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achieved. The EV is the vector difference between the intended refraction correction
(IRC) and the SIRC. NEV is equal to EV, in magnitude, but it is rotated to allow easy
visualization in the graphs. AS corresponds to the angular difference between the post-
and preoperative manifest cylinder axes. ER is the proportion of the intended correction
that was unsuccessfully treated. CR is the ratio of the correction magnitude achieved to the
required correction. EM corresponds to the arithmetical difference in the magnitudes
between SIRC and IRC, zero being the ideal result. EA measures whether the treatment
was applied at the correct axis and TEV represents the magnitude of EM and the angle of
EA containing both aspects of treatment error.
4.3.7.4 Study IV. Correction of irregular astigmatism
In Study IV, 11 eyes of 9 patients referred to the Helsinki Eye Hospital with diagnose of
map-dot-fingerprint dystrophy and irregular astigmatism that did not respond to medical
therapy were included. Our goal in this study was to call attention to the presence of
irregular astigmatism, which may depend exclusively upon the corneal epithelium. Pre-
and postoperative examination included ophthalmological examination, UCVA, BSCVA,
manifest refraction and VK. Irregular astigmatism was assessed by VK according to the
classification of Bogan et al. (Bogan et al. 1990) Irregular astigmatism was classified into
two groups based on the possibility to define steep or flattened areas in the central and
peripheral zones of the VK. The central zone was defined as an area of 3 mm. Group 1 had
a VK with easily identifiable steep or flat areas and was divided into five basic types:
Central elevation, central flat area, eccentric elevation, eccentric flat area and mixed.
Group 2 had irregular astigmatism without pattern.
4.3.7.5 Study V. PRK enhancement
Seven eyes of seven patients who had had previous LASIK for myopia were retreated at
least 2 years later by PRK. Inclusion criteria for PRK were flap-related complications
during initial LASIK or strong adherences of the flap that prevented flap-lift retreatment.
All eyes received transepithelial PTK/PRK using the Visx S4 excimer laser (Visx Co,
Sunnyvale CA, USA). PRK treatment was limited to the flap thickness in order to preserve
the residual stroma. On follow-up visits complete eye examination was performed. The
postoperative corneal haze was graded subjectively during slit-lamp examination using the
clinical grading scale (0+ to 4+) by Fantes (Fantes et al. 1990).
4.3.8 Statistical methods
In Studies I and III statistical analysis of paired outcomes was performed with the Mann-
Whitney test for nonparametric comparison and one-way analysis of variance for repeated
measures with the Bonferroni multiple comparison adjustment. The Kruskal-Wallis test
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