Anyone who is going to fit gas-permeable (GP) lenses will need to fit toric lenses. So, let’s simplify the toric lens fitting experience.
Front toric contact lenses are used when the amount of refractive astigmatism is significantly different from the corneal astigmatism and when there is no need for a toric back surface. Just incorporate the difference into the lens to create a front toric lens. If you’re in doubt about the amount of residual cylinder, place a spherical lens on the eye and perform a sphere-cylinder over-refraction. Then, add this power to the power of the diagnostic lens.
The goal is to have a well-centered lens that moves on each blink. The front toric lens needs to be stabilized so as not to rotate, which is achieved by adding prism to the lens and/or truncating the lens. If the lens still rotates after prism is increased and the lens is truncated, it will be necessary to adjust the axis of the prism. If the lens rotates to the left from the six o’clock position, using Left Add Right Subtract (LARS), add the amount of rotation to the axis. Conversely, if it rotates to the right, subtract that amount of rotation from the axis. The more minus power in the vertical meridian, the more prism is needed to minimize rotation. Prism power will vary from 1.0 prism diopter to 3.0 prism diopters.
Back toric lenses can be divided into sphere power equivalent (SPE) lenses and and cylinder power equivalent lenses (CPE). The difference is the SPE lens is bitoric—i.e., both front and back surfaces are toric—while the back toric lens has a spherical front surface. A back toric fit is needed when the corneal toricity does not allow a spherical base curve to center properly on the eye. Back toric lenses are ideal when the corneal toricity extends nearly from limbus to limbus. The goal is to have the lens touch lightly in the midperiphery of the horizontal meridian and to have about 0.75D of clearance in the vertical meridian.
When fitting an SPE lens, the power of lens of the flattest meridian is determined by the same method as used with a spherical lens. Subtract the amount by which the flattest meridian is flatter than flat K from the spherical component of the subjective refraction. To determine the power of the steeper meridian, add the amount of back surface toricity (in diopters) to the power of the flattest meridian. The flattest meridian will be the most plus (or least minus) power.
For instance, if the subjective refraction is -2.00 -2.75 x 180, and Ks are 44.00/46.75 @ 090, the base curve will be 7.67 (44.00D)/7.34 (46.00D). The lens power in the flat meridian will be -2.00D, since this meridian is fit “on K.” The steep meridian is fit 2.00D steeper than the flat meridian; therefore, the power of that meridian would be -4.00D. The lens powers shown are what will be read in the lensometer.
Back toric (only) lenses can be used when the amount of refractive cylinder is greater than the amount of corneal cylinder. For example, if the subjective refraction is -2.00 -3.50 x 180, and Ks are 44.00/46.50 @ 090, you would expect there to be 1.00D of residual astigmatism when this patient is fit with a spherical lens. This could be corrected with a front toric lens as described above or with a back surface toric lens.
The base curve of a back toric lens is determined the same way as the SPE example above. When choosing a back toric lens, the resultant power depends on the index of refraction of the lens material. The amount of refractive cylinder will be determined by this formula:
Index of refraction
of lens material Toricity of
- 1.00 x the base curve
Index of refraction of the lens
of keratometer - 1.00
If we use SGP2 material (N = 1.485) in the above example and fit the lens with 2.00D of toricity, the amount of cylinder in the lens as read on the lensometer will be:
1.485 – 1.00 x 2.00 = 2.87D
1.3375 – 1.00
In this example, the patient would be under-corrected by only 0.50D cylinder, which should not cause any problems.
But, if the patient were fit with a low index of refraction material such as Bausch + Lomb’s Boston XO (N = 1.415), the lensometer would read:
-1.415 x 2.00 = 2.45D cylinder -1.3375.
In this example, the patient would be under-corrected by more than 1.00D cylinder, which could create vision problems for the patient.