Is There Value in Correcting Ocular Aberrations With Lenses?
By Eric Papas, PHD, MCOPTOM, DIPCL, FAAO
Knowledge of the imperfect nature of the optics of the eye and the various devices available to correct it is as old as the hills, but only in the last 15 years or so have convenient methods to measure these aberrations become readily accessible. The advent of these and other “wavefront” technologies brought promises of “super vision” for all, based on the premise that vision corrections could be tailored to deal with all of the defects of the optical system, rather than just the sphere (defocus) and cylinder (astigmatism) components that traditional refraction yields.
Is it Worth the Cost?
While customized ablation to correct aberration has become a common feature of refractive surgery, the same cannot be said for contact lenses. Admittedly, the need to prevent rotation of the lens on the eye is an additional complicating factor, but given recent improvements in toric lens design and the consequent enormous increase in their successful use, this should really not be an obstacle.
Perhaps of more practical importance is the likelihood that, compared to a traditional contact lens, there will be additional costs involved, whether they are due to the extra time and equipment needed to measure the eye or to the additional complexity of the design and its manufacture. The extent to which contact lens wearers can be persuaded to put their hands into their pockets or purses would seem to depend critically on their belief—or perhaps the belief of their practitioner—that a real visual advantage would be gained from their extra spending.
Although it is possible to measure all of the aberrations of a given eye, it seems unnecessary to try to correct each and every one.
Optical scientists generally adopt a triangular categorization scheme for aberrations that places second-order effects such as defocus and astigmatism toward the top, while higher orders come lower down. Generally speaking, the higher the order (lower on the triangle), the lower the potential for visual effects. Thus, once we have dealt with the sphere and cylinder, the higher-order aberrations with the largest potential effect on image quality are the third-order effects of coma and trefoil and the fourth-order phenomenon of spherical aberration. The last of these has received considerable attention over the years with several manufacturers seeking to incorporate suitable corrections into their contact lenses.
Effects on Visual Acuity
A very interesting recent finding is that while spherical aberration alone can indeed produce quite marked image degradation effects, these can be substantially reduced by a suitable adjustment of the spherical lens power (Cheng et al, 2010). This is quite comforting from a clinical perspective because it implies that the procedure of conducting a careful, subjective, spherical over-refraction with a trial contact lens in situ goes a long way toward eliminating the influence of any spherical aberration in the lens-eye system.
Although such behavior is extremely convenient, it is most influential when high-contrast acuity is the visual metric. As is well known, this measure indicates what is happening only at high spatial frequencies, and unfortunately, the prediction is that uncorrected spherical aberration will continue to manifest effects at other points on the contrast sensitivity function.
While Cheng et al (2010) conducted experiments with a laboratory-based optical bench system and did not use “real-world” corrections, the results nicely confirm observations made in actual contact lens-wearing eyes. A number of researchers have looked at vision from lenses that are marketed with various incorporated corrections for spherical aberration, and all have failed to show visual acuity differences among the options (Efron et al, 2008; Lindskoog Petterson et al, 2011; Papas et al, 2009).
Despite this harmony, all of these studies might reasonably be criticized on the grounds that, like most clinicians in practice, they did not have the resources to tailor the aberration correction to meet the individual requirements of each wearer. Thus, any given eye would have received only the “average” correction set by each manufacturer.
Fortunately, however, one other group was able to avoid this issue and have its subjects compare vision between lenses that were either aberration-free or custommade to correct each subject's own spherical aberration. The results were in close agreement with the data from the laboratory experiments in showing no significant visual acuity differences while highlighting a contrast sensitivity advantage for the custom lens over the aberration-free lens at lower spatial frequencies in the region of 6 cycles/deg (Dietze and Cox, 2004).
Searching for “Real-World” Results
Contrast sensitivity measurements made in the laboratory or consulting room may well give us more information compared to acuity tests alone, but to what extent can we expect such differences to matter to real contact lens wearers during the course of their lives? Finding visual performance indicators that usefully replicate common tasks is a problem that has confronted many researchers in this, as well as in other, areas.
One widely used method is to have contact lens wearers rate the quality of their vision in various situations using a simple, subjective scale. When this technique was used to compare contact lenses with spherical aberrations that ranged from positive, through zero to negative values, the subjects did not indicate that they were able to perceive any significant differences (Papas et al, 2009).
It is worth remembering, however, that these subjective methods can suffer from a considerable degree of inherent variability and so may be insensitive to subtle effects.
As an alternative, some elegant recent work has attempted to assess the effect of correcting individual aberrations on tasks that the researchers suggest are representative of common activity (Sawides et al, 2010). These were things like evaluating the subjective sharpness of natural images, recognizing familiar faces, and determining the nature of facial expressions. When a correction was introduced that removed most of the aberrations in each subject's eye (including coma, trefoil, and spherical aberration), most subjects reported noticeable improvements on the first two of these measures, though facial expression recognition was unchanged.
This result offers the tantalizing prospect that such interventions can have appreciable visual impact, but it remains to be seen how well this will translate to the contact lens arena.
More Research Needed
If there is one thing that is very clear from the literature, it is how much need there is for dedicated, contact-lens-based research to support the clinical application of individual aberration correction. Until we have this support, it seems that the jury will still be out. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #202.
|Associate Professor Papas is executive director of Research & Development and director of Post Graduate Studies, Brien Holden Vision Institute and Vision Cooperative Research Centre, and senior visiting fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. The Brien Holden Vision Institute and Vision Cooperative Research Centre have received research funds from B+L, AMO, and Allergan and have proprietary interest in products from Alcon, CooperVision, and Carl Zeiss. You can reach him at firstname.lastname@example.org.|