The Changes Of Corneal Parameters And The Calculation Of Corneal Power After Refractive Surgery

Chapter1Comparison of Orbscan â…¡ and Pentacam measurements of corneal parameters before and after refractive surgeryObjectiveTraditional keratometer, keratoscope and corneal topography only analyze the anterior surface of corneal. Orbscan â…¡ and Pentacam can measure both of the corneal surface, and improve our ability to understand the cornea. Orbscan â…¡, combining the slit-scanning and Placido disc, is the first instrument capable of measuring the posterior corneal surface. Orbscan â…¡-based research found:corneal protrusion occurs after LASIK/PRK. However, the accuracy of Orbscan â…¡ to measure the posterior corneal surface has been questioned. Pentacam using a Scheimpflug imaging technology can get the topography of anterior and posterior corneal surface, the corneal thickness, anterior chamber volume, the anterior chamber depth at any point. This study was designed to take advantage of Orbscan â…¡ and Pentacam to measure the parameters of the normal corneas and the corneas after refractive surgery, analyze the reliability of two instruments and the impact of corneal refractive surgery on the corneal surface morphology according to the two instruments. MethodsThe cases of this study were the patients in refractive surgery centers of our hospital in February to April,2010. The cases were examined with A-scan, IOLMaster, Orbscan II and Pentacam. Patients of the refractive surgery group carried out such examinations before and after the surgery. Ka-3mm, Kp-3mm, Ka-central, Kp-central and CT were compared between Orbscan II and Pentacam. Because the IOLMaster measurement of anterior corneal power （Ka-I） and A-scan corneal thickness （CTA） were also involved in the analysis. Postoperative parameters of the refractive surgery group also had the statistical analysis. We compared the differences of posterior corneal power measurements of the two instruments and analyze whether the changes of the posterior corneal power （ΔKp-3mm/ΔKp-central） and RBT were correlated. Statistical methods concluded descriptive analysis, consistency analysis, paired t test and Pearson correlation analysis.ResultsThe differences between the measured values of Orbscan II and Pentacam Ka-3mm （47.89±1.37D,48.15±1.49D）, Ka-central （47.76±1.42D,43.16±1.38D）, Kp-3mm （-6.42±0.22D and-7.01±0.25D）, Kp-central （-6.33±0.43D,-6.13±0.23D） were statistically significant. Comparison with the Orbscan II and Pentacam measurements of the Ka-3mm, respectively, with the Ka-I（48.14±1.54D）, the difference of Ka₃mm-P and Ka-I was not significant, and the difference of Ka-3mm-O and Ka-I was not significant （t=-3.733, p=0.0002）. Orbscan Ⅱ（550.68±31.21μm） and the Pentacam,（548.57±30.04μm） CT measured values were of no significant difference （t=1.326, p=0.186）. Respectively compared with CTA, there are statistical differences. In the refractive surgery group, comparing of post-Ka-3mm, post-Ka_centrai, post-Kp-3mm and post-Kp-central, there were significant differences between the two instruments. Comparing Kp-3mm and Kp-central before and after surgery, Orbscan â…¡ found post-Kp-3mm and post-Kp-central were smaller （more negative, which is more steep）, while the Pentacam measured values had no significant difference. AKp-3mm-O/ΔKp-centrai-o and RBT were linearly correlated. LASIK and LASEK brought similar changes of the corneal parameters.ConclusionOrbscan II and Pentacam measurements of the anterior corneal power were of high accuracy, but the results can not replace the other. Measurement of posterior corneal power, the Pentacam measured smaller values in the normal cornea, but measured larger values after LASIK/LASEK. Orbscan II found posterior surface protrusion, while Pentacam did not. Considering the repeatability, reproducibility and principles of these instruments, Pentacam might be more accurate.Chapter2Corneal parameters and their changes after corneal refractive surgeryObjectiveTraditional keratometer, keratoscope and corneal topography can only measure the radius of the anterior corneal, then convert them to corneal refractive power with the keratometric index. An important prerequisite for the use of keratometric index is that the ratio of the anterior/posterior corneal surface is a constant. PRK/LASIK/LASEK change the ratio. Then the traditional keratometer and corneal topography is clearly inaccurate. In this study, we measured the corneal parameters with Pentacam, calculated the AP ratio, corneal refractive power by the Gaussian optics formula, and the keratometric index, and compare the variations after LASIK/LASEK.MethodsRa, Rp and CT of the normal cornea and post-surgery cornea were measured with Pentacam. The values were calculated:the AP ratio, Ka, Kp, KG, K2, keratometric index （Nc）, Km, K1.3278, K-6.17. Normality of the above parameters were tested.The averages and standard deviations of these parameters were calculated. The paired t-test compared Kg and K1.3278, Kg and K-6.17. The parameters in refractive surgery group were also calculated before and after surgery, and compared using paired t-test. The differences between post-KG and post-K1.3278,post-KG and post-K-6.17were compared. Post-KG and post-Km were linearly regressed.Results:Rp （6.36±0.22mm）,Kp （-6.29±0.22D）, CT（548.60±30.04um）, and K2（-6.17±0.22D） showed a normal distribution. The mean value of AP ratio and Nc were1.228and1.3278. Ra and Rp showed a positive correlation. Mean Kg, K1.3278and K.6.17, were41.98±1.33D. In the refractive surgery group, Rp and Kp had no significant difference before and after surgery. K2changed from-6.16±0.21D to-6.19±0.21D.There are statistical differences of AP ratio and Nc before and after surgery. The mean of post-KG and post-K1.3278were32.60±2.02D and33.83±1.79D, there was significant difference between them. The mean of post-KG and post-K.6.17were32.60±2.02D and32.63±2.14D, there was no significant difference between them.ConclusionIn this study, the AP ratio was1.228. Ra and Rp had a high linear correlation （R2=62.9%）. This was the premise of the thin-len model to calculate the corneal refractive power accurately. The keratometric index was1.3278, lower than the standard keratometric index of1.3375. K2was-6.17±0.22D, with normal distribution, a smaller variation. Two corneal refractive power calculation formulas from the above: K1.3278=0.971×Km K-6.17=1.114×Km-6.17Using of K1.3278and K-6.17formula can get more accurate corneal refractive power values. For calculations after refractive surgery, K.6.17may be more accurate. Chapter3The algorithm of corneal refractive power in intraocular lens power calculationObjectiveIn this study, K1.3278and K-6.17were used to calculate the normal and post-surgery corneal refractive power for intraocular lens implantations. The postoperative actual refractive status worked as a standard to determine the accuracy of the formulas.Methods:The normal cornea group included consecutive cases in July2011in our cataract center. In the refractive surgery group, we retrospectively analyzed9eyes of six cases after refractive surgery who needed phacoemulsification and intraocular lens implantation. The intraocular lens diopter were calculated with the following corneal refractive powers:（1） direct measurement of the A-scan or IOLMaster’s Km;（2） K1.3278;（3） K-6.17. Intraocular lens power calculation formular was the Haigis formula. The paired t-test was used to compare the differences. Analysis software was R-2.14.2.Results:In normal corneal group, the prediction error of Km, K1.3278and K₆.17were-0.02±0.24D,0.92±0.26D and0.75±0.28D. PE1-3278and PE-6.17were of no significant difference. There are significant differences between PE1.3278and PEm, PE-6.17and PEm. The rates of prediction error within±0.50were93.88%,2.04%and22.22%. The absolute errors were0.02±0.14D,0.92±0.26D and0.75±0.28D.In the refractive surgery group, the prediction error of Km, K1.3278and K-6.17were-2.23±0.47D,-1.19±0.46D and-0.60±0.40D. There are significant differences among them. The rates of prediction error within±0.50were0,11.11%and33.33%. The absolute errors were2.23±0.47D,1.19±0.46D and0.61±0.37D, and there are significant difference among them.ConclusionIn normal eyes, the intraocular lens power calculation error of K1.3278and K-6.17is significantly larger than the original corneal power value. Because the IOL formulas had been optimized, Km was accurate enough. For intraocular lens power calculation after refractive surgery, the errors of K1.3278and K-6.17were significantly less than Km, but still tend to underestimate the required intraocular lens diopters, leading to postoperative low hyperopia. Intraocular lens power calculation after corneal refractive surgery is still a thorny issue. With the combination of different intraocular lens power calculation formulas and different corneal refractive power correction method, you will get different results. The more realistic approach is to calculate with the known data in as much as possible ways, then make a overall consideration.