Accuracy of IOL Power Calculation in Eyes After Refractive Surgery Using Ray Tracing and C-Constant for Prediction of IOL Position
Narrative Responses:
Purpose
To evaluate the accuracy of IOL power calculation in eyes with prior refractive surgery using the newly described C-constant for the prediction of IOL position.
Methods
A total of 38 consecutive eyes having lens surgery after previous cornea surgery for myopia were studied. The pre-laser refraction ranged from - 6 to - 14 D. All patients had laser biometry (Lenstar) of all intraocular distances and Scheimpflug photography (Pentacam) to measure the central anterior and posterior curvatures and conic coefficients of the cornea. Both 'SimK' values and values from the more central cornea were used in the IOL power calculation which was performed with the Olsen formula using ray tracing and the C-constant concept for the prediction of the IOL position.
Results
The average prediction error (observed refraction minus observed refraction) was found to be + 0.37 D +/- 0.77 D (standard deviation), range - 1.48 D to + 1.97 D and - 0.01 D +/- 0.85 D, range - 1.48 D to + 2.03 D using the 'SimK' and the central readings from the Scheimpflug scanning, respectively. The latter mean error was not significantly different from zero.
Conclusion
Ray tracing and the C-constant concept are promising tools for unbiased IOL power calculation in post-refractive surgery eyes.