Using LaserSight Astrapro Planner 2.2 Z software in corneal topography-guided laser in situ keratomileusis for myopia with asymmetric corneal shape

AIM:To determine the clinical outcomes of laser insitu keratomileusis(LASIK)treatments using LaserSight AstraPro Planner 2.2 Z software for myopia with asymmetric corneal shape.METHODS:Four hundred and eighty-five eyes[243patients;spherical equivalent(SE),-5.93±1.88 diopters(D)]were treated with asymmetric ablations using LaserSight SLX laser(version 5.3,300Hz)were retrospectively analyzed and LaserSight AstraPro Planner2.2 Z software.Preoperative and postoperative uncorrected visual acuities(UCVA),spherical equivalent(SE)refraction,pachymetry,and corneal asphericity(Q value)and decentration were evaluated.RESULTS:At 12mo postoperatively,the decimal UCVA was 1.0 or better in 449(92.6%)eyes.Two eyes(0.4%)lost 1 line of the decimal best spectacle-corrected visual acuity(BCVA),316(65.2%)did not change,149(30.7%)gained 1 line,and 18(3.7%)gained 2 lines or more after surgery.Three hundred and thirty-two eyes(68.5%)were within 0.5 D in SE.The mean tissue saving ablation depth was 4.28±2.86(0-16)μm(median,4μm).The mean attempted remaining central corneal thickness was435.79±29.56μm,the mean postoperative pachymetry was 444.94±28.93μm.The mean preoperative Q value was-0.19±0.18,the postoperative was 0.30±0.48(P=0.000).The mean postoperative decentration was 0.39±0.19 mm.CONCLUSION:Topography-guidedLASIKwithAstraPro Planner 2.2 Z custom ablation planning software in an asymmetric ablation mode was an effective,safe,predictable,and stable refractive procedure for the myopia with asymmetric corneal topography.

Validity of autorefractor based screening method for irregular astigmatism compared to the corneal topography-a cross sectional study

AIM：To present a method of screening for irregular astigmatism with an autorefractor and its determinants compared to corneal topography.METHODS：This cross-sectional validity study was conducted in 2013 at an eye hospital in Spain.A tabletop autorefractor（test 1）was used to measure the refractive status of the anterior surface of the cornea at two corneal meridians of each eye.Then corneal topography（test 2）and Bogan’s classification was used to group eyes into those with regular or no astigmatism（GRI）and irregular astigmatism（GRII）.Test 1 provided a single absolute value for the greatest cylinder difference（Vr）.The receiver operating characteristic（ROC）were plotted for the Vr values measured by test 1 for GRI and GRII eyes.On the basis a Vr value of 1.25 D as cut off,sensitivity,specificity were also calculated.RESULTS：The study sample was comprised of 260 eyes（135 patients）.The prevalence of irregular astigmatism was 42%[95%confidence interval（CI）：36,48].Based on test 2,there were 151 eyes in GRI and 109 eyes in GRII.The median Vr was 0.75 D（25%quartile,0.5 D）for GRI and1.75 D（25%quartile,1.25 D）for GRII.The area under curve was 0.171 for GRI and 0.83 for GRII.The sensitivity of test I was 78.1%and the specificity was 76.1%.CONCLUSION：A conventional autorefractor can be effective as a first level screening method to detect irregular corneal astigmatism in places where corneal topography facilities are not available.

Influence of overnight orthokeratology lens fitting decentration on corneal topography reshaping

Background:This retrospective study was designed to investigate the sole influence of orthokeratology(OK)lens fitting decentration on the Zernike coefficients of the reshaped anterior corneal surface.Methods:This study comprised a review of 106 right eyes and measurements of corneal topography both before OK and at 1-month follow-up visit.A routine was designed to calculate local corneal surface astigmatism and assist the determination of OK lens fitting decentration from pupil center.The pupil-centered corneal Zernike coefficients of baseline(PCCB)and post-treatment(PCCP)were calculated.Meanwhile,the OK-lens-centered corneal Zernike coefficients of post-treatment(OCCP)were also calculated and considered as the presumptive ideal fitting group without decentration.Relationships between lens fitting decentration and the change of Zernike coefficients including(PCCP−PCCB)and(PCCP−OCCP)were analyzed.Results:Patients with a mean age of 11±2.36 years old had an average spherical equivalent refractive error of−3.52±1.06 D before OK.One month after treatment,OK lens fitting decentration from pupil center was 0.68±0.35 mm.RMS of 3rd-order(P<0.05),RMS of 4th-order(P<0.001)and RMS of total high order(P<0.001)corneal Zernike coefficients were increased in PCCP by comparing with OCCP,which was solely caused by lens fitting decentration.Nevertheless,no significant difference was observed in C^(0)_(2)(P>0.05).For the high order corneal Zernike coefficients in(PCCP–OCCP),radial distance of decentration was correlated with C^(−1)_(3)(r=−0.296,P<0.05),C^(1)_(3)(r=−0.396,P<0.001),and C^(0)_(4)(r=0.449,P<0.001),horizontal decentration was significantly correlated with C^(1)_(3)(r=0.901,P<0.001)and C^(1)_(5)(r=0.340,P<0.001),and vertical decentration was significantly correlated with C^(−1)_(3)(r=0.904,P<0.001).Conclusions:OK lens fitting decentration within 1.5 mm hardly influenced the change of corneal spherical power for myopia correction,but significantly induced additional corneal high order Zernike coefficients including C^(−1)_(3),C^(1)_(3),C^(0)_(4),and C^(1)_(5).

Accuracy and reliability of orthogonal polynomials in representing corneal topography

Fitting of corneal topography data to analytical surfaces has been necessary in many clinical and experimental applications,yet absolute superiority of fitting methods was still unclear,and their overfitting risks were not well studied.This study aimed to evaluate the accuracy and reliability of orthogonal polynomials as fitting routines to represent corneal topography.Four orthogonal polynomials,namely,Zernike polynomials(ZPs),pseudo-Zernike polynomials(PZPs),Gaussian-Hermite polynomials(GHPs)and Orthogonal Fourier-Mellin polynomials(OFMPs),were employed to fit anterior and posterior corneal topographies collected from 200 healthy and 174 keratoconic eyes using Pentacam topographer.The fitting performance of these polynomials were compared,and the potential overfitting risks were assessed through a prediction exercise.The results showed that,except for low orders,the fitting performance differed little among polynomials with orders
10 regarding surface reconstruction(RMSEs~0.3μm).Anterior surfaces of normal corneas were fitted more efficiently,followed by those of keratoconic corneas,then posterior corneal surfaces.The results,however,revealed an alarming fact that all polynomials tended to overfit the data beyond certain orders.GHPs,closely followed by ZPs,were the most robust in predicting unmeasured surface locations;while PZPs and especially OFMPs overfitted the surfaces drastically.Order 10 appeared to be optimum for corneal surfaces with 10-mm diameter,ensuring accurate reconstruction and avoiding overfitting.The optimum order however varied with topography diameters and data resolutions.The study concluded that continuing to use ZPs as fitting routine for most topography maps,or using GHPs instead,remains a good choice.Choosing polynomial orders close to the topography diameters(millimeters)is generally suggested to ensure both reconstruction accuracy and prediction reliability and avoid overfitting for both normal and complex(e.g.,keratoconic)corneal surfaces.

Placido disk-based topography versus high-resolution rotating Scheimpflug camera for corneal power measurements in keratoconic and post-LASIK eyes：reliability and agreement

AIM：To compare the repeatability/reproducibility of measurement by high-resolution Placido disk-based topography with that of a high-resolution rotating Scheimpflug camera and assess the agreement between the two instruments in measuring corneal power in eyes with keratoconus and post-laser in situ keratomileusis（LASIK）. METHODS：One eye each of 36 keratoconic patients and 20 subjects who had undergone LASIK was included in this prospective observational study. Two independent examiners worked in a random order to take three measurements of each eye with both instruments. Four parameters were measured on the anterior cornea：steep keratometry（Ks）,flat keratometry（Kf）,mean keratometry（Km）,and astigmatism（Ks-Kf）. Intra-examiner repeatability and inter-examiner reproducibility were evaluated by calculating the within-subject standard deviation（Sw）the coefficient of repeatability（R）,the coefficient of variation（Co V）,and the intraclass correlation coefficient（ICC）. Agreement between instruments was tested with the BlandAltman method by calculating the 95% limits of agreement（95% Lo A）.RESULTS：In keratoconic eyes,the intra-examiner and inter-examiner ICC were 〉0.95. As compared with measurement by high-resolution Placido disk-based topography,the intra-examiner R of the high-resolution rotating Scheimpflug camera was lower for Kf（0.32 vs 0.88）,Ks（0.61 vs 0.88）,and Km（0.32 vs 0.84）but higher for Ks-Kf（0.70 vs 0.57）. Inter-examiner R values were lower for all parameters measured using the high-resolution rotating Scheimpflug camera. The 95% Lo A were-1.28 to ＋0.55 for Kf,-1.36 to ＋0.99 for Ks,-1.08 to ＋0.50 for Km,and-1.11 to ＋1.48 for Ks-Kf. In the post-LASIK eyes,the intra-examiner andinter-examiner ICC were 〉0.87 for all parameters. The intra-examiner and inter-examiner R were lower for all parameters measured using the high-resolution rotating Scheimpflug camera. The intra-examiner R was 0.17 vs 0.88 for Kf,0.21 vs 0.88 for Ks,0.17 vs 0.86 for Km,and 0.28 vs 0.33 for Ks-Kf. The inter-examiner R was 0.09 vs 0.64 for Kf,0.15 vs 0.56 for Ks,0.09 vs 0.59 for Km,and 0.18 vs 0.23 for Ks-Kf. The 95% Lo A were-0.54 to ＋0.58 for Kf,-0.51 to ＋0.53 for Ks and Km,and-0.28 to ＋0.27 for Ks-Kf. CONCLUSION：As compared with Placido disk-based topography,the high-resolution rotating Scheimpflug camera provides more repeatable and reproducible measurements of Ks,Kf and Ks in keratoconic and post-LASIK eyes. Agreement between instruments is fair in keratoconus and very good in post-LASIK eyes.

Early visual function outcomes of topography-guided FS-LASIK and SMILE in treatment of myopia and myopic astigmatism

AIM: To compare the quality outcomes of vision at early phase after topography-guided femtosecond laser-assisted in situ keratomileusis(FS-LASIK-CV) and small incision lenticule extraction(SMILE) in treatment of myopia and myopic astigmatism.METHODS: Retrospective comparative analysis of 49 patients that underwent FS-LASIK(n=23) or SMILE(n=26) procedure for myopia and myopic astigmatism between April and September in 2019. Pre-and postoperative uncorrected visual acuity(UCVA), spherical equivalent refraction(SEQ), cylindrical refraction, contrast sensitivity function(CSF), and corneal higher-order aberrations(HOAs) were evaluated. Independent t-test was used for inter-group comparison, while repeated measures ANOVA was used to analyze changes at different time points. RESULTS: In both groups, 100% of the eyes obtained a UCVA of 20/20 or better at 1 wk, 1, and 3 mo postoperatively. At 1 d and 3 mo postoperatively, UCVA was better in FS-LASIKCV group than in SMILE group. At 1 wk postoperatively, SEQ was lower in SMILE group than in FS-LASIK-CV group(P=0.006). At 3 mo postoperatively, the SEQ reached target refraction in both groups. The residual astigmatism was reduced in both groups without intergroup difference(P>0.05). At 3 mo postoperatively, the spherical aberration and coma under 6 mm pupil size were higher than preoperative levels in both groups(P<0.05). However, the increase in the corneal HOAs in the FS-LASIK-CV group was less than the SMILE group(P<0.05). At 3 mo postoperatively, the log CS were better than preoperative levels under scotopic conditions without glare and scotopic conditions with glare in both groups(P<0.05). At 1 and 3 mo postoperatively, under scotopic conditions without glare and scotopic conditions with glare, FS-LASIK-CV group showed more improvement in log CS at two spatial frequencies(12.0 c/d and 18.0 c/d;P<0.05).CONCLUSION: Both FS-LASIK-CV and SMILE demonstrate to be safe, effective, and predictable in treatment of myopia and myopic astigmatism. Early postoperative improvement in UCVA and CSF at high spatial frequency under scotopic conditions were better after FS-LASIK-CV than SMILE.

Interchangeability of corneal curvature and asphericity measurements provided by three different devices

AIM: To evaluate the interchangeability of keratometric and asphericity measurements provided by three measurement systems based on different optical principles.METHODS: A total of 40 eyes of 40 patients with a mean age of 34.1 y were included. In all cases, a corneal curvature analysis was performed with IOL-Master(IOLM), iDesign 2(ID2), and Sirius systems(SIR). Differences between instruments for flattest(K1) and steepest(K2) keratometric readings, as well as for magnitude and axis of corneal astigmatism were analyzed. Likewise, differences in asphericity(Q) between SIR and ID2 were also evaluated. RESULTS: Mean differences between devices for K1 were 0.20±0.21(P<0.001),-0.12±0.36(P=0.046) and-0.32±0.36 D(P<0.001) for the comparisons IOLM-SIR, IOLM-ID2 and SIR-ID2, respectively. The ranges of agreement for these comparisons between instruments were 0.41, 0.70, and 0.70 D. For K2, mean differences were 0.31±0.33(P<0.001),-0.08±0.43(P=0.265) and-0.39±0.38 D(P<0.001), with ranges of agreement of 0.65, 0.84, and 0.74 D. Concerning magnitude of astigmatism, ranges of agreement were in the limit of clinical relevance(0.49 D, P=0.011; 0.55 D, P=0.386; 0.43 D, P=0.05). In contrast, ranges of agreement were clinically relevant for astigmatic axis(26.68o, 33.83o and 18.37o, P≥0.121) and for Q between SIR and ID2(0.16, P<0.001). CONCLUSION: The keratometric corneal power, astigmatic axis and asphericity measurements provideby the three systems evaluated cannot be considered as interchangeable, whereas measurements of corneal astigmatism obtained with SIR and ID2 can be considered as interchangeable for clinical purposes.

Edge detection and mathematic fitting for corneal surface with Matlab software

AIM：To select the optimal edge detection methods to identify the corneal surface,and compare three fitting curve equations with Matlab software. METHODS：Fifteen subjects were recruited. The corneal images from optical coherence tomography（OCT）were imported into Matlab software. Five edge detection methods（Canny,Log,Prewitt,Roberts,Sobel）were used to identify the corneal surface. Then two manual identifying methods（ginput and getpts）were applied to identify the edge coordinates respectively. The differences among these methods were compared. Binomial curve（y=Ax2＋Bx＋C）,Polynomial curve [p（x）=p1xn＋p2x（n-1）＋....＋pnx＋pn＋1] and Conic section（Ax2＋Bxy＋Cy2＋Dx＋Ey＋F=0）were used for curve fitting the corneal surface respectively. The relative merits among three fitting curves were analyzed. Finally,the eccentricity（e）obtained by corneal topography and conic section were compared with paired t-test. RESULTS：Five edge detection algorithms all had continuous coordinates which indicated the edge of the corneal surface. The ordinates of manual identifying were close to the inside of the actual edges. Binomial curve was greatly affected by tilt angle. Polynomial curve was lack of geometrical properties and unstable. Conic section could calculate the tilted symmetry axis,eccentricity,circle center,etc. There were no significant differences between ＇e＇ values by corneal topography and conic section（t=0.9143,P=0.3760 〉0.05）.CONCLUSION：It is feasible to simulate the corneal surface with mathematical curve with Matlab software. Edge detection has better repeatability and higher efficiency. The manual identifying approach is an indispensable complement for detection. Polynomial and conic section are both the alternative methods for corneal curve fitting. Conic curve was the optimal choice based on the specific geometrical properties.

Comparison of corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus

AIM:To evaluate the differences in corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus.METHODS:In our prospective clinical trial,40 patients(60 eyes)with progressive keratoconus were randomized to undergo corneal cross-linking with transepithelial(TE group,n=30)or epithelium-off(EO group,n=30)keratoconus.Examinations comprised topography,corneal biomechanical analysis and specular microscopy at 6 mo postoperatively.RESULTS:The keratometer values were not significantly different between the TE and EO corneal cross-linked groups in different periods(each P>0.05).The corneal thickness of the EO group was greater than that of the TE group at 1 wk after the operation(each P<0.05).Regarding corneal biomechanical responses,the EO group showed a longer second applanation length than TE group(P=0.003).Regarding the corneal endothelial function,standard deviation of the endothelial cell size,and coefficient of variation in the cell area,the values of EO group were larger than those of TE group at 1 wk(P=0.011,0.026),and the percentage of hexagonal cells in EO group was lower than that in TE group at 1 and 6 mo(P=0.018,0.019).CONCLUSION:Epithelium-off corneal cross-linking may strengthen corneal biomechanics better than TE procedure can.However,the TE procedure with a lower ultraviolet-A irradiation intensity would be safer for corneal endothelial function.

Visual differences in topography-guided versus wavefront-optimized LASIK in the treatment of myopia: a Meta-analysis

AIM: To investigate the potential differences between topography-guided(TG) and wavefront-optimized(WFO) laser in situ keratomileusis(LASIK) for the treatment of myopia.METHODS: A systematic literature search was performed to determine relevant trials comparing LASIK with TG and WFO from the time of library construction to August 2020, and The PubMed, Cochrane, Web of Science, EMBASE and Chinese databases(i.e. CNKI, CBM, WAN FANG and VIP) were accessed. The data on visual acuity, refractive status and wavefront aberration were retrieved and evaluated from three to six months after surgery. STATA(version 14.0) software was used for statistical analysis. A cumulative Meta-analysis was simultaneously performed.RESULTS: Eleven studies with a total of 1425 eyes were incorporated. No statistically significant differences were evident between TG and WFO ablation in the proportion of eyes achieving an uncorrected distance visual acuity(UCVA) of 20/20 or better(P=0.377), gaining one line or more(P=0.05), postoperative cylinder(P=0.40), vertical coma(P=0.593) and horizontal coma(P=0.957). After TG ablation, the proportion of the patients’ eyes of which postoperative refraction is within ±0.5 diopter of the target refraction was significantly higher than that undergoes WFO(P=0.003). As opposed to the WFO group, manifest refraction spherical equivalent(MRSE;P=0.000) was lower, and UCVA(P=0.005) was better in the TG group. The higher-order aberrations(HOAs;P=0.000), spherical aberration(P=0.000) and coma(P=0.000) were significantly lower in TG group. The cumulative Meta-analysis illustrated that the proportion of eyes achieving UCVA of 20/20 or better, postoperative refraction within ±0.5 diopter, and MRSE has steady between the two groups.CONCLUSION: Both TG-LASIK and WFO-LASIK are safe, effective, and predictable for correcting myopia. TG-LASIK may produce fewer aberration and is more precise than WFO-LASIK.

Analysis of corneal morphologic and pathologic changes in early-stage congenital aniridic keratopathy

AIM：To determine typical corneal changes of congenital aniridic keratopathy（CAK）using corneal topography and confocal systems,and to identify characteristics that might assist in early diagnosis.METHODS：Patients with CAK and healthy control subjects underwent detailed ophthalmic examinations including axial length,corneal thickness,tear film condition,corneal topography,and laser-scanning in vivo confocal microscopy（IVCM）. RESULTS：In early stage aniridic keratopathy,Schirmer I test（SIT）,break-up time（BUT）,mean keratometry（mean K）and simulated keratometry（sim K）were reduced relative to controls（P〈0.05）,while simulation of corneal astigmatism（sim A）and corneal thickness were increased（P〈0.05）. In addition,significantly more eyes exhibited flat cornea compared with the control group. Inflammatory dendritic cells were present in the aniridic epithelium,with significantly increased density relative to controls（P〈0.05）. Palisade ridge-like features and abnormal cell morphology were observed in six out of sixteen CAK cases. In central cornea area,the aniridic corneas had the increased subbasal nerve density. CONCLUSION：These changes in corneal morphology in borderline situations can be useful to confirm the diagnosis of CAK.

Three-year follow-up in advanced pediatric keratoconus:thin corneas may not have pachymetry properly assessed after crosslinking

AIM:To analyze the crosslinking(CXL)effects in pediatric keratoconus,and to identify the patients’corneal characteristics whose pachymetry could not be adequately evaluated by Scheimpflug method after procedure.METHODS:Consecutive pediatric patients with progressive keratoconus underwent CXL were included.Best-corrected visual acuity(BCVA)and spheric equivalent(SE)were measured before and after CXL.After CXL,groups 1 and 2 were divided based on the posterior surface Pentacam quality specifications(QS):"OK"(Group 1)and"not OK"(Group 2).The mean(RmF and RmB)and minimum(RminF and RminB)radius of curvatures of the anterior and posterior corneal surfaces,and the thinnest pachymetry(Pmin)were measured preoperatively at 3,6,12,24,and 36 mo.Haze was annotated.RESULTS:Twenty-six patients(14 men,mean age 14±1.8 y)and median Kmax of 59.9 D initially and 61.4 D preoperatively were treated.BCVA was not different before and 24 mo after CXL.Group 2 statistically differed to group 1 in that SE was more myopic before and with no difference 24 mo after CXL;RmF and RmB were steeper and Pmin was thinner pre-surgically.Group 2,in which pachymetric changes could not be adequately evaluated after surgery,presented with significant RmF flattening,a shift to hyperopia,and more haze after CXL.CONCLUSION:Patients whose pachymetry could not be adequately evaluated after CXL had steeper and thinner corneas before surgery.The predictive factors for impaired QS after CXL are RmF,RmB,and Pmin.In advanced keratoconus,alternative methods to analyze pachymetry and the posterior surface should be considered.

AB092.Database for the anatomopathological,functional and surgical characterization of the cornea

Background:The purpose of this infrastructure is to provide to the Network researchers a database and diverse related tools for the anatomical and functional analysis of the normal,pathological and surgical cornea.Methods:This database is composed of normal and pathological individuals,totaling more than 36,000 patients.It includes anatomical and functional imaging data,physiological optics data,psychometric and clinical data(medical history,surgical parameters,acuteness,etc.).Various corneal topography tools were added,giving the database a unique character:tools for analyzing individual maps,average map tools for the study and comparison of populations,3D modeling and visualization tools,statistical tools,etc.There are also screening tools for detecting various corneal conditions(LASIK,PRK,RK,keratoconus)and for secure data exchange between colleagues.Results:Several studies were made in recent years thanks to this common infrastructure.For example,this database has provided important information regarding the evolution of the 3D shape of the normal cornea with age and ametropia and has confirmed the mirror symmetry of corneas for the right and the left eyes(enantiomorphism).The different stages of Fuchs’dystrophy were also characterized to provide essential knowledge for surgery of the posterior layer of the cornea.Our database also allowed studying the anatomy of the wounds and the shape of the cornea before and after a transfixing transplant or an endothelial transplant(DSAEK and DSEK).The data on the characterization of experimentally transplanted corneas with corneal equivalents generated by tissue engineering and the recent addition of clinical data on the replacement of a diseased cornea with a synthetic corneal equivalent(keratoprosthesis)also resulted in several publications.More recently,the database has allowed to develop innovative algorithms to determine the optimal shape of an implant according to the clinical parameters of the recipient.On the other hand,we also demonstrated that the 3D shape of the cornea can be used as a biometric characteristic(such as fingerprints)for identification of individuals for various applications ranging from forensics to secure border crossings.Consequently,a new multimodal database(cornea+iris+eventually retina)was created for the purpose of biometric identifications.This database provides a unique set of anatomical and functional tools for the analysis of the cornea.It is characterized by the scientific quality and large quantity of accumulated information on the cornea and the high-level tools to exploit its content.Conclusions:The common infrastructure is easily accessible to all VHRN members on request.The database will also be accessible online in 2018(see http://cvl.concordia.ca for more information).

Comparative results of corneal curvature measurements with three different keratometers after radial keratotomy

Background Corneal curvatures measured by conventional instruments after refractive surgeries were greater than the real values. This study on the instruments and methods for measuring corneal curvature was carried out with the aim of determining an accurate, simple and effective method to evaluate corneal refractive power after radial keratotomy （RK）. Methods A retrospective study was carried out on 52 eyes of 42 patients with a history of RK. The postoperative measurements after RK were performed with a manual keratometer （Rodenstock, Munchen-Hamburg, Germany）, a corneal topographer （Tomey TMS-1） and an automated IOLMaster keratometer （Carl Zeiss GmbH, USA）. The results obtained from analyzing clinical histories （clinical history method, CHM） were compared to the postoperative measurements with a two-way analysis of variance. Adjusted keratometry was used to correct the corneal central refractive power （K） values of the three keratometers. The CHM results were also compared to the corrected K values obtained with the three keratometers using the same statistical method. Results The smallest mean value was found for the CHM （（36.61±2.23） diopters）. The K values obtained with the CHM were very different from the results obtained with the three different instruments. The corrected K values obtained with the IOLMaster were the closest to the K values from the CHM. Conclusions It was shown that K values from measurements with the three instruments were higher. When the power of the intraocular lens of the patients after RK was calculated, the errors of the K values measured with the instruments should be taken into consideration.

Corneal topographic analysis after excimer photorefractive keratectomy

Objective To evaluate the corneal surface changes and visual quality after excimer photorefractive keratectomy (PRK) for myopia. Methods Corneal topographic analysis was performed on 23 patients (38 eyes) with myopia after PRK at the 1 , 3 , 6 month postoperative follow up visits. Results The corneal sphericity was changed after excimer PRK. As time went on, the mean surface regularity index (SRI) and the surface asymmetry index (SAI) decreased gradually, and the corneal surface became more smooth; mean simulated keratoscope readings (SimK) showed a gradual restoration. At the 6 month postoperative examination, corneal topography showed four main patterns of ablation: round or oblong (50%), collar button (23.68%), semicircular (18.42%) and central island (7.9%). Patterns of ablation were correlated with visual acuity. Conclusion The quantitative analysis of coneal topography is essential for evaluating corneal surface changes after PRK and helpful in the surgical design of PRK and in predicting the refractive outcome with greater precision.