VERTICAL TILT AFTER PENETRATING KERATOPLA STY - COMPARISON BETWEEN NONMECHANICAL TREPHINATION WITH THE EXCIMER-LASER AND MOTOR TREPHINATION

Background Besides decentration of the graft/host trephination and ''h orizontal torsion'' ''vertical tilt'' is an important factor for reduc ed visual outcome after penetrating keratoplasty (PK). The purpose of this study was to evaluate the time course of vertical tilt in absolut e value and direction and to correlate it with functional results afte r PK. Patients and methods Fifty patients each (20 primary dystrophies , 30 keratoconus) underwent nonmechanical trephination (NMT) (excimer laser MEL60, Aesculap-Meditec, Heroldsberg, Germany) or mechanical mot or trephination (MT) (Geuder, Heidelberg, Germany) in penetrating kera toplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in keratoconus , 8 orientation teeth in NMT, double-running 10-0 nylon suture) were p erformed by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial suture removal and after complete suture remova l, corneal topography analysis (TMS-1, Tomey, Tennenlohe, Germany) was performed. After a Gram-Schmidt-orthogonalization corneal topography height data of 25 noncentric rings in 256 hemimeridians were decompose d into Zernike components of radial order n = 16 in the sense of minim izing the root mean square error. The tilt of the surface relative to the videokeratoscope axis was calculated from the Zernike components Z (1)(1) and Z(1)(-1). The meridional power at the cardinal meridians wa s derived from all parabolic Zernike terms. Tilt and the difference be tween both meridians of the Zernike representation (ZA) were correlate d with the results of Zeiss keratometry (KA), Simulated Keratometry (S imK) of the TMS-1, subjective refraction (RZ) and best-corrected visua l acuity. Results After NMT, vertical tilt of the graft was 3 degrees without significant change over time. Following MT, an equivalent time course could be observed before partial suture removal. However, afte r complete suture removal, a significant increase of the tilt was meas ured to 5 degrees (p = 0,02). No significant difference could be detec ted comparing keratoconus and Fuchs' dystrophy both in NMT and MT. The direction of the vertical tilt component piled up to the hemimeridian defined by the knot of the first running suture. At all postoperative follow-up examinations, the ZA of the Zernike decomposition showed a good correlation to the RZ, whereas the KA and the SimK did not. At th e end of the follow-up, best-corrected visual acuity after NMT was 2 d ecimal lines better than after MT. Conclusions The Zernike decompositi on of topographic height data is a suitable tool for extraction and qu antifying vertical tilt of the graft following penetrating keratoplast y. In contrast to conventional keratometry with its 4-point measuremen t, a decomposition of topographic height data into orthogonal polynomi als enables a detection of both cardinal meridians even in corneas wit h a high degree of local irregularities.