Toric GP Lens Rotation and Induced Astigmatism
BY JOHN MARK JACKSON, OD, MS, FAAO
How does the rotation of a toric GP lens affect visual acuity? The effect of axis misalignment with soft toric lenses is easy to see, because all of the cylinder correction is in the contact lens. But with GPs, the tear lens effect can make it difficult to see how much cylinder you need think about.
Front-surface torics (FSTs) are used when a spherical base curve will fit the cornea, but there is excess residual cylinder with a spherical GP. Some of the cylinder is corrected by the tear lens and some by the contact lens. Rotation of this lens won’t change the tear film power, but it does shift the axis of the lens, which induces a cross-cylinder effect.
The amount of rotation-induced cylinder is simply the cylinder power of the lens itself. This will likely be less cylinder than what that patient would have with a soft toric, so rotation would have less impact on acuity. Rotational stability with a FST can be problematic, though.
Bitorics and Back-Surface Torics
Both bitorics and back-surface torics (BSTs) have toric base curves to improve the fitting relationship on a cornea that has a large amount of regular astigmatism. This “wraps” the base curve around the toric cornea for a better fit.
Because the base curve is toric, when these lenses rotate, the tear film power does change, and that makes the optics a bit trickier to calculate. The tear film and the lens both have a cross-cylinder effect. Fortunately, the effect of all of this is easy to predict.
The amount of cylinder that rotates with a bitoric or BST is equal to the residual astigmatism that you would expect with a spherical GP on that eye. It does not make a difference how flat or steep the base curves are; this relationship will always hold true.
For example, if the refractive cylinder is –4.00 x 180, and the Ks are 42.00/45.00 @ 090 (3.00DC on the cornea), we would expect to have residual astigmatism of –1.00DC with a spherical GP. With a properly-designed bitoric for this eye, if the lens aligns with the patient’s corneal axis (doesn’t rotate), there is no induced cylinder. But, if the lens does rotate (Figure 1), it will be as though you had a –1.00DC soft toric rotating on that eye.
Figure 1. How much cylinder is induced when this bitoric lens rotates?
Putting It Together
Once you know how much cylinder power is generated by rotation, the rules for determining the induced cylinder for toric GPs are similar to those for soft torics (Jackson, 2014). For every 10° of axis misalignment, an over-refraction would pick up one-third of the rotating cylinder power. For our bitoric example above, if it were to misalign 20°, the patient would have two-thirds of –1.00DC, or –0.67DC. If he were wearing a soft toric, with that rotation, it would be two-thirds of –4.00DC, or –2.67DC. It should be clear to see that this bitoric, even with a significant amount of rotation, would provide superior vision for the patient compared to the soft toric.
I created a spreadsheet to show the bitoric over-refraction calculations. A link to this spreadsheet is available by clicking here.
For references, please visit www.clspectrum.com/references and click on document #235.
Dr. Jackson is an associate professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.