CONTINUOUS NONCONTACT CORNEAL PACHYMETRY WITH A HIGH-SPEED REFLECTOMETER

PURPOSE: We developed an instrument that permits non-contact, continuo us, high speed and high precision monitoring of corneal thickness and tested the stability and reproducibility of measurements made over ext ended time periods and under various conditions of low surface reflect ivity encountered during protracted exposure of the unmoistened cornea l surface to ambient air. METHODS: The optical pachymeter (basic compo nent of a broad-band, all-fiber Michelson interferometer) was used to monitor changes in the central corneal thickness of enucleated porcine eyes. Measurements were performed on three groups of eight eyes, each with different surface characteristics: intact epithelium, mechanical ly abraded epithelium, and 90 mu m excimer laser keratectomy. Corneal thickness was monitored continuously with values recorded every 2 to 3 minutes for periods up to 1 hour in the absence of surface rinsing. R ESULTS: The thicknesses of all unmoistened corneas could be monitored with a precision of 1 mu m (ascertained using a calibration glass plat e and a living human corneal over the entire observation period. Under ambient air conditions, deturgescence occurred in each case, and rang ed from 1 to 5 mu m/min. The rate of corneal thinning was fairly const ant during the first 15 minutes of monitoring, but was nonlinear there after. Corneas with an intact epithelium had the lowest thinning rate with only 10% of the original thickness lost during the course of 1 ho ur. Deturgescence increased to 25% in corneas that had mechanical remo val of the epithelium and to 28.5% in those that had an anterior excim er laser keratectomy, during a similar time-period. CONCLUSION: With t his new interferometric method, continuous, non-contact measurement of corneal thickness is possible to within a precision of lpm for period s up to 1 hour, even under the modified surface conditions after photo ablative keratectomy. This device may be useful for on-line monitoring of ablation depths during such procedures.