In recent years, measuring corneal thickness has become of great interest for several reasons. Measuring intraocular pressure using Goldmann applanation tonometry is influenced by corneal thickness,1 and the function and health of the endothelial cells alter the corneal thickness. Moreover, the increasing popularity of corneal refractive surgery has made precise measurement of preoperative corneal thickness mandatory.
Among the methods used to measure corneal thickness are ultrasonic pachymetry, scanning slit topography (Orbscan II; Bausch & Lomb, Rochester, NY), and rotating Scheimpflug imaging (Pentacam; Oculus Ine, Wetzlar, Germany). The most common method used to measure corneal thickness is ultrasound pachymetry, which has been considered the gold standard. However, ultrasound pachymetry is a contact method that requires topical anesthesia and considerable examiner experience, making reproducibility dependent on the examiner's experience in using this method.
Because several studies2-5 have reported Orbscan and ultrasonic pachymetry yield similar measurements of corneal thickness when the acoustic equivalent correction factor (0.92) is used, we did not measure corneal thickness with ultrasound pachymetry for this study. In contrast to ultrasound pachymetry, Orbscan and Pentacam are non-contact methods and have a high degree of comfort for the patient. In addition, there is no need for anesthesia and no risk of corneal infection with the Orbscan and Pentacam. However, it is not entirely clear whether the results of these two methods are comparable and whether they can be used interchangeably. Therefore, this prospective study was undertaken to compare central corneal thickness measurements obtained with the Pentacam rotating Scheimpflug camera and the Orbscan scanning slit corneal topography system using the acoustic equivalent correction factor, designated as Orbscan (CF) in this study.
PATIENTS AND METHODS
Ninety-one eyes of 51 volunteers (25 men and 26 women) with a refractive error ranging from -17.00 to + 5.50 diopters (D) (mean: -3.75±4.19 D) were included in the study. Mean age of participants was 32.4±8.85 years. Individuals with systemic and ocular diseases such as diabetes, connective tissue disease, dry eyes, keratoconus, uveitis, corneal and lens opacities, glaucoma, retinal pathology, previous intraocular or corneal surgery, and pregnancy that could potentially interfere with the purpose of the study were excluded.
Prior to enrollment, the purpose of the study was described to participants and informed consent was obtained. The study was performed in adherence to the Declaration of Helsinki.
Participants were asked to stop wearing contact lenses for at least 1 month before undergoing ophthalmic evaluation, which included corneal topography and a detailed ophthalmic examination with manifest and cycloplegic refraction. Cycloplegic refraction was performed during the first visit, and subjective refraction was performed 1 week later, taking into account the results of the cycloplegic refraction. When there was a discrepancy between these two methods, a duochrome (red-green) test was performed, which was used as the final refraction.
All measurements were taken at the same time of day ? between 3 and 6 PM. Measurements were obtained sequentially using the Pentacam rotating Scheimpflug camera and Orbscan II (CF) scanning slit system by two different investigators who were not aware of the results of the other method.
Pentacam uses a rotating Scheimpflug camera and a monochromatic slit-light source (blue LED at 475 nm) that rotate together around the optical axes of the eye to calculate a three-dimensional model of the anterior segment, including data from anterior and posterior corneal topography, corneal thickness, measurements of anterior chamber depth, lens opacity, and lens thickness. Within 2 seconds, the system rotates 180? and acquires 25 or 50 images (depending on the user settings) that contain 500 measurement points on the front and back corneal surfaces to draw a true elevation map. For this study, 25 images per scan were acquired. The software is almost fully automated.
After patients' data are entered, the program changes to imaging mode. Patients sit in front of the device, place their chin on a chin rest, and are asked to keep both eyes open while fixating on a blinking target in the center of the camera. Examiners see a real-time image of the eye on the computer screen. The image must be focused and centered manually by moving the Pentacam. Markings on the screen indicate the direction that operators should move the camera's joystick. As soon as the image is perfectly aligned, patients are asked not to move and keep their eye open, and the scan is started.67 The center of the cornea is measured in each of the single images of a scan; therefore, it should be possible to calculate precise values for the central corneal thickness.
Orbscan is a scanning slit topography system that uses a horizontally moving slit beam to produce multiple slit images of the anterior segment and provides data for (anterior and posterior) corneal topography, corneal thickness, and anterior chamber depth. Patients sit in front of the device, place their chin on a chin rest, and are asked to keep both eyes open. A real-time image of the eye is visible on the screen and is centered and aligned manually. Operators use the reflections of the slit-lamp light as orientation for the correct alignment.
With this device, 40 slits are projected sequentially on the eye, 20 from the left and 20 from the right side, producing multiple slit-lamp images of the eye to calculate a mathematical model of the cornea and the anterior lens surface with computer software. Any artifacts from eye movement during imaging result in an error message, and the scan has to be repeated. The corneal thickness value of the center of the cornea was obtained using the acoustic equivalent correction factor (0.92) to achieve equivalence with the ultrasonic evaluation, as recommended by the manufacturer.
All data were entered into a Microsoft Excel spreadsheet (Microsoft Corp, Redmond, Wash). Paired differences were calculated and presented as mean and maximum values, with standard deviations. The level of statistical significance and the correlation between the two methods were calculated using the Student paired t test and Pearson correlation coefficients. Bland-Altman plots were used to identify any potential dependency between differences and means of two measurements.8,9 P values <.05 were statistically significant.
Figure 1. Scatterplot showing correlation of central corneal thickness measurements in 91 eyes using Orbscan Il (CF) and Pentacam.
Figure 2. Bland-Altman plot showing correlation of central corneal thickness measurements in 91 eyes using Orbscan (CF) and Pentacam.
Central corneal thickness measurements with Orbscan II (CF) ranged from 438 to 610 µm (mean: 527.8±37.12 µm). Central corneal thickness measurements with Pentacam ranged from 472 to 634 µm (mean: 541.65±32.22 µm). The two devices showed a good correlation in central corneal thickness measurement (Bp 2=0.84) (Figs 1 and 2). The difference between Pentacam and Orbscan values ranged from -43 to +52 µm (mean: -13.98±14.79 µm) (P<.0001). No correlation was found between differences in central corneal thickness and refractive error (Fig 3).
Orbscan and ultrasonic pachymetry yield similar measurements of corneal thickness when the acoustic equivalent correction factor (0.92) is used. Without the equivalent correction factor, Orbscan measurements are greater than those of ultrasonic pachymetry.1013 This discrepancy may have different explanations. One explanation is Orbscan measures the thickness between the air-tear film interface and the posterior corneal surface, whereas in ultrasonic pachymetry, the posterior corneal reflection point might be located between Descemets membrane and the anterior chamber.2411 Another explanation could be that repeated contact by the pachymetry probe may lead to swelling of the cornea, resulting in larger corneal thickness values, even though it has been shown repeated contact with a pachymetry probe slightly reduced corneal thickness.14
In addition, measurements obtained with pachymetry depend on the exact axial placement of the probe relative to the center of the cornea. Thus, the reproducibility of measurements in ultrasonic pachymetry depends on examiner expertise. Moreover, this method requires contact, which may be uncomfortable for patients or may even lead to damage of the corneal epithelium with subsequent corneal infection.15 For this reason and because it has been demonstrated that there is good agreement between Orbscan (CF) and ultrasound, we decided to compare only the two methods that do not require corneal contact.
Figure 3. Scatterplot showing correlation between differences in central corneal thickness (CCT) measurements and refractive errors in 91 eyes.
Only two other studies have compared Orbscan and Pentacam. Buehl et al6 compared the central corneal thickness in 88 eyes of 44 healthy participants and found no statistically significant difference (P=. 81) between Pentacam and Orbscan (the report did not specify whether the Orbscan CF was used). They found no dependency between paired differences and mean values in central corneal thickness measurements. Seven of these participants were contact lens wearers who removed their lenses at least 12 hours before the study.
In another study using the Orbscan CF, Amano et al5 compared 74 eyes of 64 patients and found no statistically significant differences in the measurements between the two methods (P=. 569), with a significant linear correlation between the rotating Scheimpflug camera and scanning slit topography (r=0.930, P<.0001). They found scanning slit topography tended to underestimate corneal thickness relative to the rotating Scheimpflug camera for thickness less than or equal to 500 µm and to overestimate for thickness greater than 550 µm.
Our results showed Pentacam measurements were thicker than those obtained with Orbscan II (CF) and agree with those obtained by Amano et al5 (see Fig 1); the difference is not related to the measured corneal thickness (see Fig 2), even if the overestimation tends to decrease for corneal thickness measurements greater than 560 µm (see Fig 3). The reason for these differences is unclear, but the distinct methodologies in each device and use of a correction factor in Orbscan might induce this tendency. On the other hand, Pentacam is an optical method of corneal thickness measurement similar to that of Orbscan; it might also require correction of the raw data to match its data to those of ultrasonic pachymetry. To our knowledge, however, the manufacturer has not disclosed whether this kind of correction is performed in the machine. Another possibility is that we used the central corneal thickness as defined by the machines, and the two devices may differ in the way they define the center of the cornea.
To date, it is difficult to estimate which of the two systems is more accurate, that is, is closest to the actual value of corneal thickness. However, on the basis of our results, we can conclude the two devices cannot be used interchangeably as the differences among them can be as great as 50 µm.
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