TORIC LENSES

Toric Lenses: Keep it Simple

Astigmatism surgery is a win-win for doctors and patients because it's safe and there's a great visual payoff.

By Iqbal (Ike) K. Ahmed, MD

Toric lenses have been a great addition to our practice. Every patient who comes in with astigmatism is a candidate and the surgery is a win-win for doctors and patients because it is safe, the risks are low, and there's a great visual payoff.

Surgically Induced Astigmatism

Some surgeons don't always seek out astigmatism, but it's important to recognize it and form your surgical plan accordingly. In addition, always be aware of the amount of astigmatism you're inducing with your selected surgical technique.

Surgically induced astigmatism (SIA) is an important factor when selecting a lens that corrects for astigmatism. Monitor results trough pre-and post-keratometry and SIA calculators.

Once you have a ballpark number — most are in the range of .2D to .7D — you can move on to measuring for the lens. Remember, typically the smaller the incision, the lower the SIA will be.

Toric lenses are an extremely safe and effective way to treat astigmatism in cataract patients. Our study, which I discuss in detail below, is a binocular study that demonstrated excellent toric IOL outcomes.

When screening a patient, I look for refractive cylinder in his eyeglasses. During the initial exam, we typically perform an automated keratometry and autorefraction. This provides a good baseline of the patient's corneal cylinder. At that point, we can look a little further to see if the patient qualifies for a toric lens. I aim for .5D or less of cylinder postoperatively. In Canada, we have the T3 to T9 range of Alcon toric lenses available, so we can correct more than 4D of corneal cylinder. The extended line greatly reduces the number of limbal relaxing incisions we perform.

Good Measurements = Excellent Outcomes

Implanting a toric lens is a very straightforward procedure, but there are a few key points I would like to highlight. I prefer to mark the eye in the pre-op area with a three-ray marker. There will always be some variability in the marks, in the measurements and the final position. But if you keep it simple, clinical outcomes will be exceedingly positive and good clinical outcomes lead to happy patients.

To achieve accurate measurements, make sure the patient's corneas are reasonably lubricated. I prefer to obtain corneal measurements before any anesthetic drops are added to the eye and before any contact is made with the cornea with other testing. It's essential to obtain corneal measurements from a cornea that isn't dried out.

I typically obtain automated keratometry measurements with the IOL Master (Carl Zeiss Meditec) and use topography as well. While not absolutely essential in my opinion, corneal topography is helpful in verifying keratometry, and ruling out corneal anomalies such as subtle keratoconus, and so on.

In addition, I consider refraction and make sure the numbers jive in terms of magnitude and axis steepness. The IOL Master and keratometry measurements are the most important measurements I take. If I have discrepancies, I may do a manual keratometry for comparison, but this is very rare.

A third key factor is the use of online calculators, which I believe are a fantastic resource.

I encourage new surgeons to look up the Acrysof Toric IOL calculator (acrysoftoriccalculator.com). You simply enter your baseline measurements (the SIA, the axis and lens power) and the calculator will produce a report telling you which lens to use, the anticipated residual cylinder and the axis to put the lens in.

In the OR

Marking the steep axis for correct toric IOL alignment is one of the most critical components — and believe it or not — one of the more common sources of errors (even for seasoned veterans) in astigmatism treatment. However, I don't believe this step requires complex gadgetry or mathematics. I prefer to keep it very simple, recognizing the tolerances of each step of the process.

In our OR, the eye is marked in pre-op while the patient in sitting up and fixating ahead. I mark the 3, 6 and 9 o'clock marks to establish reference marks with the patient in the sitting position to reduce the risk of IOL malposition due to cyclotorsion while the patient is supine on the operating table. The implantation and surgical technique are straightforward.

In the OR, with the patient in position under the microscope, and before making any incisions, I mark the steep axis on the cornea. I prefer a two-handed marking technique, which means I use a Mendez ring with 5-degree graduation marks on them and align the 3, 6 and 9 o-clock marks on the ring to the already placed marks on the patient's cornea. The steep axis is then marked with a two-ray marker, providing great visualization and precision marking. I find this to be the most effective way to mark an eye. With one-handed tools, it's often hard to visualize the cornea.

At the time of implantation, I use a moderate amount of cohesive viscoelastic in the capsular bag. I then inject the lens and rotate the lens about 20º short of the mark. This is done so that when I remove the viscoelastic and the lens moves a bit, I don't over-rotate past the steep axis.

Then, I withdraw the I/A handpiece and inject BSS with a cannula. Next, I hydrate the wounds and ensure that we have a nice pressurized eye. For my final positioning, I use a 27-gauge cannula on a 3cc syringe filled with BSS solution to rotate the lens to its final position while injecting BSS solution to keep the bag expanded during the technique. Finally, I make sure there's a watertight seal.

Toric Implantation Study

In our bilateral toric implantation study,¹ postoperative IOL stability was shown to be excellent. The study was a large prospective multicenter Canadian study conducted in academic settings and in private practices. We examined visual outcomes, uncorrected visual acuity and rotational stability as well as subjective findings of 117 patients.

The lens proved to be very solid and stayed in position. To ensure this, it's necessary that the capsular bag shrink-wraps around the lens and this starts immediately with fibronectin binding between the IOL and capsule.

The design and material of the single-piece toric IOL are important factors in maintaining centration. If this lens is going to rotate, it will happen in the first few hours after surgery.

The study also demonstrated that residual refractive cylinder was very small — .4D. Of all the patients surveyed, more than 90% were within 5 degrees of IOL alignment. In summary, this bilateral study showed excellent outcomes and high patient satisfaction.

Toric lenses are a great first step in advanced IOL technology. We're hitting the mark with 95% plus accuracy, but it's important to monitor your results. There's very little downside in performing toric IOL surgery and a great amount to be gained in patient satisfaction. nMD

REFERENCE

1. Ahmed IK, Rocha G, Slomovic AR, Climenhaga H, Gohill J, Grégoire A, Ma J. Canadian Toric Study Group. Visual function and patient experience after bilateral implantation of toric intraocular lenses. J Cataract Refract Surg. 2010;36:609-616.

Cataracts and Astigmatism Patients with cataracts can be expected to have some amount of clinically significant astigmatism. The key to toric IOL implantation is the reliability and predictability of correcting the preexisting astigmatism from the clinical perspective. From the patient perspective, because this is an out-of-pocket expenditure, we also need to evaluate patient satisfaction to see if patients are happy with their vision post-operatively. In a definitive biometry study of 7500 cataractous eyes, corneal astigmatism of 0.75D to 1.50D was observed in 41% of patients. For preexisting astigmatism of more than 1.50D, the prevalence was observed to be 18% of patients. This study (Figure 1) shows the change in 164 eyes, 6 months postoperatively of corneal astigmatism after implantation, 148 eyes (90.2%) had 1.00D or less of refractive astigmatism, 140 eyes (85.4%) had 0.75D or less, 116 eyes (70.7%) had 0.50D or less, and 75 eyes (45.7%) had 0.25D or less. No eyes had an increase in cylinder (preoperative cylinder to 6-month postoperative refraction). Figure 1. Magnitude of astigmatism in 164 eyes preoperatively and 6 months postoperatively. Overall, 90% of eyes had less than 1.0D of residual astigmatism in this study. Thus, the reduction in corneal cylinder is reliable and predictable. Patients (n=78) also provided satisfaction ratings (Figure 2). Patient satisfaction was statistically significantly higher at 6 months than at baseline. Figures reprinted with permission from the Journal of Cataract and Refractive Surgery Figure 2. Vision satisfaction ratings preoperatively and 6 months postoperatively (n=78) with 1 representing patients who were completely unsatisfied and 10 representing completely satisfied.