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Spherical aberration-blocking lenses have minimal impact on correcting astigmatism
Contact Lenses and Eyewear
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Clinicians have debated whether an aberration-blocking aspheric contact lens can correct for astigmatism. According to three researchers, it is possible, but highly unlikely.
“It is unlikely that an aspheric contact lens can correct for astigmatism because spherical aberration is not astigmatism,” Pete S. Kollbaum, OD, PhD, a research associate in optometry at Indiana University in Bloomington, told Primary Care Optometry News in an interview.
Dr. Kollbaum noted there are presumably only two ways an aspheric lens could correct astigmatism. “One way is if the aspheric lens you place on the eye had levels of coma inherent to that lens,” he said, “and this lens happened to decenter on the eye in the right direction to correct for astigmatism.”
A second hypothesis involves an aspheric lens that does not completely conform to the eye, as is typically expected with soft contact lenses. “As with a gas-permeable lens, this non-conforming lens could create an astigmatic tear lens that could possibly correct for levels of corneal astigmatism,” Dr. Kollbaum said.
Dr. Kollbaum and Arthur Bradley, PhD, at Indiana University have used Shack-Hartmann wavefront aberrometry to assess the on-eye optics of five available aspheric lenses and several spherical and toric lenses in 16 human eyes. “We found that for the subjects evaluated, levels of astigmatism remained unchanged when the aspheric lenses were on the eye,” he said.
“Additionally, we evaluated the two hypotheses for how aspheric lenses may correct for astigmatism and found neither one to be true. In all instances, levels of astigmatism that were present without the lens on the eye still manifested themselves with that aspheric lens on the eye,” Dr. Kollbaum said. “It is possible, if an improvement in acutiy is noted clinically in some aspheric lens wearers that this improvement is not due to an alteration of astigmatism, but rather an alteration of spherical aberration.”
Study: Minor improvement
At least one study has demonstrated some improvement in astigmatism with spherical lenses. The authors compared a spherical lens to an aspheric lens with the Frequency 55 material (CooperVision). Investigators retrospectively reviewed about 150 patient charts over a 3-year period. Slightly less than 25% of patients had been fitted with a spherical lens, while the remaining wore an aspheric lens.
“The average amount of astigmatism that was masked with the spherical lens was about 0.40 D, which is minor,” Nimesh B. Patel, OD, an assistant professor of optometry at Northeastern State University College of Optometry in Tahlequah, Okla., said in an interview. “In comparison, we found that the aspheric lens appeared to mask about 0.70 D of astigmatism.”
Whether spheric or aspheric lenses, “I believe that thicker moduli are probably more effective than thinner moduli in masking astigmatism,” Dr. Patel, chief of optometry at Hastings Indiana Medical Center in Tahlequah, concluded.
Principle of optics
Dr. Bradley, a professor of vision science at Indiana University in Bloomington, said it is unlikely that a spherical aberration-blocking lens could correct for astigmatism. The general principle of optics makes it clear that correcting one type of optical aberration will correct only that aberration. It will not correct other varieties, he said.
“It is a bit like trying to correct astigmatism with a standard spherical soft contact lens, which is not a good correction for astigmatism,” Dr. Bradley said.
Nonetheless, clinical claims of spherical aberration-blocking lenses correcting for astigmatism require more investigation, Dr. Bradley said.
For example, just as spherical gas-permeable lenses can “correct” for corneal astigmatism, so might “aspheric” soft contact lenses if they fail to conform to the cornea.
For more information:
- Pete S. Kollbaum, OD, PhD, can be reached at Indiana University, 800 E. Atwater Ave., Bloomington, IN 47405; (812) 856-0108; fax: (812) 855-8664; e-mail: kollbaum@indiana.edu.
- Arthur Bradley, PhD, can be reached at School of Optometry, Indiana University, 800 E. Atwater Ave., Bloomington, IN 47405; (812) 855-2465; fax: (812) 855-7045; e-mail: bradley@indiana.edu.
- Nimesh B. Patel, OD, can be reached at Northeastern State University College of Optometry, 1001 N. Grand Ave., Tahlequah, OK 74464; (918) 456-5511; fax: (918) 458-2104; e-mail: patelnb@nsuok.edu. Neither Dr. Kollbaum, Dr. Bradley nor Dr. Patel has a direct financial interest in the products mentioned in this article, nor are they paid consultants or employees for any companies mentioned.
Reference:
- Masking Cylinder with Aspheric Soft Lenses. Contact Lens Specturm. July 2004;42-45.