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CB Nau, JW McLaren, BH Lee, JC Erie, DO Hodge, WM Bourne; Corneal Thickness Measurement: Disagreement between Confocal Microscopy and Ultrasonic Pachometry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):145.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Confocal microscopy should provide an accurate means of measuring corneal thickness when images of the epithelium and endothelium are visible and the distance between consecutive frames is known. However, the estimate of distance must be corrected for non-uniform spacing between consecutive frames and for refraction at the corneal surface. In this study we measured corneal thickness by using through-focus scans from a confocal microscope, and compared them to thickness measured by ultrasonic pachometry and by a non-contact optical method. Methods:Corneal thickness was measured in one eye of 24 normal subjects by using confocal microscopy (Tandem Scanning, Reston, VA). The focal plane was scanned through the cornea at a constant rate, and the number of video frames between the epithelium and endothelium were determined by examination of the images. The relationship between frame number and distance was determined from similar scans of PMMA contact lenses with thicknesses between 400 and 650 µm in 50 µm steps. Distances were corrected for refraction at the surface of the cornea and contact lens. Corneal thicknesses were also measured by using two ultrasonic pachometers (DHG-1000, DGH Technologies, Inc., Frazer, PA and Sonogage, Sonogage Inc., Cleveland, OH), and by non-contact optical pachometry (Orbscan II, Orbtek, Inc., Salt Lake City, UT; software version 3.00D, acoustic factor 0.92). Results:Corneal thickness measured by confocal microscopy was 525 31µm (mean SD). This was less than thickness measured by both ultrasonic pachometers, 554 28µm by the DGH, and 555 28µm by the Sonogage (p<0.001). Thickness measured by the Orbscan pachometer, 540 35µm, was greater than that measured by confocal microscopy (p<0.001) but less than that measured by ultrasonography (p<0.001). Conclusion:Corneal thickness measured by confocal microscopy is less than thicknesses measured by ultrasonography and by the Orbscan pachometer, although it is consistent with earlier measurements by optical pachometry. Confocal microscopy provides an accurate means of measuring corneal thickness, because the endothelial and epithelial surfaces are visible.
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