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

Background: Corneal surface irregularities may limit the visual outcome aft er penetrating keratoplasty (PK). Corneal topographers mainly render empiri cally derived and system-specific statistical indices for characterization of superficial inhomogeneities which may lack clinical evidence and make in ter-system comparisons difficult. The purpose of this study was to detect a nd 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 1 0 patients with Fuchs' dystrophy with all-sutures-out after penetrating ker atoplasty. For trephination we used the excimer laser MEL60 (Aesculap-Medit ec, 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 ophtha lmological examination including OrbScan topography analysis (Orbtec, USA) was performed. The refraction data were extracted via,,data recorder" and d ecomposed using 2-dimensional wavelet analysis methods (Daubechies-4-wavele ts on five scales of resolution). Corneal irregularities were quantified (s cale 1 = fine details to scale 5 = coarse details). Results: All detail coefficients (horizontal, vertical and diagonal) correl ated statistically significant with the ,,Irregular Astigmatism" provided b y 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 a nd 5 were significantly smaller. The horizontal and vertical detail coeffic ients correlated significantly inversely with the best-corrected visual acu ity (p<0.04). All detail coefficients were significantly lower in the patie nt group with keratoconus compared to Fuchs' dystrophy. Conclusions: Wavelet decomposition of corneal topography refraction data al lows an analytical isolation and quantification of focal corneal superficia l irregularities. This algorithm is independent of the currently used topog raphy system and allows a smoothing of the raw data set adapted to scale of resolution and data compression.