Refractive Lens Surgery

60 S. Bylsma
aphakic state. In contrast, tissue-directed
treatments such as astigmatic keratectomy
(AK) [7–11], limbal relaxing incisions (LRIs)
[12–16], laser-assisted in-situ keratomileusis
(LASIK) [17, 18], photorefractive keratectomy
(PRK) [19, 20], paired keratotomy incisions
[21, 22], and on-axis incisions [23] correct
astigmatism by altering the shape of
the cornea; the astigmatic treatment is adjunctive
to the spherical IOL correction of
aphakia. These adjunctive corneal procedures
all share the potential for postoperative
tissue remodeling, which may lead to a regression
and diminution of the treatment efficacy
over time. While LASIK and PRK offer
quite precise and relatively stable treatment
of astigmatism, their cost is prohibitive to
many refractive lens surgeons and patients.
As a result, LRIs and AK are often used with a
spherical IOL, but these may be less predictable,
require nomogram adjustments for
age- and gender-specific variation between
individuals and are known to be less reliable
for younger eyes and higher magnitudes of
corneal astigmatism [24].
In contrast, optical treatments of corneal
astigmatism have the advantages of offering a
less invasive, single-step surgery implantation
of an inert device of precise refractive
power that does not change over time. In addition,
higher magnitudes of astigmatism
may be treated optically compared to the tissue-limited
treatment. For example, various
custom toric optics have been successfully
used for cases of up to 30 diopters of astigmatism
[25–28]. Thus, the significant potential
benefits of optical rather than structural correction
of astigmatism at the time of lens refractive
surgery include simplicity, precision,
versatility, and refractive stability.
Despite these advantages, a potential
drawback of optical correction is the possibility
of the toric IOL deviating from the intended
cylinder axis after implantation. Offaxis
rotation of a toric IOL will decrease the
desired astigmatic correction in proportion
to the magnitude of deviation. If the toric IOL
is rotated off-axis by 10–20 degrees, the astigmatic
correction is decreased by about onethird,
and if off by 20–30 degrees, it is decreased
by about two-thirds [26, 29]. Beyond
30 degrees of off-axis rotation, astigmatism is
no longer corrected and may in fact increase
with more severe malpositions. Therefore, a
toric IOL must not only be implanted in the
proper corneal meridian, but also resist longterm
off-axis rotation successfully to treat
astigmatism. As we will see, early studies of
the toric IOL were characterized by intensive
investigations to determine the design that
was most rotationally stable and best suited
for development into a toric IOL. The result of
these early efforts is that the only toric IOL
that has reached Food and Drug Administration
approval in the USA to date is one of
plate-haptic design.
Thus, refractive lens surgeons confronted
with an astigmatic patient have a choice to
correct astigmatism with tissue treatment
versus optical correction. Each modality has
its inherent advantages and disadvantages. In
this chapter, we will explore the clinical use of
the only Food and Drug Administration
(FDA)-approved toric IOL available in the
USA: the Staar toric IOL (STIOL).
7.2 The Staar Toric IOL
Today, the STIOL is the only pseudophakic
IOL available in the USA for the correction of
astigmatism. The STIOL is a posterior-chamber
foldable IOL made of first-generation silicone
that employs a plate-haptic design
(Fig. 7.1). The STIOL is available in two models,
both with 6.0-mm optics (model AA-
4203-TF and model AA-4203-TL; Staar Surgical,
Monrovia, CA). The two models differ in
their overall length. Model AA-4203-TF,
which is now available in a spherical equivalent
(SE) power from 21.5 to 28.5 D (no longer
to 30.5 D), is 10.8 mm in length, while model
AA-4203-TL is available from 9.5 to 23.5 D SE
power and is 11.2 mm in overall length. The