David Ruston

Evaluation of the Eyesys Model Ii Computerized Videokeratoscope. Part Ii: The Repeatability and Accuracy in Measuring Convex Aspheric Surfaces

Purpose. To evaluate the precision and repeatability of a Placido disc-based computerized videokeratoscope, using convex surfaces of varying eccentricities and apical radii designed to simulate the range of topographical variations of the human cornea, rather than the purely spherical surfaces used in most previous studies. Methods. Form Talysurf analysis was used to verify the exact form of 12 Perspex convex surfaces. The EyeSys model II videokeratoscope was used to measure the sagittal radii of curvature twice at known points on each surface. The raw data tables were analyzed to assess the repeatability and accuracy for both central and peripheral points on each surface. The relationship between these factors and the eccentricity was investigated. Results. For central radii the instrument showed high correlation (r=0.996) between actual and measured values. There was a small instrumental bias of +0.042 mm and the 95% limits of agreement were narrow (+0.121 to -0.037 mm), indicating clinically acceptable accuracy. The accuracy decreased slightly as the p-value decreased (greater peripheral flattening). For peripheral radii, the overall accuracy compared well to central radii, with an average bias of +0.022 mm and maximum error in 95% of cases 0.083 mm (bias +1.96 x SD). However, for surfaces where p=0.50, the bias was +0.049 and maximum error in 95% of cases 0.110 mm. Repeatability for the aspheric surfaces was shown to be high (SD ±0.01 mm in all quadrants). Conclusions. The accuracy of the EyeSys corneal analysis system (CAS) in measuring central and peripheral radius of curvature was shown to be dependent on the shape of the surface to be measured. For more rapidly flattening surfaces, a decrease in accuracy was found for both central and peripheral radius of curvature, which in clinical terms is thought acceptable.

Evaluation of the EyeSys Model II Computerized Videokeratoscope. Part I: Clinical Assessment

Purpose. To compare the agreement and repeatability of keratometry measurements with simulated keratometry measurements from the EyeSys (model II) corneal analysis system (CAS). Furthermore, to establish any relationship between the repeatability of peripheral corneal measurements and the spatial location at the cornea. Methods. Measurements using a Bausch & Lomb keratometer in 19 eyes (19 subjects) were compared with the simulated keratometry values using the EyeSys CAS. Repeatability of the keratometer (14 eyes, 14 subjects) and the EyeSys simulated keratometry values (10 eyes, 10 subjects) was assessed by comparing the difference of two measurements. Peripheral repeatability of the EyeSys CAS was compared in a similar manner in a sample of 10 eyes (10 subjects). Results. The 95% confidence limits (—0.529 to +0.149 D) showed a lack of agreement between both instruments. With respect to the repeatability, both the keratometer (SD=±0.103 D) and the EyeSys (SD=±0.072 D) were found to exhibit approximately similar degrees of repeatability. The repeatability of peripheral corneal measurements using the EyeSys CAS showed changes in repeatability dependent on the corneal meridian and the distance from the point of alignment. Measurements along the superior and nasal meridians showed poorest repeatability. Repeatability was also found to deteriorate away from the point of alignment. Conclusions. EyeSys simulated keratometry values were not interchangeable with the keratometer. This could be due to differences in the method of alignment or measurements from different areas on the corneal surface. Repeatability of both the Bausch & Lomb keratometer and the EyeSys CAS was found to be similar. The spatial dependency of peripheral corneal radius repeatability measurements could be attributed to interference of the ocular adnexa in those areas of measurement.