IOL FORMULAS

A new formula for calculating IOL power may offer greater precision for biometric calculations by incorporating the demographic specifics of individual patients based on race and gender. Kenneth J Hoffer MD, clinical professor of ophthalmology at Stein Eye Institute, University of California, said that human average values for biometric measurements are used in the fourth-generation formulas, specifically the Holladay 2 and the Hoffer H. The new formula, known as the Hoffer H-5, uses the basic structure of the widely used Holladay 2 formula but adds race and gender parameters to its logarithm of mean human biometry averages, he told a session of the XXXI Congress of the ESCRS in Amsterdam.

“I discovered that the biometry of women and men are different, with males having an axial length of about 0.50mm longer than women and having a cornea that is flatter by about 0.50 D. There are also racial differences in biometry values, so the new formula aims to take account of these important differences for even greater precision.”

The new Hoffer H-5, the “H” of which pays tribute to the work of Jack Holladay MD, represents the first fifth-generation formula available. “We have gone through various generations over the years by adding different parameters to help us predict where the IOL will sit in the eye. The first-generation formulas used a constant for the anterior chamber depth and then the second-generation incorporated data on the axial length of the patient’s eye. The third-generation saw the introduction of keratometry values and the fourth-generation used lens thickness and corneal diameter and other factors,” he said.

Starting point

The starting point of the Hoffer H-5 formula was to perform a search of the published scientific literature looking at the biometric measurements of human eyes in large series. Based on data from 212,740 eyes, Dr Hoffer was able to produce average values for different populations based on race and gender.

“We found a consistent difference between male and female biometry in every racial group in terms of axial length, K readings and anterior chamber depth. The average lens thickness, however, showed very little difference between genders,” he said.

Dr Hoffer also found statistically significant differences in biometry between Caucasian, Latino, Asian-Indian, Oriental, and Middle Eastern eyes. “Obviously the next step is to test the formula with much larger patient numbers and more diverse population groups which we are doing now. We have started a global collaborative study that has now collected 3,000 eyes from surgeons from around the world and are still amassing more data. We think that this new formula definitely has the potential to improve on existing formulas,” he concluded.

Kenneth Hoffer: Khoffermd@aol.com