[Interpretation of corneal topography after penetrating keratoplasty with wave-front parameters--comparison between non-mechanical trepanation with excimer laser and motor trepanation]

Abstract

Background: Besides irregular astigmatism characterized by the asymmetric components of the corneal surface, the aberration of the cornea from an ideal sphere degrades the optical performance of the "optical system eye". Best-corrected visual acuity may be markedly decreased with an increasing aperture diameter. The purpose of this study was to evaluate the time course of the symmetrical part of the aberration from an ideal sphere and to correlate it with functional results after penetrating keratoplasty (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) 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 symmetrical part of the deviation from an ideal spherical surface was calculated from the Zernike components Z4(0), Z6(0), ..., Z16(0). From the Zernike components, the longitudinal focus distribution and its standard deviation (SDF) was determined. SDF was correlated with the surface asymmetry index (SAI), the surface regularity index (SRI), the potential visual acuity (PVA) of the TMS-1 and the spectacle-corrected visual acuity.

Results: In the time course after PK, SDF decreased from the 6 weeks follow-up examination to the end of the follow-up 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 central 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 suture 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 after the 6 months follow-up the SDF within the 3 mm central area correlated inversely with the best-corrected visual acuity, more with NMT than with MT (p = 0.005 and p = 0.04 after suture removal). Best-corrected visual acuity was approximately 2 decimal 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 decomposed due to the orthogonality condition. Following NMT, SDF was markedly lower after suture removal.