Consultant's Corner

Fitting Against-the-Rule Astigmatism

by Michael A. Johnson, FCLSA
Director of Consultation Services

One of the more difficult GP lens patients is the “against-the-rule” astigmat who requires a prism ballast, front surface, cylinder lens design.

These patients will typically show a diopter or more of against-the-rule spectacle cylinder with little or no corneal cylinder. This will generally result in residual cylinder, causing visual difficulties in a basic spherical lens design. You can possibly avoid having to prescribe a front toric if your patient has a moderate to high myopic correction and there is 1.0 diopter or less of residual cylinder present over a spherical lens power. In this case, you can attempt to use a spherical equivalent power. The acuity obtained and the patient acceptance of that acuity will be the determining factor to the success of the spherical equivalent. You may also attempt to use a panofocal front surface along with the spherical equivalent power to aid the acuity. A panofocal front surface is a form of an aspheric effect which can be applied to the front surface of spherical rigid lenses. It helps to redirect spherical and chromatic aberrations from the peripheral margins of the lens, which can sharpen the acuity for the patient. A panofocal effect does not work all the time and is much more beneficial when there is .75 diopter or less of residual cylinder present.

When all other attempts to avoid using a front toric lens fail, then you are left with only two choices for contact lenses. An oxygen permeable prism ballast front surface cylinder or a soft toric lens design. When you choose to use an RGP lens design, I recommend using the following guidelines:

Fitting Step 1
Use a spherical trial set to determine the best fit relationship for base curve and diameter. A diameter no smaller than 8.8 and averaging between 9.2 to 9.6 is recommended due to the base down prism which will be required to stabilize the cylinder axis of the front toric design.

Fitting Step 2
Do a complete over-refraction for sphere and cylinder over the best fit trial lens to determine the power required for the front toric. There is a natural tendency for prism ballasted lenses to rotate nasally 5 to 15 degrees. For this reason, change the cylinder axis you come up with ten degrees temporally. On the OD subtract 10 degrees and on the OS add 10 degrees to the cylinder axis.

Example
OD power is -200-100×090; change axis 10 degrees temporal to make final power -200-100×080. OS power is -200-100×090; change axis 10 degrees temporal to make the final power -200-100×100.

Fitting Step 3
The standard amount of prism generally used is 1.5 diopters. You can use less prism (1.0 to 1.25 diopters) if you have a low minus or any plus sphere power, and you may need more prism (1.75 to 2.0 diopters) for high minus sphere powers (-7.00 or greater) to maintain the cylinder axis stability.

Fitting Step 4
You are now ready to order your front toric lens by using the base curve and diameter from your trial lens, the power obtained from the over-refraction with the cylinder axis shifted 10 degrees temporally and the proper prism amount to stabilize the cylinder axis.

Art Optical will automatically place a red dot at the base of the prism so you can observe how the lenses are orientating on the patient. You should observe the red dot positioned nasally approximately 10 degrees.

After you have designed, ordered and received your front toric lenses, you should first verify the specifications prior to preparing the lenses for dispensing. I recommend using the following procedure to verify the power:

Verification Step 1
Preset your lensometer to aid you in reading a prism ballasted front surface lens power. Note the power you have ordered and preset the cylinder axis on the lensometer to what the lens should read. Set the power wheel at the spherical power prescribed for the lens.

Example
Lens power is listed at -2.00-1.00×075. Set the axis wheel on your lensometer to axis 075, set the power wheel on your lensometer to -2.00.

Verification Step 2
Now place the contact lens into the lensometer with the concave side facing the lensometer and the convex side facing you. Rotate the lens until the spherical power line comes in clearly. When the spherical power is clear, the prism in the lens will be base down and should be at 90 degrees. Turn the power wheel clockwise to verify the cylinder amount. If your spherical power was -2.00 and you have specified -1.00 cylinder then the cylinder power lines (the three wide spaced lines) will come in to clear focus at -3.00 on your power wheel. The difference between the -2.00 spherical line and the -3.00 cylinder lines equals -1.00 of cylinder. The same procedure will hold true if you specified the power in plus cylinder form with the exception of turning the power wheel counter-clockwise to find your cylinder power.

After verifying your lens powers, clean the lenses one more time and store them in a soaking solution for at least four hours prior to dispensing. I would recommend soaking them overnight to assure you have a good pretreated and wettable surface.

When you place the lenses on the patient at the time of dispensing, allow approximately 15 to 30 minutes for the lenses to settle down before checking the acuity and examining the fit. Art Optical will place a red dot at the base of the prism to help you observe the prism alignment. By using the slit lamp you may observe the red dot and verify if the prism is base down at 90 degrees or rotating slightly nasally as usually predicted.

If the patient is not obtaining good acuity, then do a sphere and cylinder over-refraction over the lens. If the patient needs just a spherical power change over the lens you can have the existing lens modified or order a new lens with the corrected power. If the patient requires additional cylinder over the lens, then note the axis where the cylinder is showing through. If the cylinder is showing through at the same axis as was specified for the lens power, then add the cylinder amount to the existing power and reorder the lens.

Example
Lens power is -2.00-1.50×090, over-refraction is -0.25-0.50×090, then the new power should be -2.25-2.00X090. If the axis is showing up 90 degrees away from the specified axis then the over-refraction should be transposed before adding to the existing power.

Example
Lens power is -2.00-1.50×090; over-refraction is plano -0.50×180. Transpose the over-refraction to -0.50+0.50×090. The new power should then be -2.50-1.00×090.

The lenses will be expected to ride slightly inferior and rest near the lower lids. If the lenses are riding below the inferior limbus, then you will more than likely have to alter the fit. You may want to go to a larger diameter with a little less prism to get the lenses to come up and you may also want to incorporate a lenticular design with a slight minus carrier to get better centration. If the lenses ride too high, then you can increase the base down prism amount, and if necessary, use a lenticular design with a plus edge to reduce the upper lid lift.

If the physical fit of the lenses are good and the acuity is acceptable, then the lenses can be dispensed to the patient. Start the patient out on a slower wearing time schedule to help with the comfort factor. Have them start out at 2-3 hours the first day and build up one hour every other day until they reach eight hours and then they can build up one hour everyday until they reach full waking hours.

As always, consultation assistance is immediately available calling our toll-free, Consultation Direct Line at (800) 566-8001.


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