prescribing
for astigmatism
Toric Torque
BY
TIMOTHY B. EDRINGTON, OD, MS, FAAO, & LONG D. TRAN, OD
Rotational stability is essential for successful
toric soft lens wear. Visual clarity decreases when the correcting cylinder positions
or rotates away from the astigmatism axis. Snyder (1989) showed that 30 degrees of axis misalignment will result in the full amount of the correcting
cylinder appearing in the over-refraction (OR), 15 degrees of misalignment results
in one-half of the correcting cylinder in the OR, and 10 degrees misalignment results
in one-third of the correcting cylinder in the OR.
For example, if the correcting cylinder aligns with the refraction
cylinder axis, you can expect an OR of plano DS; but if the correcting cylinder
of the toric lens is 3.00D and the lens axis misaligns by 15 degrees, 1.50D of cylinder
at an oblique axis will appear in the OR.
Watermelon Seeds
Manufacturers have incorporated many methods of stabilization
into toric soft lens designs since the 1970s. Prism-ballast has been the primary
method. Other methods have included back-surface toricity, truncation, thin-zones,
eccentric lenticulation and designs that combine more than one of these strategies.
The interaction between the eyelids and the lens edge profile
is more critical in stabilizing the rotational position of a prism-ballasted contact
lens than is gravity. To demonstrate this, Dr. Tom Brungardt instructed optometry
students to lay down on a table top with their heads positioned upside-down over
the edge of table. He would then apply a prism-ballasted rigid lens with the prism
base up towards the ceiling and ask the students to blink normally. The prism base
remained toward the ceiling while the students blinked, then rotated so the base
positioned towards the floor when the students stopped blinking (or fainted). Dr.
Tony Hanks referred to this stabilization effect of the upper eyelid as the watermelon
seed effect. If you place a juicy watermelon seed between your thumb and index finger
and squeeze, the rounded portion of the seed tends to precede the apex of the seed
as it leaves your fingers. This demonstrates that upper eyelid pressure functions
to control the rotational orientation of a toric soft contact lens by squeezing
the thinnest portion of the lens periphery. Thin zones, eccentric lenticulation
and prism-ballast designs utilize this theory to stabilize lens rotation and position.
A New Approach
Recently, Vistakon introduced its Accelerated Stabilization Design
in the Acuvue Advance for Astigmatism silicone hydrogel lens. This design utilizes
four active zones near the midperiphery of the lens to minimize lens rotation. These
stabilization zones have a thicker profile than the rest of the lens. By rapidly
increasing the slope of these active zones, the lens employs eyelid dynamics to
quickly reorient when misaligned. Be aware that the active zones of the Acuvue
Advance for Astigmatism lens are aligned within the interpalpebral fissure. Angled
eyelids may require additional adjustment of the axis.
The Take Home
Eyelid dynamics and lens design play equally important roles in
successful toric lens prescribing. When fitting a patient, consider eyelid tone,
eyelid position and aperture size. Also, it's not necessary for the base down marking
to position at 6 o'clock. What is important is consistent location of the correcting
cylinder axis and the rotational stability of the lens.
To expedite the correct rotational position of a toric soft contact
lens, apply the lens base down (or thin zone down). Many of today's toric soft lenses
have markings to indicate the base of the prism. Or you and your patients might
be able to determine the thicker prism base portion of the lens by feel or by visual
inspection.
For references, please visit
www.clspectrum.com/references.asp
and click on document #129.
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: August 2006