The Unevenness and Non-orthogonal State of Distribution of Corneal Thickness and the Influence on Correction of Myopic Astigmatism by LASEK

Abstract

To observe and calculate the unevenness and the non-orthogonal state of distribution of corneal thickness and the relationshipbetween them using Pentacam and to investigate the influence of unevenness and the non-orthogonal state on correction of myopic astigmatism by laser subepithelial keratomileusis (LASEK). 230 eyes with myopic astigmatism treated with LASEK were divided into two groups: 114 eyes as the low astigmatism group (-0.25 to -0.75 DC) and 116 eyes as the midrange-high astigmatism group (-1.00 to -4.50 DC). With the help of the diagram of keratoconus evaluation program of the Pentacam, the D 3.0 and D 6.5 were calculated for 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 (M 90) for the index of non-orthogonal states. The correction of myopic astigmatism by LASEK was based on standard vector analysis and power vector analysis. The follow-up period was for 3months. The preoperative M 90 was 22.14°±20.87°, D 6.5 was 58.66±21.32μm, and D 3.0 was 16.11±4.28μm for the 230 eyes that were tested. The D 6.5 of low astigmatism group (55.62±20.81)μm was significantly lower than that of midrange-high astigmatism group (61.65±21.48)μm (P<0.05). Of the 230 eyes, the M 90 was positively correlated with D 6.5 (r=0.37, P<0.001), and D 6.5 was positively correlated with D 3.0 (r=0.56, P<0.001). 3months postoperatively, the absolute error vector (|EV|) of low astigmatism group (0.46±0.34) was significantly lower than that of midrange-high astigmatism group (0.53±0.29) (P<0.01). The error of magnitude of low astigmatism group (-0.10±0.31) was significantly lower than 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 significantly higher than that of midrange-high astigmatism group (9.99±17.32) (P<0.001). The correction ratio of low astigmatism group (1.45±1.21) was significantly 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 significantly higher than that of midrange-high astigmatism group (0.42±0.27) (P<0.001). In low astigmatism group, M 90 was positively correlated with |EV| (r=0.30, P<0.001). In midrange-high astigmatism group, M 90 was positively correlated with ER (r=0.31, P<0.001) and D 6.5 was positively correlated with |EV| and B, respectively (r=0.34, 0.33, P<0.001). The relationship between unevenness and non-orthogonal state of distribution of corneal thickness could influence the correction of astigmatism by LASEK. Therefore, more attention should be paid to the correction of midrange-high astigmatism group by excimer surgery.