EyeWorld India June 2021 Issue

Supported by Mastering the Barrett Toric Calculator Dr. Graham Barrett of Australia presented “Mastering the Barrett Toric Calculator” and discussed the finer points of using the Barrett formulae. Toric IOLs play a key role in improving refractive outcomes and thus accurate- ly predicting the required toric cylinder value is essential to the success of refractive cataract surgery. Improvements in tech- nology have allowed new and more accurate formulae to be developed. One formula that is widely used is the Barrett Uni- versal II formula (BUII ), a ver- gence formula based on paraxial ray tracing. The BUII, available for use online on the APACRS website, calculates the principle planes for the predicted IOL and can be used with the measured posterior cornea. The BUII for- mula is identical to the Barrett TK formula that is available for use on the IOLMaster 700. The online calculator for the BUII formula allows a practitioner to enter data for both eyes as well as the lens model. Lens constants will be hika also found that the largest rotation of the IOL occurred from the end of cataract surgery to 1 hour after surgery. After 1 hour, the toric IOL stayed very stable. Dr. Oshika explained that waiting to perform repositioning surgery is effective because the capsule bag has not completely under- gone fibrosis immediately after surgery, and the lens itself may stick to the capsule bag. Thus, the time period immediately after surgery is the most critical period for maintaining rotational stability of toric lens. Dr. Oshika’s advice to reduce misalignment is to wait until the IOL has completely un- folded, stating that “some lenses are very slow to open, depending on the type of lens.” He also ad- vises surgeons to instruct pa- tients to rest and avoid walking around for at least 1 hour after surgery. As for the following days and weeks after surgery, discus- sion from the panel agreed that high impact exercising, especial- ly the up and down movement of jogging, should be avoided for 2-3 weeks. Tetsuro OSHIKA Japan automatically entered, but one can also enter a specific lens factor or the A constant. Once calculated, a table will display IOL powers for the predicted spherical equivalent refraction. Another formula to consider is the Barrett Toric Calculator, which is based on the BUII and provides the same spherical power prediction as well as the predicted toric cylinder value. The calculator predicts the pos- terior corneal power for each eye based on a theoretical model. Surgically induced astigmatism (SIA) should be listed as the centroid value, which is typically in the range of 0.1-0.12 for a 2.4 mm temporal incision. Dr. Bar- rett recommends a lens factor of 2.09, although a personal con- stant can be entered if preferred. The integrated K calculator and measured PCA options are quite valuable and are available after data has been entered and cal- culated. One important consid- eration is deriving a vector mean or median K, which is helpful in measuring corneal astigmatism for toric IOL calculations. This is because corneal astigmatism measurements are not always repeatable when using measures such as the Warren Hills vali- dation criteria due to the poor correlation for the keratometric cylinder when measuring the axi- al length from different devices. Dr. Barrett then explained the reason he developed the True K formula: to improve outcome prediction in eyes that have had previous refractive surgery. The True K formula has shown to be more accurate than other methods for patients who have undergone myopic LASIK when their refractive history is known and for patients who have under- gone laser correction for hyperopia. Dr. Barrett’s True K TK formula, created last year, modified the True K formula and allows for the incorporation of the measured posterior cornea. Studies con- ducted by Dr. Barrett confirm that the True K with inclusion of the posterior cornea provided the most accurate and repeat- able option in both myopic and hyperopic patients undertaking cataract surgery without prior refractive information. Dr. Barrett also pointed out that there is a need for a custom toric calculator when selecting a Toric IOL in eyes that have undergone previous refractive surgery. After modifying the True K formula to create the True K TK formula, Dr. Barrett also updated the True K Toric Calculator which can now be used with the predicted posterior cornea or a measured option for posterior corneal astigmatism. Additionally, this True K Toric Calculator includes the K Calculator, allowing the user to enter up to 3 different values for the anterior cornea. The True K Toric Calculator will then calculate a new integrated K or median vector value which is used for the final calculation. The last consideration for mas- tering Toric IOLs is learning how to manage unexpected outcomes in patients, whether by exchang- ing implanted lenses, adding a piggyback IOL, or rotating an existing Toric IOL. Formulas such as the Holladay R or Astigma- tism Fix can provide solutions Graham BARRETT Australia Ronald YEOH Singapore Hungwon TCHAH South Korea Dr. Graham Barrett discusses use of the Barrett Toric Calculator. Copyright 2021 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. All other trademarks are the intellectual property of their respective owners. © Johnson & Johnson Surgical Vision, Inc. 2021 PP2021CT4836