Refractive Lens Surgery

50 L.D. Nichamin
surgeon the ability to reduce cylinder error
effectively, safely, and reproducibly to acceptable
levels of 0.50 D or less.
6.2 Surgical Options
Options to reduce astigmatism include manipulation
of the main incision, supplemental
peripheral relaxing incisions, use of a toric
intraocular lens (IOL) or Bioptics. Limbal relaxing
incisions (LRIs) are the most commonly
employed approach and have proven
to be safe, successful and cost-effective. Lens
rotation with toric IOLs remains a consideration
with their use. Bioptics with the excimer
laser affords exquisite accuracy to reduce
residual astigmatism and spherical error.
Several different approaches may be taken
to reduce or eliminate pre-existing astigmatism
either at the time of,or following lens exchange
surgery. Perhaps the simplest method
is to manipulate the main surgical incision in
order to achieve a particular degree of astigmatic
reduction. This is accomplished by centering
the incision upon the steep corneal
meridian (or positive cylinder axis) and then,
by varying its size and design, one may effect
a certain amount of wound flattening [1, 2].
This approach, however, presents logistical
challenges, including movement around the
surgical table, often producing awkward
hand positions. In addition, varying surgical
instrumentation may be required along with
a dynamic mindset. For these reasons, this
approach has largely been supplanted by other
techniques, most notably through the use
of additional relaxing incisions, as described
in detail below.
Another viable means to reduce astigmatism
is through the use of a toric IOL [3]. This
option has the advantage of avoiding additional
corneal surgery, at least for modestto-moderate
levels of cylinder, or may be
combined quite effectively with additional
keratorefractive techniques to reduce high
levels of astigmatism [4]. This alternative, although
effective, has seen somewhat limited
acceptance, at least within the USA. This may
be due to Food and Drug Administration
(FDA) approval of only one toric implant thus
far – a single-piece plate-haptic design comprised
of an early-generation silicone elastomer
with a relatively low index of refraction
– a design that seems to have generated only
modest interest at this time.
This particular implant, Staar Surgical’s
model AA-4203, is available in two toric powers
of 2.0 and 3.5 D that will correct 1.4 and
2.3 D, respectively, at the corneal plane. The
lens is manufactured in two overall lengths:
the TF version, which is 10.8 mm and available
in spherical powers of 21.5–28.5 D, and
the TL version, which is 11.2 mm in length
and runs from 9.5 up to 23.5 D. The most
widely encountered problem with this device
is postoperative rotation. Euler’s theorem reminds
us that axis misalignment of 5, 10, and
15 degrees will result in 17, 33, and 59% reduction,
respectively, of surgical effect [5].
Reports of significant rotation with this implant
vary from 9.2 to 18.9% [3, 6]. Optimal
timing for repositioning would appear to be
between 1 and 2 weeks postoperatively, just as
capsular bag fibrosis is beginning to take
place. The use of toric implants will likely increase
as newer designs reach the marketplace.
An additional keratorefractive option to
reduce astigmatism in association with implant
surgery exploits the advanced technology
of the excimer laser. This is generally performed
subsequent to the lens exchange
procedure, similar to its use with myopic phakic
implants as first described by Zaldivar,
and is now widely referred to as Bioptics [7].
More recently approved modalities such as
conductive keratoplasty are now being studied
in an off-label fashion to enhance both hyperopic
spherical and astigmatic error [8].