Besides decentration of the graft/host
trephination and "horizontal torsion" "vertical tilt" is an important factor for reduced visual outcome after
penetrating keratoplasty (PK). The purpose of this study was to evaluate the time course of vertical tilt in absolute value and direction and to correlate it with functional results after 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 motor
trephination (MT) (Geuder, Heidelberg, Germany) in
penetrating keratoplasty. All procedures (7.5 mm in dystrophies, 8.0 mm in
keratoconus, 8 orientation teeth in NMT, double-running 10-0
nylon suture) were performed by one surgeon (GOHN). At a postoperative gate of 6 weeks, 6 months, before partial
suture removal and after complete
suture removal, 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 decomposed into Zernike components of radial order n = 16 in the sense of minimizing the root mean square error. The tilt of the surface relative to the videokeratoscope axis was calculated from the Zernike components Z1(1) and Z1(-1). The meridional power at the cardinal
meridians was derived from all parabolic Zernike terms. Tilt and the difference between both
meridians of the Zernike representation (ZA) were correlated with the results of Zeiss keratometry (KA). Simulated Keratometry (SimK) of the
TMS-1, subjective refraction (RZ) and best-corrected visual 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, after complete
suture removal, a significant increase of the tilt was measured to 5 degrees (p = 0.02). No significant difference could be detected 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 the end of the follow-up, best-corrected visual acuity after NMT was 2 decimal lines better than after MT.
CONCLUSIONS: The Zernike decomposition of topographic height data is a suitable tool for extraction and quantifying vertical tilt of the graft following
penetrating keratoplasty. In contrast to conventional keratometry with its 4-point measurement, a decomposition of topographic height data into orthogonal polynomials enables a detection of both cardinal
meridians even in corneas with a high degree of local irregularities.