Improving the Accuracy of Lens Calculations in Cataract Surgery
Uday Devgan, MD
Cataract surgery is meant to correct cataracts, but we can also adjust the refractive state of the eye at the same sitting. Though LASIK (laser-assisted in situ keratomileusis) seems to be in the refractive surgery spotlight, our most powerful procedure to correct just about any degree of refractive error is modern-day cataract surgery.
Many decades ago, when intraocular lenses (IOLs) were becoming more popular, ophthalmologists realized that the IOL power could be tailored to the eye to help provide a specific refractive outcome. At first, there were tables based on the patient’s preoperative refraction. These provided some guidance, but very few patients were within 0.5 diopters (D) of the intended refractive target. Lens calculation pioneers Drs Sanders, Retzlaff, and Kraff analyzed patient data and came up with a simple regression formula: P = A -2.5L -0.9K, where P is the IOL power to be implanted, A is the A-constant of the IOL, L is the axial length of the eye, and K is the average keratometry. For most average-sized eyes, this worked reasonably well, but for longer or shorter eyes, an adjustment was needed, so the SRK-II formula was developed.
These regression methods were reasonable but were supplanted by the theoretical methods such as the SRK/T (T for theoretical), Holladay 1, Hoffer Q, and other formulas which used the patient’s axial length and keratometry to estimate the effective lens position (ELP). This resulted in more consistent outcomes than the regression methods. Our biometry improved as well, with more advanced methods to measure the axial length, anterior segment dimensions, and corneal power. The results kept improving and newer formulas such as the Holladay 2 and Haigis used more input data, beyond just the axial length and keratometry, to estimate the ELP. With these methods and careful attention to detail, surgeons could now help 80% or more of their patients achieve an accuracy of +/- 0.5 D.
But can we do better? For LASIK, we can reliably achieve +/- 0.5 D in 95% or more of patients after the initial surgery—can we do that in at least 90% for cataract surgery patients? The primary ways that we can do this are (1) preoperatively, to increase the accuracy of our lens power calculations; (2) intraoperatively, to use intraoperative aberrometry; and (3) postoperatively, to hone the refractive outcome by adjusting the IOL power or the corneal power.
Preoperative IOL calculations are improving, with new methods becoming available every year. The Barrett Universal II formula (accessible at www.apacrs.org) attempts to increase the accuracy by improving the theoretical calculations. The Hill-RBF method (accessible at www.ascrs.org) uses a large volume of data to create a type of artificial-intelligence way of improving accuracy. The Ladas Super Formula (also accessible at www.IOLcalc.com) does both by incorporating the sweet spots of the best available formulas as well as using a big-data approach to crowdsource huge numbers of eyes to capture every subset of eye types. All of these methods show promise for achieving an unprecedented level of accuracy in the calculations that we do before surgery.
Intraoperative aberrometry also holds promise because it allows us to measure an aphakic eye during surgery. It doesn’t yet solve the issue of predicting a final resting ELP, and there is some concern that corneal measurements during surgery may not be reflective of the final postoperative corneal state. Every subsequent generation of devices will improve, and I foresee a day when our best IOL calculation formulas will be built into the aberrometers and we will no longer need to do any preoperative IOL calculations.
Finally, we have the ability to adjust the refractive state of the eye by adjusting the corneal power or the IOL power. The corneal power adjustment is easily done with an excimer laser ablation in the form of LASIK or photorefractive keratectomy (PRK). We can also use a femtosecond corneal laser to perform relaxing incisions for astigmatism or lenticule extraction treatments for myopia. We can also adjust the power of the IOL by doing an IOL exchange or adding a piggyback IOL. New IOL technologies will allow us to exchange just an optic while leaving the lens base and haptics in place or to adjust the anterior-posterior optic position by mechanical means. There are also experimental ways to adjust the IOL power from outside the eye, such as using a femtosecond laser to alter the optic shape allowing change in spherical and cylindrical power or using tunable lasers at specific frequencies to alter the refractive index, and thus power, of the lens. These methods have major advantages over the IOLs which require UV light for power adjustment (not available in the United States).
We will continue to improve the refractive accuracy and to make cataract surgery so much more than simply correcting the cataract.