Achieving an excellent refractive result after cataract surgery requires that we surgeons nail both the sphere and the cylinder of the postoperative refraction. This means addressing the preoperative corneal astigmatism at the time of cataract surgery. The most powerful and accurate surgical tool for neutralizing corneal astigmatism is a toric IOL.
STEP No. 1. MEASURE
At the preoperative examination, the amount and meridian of corneal astigmatism must be accurately measured. Note that just the corneal astigmatism is desired, not the refractive astigmatism of the whole eye, because the latter includes lenticular astigmatism induced by the cataract. The best candidates for toric IOLs have regular, symmetric, stable corneal astigmatism.
You can use many different biometers to determine the amount and the axis of corneal astigmatism, including manual keratometers, corneal topographers, autokeratometers, or the keratometer built into your optical coherence biometry unit. Corneal topography accurately identifies the axis as well as the degree of regularity. Measuring the posterior corneal astigmatism is more challenging, because it requires a tomography unit such as the dual Scheimpflug analyzers.
STEP No. 2. CALCULATE
The clear corneal incision used during phacoemulsification has an effect on the corneal astigmatism, typically a flattening at the meridian of the incision of about 0.30 to 0.50 D, depending on the size of the incision. You must take this factor into account when implanting a toric IOL. Placing the incision on the same axis as the astigmatism will enhance the effect of the toric IOL, whereas placing it 90º away will reduce the effect. To take into account the average posterior corneal astigmatism as well as plan for a mild against-the-rule shift with age, for most eyes, it is beneficial to target a postoperative goal of 0.25 to 0.50 D of with-the-rule anterior corneal astigmatism.
STEP No. 3. MARK
You must accurately mark the steep corneal axis in order to properly align the toric IOL (Figures 1-3). Marking is best accomplished with the patient sitting up so as to avoid cyclotorsion of the eye, which happens when patients are supine on the OR bed. One option is using a surgical marking pen at the superior and inferior 90º meridians (12- and 6-o'clock positions) before surgery and then marking the steep corneal axis intraoperatively with a Mendez Degree Gauge. Another technique is to mark the actual steep axis before surgery while the patient is seated at the slit-lamp microscope.
Accurate intraoperative aberrometers and biometric devices that may increase the accuracy of marking the steep axis as well as alignment of the toric IOL are also available.
STEP No. 4. ALIGN
Misalignment of a toric IOL will reduce the effectiveness of the cylinder treatment and may even induce distortion and a new axis of astigmatism, which the patient may not tolerate.
A capsulorhexis that is smaller than the optic helps to securely hold the toric IOL in position during the postoperative period. Viscoelastic should be removed from behind the toric IOL. While keeping the eye inflated with the I/A probe, you can nudge the IOL to the correct alignment. Because of the tacky nature of the hydrophobic acrylic toric IOLs, a light tapping motion will help to secure the IOL in position against the posterior capsule.
To ensure correct alignment, simply line up the toric IOL's orientation dots with the corneal marks (Figure 4). During the first postoperative examination, you can confirm the IOL's position by noting this same alignment. If the toric IOL has rotated or become misaligned, reposition it in a timely manner.
Correcting astigmatism at the time of cataract surgery is important to give patients the best vision possible. This step can be achieved both with incisional methods and with toric IOLs. For patients with more than 1.00 D of corneal astigmatism, the toric IOLs are my treatment of choice. In the United States, the only toric IOLs currently approved by the FDA are monofocal. In Europe and Asia, however, the options include multifocal toric IOLs and piggyback toric IOLs. It is time to take your cataract surgery technique to the next level by addressing corneal astigmatism and incorporating toric IOLs into your practice.
Uday Devgan, MD, FRCS(Glasg), is in private practice at Devgan Eye in Los Angeles, chief of ophthalmology at Olive View UCLA Medical Center, and associate clinical professor at the UCLA School of Medicine. Dr. Devgan is a speaker for Accutome, Inc., and a stockholder in Alcon Laboratories, Inc., and he has a financial interest in the Devgan Axis Marker. Dr Devgan may be reached at (800) 337-1969; firstname.lastname@example.org.