Methods
The theoretical volumes of the cornea, flap and ablated corneal lenticules for spherical and cylindrical myopic corrections were calculated by mathematical approximations based on a simplified geometric model and Munnerlyn-based ablations. These results were then compared for various optical zone diameters (Z), dioptric corrections (D), and the percentage of the volumes altered (PVA) with the percentage of tissue altered (PTA).
Results
The volumes of the LASIK flap and laser ablated tissue increase exponentially with their respective diameter zones. \r\nFor the same depth of ablation, the volume of tissue ablated can vary significantly, depending on the magnitude of the correction and the optical zone diameter, and the presence of a negative cylindrical ablation. As a result, the uni axial PTA calculation is not predictive of the actual percentage of three dimensional tissue volume altered (PVA).
Conclusion
Non-axial but zonal parameters such as the respective diameters of flap and laser ablation zone are the most important determinant of the volume of tissue excised during excimer laser surgery. New models estimating the volume of the flap and corneal tissue may be necessary to determine their influence on corneal stability and procedure’s outcome.