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J.A. Izatt, B. Applegate, B. Dodge, R. Nappi, C. Toth; Novel Non–Contact Optical Pachymeter . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2762.
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Purpose: Accurate in–vivo measurement of human corneal thickness (pachymetry) is an important diagnostic procedure in ophthalmology. Pachymetry is an important precursor to refractive surgery procedures such as LASIK, and the recently published Ocular Hypertension Treatment Study (OHTS) found that corneal thickness is an important risk factor for the development of glaucoma. Ultrasound pachymeters are the current clinical standard, however they have significant drawbacks including the difficulty of precise visualization and placement of the measurement site, the need for topical anesthetic, and the potential for corneal distortion during measurement. The purpose of this work was to demonstrate operation and initial evaluation of a novel optical pachymeter which overcomes many of these drawbacks. Methods: We have developed a novel optical pachymeter based on common–path low–coherence spectral interferometry which is of substantially simpler design and use than optical pachymeters based on scanning interferometry (such as is used in optical coherence tomography). The spectral interferometer has no moving parts and does not depend upon precise positioning of the patient’s eye with respect to the pachymeter probe, thus hand–held operation is readily achieved. The pachymeter has 1cm working distance from the probe tip to the cornea, a spot size of 18 microns on the cornea, and depth sampling resolution of 3.6 micrometers over a 3.25 mm scan depth range. Complete A–scans are acquired over the scan depth at 20Hz and displayed in real time on a computer display. Software was written to automatically detect and report corneal thickness strong reflections from the anterior and posterior corneal surfaces. Initial corneal depth reproducibility measurements were estimated from the standard deviation of 12 repeated central corneal thickness measurements on each of 2 volunteers. Results: The pachymeter was found to allow for rapid, non–contact acquisition of high quality central corneal A–scans allowing for delineation of epithelial and corneal thickness. The mean and standard deviation of corneal thickness measured over a period of a few minutes on a volunteers with normal vision was 561.7 +/– 1.2 microns. Conclusions: The novel non–contact optical pachymeter based on a common–path spectral interferometer design features an axial reproducibility typical of optical pachymetry and an ease of use typical of ultrasound pachymetry. This instrument may find use in glaucoma screening as well as pre– and intra–surgical applications in corneal refractive surgery.
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