Effects of error in radius of curvature on the corneal power measurement before and after laser refractive surgery for myopia

PURPOSE

To investigate the sources of error in corneal power measurement before and after corneal refractive surgery for myopia.

METHODS

The study comprised 28 eyes of six males and eight females with a mean age of 26 (range 18-39 years). The radius of curvature of anterior and posterior corneal surface, Q-Values of anterior and posterior corneal surface and corneal central thickness were measured by rotating Scheimpflug imaging (Pentacam). The true net power F(g), back vertex power F(v), and keratometric power SimK, were calculated respectively at the apex and at a paracentral area on the 3 mm ring.

RESULTS

For virgin eyes, the overestimation (0.53 ± 0.11 D) of the corneal power by using a keratometric index of 1.3375 was balanced by the underestimation (-0.21 ± 0.09 D) of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error with a mean value of 0.33 ± 0.11 D. With the Q-value changing from -0.09 to -0.41, the percentage balanced by the error in radius of curvature increased from 16% to 73%. However, for eyes after laser refractive surgery, the radius of curvature error lead to an overestimation (0.54 ± 0.16 D) of the corneal power and the keratometric index of 1.3375 again overestimated (1.59 ± 0.26 D) the corneal power, resulting in a large measurement error with a mean value of 2.12 ± 0.40 D. With the Q-value changing from 0.35 to 1.89, the percentage added by the error in radius of curvature increased from 14% to 32%.

CONCLUSIONS

For virgin eyes, the overestimation of the corneal power by using a keratometric index of 1.3375 is balanced by the underestimation of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error. However, for eyes after laser refractive surgery, the flatter anterior corneal surface means that the use of a keratometric index of 1.3375 significantly overestimates the corneal power and the radius of curvature error now adds to this overestimation and results in a large measurement error.

Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery

PURPOSE

To present the prospective application of the Orbscan II central 2-mm total-mean corneal power obtained by quantitative area topography in intraocular lens (IOL) calculation after refractive surgery.

METHODS

Calculated and achieved refraction and the difference between them were studied in 77 eyes of 61 patients with previous radial keratotomy (RK), RK and additional surgeries, myopic LASIK, myopic photorefractive keratectomy (PRK), or hyperopic LASIK who underwent phacoemulsification without complications in 3 eye centers. All IOL calculations used the average from the central 2-mm Orbscan II total-mean power of maps centered on the pupil without the use of previous refractive data. Six IOL styles implanted within the bag were used.

RESULTS

Using the SRK-T formula, the overall calculated refraction was -0.64+/-0.93 diopters (D). The overall achieved spherical equivalent refraction (-0.52+/-0.79 D; range: -3.12 to 1.25 D; 95% confidence interval [CI]: -0.70/-0.34 D) was +/-0.50 D in 53% of eyes, +/-1.00 D in 78% of eyes, and +/-2.00 D in 99% of eyes. The overall difference between the calculated and achieved refraction (0.12+/-0.93 D, P=.27; range: -2.18 to 2.62 D; 95% CI: 0.09/0.33 D) was +/-0.50 D in 39% of eyes, +/-1.00 D in 77% of eyes, and +/-2.00 D in 96% of eyes. This difference was +/-1.00 D in 77% of eyes with RK (P=.70), 82% of eyes with myopic LASIK (P=.34), and 90% of eyes with myopic PRK (P=.96). In eyes with RK followed by LASIK, a trend toward undercorrection was noted (P=.03). In eyes with hyperopic LASIK, a trend toward overcorrection was noted (P=.005).

CONCLUSIONS

In eyes with previous corneal refractive surgery, IOL power calculation can be performed with reasonable accuracy using the Orbscan II central 2-mm total-mean power. This method had better outcomes in eyes with previous RK, myopic LASIK, and myopic PRK than in eyes with hyperopic LASIK or RK with LASIK.