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-1, precision < 0.25 D). The accuracy and reproducibility of all 25 rings of the TMS were studied as a function of focus (250 to 500 microns), centering (100 to 300 microns), 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 microns 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.