prescribing for astigmatism
Choosing the Right GP Toric
BY JULIE A. SCHORNACK, OD, MED
Although I wrote about the benefits of bitoric GP lenses in my October column, I feel that I did not give the subject the attention it deserves. This month I will delve a bit more into the as pects of fitting bitoric lenses on appropriate patients.
Whenever a patient presents in the office with 1.50D or more of corneal toricity, my GP toric design alarm bells start to go off. A careful analysis of corneal toricity and the patient's subjective refraction will often provide the key to toric lens design selection.
This is not to suggest that a spherical GP contact lens cannot be fit on toric corneas in excess of 1.50D. However, a spherical GP lens on increasingly toric corneas is more likely to cause lens awareness, result in fluctuating vision due to an excessively high- or low-riding lens, or cause dryness and staining.
Stealing liberally from lessons that I've learned from Tim Edrington, OD, MS, FAAO, at the Southern California College of Optometry, I use the following guidelines (Table 1) when considering the appropriate GP toric lens design.
SPE Versus CPE
When the amount of corneal toricity is 1.50D or greater and a spherical GP diagnostic contact lens yields less than 0.75D of residual cylinder in the over-refraction, consider a spherical power effect (SPE) GP design. What will tip the scales in favor of this lens design is if the spherical diagnostic lens demonstrates an excessive amount of movement on each blink or if it rides excessively low or high. A toric back surface on the contact lens will considerably improve centration and comfort.
A toric back surface induces cylinder (remember the 1-2-3 rule?) in the contact lens power that is not needed because the over-refraction cylinder is close to negligible. That is why the lens is designed to have a spherical power effect in which the front-surface toricity cancels this induced cylinder.
In comparison, when you place a spherical diagnostic contact lens on a patient who has 1.50D or more of corneal toricity and the amount of over-refraction cylinder is greater than 0.75D, then a cylindrical power effect (CPE) lens may be the best option. In this circumstance, the toricity on the back surface of the contact lens helps with fit and comfort, but the induced cylinder is not enough to correct the patient's refractive needs. Therefore, the remainder of the required cylinder is obtained though toric power on the front surface of the contact lens.
Base Curve Toric Designs
Select a base curve toric lens design when the 1-2-3 rule works in your favor. In these circumstances, the patient exhibits 1.50D or greater of corneal toricity. At this point you know that you may need a toric back surface to improve the fit and comfort of the GP lens. If during the over-refraction on a spherical GP diagnostic lens you find that the over-refraction cylinder is approximately one-half the value of the corneal toricity at approximately the same axis as the corneal toricity, then the base curve toric design is perfect for this patient.
The beauty of a base curve toric design is that there is toricity only on one surface of the contact lens, so the cost of the lens is often less then what we see with bitorics. CLS
Dr. Schornack is the associate dean of Clinical Education and serves in the Cornea and Contact Lens Service at the Southern California College of Optometry.