[Wavelet representation of corneal topography data after nonmechanical penetrating keratoplasty--a clinical study].

BACKGROUND

Corneal surface irregularities may limit the visual outcome after penetrating keratoplasty (PK). Corneal topographers mainly render empirically derived and system-specific statistical indices for characterization of superficial inhomogeneities which may lack clinical evidence and make inter-system comparisons difficult. The purpose of this study was to detect and quantify focal surface irregularities of the cornea after nonmechanical PK by 2-dimensional wavelet decomposition based on corneal topography data.

PATIENTS AND

METHODS

Our study included 15 patients with keratoconus and 10 patients with Fuchs' dystrophy with all-sutures-out after penetrating keratoplasty. For trephination we used the excimer laser MEL60 (Aesculap-Meditec, Germany) (7.5/7.6 mm diameter in dystrophies, 8.0/8.1 mm in keratoconus, double-running 10-0 nylon suture). After suture removal a complete ophthalmological examination including OrbScan topography analysis (Orbtec, USA) was performed. The refraction data were extracted via "data recorder" and decomposed using 2-dimensional wavelet analysis methods (Daubechies-4-wavelets on five scales of resolution). Corneal irregularities were quantified (scale 1 = fine details to scale 5 = coarse details).

RESULTS

All detail coefficients (horizontal, vertical and diagonal) correlated statistically significant with the "Irregular Astigmatism" provided by the OrbScan-system (p < 0.05). In scale 3 and 4 a relative maximum of the wavelet detail coefficients occurred, whereas the coefficients at scale 2 and 5 were significantly smaller. The horizontal and vertical detail coefficients correlated significantly inversely with the best-corrected visual acuity (p < 0.04). All detail coefficients were significantly lower in the patient group with keratoconus compared to Fuchs' dystrophy.

CONCLUSIONS

Wavelet decomposition of corneal topography refraction data allows an analytical isolation and quantification of focal corneal superficial irregularities. This algorithm is independent of the currently used topography system and allows a smoothing of the raw data set adapted to scale of resolution and data compression.