High-index lenses aren’t just for eyeglasses anymore. New high-index gas permeable (GP) contact lens materials now make GP contacts thinner and lighter, especially for people with high prescription powers.
Like high-index eyeglass lenses, high-index gas permeable contacts bend (refract) light more efficiently, so less lens material is needed for a given prescription power.
In the June 2010 issue of Contact Lens Spectrum, Ronald K. Watanabe, OD, associate professor of optometry at New England College of Optometry, outlines the benefits of high-index GP contacts. These include:
- Because high-index lenses are thinner than regular GP lenses, they fit more easily under the upper eyelid, which can improve comfort, especially in cases of high myopia.
- High-index GP lenses also are lighter, which can improve the upper eyelid’s ability to hold them properly in place on the eye.
- High-index materials also can increase the effective “add” power of aspheric GP multifocal contacts, potentially providing a better correction of presbyopia for contact lens wearers over age 40.
On the downside, high-index gas permeable contacts may not stay as moist as conventional GP lenses, possibly making them less comfortable for someone with dry eyes.
Also, high-index lenses have a lower oxygen permeability than some other GP lenses, which may be a problem for wearers whose corneas cannot tolerate a reduced oxygen supply, according to Dr. Watanabe.
The front surface of the eye returns to its original shape relatively quickly after discontinuation of overnight orthokeratology, according to a study published in the British Journal of Ophthalmology.
Researchers in Japan conducted the study to investigate the reversibility of the orthokeratology procedure used for the temporary non-surgical correction of myopia. The procedure, also called “ortho-k,” reshapes the cornea (the front surface of the eye) with specially designed rigid gas permeable contact lenses worn during sleep.
The prospective study followed 17 subjects who underwent the ortho-k procedure for a period of 12 months.
Prior to the procedure, several baseline eye measurements were taken, including refraction, corneal topography, wavefront aberrometry, uncorrected visual acuity (UCVA) and contrast sensitivity. These tests were then repeated 12 months after commencement of ortho-k, and one week and one month after discontinuation of the procedure.
The researchers found that all measures of corneal shape, refractive error and visual performance returned to baseline levels within one week of discontinuation of overnight ortho-k.
They concluded the effects of orthokeratology are completely reversible, and that the cornea fully recovers its original parameters relatively quickly after ortho-k is discontinued.
Source: Recovery of corneal irregular astigmatism, ocular higher-order aberrations, and contrast sensitivity after discontinuation of overnight orthokeratology. British Journal of Ophthalmology. February 2009.
Ortho-k is an alternative to LASIK surgery for nearsighted individuals who want to see clearly without glasses or contacts and are too young for laser vision correction or for some other reason are not good candidates for refractive eye surgery.
Researchers in Japan recently evaluated the reversibility of orthokeratology. Reversibility is desirable, as teenagers who undergo ortho-k may want to have LASIK surgery later, and it is important for eyes undergoing laser vision correction to be healthy and have a stable refractive error. Also, the researchers wanted to know if ortho-k causes any long-term loss of best-corrected visual acuity after the treatment is discontinued.
The study evaluated 17 nearsighted subjects who underwent ortho-k for a period of 12 months. The following measurements were taken prior to ortho-k, 12 months after commencement of the procedure, and one week and one month after ortho-k was discontinued:
- Refractive error
- Corneal topography
- Wavefront aberrometry to evaluate higher-order aberrations
- Visual acuity with a standard eye chart
- Contrast sensitivity
The study showed that after one week of discontinuing wear of the ortho-k lenses, all measurements returned to the pre-treatment levels. The researchers concluded that orthokeratology is completely reversible, as determined by measurements of corneal shape, refractive error, visual acuity and quality of vision.
Source: Recovery of corneal irregular astigmatism, ocular higher-order aberrations and contrast sensitivity after discontinuation of overnight orthokeratology. British Journal of Ophthalmology. February 2009.