Purpose: To determine the exact theoretical asphericity after refractive surgery when the parabolic approximation of the Munnerlyn formula is utilized.
Method: From a least-square analysis and considering the conicoid model for anterior cornea, we calculated the final asphericity (p'-factor) from the preoperative asphericity and the radii before and after surgery.
Results: The final p'-factor is given by p'=p(R'3/R3); R' is the final radius of curvature; R is the preoperative radius; and p is the preoperative p'-factor.
Conclusions: This equation for postoperative asphericity may be useful in studying the influence on asphericity of different surgical variables. It allows comparisons of real asphericity with the asphericity that should theoretically result. It also explains the increased spherical aberration after refractive surgery and the deterioration of retinal image quality that some authors have found experimentally.