Adding to Your Bifocal Options

BY DOUGLAS P. BENOIT, O.D., F.A.A.O.
APR. 1996

With an estimated 77 million people in the United States who are over age 40, your skill in fitting complex bifocal lenses will help keep you active in the growing presbyopic market.

As more and more baby boomers enter the age of presbyopia, they're constantly looking for opportunities to rid themselves of bifocal spectacles. This quest has placed renewed emphasis on bifocal contact lenses, and manufacturers are striving to respond to this need. In this report, we'll focus on a new aspheric soft multifocal.

A SUCCESSFUL FIT

Sylvia, a 52-year-old presbyope, didn't want to wear bifocal spectacles but when she tried bifocal contact lenses in the past, she experienced inadequate vision and poor depth perception. (She had tried monovision and the Echelon diffractive bifocal.) She was in good general health and was taking estrogen on a daily basis.

Best corrected vision was 20/20 at distance and near with the following prescription: OD +2.25 -0.50 x 100, OS +3.00 -0.50 x 97; add +1.75 OU. The right eye was dominant on testing. Slit lamp examination revealed normal lids, meibomian orifices and lashes. The conjunctiva evidenced small pingueculi OU. The tear meniscus was of normal height and width and was free of debris. Tear break up time was greater than 11 seconds for each eye. Keratometric readings were: OD 43.25/43.50 @ 90; OS 43.25/ 43.25. The mires were clear and regular OU.

After a thorough fitting evaluation, I dispensed the following lenses: OD Esstech PS-D, 8.70 base curve, 14.0 diameter, +3.50 power; OS Esstech PS, 8.70 base curve, 14.0 diameter, +4.50 power. These lenses provided distance vision of 20/20 OD and 20/25 OS, with near vision of 20/25+ OU. Fit and comfort were optimal.

At her one-week follow-up, Sylvia reported she could perform her daily visual activities easily without fatigue at day's end. She felt her vision with the new contact lenses was better than with her glasses, and she reported no difficulty with depth perception. She noted slight dryness at work, which was remedied by rewetting, but she had no dryness outside the office. Sylvia continues to do very well and because of her success with the Esstech lens, she has sent me many referrals.

MORE ADD, THIN DESIGN

The Esstech PS Multifocal from Blanchard Contact Lens, Inc., is a relatively new entry into the bifocal market. I believe this design incorporates the best features of aspheric optics in a lens that's easy to work with, comfortable on the eye and visually satisfying to patients. This lens is a refinement of the PS 45 lens, having more overall add power in a thinner design. Table 1 lists the parameters for the original Esstech PS lens with the +2.00 add and the newer Esstech PS-D, which has a +1.50 add. Note: the power designation represents the combined distance and add powers. For example, a patient with a +2.00 distance prescription and patient with a +2.00 add would need a +4.00 lens power to start in the Esstech PS.

Centration with this lens is similar to that of all simultaneous image designs. However, the Esstech lens provides a larger range of acceptable deviations from center. This is due in part to the power block or central 2.5mm zone. Within this area, the lens undergoes a power change of 1.50 or 2.00 diopters, which constitutes the near addition and is a fixed value. As the edge of the lens is approached, there is a progressive decrease in plus power. As the pupil enlarges when looking from near to far, more of the power band devoted to distance is used. Since the lens has no distinct lines where the power changes, patients get a smooth transition when switching gaze and are unaware of crossing any power boundaries. The central near design, combined with the normally smaller pupil at near, affords a sharper image and good contrast when reading.

All of these attributes are the result of the "S" curve aspheric front surface. A unique lathing process cuts curves at infinitely variable rates of flattening or steepening to produce an anterior surface that is constantly flattening as you move away from the center. Usable power extends across most of the lens with an average optic zone of 10.5mm. Also, this design enhances vision by controlling aspherical aberration.

The same front curve design that gives good vision can cause a degradation of vision if the lens is not fit properly. The standard 8.70 base curve will work well on corneal curves from 42.75D to 45.50D. Corneas with K's flatter than 42.75 should be fit with the 9.10 base curve. Conversely, a corneal value steeper than 45.50 should start with the 8.30 lens. The reasoning is straightforward. If the lens fit is too steep for the cornea, the back curve is pushed forward, creating a situation of relative over-plussing. Additional minus in the overrefraction gives better distance vision but the near vision suffers. The reverse is true if the lens is too flat. Switching to the appropriate base curve should restore good vision at all distances with this or any bifocal lens.

Modified monovision using various bifocal and/or spherical lenses to arrive at the desired vision is another option with the Esstech lens. For example, a patient with a +1.75 add and a close working distance may not be happy with the near VA provided by the Esstech PS-D. The higher add Esstech PS may blur the distance vision to unacceptable levels, yet give great near vision. Using the Esstech PS-D on the dominant eye and biasing it for distance, while using the standard add lens with a little extra plus on the fellow eye, generally produces a better result.

SUMMARY

The key to successful fitting these or any bifocal lenses is to become familiar with the lenses and fit as many patients as possible. Only through experience will you learn the intricacies of varying the lens-to-cornea fitting relationship, using dissimilar adds or even combinations of single vision and bifocal designs. This will ultimately increase your efficiency, and increased success rates will garner more word-of-mouth referrals from satisfied and loyal patients. CLS

Dr. Benoit is chief of Contact Lens Services with Concord Ophthalmologic Associates in Concord, N.H. He is also a diplomate of the Cornea and Contact Lens Section and a fellow of the AAO.