Today’s intraocular lens (IOL) calculation formulas have evolved in many ways, including not only the use of optics but also ray tracing, mathematics and even artificial intelligence. Another shift is reflected in the increasing number of variables included in these formulas. Traditional formulas use only two variables — axial eye length and corneal curvature — to calculate lens power, while updated formulas now account for other differences in eye anatomy that need to be considered for optimum patient outcomes.

As ophthalmic technicians, we need to understand the calculation process, the latest formulas, and where we fit into the equation.

The calculations

Overview

To put it simply, in our calculations we are trying to predict where the IOL will situate in the bag post-operatively, or the distance between the IOL plane and the retinal plane. If we know that distance, we can easily determine the correct lens power needed to bend light to a focus on the retina. But we now know that this distance will vary due to differences in eye anatomy that go beyond just axial length and corneal curvature.

For example, two people can have the same axial length and keratometric readings (or K-readings) but significantly different anterior chamber depths, lens thicknesses, or horizontal white-to-white measurements that could cause their implants to situate at different planes and therefore different distances from the retina. This will result in them needing different IOL powers. Using traditional, two-variable formulas, however, would result in the same calculated power for the same implant model. Current multivariable formulas utilizing these additional eye anatomy measurements are therefore more accurate and better predict post-op lens position, which should result in better patient outcomes.

Current generation formulas

Today’s technicians primarily use optical biometers with laser technology to measure these variables. But as I hold my courses in practices around the country, I find that some doctors are still having them run the calculations with older, two-variable formulas.

Some technicians think that because they measured so many variables that any formula chosen will use them all, and that is just not the case. We must all understand which formulas use which variables so that we can adopt the most appropriate ones to get the most accurate results.

Some formulas have been found to be more accurate than others.

Here are some examples:

• The Barrett Universal II formula is one of the best we have today. It is on virtually all optical biometers and is easily accessible online.
• The Hill-RBF is only available on the LenStar biometer but is also accessible online for those who use other biometers.
• Other newer online formulas perform well and are easily accessible, including Kane, EVO, Pearl DGS, and Hoffer QST, so we really have no excuse to use technologically advanced IOLs and biometers yet run calculations with decades-old formulas.

When not to use advanced formulas

We must keep in mind that these new formulas were designed only for standard, cataractous eyes. It is therefore not appropriate to use them for procedures such as IOL exchanges or aphakic eyes needing secondary IOLs.

• The pseudophakic eye has a much deeper anterior chamber depth (ACD) and a much thinner lens than the phakic eye, and the aphakic eye has no ACD or lens thickness at all, so this will throw off multi-variable formulas.
• In these rare cases, it is best to use a two-variable formula or a program designed for these situations, such as the Holladay II IOL Consultant (PC version) or the Barrett Rx formula. We also have special formulas designed for eyes with abnormally shaped corneas like those with keratoconus, such as Barrett True K KCN or Kane KCN, or have had refractive surgery, such as the Barrett True K, and we must keep up with these as well.

Techs and the calculation process

A team effort

Whenever I speak to cataract surgeons, I emphasize that achieving the best results for our cataract patients is a team effort and should be approached as such within the practice. This requires an investment in technician education to help them better understand the measurement and calculation process vs simply being “button-pushers.”

Some physicians seem to believe that the ease of use of automated equipment like our current biometers means that they don’t require much education but, to paraphrase Warren Hill, MD, “Any instrument is only as good as it’s operator.” As good as our optical biometers are today, they can still give us erroneous readings from time to time, and the user must be educated enough to recognize and address these errors.

The physicians must not only perform good surgery, but they must also be willing to adopt the latest calculation formulas, just as they’re willing to adopt the latest implants and surgical technologies.

It falls on all of us to remain up to date and change as needed.

The ophthalmic technician’s role

Of course, no formula will perform well without the input of good measurements.

Technicians must obtain the most precise measurements possible, recognize potential errors or improper caliper placement, be very particular about standard deviations, verify that the measurements make sense for each patient, and problem-solve the challenging cases. They must also know how to perform precise immersion A-scan biometry — not only for the dense cataracts but also to verify borderline measurements as indicated by our biometers and those with asymmetry greater than 0.3 mm.

Once those measurements are obtained, they must be input into the most accurate formulas for the calculations.

How can we effect change?

The SRK/T formula is about 32 years old, and the Hoffer Q formula is now 41 years old. I doubt any cataract surgeon today would utilize any surgical equipment or implant that antiquated. At ASCRS this year, even Dr. Hoffer himself said “it is time to bury the Hoffer Q!”

Yet sadly, I hear technicians say all too often that their surgeons have heard of the latest formulas, but they still have techs run calculations with the older formulas. This puts some technicians in a predicament.

For practices that may be hesitant to adapt, my suggestion is to initially run the calculations with both the older and newer formulas so the surgeons can compare the results. I think this is only fair, especially since change can be scary for those who feel their results with the older ones have been good. This comparison will demonstrate that the difference in recommended power isn’t enormous and often even the same, so they won’t be as reluctant to switch to the new formula.

Sometimes it just takes that one case in which the surgeon implants the power recommended by the older formula and has an undesirable patient outcome. Then, once they realize the outcome would have been better had they gone with the newer formula, they will likely decide you no longer need to run calculations with the older ones.

It also helps if they review current journal articles on the topic and attend lectures and paper sessions on IOL calculations at major conferences — even attend them together if possible so everyone hears the same thing!

Conclusion

As biometry technicians, we have a lot of responsibility, and it is critical for us to stay educated and up to date.

We must all maintain best practices in our measurement and calculation protocols so our patients have the outcomes they deserve. OP