prescribing for astigmatism
Toric Silicone Hydrogels
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & LONG D. TRAN, OD
At the Southern California College of Optometry, we've embraced silicone hydrogel lenses. We generally recommend silicone hydrogels to patients wearing traditional soft lens materials when parameter availability permits, particularly when a patient regularly sleeps in his lenses or shows clinical signs of corneal hypoxia such as neovascularization.
Patients requiring thicker lens profiles including high myopes, high hyperopes and astigmats would particularly benefit from wearing silicone hydrogel contact lenses. Eghbali et al (1996) reported on the amount of oxygen transmissibility (Dk/t) through prism-ballasted toric soft contact lenses made of traditional materials. In their sample, measurements of Dk/t at 3mm superior to the lens center allowed on average 13 � 4x10-9, whereas Dk/t through a point 3mm below the lens center was 6 � 1x10-9. At 6mm from the lens center the difference was even greater, revealing that the inferior cornea receives only one-half to one-third the amount of oxygen through toric soft lenses made in traditional materials.
Current Toric Silicone Hydrogel Parameters
|LENS||DK||WATER CONTENT||REPLACEMENT SCHEDULE||BASE CURVE/DIAMETER (MM)||SPHERE POWER (D)||CYLINDER POWER (D)|
|Acuvue Advance for Astigmatism||60||47%||two weeks||8.6/14.5||+6.00 to �9.00||�0.75 to �2.25 0.50D increments|
|PureVision Toric||101||36%||one month||8.7/14.0||+6.00 to �9.00||�0.75 to �2.75 0.50D increments|
|O2Optix Toric||110||33%||two weeks||8.7/14.5||�1.00 to �6.00||�0.75 and �1.25 for Astigmatism|
Toric Silicone Hydrogel Options
Power parameters for available silicone hydrogel torics are currently somewhat limited, but extended parameters become available on a regular basis. Also, additional manufacturers and lenses will soon enter the market.
To date, three toric silicone hydrogel products are distributed in the United States. Table 1 summarizes the current available parameters of these lenses.
The Acuvue Advance for Astigmatism (Vistakon) lens utilizes an Accelerated Stabilization Design in which four active zones of added lens thickness located in the midperiphery of the lens help to enhance lens rotational stability. You may need to alter the axis of the correcting cylinder more for patients who have angled eyelids. The prism base markings are located at 6 o'clock and 12 o'clock, and Vistakon suggests that you need only three minutes before assessing lens rotation and rotational stability.
The PureVision Toric (Bausch & Lomb) lens has a design similar to the company's SofLens 66 Toric, but in the higher-Dk balafilcon A material. It has FDA approval for up to 30 days of continuous wear.
The O2Optix Toric for Astigmatism (CIBA Vision) is not widely distributed in the United States at this time. This new design incorporates prism at the 4 o'clock and 8 o'clock lens positions, leaving the 6 o'clock position thin to minimize interaction with the lower eyelid. Scribe marks are located at 3 o'clock, 6 o'clock and 9 o'clock. It has FDA approval for up to six nights of extended wear.
For references, please visit www.clspectrum.com/references.asp and click on document #133.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org. Dr. Tran is an assistant professor at the Southern California College of Optometry. E-mail him at email@example.com.