Cataract surgery is the most commonly performed refractive surgery today. The last decade has seen a paradigm shift as phacoemulsification, better surgical skills, better machines, and more accurate biometric calculations have transformed cataract surgery from a simple vision-restoring surgery to a highly demanding refractive surgery. As more and more surgeries are being performed on patients having good preoperative best-corrected visual acuity, the expectations of patients have also increased exponentially, and spectacle independence remains a common goal of surgery for surgeons and patients alike.
The past few years has seen a booming industry of premium intraocular lenses (IOLs) such as multifocal IOLs, trifocal IOLs, and toric IOLs, all targeting the patient’s desire for spectacle independence. Femtosecond laser-assisted cataract surgery (FLACS) has also emerged as a promising technology aiming to achieve predictable circular and central capsular opening with perfect IOL overlap to provide better centration. However, all these technologies fail to prevent refractive surprise if the biometry calculations have not been accurate. Hence, the search for the ideal biometry formula remains the holy grail even today.
There have been multiple formulae over the years, and physicians and mathematicians have moved from theoretical to regression formulae, and then back again to theoretical formulae, trying to understand the optics and anatomy of the complex human eye. Modern formulae even incorporate ray tracing aberrometry and artificial intelligence to reduce the prediction error further. But the interrelationship between the various components of the optical system of the eye and its changing dynamics from the phakic to pseudophakic state has been difficult to accurately predict. Moreover, these equations change in a nonlinear way from the hypermetropic to the myopic eye, and in extreme ranges of ametropia, most formulae lose their accuracy.
As a result, till now, the most reliable method was to use different formulae for different range of axial length (AL). In India, most ophthalmologists still prefer to use the modified SRK-II formula for the 22- to 24.5-mm range. SRK/T remains the formula of choice for high axial myopia patients (AL >24.5 mm), while Hoffer Q is used most commonly for hypermetropic eyes. One of the prime reasons for the widespread use of these formulae is the ease of their use. All of them require only two variables (corneal curvature and AL) for calculation, which are easily calculated by commonly available machines. The Royal College of Ophthalmologists have set the standards at 55% and 85%, respectively, for postoperative refraction of ±0.5 D and ±1.0 D, which was achievable by these formulae.[1]
However, as we said before, times are changing, and modern cataract surgery has set the bar higher than before. Of all the newer generation formulae developed, Barrett Universal II formula has stood the test of time most firmly. It has been proved to be highly accurate over a wide range of ALs, and hence the name “universal.”[2] The formula is based on a theoretical model of the eye in which the anterior chamber depth (ACD) is related to the AL and corneal curvature (K). The effective lens position (ELP) is calculated with the help of ACD and a lens factor (LF), which itself is dependent on five variables: K, AL, ACD, lens thickness (LT), and white-to-white (W-W). It is freely accessible on www.apacrs.org.
Over the past few years, evidence from literature has surfaced from various parts of the world, proving the accuracy of this formula over a wide range of AL. Melles et al. from California, USA, published a large series of 18,501 eyes in 2018, in which they established that the Barrett Universal II formula had lower prediction error when compared with SRK/T, Haigis, Hoffer Q, Holladay 1, Holladay 2, and Olsen formulae for SN60WF and SA60AT IOLs (Alcon Laboratories, Inc., Fort Worth, TX, USA).[3] Roberts et al. from New South Wales, Australia, published their series on a cohort of 400 patients with FLACS and SN60WF implant in 2018, again showing that the Barrett Universal II formula outperformed the Holladay 2, SRK/T, Hoffer, and Hill RBF formulae with the lowest rates of refractive surprise.[4] And now, Kuthirummal et al. have presented the first set of data on 244 Asian Indian eyes which once again shows better postoperative refraction accuracy over a wide range of ALs with the Barrett Universal II formula when compared with the modified SRK-II, SRK/T, and Olsen formulae.[5]
The high accuracy of the Barrett Universal II formula is a result of its unique mathematical model which calculates the ELP very precisely and even takes into account the varying principal planes of the differently shaped IOLs. Other new-generation formulae like the Holladay 2, Olsen, and Hill-RBF have also performed exceedingly well albeit over lesser range of ALs, and these together have revolutionized the field of biometry and cataract surgery. One major hindrance to the widespread adoption of these formulae remains the fact that most of them require the use of optical biometry, which is not widely available. Also, the inability of optical biometry to measure the AL and ACD in eyes with dense cataracts remains a major limitation at present. However, with the advent of newer optical biometry machines incorporating swept-source optical coherence tomography technology, such as the IOL Master 700 (Carl Zeiss Meditec AG, Jena, Germany) and OA-2000 (Tomey, Nagoya, Japan), this limitation will be overcome to some extent.[6]
It is imperative to change with the changing times, and we feel that adopting these newer formulae into practice has become essential for every ophthalmologist in the country to deliver the best of technology to our patients.