All the eyes underwent routine preoperative examinations including best-corrected visual acuity (BCVA), intraocular pressure (IOP), cycloplegic and manifest refraction, anterior segment examination by slit-lamp, corneal topography and tomography measurements. The eyes were divided into three groups according to the manifest SE: low myopia (− 3.00 D < SE ≤ − 0.50 D), moderate myopia (− 6.00 D < SE ≤ − 3.00 D) and high myopia (SE ≤ − 6.00 D). The eyes were also divided into five groups according to the MA: MA < 0.50 D, 0.50 D ≤ MA < 1.00 D, 1.00 D ≤ MA < 2.00 D, 2.00 D ≤ MA < 3.00 D, MA ≥ 3.00 D.
Pentacam examination was performed by experienced technicians. The Pentacam instrument (Pentacam HR, Oculus GmbH, Wetzlar, Germany) was calibrated regularly on a weekly basis. The patients were positioned in front of the Pentacam instrument with the forehead and chin properly supported and both lateral canthi aligned with the marks. The patients were asked to blink 2–3 times to have the tear film evenly distributed on the cornea and then open the eyes wildly to stare at the fixation target while the instrument was proceeded to the cornea. Once the red cross on the screen coincided the red circle at the pupil center, the instrument automatically captured 50 rotational Scheimpflug images within 2 s. The power and axis of the keratometric astigmatism and PCA were measured within the central 3 mm using a default refractive indexes for the cornea (1.376) and aqueous humor (1.336). The procedure was performed again if the patient’s eye blinked during the measurement or quality of the scan was poor (comment on the display marked yellow or red). Only image covered at least central 8.0 mm of corneal surface and image quality labelled with ‘OK’ on the display was accepted. Pentacam data of the eyes were retrieved from the machine and only results with image quality labelled with ‘OK’ were included.
ACA was defined as WTR when the steepest meridian was 90° ± 30°, as ATR when the steepest meridian was between 0–30° or 150–180°, and as OBL when the steepest meridian > 30° and < 60°, or > 120° and < 150°. Because the posterior corneal surface has negative refractive power (divergent), we used other classification labels for the PCA to avoid confusion with the ACA. The PCA was defined as SMV when the steepest meridian was 90° ± 30°, as SMH when the steepest meridian was between 0°–30° and 150°–180°, and as SMO in the rest. Aggregate PCA was calculated as previously described30 (link).
Pentacam examination was performed by experienced technicians. The Pentacam instrument (Pentacam HR, Oculus GmbH, Wetzlar, Germany) was calibrated regularly on a weekly basis. The patients were positioned in front of the Pentacam instrument with the forehead and chin properly supported and both lateral canthi aligned with the marks. The patients were asked to blink 2–3 times to have the tear film evenly distributed on the cornea and then open the eyes wildly to stare at the fixation target while the instrument was proceeded to the cornea. Once the red cross on the screen coincided the red circle at the pupil center, the instrument automatically captured 50 rotational Scheimpflug images within 2 s. The power and axis of the keratometric astigmatism and PCA were measured within the central 3 mm using a default refractive indexes for the cornea (1.376) and aqueous humor (1.336). The procedure was performed again if the patient’s eye blinked during the measurement or quality of the scan was poor (comment on the display marked yellow or red). Only image covered at least central 8.0 mm of corneal surface and image quality labelled with ‘OK’ on the display was accepted. Pentacam data of the eyes were retrieved from the machine and only results with image quality labelled with ‘OK’ were included.
ACA was defined as WTR when the steepest meridian was 90° ± 30°, as ATR when the steepest meridian was between 0–30° or 150–180°, and as OBL when the steepest meridian > 30° and < 60°, or > 120° and < 150°. Because the posterior corneal surface has negative refractive power (divergent), we used other classification labels for the PCA to avoid confusion with the ACA. The PCA was defined as SMV when the steepest meridian was 90° ± 30°, as SMH when the steepest meridian was between 0°–30° and 150°–180°, and as SMO in the rest. Aggregate PCA was calculated as previously described30 (link).
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