Three-dimensional representation and qualitative comparisons of the amount of tissue ablation to treat mixed and compound astigmatism.

PURPOSE To compare the shape and volume of the lenticules of corneal tissue ablated for the correction of spherical, cylindrical, and spherocylindrical refractive errors using Boolean operations of theoretical 3-dimensional (3-D) surfaces. SETTING Department of Ophthalmology, Rothschild Foundation, Paris, France. METHODS Digital modeling software was used to perform graphic representations of ablated lenticules on 3-D virtual surfaces. Various Boolean operations were performed between different preoperative and postoperative surfaces, and the additional and subtractive properties of ablated theoretical lenticules were analyzed to determine profiles of ablated lenticules for mixed and compound myopic and hyperopic astigmatism. RESULTS Negative-cylindrical treatment, used to treat simple myopic astigmatism, was equivalent to the combination of a positive-cylindrical and a negative-spherical treatment of the same magnitude. Combining a pure negative-cylindrical and a positive-spherical treatment in a sequential strategy when treating compound astigmatism resulted in redundant ablation (plano lenticule), leading to an unnecessary increase in the amount of tissue ablation. CONCLUSIONS Negative-cylindrical treatments result in greater tissue ablation than corresponding positive-cylindrical treatments. For any given compound astigmatic error, the strategy using the greater magnitude of positive cylinder incurs the minimal amount of tissue ablation.