A History of Toric IOL Calculations

The first US FDA–approved toric IOL, the STAAR Toric IOL (STAAR Surgical), was introduced in 1998. It was available in only two toric powers, 3.50 and 2.00 D, and we surgeons did not use a toric calculator for IOL selection. Even our spherical IOL calculations were rudimentary at the time. We primarily used contact ultrasound and autorefractor keratometry readings. For patients with high astigmatism, however, the STAAR Toric IOL was a major advance.  

The earliest toric calculators were introduced around 2005 with the launch of the AcrySof Toric IOL (Alcon). By then, the IOLMaster (Carl Zeiss Meditec) was available, but refractive results, especially with 1.50 D toric IOL powers, were highly unpredictable. 

I remember hearing Douglas Koch, MD, describe his findings on posterior corneal astigmatism at a national conference. His talk changed everything. Today, it is hard to imagine using multifocal IOLs without accurate toric calculators. 

This article presents the origin story of modern toric calculations.

– Tal Raviv, MD

When toric lenses were introduced in the late 1990s, toric IOL calculations were based on measurements of anterior corneal astigmatism alone. This article describes how the development of nomograms that account for additional contributors to ocular astigmatism improved refractive outcomes and discusses how to maximize success with these lenses.

A FLAWED ASSUMPTION 

Early toric IOL calculations assumed that only the anterior cornea contributed to astigmatism in an aphakic eye. In 2017, Baylor College of Medicine acquired a Galilei (Ziemer Ophthalmic Systems), and I proposed using the device to measure the posterior cornea—something I had not heard others discuss. 

At that time, I was encountering toric IOL patients with perplexing results. For example, one of these individuals presented with 1.50 D of with-the-rule (WTR) astigmatism preoperatively, underwent cataract surgery with a toric IOL to correct that amount of cylinder, and returned with 1.00 D of against-the-rule (ATR) astigmatism. A second patient with 1.00 D of ATR astigmatism underwent cataract surgery with a 1.00 D toric IOL (at the corneal plane) and ended up with a 1.00 D undercorrection. I made the same mistake in the patient’s second eye because I did not believe my numbers.

As my colleagues and I compiled data on the posterior cornea, we were astonished to find that it was an important contributor to astigmatism and that the amount of posterior corneal astigmatism (PCA) also varied depending on the anterior corneal curvature.

These discoveries prompted us to examine the peer-reviewed literature. We found that we were by no means the first to suspect that there might be more than one contributor to astigmatism. In the 1800s, Louis Émile Javal, MD, the French ophthalmologist known for his work in physiologic optics and strabismus, determined that the anterior cornea did not accurately predict refractive astigmatism. Several years before our investigation, other researchers had documented PCA,¹ but no one else had tied these findings to toric IOL planning or realized that the amount of PCA varied according to the anterior corneal curvature.

My colleagues and I developed the Baylor nomogram, which divided eyes according to whether they had WTR, ATR, or oblique astigmatism and estimated how much PCA was present based on their anterior corneal curvature.² When I presented these findings at the 2012 annual meeting of the ASCRS,³ it was an aha moment for the audience. Cataract surgeons were encountering refractive surprises with toric IOLs, but they had not quite figured out why. Industry colleagues recognized that PCA was a key factor in their suboptimal outcomes and subsequently began using our findings to redo their own nomograms.

NOMOGRAM REFINEMENT 

Adi Abulafia, MD, studied the Baylor nomogram and broke astigmatism down into vectors. He found that the amount of PCA could be predicted based on the x-vector alone. The Abulafia-Koch formula was a major step forward in improving refractive accuracy.⁴

Graham Barrett, MD, was also developing his toric IOL formula at the time. Soon thereafter, Jack Kane, MD, developed his toric calculator. 

These three formulas are the most commonly used today.

OBJECTIVE MEASUREMENTS 

A reasonable question is whether direct measurements of PCA and total keratometry could meaningfully improve the accuracy of toric IOL formulas. My colleagues and I compared results when measured versus calculated PCA values were entered into the Barrett Toric Calculator. The benefit of using measured PCA was found to be marginal, which surprised us.⁵ The cause is noise in the system. This noise includes variability in corneal power measurements from the front and back of the cornea, toric IOL alignment, and the postoperative refraction plus a newly recognized factor: IOL tilt. In other words, several factors contribute to refractive accuracy with these formulas—even with posterior corneal measurements.

LINGERING MISCONCEPTIONS

No. 1: The Amount of Postoperative Astigmatism Remains Constant Over Time

One of the most persistent misconceptions regarding astigmatism pertains to the target correction. During my 2012 Charles D. Kelman, MD, Innovator’s Lecture at the ASCRS Annual Meeting,³ I discussed a finding by Ken Hayashi, MD, that corneal astigmatism undergoes a long-term ATR shift of approximately three-eighths of a diopter after cataract surgery.⁶ Given the choice, cataract surgeons’ natural preference—one particularly prevalent among beginning surgeons—is to leave patients with 0.10 D of ATR astigmatism rather than, for example, 0.30 D of WTR astigmatism. Even that small amount of ATR astigmatism, however, could become 1.00 of ATR astigmatism in the next 2 decades.

Accounting for the long-term ATR shift in patients’ astigmatism is essential to maximizing their long-term satisfaction. In my experience, even individuals who receive a trifocal IOL can tolerate up to 0.50 D of WTR astigmatism after cataract surgery. 

No. 2: Measuring Once Is Sufficient

To maximize outcomes, the ocular surface should be optimized, and at least two but ideally three corneal measurements should be obtained—preferably on 2 different days. One of the measurements should be topography or tomography. I am a huge advocate of Placido imaging with fine mires because of the exquisite detail it provides of the corneal surface. 

Consistency among these preoperative measurements is critical to achieving optimal surgical outcomes. 

No. 3: Toric IOL Alignment Is Accurate Enough

The Table indicates how precise alignment must be as a function of the IOL’s toricity. Close attention to alignment, whether with manual marks or digital imaging, is essential, especially when 1.50 D or more of astigmatism is being corrected.