Consultant's Corner
Utilizing Back Toric Lens Designs in Your GP Practice
by Michael A. Johnson, FCLSA
Director of Consultation Services
Back toric lens designs are still somewhat controversial; however, Art Optical Contact Lens, Inc. utilizes back toric lens designs for approximately 85% of the high toric corneas we assist in fitting. The majority of these fits (80% or greater) end up successful on the first pair of lenses designed.
Our best success comes with first time lens wearers, free from pathological problems, who have 3 diopters or greater of corneal astigmatism and a near equal amount of spectacle cylinder. We have also achieved success on corneas with as high as 8 diopters of cylinder.
The design Art Optical utilizes is the two-thirds principle which requires using 2/3 of the spectacle cylinder as the difference between the flat and steep base curve.
The theory is based on the known value that the dioptric difference between the two base curves of a back toric lens multiplied by the refractive index of the material used will create a certain in-air cylinder.
Example: Base curves of a known back toric lens are 8.03/7.67 or (42.00/44.00) which is 2.0 diopters of cylinder. Two diopters of cylinder multiplied by the refractive index of the material (1.5) or 2.0×1.5 = 3.00 diopters of in-air cylinder.
By reversing this formula you can determine how much toricity to use on a back toric lens design to neutralize a known amount of spectacle cylinder. Take the spectacle cylinder and divide it by the refractive index of the material. This sum is the dioptric difference you must have between the flat and steep base curve to neutralize the spectacle cylinder.
Example: K’s 42.00/45.00 RX -3.00-3.00 X 180.
Take the spectacle cylinder (-3.00) and divide it by the refractive index (1.5). -3.00 divide 1.5 = 2.0 diopters of toricity needed between the flat and steep base curves. The resultant base curves for the back toric lens design should be 8.03/7.67 (42.00/44.00). I recommend typically staying on flat K for the primary base curve and adding the difference to get the steep base curve.
No cylinder power need be specified in a back toric lens design. The cylinder generated is totally produced by the back surface toricity. Adjust your sphere power for any vertex change necessary.
A back toric lens design will work as long as there is enough corneal cylinder present to equal 2/3 of the spectacle cylinder. When there is a need to generate more cylinder than what is present in the K’s, then a bi-toric design should be utilized.
For assistance, call our Consultation Direct Line at (800) 566-8001.
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