屈光手术后角膜参数的改变及角膜屈光力的计算方法
|
摘要
第一章Orbscan Ⅱ和Pentacam测量屈光手术前后角膜参数的比较
目的：
传统的角膜曲率计、角膜镜及角膜地形图仪仅仅能分析角膜前表面的形状和光学质量。最近推出的Orbscan Ⅱ和Pentacam能够同时获得角膜前后表面的信息,提高了人们了解角膜特性的能力。Orbscan Ⅱ眼前节分析仪结合了裂隙扫描技术和Placido盘,是最早的能测量角膜后表面的仪器。基于Orbscan Ⅱ的研究指出：LASIK和PRK术后常规出现微小的角膜前突。但是Orbscan Ⅱ测量角膜后表面的精确性已经被质疑。Pentacam眼前节分析仪使用的是Scheimpflug成像技术,可获得全角膜的前表面、后表面地形图,各点角膜厚度,前房容积,前房任意点深度,自动测量晶体密度。本研究旨在利用Orbscan Ⅱ和Pentacam对正常角膜和角膜屈光手术前后角膜的参数进行测量,分析两种仪器的可靠性,及角膜屈光手术对角膜后表面形态的影响,为这两种仪器在角膜屈光手术的术前筛查、术后随访中的应用提供参考,为手术方式的选择提供依据。
方法：
本研究的病例来源为2010年2月至4月在我院屈光手术中心就诊的连续病例。正常角膜组病例均在分别检查A超中央角膜厚度,IOLMaster, Orbscan Ⅱ及Pentacam检查,屈光手术组患者在术前及术后17～24周分别进行上述检查。正常角膜组比较Orbscan Ⅱ及Pentacam测量角膜前后表面3mm平均轴向屈光力(Ka-3mm和Kp-3mm、中央轴向屈光力(Ka-central和(Kp-central)、角膜厚度(CT)(两种仪器均选用最薄点角膜厚度)有无统计学差异,研究两者的一致性。由于IOLMaster测量的角膜前表面屈光力Ka-I和A超的角膜厚度测量值CTA也参与分析。屈光手术组术后参数也做上述统计分析。比较屈光手术组后表面3mm平均轴向屈光力(Kp-3mm)、中央轴向屈光力(Kp-central)在手术前后有无统计学差异。两种仪器的测量结果有无差异。后表面屈光力的变化值(ΔKp-3mm-O和ΔKp-central-O)与残余基质层厚度(RBT)有无相关性。本数据采用R-2.14.2分析软件进行数据分析和统计。统计方法主要有：描述性分析,一致性分析,配对t检验,pearson相关分析。
结果：
正常角膜组共143例286眼,Orbscan Ⅱ及Pentacam的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,-7.01±0.25D)、Kp-central (-6.33±0.43D,-6.13±0.23D)测量值之间的差异,均有统计学意义。Orbscan Ⅱ及Pentacam测量的Ka-3mm分别与Ka-1(48.14±1.54D)比较,Pentacam测量值与Ka-I相比无统计学差异(t=0.478,p=0.633),而OrbscanⅡ测量值与Ka-I相比存在统计学差异(t=-3.733, p=0.0002)。 Orbscan Ⅱ(550.68±31.21μm)和Pentacam (548.57±30.04μm) CT测量值两者之间无统计学差异(t=1.326,p=0.186)。两者分别与CTA比较,均存在统计学差异。
屈光手术组共69例136眼。Orbscan Ⅱ及Pentacam测量术后Ka-3,、Kp-central Kp-3mm、Kp-central的结果,两种仪器各个测量值之间的差异均有统计学意义。Orbscan Ⅱ(t=81.26, p<0.001)及Pentacam(t=56.98, p<0.001)的post-Ka-3mm测量值与post-Ka-Ⅰ(38.58±1.97D)相比存在统计学差异。术后Orbscan Ⅱ(450.92±50.73μm)与Pentacam (467.36±40.47μm) CT测量值之间存在统计学差异(t=6.76,p<0.001)。两者分别与post-CTA比较,post-CTO与post-CTA之间均存在统计学差异,post-CTP与post-CTA之间不存在统计学差异。比较手术前后Kp-3mm和Kp-central,OrbscanⅡ测量值在术后均变小(更负,即更陡峭),而Pentacam测量值无统计学差异。Orbscan II测量的Kp-3mm和Kp-central变化值与RBT存在线性相关,Pentacam测量结果显示：与RBT无线性关系。按照不同手术方式分组后,LASIK组和LASEK组的手术前后Kp-3mm/Kp-central变化量,两组之间无统计学差异。
结论：
Orbscan Ⅱ和Pentacam测量角膜前表面屈光力准确性高,但两者的结果不能相互替代使用。测量角膜后表面屈光力,在正常角膜中Pentacam测量值较小,即Pentacam测量的正常角膜后表面较陡峭,但在LASIK/LASEK术后则相反。Pentacam并未发现Orbscan Ⅱ得到的屈光手术后后表面前突的结论。从仪器的重复性、再现性和原理考虑,Pentacam测量值可能较为精确。
第二章Pentacam对角膜参数及其在角膜屈光手术后变化的研究
目的：
角膜曲率计、角膜镜和常规的角膜地形图仪,仅能测量前表面的曲率半径,而无法测量角膜后表面,只能根据角膜屈光指数将角膜前表面曲率半径转换为角膜屈光力。使用角膜屈光指数的重要前提是：角膜前、后表面曲率半径之间存在线性关系。在准分子激光屈光性角膜切削术(PRK)、准分子激光原位角膜磨镶术(LASIK)、准分子激光上皮下角膜磨镶术(LASEK)等角膜屈光手术后,这种线性关系发生了改变。这时用角膜曲率计和常规的角膜地形图仪来测量角膜屈光力显然是不准确的。本研究通过Pentacam对角膜前、后表面的参数进行了研究,利用高斯光学公式计算角膜总的屈光力,计算前后表面曲率半径之比AP比,计算角膜屈光指数,探讨角膜的几何形态与屈光特点,建立角膜屈光力的计算方法,并比较上述角膜参数在LASIK/LASEK术后的变化规律。
方法：
采用Pentacam测量的正常角膜组角膜前后表面中央3mm平均轴向曲率半径(Ra、Rp)、瞳孔中心角膜厚度(CT)的数值分别计算：角膜前后表面曲率半径之比(AP比)、前表面屈光力(Ka)、后表面屈光力(Kp)、根据高斯光学公式计算的角膜总屈光力(KG)、K2(角膜总屈光力中除Ka以外的部分)、角膜屈光指数(Nc)、由标准角膜屈光指数1.3375计算的角膜屈光力测量值(Km)、由矫正的角膜屈光指数1.3278计算的角膜屈光力K1.3278、由K2均值-6.17计算的角膜屈光力K-6.17.分别对以上参数进行正态性检验,计算各参数的平均数、标准差,直线回归分析Ra与Rp的关系。配对样本t检验比较KG与K1.3278的差异并对两者进行直线回归分析,配对样本T检验比较KG与K-6.17的差异并对两者进行直线回归分析。屈光手术组也分别计算出手术前后的各参数,用配对t检验进行比较。比较术后KG与术后K1.3278的差异并对两者进行直线回归分析,比较术后KG与术后K-6.17的差异并对术后KG与术后K-6.17进行直线回归分析。切削深度与术后Nc、术后K2进行直线回归分析。术后KG与术后Km进行直线回归分析。
结果：
正常角膜组各角膜参数中Rp (6.36±0.22mm)、Kp (-6.29±0.22D)、CT (548.60±30.04μm)、K2(-6.17±0.22D)值呈正态分布,其余各变量均不符合正态分布。AP比的均值为1.228,Nc的均值为1.3278。Ra与Rp呈正相关。KG与K1.3278的均值分别为41.98±1.33D和41.98±1.33D,t=0.09,p=0.928,两者之间不存在统计学差异。建立KG与K1.3278的回归模型,回归方程：KG=1.0161×K1.3278-0.6736,回归系数b=1.016,p<0.001,决定系数R2=99.2%。KG与K-6.17均值分别为41.98±1.33和41.98±1.33,t=0.1,p=0.99,两者之间不存在统计学差异。建立KG与K-6.17的回归模型,回归方程：KG=0.8859×K-6.17+4.79,回归系数b=0.88,p<0.001。决定系数R2=99.0%。
屈光手术组Rp、Kp在手术前后无统计学差异。K2在手术前后分别为-6.16±0.21D和-6.19±0.21D,两者之间存在统计学差异。AP比和Nc手术前后相比较,均存在统计学差异。屈光手术组post-KG与post-K1.3278的均值分别为32.60±2.02D和33.83±1.79D,两者之间存在统计学差异,t=-62.79,p<0.001。建立post-KG与post-K1.3278的回归模型,回归方程：post-KG=1.079×post-Ki.3278-3.911,回归系数b=1.079,p<0.001,决定系数R2=99.3%。post-KG与Post-K-6.17的均值分别为32.60±2.02D和32.63±2.14D,两者之间不存在统计学差异,t=-1.6005,p=0.11。建立post-KG与post-K-6.17的回归模型,回归方程：post-KG=0.941×post-K-6.17+1.89,回归系数b=0.94,p<0.001,决定系数R2=99.29%。切削深度与术后N。存在负相关,与术后K2无线性相关。术后KG与术后Km进行直线回归,post-KG=-5.21×post-Km-0.15,决定系数R2=31%。
结论：
本研究利用Pentacam测量的角膜前后表面中央3mm平均轴向曲率半径和瞳孔中心角膜厚度,计算得到AP比为1.228。Ra与Rp呈直线相关,且具有较高的相关性(R2=62.9%)且AP比的变异较小,这种前后表面曲率半径的线性关系是薄透镜模型准确计算角膜屈光力的前提。计算出角膜屈光指数为1.3278,低于标准角膜屈光指数1.3375。计算出正常角膜的K2为-6.17±0.22D,符合正态分布,变异较小。由上述结论得到两个角膜屈光力的计算公式：K1.3278=0.971×Km K-6.17=1.114×Km-6.17
利用K1.3278和K-6.17计算公式可以获得较精确的角膜屈光力数值。用于屈光手术后角膜屈光力时,K1.3278的准确性,可能随着切削深度的增加而下降。而K-6.17公式较为准确。
第三章角膜屈光力计算方法在人工晶状体度数计算中的应用
目的：
本研究利用已推导出的角膜屈光力计算公式计算出角膜总屈光力,用于正常角膜眼及角膜屈光手术后的人工晶状体度数计算,并以术后实际的屈光状态作为标准,判断角膜屈光力计算公式的准确性。
方法：
正常角膜组为2011年7月在我院白内障中心就诊并接受白内障超声乳化联合人工晶状体植入术的连续病例,进行前瞻性研究。共38例49眼。屈光手术组回顾性分析了6例9眼屈光手术后的白内障超声乳化联合人工晶状体植入术。正常角膜组和屈光手术组分别用以下的角膜屈光力计算值来计算人工晶状体度数：(1)直接测量的A超或IOLMaster的角膜曲率Km；(2)K1.3278；(3)K-6.17。采用的人工晶状体度数计算公式为Haigis公式。分别计算各种角膜屈光力计算方法在计算人工晶状体度数时产生的误差,即Km、K1.3278和K-6.17的预测误差和绝对误差。配对样本t检验比较不同屈光力计算方法之间的差异。采用的统计软件为R-2.14.2分析软件。
结果：
正常角膜组Km、K1.3278和K-6.17的预测误差分别-0.02±0.24D、0.92±0.26D和0.75±0.28D,PE1.3278和PE-6.17之间无统计学差异,PE1.3278与PEm、PE-6.17与PEm之间均存在统计学差异。三种角膜屈光力计算方法的预测误差在±0.50之内的比例分别为93.88%、2.04%和22.22%。三者的绝对误差分别0.02±0.14D、0.92±0.26D和0.75±0.28D,AE1.3278和AE-6.17之间无统计学差异,AE1.3278与AEm,AE-6.17与AEm之间均存在统计学差异。
屈光手术组Km、K1.3278和K-6.17的预测误差分别-2.23±0.47D、-1.19±0.46D和-0.60±0.40D,PEm、PE1.3278和PE-6.17之间均存在统计学差异。三种角膜屈光力计算方法的预测误差在±0.50之内的比例分别为0、11.11%和33.33%,预测误差在±1.00之内的比例分别为0、33.3%和88.89%,预测误差在±2.00之内的比例分别为33.3%,100%和100%。三者的绝对误差分别2.23±0.47D、1.19±0.46D和0.61±0.37D,AEm、AE1.3278和AE-6.17之间均存在统计学差异。
结论：
K1.3278和K-6.17用于正常眼的人工晶状体度数计算,误差明显大于使用原始的角膜屈光力测量值。分析其原因是：所有的人工晶状体度数公式均由实际手术效果发展而来,并针对以往传统角膜屈光力的误差,按照人工晶状体植入术后的实际屈光状态进行了优化。因此,如果不对人工晶状体度数计算公式进行优化,而在正常角膜眼的人工晶状体度数计算中,直接使用矫正的角膜屈光力值将引入新的误差。
K1.3278和k-6.17用于屈光手术后的人工晶状体度数计算,误差明显小于使用原始的角膜屈光力测量值,但仍趋向于低估所需的人工晶状体度数,可能导致术后低度远视。K-6.17公式的预测结果相对较好。角膜屈光手术后的人工晶状体度数计算仍然是一个棘手的问题。不同的角膜屈光力矫正方法,结合不同的人工晶状体度数计算公式,会得到不同的结果。较为现实可行的办法是：利用已知数据用尽可能多的方法来计算,综合评估后再做决定。
Chapter1Comparison of Orbscan Ⅱ and Pentacam measurements of corneal parameters before and after refractive surgery
Objective
Traditional keratometer, keratoscope and corneal topography only analyze the anterior surface of corneal. Orbscan Ⅱ and Pentacam can measure both of the corneal surface, and improve our ability to understand the cornea. Orbscan Ⅱ, combining the slit-scanning and Placido disc, is the first instrument capable of measuring the posterior corneal surface. Orbscan Ⅱ-based research found:corneal protrusion occurs after LASIK/PRK. However, the accuracy of Orbscan Ⅱ 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 Ⅱ 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.
Methods
The 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.
Results
The 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 Ka_3mm-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 Ⅱ 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.
Conclusion
Orbscan 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 surgery
Objective
Traditional 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.
Methods
Ra, 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.
Conclusion
In 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-6.17
Using 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 calculation
Objective
In 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 K_6.17were-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.
Conclusion
In 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.
目的：
传统的角膜曲率计、角膜镜及角膜地形图仪仅仅能分析角膜前表面的形状和光学质量。最近推出的Orbscan Ⅱ和Pentacam能够同时获得角膜前后表面的信息,提高了人们了解角膜特性的能力。Orbscan Ⅱ眼前节分析仪结合了裂隙扫描技术和Placido盘,是最早的能测量角膜后表面的仪器。基于Orbscan Ⅱ的研究指出：LASIK和PRK术后常规出现微小的角膜前突。但是Orbscan Ⅱ测量角膜后表面的精确性已经被质疑。Pentacam眼前节分析仪使用的是Scheimpflug成像技术,可获得全角膜的前表面、后表面地形图,各点角膜厚度,前房容积,前房任意点深度,自动测量晶体密度。本研究旨在利用Orbscan Ⅱ和Pentacam对正常角膜和角膜屈光手术前后角膜的参数进行测量,分析两种仪器的可靠性,及角膜屈光手术对角膜后表面形态的影响,为这两种仪器在角膜屈光手术的术前筛查、术后随访中的应用提供参考,为手术方式的选择提供依据。
方法：
本研究的病例来源为2010年2月至4月在我院屈光手术中心就诊的连续病例。正常角膜组病例均在分别检查A超中央角膜厚度,IOLMaster, Orbscan Ⅱ及Pentacam检查,屈光手术组患者在术前及术后17～24周分别进行上述检查。正常角膜组比较Orbscan Ⅱ及Pentacam测量角膜前后表面3mm平均轴向屈光力(Ka-3mm和Kp-3mm、中央轴向屈光力(Ka-central和(Kp-central)、角膜厚度(CT)(两种仪器均选用最薄点角膜厚度)有无统计学差异,研究两者的一致性。由于IOLMaster测量的角膜前表面屈光力Ka-I和A超的角膜厚度测量值CTA也参与分析。屈光手术组术后参数也做上述统计分析。比较屈光手术组后表面3mm平均轴向屈光力(Kp-3mm)、中央轴向屈光力(Kp-central)在手术前后有无统计学差异。两种仪器的测量结果有无差异。后表面屈光力的变化值(ΔKp-3mm-O和ΔKp-central-O)与残余基质层厚度(RBT)有无相关性。本数据采用R-2.14.2分析软件进行数据分析和统计。统计方法主要有：描述性分析,一致性分析,配对t检验,pearson相关分析。
结果：
正常角膜组共143例286眼,Orbscan Ⅱ及Pentacam的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,-7.01±0.25D)、Kp-central (-6.33±0.43D,-6.13±0.23D)测量值之间的差异,均有统计学意义。Orbscan Ⅱ及Pentacam测量的Ka-3mm分别与Ka-1(48.14±1.54D)比较,Pentacam测量值与Ka-I相比无统计学差异(t=0.478,p=0.633),而OrbscanⅡ测量值与Ka-I相比存在统计学差异(t=-3.733, p=0.0002)。 Orbscan Ⅱ(550.68±31.21μm)和Pentacam (548.57±30.04μm) CT测量值两者之间无统计学差异(t=1.326,p=0.186)。两者分别与CTA比较,均存在统计学差异。
屈光手术组共69例136眼。Orbscan Ⅱ及Pentacam测量术后Ka-3,、Kp-central Kp-3mm、Kp-central的结果,两种仪器各个测量值之间的差异均有统计学意义。Orbscan Ⅱ(t=81.26, p<0.001)及Pentacam(t=56.98, p<0.001)的post-Ka-3mm测量值与post-Ka-Ⅰ(38.58±1.97D)相比存在统计学差异。术后Orbscan Ⅱ(450.92±50.73μm)与Pentacam (467.36±40.47μm) CT测量值之间存在统计学差异(t=6.76,p<0.001)。两者分别与post-CTA比较,post-CTO与post-CTA之间均存在统计学差异,post-CTP与post-CTA之间不存在统计学差异。比较手术前后Kp-3mm和Kp-central,OrbscanⅡ测量值在术后均变小(更负,即更陡峭),而Pentacam测量值无统计学差异。Orbscan II测量的Kp-3mm和Kp-central变化值与RBT存在线性相关,Pentacam测量结果显示：与RBT无线性关系。按照不同手术方式分组后,LASIK组和LASEK组的手术前后Kp-3mm/Kp-central变化量,两组之间无统计学差异。
结论：
Orbscan Ⅱ和Pentacam测量角膜前表面屈光力准确性高,但两者的结果不能相互替代使用。测量角膜后表面屈光力,在正常角膜中Pentacam测量值较小,即Pentacam测量的正常角膜后表面较陡峭,但在LASIK/LASEK术后则相反。Pentacam并未发现Orbscan Ⅱ得到的屈光手术后后表面前突的结论。从仪器的重复性、再现性和原理考虑,Pentacam测量值可能较为精确。
第二章Pentacam对角膜参数及其在角膜屈光手术后变化的研究
目的：
角膜曲率计、角膜镜和常规的角膜地形图仪,仅能测量前表面的曲率半径,而无法测量角膜后表面,只能根据角膜屈光指数将角膜前表面曲率半径转换为角膜屈光力。使用角膜屈光指数的重要前提是：角膜前、后表面曲率半径之间存在线性关系。在准分子激光屈光性角膜切削术(PRK)、准分子激光原位角膜磨镶术(LASIK)、准分子激光上皮下角膜磨镶术(LASEK)等角膜屈光手术后,这种线性关系发生了改变。这时用角膜曲率计和常规的角膜地形图仪来测量角膜屈光力显然是不准确的。本研究通过Pentacam对角膜前、后表面的参数进行了研究,利用高斯光学公式计算角膜总的屈光力,计算前后表面曲率半径之比AP比,计算角膜屈光指数,探讨角膜的几何形态与屈光特点,建立角膜屈光力的计算方法,并比较上述角膜参数在LASIK/LASEK术后的变化规律。
方法：
采用Pentacam测量的正常角膜组角膜前后表面中央3mm平均轴向曲率半径(Ra、Rp)、瞳孔中心角膜厚度(CT)的数值分别计算：角膜前后表面曲率半径之比(AP比)、前表面屈光力(Ka)、后表面屈光力(Kp)、根据高斯光学公式计算的角膜总屈光力(KG)、K2(角膜总屈光力中除Ka以外的部分)、角膜屈光指数(Nc)、由标准角膜屈光指数1.3375计算的角膜屈光力测量值(Km)、由矫正的角膜屈光指数1.3278计算的角膜屈光力K1.3278、由K2均值-6.17计算的角膜屈光力K-6.17.分别对以上参数进行正态性检验,计算各参数的平均数、标准差,直线回归分析Ra与Rp的关系。配对样本t检验比较KG与K1.3278的差异并对两者进行直线回归分析,配对样本T检验比较KG与K-6.17的差异并对两者进行直线回归分析。屈光手术组也分别计算出手术前后的各参数,用配对t检验进行比较。比较术后KG与术后K1.3278的差异并对两者进行直线回归分析,比较术后KG与术后K-6.17的差异并对术后KG与术后K-6.17进行直线回归分析。切削深度与术后Nc、术后K2进行直线回归分析。术后KG与术后Km进行直线回归分析。
结果：
正常角膜组各角膜参数中Rp (6.36±0.22mm)、Kp (-6.29±0.22D)、CT (548.60±30.04μm)、K2(-6.17±0.22D)值呈正态分布,其余各变量均不符合正态分布。AP比的均值为1.228,Nc的均值为1.3278。Ra与Rp呈正相关。KG与K1.3278的均值分别为41.98±1.33D和41.98±1.33D,t=0.09,p=0.928,两者之间不存在统计学差异。建立KG与K1.3278的回归模型,回归方程：KG=1.0161×K1.3278-0.6736,回归系数b=1.016,p<0.001,决定系数R2=99.2%。KG与K-6.17均值分别为41.98±1.33和41.98±1.33,t=0.1,p=0.99,两者之间不存在统计学差异。建立KG与K-6.17的回归模型,回归方程：KG=0.8859×K-6.17+4.79,回归系数b=0.88,p<0.001。决定系数R2=99.0%。
屈光手术组Rp、Kp在手术前后无统计学差异。K2在手术前后分别为-6.16±0.21D和-6.19±0.21D,两者之间存在统计学差异。AP比和Nc手术前后相比较,均存在统计学差异。屈光手术组post-KG与post-K1.3278的均值分别为32.60±2.02D和33.83±1.79D,两者之间存在统计学差异,t=-62.79,p<0.001。建立post-KG与post-K1.3278的回归模型,回归方程：post-KG=1.079×post-Ki.3278-3.911,回归系数b=1.079,p<0.001,决定系数R2=99.3%。post-KG与Post-K-6.17的均值分别为32.60±2.02D和32.63±2.14D,两者之间不存在统计学差异,t=-1.6005,p=0.11。建立post-KG与post-K-6.17的回归模型,回归方程：post-KG=0.941×post-K-6.17+1.89,回归系数b=0.94,p<0.001,决定系数R2=99.29%。切削深度与术后N。存在负相关,与术后K2无线性相关。术后KG与术后Km进行直线回归,post-KG=-5.21×post-Km-0.15,决定系数R2=31%。
结论：
本研究利用Pentacam测量的角膜前后表面中央3mm平均轴向曲率半径和瞳孔中心角膜厚度,计算得到AP比为1.228。Ra与Rp呈直线相关,且具有较高的相关性(R2=62.9%)且AP比的变异较小,这种前后表面曲率半径的线性关系是薄透镜模型准确计算角膜屈光力的前提。计算出角膜屈光指数为1.3278,低于标准角膜屈光指数1.3375。计算出正常角膜的K2为-6.17±0.22D,符合正态分布,变异较小。由上述结论得到两个角膜屈光力的计算公式：K1.3278=0.971×Km K-6.17=1.114×Km-6.17
利用K1.3278和K-6.17计算公式可以获得较精确的角膜屈光力数值。用于屈光手术后角膜屈光力时,K1.3278的准确性,可能随着切削深度的增加而下降。而K-6.17公式较为准确。
第三章角膜屈光力计算方法在人工晶状体度数计算中的应用
目的：
本研究利用已推导出的角膜屈光力计算公式计算出角膜总屈光力,用于正常角膜眼及角膜屈光手术后的人工晶状体度数计算,并以术后实际的屈光状态作为标准,判断角膜屈光力计算公式的准确性。
方法：
正常角膜组为2011年7月在我院白内障中心就诊并接受白内障超声乳化联合人工晶状体植入术的连续病例,进行前瞻性研究。共38例49眼。屈光手术组回顾性分析了6例9眼屈光手术后的白内障超声乳化联合人工晶状体植入术。正常角膜组和屈光手术组分别用以下的角膜屈光力计算值来计算人工晶状体度数：(1)直接测量的A超或IOLMaster的角膜曲率Km；(2)K1.3278；(3)K-6.17。采用的人工晶状体度数计算公式为Haigis公式。分别计算各种角膜屈光力计算方法在计算人工晶状体度数时产生的误差,即Km、K1.3278和K-6.17的预测误差和绝对误差。配对样本t检验比较不同屈光力计算方法之间的差异。采用的统计软件为R-2.14.2分析软件。
结果：
正常角膜组Km、K1.3278和K-6.17的预测误差分别-0.02±0.24D、0.92±0.26D和0.75±0.28D,PE1.3278和PE-6.17之间无统计学差异,PE1.3278与PEm、PE-6.17与PEm之间均存在统计学差异。三种角膜屈光力计算方法的预测误差在±0.50之内的比例分别为93.88%、2.04%和22.22%。三者的绝对误差分别0.02±0.14D、0.92±0.26D和0.75±0.28D,AE1.3278和AE-6.17之间无统计学差异,AE1.3278与AEm,AE-6.17与AEm之间均存在统计学差异。
屈光手术组Km、K1.3278和K-6.17的预测误差分别-2.23±0.47D、-1.19±0.46D和-0.60±0.40D,PEm、PE1.3278和PE-6.17之间均存在统计学差异。三种角膜屈光力计算方法的预测误差在±0.50之内的比例分别为0、11.11%和33.33%,预测误差在±1.00之内的比例分别为0、33.3%和88.89%,预测误差在±2.00之内的比例分别为33.3%,100%和100%。三者的绝对误差分别2.23±0.47D、1.19±0.46D和0.61±0.37D,AEm、AE1.3278和AE-6.17之间均存在统计学差异。
结论：
K1.3278和K-6.17用于正常眼的人工晶状体度数计算,误差明显大于使用原始的角膜屈光力测量值。分析其原因是：所有的人工晶状体度数公式均由实际手术效果发展而来,并针对以往传统角膜屈光力的误差,按照人工晶状体植入术后的实际屈光状态进行了优化。因此,如果不对人工晶状体度数计算公式进行优化,而在正常角膜眼的人工晶状体度数计算中,直接使用矫正的角膜屈光力值将引入新的误差。
K1.3278和k-6.17用于屈光手术后的人工晶状体度数计算,误差明显小于使用原始的角膜屈光力测量值,但仍趋向于低估所需的人工晶状体度数,可能导致术后低度远视。K-6.17公式的预测结果相对较好。角膜屈光手术后的人工晶状体度数计算仍然是一个棘手的问题。不同的角膜屈光力矫正方法,结合不同的人工晶状体度数计算公式,会得到不同的结果。较为现实可行的办法是：利用已知数据用尽可能多的方法来计算,综合评估后再做决定。
Chapter1Comparison of Orbscan Ⅱ and Pentacam measurements of corneal parameters before and after refractive surgery
Objective
Traditional keratometer, keratoscope and corneal topography only analyze the anterior surface of corneal. Orbscan Ⅱ and Pentacam can measure both of the corneal surface, and improve our ability to understand the cornea. Orbscan Ⅱ, combining the slit-scanning and Placido disc, is the first instrument capable of measuring the posterior corneal surface. Orbscan Ⅱ-based research found:corneal protrusion occurs after LASIK/PRK. However, the accuracy of Orbscan Ⅱ 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 Ⅱ 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.
Methods
The 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.
Results
The 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 Ka_3mm-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 Ⅱ 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.
Conclusion
Orbscan 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 surgery
Objective
Traditional 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.
Methods
Ra, 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.
Conclusion
In 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-6.17
Using 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 calculation
Objective
In 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 K_6.17were-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.
Conclusion
In 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.
引文
[1]Holladay JT. Consultations in refractive surgery [comment]. Refract Corneal Surg 1989; 5:203.
[2]Odenthal MTP, Eggink CA, Melles G, et al. Clinicalandtheoretical results of intraocular lens power calculation for cataract after photorefractive keratectomy for myopia. Arch Ophthalmol 2002; 120:431-438.
[3]Hamed AM, Wang L, Misra M, et al. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109:651-658.
[4]Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg 2003; 29:2063-2068.
[5]Shammas HJ, Shammas MC, Garabet A,et al. Correcting the corneal power measurements for intraocularlens power calculations after myopic laser in situ keratomileusis. Am J Ophthalmol 2003; 136:426-432.
[6]Savini G, Barboni P, Zanini M. Correlation between attempted correction and keratometric refractive index of the cornea after myopic excimer laser surgery. J Refract Surg 2007; 23:461-466.
[7]Haigis W, Lege BAM. IOL-Berechnung nach refraktiver Laserchirurgieaus aktuellen Messwerten. In:Kohnen T, Auffarth GU, Pham DT, eds,20. Kongreβ der Deutschsprachigen Gesellschaft fu" r Intraokularlinsen-Implantation und Refraktive Chirurgie, Heidelberg, Germany, 3 und 4 Marz 2006. Koln, Germany, Biermann Verlag,2006; 383-387.
[8]Kawamorita T, Uozato H, Kamiya K,et al. Repeatability,reproducibility, and agreement characteristics of rotating Scheimpflug photography and scanning-slit corneal topography for corneal power measurement. J Cataract Refract Surg 2009; 35:127-133.
[9]Perez-Escudero A, Dorronsoro C, Sawides L,et al. Minor influence of myopic laser in situ keratomileusis on the posterior corneal surface. Invest Ophthalmol Vis Sci.2009 Sep;50 (9):4146-54.
[10]Chen YA, Hirnschall N, Findl O. Evaluation of 2 new optical biometry devices and comparison with the current gold standard biometer. J Cataract Refract Surg.2011 Mar;37 (3):513-7.
[11]Lackner B, Schmidinger G, Pieh S, et al. Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005; 82:892-899.
[12]Buehren T, Collins MJ, Iskander DR, et al. The stability of corneal topography in the post-blink interval. Cornea 2001; 20:826-833.
[13]Marsich MW, Bullimore MA. The repeatability of corneal thickness measures.Cornea 2000; 19:792-795.
[14]何燕玲,黎晓新,鲍永珍,等Pentacam系统与A超角膜测厚仪测量瞳孔中心角膜厚度的比较.中华眼科杂志2006：42(11)：985-988.
[15]Fakhry MA, Artola A, Belda JI, et al. Comparison of corneal pachymetry using ultrasound and Orbscan Ⅱ. J Cataract Refract Surg 2002; 28:248-252.
[16]Christensen A, Narvaez J, Zimmerman G.Comparison of central corneal thickness measurements by ultrasound pachymetry, konan noncontact optical pachymetry, and orbscan pachymetry. Cornea 2008; 27:862-865.
[17]Suzuki S, Oshika T, Oki K, et al. Corneal thickness measurements: scanning-slit corneal topography and noncontact specular microscopy versus ultrasonic pachymetry. J Cataract Refract Surg 2003; 29:1313-1318.
[18]Hashemi H, Mehravaran S. Central corneal thickness measurement with Pentacam, Orbscan Ⅱ, and ultrasound devices before and after laser refractive surgery formyopia. J Cataract Refract Surg 2007; 33:1701-1707.
[19]Prospero Ponce CM, Rocha KM, Smith SD, et al. Central and peripheral corneal thickness measured with optical coherence tomography, Scheimpflug imaging, and ultrasound pachymetry in normal, keratoconus-suspect, and postlaser in situ keratomileusis eyes. J Cataract Refract Surg 2009; 35: 1055-1062.
[20]Buehl W, Stojanac D, Sacu S,et al. Comparison of three methods of measuring corneal thickness and anterior chamber depth. Am J Ophthalmol 2006; 141:7-12.
[21]Amano S, Honda N, Amano Y, et al. Comparison of central corneal thickness measurements by rotating Scheimpflug camera, ultrasonic pachymetry, and scanning-slit corneal topography. Ophthalmology 2006; 113:937-941.
[22]Rosa N, Lanza M, Borrelli M, et al. Comparison of central corneal thickness measured with orbscan and pentacam. J Refract Surg 2007;23:895-899.
[23]Kim SW, Byun YJ, Kim EK, et al. Central corneal thickness measurements in unoperated eyes and eyes after PRK for myopia using Pentacam, Orbscan Ⅱ and ultrasonic pachymetry. J Refract Surg 2007; 23:888-894.
[24]Matsuda J, Hieda O, Kinoshita S. Comparison of central corneal thickness measurements by Orbscan Ⅱ and Pentacam after corneal refractive surgery. Jpn J Ophthalmol 2008; 52:245-249.
[25]Doors M, Cruysberg LP, Berendschot TT, et al. Comparison of central corneal thickness and anterior chamber depth measurements using three imaging technologies in normal eyes and after phakic intraocular lens implantation. Graefes Arch Clin Exp Ophthalmol 2009; 247:1139-1146.
[26]Hashemi H, Mehravaran S. Corneal changes after laser refractive surgery for myopia:comparison of Orbscan Ⅱ and Pentacam findings. J Cataract Refract Surg 2007;33:841-847.
[27]Yoshida T, Miyata K, Tokunaga T, et al. Difference map or single elevation map in the evaluation of corneal forward shift after LASIK. Ophthalmology 2003; 110:1926-1930.
[28]Miyata K, Tokunaga T, Nakahara M, et al. Residual bed thickness and corneal forward shift after laser in situ keratomileusis. J Cataract Refract Surg 2004; 30:1067-1072.
[29]Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of Orbscan II post-LASIK:apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures. Clin Exp Ophthalmol 2005; 33:147-152.
[30]Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 106:406-409; discussion by RK Maloney,409-410.
[31]Sharma N, Rani A, Balasubramanya R, et al. Posterior corneal topographic changes after partial flap during laser in situ keratomileusis.Br J Ophthalmol 2003; 87:160-162.
[32]Twa MD, Roberts C, Mahmoud AM, et al. Response of the posterior corneal surface to laser in situ keratomileusis for myopia. J Cataract Refract Surg 2005; 31:61-71.
[33]Herna'ndez-Quintela E, Samapunphong S, Khan BF, et al. Posterior corneal surface changes after refractive surgery. Ophthalmology 2001; 108:1415-1422.
[34]Randleman JB, Woodward M, Lynn MJ, et al. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology.2008;115:37-50.
[35]Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg 2006; 32:1426-1431.
[36]Vicente D, Clinch TE, Kang PC. Changes in posterior corneal elevation after laser in situ keratomileusis enhancement. J Cataract Refract Surg.2008;34 (5):785-788.
[37]Ha BJ, Kim SW, Kim SW, et al. Pentacam and Orbscan Ⅱ measurements of posterior corneal elevation before and afterphotorefractive keratectomy. J Refract Surg.2009;25:290-295.
[38]Sun HJ, Park JW, Kim SW. Stability of the posterior corneal surface after laser surface ablation for myopia. Cornea.2009;28:1019-1022.
[39]Seitz B, Torres F, Langenbucher A, et al. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology 2001; 108:666-672.
[40]Seiler T, Koufala K, Richter G.Iatrogenic keratectasia after laser in situ keratomileusis [J].J Refract Surg,1998,14 (3):312-317.
[41]Guirao A.Theoretical elastic response of the cornea to refractivesurgery:risk factors for keratectasia [J]. J Refract Surg,2005 (2),21:176-185.
[42]Lowe RF, Clark BAJ. Posterior corneal curvature; correlations in normal eyes and in eyes involved with primary angle-closure glaucoma. Br J Ophthalmol 1973; 57:464-470.
[43]Royston JM, Dunne MCM, Barnes DA. Measurement of posterior corneal surface toricity. Optom Vis Sci 1990; 67:757-763.
[44]Royston JM, Dunne MCM, Barnes DA. Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques. Ophthalmic Physiol Opt 1990; 10:385-388.
[45]Edmund C. Posterior corneal curvature and its influence on corneal dioptric power. Acta Ophthalmol (Copenh) 1994;72:715-720.
[46]Garner LF, Owens H, Yap MKH, et al. Radius of curvature of the posterior surface of the cornea. Optom Vis Sci 1997; 74:496-498.
[47]Dubbelman M, Weeber HA, van der Heijde RGL,et al. Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography. Acta Ophthalmol Scand 2002; 80:379-383.
[48]Dubbelman M, Sicam VADP, van der Heijde GL. The shape of the anterior and posterior surface of the aging human cornea.Vision Res 2006; 46:993-1001.
[49]Lim K-L, Fam H-B. Relationship between the corneal surface and the anterior segment of the cornea:an Asian perspective.J Cataract Refract Surg 2006; 32:1814-1819.
[50]Fam H-B, Lim K-L. Validity of the keratometric index:large population-based study. J Cataract Refract Surg 2007; 33:686-691.
[51]Ho JD, Liou SW, Tsai RJ, et al.Estimation of the effective lens position using a rotating Scheimpflug camera. J Cataract Refract Surg.2008 Dec;34 (12):2119-27.
[52]Tang M, Li Y, Avila M, et al. Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography. J Cataract Refract Surg 2006;32:1843-1850.
[53]Koch D, Wang L. Calculating IOL power in eyes that have had refractive surgery [editorial]. J Cataract Refract Surg 2003;29:2039-2042.
[54]Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17-29.
[55]Smith RJ, Chan W-K, Maloney RK. The prediction of surgically induced refractive change from corneal topography. Am J Ophthalmol 1998; 125:44-53.
[56]Awwad ST, Manasseh C, Bowman RW, et al. Intraocular lens power calculation after myopic laser in situ keratomileusis:estimating the corneal refractive power. J Cataract Refract Surg 2008; 34:1070-1076.
[57]Ferrara G, Cennamo G, Marotta G,et al. New formula to calculate corneal power after refractive surgery. J Refract Surg 2004; 20:465-471.
[58]Rosa N, Capasso L, Lanza M, et al. Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery. J Cataract Refract Surg 2005;31:1020-1024.
[59]Tang Q, Hoffer KJ, Olson MD,et al. Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery. J Cataract Refract Surg.2009 Jul;35 (7):1198-203.
[60]Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery:BESSt formula. J Cataract Refract Surg 2006; 32:2004-2014.
[61]Jin H, Holzer MP, Rabsilber T,et al. Intraocular lens power calculation after laser refractive surgery:corrective algorithm for corneal power estimation. J Cataract Refract Surg.2010 Jan;36 (1):87-96.
[62]Cheng AC, Ho T, Lau S,et al. Introducing a new ratio useful in the estimation of preoperative corneal power in patients with myopic LASIK based on postoperative corneal data. Cornea.2009 Jan;28 (1):1-4.
[63]Ianchulev T, Salz J, Hoffer K, et al. Intraoperative optical refractive biometry for intraocular lens power estimation without axial length and keratometry measurements. J Cataract Refract Surg 2005; 31:1530-1536.
[64]Mackool RJ, Ko W, Mackool R. Intraocular lens power calculation after laser in situ keratomileusis:aphakic refraction technique.J Cataract Refract Surg 2006; 32:435-437.
[65]Kenneth J. Hoffer. Intraocular lens power calculation after previous laser refractive surgeryJ Cataract Refract Surg 2009; 35:759-765.
[2]Odenthal MTP, Eggink CA, Melles G, et al. Clinicalandtheoretical results of intraocular lens power calculation for cataract after photorefractive keratectomy for myopia. Arch Ophthalmol 2002; 120:431-438.
[3]Hamed AM, Wang L, Misra M, et al. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109:651-658.
[4]Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg 2003; 29:2063-2068.
[5]Shammas HJ, Shammas MC, Garabet A,et al. Correcting the corneal power measurements for intraocularlens power calculations after myopic laser in situ keratomileusis. Am J Ophthalmol 2003; 136:426-432.
[6]Savini G, Barboni P, Zanini M. Correlation between attempted correction and keratometric refractive index of the cornea after myopic excimer laser surgery. J Refract Surg 2007; 23:461-466.
[7]Haigis W, Lege BAM. IOL-Berechnung nach refraktiver Laserchirurgieaus aktuellen Messwerten. In:Kohnen T, Auffarth GU, Pham DT, eds,20. Kongreβ der Deutschsprachigen Gesellschaft fu" r Intraokularlinsen-Implantation und Refraktive Chirurgie, Heidelberg, Germany, 3 und 4 Marz 2006. Koln, Germany, Biermann Verlag,2006; 383-387.
[8]Kawamorita T, Uozato H, Kamiya K,et al. Repeatability,reproducibility, and agreement characteristics of rotating Scheimpflug photography and scanning-slit corneal topography for corneal power measurement. J Cataract Refract Surg 2009; 35:127-133.
[9]Perez-Escudero A, Dorronsoro C, Sawides L,et al. Minor influence of myopic laser in situ keratomileusis on the posterior corneal surface. Invest Ophthalmol Vis Sci.2009 Sep;50 (9):4146-54.
[10]Chen YA, Hirnschall N, Findl O. Evaluation of 2 new optical biometry devices and comparison with the current gold standard biometer. J Cataract Refract Surg.2011 Mar;37 (3):513-7.
[11]Lackner B, Schmidinger G, Pieh S, et al. Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005; 82:892-899.
[12]Buehren T, Collins MJ, Iskander DR, et al. The stability of corneal topography in the post-blink interval. Cornea 2001; 20:826-833.
[13]Marsich MW, Bullimore MA. The repeatability of corneal thickness measures.Cornea 2000; 19:792-795.
[14]何燕玲,黎晓新,鲍永珍,等Pentacam系统与A超角膜测厚仪测量瞳孔中心角膜厚度的比较.中华眼科杂志2006：42(11)：985-988.
[15]Fakhry MA, Artola A, Belda JI, et al. Comparison of corneal pachymetry using ultrasound and Orbscan Ⅱ. J Cataract Refract Surg 2002; 28:248-252.
[16]Christensen A, Narvaez J, Zimmerman G.Comparison of central corneal thickness measurements by ultrasound pachymetry, konan noncontact optical pachymetry, and orbscan pachymetry. Cornea 2008; 27:862-865.
[17]Suzuki S, Oshika T, Oki K, et al. Corneal thickness measurements: scanning-slit corneal topography and noncontact specular microscopy versus ultrasonic pachymetry. J Cataract Refract Surg 2003; 29:1313-1318.
[18]Hashemi H, Mehravaran S. Central corneal thickness measurement with Pentacam, Orbscan Ⅱ, and ultrasound devices before and after laser refractive surgery formyopia. J Cataract Refract Surg 2007; 33:1701-1707.
[19]Prospero Ponce CM, Rocha KM, Smith SD, et al. Central and peripheral corneal thickness measured with optical coherence tomography, Scheimpflug imaging, and ultrasound pachymetry in normal, keratoconus-suspect, and postlaser in situ keratomileusis eyes. J Cataract Refract Surg 2009; 35: 1055-1062.
[20]Buehl W, Stojanac D, Sacu S,et al. Comparison of three methods of measuring corneal thickness and anterior chamber depth. Am J Ophthalmol 2006; 141:7-12.
[21]Amano S, Honda N, Amano Y, et al. Comparison of central corneal thickness measurements by rotating Scheimpflug camera, ultrasonic pachymetry, and scanning-slit corneal topography. Ophthalmology 2006; 113:937-941.
[22]Rosa N, Lanza M, Borrelli M, et al. Comparison of central corneal thickness measured with orbscan and pentacam. J Refract Surg 2007;23:895-899.
[23]Kim SW, Byun YJ, Kim EK, et al. Central corneal thickness measurements in unoperated eyes and eyes after PRK for myopia using Pentacam, Orbscan Ⅱ and ultrasonic pachymetry. J Refract Surg 2007; 23:888-894.
[24]Matsuda J, Hieda O, Kinoshita S. Comparison of central corneal thickness measurements by Orbscan Ⅱ and Pentacam after corneal refractive surgery. Jpn J Ophthalmol 2008; 52:245-249.
[25]Doors M, Cruysberg LP, Berendschot TT, et al. Comparison of central corneal thickness and anterior chamber depth measurements using three imaging technologies in normal eyes and after phakic intraocular lens implantation. Graefes Arch Clin Exp Ophthalmol 2009; 247:1139-1146.
[26]Hashemi H, Mehravaran S. Corneal changes after laser refractive surgery for myopia:comparison of Orbscan Ⅱ and Pentacam findings. J Cataract Refract Surg 2007;33:841-847.
[27]Yoshida T, Miyata K, Tokunaga T, et al. Difference map or single elevation map in the evaluation of corneal forward shift after LASIK. Ophthalmology 2003; 110:1926-1930.
[28]Miyata K, Tokunaga T, Nakahara M, et al. Residual bed thickness and corneal forward shift after laser in situ keratomileusis. J Cataract Refract Surg 2004; 30:1067-1072.
[29]Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of Orbscan II post-LASIK:apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures. Clin Exp Ophthalmol 2005; 33:147-152.
[30]Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 106:406-409; discussion by RK Maloney,409-410.
[31]Sharma N, Rani A, Balasubramanya R, et al. Posterior corneal topographic changes after partial flap during laser in situ keratomileusis.Br J Ophthalmol 2003; 87:160-162.
[32]Twa MD, Roberts C, Mahmoud AM, et al. Response of the posterior corneal surface to laser in situ keratomileusis for myopia. J Cataract Refract Surg 2005; 31:61-71.
[33]Herna'ndez-Quintela E, Samapunphong S, Khan BF, et al. Posterior corneal surface changes after refractive surgery. Ophthalmology 2001; 108:1415-1422.
[34]Randleman JB, Woodward M, Lynn MJ, et al. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology.2008;115:37-50.
[35]Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg 2006; 32:1426-1431.
[36]Vicente D, Clinch TE, Kang PC. Changes in posterior corneal elevation after laser in situ keratomileusis enhancement. J Cataract Refract Surg.2008;34 (5):785-788.
[37]Ha BJ, Kim SW, Kim SW, et al. Pentacam and Orbscan Ⅱ measurements of posterior corneal elevation before and afterphotorefractive keratectomy. J Refract Surg.2009;25:290-295.
[38]Sun HJ, Park JW, Kim SW. Stability of the posterior corneal surface after laser surface ablation for myopia. Cornea.2009;28:1019-1022.
[39]Seitz B, Torres F, Langenbucher A, et al. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology 2001; 108:666-672.
[40]Seiler T, Koufala K, Richter G.Iatrogenic keratectasia after laser in situ keratomileusis [J].J Refract Surg,1998,14 (3):312-317.
[41]Guirao A.Theoretical elastic response of the cornea to refractivesurgery:risk factors for keratectasia [J]. J Refract Surg,2005 (2),21:176-185.
[42]Lowe RF, Clark BAJ. Posterior corneal curvature; correlations in normal eyes and in eyes involved with primary angle-closure glaucoma. Br J Ophthalmol 1973; 57:464-470.
[43]Royston JM, Dunne MCM, Barnes DA. Measurement of posterior corneal surface toricity. Optom Vis Sci 1990; 67:757-763.
[44]Royston JM, Dunne MCM, Barnes DA. Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques. Ophthalmic Physiol Opt 1990; 10:385-388.
[45]Edmund C. Posterior corneal curvature and its influence on corneal dioptric power. Acta Ophthalmol (Copenh) 1994;72:715-720.
[46]Garner LF, Owens H, Yap MKH, et al. Radius of curvature of the posterior surface of the cornea. Optom Vis Sci 1997; 74:496-498.
[47]Dubbelman M, Weeber HA, van der Heijde RGL,et al. Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography. Acta Ophthalmol Scand 2002; 80:379-383.
[48]Dubbelman M, Sicam VADP, van der Heijde GL. The shape of the anterior and posterior surface of the aging human cornea.Vision Res 2006; 46:993-1001.
[49]Lim K-L, Fam H-B. Relationship between the corneal surface and the anterior segment of the cornea:an Asian perspective.J Cataract Refract Surg 2006; 32:1814-1819.
[50]Fam H-B, Lim K-L. Validity of the keratometric index:large population-based study. J Cataract Refract Surg 2007; 33:686-691.
[51]Ho JD, Liou SW, Tsai RJ, et al.Estimation of the effective lens position using a rotating Scheimpflug camera. J Cataract Refract Surg.2008 Dec;34 (12):2119-27.
[52]Tang M, Li Y, Avila M, et al. Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography. J Cataract Refract Surg 2006;32:1843-1850.
[53]Koch D, Wang L. Calculating IOL power in eyes that have had refractive surgery [editorial]. J Cataract Refract Surg 2003;29:2039-2042.
[54]Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17-29.
[55]Smith RJ, Chan W-K, Maloney RK. The prediction of surgically induced refractive change from corneal topography. Am J Ophthalmol 1998; 125:44-53.
[56]Awwad ST, Manasseh C, Bowman RW, et al. Intraocular lens power calculation after myopic laser in situ keratomileusis:estimating the corneal refractive power. J Cataract Refract Surg 2008; 34:1070-1076.
[57]Ferrara G, Cennamo G, Marotta G,et al. New formula to calculate corneal power after refractive surgery. J Refract Surg 2004; 20:465-471.
[58]Rosa N, Capasso L, Lanza M, et al. Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery. J Cataract Refract Surg 2005;31:1020-1024.
[59]Tang Q, Hoffer KJ, Olson MD,et al. Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery. J Cataract Refract Surg.2009 Jul;35 (7):1198-203.
[60]Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery:BESSt formula. J Cataract Refract Surg 2006; 32:2004-2014.
[61]Jin H, Holzer MP, Rabsilber T,et al. Intraocular lens power calculation after laser refractive surgery:corrective algorithm for corneal power estimation. J Cataract Refract Surg.2010 Jan;36 (1):87-96.
[62]Cheng AC, Ho T, Lau S,et al. Introducing a new ratio useful in the estimation of preoperative corneal power in patients with myopic LASIK based on postoperative corneal data. Cornea.2009 Jan;28 (1):1-4.
[63]Ianchulev T, Salz J, Hoffer K, et al. Intraoperative optical refractive biometry for intraocular lens power estimation without axial length and keratometry measurements. J Cataract Refract Surg 2005; 31:1530-1536.
[64]Mackool RJ, Ko W, Mackool R. Intraocular lens power calculation after laser in situ keratomileusis:aphakic refraction technique.J Cataract Refract Surg 2006; 32:435-437.
[65]Kenneth J. Hoffer. Intraocular lens power calculation after previous laser refractive surgeryJ Cataract Refract Surg 2009; 35:759-765.