| Abstract: | Background : This investigation compares the reliability and validity of two computer-assisted corneal topographers by analysing eccentricity and central curvature measurements, which are the main indices for corneal shape used to estimate optimum fit in orthokeratology. Method : Repeated measurements of human corneas and a calibration ball have been made with the TMS-2 and EyeSys corneal topographers. Results : Significant differences between instruments were found for central curvature (TMS-2 averaging 0.5 D steeper) and for eccentricity (EyeSys averaging 0.15 lower eccentricity). Calibration ball measurements (TMS-2 found to be an average 0.27 D steeper) provide at least a partial explanation for the central curvature differences. Conclusions : Results obtained may be partly explained by variations in keratoscopic design which mean that the TMS-2 measures curvature closer to the visual axis where curvature is steeper. The significant differences found for eccentricity and central curvature are only sometimes more or less nullified when VMC design lenses are calculated. Recourse to careful examination of alignment, movement and centration of diagnostic lenses using fluorescein patterns is indicated before prescribing or modifying fit, especially as clinically significant inter-instrument and intra-instrument variations in readings can occur. |
