POST-LASIK CATARACT PATIENTS

The best available techniques for calculating IOL power in patients who have undergone previous corneal refractive surgery can achieve the required minimal standards for predictive accuracy, but they will not match the predictability which normal cataract patients and their surgeons expect of modern cataract surgery, said Douglas D Koch MD, Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, US.

The sources of errors in IOL calculations in eyes that have undergone previous corneal refractive surgery include flawed assumptions in the conventional eye model, the changed asphericity of the cornea, the changed relationship between the cornea’s anterior and posterior curvature, and difficulties in calculating effective lens position, Dr Koch told the XXIX Congress of the ESCRS.

“Standard methods of calculating net corneal refractive power assume a refractive index of the cornea of 1.3315, which is in turn based on the assumption that there is a fixed ratio between the front and the back. However, LASIK and PRK alter the ratio of anterior corneal curvature to posterior curvature, and therefore use of 1.3315 as the standard effective refractive index of cornea is no longer valid,†he said.

Furthermore, he and his associates calculated the actual standardised index of refraction for different types of eyes conducted using the Galilei™ combined placido and dual Scheimpflug topography system (Ziemer) the calculated index of refraction was 1.3278 in normal eyes that had not undergone any surgery, 1.346 in eyes that had undergone myopic LASIK or PRK, and 1.3302 in eyes that had undergone hyperopic LASIK or PRK – very near the classical Gullstrand eye value of 1.3315 (Wang et al, IOVS 2011; 52:1716-1722).

There is currently a broad range of approaches to correcting IOL calculations for the changes induced by corneal refractive surgery. They fit into three main categories. First, there are those approaches that involve measuring corneal power and then modifying it based on the amount of refractive correction using the clinical history method based on regression analysis as in the case of Haigis-l and Shammas methods, Dr Koch said.

In the second category are those formulas which use corneal measurements that are obtained when the patient presents for cataract surgery and then modifies them by some fraction of the LASIK-induced refractive change. The adjustments are made either to the corneal values used in the IOL calculations or in the IOL power after it is calculated in the standard way.

The third category uses no historical data – only measurements obtained at the time of presentation for cataract surgery.

Most methods also use the “double K†calculation described by Jack Holladay and Jaime Aramberri, which aim to eliminate errors in effective lens position by using preoperative keratometry for effective lens position and postoperative keratometry for calculating lens power.

New Internet resource

In order to help cataract surgeons decide which type of IOL calculation formula to use in eyes that have undergone previous refractive surgery, Dr Koch, Dr Li Wang, and Dr Warren Hill have created a new website, the ASCRS IOL calculator, that performs IOL calculations using all of the formulas individually as well as an average value of the calculations.

To make the IOL calculations, the surgeon or technician enters data such as the type of refractive procedure performed, the amount of correction, preoperative refraction and keratometry, and the type of topography device used and the measurements obtained. Once the data are entered, the online program performs the calculations using all of the different formulas. Clicking on the yielded value of each formula provides an explanation of the calculation and its clinical reference.

To evaluate the ASCRS IOL calculator, Dr Koch and his associates conducted a two-centre study involving 72 eyes of 57 consecutive patients who were undergoing cataract surgery after having undergone myopic LASIK (Wang et al. J Cataract Refract Surg., 2010;36(9):1466-73). The patients’ ages at the time of cataract surgery ranged from 42 to 77 years and had undergone LASIK correction of myopia ranging from 0.98 D to 11.21 D.

The researchers found that methods using surgically induced change in refraction and methods using no previous data gave better results than methods using pre-LASIK/PRK K values and surgically induced change in refraction. For example, the clinical history methods were accurate to within half a dioptre in less than 50 per cent of cases, compared to 57 per cent to 67 per cent with the induced refraction and no previous data groups. The figures for accuracy which were within 1.0 D were only 60 per cent to 69 per cent with the clinical history methods, compared to 86 per cent to 94 per cent with the induced refraction and no previous data methods.

Dr Koch noted that, although the predictability achieved with the formulas using induced refractive change or no previous data met the proposed UK NHS benchmark of 85 per cent being within 1.0 D of target and 55 per cent being within 0.5 D of target refraction, the predictability was below that which surgeons have come to expect in the treatment of normal cataract patients.

“We meet those criteria, but I would submit that those criteria are really very loose. Current standards are much higher, and those are the standards we have to give our patients,†he said.

Future advances which may bring IOL calculation in refractive surgery patients up to standard include the development of technologies that will provide a more accurate measure of corneal power and effective lens position, better IOL calculation formulas, new technologies for determining IOL power intraoperatively, and lenses with postoperatively adjustable refractive power, as in the case of the light-adjustable lens, Dr Koch said.