Regularity of corneal topography after penetrating keratoplasty--comparison between non-mechanical (excimer laser 193 nm) and mechanical trepanation

Abstract

"Vertical tilt" of the graft caused by incongruent cut angles, "horizontal torsion" caused by a asymmetric suturing and decentration of the wound seem to be the most important reasons for high and/or irregular postoperative astigmatism and deterioration of visual outcome after penetrating keratoplasty. We studied the time course of qualitative and quantitative topographic criteria after nonmechanical and mechanical trephination in correlation with visual outcome. Fifty patients each (30 keratconus, 20 Fuchs' dystrophies) underwent nonmechanical trephination (excimer laser MEL60, Aesculap-Meditec, Heroldsberg) and mechanical motor-trephination in penetrating keratoplasty. All procedures (7.5 mm trephination diameter in Fuchs, 8.0 mm in keratoconus, double-running 10-0 nylon suture) were done by one surgeon. Pre-, intra- and postoperative treatment were identical. At the follow-up examinations, the keratometric astigmatism, qualitative and quantitative criteria of the automatic videokeratography, visual acuity and refraction were assessed. In addition, vector-corrected astigmatism (Jaffe-model) was evaluated in the postoperative time course, particularly after suture removal. After a mean follow-up of 1.1 +/- 0.8 years, keratometric net astigmatism was 4.0 and 4.2 diopters after nonmechanical and mechanical trephination. Corneal topography analysis showed a higher orthogonality of the bowtie shape and less asymmetry between opposite hemimeridians. Vector-corrected changes of corneal astigmatism were less after complete suture removal in nonmechanical trephination. After nonmechanical trephination, visual acuity was 2 decimal lines better. Nonmechanical trephination with the excimer laser 193 nm has the potential to improve the visual outcome by creating smooth and congruent cut edges parallel to the optical axis and the application of "orientation teeth". Since corneal net astigmatism does not differ significantly, a higher regularity of corneal topography after excimer laser trephination is supposed to be a main reason for this finding.