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Q: Can I use the "LARS" method to compensate for axis rotation of
ACUVUE TORIC?
A: Yes, you can use the "LARS" method to compensate for axis rotation of ACUVUE® TORIC. However, with the scribe marks in the horizontal meridian, 'left' vs. 'right' rotation can be confusing. Therefore, thinking "clockwise" and "counterclockwise" may be easier. Clockwise (into the future) means adding to the refractive axis, and counterclockwise (back in time) means taking away from the refractive axis.
Example: Spectacle Rx: -3.00-1.25X 10. After stabilization, the initial ACUVUE® TORIC diagnostic lens rotates counterclockwise 20 degrees. The compensated lens should be: -3.00-1.25 X 170.
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Q: Why are the ACUVUE® TORIC orientation marks at 3 and 9 o'clock?
A: The dual thin-zone design of ACUVUE® TORIC provides the highest oxygen transmissibility of any soft toric contact lens. For your patients, this means healthier corneal physiology. Since the thin zones are in the vertical meridian, the thicker portions in the horizontal meridian lend themselves to carrying the scribe marks. This also provides the practitioner with the ability to observe orientation and lens stability without manipulation of the eyelids.
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Q: What is meant by "rotational stability?"
A: While the vast majority of ACUVUE® TORIC fits rotate minimally (less than 10 degrees), the lens may rotate more on a given eye. However, the rotation will remain the same for each subsequent lens, providing the practitioner with the ability to simply adjust the correcting axis and prescribe the ACUVUE® TORIC with confidence that the lens will continue to orient itself just like the initial diagnostic lens.
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Q: Is there a specific top and bottom to the ACUVUE® TORIC, or can it be inserted either way?
A: There is not a "top" or "bottom" to ACUVUE® TORIC:
either thin zone can be stabilized by the upper lid and the orientation of the
optics will remain the same.
By comparison, prism-ballast lenses do have a "top" and "bottom." When the lens placed on the patient's eye with the area of maximum thickness, or prism, in the inferior (6 o'clock) position, it quickly settles. If the lens is not applied in this way, it will often orient correctly after several blinks.
Inserting ACUVUE® TORIC with the scribe marks in the horizontal meridian may hasten the settling time, however it is not necessary to place it in the eye in any particular orientation. No matter which way the lens is placed upon the cornea, the lens should orient properly within a short time.
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Q: Should patients expect good comfort initially when a toric soft lens is applied?
A: A few patients may notice greater "awareness" when switching from one brand of toric lens to another. Similar to first time toric lens wearers changing from a spherical design, some patients changing from a prism ballast design to a thin-zone design may experience increased awareness ("old shoe/new shoe" analogy). The Eye Care Professional should assure the patient that they should feel completely comfortable with their contact lenses within forty-eight hours.
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Q: Should we assume that the lens rotates nasally or temporally when we insert our first diagnostic lens, or should we just pick the axis closest to the spectacle Rx?
A: You cannot assume that a soft toric lens will rotate at all. So your first diagnostic lens choice should be as close as possible to the spectacle Rx. Do not make any assumptions about a clockwise or counterclockwise rotation before inserting the first diagnostic lens.
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Q: Which way do I adjust the rotation: from my observation point or the patient's?
A: The adjustment for any toric lens is made from the observer's
position.
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Q: If I need to adjust for vertex compensation, should I adjust that amount in the cylinder component of the Rx as well?
A: The most accurate way to adjust for vertex distance in
a toric prescription is to draw an optical cross and consider the total power
in each principal meridian.
Example: -5.00 - 1.25 X 180 spectacle Rx. The -5.00 vertex
compensation is -4.75; the vertex compensation for the -6.25 in the other meridian
is -5.75. Thus, the resulting contact lens Rx would be -4.75 - 1.00 X 180 (assume
12 mm vertex distance).
Both meridians do not always require compensation.
Example: -1.00 - 4.50 X 180. In this example, the -1.00 meridian
does not need to be compensated due to its low power, but the other meridian
is -5.50 total power and would be reduced to -5.00, with a final CL Rx of -1.00
-4.00 X 180.
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Q: Do I need to perform a spherocylindrical over-refraction?
A: If the lens axis is stable and the rotation is consistent
with the refractive cylinder axis, it is not routinely necessary to perform
a cylindrical over-refraction with today's soft toric lenses. However, if rotation
is observed, it is safe to assume that any resultant cylinder is being induced
by the rotation. It is therefore recommended that after a spherical over-refraction
is performed, the rotation be compensated based on the amount of rotation observed.
Example: To determine if the appropriate amount of astigmatic
power in the contact lens was selected, it may be valuable to test 0.50D of
cylinder power on axis and 90 degrees away from the axis. If the patient accepts
the 0.50D on axis, then they may need additional cylinder power. If they prefer
their vision with the 0.50D 90 degrees away, they may benefit from a reduction
in cylinder power.
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Q: Should I perform a Jackson cross-cylinder and/or Vectographic conversion for resultant cylinder?
A: Some manufacturers have attempted to simplify the fitting
process by offering "computer assisted" conversion to calculate cross-cylinder
problems. These devices are currently being challenged in terms of their accuracy.
Modern soft toric contact lenses such as ACUVUE® TORIC, have proven
to be so stable in rotation, as well as in optical quality, that a simple adjustment
in rotation will generally eliminate the need for additional calculative conversion.
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Q: Once I've compensated for the axis rotation of the diagnostic lens, where should I expect the patient's new lens to orient?
A: With ACUVUE® TORIC, in which the stabilization
system is independent of the optical system, you should expect the patient's
new lens to orient in the same manner as the diagnostic lens, even after you've
compensated for axis rotation.
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Q: What are the benefits of a disposable toric soft lens?
A: The benefits of disposability are similar for spherical
or toric patients. More frequent replacement of soft lenses has been demonstrated
clinically to reduce the incidence of Giant Papillary Conjunctivitis (GPC).
In addition, more frequent replacement has also been shown to improve patient
comfort and performance due to cleaner, more wettable surfaces.
An additional consideration with torics, which unlike sphericals
do not rotate in the eye, is that certain areas of the lens build up deposits
more quickly than others. An uneven build up of deposits may impact on the rotational
stability of the lens.
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