Seeing Is Believing

A pervasive adage within cataract and refractive surgery circles is this: for every degree of toric IOL rotation, three percent of the toric effect is lost. At 30°, the entire effect of the toric IOL is lost, and beyond 30°, an additional cylinder is induced. The basis for this axiom lies within a landmark study from 1994, which found that the maximum amount of post-operative rotation before a complete negation of the astigmatic effect was 30° (1). Though this article provided a guiding principle for rotational tolerance of toric IOLs, there are nuances that are worth discussing.

The first thing to consider is that the traditional process of placing a toric IOL introduces a number of opportunities for error. Patients are often marked before surgery by hand, which involves estimating the 0°, 90°, and 180° positions. Even in the most accurate surgeon’s hands, the pen marks span at least 3°. Next, degree gauge rings are used to mark the toric axis. These rings must line up with the original pre-op marks, and then the surgeon must estimate the exact position of the axis as the rings typically have increments of 5° – which again involves making a mark that can also span around 3°.

When the toric IOL is finally placed, the marks of the toric are compared with the cornea. Like the rest of cataract surgery, this step is subject to parallax distortion. The toric marks on the IOL also must align with the corneal marks. Because the corneal marks may be 3° wide, this introduces the possibility of 3° of alignment error.

Based on the original adage, given almost 10° of potential cumulative error – assuming every step was completed perfectly – we end up with a 30 percent reduction in astigmatic correction from the toric IOL. Clinical experience, however, reveals consistently excellent visual acuity after toric IOL placement, which means either the eye is more tolerant to toric IOL rotation than the study suggests, the relationship is nonlinear, or there are other considerations (2, 3). Of course, these problems are addressed by the use of femtosecond technology and a precise toric mark on the capsulorhexis, but studies have shown the visual outcomes for conventional versus femtosecond toric IOL placement in cataract surgery are equivalent (4).

So, what is the missing piece? Maybe the answer is just that it all comes down to the patient’s visual experience. “Clinically significant” astigmatism is generally understood as somewhere in the 0.75 D–1.00 D range (5, 6). Therefore, a patient with around 1.50 D of astigmatism should have excellent vision as long as they are at or below that range post-op. If you consider a T3 toric IOL, which corrects 1.50 D of astigmatism at the IOL plane and 1.03 D at the corneal plane. Using the original math of 1° for three percent of the effect, the IOL should be able to rotate 20° before it loses 60 percent of its corrective power. This would leave 40 percent, or 0.60 D, of the original 1.50 D which would land the eye under the 1.00 D cylinder mark. The conclusion that can be made from this is that the T3 IOL has a 40° window of rotation before the eye experiences a “clinically significant” effect. This analysis disregards the vector of astigmatism as with-the-rule astigmatism tends to be the best tolerated and oblique the least. However, studies have demonstrated that the cylinder of any axis minimally impacts visual acuity under a magnitude of 0.75 D (7, 8).

Now consider a T6 IOL, which has 3.75 D of cylinder at the IOL plane and 2.57 D at the corneal plane. To retain 1.75 D of cylinder correction equates to 74 percent of its effect. A T6 then has 26 percent of wiggle room, or conservatively 8°. Considering both directions, a T6 has around a 16° window before the eye experiences a “clinically significant” effect. By the same math, a T9 would have a 12° window.

It goes without saying that surgeons should always strive for perfection, but if it ever seems that actual outcomes are better than the old “1° for three percent” rule suggests, our thinking above could be the reason why. Even high-powered toric IOLs have more than a 10° window where the outcome likely will be acceptable from a subjective visual perspective. For those without access to femtosecond guided technology, calculating the “window of tolerance” for toric IOLs before surgery may prove to be an interesting exercise and also create additional situation awareness. One easy way to do this is to use the toric Results Analyzer created by John Berdahl and David Hardten. For example, use a T3 and make the current axis, calculated axis, and current refraction plano axis all 90° (basically, input a perfect result). The website graphically demonstrates how much the IOL can rotate and what effect will result. In this case, anywhere from 70° to 110° degrees gives < 0.75 D of residual astigmatism.

Does this mean that surgeons should stop passing the “one for three” rule down to future generations? Not necessarily, especially since it is a principle that looks to instill a mindset of caution and precision when performing conventional toric IOL placement. Instead, this analysis looks to provide an interesting perspective on the real-world tolerances of placing toric IOLs and shines a light on why, when it comes to a patient’s sight, their perception is their reality – even when the math might suggest otherwise.