'OK' contact lenses work by flattening front of cornea, not the entire cornea...

(Press-News.org) Philadelphia, Pa. (March 04, 2013) - A contact lens technique called overnight orthokeratology (OK) brings rapid improvement in vision for nearsighted patients. Now a new study shows that OK treatment works mainly by flattening the front of the cornea, reports a recent study, "Posterior Corneal Shape Changes in Myopic Overnight Orthokeratology", appearing in the March issue of Optometry and Vision Science, official journal of the American Academy of Optometry. The journal is published by Lippincott Williams & Wilkins, a part of Wolters Kluwer Health.

"This study appears to show that it is only, or primarily, the very front surface layers of the cornea that are altered by OK contact lens treatment," comments Anthony Adams, OD, PhD, Editor-in-Chief of Optometry and Vision Science. The study was performed by Jeong Ho Yoon, PhD, of University of Choonhae Health Science, Ulsan, Republic of Korea, and Helen A. Swarbick, PhD, FAAO, of University of New South Wales, Sydney, Australia.

Overnight Orthokeratology Works—But How?
Orthokeratology is a clinical technique to reduce nearsightedness (myopia) using specially designed rigid contact lenses to manipulate the shape of the cornea—the transparent front part that lets light into the eye. Dr Adams likens OK therapy to orthodontic treatment using braces to change the alignment of the teeth. He explains "Wearing these lenses overnight for about six hours is currently the treatment approach for most clinicians who use OK for the temporary correction of low to moderate myopia." But it has been unclear exactly how OK works to reduce myopia: Do the contact lenses reshape just the front surface of the cornea, or do they bend and flatten the entire cornea?

To find out, Drs Yoon and Swarbick had 18 young adults with "low" (relatively mild) myopia wear OK lenses overnight for 14 days. Using sophisticated techniques, the researchers made detailed measurements of corneal shape and thickness before, during, and after treatment.

As in previous studies, wearing OK contact lenses led to reduced myopia, thus improving vision. The changes were significant after the first night wearing OK lenses, By 14 days, myopia was almost completely eliminated and the participants had near-normal uncorrected (without glasses) visual acuity.

Results 'Achieved Primarily through Remodeling of Anterior Cornea'
These changes were linked to significant flattening of the anterior (front) portion of the cornea. Like the vision changes, the change in corneal shape was significant after the first night wearing OK lenses. Although corneal flattening continued throughout the 14-day treatment period, about 80 percent of the change occurred in the first four days. In contrast, OK lenses caused only a small and temporary change in the shape of the posterior (rear) cornea, and only slight thinning of the central cornea. "Overall, our results support the current hypothesis that the OK refractive effect is achieved primarily through remodeling of the anterior corneal layers, without overall corneal bending," according to Drs Yoon and Swarbick.

Overnight OK contact lenses provide an effective treatment for relatively mild myopia. It requires custom-made rigid contact lenses designed to accommodate the shape and desired change of the patient's eye. While OK lenses provide an alternative to LASIK surgery for correction of myopia, the patient must continue nightly contact lens wear to maintain the improvement.

The authors hope their results "will provide a more complete picture of overall corneal changes during myopic OK." In particular, the study demonstrates that overnight OK lenses don't change the curvature of the posterior cornea—"at least in the first two weeks of lens wear."

### To read the article "Posterior Corneal Shape Changes in Myopic Overnight Orthokeratology", please visit http://journals.lww.com/optvissci/fulltext/2013/03000/Posterior_Corneal_Shape_Changes_in_Myopic.3.aspx

About Optometry and Vision Science
Optometry and Vision Science, official journal of the American Academy of Optometry, is the most authoritative source for current developments in optometry, physiological optics, and vision science. This frequently cited monthly scientific journal has served primary eye care practitioners for more than 75 years, promoting vital interdisciplinary exchange among optometrists and vision scientists worldwide. About the American Academy of Optometry
Founded in 1922, the American Academy of Optometry is committed to promoting the art and science of vision care through lifelong learning. All members of the Academy are dedicated to the highest standards of optometric practice through clinical care, education or research. About Lippincott Williams & Wilkins
Lippincott Williams & Wilkins (LWW) is a leading international publisher of trusted content delivered in innovative ways to practitioners, professionals and students to learn new skills, stay current on their practice, and make important decisions to improve patient care and clinical outcomes. LWW is part of Wolters Kluwer Health, a leading global provider of information, business intelligence and point-of-care solutions for the healthcare industry. Wolters Kluwer Health is part of Wolters Kluwer, a market-leading global information services company with 2012 annual revenues of €3.6 billion ($4.6 billion).