EyeWorld Asia-Pacific March 2021 Issue

W hat is the best approach to im- proving refractive outcomes after cataract surgery? In regards to methods of biom- etry, there are several factors that have changed over the years allowing clinicians to manage IOL power calculations and astigma- tism more appropriately. And in today’s ever-changing landscape of refractive cataract surgery, there are new parameters such as Total Keratometry that need to be considered. Advancement of biometry over the years Looking back at the history of bi- ometry, there are several milestones that are worth reflecting. The field of biometry, measuring the human eye, began in the 19th century with the development of the Javal keratome- ter in 1890 which allowed clinicians to measure the front surface of the cornea. In 1949, Harold Ridley im- planted the first intraocular lens (IOL) with quite a few achievements - he inserted a round lens in the posterior chamber and his methods of steril- ization worked well. However, there was still room for improvement with refractive outcomes. The first IOL pa- tient had a postoperative refraction of -24.0 D with 6.0 D of astigmatism at 30 degrees using the first IOL formula (IOL Power = (1.25 × preoperative spherical equivalent)). In 1970, the first A-scan ultra- Optimizing Refractive Outcomes: Supplement to EyeWorld Asia-Pacific March 2021 APACRS sound was performed. Karl Ossoinig and Jackson Coleman found a meth- od to bounce an ultrasonic pulse off the surface of the retina. By measur- ing the time for that pulse to bounce back, one could determine the axial length. Ultrasound also allowed for the anterior chamber depth (ACD) to be measured. This discovery helped improve refractive outcomes further. The next landmark discovery was in 1999 with the introduction of partial coherence laser interferometry (PCI) in the ZEISS IOLMaster, largely due to the work of Adolf Fercher and Christoph Hitzenberger. By capturing the movement of the reference mirror that changes the interference pattern, axial length can be measured. In 2014, the ZEISS IOLMaster 700 introduced SWEPT Source opti- cal coherence tomography (SS-OCT) technology to biometry. This allows for a precise imaging-based measure- ment of biometry data such as axial length, corneal thickness, anterior chamber depths, and lens thickness. With this technology, the speed of acquisition improved, meaning 2,000 scans could be acquired per sec- ond. Additionally, the IOLMaster 700 provides direct measurements for the posterior corneal surface and, combined with the telecentric ker- atometry measurement of the anterior corneal surface, provides precise Total Keratometry (TK) 1 and allows for measurement of Central Topography. 2 Accounting for the posterior corneal surface and thickness is import- ant because it has an effect on the total corneal power. Most biometric formulas rely on standard K readings from the anterior cornea surface, while assuming the posterior corne- al radius, based on a keratometric index, and this will cause inaccurate results in some cases where the cor- nea is not "normal." 3 New formulas are now trying to predict the effect of the posterior cornea power. This way, clinicians are able to better solve the IOL puzzle with greater precision but are still relying on assumptions rather than measurements. The question then turns to how clinicians can best utilize the new information on the actual measured posterior corneal surface that we now have. Accurate IOL power calculations utilizing both anterior and posterior corneal surface With the ability to measure both the anterior and posterior corneal surface, it is also important to choose a formula that will not only deal with real measured values but also provide the most accurate IOL power. Profes- sor Graham Barrett of the Lions Eye Institute has developed formulas that are utilized in the IOLMaster 700. The Barrett Suite comprises of the Barrett Universal II, Barrett Universal II TK, Barrett Toric, the Barrett Toric TK, the Barrett True K, and the Barrett True TK formulas. The Barrett Universal II formula uses up to five different vari- ables: axial length, keratometry, an- terior chamber depth, lens thickness, and white to white measurement. Dr. Barrett himself says, “For every lens that you implant, [the Barrett Univer- sal II] works out the principle plane [of refraction] for those optics. This is the reason why it maintains its accuracy in unusual situations such as high myopia.” The Barrett Toric Calculator, on the other hand, gives the option of using the predicted posterior cornea power by implementing nomograms or directly using the measured pos- terior cornea power. Clinicians need to take into consideration that the original Barrett Toric Calculator does indeed use standard K and includes the standard nomogram to address posterior corneal surface effects. Using TK values with this calculator would compensate twice for the pos- terior corneal astigmatism and thus may result in inaccurate outcomes. If the TK and posterior keratometry (PK) values are available as measured with the IOLMaster 700, they can be used directly in the online calculator though, by choosing and entering measured instead of predicted values. The online calculator will then replace the posterior surface nomograms with the actual measurements. The news magazine of the Asia-Pacific Association of Cataract & Refractive Surgeons “ The Barrett True K with TK formula elevates post corneal refractive surgery IOL power calculation to the next level. ” Professor Graham Barrett, Australia , Copyright 2020 APACRS. All rights reserved. The views expressed here do not necessarily reflect those of the editor, editorial board, or publisher, and in no way imply endorsement by EyeWorld, Asia-Pacific or APACRS.