Computer-Assisted Corneal Topography: Accuracy and Reproducibility of the Topographic Modeling System

BACKGROUND: Assessment of the efficacy and stability of novel refractive surgery techniques and comparative studies require accurate and reproducible corneal mapping technology, ideally to ±0.25 D. A vertically mounted Topographic Modeling System (TMS) system was evaluated. MATERIALS AND METHODS: Six calibrated PMMA spheres (30.00, 35.00, 43.00, 50.00, 55.00, and 60.00 D) were used. Two examiners performed three measurements on each test sphere and compared appropriate data with a previously tested and calibrated digital auto-keratometer (SK-I, precision <0.25 D). The accuracy and reproducibility of all 25 rings of the JMS were studied as a function of focus (250 to 500 µm), centering (100 to 300 µm), and integrity of the inner and outer reflected placido images to mimic tear breaks at the corneal apex and peripheral vignetting and shadows produced by eyelids and eyelashes. RESULTS: The accuracy of the 25 rings decreased from the center to the periphery. The deviation scores (mean difference of three readings between measured and calibrated surface powers) of rings 3 through 25 were 100% within ±0.25 D only for the 43.00-diopter sphere. They were 56%, 68%, 43%, 13%, and 17% within ±0.25 D respectively for the 30.00-, 35.00-, 50.00-, 55.00-, and 60.00 -diopter spheres. For 500 µm of defocusing, we observed a mean deviation of 0.35 D for the flattest sphere to 2.30 D for the 60.00-diopter sphere. For the same amount of defocusing, the error produced by the TMS increased progressively with the sphere dioptric power. Masking the first four and six rings, one by one, showed topographic maps to be valid only when all four central placido images were fully read by the computer. CONCLUSIONS: The TMS clinical performance could be improved by 1) increasing focus depth of field, 2) blocking data processing when central rings are incomplete or missing, 3) increasing the accuracy over the 30.00- to 60.00-diopter range, and 4) providing the user with calibration spheres. (Refract Corneal Surg 1993;9:347-357.) RÉSUMÉ INTRODUCTION: L'évaluation de l'efficacité et de la stabilité de nouvelles techniques de chirurgie refractive comme l'élaboration d'études comparatives nécessitent l'utilisation de système d'analyse topographique corneen dont la précision et la reproductibilité doivent être connues et idéalement de Tordre de ±0,25 D. Les auteurs évaluent le domaine de validité du système vidéokératoscopique TMS-I. MÉTHODES: Six sphères calibrées de (30, 35, 43, 50, 55, 60 D) en PMMA sont utilisées. Deux expérimentateurs ont effectués chacun 3 mesures sur chaque sphère et comparent les résultats avec ceux obtenus dans les mêmes conditions à l'aide d'un réfractomèfre automatique (SK-I) dont le domaine de validité avait été préalablement évalué (précision connue <0,25 D). La précision des 25 anneaux du TMS est étudiée et le retentissement d'une erreur de mise au point (250 et 500 µm) et de centrage (100 à 300 µm) analysé. L'importance relative d'une erreur de lecture de l'un où de plusieurs anneaux au centre où en périphérie est appréciée.