prescribing for astigmatism
Calculate the GP Toric Design That's Right for Your Patients
BY THOMAS G. QUINN, OD, MS, FAAO
Ever encountered a high astigmat who's a great candidate for toric GP lenses, but you wonder if you should fit him with a back or bitoric lens? Relax. This is not something you need to decide upfront. Fit the patient with the ideal fit and adjust power based on any tear lens effects, and the answer will present itself to you!
Where Do You Start?
Begin by looking at the patient's upper lid. If it covers the upper limbus (which on most patients it does), then select a large diameter (9.6mm) so the lens will tuck up under the lid. This approach provides maximum patient comfort and fit stability. If the upper lid lies above the superior limbus, then choose a small diameter (8.6mm) that will fit within the palpebral aperture.
|TABLE 1 Calculating the Lens Design|
Base curves: 8.22mm/7.85mm
Difference between base curves (BC): 2.00 diopters
Difference between powers (BVP): 3.00D
Calculate: 2.00 diopters x 1.5 = 3.00D
Therefore, the lens is a back-surface toric.
Choosing the Base Curves
Choose a base curve for the horizontal meridian that will provide a near alignment fitting relationship. For large lenses, choose a base curve that is 0.25 diopter to 0.50 diopter flatter than the horizontal K reading. For small lenses, choose a base curve that is 0.25 diopter to 0.50 diopter steeper than K.
Choose a base curve for the vertical meridian that will provide a slightly flat fitting relationship (0.75 diopter flatter than the fitting approach taken in the horizontal meridian). This helps promote good lens movement with the blink.
Tear Power When a base curve does not match the K reading, this introduces a tear lens that contributes to the optical system. Base curves flatter than K contribute minus power, and steeper than K base curves contribute plus power.
Lens Power Consider the spectacle correction needed in each meridian, adjust for vertex effects if necessary, then adjust for the tear lens to arrive at the final lens powers. You have now determined all of the lens parameters and can place your order.
But wait -- how do you know if it's a back or bitoric design?
Revealing the Answer
One simple step remains to unveil the lens design. Calculate the difference between the base curves (in diopters) and compare that to the difference in the powers between the two meridians. If the difference in power is about one and a half times* the difference between the base curves, then it's a back-toric design.
If this somewhat unusual relationship does not exist, then the lens also needs a toric curve on the front surface to achieve the desired powers, creating a bitoric design. (Table 1 shows an example of how to make these calculations.)
Why Do We Care?
When you know that a lens is a back toric, you have the option of adding minus sphere power to the lens if indicated, which could save you from ordering a new lens from the lab. Also, many labs charge less for a back-toric GP lens because they need to fabricate a toric surface on only one side of the contact lens. So if you know that a lens is a back toric, it will literally pay you in return!
*1.3 or 1.4 may be a more precise conversion factor for many of the new, lower-index GP materials.
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.