Effects of error in radius of curvature on the corneal power measurement before and after laser refractive surgery for myopia

摘要

Purpose: To investigate the sources of error in corneal power measurement before and after corneal refractive surgery for myopia. Methods: The study comprised 28 eyes of six males and eight females with a mean age of 26 (range 18-39years). The radius of curvature of anterior and posterior corneal surface, Q-Values of anterior and posterior corneal surface and corneal central thickness were measured by rotating Scheimpflug imaging (Pentacam). The true net power F g, back vertex power F v, and keratometric power SimK, were calculated respectively at the apex and at a paracentral area on the 3mm ring. Results: For virgin eyes, the overestimation (0.53±0.11D) of the corneal power by using a keratometric index of 1.3375 was balanced by the underestimation (-0.21±0.09D) of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error with a mean value of 0.33±0.11D. With the Q-value changing from -0.09 to -0.41, the percentage balanced by the error in radius of curvature increased from 16% to 73%. However, for eyes after laser refractive surgery, the radius of curvature error lead to an overestimation (0.54±0.16D) of the corneal power and the keratometric index of 1.3375 again overestimated (1.59±0.26D) the corneal power, resulting in a large measurement error with a mean value of 2.12±0.40D. With the Q-value changing from 0.35 to 1.89, the percentage added by the error in radius of curvature increased from 14% to 32%. Conclusions: For virgin eyes, the overestimation of the corneal power by using a keratometric index of 1.3375 is balanced by the underestimation of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error. However, for eyes after laser refractive surgery, the flatter anterior corneal surface means that the use of a keratometric index of 1.3375 significantly overestimates the corneal power and the radius of curvature error now adds to this overestimation and results in a large measurement error.