Lens power calculation

One of the crucial steps in cataract surgery is the implantation of an intraocular lens. We have already discussed the multiple types of lenses possible and available to both the doctor and the patient: monofocal, EDOF, trifocal, toric or not… But each lens model also has a range of possible prescriptions, and today we will explain how the lens power calculation works.

If we want to implant the monofocal SN60WF lens from Alcon, one of the most commonly used lenses in our environment, we will have prescriptions available from 6 diopters (D) to 30 diopters, in intervals of 0.50 D: 6D, 6.50D, 7D, 7.50D, 8D, 8.5D…

The immediate question that should arise is: How do they choose the lens power?

Biometry

It is a routine examination that we perform in consultations where we measure some segments of the eye. Specifically, biometry focused on cataracts measures at least 3 data of our eye: the axial length (how long it is), the curvature of the cornea, and the space between the cornea and the crystalline lens.

The reference device, currently, to perform these measurements is the IOLMaster by Zeiss. This instrument repeatedly measures all previous values to ensure they are correct. It comes with a database of installed lenses and mathematical formulas. In a few steps, the professional can make the measurements and the machine automatically recommends the lens prescription to use. While the professional has the final word, the machine recommends the lens power to be implanted.

The formulas

Once we have obtained the biometry, we have to enter those values into different mathematical formulas to obtain the ideal prescription. There are more than a dozen of them and added modifications, ranging from very primitive formulas to more innovative proposals based on artificial intelligence. Classic options usually come already installed in the biometer. For example, IOLMaster includes the formulas of Holladay I and II, SRK/T, Haigis and Hoffer Q.

And if there are so many formulas, it is because none of them is perfect. We could say that SRK/T and Haigis are the most versatile of the classic options. But both fail in small eyes (hypermetropes), so Hoffer Q is better. Both also fail if the cornea is irregular, where Holladay II gives better results.

In addition, all of them fail in extremely myopic patients, so other authors have published modifications on these formulas to adapt them to high myopia. Newer formulas have been released with a lower error rate, such as the Barrett calculator and the Kane formula. In fact, from our application, CataracTools, you can access the Kane calculator.

Often not all formulas agree on recommending one lens or another, and we usually choose the prescription in which there is consensus among different formulas.

Furthermore, toric lenses need variations on the previous formulas to be calculated.

Prior LASIK surgery

A separate case are patients who have undergone refractive surgery (LASIK, PRK, radial keratotomy…). Typical biometers are not powerful enough to study the corneas of these patients. We need a specific device for the study of the altered cornea.

As you may have predicted, these cases indeed need different formulas to be able to calculate the lens. Even with a specific device and special adapted formulas, calculation errors are abundant. The margin of error of these formulas has improved over the years, but errors still occur due to mathematical mistakes.

The refractive surprise

Calculation errors in practice lead to a very specific and typical situation: the surgery has taken place normally, the lens implanted was theoretically correct, the evolution of the eye has been uneventful but the patient has an unsatisfactory vision. This is due to an error in the formula that caused us to implant an incorrect lens by one diopter (for example). Remember that one diopter of myopia causes the patient to lose 50% of the definition for distant vision.

In fact, in my practice, I have encountered a case of a refractive surprise of 3 diopters. The patient had very poor vision, and we proceeded to replace the lens immediately. Fortunately, this was an extreme case, but refractive surprises of about half a diopter are frequent, especially in patients who have undergone corneal refractive surgery.

The solution for severe cases is, indeed, to replace the lens in a second surgery. In mild cases, glasses can be used or a touch-up of the cornea can be done with LASIK. However, this in turn has a limitation: patients who have had LASIK may have problems if they are retouched. That is, those patients with a higher probability of having a refractive surprise are also those who have a worse solution.

This is why trifocal lenses are partially discouraged in patients who have had LASIK.

The outcome

All being well, implanted lenses’ success greater than 90%. However, this percentage is decreases in some cases, and we may be forced to use glasses. But it is important to remember that in most cases, it is not the fault of the surgeon or the optometrist. The formula failed.