Baffled by Bitorics?
BY JOHN MARK JACKSON, OD, MS, FAAO
I recently saw a patient who wanted to try soft toric lenses because she'd tried GP lenses before and said that they didn't work for her they were uncomfortable and unstable, though her vision with them was very good. While not all patients can adapt to GPs, be sure to check the old lenses if possible before abandoning the effort.
Figure 1. Poor fit with a spherical lens on a highly toric cornea.
Recognize When You Need One
My patient's previous practitioner had fit her in spherical GP lenses. Her current findings showed a spectacle prescription of OD +1.00 –4.00 x025, OS +1.25 –4.25 x170 and Ks of OD 42.25/46.25 @ 110 and OS 43.00/47.00 @ 080. Although a spherical GP lens will correct her vision well (because the spectacle cylinder and the K cylinder are very close), the cornea has far too much toricity for a spherical lens to fit well. Her previous doctor should have fit bitorics!
Do bitoric GP lenses intimidate you? They shouldn't. All you're doing when you prescribe them is providing a tailored fit to each corneal meridian to minimize the wobble you'd get with a spherical base curve. My rule-of-thumb is that if the corneal cylinder is more than 3.00D, I definitely prescribe a bitoric. Between 2.00DC and 2.75DC I might need one if a spherical lens is too unstable. Figure 1 shows a poor fitting relationship of a spherical contact lens on a highly toric cornea.
Easier to Fit than You Think
Figure 2. Improved fit with a bitoric lens.
For my patient, I fit her with 9.6mm-diameter bitoric lenses with these base curves: OD 41.75/44.75, OS 42.50/45.50. Notice that I fit the flat and steep meridians a little differently; each lens is 0.50D flatter than K in the flat meridian and 1.50D flatter than K in the steep meridian. This type of bitoric fit is called low toric simulation because it mimics the way a spherical lens would fit on a cornea that has 1.00D corneal cylinder, which is ideal: It has just enough of a rocking movement for good tear exchange and smooth vertical movement with the blink. Figure 2 shows how a bitoric lens looks just like a spherical lens would on a lower-cylinder cornea (though the lens in this photo is a little steep).
Once you choose the base curves, calculate the power by using the tear film optics. Because the lenses are flatter than the cornea, we add plus to the lens power to compensate for the minus tear film. The lens powers are OD +1.50/–1.50 and OS +1.75/–1.75.
Other Bitoric Resources
If all this seems too confusing, you can always use a "cookbook" method like the classic Mandell-Moore fitting guide. You can find this on the GPLI Web site at www.gpli.info/mandell.htm.
But the method of fitting bitoric GP contact lenses isn't as important as recognizing when you need to fit one. Go the extra mile and you'll find fitting these lenses isn't really difficult at all.
Dr. Jackson is an assistant professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.