An indirect method to compare the reference centres for corneal measurements.

PURPOSE: Corneal measurements are commonly presented with respect to a specific reference centre, but the location of the reference centre on the corneal surface could vary from one diagnostic modality to another. This study aimed to develop a method for comparing reference centres used by corneal measurement systems. METHODS: An indirect method was developed to compare reference centres by making use of the pupil centre and its offset from the reference centre. Reference centres in a Scheimpflug imaging system, the Pentacam HR, and a Placido-ring corneal topography system, the ATLAS Corneal Topography System, were compared for the right and left eyes of 30 subjects. The subjects all had similar pupil sizes when measured by the two systems. Differences and correlations of the pupil centre offsets between the two systems were statistically tested and compared by Bland-Altman analyses. RESULTS: There were no significant differences in mean pupil offsets between the two systems for either the right or left eyes (p > 0.05). There were strong correlations of the pupil centre offsets between the two systems for each eye (right eye x-axis: r = 0.95, p < 0.0001; right eye y-axis: r = 0.98, p < 0.0001; left eye x-axis: r = 0.96, p < 0.0001, left eye y-axis: r = 0.93, p < 0.0001). Bland-Altman analyses revealed no significant differences in pupil centre offsets between the two systems. CONCLUSIONS: The Pentacam HR system and the ATLAS system have very similar reference centres. Thus it is possible to directly analyze data from the Pentacam HR and other instruments using the corneal vertex or the pupil centre as the reference centres due to the similarity in the reference centre settings between the two systems.