small pupils.
In young people the crystalline lens counteracts most but not all of the positive corneal SA by providing a negative SA and, as a result, total ocular positive SA is low. With age, the crystalline lens undergoes changes and the SA induced by the lens becomes progressively more positive. Although there is inter-patient variability, on average by ages 40–50 years lenticular SA has risen such that total ocular SA is greater than zero, with lenticular and total ocular SA growing to progressively higher positive values with increasing age.97 Typical spherical IOLs act in a similar manner to the aged crystalline lens in that they induce a positive SA by over-refraction of rays of light at the lens periphery. The SA induced by a given spherical IOL is proportional to its power99
and increases with pupil dilation. For this
reason, spherical IOLs can be expected to reduce vision performance to below optimum levels post-operatively.
Aspheric IOLs are different; through a modification of one or both of the IOL surfaces, aspheric IOLs can be manufactured so that they
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induce variable amounts of negative SA or no SA, allowing them to perform in similar fashion to the crystalline lens in young people. Some manufacturers have designed IOL platforms to completely negate the average corneal SA by inducing a negative SA at 6mm of -0.27µm while others induce no SA, providing a neutral asphericity factor, leaving the corneal SA unbalanced but not added to by the typical amounts seen with spherical IOLs. The AcrySof® IQ IOL, Alcon’s designation for its aspheric IOL, features an aspheric modification to the posterior optic surface that provides -0.20µm of negative SA to the eye measured with a 6mm pupil. This only partially corrects corneal SA, leaving the average patient’s pseudophakic ocular system with a very slight residual positive SA. This small amount of defocus produced by the small residual SA has been shown to provide a slightly increased best-corrected acuity, depth of field, and some degree of multifocality, allowing patients to potentially better tolerate residual ametropia and experience better uncorrected near vision and, potentially, intermediate vision.101–106
Contrast sensitivity measurements, more than visual acuity, have been shown to predict functional vision and visual performance for a range of object scales. Aspheric IOLs may slightly improve photopic contrast sensitivity, especially at lower spatial frequencies.107–115 However, photopic contrast sensitivity measurements are usually made at a luminance of 85cd/m2 with a resultant average pupil diameter of approximately 3mm110,113,116–120
and, at this level,
aspheric IOLs have not been shown to significantly reduce ocular HOAs. Mesopic contrast sensitivity measurements are usually performed at a median luminance of 3–6cd/m2 where the average pupil diameter of elderly pseudophakic patients would be about 4mm.108,110,113,116,118,120–123
At this larger pupil diameter there is more of a
potential benefit available by correcting SA. Contrast sensitivity improved significantly under these conditions, with a large majority of studies108–115,120,121,124–129
over spherical counterparts. showing a benefit for aspheric IOL performance
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