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EyeNet
Magazine >> Cataract
Cataract
Pearls for
Implanting the Staar Toric IOL
By David F. Chang, MD, Special to EyeNet Magazine Toric
IOLs are an excellent adjunct or complement to corneal
astigmatic incisions, particularly in those cases where
limbal-relaxing incisions aren't powerful or predictable
enough. However, success with the toric IOL depends upon
avoiding early postoperative rotation of the lens, which
reduces the amount of astigmatic correction. In turn, proper
alignment depends upon proper IOL sizing–as my experience
demonstrates.
Toric IOL
Indications
Like many ophthalmologists, I have found
limbal-relaxing incisions (LRIs) to be effective and reliable
in reducing lower amounts of astigmatism (< 2.00 D). I use
a preset 600-micron blade and the Gills nomogram. While this
method doesn't adjust for individual variation in peripheral
corneal thickness and corneal diameter, the dominant variable
is the patient's age. As with all incisional keratotomy,
increasing age amplifies the effect of a given incision
length. However, the greater the amount of targeted
astigmatism correction, the more variable are the results of
LRIs. For these reasons, I have found the toric IOL to be
particularly appropriate for two groups of patients:
Cataract patients who are younger than 65 and who have
>2.00 D of astigmatism.
Their younger age significantly reduces the attainable effect
from incisional keratotomy.
Patients with the largest degrees of astigmatism (>
3.00 D). In addition to more unpredictability, against the
rule astigmatism in these patients requires combining the
temporal clear corneal incision with a temporal LRI. Slight
override of the incision edges can cause significant
discomfort and foreign-body sensations for these
patients.
Toric IOL
Specifications
The Staar toric plate haptic IOL has
the large haptic fenestrations and comes in two astigmatic
powers and two lengths:
The +2.00 toric lens corrects approximately 1.50 D of
keratometric astigmatism, while the +3.50 toric corrects
approximately 2.25 D.
The "TF" lens has an overall length of 10.8 mm. The longer
11.2-mm "TL" model was later released for spherical powers of
< 23.5 D. The haptics of the longer lens also have a matte
finish to make them less slippery.
The spherical power
of the toric IOL is calculated in the same way as for a
conventional IOL. Next, the astigmatic power is selected
without having to adjust the spherical power. Hash marks on
the IOL allow this cylindrical power to be surgically aligned
with the steeper "plus" axis of astigmatism.
To avoid
any potential influence of lenticular astigmatism, one should
use the keratometric reading, rather than the refraction, for
selecting the IOL toric power and axis. Rigid contact lens
wear should be discontinued for one week to obtain more
accurate kerotometry readings. These patients should be told
that any residual postoperative astigmatism can be corrected
only with spectacles or a toric soft contact
lens.
Personal Learning
Curve
In February 1999, I implanted my first six
"TF" (10.8-mm) toric IOLs in four patients. The longer "TL"
(11.2-mm) lens had not yet been announced or released. I used
Viscoat (chondroitin sulfate plus sodium hyaluronate) in these
uncomplicated cases under topical anesthesia, and proper IOL
orientation was confirmed each time by the operative video.
In three eyes, the axis remained within ±15 degrees of
the targeted axis postoperatively. However, one patient had
misalignments of 70 degrees and 85 degrees in each of her eyes
by the first postoperative day. Each time, the IOL was
immediately repositioned, as the doubling of her astigmatism
was intolerable. Both lenses have remained perfectly aligned
since then.
A third IOL was 30 degrees misaligned by
the first postoperative day, and I waited the recommended two
weeks to surgically realign it. At surgery, I discovered that
the anterior capsule was already fibrotic and the anterior and
posterior capsules had already fused peripheral to the IOL.
Use of a spatula and viscodissection were needed to reopen the
capsular bag so that the IOL could be rotated. The lens didn't
move easily, but proper and stable alignment eventually was
achieved.
Staar made the first TL lenses in the United
States available to me in March 1999. I have used only these
longer IOLs in the next 37 consecutive cases, and none have
needed to be repositioned. All have been with the higher +3.50
D toric power.
Pearls for
Implantation
My current technique is as
follows:
Always use the longer TL IOL, if available, in the desired
power.
Use a cohesive viscoelastic (such as Healon, Provisc or
Biolon), which is less likely than dispersive viscoelastics to
coat and lubricate the IOL surface.
Remove viscoelastic trapped behind the IOL with
irrigation-aspiration to maximize contact between the IOL and
the posterior capsule.
Don't overinflate the eye at the end of surgery. Leaving
the eye somewhat "softer" than usual probably allows the
flaccid capsular bag to collapse around the IOL more quickly.
Use an astigmatically neutral, temporal clear corneal
incision. Preoperatively, I inform patients that surgical
repositioning may be needed.
Postoperatively, I
perform a dilated exam on the first and sixth postop days to
check the IOL axis. Any rotation tends to occur between the
time of surgery and the first postop morning. I have yet to
record any significant change from the first postop day axis.
Based upon my experience with three badly misaligned
cases, I would perform repositioning by one week or earlier.
Once the capsular bag fully contracts (this occurred by two
weeks with my third case), more force is required to rotate
the IOL, and this might increase the chance of tearing the
capsular bag.
Excellent rotational stability was
achieved with this technique in my series of 37 consecutive TL
toric implants. Sixty-seven percent were within 5 degrees of
the target axis, while 89 percent were within 10 degrees and
100 percent were within 15 degrees. This was significantly
better than the FDA study cohort receiving the shorter IOL
(see sidebar). My repositioning rate was 0 percent with the
longer TL IOL, vs. 50 percent with the shorter TF IOL.
Importance of Rotational
Stability
The Staar plate haptic toric IOL
became the first FDA-approved toric lens in November 1998.
Studies suggested that the plate haptic design is superior to
that of curved haptic designs in terms of long-term rotational
stability.1,2 Using serial
digital photographs, Patel demonstrated that late
postoperative rotation was unusual with plate haptic lenses
but more frequent with three-piece designs.
However,
Patel's study also confirmed that both lens designs could
rotate during the immediate postoperative period–and that the
plate haptic design did so more frequently.2 Presumably, it isn't until the
capsular bag contracts that the corners of the plate haptic
lens are able to resist natural rotational forces.
It's important to note that the Patel study used a
10.5-mm plate haptic IOL, which is shorter than the two sizes
(10.8 and 11.2 mm) available from Staar.
Misalignment
of the toric IOL axis reduces the amount of astigmatism
correction. With 10 degrees of axis deviation, one-third of
the effect is lost. With 20 degrees of axis deviation,
two-thirds of the effect is lost. Lens misalignment greater
than 30 degrees produces a net increase in astigmatism.
In the U.S. FDA study data, 24 percent of the toric
IOLs ended up more than 10 degrees off axis–12 percent were
more than 20 degrees off, 8 percent were more than 30 degrees
off and 5 percent were more than 45 degrees off. The FDA study
used the shorter 10.8-mm toric size.
So far,
significant late rotation of the lenses has not been reported,
consistent with the published Patel findings.3 Thus, the key to success with
the toric IOL is in avoiding early postoperative rotation of
the lens. This depends upon having an adequately long IOL for
the patient's capsular bag diameter. The learning curve for my
first 43 cases underscores this point.
1 K. Shimizu et
al. J Cataract Refract Surg
1994;20:523-526.
2 C. K. Patel et al. Ophthalmology
1999;106:2190-2196.
3 I. Ruhswurm et al. J Cataract Refract Surg
2000;26:1022-1027.
Dr.
Chang is clinical professor of ophthalmology at the University
of California, San Francisco, and is in private practice in
Los Altos, Calif. He has no financial interest in any of the
products mentioned.
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