RGP insights

Bitorics Are Simple ... Optical Cross My Heart!

BY TIMOTHY B. EDRINGTON, OD, MSED

Patients are too often told that they're not good candidates for contact lenses because they have too much astigmatism. Now more high astigmats are successfully fitted with toric soft lenseslenses, although vision might not always be optimal. A rotational misalignment of 15 degrees through a toric soft contact lens will induce an unwanted overrefraction cylinder of approximately one-half of the prescribed cylinder.

Optimizing Toric Vision

Back surface toric and bitoric RGP lens designs optimize vision. You can design bitoric RGPs by using spherical base curve trial lenses or keratometry and manifest refraction data with a bitoric lens guide like the Mandell-Moore guide.

I prefer to use spherical power effect (SPE) bitoric diagnostic lenses before prescribing. With the SPE bitoric design, the flat and steep meridians of the lens and cornea will align with one another. With a bitoric trial lens, I can evaluate the fluorescein pattern to fine-tune the fit and perform a spherocylinder overrefraction to fine-tune vision. If there's 0.50D or less cylinder in the overrefraction, you may add the spherical equivalent of the overrefraction to the powers of both meridians of the diagnostic lens. If the overrefraction reveals a clinically unacceptable amount of cylinder, place the diagnostic lens parameters on an optical cross and add the overrefraction findings from each meridian to obtain contact lens powers. For example: you apply a 44.50D/ 41.50D, -4.00D/-1.00D SPE bitoric diagnostic lens to your patient's with-the-rule cornea and obtain a -0.50 -1.75 x 180 overrefraction. Place these values on (the dreaded) optical crosses to obtain resultant powers for each meridian (see above). Assuming the fluorescein pattern reveals an optimal lens-to-cornea fitting relationship, the resulting prescription for base curves and powers would be: 44.50D/41.50D, -6.25D/-1.50D. If the base curves need to be adjusted, including modifying the amount of toricity, change the power in each meridian by the amount you change the base curve in the same meridian. If you steepen the base curves in both meridians, add the same amount of minus in both meridians to compensate for the change in the tear lens. In the above example, let's assume that the resultant lens provides an alignment fitting relationship and a plano sphere overrefraction.

However, let's say you want to prescribe the base curve toric by 3.50D instead of 3.00D. The alignment fluorescein pattern indicates that the average of the two base curves (43.00D) need not be changed. Maintain the 43.00D average base curve and increase the toricity to 3.50D. This results in base curves of 41.25D and 44.75D. Alter the contact lens powers by the amount of the change in base curve in each meridian: 44.75D/ 41.25D, -6.50D/-1.25D. Note that axes are not specified for bitoric lens designs. You may order the remaining parameters using your standard RGP design philosophy. While the first bitoric fit can be intimidating, your RGP lab is willing to assist you, and so am I. Don't be shy,
e-mail: tedrington@scco.edu.

Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry.