| Purpose: To compare the impact of different source and type ofmyopic astigmatism on the visual quality including visual acuity, refraction,index of astigmatism correction success (postoperative whole eyeastigmatism/preoperative whole eye astigmatism）, whole eye wavefrontaberration, anterior corneal wavefront aberration and contrast sensitivity,after correction by LASIK.Methods：(1)96eyes of35consecutive patients combined withmyopia and astigmatism were included in the retrospective study.Preoperative and postoperative magnitude and axis of whole eyeastigmatism, anterior corneal astigmatism and ocular residualastigmatism(ORA, which is the intraocular astigmatism) were compared tolearn the change of astigmatism component after LASIK.(2) Eyes weredivided into5groups according to the ratio of preoperative ORA to wholeeye astigmatism: Group1：Ratio≤1、Group2：1＜Ratio≤1.5、Group3：1.5＜Ratio≤2、Group4：2＜Ratio≤2.5、Group5：Ratio＞3. Comparison ofvisual quality within groups was accomplished to learn the the impact of astigmatism source on visual quality after LASIK.(3)18eyes were pickedfrom WTR astigmatism which match the spheral and cylinder of the eyes ofATR astigmatism. Comparison of visual quality between WTR astigmatismand ATR astigmatism was made to learn the impact of astigmatism type onvisual quality after LASIK.Results:1. Preoperative whole eye astigmatism of the corneal plane was-0.86±0.62D and the anterior corneal astigmatism was-1.35±0.89D. The meanof ORA was-0.55±1.98D and the absolute value of it was1.56±1.33D,among which were57eyes exceeding the value of1.00D, accounting for59.3%of the whole eyes, which indicating that there is a large proportionof ORA in myopic astigmatism population and that the ORA compensatefor the anterior corneal astigmatism. The whole eye astigmatism of thecorneal plane and anterior corneal astigmatism decreased to-0.27±0.24Dand-0.94±0.50D respectively after operation whereas the ORA didn’tchange too much(-0.41±1.10D postoperatively).2.The whole eye astigmatism was primarily WTR(78.1%) but changedto ATR postoperatively(47.8%). Percentage of OBL astigmatism increasedto25.4%postoperatively. Anterior corneal astigmatism was primarilyWTR(82.3%) and remained the same after operation(68.8%). ORA wasprimarily ATR(77.1%) and didn’t change orientation after surgery (74%).3. In the study of astigmatism source, UCVA of each group increased to about1.2postoperatively which exceeded the preoperative mean valueof BUVA of1.0, but there was no significant difference within5groups.There were no difference of spheral and cylinder within groups except thatthe sphere of group5was bigger than group3. The IOS(index of success)from group1to group5were0.30±0.24，0.39±0.29，0.41±0.48，0.36±0.36，0.40±0.48respectively and there was no significant difference.4. In the study of astigmatism source, preoperative whole eye totalaberration of the five groups were dominated by lower order aberrations.Total higher order aberration only account for a smaller proportion whichwas given priority to with coma and trefoil. Total higher order aberrationssignificantly increased in each group after LASIK, which were1.56、1.69、1.43、2.04、1.44times of preoperative values respectively. Among them,coma increased most significantly followed by spherical aberration. Therewas no statistically significant difference of postoperative total aberration,total higher-order aberration, spherical aberration or coma within5groupsexcept that group4had more increase in trefoil than the other four groups,which had significant difference.5.In the study of astigmatism source, anterior corneal aberrationincreased significantly after operation. The increase of higher orderaberrations was dominated by spherical aberration followed by coma. Theamount of total aberration increasing decreased from group1to group5with RMS values of1.22±1.43μm、0.73±1.28μm、0.72±0.79μm、 0.49±0.72μm、0.33±0.71μm respectively. Among them there wassignificant difference between group1and group4, group1and group5.The increase of trefoil in group1was greater than group2, group3andgroup5which had significant statistical difference. There was nostatistically significant difference within groups of postoperative anteriorcorneal total higher order aberrations, spherical aberration, coma orquadrafoil.6.Correlation analysis found that Ratio had a positive correlation withpreoperative anterior corneal total aberration and anterior cornealastigmatism which indicating that eyes with high intraocular astigmatismare often combined with higher anterior corneal astigmatism and anteriorcorneal total aberration.7.In the study of astigmatism source, Goup5had more increase ofscotopic contrast sensitivity in frequency of6c/d than Goup1and moreincrease of glare contrast sensitivity in frequency of18c/d than Group4.8. The mean UCVA of with-the-rule(WTR) astigmatism group andagainst-the-rule(ATR) astigmatism group were1.27±0.29and1.24±0.20after operation respectively.The spheral of the two groups were0.57±0.37D and0.69±0.36D and the cylinder were-0.21±0.20D、-0.24±0.18D respectively.The IOS of the two groups were0.33±0.34and0.45±0.37. There were no statistically significant differences between groups ofthe indexes above. 9. There were no statistically significant differences between WTRgroup and ATR group of postoperative variation of whole eye totalaberrations, total higher order aberrations, spherical aberration, coma ortrefoil.10. The preoperative anterior corneal total aberration of WTR groupwas significantly larger than ATR group which were1.74±0.65μm and0.99±0.30μm respectively but there were no statistically significantdifferences between two groups of anterior corneal total higher orderaberrations, spherical aberration, coma or trefoil which indicated thatanterior corneal defocus and astigmatism were relatively small in oursample.11.WTR group had significantly less gain of anterior corneal totalaberration (0.59±0.57μ m） than ATR group （0.93±0.41μ m）postoperatively and the gain of anterior corneal total higher orderaberrations, spherical aberration, coma, trefoil or quadrafoil were slightlymore than ATR group but there were no statistically significantdifference between two groups.12. Though both WTR group and ATR group had gain of photopicand glare contrast sensitivity in some part of spatial frequency afteroperation while the change of scotopic contrast sensitivity was not obvious,but there were no statistically significant difference between the two groupsof contrast sensitivity variation at three conditions. Conclusions:1.There is a large proportion of ORA in myopicastigmatism population and that the ORA compensates for anterior cornealastigmatism. The whole eye astigmatism changed from WTR to ATR afterLASIK while the anterior corneal astigmatism remained WTR.2. Whateverthe astigmatism is dominated by corneal astigmatism or ORA, there was nostatistically significant difference of visual quality after correction ofmyopic astigmatism by LASIK. Ratio had a positive correlation withpreoperative anterior corneal total aberration and anterior cornealastigmatism which indicating that eyes with high intraocular astigmatismare often combined with higher anterior corneal astigmatism and anteriorcorneal total aberration.3.The type of astigmatism (WTR or ATR) had noimpact on visual quality after correction of myopic astigmatism by LASIKbut the result may be influenced by sample size. The anterior corneal totalaberration of ATR group was found to be significantly less than WTRgroup but there were no statistically significant differences between twogroups of anterior corneal total higher order aberrations and whole eyeaberrations. Therefore we infer that the formation of ATR astigmatism ismainly due to the effect of ORA. More deeper study is needed to reveal thetruth. |
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