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What guides a surgeon's choise?
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
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:
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.