For patients, another post has been uploaded with an introduction to the field. This page will elaborate on a more professional approach to biometry formulas.
Long story short
|Length <22.5mm||Hoffer Q||Barrett|
|Length 22.5-26mm||SRKT/Holaday I/Haigis||T2 formula|
|Prior LASIK surgery||Haigis-L||Barrett True-K|
|Toric lenses||–||Barrett Toric calculator|
Propietary lens calculator
Most common formulas include Hoffer Q, SRK and SRKT, Haigis and Holladay. These formulas ara available in most biometers (such as IOLMaster, Lenstar, Anterior…). Unfortunately they lack precision when it comes to “other than average” eyes.
Generally speaking, early traditional formulas require some biometry data (such as axial length and keratometry) to guess the best lens. Later formulas also require some extra data (i.e. anterior chamber depth and white-to-white distance) in order to guess the elective lens position (ELP). Indeed, the final lens position in the eye plays a major role in estimating its power. Below these lines
Many authors have developed their own formulas in other to achieve the perfect result. However, the perfect formula is still an oximoron.
Dr. Graham Barrett has developed his own suite of formulas, which are available at the website of the Asia-Pacicific Association of Cataract & Refractive Surgeons:
–Barrett Universal II Formula 1.05
–Barrett Toric Formula
–Barrett True-K Formula
–Barrett True-K Toric Formula
As you can see in the graph above this line, Barrett Universal Formula consistently gives better results than the traditional formulas. The further from the average eye, the greatest the improvement. Though this study focused on the Univesal Formula, similar results are to be expected with the other formulas.
Dr. Kane‘s formula has proven to improve on the results from previous formulas. Small variations of the same form are used for this formula, so only one link is necessary. As you can see from the form, the biological gender of the patient is required, since small changes are applied. Feel free to perform some random calculations and compare mare and female patients.
According to the site, Multiple published clinical studies have demonstrated that the Kane formula is more accurate than all currently available IOL formulas (including Hill-RBF 2.0, Barrett Universal 2, Olsen, Haigis, Hoffer Q, Holladay 1, SRK/T, EVO and Holladay 2). Unfortunately, some of the articles shown include Dr. Kane as an author or co-author. Though it doesn’t directly imply a bias, we’d like to see more studies from third parties.
Dr. Warren E. Hill published the Hill-RBF formula, optimised for Lenstar LS900 and Alcon SN60WF and Alcon MA60MA lenses. So, if your clinic is routinelly using these tools, make sure to check the Hill-RBF formula.
This formula features a different approach: machine learning. The Pearl-DGS fromula has been optimized for PhysIOL Finevision lenses, so be sure to check it out if these lenses are among daily routine. Furthermore, it includes a calculator for second eyes, compensating for the errors of the first surgery (if the refractive result wasn’t already perfect enough).
Nor Barrett, Kane or Hill-RBF have been publicly released. Only the general approach to each formula has been stated in conferences or articles, but the full content of each formula has not been published.
Personal clinical approach on choosing a biometry formula
During my daily routine, I use the combination of SRKT, Haigis and Holaday I for average eyes. Eyes that aren’t short or too long or have a very narrow anterior chamber fit in this category. Since these are three formulas, I choose among the suggested result. If all three formulas suggest the same lens, it’s obvious. When one formula disagrees with the other two, I leave the odd one out. In the rare cases where each formula suggests a different lens, I tend to check with an external formula.
For unusual eyes, I always use external formulas. And, most often than not, both Barrett and Kane.
For further information, we recommend the article in Eyewiki. By far the best source of information on this topic: publicly available, updated and maintained by the AAO. Most affirmations are backed by strong sources and referenced within the article. Lack of evidence is salso stated when information is scarce. All in all, the best of the best.