Placido-disk Corneal Topography Research Articles

Effect of femtosecond laser–created clear corneal incision on corneal topography

To evaluate changes in anterior corneal topography after femtosecond laser-created clear corneal incisions (CCIs). Fondazione G.B. Bietti IRCCS, Rome, Italy. Comparative case series. Phacoemulsification was performed through a 2.75 mm incision. In the study group, a 3-plane CCI was created using a 150 kHz femtosecond laser. In the control group, a single-plane angled CCI was created using disposable knives. Placido disk corneal topography was performed preoperatively and 1 week and 1 month postoperatively. Central curvature (simulated keratometry [K]) and local changes in anterior corneal topography at the CCI site were evaluated and compared between groups. No significant changes in the mean simulated K values were found in either group postoperatively (P>.05). The preoperative to 1-month postoperative mean simulated K ranged between 43.30 diopters (D)±0.82 (SD) and 43.17±0.82 D in the study group and between 43.34±1.44 D and 43.29±1.48 D in the control group. The method of CCI creation significantly influenced the changes in the anterior central topography during the first month postoperatively (P<.001). Greater steepening of the incision edges was found in the control group (mean 36.60±4.13 D to 37.74±2.39 D) than in the study group (mean 38.81±2.91 D to 39.15±4.48 D). The 2.75 mm 3-plane femtosecond CCIs induced slight changes in the anterior central corneal topography, similar to single-plane angled manual incisions. Topographic differences between the femtosecond method and manual method were found at the CCI site. No author has a financial or proprietary interest in any material or method mentioned.

Precision of a new Scheimpflug and Placido-disk analyzer in measuring corneal thickness and agreement with ultrasound pachymetry

To assess the precision of corneal thickness measurements obtained by a new Scheimpflug camera combined with Placido-disk corneal topography (Sirius) and compare the measured values with those obtained by ultrasound (US) pachymetry. Eye Hospital of Wenzhou Medical College, Wenzhou, China. Comparative evaluation of a diagnostic test or technology. Eyes of healthy subjects were examined with the Scheimpflug-Placido topographer. Central (CCT) and thinnest (TCT) corneal thickness were recorded after 3 consecutive measurements. For US pachymetry, only CCT was measured. Measurements were repeated within 1 week. The within-subject standard deviation (S(w)), test-retest repeatability, coefficient of variation (CoV), and intraclass correlation coefficient (ICC) were calculated to evaluate intrasession repeatability and intersession reproducibility. Interdevice comparison was analyzed with paired t tests and Bland-Altman plots. The intrasession repeatability of Scheimpflug-Placido measurements was high, with test-retest and CoV close to 9 μm and 0.6% for CCT and TCT, respectively. The intersession test-retest and CoV were close to 10 μm and 0.7%, respectively. The ICC was higher than 0.98 for repeatability and reproducibility. High agreement was found between Scheimpflug-Placido and US pachymetry measurements, with narrow 95% limits of agreement. The Scheimpflug-Placido instrument showed excellent intrasession repeatability and intersession reproducibility of CCT and TCT measurements in healthy eyes. High agreement and lack of statistically significant difference suggest that the instrument's TCT and the US pachymetry-CCT measurements can be used interchangeably in subjects with normal corneal thickness. No author has a financial or proprietary interest in any material or method mentioned.

Accuracy of corneal power measurements by a new Scheimpflug camera combined with Placido-disk corneal topography for intraocular lens power calculation in unoperated eyes

To assess the accuracy of the corneal power measurements with a new Scheimpflug camera combined with Placido-disk corneal topography (Sirius) (combined Scheimpflug camera-topographer) for intraocular lens (IOL) power calculation in unoperated eyes and compare the results with those by a validated corneal topographer (Keratron) (validated topographer). Private practice. Case series. Consecutive patients having phacoemulsification and in-the-bag IOL implantation were studied. Intraocular lens power was calculated using the Hoffer Q, Holladay 1, and SRK/T formulas; the axial length, as measured by ultrasound immersion biometry; and 3 corneal power measurements: validated topographer simulated keratometry (K); combined Scheimpflug camera-topographer simulated K (derived from anterior corneal curvature only); combined Scheimpflug camera-topographer mean pupil power (derived from anterior and posterior corneal curvatures through ray tracing). The prediction error was calculated as the difference between the predicted refraction and the refraction measured 1 month postoperatively. When the corneal power measurements from the combined Scheimpflug camera-topographer were used, the mean absolute error (MAE) ranged between 0.23 diopter (D) ± 0.24(SD) (simulated K and Hoffer Q formula) and 0.33 ± 0.23 D (mean pupil power and SRK/T formula). There were no statistically significant differences between the MAE generated by the simulated Ks of the 2 devices with any of the 3 formulas. Both corneal power measurements (simulated K and mean pupil power) provided bythe new combined Scheimpflug camera-topographer were successfully entered into third-generation IOL power calculation formulas in unoperated eyes. Dr. Hoffer is the author of the Hoffer Q formula and owns the EyeLab, which sells Hoffer Programs. No other author has a financial or proprietary interest in any material or method mentioned.