Toric intraocular lenses are emerging today in cataract practice as one of the standards of quality as they allow control
not only of the spherical correction but also the astigmatic preoperative condition of the patient on an accurate basis. Even
though these lenses have been on the market for at least six years, it is not until recently that many surgeons have begun
to incorporate them into their practices. The reason for that is the initial apparent complexity of the use concerning preoperative
and interoperative management of the patient, selection of the IOL, selection of the adequate lenses for purpose and knowledge
about the postoperative outcomes.
Today, toric pseudophakic intraocular lenses are finding an excellent environment for their use; as the refractive outcome
of cataract surgery is becoming more and more important and the incision size has been decreased to mini-incisions (2.2 mm)
or microincisions either biaxial or coaxial (sub 1.8 mm). It is clear that without adequate control of the astigmatic outcome
of cataract surgery, toric lenses do not have a precise role that is good enough to compensate for the burden of higher cataract
surgical costs both for patients and medical organizations.
Astigmatism as a refractive error is a visually disabling problem affecting the general population, most especially those
afflicted with cataracts. Around 15% to 20% of cataract patients have at least 1.5 diopters (D) of corneal or refractive astigmatism.1
In contrast to normal methods for the correction of astigmatism during cataract surgery, such as limbal relaxing incisions
in which many variables are involved in the outcome and are not precise enough, toric IOL implantation offers a predictable,
stable and safer way to reduce pre-existing astigmatism. Combined with small incisions, cataract surgery techniques can provide
a greater opportunity to correct cylindrical errors intraoperatively, thus improving visual quality leading to spectacle independence.
The concept of neutralizing congenital corneal astigmatism using a rigid PMMA toric intraocular lens was first developed by
Shimizu in 1992, the same year in which Grabow and Shepherd implanted the first foldable silicone toric plate haptic IOL.
Indications and preoperative assessments of candidates for toric IOLs
Except for the specific contraindications detailed below, there are no other reasons besides financial ones for not implanting
toric IOLs in eyes suffering corneal astigmatism greater than 1.5D in the 'non-astigmatic cataract surgery age.'
These contraindications can include:
- Irregular corneal astigmatism.
- Any condition which could lead to intra or postoperative misalignment of the axis or inadequate centring of the optics such
as: Evidence or suspicion of zonullar instability. Crystalline lens subluxation. Sulcus implant (if specific adequate models
of toric IOLs for sulcus implantation are not available).
- Important ectopic pupil (pupiloplastia should be performed intraoperatively).
- Refractive but not corneal astigmatism (lens induced preoperative cylinder).
- Extracapsular extraction or surgical techniques causing unpredictable surgical induced astigmatism.
Preoperative assessment does not differ from the routine exams to be done before cataract surgery except for the need to perform
manual keratometry or topographic exam, discarding values obtained from automatic keratometers.
Models of pseudophakic toric IOLs
There are two models of toric IOLs with fixed torus available for the surgeon on the market. However, this obviously limits
the accuracy for correcting, with precision, the huge amount of possible spherocylindrical combinations.
Toric IOLs with fixed torus
Staar Toric IOL STAAR AA4203TF (StaarSurgical, Monrovia, California)
The Staar Surgical Toric Lens was the first toric intraocular lens approved by the US Food and Drugs Administration (FDA)
for use in the United States (1998). This plate haptic silicone lens provides a full range of spherical powers but only two
cylinder power options: 2.00 and 3.50D of astigmatism at the IOL plane which corresponds to a correction of 1.54 or 2.30 respectively
at the height of the cornea.
Acrysof Toric IOL (Alcon Laboratories Inc., Fort Worth, Texas)
The Acrysof Toric lens is composed of an acrylic polymer that has UV and blue-light absorbers. The lens is built on the same
platform as the Acrysof Single-Piece monofocal models SA60AT and SN60AT IOLs (Alcon Laboratories Inc.) and can be folded or
injected to be inserted inside the eye.
Currently, the IOL is available in powers of +1.50D (SN60T3), +2.25D (SN60T4) and +3.00D (SN60T5), which are supposed to correct
1.03, 1.55 and 2.06 dioptres respectively at the corneal plane.
Greater magnitude of astigmatism can be corrected with SN60T6, SN60T7, SN60T8 and SN60T9 (which would correct 3.75, 4.5, 5.25
and 6 D at the plane of the IOL and thus 2.57, 3.08, 3.60 and 4.11 respectively at the corneal plane), but these models are
still pending FDA approval.
The AcrySof IQ Toric IOL has recently been designed with an aspherical profile.