Eyeworld

I N FOCUS 42 | EYEWORLD | AUGUST 2019 CATARACT SURGERY POST-LVC Contact information Al-Mohtaseb: zaina1225@gmail.com Devgan: devgan@gmail.com Gatinel: gatinel@gmail.com Koch: dkoch@bcm.edu by Chiles Aedam Samaniego EyeWorld Contributing Writer which uses the measured keratometry for the vergence calculation but a more normal K value for the ELP determination. We can also use newer formulae that will place more emphasis on a measured anterior chamber depth." Dr. Gatinel also pointed out how predicting ELP loses accuracy in post-refractive surgery eyes, complicating IOL power calculations, but added that other factors affecting IOL power calculations include tear film changes altering topography readings, irregular astigmatism, subtle refractive index changes from epithelial hyperplasia, biomechanical instability following radial keratotomy, and some "peculiar effects" cataract surgery may have on corneal wall ge- ometry and hydration. There are many factors to consider, and physicians do not yet have a perfect understand- ing of how everything fits together. "This is an issue that hasn't been thoroughly worked out; factors such as altered higher order aberrations have yet to be understood and incorporated," Dr. Koch said. Role of diagnostics Dr. Gatinel noted that the accuracy and pre- cision of IOL power calculations depend on the quality of preop measurements. He said that accessing epithelial and posterior corneal mapping could, in theory, improve the corneal power estimation. Optical biometry has also "tremendously improved" in accuracy, and 2-D and eventually 3-D reconstructions of the ante- rior segment and even the whole eye "may bring another level of precision and enable some fancy ray tracing techniques," he said. Dr. Al-Mohtaseb is optimistic about devel- opments in diagnostics and the effect they will have on IOL power calculations. "The accuracy of biometry and topography are continuing to improve. As we get more accurate measure- ments of total corneal power even in post-re- fractive eyes, we will have an increase in accura- cy of IOL power calculations," she said. Dr. Koch is more cautious, highlighting how imperfectly all the factors affecting IOL power calculations are currently understood. "I think that the most promising option is A ny IOL power calculation has three key ingredients, according to Uday Devgan, MD: (1) the power of the cornea, (2) the axial length of the eye, and (3) the effective lens posi- tion (ELP). This applies to all eyes, whether or not their corneas had previously undergone surgery, such that Zaina Al-Mohtaseb, MD, said that the principles for achieving good refractive outcomes "are the same as for virgin corneas as patients who have received prior corneal procedures, which include accurate biometry and keratometry, optimizing IOL calculations, and ruling out ocular pathology"—particularly corneal and retinal conditions such as ocular surface disease, epithelial basement membrane dystrophy (EBMD), and epiretinal membranes (ERM). However, prior corneal refractive proce- dures definitely influence outcomes, affect- ing the accuracy of IOL power calculations. EyeWorld consulted Dr. Al-Mohtaseb and Dr. Devgan, as well as Damien Gatinel, MD, and Douglas Koch, MD, on improving the accuracy of IOL power calculations in post-refractive surgery patients. Post-refractive challenges According to Dr. Al-Mohtaseb, IOL power calculations become more challenging primarily for two reasons: (1) inaccurate calculation of total corneal refractive power due to the ratio change between the anterior and posterior cor- neal curvature, and (2) the increased difficulty of predicting ELP. Regarding the first problem, Dr. Devgan noted that "when we change the front curva- ture of the cornea with LASIK but not the back curvature, the biometer does not read as accurately." On the second problem, Dr. Devgan ex- plained, "With prior corneal refractive surgery, like LASIK, we have significant changes to the corneal power. Since the IOL power formulae tend to use the corneal power to determine the ELP, this is also thrown off. We can minimize this by using the Aramberri double-K method, IOL power calculation after refractive surgery At a glance • Corneal power, axial length, and ELP are key ingredients of current IOL power calculations. • Improving the accuracy of diagnostics improves the ac- curacy of power calculations, but a better understanding than physicians currently have of all the factors affecting the refractive power of the eye is also necessary. • With no one device and IOL power calculation formula able to provide the best results in all cases, surgeons should use multiple devices and formulas along with their best judgment to optimize outcomes.

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