| Objective To observe and calculate the unevenness and the non-orthogonal state of distribution of corneal thickness and the relationship between them using Pentacam, and using vector analysis to investigate the influence of unevenness and the non-orthogonal state on correction of myopic astigmatism by laser subepithelial keratomileusis (LASEK).Methods 230 myopic astigmatism eyes of 64 male(48.5%) and 68 female(51.5%) patients treated with LASEK, were divided into two groups, low astigmatism group (-0.25DC--0.75DC) 114 eyes and midrange-high astigmatism group (-1.00DC～-4.50DC) 116 eyes. To be included in the study, patients had to be suitable for LASEK and free of other pathologic ocular conditions and relevant systemic disease. All excimer laser surgeries were performed with ALLEGREETTO WAVE EYE-Q (Wavelight, LTD. Germany). The diameter of optical zone of ablation was 6.5mm. The target postoperative refraction of myopia and astigmatism was zero diopter in all eyes. Astigmatism is mainly derived from cornea, which is caused by the difference of curvature of the cornea on different meridians, and there may have difference between the curvature of the anterior and the posterior surface on the same meridian. The astigmatic cornea is a concave toric lens at which center is the thinnest point of the cornea, and the astigmatism of the posterior corneal surface could result in compensation of the astigmatism of the anterior corneal surface. If the values of difference of the cornea curvature between the anterior surface and posterior surface of the same meridian were smaller among the different meridians, the astigmatism of posterior surface compensate for the astigmatism of anterior surface were weaker; on the contrary, it were stronger; and the larger the value of the difference of the cornea curvature between the anterior surface and posterior surface of some meridian was, the faster the thickness of the cornea from the thinnest point to periphery of this meridian increased; on the contrary, it were slower. This made the distribution of the corneal thickness various among the different meridians, which resulted in uneven distribution of the corneal thickness. The effect of the astigmatism of posterior surface compensate for the astigmatism of anterior surface could be estimated by the result of the unevenness measured in a quantitative way. In addition, the two principal meridians of astigmatism of anterior corneal surface could be non-orthogonal, so the two principal meridians of the astigmatism of posterior surface compensate for that of anterior surface could also be non-orthogonal. However, the correction of astigmatism by Excimer laser refractive surgery conventionally performed in orthogonal way. So the unevenness and non-orthogonal state of distribution of corneal thickness might have an impact on the correction of astigmatism. Together with the change in biomechanics of cornea postoperatively, thus may further influence the correction of astigmatism. Pentacam is a three-dimensional anterior segment tomography based on a rotating Scheimpflug camera imaging, which can detect the elevation of anterior and posterior surfaces of the cornea, and the corneal thickness and distribution of corneal thickness could be calculated and analyzed according to the elevation. The patient was instructed to watch the blue light with the black fixation target in the middle of it when detected by Pentacam in automatic mode. Only scans that had an examination quality specification graded as "OK" were saved. In the diagram of keratoconus evaluation program of the Pentacam, with thinnest point of the cornea as the center of the circle, the concentric circle with radius of 1.50mm, i.e. diameter of 3.0mm and radius of 3.25mm, i.e. diameter of 6.5mm of optical district for ablation was made. In the Cartesian coordinates of which the corneal apex was the origin, We using the trigonometric function to positioning the coordinates (x, y) of the intersection with which the two concentric circles mentioned above and the maximum and minimum progression-index meridian met, and measured and calculated the difference of the two intersections at the same concentric circle in the pachymetry map separately. The results of the difference were represented by D3.0 and D6.5 and as the index of distribution of unevenness of the corneal thickness; and the absolute value of the angle between the maximum and minimum progression-index orientation(less than 180 degrees) minus 90(M90) as the index of non-orthogonal states. Noncycloplegic automated refraction was performed with an autorefractmeter (RM8800; Topcon; Japan) before and 3 month after surgery. Three readings were obtained in each eye with the vertex distance was 12mm, and the average of these values was used for analysis. The evaluation and calculation of astigmatism was by using standard vector analysis which was recommended by American National Standards Institute and the power vector analysis of Thibos’. The terms in involved in this study was including intended refractive correction (IRC), surgically induced refractive correction (SIRC), error vector (EV), error of angle (EA), error of magnitude (EM), error ratio (ER), correction ratio(CR) and blur strength(B). Cylinder data are presented in negative cylinder form throughout this study. After the data were compiled and calculated by vector analysis, they were statistically analyzed with SPSS software (version 17.0; SPSS, Inc, Chicago, IL). The mean value of normality data compared and analyzed with two independent samples t test, and abnormality data with Wilcoxon signed-rank test. Correlations analysis was examined using Spearman rank correlation tests. A significance level of a=0.05 was used for all tests.Results After LASEK, no serious complications appeared during the period of follow-up, few of patients appeared dry eyes or visual fatigue. Spherical equivalent had no significant difference between low (-6.46±2.37)D and midrange-high (-6.87±1.89) D astigmatism group preoperatively (P>0.05).The M90 was (22.14±20.87) °, D6.5 was (58.66±21.32) μm and D3.0 was (16.11±4.28) μm of 230 eyes preoperatively. The D6.5 of low astigmatism group(55.62±20.81)μm was significant lower than that of midrange-high astigmatism group(61.65±21.48)μm(P< 0.05). Of 230 eyes, the M90 was positively correlated with D6.5 (r=0.37, P<0.001), and D6.5 was positively correlated with D3.0(r=0.56, P< 0.001).3 months postoperatively, the absolute error vector (|EV|) of low astigmatism group(0.46±0.34) was significant lower than that of midrange-high astigmatism group(0.53±0.29) (P< 0.01). The error of magnitude (EM) of low astigmatism group(-0.10±0.31) was significant lower that of midrange-high astigmatism group(0.08±0.41)(P< 0.001). The absolute error of angle (|EA|) of low astigmatism group(26.10±27.24) was significant higher than that of midrange-high astigmatism group(9.99±17.32) (P< 0.001). The correction ratio(CR) of low astigmatism group(1.45±1.21) was significant higher than that of midrange-high astigmatism group(0.94±0.33) (P<0.01). The error ratio(ER) of low astigmatism group(1.34±1.40) was significant higher than that of midrange-high astigmatism group(0.42±0.27) (P< 0.001). In low astigmatism group, M90 was positively correlated with|EV|(r=0.30, P<0.001). In midrange-high astigmatism group, M90 was positively correlated with ER(r=0.31, P<0.001), and D6.5 was positively correlated with|EV| and B respectively(r=0.34,0.33, P<0.001).Conclusion There had relationship between unevenness and non-orthogonal state of distribution of corneal thickness, which could have influence on correction of astigmatism by LASEK. More attention should be kept on the correction of midrange-high by excimer surgery. |
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