EyeWorld India March 2015 Issue

March 2015 IOL Calculations 29 EWAP SECONDARY FEATURE Views from Asia-Paci c Adi ABULAFIA, MD Director of Cataract Surgery Ophthalmology Department, Asaf Harofeh Medical Center (af liated to the Tel-Aviv University) Tsri n, 70300, Israel And Ein-Tal Eye Center, Tel-Aviv, Israel Tel. no. +972-545577333 adi.abula a@gmail.com T oday, patient’s expectations for excellent refractive outcomes following cataract surgery are high. Accurate prediction, however, for the high and extreme axial myopic eyes remains a challenge. Using third-generation formulas with standard IOL constants in long eyes can reportedly lead to unexpected postoperative hyperopic outcomes. The reason for those results is still a matter of debate. IOL power calculations for IOLs with a lens power below 6 D are more challenging. Several solutions were suggested to address IOL calculations in extreme axial myopic eyes. The most widespread choice is apparently to aim for a myopic refraction that would avoid an unwanted postoperative hyperopic refraction. One approach is to adjust the IOL constants. The anatomic position of an IOL does not necessarily re ect the principal optical plane of an IOL. Haigis pointed out that low-power and negative-power IOLs have a different geometry. If an IOL is not symmetrically biconvex, the principal plane can vary with different diopters. The principal plane of low-power IOLs is far from the geometric IOL position, and it switches sides when the IOL power sign is changed. The MA60MA (Alcon Acrysof) negative-diopter IOL and the low-powered IOLs less than 6 D have a meniscus con guration unlike positive-diopter IOLs 6 D and greater which are biconvex. This may also affect IOL calculations. Petermeier et al. suggested a different constant for low power plus and minus MA60MA IOLs (an A constant of 126.6 and 104.4, respectively). This approach was challenged by Preussner, who suggested that an axial length (AL) adjustment would be more appropriate. Wang and Koch had developed an AL regression equation for each of the standard formulas. Their method is based on the theory that AL measurements by optical coherence biometry have a systematic error that grows in a linear fashion. Since optical coherence biometry assigns a single, xed index of refraction to all eyes, the proportion of the vitreous part increases in long eyes, which may result in overestimation of the AL. These adjustments are also available as a part of the Holladay IOL consultant software. The Barrett Universal II formula, available on the APACRS and on the Lenstar (HS) device is based on paraxial ray tracing (Gaussian/thick lens), which takes into account the change in the principal planes encountered with different-powered IOLs. The problem experienced with other formula is that they do not consider the change in optical con guration or address the change in vergence required for a negative powered IOL. The Barrett Universal II therefore does NOT require adjustment of axial length or unusual A constants—the manufacturer’s recommended A constant of 119.2 is appropriate for calculating both low dioptric and negative IOLs MA60MA. Another factor to consider in high myopic eyes is that the effective toric power of a lens implant is less when you have a low-powered IOL that is going to t deep in the eye. The Barrett Toric calculator, available at the ASCRS & APACRS web sites and on the Lenstar device, incorporates the Barrett Universal II formula and gives a comprehensive solution for myopic eyes with astigmatism. In a recent study published by our group, we evaluated and compared the accuracy of formulas and methods for calculating IOLs in eyes with AL>26 mm, using Ein-Tal Eye Center database. The formula that provided the best overall prediction for eyes with AL>26 mm including low dioptric and negative powered IOLs was the Barrett Universal II. Editors’ note: Dr. Abula a has no nancial interests related to his comments 0.8544 x IOLMaster AL + 3.7222. Finally, the Hoffer Q optimized axial length = 0.853 x IOLMaster AL + 3.5794. Using the formulas, practitioners first take traditional axial length measurements, Dr. Wang explained. “Then we use the formula to adjust the measured axial length to get a new axial length—that is the optimized axial length.” The optimized axial length is then plugged into the biometry machine. “The user runs the calculation again and gets a new outcome for IOL power,” she said. This axial length adjustment should only clinically be used for eyes 25 mm or longer, Dr. Wang advised. Practical pearls When using the adjustment, Dr. Koch recommended picking the IOL that is just on the minus side of zero. “Also, you should advise patients that calculations [in long eyes] are more challenging,” he said. “The only other factor to consider is that if you’re using a toric lens in these patients, you have to bear in mind that the effective toric power of a lens implant is less when you have a low-powered IOL that’s going to fit deep in the eye.” It can be as much as 0.5 D less, Dr. Koch noted. “You need to think about adjusting your toricity if you’re adding that to your calculation.” “The Holladay 2 also includes the adjustment of the effective IOL toricity as a function of anterior chamber depth and axial length,” Dr. Koch added. Dr. Wang pointed out that the Holladay 2 consultant program now includes the adjustment. “The Holladay 2 consultant program incorporated our formula,” she said. “So if users select that for eyes longer than 25 mm, the program will automatically calculate the IOL power, with the adjustment formula.” While using these formulas should help most prevent hyperopic surprise, Dr. Wang stressed that practitioners may want to fine-tune these for their own patients. “If [practitioners] have a number of [long] eyes, it would be good to optimize their lens constant,” she said. “That might improve the accuracy.” Meanwhile, Dr. Koch foresees a day when such a regression formula will no longer be needed. “Ideally what we’re going to have is a situation where we can accurately measure each compartment of the eye; with that in mind, this modification will no longer be necessary,” he said. “We’re all working toward that goal.” EWAP Editors’ note: Drs. Koch and Wang have no financial interests related to their comments. Contact information Koch: dkoch@bcm.tmc.edu Wang: liw@bcm.tmc.edu