INTERPRETATION OF CORNEAL TOPOGRAPHY WITH WAVE-FRONT PARAMETERS AFTERPENETRATING KERATOPLASTY - COMPARISON BETWEEN NONMECHANICAL TREPHINATION WITH THE EXCIMER-LASER AND MOTOR TREPHINATION

Background. Besides irregular astigmatism characterized by the asymmet ric components of the corneal surface, the aberration of the cornea fr om an ideal sphere degrades the optical performance of the ''optical s ystem eye''. Best-corrected visual acuity may be markedly decreased wi th an increasing aperture diameter. The purpose of this study was to e valuate the time course of the symmetrical part of the aberration from an ideal sphere and to correlate it with functional results after pen etrating keratoplasty (PK). Patients and methods. Fifty patients each (20 primary dystrophies, 30 keratoconus) underwent nonmechanical treph ination (NMT) (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Ger many) or mechanical motor trephination (MT) in penetrating keratoplast y. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus, 8 ori entation teeth in NMT, double-running 10-0 nylon suture) were performe d by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture removal, corn eal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) was perfor med. After a Gram-Schmidt-orthogonalization, corneal topography height data of 25 noncentric rings in 256 hemimeridians were decomposed into Zernike components of radial order n = 16 in the sense of minimizing the root mean square error. The symmetrical part of the deviation from an ideal spherical surface was calculated from the Zernike components Z(4)(0), Z(6)(0),..., Z(16)(0). From the Zernike components, the long itudinal focus distribution and its standard deviation (SDF) was deter mined. SDF was correlated with the surface asymmetry index (SAI), the surface regularity index (SRI), the potential visual acuity (PVA) of t he TMS-1 and the spectacle-corrected visual acuity. Results. In the ti me course after PK, SDF decreased from the 6 weeks follow-up examinati on to the end of the followup from 1.27 mm to 0.77 mm in NMT (p = 0.01 ) and from 1.29 mm to 1.20 mm following MT (p = 0.24) within the centr al corneal region of 3 mm in diameter. The SAI did not depend on SDF, whereas the SRI correlated significantly inversely with the SDF within the 3 mm zone immediately before (p = 0.01 and p = 0.02) and after su ture removal (p = 0.01 each) after NMT. After MT, only a mild inverse correlation was observed before (p = 0.05) and after suture removal (p = 0.04). In the time course af ter the 6 months follow-up the SDF wit hin the 3 mm central area correlated inversly with the best-corrected visual acuity, more with NMT than with MT (p = 0.005 and p = 0.04 afte r suture removal). Best-corrected visual acuity was approximately 2 de cimal lines better following NMT. Conclusions Zernike decomposition of corneal topography height data allows a separation and quantification of aberration of corneal graft surface from an ideal sphere. Although corneal surfaces with a high degree of local irregularities can be de composed due to the orthogonality condition. Following NMT, SDF was ma rkedly lower after suture removal.