EyeWorld India March 2017 Issue

March 2017 18 EWAP FEATURE Tips for selecting the best keratometry values for IOLs by Rich Daly EyeWorld Contributing Writer Pearls for improving the selection of keratometry values when determining IOL calculations and astigmatism management L ess than 1% of surgeons are able to achieve refractive outcomes within 0.50 D of the target sphero-equivalent power among at least 90% of their patients. 1 A key to their success is good keratometry. Jonathan Rubenstein, MD , Deutsch Family Professor and vice chairman, Department of Ophthalmology, Rush University Medical Center, Chicago, said there are roles for autokeratometry, manual keratometry, topography, and LENSTAR (Haag-Streit, Koniz, Switzerland)/IOLMaster (Carl Zeiss Meditec, Jena, Germany) in determining corneal curvature and corneal power. “As a rule, I suggest that physicians measure the cornea AT A GLANCE • Most devices should provide accurate measurements for normal healthy corneas, but not all yield identical keratometry values. • To treat eyes with previous corneal surgery, several corneal topographers have developed indices that form an average curvature of the central cornea. • When biometry measurements are unreliable, treat patients aggressively with artificial tears for a week or two and then repeat all measurements. • Toric IOLs are much more precise than LRIs due to the variability in wound healing. Irregular astigmatism examples Location of standard deviation of corneal radii on LENSTAR printout Source (all): Jack Holladay, MD Location of standard deviation of corneal radii on IOLMaster printout with every modality that they have access to,” Dr. Rubenstein said. Dr. Rubenstein relies on manual keratometry combined with IOLMaster and LENSTAR Ks for corneal power, and IOLMaster or LENSTAR values to determine corneal axis, with the corneal map providing verification. By performing the keratometry test himself, Dr. Rubenstein can evaluate the quality of the corneal mires to see if there are crisp corneal mires or irregular mires. “Therefore, this is a method of determining the health of the ocular surface and the presence of irregular astigmatism,” Dr. Rubenstein said. “If the mires are not crisp and clean, the validity of all the other corneal measurements are diminished.” Since astigmatism is a vector, any device that measures astigmatism will need to measure both the magnitude and direction of astigmatism, said Richard Tipperman, MD , attending surgeon, Wills Eye Hospital, Philadelphia. “Autokeratometers and manual keratometers can be quite accurate for both the magnitude and direction of astigmatism,” Dr. Tipperman said. “The ‘sim Ks,’ or simulated keratometry readings, provided by topography are not accurate for IOL planning; however, the topography can be very helpful for determining the true axis of astigmatism with what has been termed ‘the credit card test.’ In this test, the surgeon places the long axis of a credit card on what is visually determined as the steepest meridian on topography and can read the axis of astigmatism directly.” Dr. Tipperman noted that the K values will always differ since disparate devices measure in varying ways and at different corneal locations. “The surgeon should analyze the data, and they should ‘make sense’ so that in most cases— with the exception of “lenticular astigmatism”—a patient with 2 D of astigmatism at axis 90 in his or her spectacles would be expected to have this approximate continued on page 20