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.
|
TABLE
1 |
|
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 tedrington@scco.edu. Dr. Tran is an assistant professor
at the Southern California College of Optometry. E-mail him at
ltran@scco.edu.
Contact Lens Spectrum, Issue: December 2006