EyeWorld Asia-Pacific March 2021 Issue

Using an Improved TK Formula for Cataract Surgery Another option is to use the Bar- rett TK formulas, which utilize anterior and posterior corneal measurements instead of nomograms, directly on board the IOLMaster 700. TK may im- prove spherical power prediction and toric cylinder (astigmatism) prediction, and additional improvements rather than values for TK are expected in spherical, toric, and post refractive prediction. Initial clinical results have already shown improved accuracy when using Total Keratometry for the Haigis-T and Barrett TK Toric Calcula- tor formulas. 4,5 It is important to know the toric cylinder prediction in planning toric lenses as Barrett’s method uses a theoretical model based on the observation that the cornea is elliptical. The horizontal diameter is almost always wider than the vertical diameter.When a clinician measures the keratometry of the anterior sur- face with a keratometer, the clinician centers on the visual axis while the optical elements of the eye are aligned on the optical axis. With the IOLMaster 700, the lens is not aligned tangentially and often appears tilted. However, it is an apparent tilt due to the displaced fovea. In fact, the angle between the optical axis and the visual axis is about 5 degrees and produces additional astigmatism. When using the Barrett toric calcu- lator, there is an algorithm for IOL tilt that corrects this misalignment of the optical axis. Using the Barrett Toric Calculator thus yields the lowest astigmatic prediction errors. Factors affecting post- refractive calculations When measuring the cornea and cal- culating IOL power in post refractive surgery patients, there are three con- founding factors that affect post-re- fractive calculations. First, when a clinician measures the cornea, he or she is in fact not measuring the actual central cornea but instead extrapo- lating from the keratometry measure- ment. This measurement leads to the second factor: overestimation of the “True” K value resulting in a hyperopic result. The power of the posterior cor- nea is inferred from the keratometry index and assumes a normal anterior to posterior ratio. After myopic laser treatment such as LASIK, however, the central cornea tends to be flatter while after hyperopic laser, the central cornea tends to be steeper. The normal ratio of the posterior to the an- terior cornea (82.5%; refractive index 1.3375) has been disrupted. Further- more, this disruption of anterior and posterior curvature ratio after LASIK and other corneal refractive surgeries leads to a false prediction of corneal heights and subsequently effective lens position (ELP). If this is not com- pensated for, one will also get an error in post-refractive calculation. This is known as the double K issue. Dr. Barrett explains that there are three basic categories of formula for post-refractive patients: patients with a full clinical history in which preoperative keratometry is known, patients with no history at all (most often the case), and patients with a partial history (knowing what the change in refraction produced by the refractive procedure is). With Bar- rett’s own formula being based on the Barrett Universal II formula, it has an integrated solution for the double K issue, allowing for use in the three mentioned categories including myo- pic LASIK/PRK and hyperopic LASIK/ PRK patients. IOL power calculations for patients with differing demands In a recent webinar hosted by ZEISS, “How to do precise IOL power calcu- lations for your demanding (refractive) patients,” Dr. Ahmed Assaf from Ains Shams University in Cairo, Egypt further explored specific subsets of patients. Most laser vision correction (LVC) patients with cataracts present as sensitive to reduced vision due to added lenticular higher order aberra- tions [HOAs] on top of corneal HOAs present after LVC. Other LVC patients with cataracts may have undergone surgery at an earlier age or have high- er expectations from LVC (expecting the same refractive results enjoyed after normal refractive surgery). With all these patients, the burden on clini- cians lies in the difficulty of estimating the correct IOL power. This is due to a variety of reasons. Media placement sponsored by Carl Zeiss Meditec AG Figure 1. The Barrett True K Calculator can be found on the APACRS website and provides the option of selecting “Predicted PCA” or “Measured PCA” for patients with prior myopic or hyperopic laser vision correction. Not all products, services or offers are approved or offered in every market and approved labeling and instructions may vary from one country to another. The statements of the authors of this supplement reflect only their personal opinion and experience and do not necessarily reflect the opinion of Carl Zeiss Meditec AG or any institution with whom they are affiliated. Carl Zeiss Meditec AG has not necessarily access to clinical data backing the statements of the authors.The statements made by the authors may not yet been scientifically proven and may have to be proven and/or clarified in further clinical studies. Some information presented in this supplement may only be about the current state of clinical research and may not be part of the official product labeling and approved indications of the product.The authors alone are responsible for the content of this supplement and any potential resulting infringements resulting from, in particular, but not alone, copyright, trademark or other intellectual property right infringements as well as unfair competition claims. 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