Correlation between refractive and measured corneal power changes after myopic excimer laser photorefractive surgery.

PURPOSE

To determine the correlation between the refractive and measured corneal power changes after myopic photorefractive surgery.

SETTING

Department of Ophthalmology, National Taiwan University Hospital, Taipei, Taiwan.

METHODS

Eighty-six eyes that had myopic photorefractive surgery were analyzed. The data included preoperative and 1-year postoperative subjective refraction, standard automated keratometry, and computerized videokeratography. Statistical analysis was performed to determine the relationship between the changes in subjective refraction in the corneal plane (Delta SEQco) and in 4 corneal power measurements including the power measured by automated keratometry (Delta Auto K), topographic-simulated keratometric power (Delta Sim K), the power of the first photokeratoscopic ring on videokeratography (Delta Central K), and the average videokeratographic power on the pupil margin (Delta Pupil K).

RESULTS

The measured corneal power always underestimated the Delta SEQco, with Delta SEQco > Delta Central K > Delta Sim K > Delta Pupil K > Delta Auto K. All the changes in measured corneal power could predict the Delta SEQco with more than 90.00% (90.19% to 92.31%) reliability at 1 year as calculated by the regression formulas (P <.001). The underestimation of measured corneal power changes was correlated with the amount of myopic correction, especially the Auto K (all P <.001).

CONCLUSIONS

Direct corneal power measurements using automatic keratometry underestimated the actual corneal flattening after photorefractive surgery, which could be adjusted by a linear regression formula. Measuring the power of the first photokeratoscopic ring on videokeratography might provide a better estimation of actual corneal flattening after photorefractive surgery.