Posters

Comparison of corneal astigmatism between autokeratometer and total corneal refractive power of the Pentacam before and after myopia-correcting keratorefractive surgery

Poster Details

First Author: S.Nam SOUTH KOREA

Co Author(s):    C. Im   H. Yang   K. Seo        

Abstract Details

Purpose:

Total corneal refractive power for 4-mm apex center zone of the Pentacam (Oculus, Inc., Wetzlar, Germany) (TCRP4) can be used for intraocular lens (IOL) power calculation for cataract surgery in eyes after myopia correcting keratorefractive surgeries (Nam SM et al., in press). Because TCRP4 measures total corneal astigmatism with the anterior and posterior curvatures, the significance of posterior astigmatism before and after keratorefractive surgery can be evaluated in comparison of keratometric astigmatism. Based on this evaluation, TCRP4 astigmatism might be introduced for toric IOL power calculation in the cornea with accidentally high astigmatism after keratorefractive surgery.

Setting:

Patients had laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) at B&VIIT eye center, Seoul, Republic of Korea from 2009 to 2011. The Allegretto Wave Eye-Q laser (WaveLight AG, Erlangen, Germany) or the Amaris excimer laser (SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany) was used for the surgery. The corneal power was measured with a autokeratometer (keratometric index of 1.3375) and the Pentacam. The measurements were analyzed at CHA Bundang Medical Center, Gyeonggi-do, Republic of Korea.

Methods:

One eye of 20 patients (10 for LASIK and 10 for PRK) were randomly included. For all cases, preoperative and postoperative best corrected visual acuities were 20/20 and no postoperative complication was accompanied. The corneal power was measured before and after the surgery with a autokeratometer and the Pentacam. Astigmatisms of TCRP4 and front refractive power for 4-mm apex center zone (RP4) were read from ‘power calculations in actual zone' on the power distribution display of the Pentacam. The corneal back astigmatism and the mean cornea front power were read on topometric display of the Pentacam. For all included cases, the mean cornea front power was within 0.5 D of the mean keratometric power of the autokeratometer. The statistical analysis of astigmatism was mathematically and graphically performed with the polar values and bivariate analysis.

Results:

Mean age of patients was 26 ± 5 years and median time of postoperative measurement was 330 days (minimum 203 and maximum 813 days). The preoperative average net corneal astigmatism was different between the keratometry (1.29 D @ 90°) and TCRP4 (1.1 D @ 91°). In contrast, the average net corneal astigmatism of front RP4 (1.4 D @ 91°) was the same with the keratometry. The postoperative average net astigmatism was different between the keratometry (0.67 D @ 91°) and TCRP4 (0.3 D @ 85°). The postoperative average difference in net astigmatism between the keratometry and TCRP4 (0.4 D @ 94°) was greater than the preoperative (0.2 D @ 82°). However, the postoperative average net corneal astigmatism of front RP4 (0.6 D @ 87°) was not different from the keratometry. The net cornea back-astigmatism was not changed by the surgery (preoperatively - 0.4 D @ 91° and postoperatively - 0.4 D @ 89°).

Conclusions:

The corneal astigmatisms of TCRP4 was different from the keratometry at preoperative and postoperative time. The degree of difference increased postoperatively. Because the astigmatism of front RP4 was equaled to the keratometry before and after the surgery, the discrepancy between TCRP4 and the keratometry would stem from the corneal back-astigmatism. The corneal back-astigmatism was not changed by the surgery. Therefore, anterior change in corneal astigmatism by the surgery would cause additional difference between total corneal astigmatism of TCRP4 and the keratometry. The postoperative keratometry astigmatism may not be as accurate as the preoperative if the corneal back astigmatism is not considered. FINANCIAL INTEREST: NONE