evaluation of corneal thickness and topography in normal ...· evaluation of corneal thickness and
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Evaluation of corneal thickness and topography innormal eyes using the Orbscan cornealtopography system
Zuguo Liu, Andrew J Huang, Stephen C Pflugfelder
AbstractAimsTo map the thickness, elevation(anterior and posterior corneal surface),and axial curvature of the cornea innormal eyes with the Orbscan cornealtopography system.Methods94 eyes of 51 normal subjectswere investigated using the Orbscan cor-neal topography system. The anterior andposterior corneal elevation maps wereclassified into regular ridge, irregularridge, incomplete ridge, island, and un-classified patterns, and the axial powermaps were grouped into round, oval, sym-metric bow tie, asymmetric bow tie, andirregular patterns. The pachymetry pat-terns were designated as round, oval,decentred round, and decentred oval.ResultsThe thinnest point on the corneawas located at an average of 0.90 (SD 0.51)mm from visual axis and had an averagethickness of 0.55 (0.03) mm. In 69.57% ofeyes, this point was located in the infero-temporal quadrant, followed by the su-perotemporal quadrant in 23.91%, theinferonasal quadrant in 4.35%, and thesuperonasal quadrant in 2.17%. Amongthe nine regions of the cornea evaluated(central, superotemporal, temporal, in-ferotemporal, inferior, inferonasal, nasal,superonasal, and superior) the centralcornea had the lowest average thickness(0.56 (0.03) mm) and the superior corneahad the greatest average thickness (0.64(0.03) mm). The mean simulatedkeratometry (SimK) was 44.24 (1.61)/43.31(1.66) dioptres (D) and the mean astigma-tism was 0.90 (0.41) D. Island (71.74%) wasthe most common elevation pattern ob-served in the anterior corneal surface, fol-lowed by incomplete ridge (19.57%),regular ridge (4.34%), irregular ridge(2.17%), and unclassified (2.17%). Island(32.61%) was the most common topo-graphic pattern in the posterior cornealsurface, following by regular ridge(30.43%), incomplete ridge (23.91%), andirregular ridge (13.04%) patterns. Sym-metric bow tie was the most common axialpower pattern in the anterior cornea(39.13%), followed by oval (26.07%), asym-metric bow tie (23.91%), round (6.52%),and irregular (4.53%) patterns. In thepachymetry maps, 47.83% of eyes had anoval pattern, and round, decentred oval,
and decentred round were observed in41.30%, 8.70%, and 2.18% of eyes, respec-tively.ConclusionThe information on regionalcorneal thickness, corneal elevation andaxial corneal curvature obtained with theOrbscan corneal topography system fromnormal eyes provides a reference for com-parison with diseased corneas. The Orb-scan corneal topography system is a usefultool to evaluate both corneal topographyand corneal thickness.(Br J Ophthalmol 1999;83:774778)
Measurement of shape, refractive power, andthe thickness of the cornea are very importantfor designing vision correction surgeries anddiagnosing corneal diseases. The majority ofthe commercial computerised videokeratogra-phy instruments measure the topography ofthe anterior corneal surface, which accountsfor the majority of the refractive power of thecornea. However, the posterior surface of thecornea also contributes to its refractive power.1
Corneal thickness is an important factor toevaluate corneal barrier and endothelial pumpfunction.24 Accurate measurement of cornealthickness is helpful in the diagnosis of cornealdiseases and avoidance of keratorefractive sur-gery complications. Hence, an instrument thatevaluates all of these factors will be very usefulfor clinical practice.
The Orbscan corneal topography system is arecently developed device that evaluates ante-rior and posterior corneal surface topographyas well as the thickness of the entire cornea.This instrument has the potential to greatlyincrease our knowledge about corneal shapeand function in health and disease. Thepurpose of this study was to establish normalvariables for elevation and curvature topogra-phy of the anterior and posterior cornea, aswell as thickness of the entire cornea with theOrbscan corneal topography system.
Subjects and methodsThis study evaluated 94 eyes (46 right eyes, 48left eyes) of 51 subjects (24 male and 27female). Normal subjects were defined as thosewho had no complaints of ocular irritation, nohistory of contact lens use, no anterior segmentabnormality on biomicroscopic examination,and a manifest refractive error of less than6.00 dioptres of myopia and less than 2.00dioptres of astigmatism with a best correctedvisual acuity of 20/20 or better. They wererecruited from employees of the Bascom
Br J Ophthalmol 1999;83:774778774
Ocular Surface andTear Center, BascomPalmer Eye Institute,University of MiamiSchool of Medicine,Miami, USAZ LiuA J HuangS C Pflugfelder
ZhongshanOphthalmic Center,Sun Yat-sen Universityof Medical Sciences.Guangzhou 510060,P R ChinaZ Liu
Correspondence to:Stephen C Pflugfelder,Bascom Palmer Eye Institute,900 NW 17th Street, Miami,FL 33136, USA.
Accepted for publication13 January 1999
on 27 July 2018 by guest. Protected by copyright.
r J Ophthalm
ol: first published as 10.1136/bjo.83.7.774 on 1 July 1999. Dow
Palmer Eye Institute and patients presentingfor routine eye examination.
The Orbscan corneal topography system(Orbscan, Inc, Salt Lake City, UT, USA) wasperformed by one individual in the followingmanner. The subjects chin was placed on thechin rest and the forehead rested against theforehead strap. The subject was instructed tolook at a blinking red fixation light. The exam-iner adjusted the optical head using a joystickto align and focus the eye so that the corneawas centred on the video monitor and both halfslits were seen on the cornea. The video imagewas then captured.
The Orbscan corneal topography systemmeasures anterior and posterior corneal eleva-tion (relative to a best fit sphere), surfacecurvature, as well as corneal thickness using ascanning optical slit device. The optical acqui-sition head scans the eye using light slits thatare projected at a 45 degree angle. Twenty slitsare projected sequentially on the eye from theleft and 20 slits from the right side for a total of40 slits. The instruments software analyses upto 240 data points per slit and calculates theaxial curvature (mm or dioptres) of theanterior and posterior corneal surface. It alsocalculates the elevation of the anterior andposterior surface of the cornea (relative to abest fit sphere) as well as the corneal thicknessof the entire cornea.
Pachymetry is determined by this instru-ment from the diVerence in elevation between
the anterior and posterior surface of thecornea. This instrument averages pachymetryin nine circles of 2 mm diameter that arelocated in the centre of the cornea and at eightlocations in the mid-peripheral cornea (supe-rior, superotemporal, temporal, inferotempo-ral, inferior, inferonasal, nasal, superonasal),each located 3 mm from the visual axis. TheOrbscan corneal topography system softwarealso identifies the thinnest point on the corneaand marks its distance from visual axis and itsquadrant location (superotemporal, inferotem-poral, superonasal, and inferonasal).
The colour coded maps showing the eleva-tion pattern of the anterior and posteriorsurface of cornea were classified according to aclassification scheme for anterior elevationmaps of normal corneas made with the PARcorneal topography system.5 This scheme clas-sifies maps into regular ridge, irregular ridge,incomplete ridge, island, and unclassifiedpatterns as shown in Figure 1. The axial curva-ture maps of the anterior corneal surface wereclassified into round, oval, symmetric bow tie,asymmetric bow tie, and irregular patterns,using a proposed classification scheme ofcorneal topography obtained with the TMS-1corneal topography system.6
Because there was no existing method forclassifying corneal pachymetry maps, thefollowing classification system was devised.The warmest colour in the pachymetry map,identifying the thinnest area of the cornea, wasused to designate one of four diVerentpatterns: round, oval, decentred round, anddecentred oval. These patterns are depicted inFigure 2. The following objective criteriadefined the categories used for this classifi-cation:
Round: the ratio of the shortest to the longestdiameter of the warmest colour zone was twothirds or greater, and the warmest colour waseither entirely located at the centre of the mapor more than half of the area of the warmestcolour entered the central 3 mm diameterzone.
Oval: the ratio of the shortest to the longestdiameter at the warmest colour zone was lessthan two thirds, and the warmest colour waseither entirely located at the centre of map ormore than half of area of the warmest colourentered the central 3 mm diameter zone.
Decentred round: the warmest colour wasround, and more than half of its area was out-side of the central 3 mm diameter zone or itwas entirely located in the peripheral cornea.
Decentred oval: the warmest colour was oval,and more than half of its area was outside thecentral 3 mm diameter zone or it was entirelylocated in the peripheral cornea.
Statistical analysis was performed by MrWilliam Feuer, a biostatistician in our depart-ment. The mean and standard deviation of themean was used for various descriptive quanti-ties. DiVerence in topographic measurementsbetween eyes was calculated using t test.
Figure 1 Elevation patterns of the posterior corneal surface: (A) regular ridge; (B)irregular ridge; (C) incomplete ridge; (D) island; (E) unclassified.
Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system 775
on 27 July 2018 by guest. Protected by copyright.
r J Ophthalm
ol: first published as 10.1136/bjo.83.7.774 on 1 July 1999