technologies for anatomical and geometric … · corneal and anterior segment imaging techniques...

Seminars in Ophthalmology, Early Online, 1–10, 2013

! Informa Healthcare USA, Inc.

ISSN: 0882-0538 print / 1744-5205 online

DOI: 10.3109/08820538.2013.835844

REVIEW ARTICLE

Technologies for Anatomical and GeometricCharacterization of the Corneal Structure and

Anterior Segment: A Review

David P. Pinero1,2,3Q3

1Department of Ophthalmology (Oftalmar), Medimar International Hospital, Alicante, Spain, 2Foundation forthe Visual Quality (FUNCAVIS), Fundacion para la Calidad Visual, Alicante, Spain, and 3Department of

Optics, Pharmacology and Anatomy, University of Alicante, Alicante, Spain

ABSTRACT

Corneal and anterior segment imaging techniques have become a crucial tool in the clinical practice ofophthalmology, with a great variety of applications, such as corneal curvature and pachymetric analysis,detection of ectatic corneal conditions, anatomical study of the anterior prior to phakic intraocular lensimplantation, or densitometric analysis of the crystalline lens. From the Placido-based systems that allow only acharacterization of the geometry of the anterior corneal surface to the Scheimpflug photography-based systemsthat provide a characterization of the cornea, anterior chamber, and crystalline lens, there is a great variety ofdevices with the capability of analyzing different anatomical parameters with very high precision. To date,Scheimpflug photography-based systems are the devices providing the more complete analysis of the anteriorsegment in a non-invasive way. More developments are required in anterior segment imaging technologies inorder to improve the analysis of the crystalline lens structure as well as the ocular structures behind the iris in anon-invasive way when the pupil is not dilated.

Keywords: Corneal topography, optical coherence tomography, orbscan, scheimpflug photography,ultrasonography

INTRODUCTION

The anatomical and geometric characterizations of thecorneal structure and anterior segment have become acrucial analysis in the clinical practice of ophthalmol-ogy for anterior segment specialists. Due to themultiple applications of the analysis of the cornealand anterior segment structures in ophthalmologyand their relevance for optimizing corneal andintraocular refractive surgery procedures, differentimaging techniques have been developed rapidly inthe last few years, mainly because of recent advancesin refractive surgery.1–11 All of these imagingtechniques provide a specific type of analysis,with some limitations and advantages over theremaining techniques developed. Specifically, the

great majority of imaging devices provide a more orless precise and accurate analysis of a specificanatomical and geometric parameter or parameters,with few instruments attempting to provide anintegral analysis and evaluation of the anteriorsegment, including curvature, asphericity, and thick-ness analysis of cornea and crystalline lens as well asthe measurement of the dimensions of the anteriorsegment space.

The purpose of the present review is to provide ageneral overview of the available technologies for theanatomical and geometric characterization of thecorneal structure and/or anterior segment, showingthe advantages and disadvantages of each of them aswell as the scientific evidence of their precision andclinical applicability.

Correspondence: Dr. David P. Pinero, Oftalmar, Department of Ophthalmology, Medimar International Hospital, Avda. Denia, 78, 03016Alicante, Spain. E-mail: [email protected]

Received 29 March 2013; accepted 7 August 2013; published online 2 2 2

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CORNEAL TOPOGRAPHY

Corneal topography is a non-invasive imaging tech-nique for mapping the anterior surface of the cornea(Figure 1) and, in some cases, of the posterior cornealsurface.12 Considering that the cornea is responsiblefor over two-thirds of the total eye’s refractive power,its shape is of critical importance in determining thequality of the ocular optical system and therefore thequality of vision. Very small changes in corneal shaperesulting from surgery or disease can have a dramaticeffect on the focus of the retinal image. For this reason,understanding and quantifying the corneal contourhas become an essential preoperative measurement fordesigning surgical intervention in refractive surgery,for assessing the outcomes of keratoplasty, as well ascorneal transplantation, or for evaluating the opticalperformance of the eye. Several technologies formeasuring corneal topography have been developedbased on different physical basis that used the follow-ing concepts for characterizing the corneal geometry:� Height or Elevation: distance of each point of a

surface from a reference surface. Fine details ofthe cornea can be obtained taking the sphere as

the reference surface. Once the shape of thecorneal surface has been expressed in terms ofheight, other parameters such as slope, curvature,and power can be calculated from it.

� Radius of curvature: the radius of curvature isusually expressed in millimeters and is a way ofcharacterizing the curvature of the anterior sur-face of the cornea. Corneas with a steep surfaceslope have a small radius of curvature, whilethose with a flatter surface slope have a largerradius of curvature.

� Power: the power is an optical property expressedin diopters (D) that depends on the shape of thesurfaces and the variation of refractive indexbetween both sides of the surface. The kerato-metric diopter is calculated using the radiusof curvature and the standard keratometricindex of refraction (SKI = 1.3375), an approxima-tion derived from some assumptions. We canexpress this relation with the following formula:

PðDÞ ¼SKI ÿ 1

r¼

337, 5

rðmmÞð1Þ

FIGURE 1. Corneal topography in a keratoconus eye.

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Seminars in Ophthalmology

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This concept is a simplification ignoring the factthat the refracting surface is air-tear interface, and itdoes not account for the oblique incidence of incom-ing light in the corneal periphery. As a result, itmiscalculates a true corneal refractive index of 1.376to 1.3375 to correct for some of these factors. That iswhy these diopters more correctly are termed kerato-metric diopters to distinguish them from the dioptersexpressing more precisely the true refractive power ata certain corneal point.

The evolution of the quantitative assessment of theanterior corneal curvature has been progressive andits range of measurement has been extended fromfour points a few millimeters apart measured bykeratometers to a grid of thousands of points coveringalmost the entire cornea, measured by computerizedcorneal topography.

Keratometer

The keratometer uses the ability of the anteriorcorneal surface to behave as a convex mirror. TheHelmholtz keratometer projects four points onto thecornea, creating a reflected image, which can beanalyzed and converted to corneal radius data usingan equation that considers distance from mire tocornea, image, and mire size. Although keratometersare still used commonly in clinical practice, they haveimportant limitations:� They perform measurements of the central 3mm,

accounting only for 6% of the corneal surface.� They assume that the cornea has a perfect

spherocylindrical shape, which is not true.� Information from the periphery is not provided.� They give no information about the central zone

inside the four points measured.

Placido-Based Systems

The patient is positioned sitting down and facing abowl containing the projected pattern, which isfocused on the anterior surface of the patient’scornea. The pattern reflected off from the cornea isanalyzed by a computer that provides data about thegeometric configuration of the cornea in differentkinds of numerical and graphical formats (Figure 2).Some of the errors that examiners can make using aPlacido-based system are the following: focusingerrors, alignment and fixation errors (that couldinduce wrong levels of astigmatisms), wrong calcula-tion of the position of the center from the small centralrings, and increased inaccuracy toward the peripherydue to the lack of accuracy of the preceding points.We can conclude that critical points for a precisemeasurement are accurate alignment, centering andfocusing. These issues depend on the ability of the

examiner to take a good measurement. It is essentialfor obtaining a good exam that the tear film forms asmooth layer over the irregular corneal epithelium.Tear film break-up causes mistracking of the miresand artifacts in the corneal map that appear asirregularity areas or false irregular astigmatism. Forexample, a dry patch could be associated with an areaof focal flattening in the corneal map. In order toavoid disturbing the tear film, corneal topographyshould be performed before giving dilating drops andtaking intraocular pressures.

In spite of being the most widely used, the Placido-based systems present some limitations13,14:� The central circle is dark and therefore no real

information from this central area is obtained.� They are designed to capture information along

meridians radially, not providing direct informa-tion of the corneal geometry circumferentially

� Approximations based on specific algorithms areused which are not very appropriate when highlyirregular corneas are analyzed. Specifically, errorsgreater than 4D may occur in very steep or flatcorneas, keratoconus with local steepening, sharptransition zones after uncomplicated refractivesurgeries, diffusely irregular surfaces after pene-trating keratoplasty, and complex surfaces afterdecentered ablations in refractive surgery orcentral islands.

Scanning-Slit Systems

In the slit-scan systems, a slit is projected sequentiallyonto the cornea at different angles. This is the basisused by the Orbscan II corneal topography systemfrom Bausch & Lomb (Figure 3). A high-resolutionvideo camera captures 40 light slits projected onto thecornea. The diffuse reflection is obtained from the

FIGURE 2. Placido rings image.

Corneal Structure and Anterior SegmentQ1 3

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cornea, iris, and lens. By triangulation, data isobtained from the anterior and posterior surfaces ofthe cornea and from other structures. These issuespermit this instrument to calculate anterior chamberdepth or full pachymetry of the cornea.12 In addition,this device provides data of the elevation and curva-ture of the posterior corneal surface.

The measurement with this device is significantlydependent on many factors, such as movement of thepatient’s eye, stability of tear film, ability of patients tokeep the eyes wide open, corneal transparency, andthe presence of corneal abnormalities. One of the mainlimitations of this scanning-slit system is the longertime of image acquisition in comparison with anyof the other commercially available instruments.In addition, there are some controversies about thevalidity of some measurements of the posteriorcorneal surface provided by the Orbscan system,especially after some types of keratorefractive surgicalprocedures.15 There are numerous scientific studiesshowing the poor reliability of some measurements ofthe posterior corneal surface provided by the Orbscansystem15–19 and articles reporting the error of under-estimation of the corneal pachymetry after laser-assisted in-situ keratomileusis, which is assumed tobe related to inaccurate detection and location of theposterior corneal surface.20–25

Scheimpflug Photography-Based Systems

Scheimpflug imaging is based on the Scheimpflugprinciple, which occurs when a planar subject is notparallel to the image plane. In this scenario, an obliquetangent can be drawn from the image, object, and lensplanes, and the point of intersection is theScheimpflug intersection, where the image is in bestfocus.26 The Pentacam from Oculus is one of the mostrecognized systems based on this principle.Specifically, this system uses a rotating Scheimpflugcamera to obtain 50 Scheimpflug images of theanterior segment in less than two seconds. Each

image has 500 true elevation points for a totalof 25,000 true elevation points for the surface ofthe cornea (Figure 4). Advantages of the Pentacaminclude the following: (1) high resolution analysisof the entire cornea, including the center of thecornea; (2) ability to measure corneas with accur-acy with severe irregularities, such as keratoconus,which may not be amenable to Placido imaging;and (3) ability to calculate pachymetry from limbus tolimbus.

The Pentacam system allows the study of boththe anterior and posterior corneal surfaces and pro-vides more repeatable and reproducible anterior andposterior measurements (Figure 5) of cornealpower than scanning-slit technology. Kawamoritaet al.27 reported 0.19 of agreement for within-raterconsecutive measurements of posterior corneal powermeasurements of diopter (D) with a Scheimpflug-based system and 0.96 D with a scanning-slit systemand for between-rater measurements, of 0.56 D and1.58 D, respectively.

Other systems based on Scheimpflug photographyhave been developed and released commercially, suchas the Sirius or Galilei systems. The Sirius system(CSO, Florence, Italy) is a topography device whichuses the principles of Scheimpflug photography andenables the acquisition and processing of 25 radialsections of the cornea and anterior chamber in veryfew seconds. The combination between two mono-chromatic 360�-rotating Scheimpflug cameras and aPlacido disk allows a full analysis of the cornea andanterior segment, providing tangential and axialcurvature data of anterior and posterior cornealsurfaces, and the global refractive power of thecornea, as well as a biometric estimation of variousstructures, and a corneal wavefront map with ananalysis of visual quality and corneal pachymetrymaps. Specifically, this system allows a measurementof 35,632 points for the anterior corneal surface and30,000 for the posterior corneal surface on high-resolution mode in 1–2 seconds.28 This device hasbeen shown to provide in normal and even inkeratoconus eyes consistent measurements of anterior

FIGURE 3. Scanning-slit system Orbscan II (Bausch & Lomb).

FIGURE 4. Scheimpflug image of the anterior segmentobtained with the Pentacam system (Oculus).

4 D. P. Pinero


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and posterior corneal curvature, pachymetry, andanterior chamber depth.28–30 Agreement analysesthat have been performed to this date suggest thatSirius and Pentacam should not be used interchange-ably.31 The Galilei Dual Scheimpflug Analyzer fromZiemer is a high-precision optical system for cornealtopography and three-dimensional analysis of theanterior eye segment based on a revolving dual-channel Scheimpflug camera and a Placido disk. Thisdevice combines the advantages of two technologies:Placido imaging furnishes high-accuracy curvaturedata and Scheimpflug imaging is responsible forcapturing precise elevation data. Like the Sirius andthe Pentacam systems, the Galilei provides repeatablemeasurements of corneal curvature and of otheranterior segment anatomical parameters.32 However,the agreement in the measurements obtained withthese three Scheimpflug photography-based devicesis questionable.30,33 The main inconvenience of allthese systems based on the use of a rotatingScheimpflug camera is the period of time requiredto make the complete rotation and therefore tocomplete the acquisition of the data. This makes themeasurements susceptible to tear film stability duringsuch a period.

ANTERIOR SEGMENTANALYSIS

Scanning-Slit Systems

The Orbscan II corneal topography system fromBausch & Lomb, based on this technology, is able toprovide a pachymetric analysis as well as analysis ofthe anterior chamber depth (Figure 6). However, a

significant limitation in the reliability of the pachy-metric measurements is present with these devices,34

especially in those corneas with their transparencyaffected and therefore with an inherent complicationin the detection of the posterior corneal surface.Anterior chamber depth measurements with theOrbscan system have been shown to be repeatableand in agreement with those obtained with thePentacam system.36

Scheimpflug Photography-Based Systems

These systems allow a comprehensive analysis of theanatomy of the anterior segment, including the fol-lowing analyses: topographic pachymetry (Figure 7),anterior chamber depth, corneal volume, anteriorsegment diameter (angle-to-angle distance), and crys-talline lens densitometry. Several studies have con-firmed and validated the consistency of all thesemeasurements.28,32–37 However, these systems, inspite of obtaining an image of the crystalline lens(Figure 8), do not provide an analysis of the curvatureof the anterior and posterior surfaces of the crystallinelens. This data would be crucial for some calculationsfor optimizing a great variety of refractive surgerytechniques.

Optical Coherence Tomography

Optical coherence tomography is an optical method ofcross-sectional scanning based on reflection and scat-tering of light from the structures within the cornea.Measuring different reflectivity from structures

FIGURE 5. Anterior (left map) and posterior (right map) sagittal curvature maps obtained with the Scheimpflug-photography-basedsystem Pentacam in an eye that had undergone photorefractive keratectomy for the correction of myopia.



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within the cornea by a method of optical interferom-etry produces the cross-section image of the corneaand other anterior segment structures.38 In opticalinterferometry, the light source is split into thereference and measurement beams. The measurementbeam is reflected from ocular structures and interactswith the reference light reflected from the referencemirror, a phenomenon called interference. The coher-ent or positive interference characterized by anincreased resulting signal is measured by the inter-ferometer and, subsequently, the position of thereflecting structure of the eye can be determined.38

In this way, the structures of the anterior segment canbe visualized with a high degree of resolution (cur-rently 18 microns axial and 60 microns transverse)(Figure 9). This allows imaging of fine structures suchas LASIK flaps, Schlemm canal, and lenticles fromlamellar keratoplasty surgery to be easily obtained.

The systems based on this technology have thefollowing two main advantages: the measuring pro-cedure is fast and easy for the examiner and patient,and no corneal touch or specific interphase is neededfor measuring. They provide a more comfortableprocedure of measurement for the patient. All anteriorsegment structures in front of the iris and posteriorsegment structures visible through the pupillary

aperture can be analyzed precisely with this technol-ogy.39 Several applications have been described forthese devices, including analysis of corneal path-ology,40 corneal refractive surgery changes,41 theposition of phakic intraocular lenses42 (Figure 10),the configuration of the iridocorneal angle area, andchanges in the central part of the lens during accom-modation.43 However, AS-OCT has an importantlimitation: it cannot detect structures behind the iris.Therefore, it is not possible to estimate the sulcus-to-sulcus distance or to analyze the areas of the crystal-line lens not visible through the iris. In addition, thedevice cannot analyze any object behind any opaquestructure. It should also be mentioned that this type ofdevice does not provide measurements of the cornealcurvature, only an anatomical analysis of the anteriorsegment.

Very High-Frequency Ultrasonography

Ultrasonic systems allow visualization of anteriorsegment structures, even in the presence of opticalopacities. In general, the resolution and depth ofpenetration are affected by transducer frequency. Thetraditional ultrasonography of the whole eye uses a

FIGURE 6. Anterior segment analysis provided by the scanning-slit system Orbscan II (Bausch & Lomb). The maps represent theanterior corneal elevation (top left), the posterior corneal elevation (top right), the anterior tangential curvature (down left) andpachymetry (down right). Additional anatomical data are listed in the central column of the screen.

6 D. P. Pinero


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10MHz transducer with approximately 150 mm reso-lution. Higher frequency of a 50MHz transducerincreases the tissue resolution to 50 mm but only at theexpense of decreasing tissue penetration depth of4–5mm, sufficient to image the anterior segment(Figure 11). Scanning of the cornea is possible with a100MHz transducer that increases the tissue reso-lution to less than 20 mm.

Different applications have been described for thisophthalmic technology; these include intraoculartumours analysis,44 determination of the position ofa phakic intraocular lens,45 planning for refractivesurgery retreatment,46 microkeratome cut analysis inLASIK,47 study of intraocular pathology,48 and

FIGURE 7. Comprehensive pachymetric analysis provided by the Scheimpflug-photography-based system Pentacam.

FIGURE 8. Crystalline analysis by Scheimpflug imaging in acase of posterior lenticonus.

FIGURE 9. Image of the anterior segment obtained with the Visante OCT system (Zeiss).



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analysis of some posterior segment structures.49

As resolution normally improves with frequency,VHF waves are used for most anterior segmentimaging, providing an axial resolution of less than40 mm.50 A main advantage of this technology is itsaccuracy and repeatability.51 The problems arise fromthe measuring procedure. Most ultrasound imagingdevices require physical contact between the corneaand the probe, which can be uncomfortable for somepatients. As example, the Artemis 2 system (ArcScan)ultrasound device does not require cornea–probecontact; rather, an interface eye transducer of salineis used as the acoustic coupling medium between thecornea and probe. However, the position of the headis uncomfortable for patients and the procedurerequires a very experienced examiner. It should alsobe mentioned that this type of device does not providemeasurements of the corneal curvature, only ananatomical analysis of the anterior segment.

CONCLUSIONS

The analysis of the anterior segment is a very usefulprocedure, especially in the field of refractive surgery,allowing a better planning of the surgery, a morecomprehensive follow-up, and a better understandingof some postoperative complications. In summary,there are a few main clinical applications of theanterior segment imaging technology:� Corneal curvature and pachymetric analysis to

evaluate the possibility of performing with safety

keratorefractive surgery with excimer laser in aspecific case.

� Detection of ectatic corneal conditions, especiallyin the most incipient stages, in order to avoid theperformance of laser refractive surgery in suchcases and to analyze and prevent the progressionof this type of corneal disease.

� Accurate planning of enhancements or retreat-ments of corneal refractive surgery procedures.

� Analysis of the anatomical outcomes of thedifferent techniques of corneal transplantation.

� Anatomical study of the anterior segment toperform a precise and accurate planning of theimplantation of phakic intraocular lenses for thecorrection of moderate to high ametropia.

� In-vivo evaluation of the real position of ananterior and even posterior segment phakicintraocular lens and the interaction with theadjacent ocular structures.

� Densitometric analysis of the crystalline lens tocontrol and evaluate its transparency and theneed for cataract surgery.

� Precise control and monitoring of corneal andanterior segment pathological conditions.

� Analysis of the iridocorneal angle configuration inglaucoma patients.

There is a great variety of technologies availableallowing the clinician to perform a characterization ofdifferent anterior segment structures. Each technologyhas different features and is useful for a specific taskin the analysis of the anterior segment and cornea.From the Placido-based systems that allow only acharacterization of the geometry of the anteriorcorneal surface to the Scheimpflug photography-based systems that provide a characterization of thecornea, anterior chamber, and crystalline lens, there isa variety of devices with the capability of analyzingdifferent anatomical parameters with very high pre-cision. It is important to know the capabilities andlimitations of each technology for performing anoptimized use of it. Scheimpflug photography-basedsystems are to date the devices providing the morecomplete analysis of the anterior segment in a non-invasive way, allowing the characterization of the

FIGURE 10. Visualization of the position of a posterior chamberpIOL (Phakic Refractive Lens, PRL, Zeiss) to the anteriorsurface of the crystalline lens by means of spectral-domainAS-OCT.

FIGURE 11. Analysis of the cornea by means of a very high-frequency ultrasonographer with a transducer of 50MHz.

8 D. P. Pinero


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geometry of anterior and posterior corneal surfaces,the anterior chamber depth and pachymetric distri-bution, and the densitometry and thickness of thecrystalline lens. However, there is no system provid-ing a characterization of the geometry of the twocorneal surfaces of the crystalline lens. More devel-opments are required in anterior segment imagingtechnologies in order to improve the analysis of thecrystalline lens structure as well as the ocular struc-tures behind the iris when the pupil is not dilated.

DECLARATION OF INTEREST

The authors report no conflicts of interest. The authorsalone are responsible for the content and writing ofthe paper.Q2

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technologies for anatomical and geometric … · corneal and anterior segment imaging techniques...

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