Difference Between an 8.2 & 8.6 Contact Lens

Contact lenses are made to fit over the cornea of your eye. Because individual eyes vary in size and shape, when being fitted for contact lenses, your ophthalmologist or optician must take a measurement to obtain the curvature of your cornea. He does this with an instrument called a keratometer or ophthalmometer. An accurate measurement assures a comfortable fit, maximizes your vision and prevents damage to your cornea.

Steeper Base Curve

• The base curves for contact lenses range from 8.0 to 10.0mm, however, the full range of base curves are not available in all brands of contact lenses. An 8.2 contact lens is more concave or deeper in the center and has a tighter fit than the flatter 8.6 contact lens. A contact lens that is too tight cannot move properly on your eye and causes irritation around the rim of the lens. It also prevents the tear film from flowing beneath the lens. Within a few hours, or sooner, the lens become too uncomfortable to wear. Other signs of an overly tight contact lens are redness and irritation, sensitivity to light and blurred vision. If you experience any of these symptoms with your contact lenses, speak with your eye care specialist about trying lenses with a higher base curve.

Flatter Base Curve

• An 8.6 lens is flatter than an 8.2 lens and provides a looser fit. However, if the curve of a loose-fitting contact lens is flatter than the curve of your cornea, you have a problem with too much movement of the lens on your eye. As the lens slips, it irritates the center of the cornea by rubbing against it, or may fall out of your eye. The lens soon becomes uncomfortable and continuing to wear it does not improve the fit. If you experience this problem with your contact lenses, speak with your eye care specialist about trying lenses with a lower base curve.

Tear Flow

• Adequate tear flow under your contact lenses helps remove debris, inflammatory cells, bacteria and other irritants that cause infections or inflammation, according to Kenneth Polse Laboratory. Tear flow keeps eyes healthy by carrying oxygen to the cornea.

Oxygen Transmission

• Research reported in "Investigative Ophthalmology & Visual Science", 2002, indicates a higher risk of corneal hypoxia -- a deficiency in oxygen delivery to the cornea -- with extended wear soft contact lenses. Tear flow slows down at night when your eyes are closed. The reduction in tear flow causes the lenses to dry out and stick to the cornea. The cornea swells, the lenses become tight and further reduces tear flow, which deprives the cornea of oxygen. Deprivation of oxygen results in the death of corneal cells. Lenses worn overnight must provide a high rate of oxygen transmissibility and adequate tear flow to avoid corneal damage.