[Measurement of the central corneal power after myopic LASIK]

Intrasession repeatability and agreement of the anterior corneal assessment provided by a multidiagnostic device

CLINICAL RELEVANCE

Multidiagnostic systems have recently appeared on the market. Knowledge of the repeatability and validity of any instrument is mandatory before its introduction in clinical practice.

BACKGROUND

The aim of this work is to examine the intrasession repeatability of anterior pole measurements provided by the multidiagnostic device Wave Analyzer Medica 700 (WAM700) and agreement with Pentacam measurements in normal eyes.

METHODS

In the right eyes of 113 participants, three repeat measurements of central keratometry, central corneal thickness, anterior chamber depth and corneal eccentricity were made with the WAM700 and Pentacam in random order. Intrasession repeatability and agreement were determined.

RESULTS

Employing WAM700, intrasession repeatability for keratometry, central corneal thickness and anterior chamber depth was good (ICCs ≥ 0.992; CV 0.48-0.98%), yet worse than the values obtained for the Pentacam (ICCs ≥ 0.998; CV 0-0.33%). WAM700 showed excellent intrasession repeatability when used to measure the anterior chamber depth (Sw 0.03 mm). However, the repeatability of this device was inferior for central corneal thickness (Sw 4.24 μm) and keratometry measurements (Sw < 0.21 D) and was poor for corneal eccentricity (Sw 0.07; ICC 0.908; CV 14.58%). Agreement between WAM700 and Pentacam showed a high ICC for the keratometry measurements, central corneal thickness and anterior chamber depth (>0.972) but lower for corneal eccentricity (ICC 0.762).

CONCLUSIONS

In healthy eyes, the WAM700 multidiagnostic device showed good intrasession repeatability for keratometry, central corneal thickness and anterior chamber depth measurements. Agreement between WAM700 and Pentacam was good for the anterior chamber depth measurement. However, these instruments cannot be considered interchangeable for keratometry, central corneal thickness and eccentricity readings.

Predictability of Sirius dual-scanning corneal tomography in the measurement of corneal power after photorefractive surgery

Determining an accurate central corneal power (K) measurement is crucial for calculating the intraocular lens power in patients who are undergoing cataract extraction. The ideal method for measuring K is to use a device that works independently of the refractive surgery information. The Scheimpflug camera system offers a promising means of measuring the true corneal power after keratorefractive surgery. In this study, we investigated the accuracy of this system in measuring central corneal power after photorefractive corneal surgery by comparing it to the theoretically derived central corneal power by history method. A total of 120 eyes of 65 (35 females and 30 males) patients were included in this study. The mean change of refraction at the spectacle plane was 3.75 D, whereas the mean change of refraction at the corneal plane was 3.37 D. Using the Sirius dual-scanning corneal tomography, the mean change in corneal power was 3.96 D. No significant differences were detected between the mean post-operative corneal power measured by the Sirius tomographer and the mean change in refraction at the corneal plane calculated clinically (P = 0.076) and the correlation was found to be high (0.913). This study suggests that Sirius dual-scanning corneal tomography offers high predictability when measuring the central 5 mm corneal power in patients who have had myopic corneal photorefractive surgery.

Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery

PURPOSE

To determine the accuracy of Pentacam Scheimpflug system Holladay equivalent keratometry (K) readings (EKRs) in calculating intraocular lens (IOL) power after corneal refractive surgery, including laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and radial keratotomy (RK).

SETTING

Jules Stein Eye Institute, David Geffen School of Medicine at University of California, Los Angeles, Los Angeles, California, USA.

METHODS

In this combined retrospective and prospective clinical study, patients who had cataract surgery after corneal refractive surgery were recalled to have Scheimpflug imaging of the operated cornea and Holladay EKR determination. The Holladay EKR was compared with a gold-standard K value, which was the back-calculated value using the original Hoffer formula based on the actual surgical outcomes. Eyes without a history of refractive surgery served as controls.

RESULTS

Twenty-seven patients (41 eyes) were evaluated; 26 eyes had previous LASIK or PRK and 15, previous RK. Forty-one eyes served as controls. The mean error of the Holladay EKR in eyes with previous LASIK or PRK was +1.84 diopters (D) (range +0.66 to +4.94 D). The mean error in eyes with previous RK was +2.17 D (range +0.48 to +3.09 D). In the control eyes, the mean EKR error was +1.38 D (range -0.17 to +2.54 D).

CONCLUSIONS

The Holladay EKR calculated using version 1.16r04 of the Scheimpflug system software was inaccurate in virgin corneas and in those with a history of LASIK, PRK, or RK using current IOL power calculation formulas. The Scheimpflug power measurements were consistently steeper than the true corneal power.