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Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry

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Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry
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  Central and peripheral corneal thicknessmeasurement with Orbscan II and topographicalultrasound pachymetry   Jose Manuel Gonza´lez-Me´ijome, OD, Alejandro Cervin˜o, OD, Eva Yebra-Pimentel, OD, PhD,Manuel A. Parafita, MD, PhD  Purpose:  To compare thickness measurements of the central 6.0 mm of the cor-nea obtained with the Orbscan   II topography system and topographical ultra-sound pachymetry. Setting:  School of Optometry, University of Santiago de Compostela, Galicia,Spain.  Methods:  In 24 right eyes, pachymetric measurements were taken at the centerand 1.2 mm and 3.0 mm on the superior and inferior hemimeridians. A 1-sample t   test was applied to assess the significance of the relationship between OrbscanII and ultrasound methods. The relationship between the 2 was assessed by ana-lyzing regression and plotting the differences against the mean corneal thickness.Orbscan II data were analyzed in 3 ways: (1) without the application of an acous-tic equivalent correction factor; (2) with a correction factor of 0.92, as recom-mended by the manufacturer; (3) with correction using the equations derived inthis study. The data were systematically compared with those of ultrasoundpachymetry.  Results:  Before the correction factor was applied, the Orbscan II overestimatedthe corneal thickness at all locations, with the mean difference (48.15  m  33.74 [SD]) significantly different from zero (  P  .001). Differences increased to-ward the periphery, and the reliability of Orbscan II readings seemed to decreasewith thicker corneas. After the acoustic equivalent was applied, the differenceswere significantly less; however, this effect did not seem clinically significant aslarge differences remained. When specific corrective equations were applied foreach corneal location, the level of agreement between Orbscan II and ultrasoundpachymetry improved substantially; the mean (–0.11  15.22  m) was not statis-tically different from zero (  P  .05). Conclusions:  The acoustic equivalent correction factor proposed by the manu-facturer to obtain corneal thickness measurements with the Orbscan II comparedto those from ultrasound pachymetry was not valid for all corneal topography po-sitions. Orbscan II measurements agreed better with those of ultrasound pachym-etry when equations for the central and each peripheral location across thetopography were applied.  J Cataract Refract Surg 2003; 29:125–132 © 2003 ASCRS and ESCRS C orneal thickness measured by pachymetry is a sen-sitive indicator of corneal health. One of the mostcommon approaches to corneal pachymetry is ultra-sound technology. Corneal thickness measured by pachymetry is now considered essential in the diagnosisof multiple corneal diseases. Its use can avoid complica-tions that may result from refractive surgery procedures.However,themostcommonlyusedmethodsofmeasur-ing corneal thickness—ultrasound and optical pachym-etry—may be adversely influenced by changes in tissuehydration caused by ultrasound speed through the cor-nea. 1,2 This can decrease corneal transparency, creating  © 2003 ASCRS and ESCRS 0886-3350/03/$–see front matterPublished by Elsevier Science Inc. doi:10.1016/S0886-3350(02)01815-1  a significant handicap to performing accurate measure-ments with optical pachymetry in some situations. 3,4 Today,moresophisticateddevicesareavailableformea-suring central and peripheral corneal thickness. 5 – 10 Earlier comparisons of optical and ultrasound cor-neal thickness measurements studied only central mea-surements, and there was a slight tendency forultrasound methods to underestimate corneal thick-ness. 2,11 However, ultrasound pachymetry is less af-fected by error than optical measurement and has thebenefit of portability. 12,13 Both measurement principles have been applied todevelop modern devices that map the thickness of theentire cornea. These include high-frequency ultrasoundbiomicroscopy and Orbscan ® topography, which aremainly used for screening and follow-up after refractivesurgery procedures. The Orbscan II topography system(Orbtek, Bausch & Lomb Surgical) uses the lateral dis-placement of 2 slit beams and a video camera. It takesseveral images of different corneal sections for 3-dimen-sional reconstruction of corneal tissue, mapping the an-terior and posterior corneal surfaces as well as the fullcorneal thickness. A study comparing Orbscan and ultrasoundpachymetry found systematic differences in central cor-neal measurements, with ultrasound readings being 23to 28  m smaller. 10 However, that study used the firstversion of the apparatus, Orbscan I, which did not usethe acoustic equivalent to convert the actual measure-ment of the Orbscan system to values that could becomparable to those obtained with ultrasound pachym-etry. Similar results were recently reported when Orb-scan measurements were compared with optical andultrasound pachymetric measurements; although thestudy did not tell which Orbscan version was used, itseems as though it was the first. 14 In contrast, a study comparing Orbscan II with ultrasound pachymetry found that ultrasound pachymetry was consistently thickerwhenanacousticequivalentof0.92wasusedforactual Orbscan II readings. 15  A more recent study showedthatOrbscanIIandultrasoundcentralpachym-etry were comparable when the acoustic factor wasconsidered. 3 These controversies must be resolved as inaccuratedetermination of corneal thickness could lead to seriouscomplications in refractive surgery procedures. Under-estimating corneal thickness can lead to the exclusion of patients who may be candidates for primary proceduresor enhancements in refractive surgery. Overestimating corneal pachymetry can lead to inadvertent thinning of the stromal bed, increasing the risk of secondary keratectasia. 16 These comparative studies considered only the cen-tral cornea; however, clinical procedures such as refrac-tive surgery affect part of the peripheral cornea. Today,many refractive surgeons rely only on Orbscan pachym-etry measurements to determine a patient ’ s suitability for laser in situ keratomileusis. Although the main objective of the new pachymet-ric devices is to offer complete corneal-thickness data,the precision and accuracy of peripheral measurementshave not been fully investigated. It is also important toknowtheimpactofcornealthicknessonOrbscanIIandultrasoundpachymetrymeasurementstodetectwhetherone instrument is biased in favor of thinner corneas orthicker corneas.Thus, we compared peripheral corneal thicknessmeasurements taken with the Orbscan II with thosetaken by topographical ultrasound pachymetry. Patients and Methods Twenty-four subjects participated in the study. The 10men and 14 women ranged in age from 21 to 26 years. Inclu-sion criteria were no ocular disease, no complaints of ocularirritation, no history of contact lens wear, and no cornealfluorescein staining or anterior segment abnormality onbiomicroscopic examination. After the procedures were ex-plained, the subjects signed an informed consent form and were enrolled in the study.  Accepted for publication August 21, 2002.From the Department of Physics, School of Sciences, University of   Minho, Braga, Portugal (Gonza´lez-Me ´ijome), and the Departments of  Surgery (Cervin˜o, Parafita) and Applied Physics (Yebra-Pimentel),School of Optics and Optometry, University of Santiago de Compostela,Galicia, Spain.Presented in part at the 32nd Congress of the European Society of Con-tact Lenses, Santander, Spain, June 2001.Supported by MEC (Spain)–PGC under contract PM 98–0225.Noneoftheauthorshasafinancialorproprietaryinterestinanymaterial or method mentioned.Reprint requests to Prof. Dr. Manuel Parafita, Escuela de O ´  ptica y Optometrı´a, Universidad de Santiago de Compostela, 15782 Santiagode Compostela, Galicia, Spain. E-mail: ciparaft@usc.es. ORBSCAN II VERSUS PERIPHERAL ULTRASOUND PACHYMETRY   J CATARACT REFRACT SURG — VOL 29, JANUARY 2003 126  Because conclusions from the analysis of 1 meridiancouldbeextendedtoanymeridianinthecornea,onlyverticalmeasurements where taken to simplify the test for the sub- jects. Corneal thickness was assessed at 7 locations along the vertical meridian of the right cornea (visual center and1.0 mm, 2.0 mm, and 3.0 mm superiorly and inferiorly from center) with the Orbscan II and topographical ultra-soundpachymetry(Paxis ® ,BVIVision).AftertheOrbscanIImeasurements,thecorneawasanesthetizedwithtopicaltetra-caine solution and topographical ultrasound pachymetry wasdone. Each result was the mean of 3 consecutive measure-ments. The ultrasound probe was sterilized and applied asperpendicular as possible to the central cornea. A slitlampexamination was performed to ensure that no corneal damageresulted from the application of the probe. To ensure preci-sion, all measurements were done by the same investigator(J.M.G.M.) and ultrasound measurements were obtained with the aid of a fixation panel. 17  As corneal thickness measurements taken by ultrasoundpachymetry can be adversely affected by fluctuations in tissuehydration, contact lens users were excluded from the study and all measurements were done between 4:00 and 8:00  PM .This is considered the time of day when the eye is most phys-iologically stable so that hydration, which affects US speed, would be more homogeneous.For Orbscan II measurements, no acoustic equivalentcorrection was used. However, a correction factor of 0.92 wasapplied during a subsequent analysis to allow comparison of the results with those in other studies and to estimate theclinical relevance of the factor.Data were analyzed using SPSS (version 10.0). The dif-ferencesbetweenOrbscanIIandultrasoundpachymetermea-surements were tested for statistical significance using theStudent paired  t   test and independent-samples  t   test againstzero. The level of significance was  P   .05. Linear regression was used to quantify the correlation between the 2 methodsand to extract a valid equation for deriving accurate values of corneal thickness from the Orbscan II; this analysis was per-formed separately for the central area and the superior andinferior areas 1.0 mm, 2.0 mm, and 3.0 mm from the center.Thedifferenceswereplottedagainstthemeanstoestimatetheinfluence of the measuredvalue on the difference between themethods. Results Table 1 shows the means and standard deviationsfor the 7 corneal positions before and after the applica-tion of the correction value to obtain the acoustic equiv-alent for the Orbscan II measurement. The meandifferences are also shown. Without applying the acoustic equivalent correc-tion, the Orbscan measurements were systematically higher than ultrasound pachymetry measurements andthe differences between the methods were larger the far-ther from the center of the cornea. When the acousticequivalent correction was applied, the mean differencedecreased between 45 and 55  m. In this case, the Orb-scan II gave lower values than ultrasound at the centerand at 1.0 mm. Although there was no statistically significant difference at 2.0 mm, the Orbscan IIhad a tendency to overestimate corneal thickness at3.0 mm. Table 1.  Mean (  SD) ultrasound pachymetry values and Orbscan II pachymetry values with and without the manufacturer-recommendedcorrection factor. CornealLocationMean Corneal Thickness (   m)Without FactorMean Corneal Thickness (   m)with Factor*US Orbs Mean Diff Orbs Mean Diff Superior3.0 mm 558  40 683  49 95 629  46 402.0 mm 566  42 620  48 54 571  45 41.0 mm 551  38 574  49 23 528  45   23Center 545  40 560  48 15 515  45   30Inferior3.0 mm 548  39 572  48 24 527  44   212.0 mm 562  43 608  47 45 559  43   31.0 mm 577  45 659  47 81 606  43 28US  ultrasound pachymetry; Orbs  Orbscan II pachymetry; Diff   difference*0.92  Orbscan II thickness ORBSCAN II VERSUS PERIPHERAL ULTRASOUND PACHYMETRY   J CATARACT REFRACT SURG — VOL 29, JANUARY 2003  127  Figure 1A shows the relationship between the 2methods for superior and inferior measurements beforetheapplicationoftheacousticequivalentcorrectionfac-tor of 0.92 and Figure 1B, after the application of thefactor. The linear regression lines illustrate the resultsshown in Table 1. As an acoustic equivalent factor wasapplied to all data, linear regression was numerically equal in terms of the coefficient of determination ( r  2 ), with a different equation for the linear regression line.Thus, the lines approach the 45-degree line, which rep-resents a perfect correlation between the pachymetricmethods. This is particularly evident at the 1.0 mm in-feriorposition,whichshowsabettercorrelationthanthecentralpositionwithahighcoefficientofdetermination( r  2  0.935). Except at the most superior positions, the r  2 values indicated a good correlation between ultra-sound and Orbscan II pachymetry.Figures 1A and 1B also show the distribution of thecorneal thickness in all 24 eyes. Using ultrasoundpachymetry as a reference, closer analysis revealed a  Figure 1A.  (Gonza´lez-Me´ijome) Regression lines between ultra-sound and Orbscan II pachymetry at superior (  A  ) and inferior ( B  )locations. Figure 1B.  (Gonza´lez-Me´ijome) Regression lines between ultra-sound and Orbscan II pachymetry after the application of an acousticequivalent correction factor of 0.92 at superior (  A  ) and inferior ( B  )locations. ORBSCAN II VERSUS PERIPHERAL ULTRASOUND PACHYMETRY   J CATARACT REFRACT SURG — VOL 29, JANUARY 2003 128  group with central ultrasound pachymetry below 527  m (n  14) and another with central ultrasoundpachymetry over 576  m (n  10).Figure 2 shows the difference between methodsplottedagainstthemeanvalues.Thethinnerandthickercorneas had a different distribution of thickness. Figure2,  A  and  B  , shows a comparison of values before andafter application of the acoustic equivalent correction toOrbscan measurements, respectively. Most values were within the 95% limit of agreement between the 2 meth-ods. The mean difference (48.15  m) was significantly different from zero when no acoustic equivalent wasconsidered ( t     18.5;  P     .001, independent  t   test).The mean difference ( – 0.72   m) was not statistically different from zero when the acoustic equivalent correc-tion factor of 0.92 was considered ( t     0.31;  P    .757).Thedistributionofdatachangedlittleafterthecon-version of Orbscan II values, with a slight displacementof all data around 50   m on the vertical axis, corre-spondingtotheeffectoftheacousticequivalentfactorof 0.92, and around 25  m on the horizontal as a result of the average between corrected Orbscan measurementsand ultrasound pachymetry. Thus, the 95% confidenceinterval (114.28 to   17.98) was less before acousticequivalent correction than after correction ( – 59.23 to57.87).Therewasgreaterdispersionofdatainthegroup with thicker corneas, which indirectly means that thelevel of agreement between the 2 pachymetric tech-niques could depend on the magnitude of thickness tobe measured. Table 2 shows a numerical estimation of this effect.Before applying the corneal factor, the differencesbetween the 2 instruments were greater with thickercorneas (greater error). After the factor was applied, theerrorsforthickerandthinnercorneasbecomemoresim-ilar (thickness measured factor becomes less important). Although differences between thinner and thicker cor-neas for the 2.0 mm and 3.0 mm peripheral locationsdid not change much, the central and 1.0 mm measure-ments benefited with the conversion, resulting in fewerdifferences in thicker corneas. As bias at different corneal locations was substan-tially different, a point-to-point analysis of regression was done to obtain expressions that would make data between Orbscan II and ultrasound pachymetry morecomparable. Orbscan values II were used as a refer-ence. When no acoustic equivalent was taken intoconsideration, the analysis provided the equationsshown in Table 3. When these equations were appliedto srcinal data, a new parameter — the Orbscan ul-trasound equivalent —  was obtained. The results show a closer correlation between techniques, with a meanthickness of 562.62  m  40.23 (SD) for the Orb-scan II ultrasound equivalent (610.89    64.08   mbefore correction) and 562.73  43.08  m for ultra-sound pachymetry. Moreover, the mean difference Figure 2.  (Gonza ´  lez-Me ´  ijome) Plots of the differences in cornealthickness between ultrasound pachymetry and Orbscan II before (  A  )and after ( B  ) the application of the acoustic equivalent correctionfactor recommended by the manufacturer. ORBSCAN II VERSUS PERIPHERAL ULTRASOUND PACHYMETRY   J CATARACT REFRACT SURG — VOL 29, JANUARY 2003  129
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