When Flexure is a Good Thing

BY DR. THOMAS G. QUINN, O.D., M.S.
MAR. 1996

Riddle me this. Patient Refer L. Source, a 35-year-old accountant, is interested in wearing contact lenses. As an avid camper, he'd like the option of occasional overnight wear for outings. His refractive findings are: -3.00 -0.25 x 180 O.D.; -3.00 -1.50 x 180 OS. Keratometry readings are 43.00@180/44.50@090 OU. His cornea and lids are clear and tear physiology appears normal. What is his optimal contact lens option?

The right eye has corneal toricity that's not reflected in the nearly spherical refraction. I'm guessing that many of you concluded the right eye would do well in a spherical soft lens. A soft lens would drape the cornea and avoid the unwanted astigmatic tear lens that would result from a standard gas permeable lens.

What about the left eye? Most would agree that a standard spherical gas permeable lens would work beautifully on this eye. The spherical lens on the toric cornea would create a tear lens equal to the refractive astigmatic error. But it's confusing to care for a gas permeable lens on one eye and a soft lens on the other.

So, if you fit a spherical soft lens on the right eye, a toric soft lens on the left eye makes sense. The uncertainty with this approach is whether or not acuity in the left eye will be adequate to meet the visually demanding tasks of an accountant, especially if the left eye is dominant.

Another way to assess potential sensitivity to mislocation of the astigmatic correction is to rotate the best subjective cylinder result in the phoropter until you elicit a response of just noticeable blur. Dr. Doug Becherer has found that blur within ±5 degrees suggests high sensitivity, thus making satisfactory toric soft lens correction challenging.

If you attempt occasional overnight wear, as this patient has requested, achieving adequate oxygen levels through a toric soft lens will be difficult. All toric soft lenses currently approved for flexible or extended wear are prism ballasted. The thickness of this design limits oxygen availability to the cornea.

WHAT ABOUT RGPS?

Again, a standard gas permeable lens would serve the left eye nicely. The problem lies with the right eye. Corneal cylinder is not reflected in the subjective refractive finding. In such a case, the astigmatic error of the cornea is compensated by a toric surface in the eye.

Some might suggest fitting a bitoric gas permeable lens on the left eye. You could select base curves to align with the major meridians of the cornea. A toric front surface would compensate for the unwanted cylinder induced by the toric back curve. This is workable, but there's a simpler option -- a flexing lens.

Although we go to great lengths to ensure that a gas permeable lens won't flex on the eye, lens flexure is desirable in such cases as this patient's right eye to avoid creating an unwanted astigmatic tear lens.

How do we promote lens flexure? A thin center thickness will limit this approach to myopic corrections. Added advantages of this lens include improved comfort, better centration and greater oxygen transmissibility. Even in standard thickness, RGPs provide superior oxygen levels to the cornea.

A steep bias on the base curve-to-cornea relationship further promotes flexure. A lens fit 0.25 to 0.50 diopter flatter than flat K is least likely to flex.

As a general rule, higher Dk silicone-acrylate materials will flex more, but not so with the newer fluorosilicone-acrylate materials.

Custom designing gas permeable lenses for this patient is simple, economical, visually satisfying and safe for flexible wear. Sometimes it pays to be flexible. CLS

Dr. Quinn has served as a faculty member and research associate at the OSU College of Optometry. He is in group practice in Athens, Ohio.