When Should I Fit a Toric GP Lens?
A GP toric lens design should be used when corneal and/or residual astigmatism is present so as to compromise the fit or vision of a spherical GP or the vision of a soft lens. There are three types of designs:
Bitoric GP Lens
Bitoric lenses are typically indicated when the corneal cylinder is ≥ 2.50D, though if limbus-limbus toricity is exhibited via topography, individuals with as little as 2D of toricity could benefit.
These designs have toric back-surface base curve radii and toric front-surface powers to optimize both the lens-to-cornea fitting relationship and the quality of vision.
Back-Surface (Only) Toric Lens
These designs have similar indications as bitoric designs (i.e., high corneal astigmatism), and the back surface is identical to that of bitoric designs. However, back toric use is limited, as all back toric designs induce a residual astigmatism equal to anywhere from one-third to one-half of the back-surface toricity of the lens (amount depends upon the refractive index of the material).
For example, if the base curve radii are 42D (8.04mm) x 45D (7.50mm) on a with-the-rule astigmatic patient, the amount of astigmatism that will result when over-refracting is -1.00 x 180 to -1.50 x 180. When this is corrected on the front surface (i.e., plus cylinder x 180), a bitoric design has been created, which would not result, in theory, in any cylindrical over-refraction, and the vision is optimized.
An indication for back toric lenses is when the refractive cylinder is 33-50 percent greater than the corneal toricity. In this case, the induced cylinder will correct for the residual astigmatism.
Front Toric Design
These designs are used for the correction of residual astigmatism (i.e., when a difference between refractive and anterior corneal cylinder exists) present when a spherical GP is worn.
A minimum of 0.75D of residual cylinder is often present before visual acuity is reduced sufficiently for a front toric design to be considered. However, with the improvements in soft toric lenses in common use today, front toric GP use is often limited to soft toric lens failures, GP spherical wearers who could achieve better vision, and bitoric lens wearers with residual astigmatism.
The limitation to these designs is the need for prism ballast to stabilize the lens, which has the plus cylinder corrected on the front surface. The weight will often result in inferior decentration. In addition, vision can be variable due to blink-induced rotation.
Useful References
GP Lens Institute
- GP Case Grand Rounds Troubleshooting Guide
- Contact Lens Clinical Pocket Guide
- GP Lens Management Guide
Publications
- Rinehart J. Torics Decoded.
- Newman C. The Basics of Residual Astigmatism and GP Lenses.
Acknowledgements
GP Lens Institute Advisory Committee members: Bruce Anderson OD, Marlane Brown OD, Carmen Castellano OD, Walter Choate OD, S. Barry Eiden OD, John Laurent OD, PhD, Derek Louie, OD, MS, Joe Shovlin OD, Frank Weinstock MD, Bruce Williams OD.