Imaging of the Cornea: Topography vs Tomography Renato Ambrósio, Jr, MD, PhD; Michael W. Belin, MD
t times technical advancements are significant enough to warrant a change or revision in terminology. For example, the classification and proper terminology of laser corneal surgery have been previously addressed so that surgeons and investigators around the world would “make sense in keratospeaking.”1 This is also the case when newer diagnostic instrumentation offers either more or different information than was previously available. It is our belief that we have reached that point with newer methods of imaging the cornea and anterior segment, thereby, revisiting the “keratospeak” regarding the terminology for corneal characterization.2 The era of computerized corneal surface analysis known as “topography” was introduced in the mid1980s, when algorithms for surface reconstruction of the acquired reflection image from Placido photokeratoscope images were developed. Stephen D. Klyce, PhD, is credited with developing color-coded maps and indices from quantitative analysis of numerous points on the corneal surface.3,4 The term “corneal topography,” however, is a misnomer, in that most of these systems measured slope and produced a derived curvature map. The term is so ingrained that we will continue to use it here to refer to standard reflective Placido-based systems. Corneal topography represented a true revolution in the diagnosis and management of corneal disease. One of the most important applications of corneal topography was in screening refractive surgical patients, as From Instituto de Olhos Renato Ambrósio and Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil (Ambrósio); and the Department of Ophthalmology & Visual Science, University of Arizona, and Southern Arizona VA Health Care System (Belin), Tucson, Arizona. Drs Ambrósio and Belin are consultants for Oculus Optikgeräte GmbH, Wetzlar, Germany. Dr Ambrósio is a speaker for Reichert Ophthalmic Instruments, Depew, New York. Correspondence: Renato Ambrósio, Jr, MD, PhD, Instituto de Olhos Renato Ambrósio, Rua Conde de Bonfim 211 - Grupo 712, Rio de Janeiro - RJ 20520-050. Tel: 55 21 2234 4233; E-mail: email@example.com doi:10.3928/1081597X-20101006-01
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well as evaluating and improving the results of corneal surgical procedures such as LASIK and photorefractive keratectomy (PRK).4,5 Corneal topography has been found to be sensitive for detecting subtle changes on the anterior corneal surface secondary to ectatic disorders prior to loss of corrected distance visual acuity and the development of typical slit-lamp microscopy findings (ie, Fleischer corneal epithelial iron ring, Munson sign, Rizzuti sign, or Vogt striae).6,7 In addition to Placido reflection–based corneal topography, other methods, such as the PAR Technology Corneal Topography System (PAR CTS; PAR Technology Corp, New Hartford, New York), which utilizes the technique of raster photogrammetry to define elevation points by analyzing a projected grid on the corneal surface, were also available. The PAR CTS provided highly accurate and reproducible elevation characterization of the corneal surface and was less sensitive to decentration.8 However, corneal measurement by either Placido reflection or raster-photogrammetry was limited exclusively to the front or anterior corneal surface. Slit-imaging technologies were a further improvement in corneal imaging because of the ability to measure not only the anterior corneal surface but the posterior surface and to define the spatial relationship between the two (thickness map), and subsequently to characterize corneal architecture in three dimension. This reconstruction of the cornea provides significantly more information, therefore, new terminology, such as “tomography,” should be considered, which is more reflective of the three-dimensional characterization of the cornea and should be used to distinguish “tomography” from surface “topography.” Topography derives from the Greek words “to place” (topo) and “to write” (graphein), which means to describe a place. This is classically related to the study of Earth’s surface shape and features or those of planets, moons, and asteroids. Tomography derives from the Greek words “to cut or section” (tomos) and “to write” (graphein). In medicine, the classic term computed tomography (CT) scanning is used for referring to the 847
Guest Editorial/Ambrósio & Belin
Methods and Instruments for Corneal Tomography Method
Orbscan IIz* (Bausch & Lomb, Rochester, New York)
Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) Oculyzer (Wavelight AG, Erlangen, Germany) Preciso (iVIS Technologies, Taranto, Italy) Galilei* (Ziemer, Port, Switzerland) TMS-5* (Tomey Corp, Nagoya, Japan) SIRIUS* (CSO, Florence, Italy)
Rotating optical coherence tomography (OCT)
Visante† (Carl Zeiss Meditec, Jena, Germany) SS-1000 CASIA (Tomey Corp)
Arc scanning with very high-frequency ultrasound
Artemis 3 (ArcScan, Morrison, Colorado)
*Hybrid systems with a Placido disk topographer. †Integrated to the ATLAS topographer in the Visante OMNI.
Figure. Three-dimensional tomographic reconstruction of the cornea and anterior chamber from the Pentacam (Oculus Optikgeräte GmbH) examination.
radiographic technique for imaging a section of an internal solid organ, producing a three-dimensional image. Corneal tomography should be used for the examination of the front and back surfaces of the cornea, along with pachymetric mapping, considering it computes a three-dimensional image of the cornea (Fig). Different technologies, such as horizontal slit scanning, rotational Scheimpflug imaging, arc scanning with very high-frequency ultrasound, and optical coherence tomography, are available in many commercial instruments (Table). Of course, as for any diagnostic system, repeatability and accuracy issues are critical to ensure the value of the data generated. Limitations of each technology also must be considered.9,10 For example, forward protrusion or displacement of the posterior corneal surface, an important landmark of ectasia after LASIK, 848
has been found in many otherwise normal postoperative LASIK eyes using the Orbscan (Bausch & Lomb, Rochester, New York).11,12 Although Dupps and Roberts13 described bulging of the posterior cornea after otherwise uneventful LASIK in normal individuals, its magnitude in stable cases should be extremely limited. Interestingly, in a later study with a mean follow-up of 14 months after LASIK, no patient demonstrated significant forward protrusion of the posterior corneal surface on Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany).14 In addition, another recent study demonstrated Pentacam and Orbscan had no correlation in posterior elevation measurements although good correlations were noted for corneal thickness, anterior elevation, and anterior chamber depth after refractive surgery.15 Scientific studies for characterizing normalcy and differentiating disease states, as well as demonstrating the sensitivity and specificity of novel tomographic parameters are required. Thickness profile16 and elevation maps17 should be properly interpreted to ensure the clinical benefit of the advanced technology. This is critically necessary in many areas, such as screening for ectasia risk among refractive surgery candidates.18,19 The occurrence of ectasia after uneventful LASIK with no recognized risk factors detected by traditional screening criteria, based on clinical parameters (refraction, age, residual stromal bed), along with single-point thickness measurements (pachymetry) and Placido-disk corneal topography, has demonstrated the need for new technologies.20 More data are needed to demonstrate whether corneal tomography enhances the sensitivity for detecting cases at risk for corneal ectasia after LASIK (see our article in this issue21). Although case control and ideally prospective studCopyright © SLACK Incorporated
Guest Editorial/Ambrósio & Belin
ies with rigorous statistical analysis are needed for definitively demonstrating the benefit of novel diagnostic tools for detecting ectasia risk, it is important to distinguish three-dimensional reconstruction methods from corneal topography. The terms corneal tomography and corneal topography are useful, because they will distinguish between the two different types of examination, which will likely coexist and be complementary. Topography can evaluate the tear film, which is not examined by slit systems and provides useful information for screening cases at risk for dry eye after refractive surgery.22 Tomography characterizes the elevation of the front and back surfaces of the cornea and reconstructs pachymetric mapping, which we have found critical to enhance the sensitivity and specificity for detecting ectasia. REFERENCES 1. Waring GO III. Classification and terminology of laser corneal surgery: making sense of keratospeak III. Refract Corneal Surg. 1990;6(5):318-320. 2. Waring GO III. Making sense of keratospeak II: proposed conventional terminology for corneal topography. Refract Corneal Surg. 1989;5(6):362-367. 3. Klyce SD. Computer-assisted corneal topography. High-resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci. 1984;25(12):1426-1435. 4. Wilson SE, Ambrósio R. Computerized corneal topography and its importance to wavefront technology. Cornea. 2001;20(5):441454. 5. Ambrósio R Jr, Klyce SD, Wilson SE. Corneal topographic and pachymetric screening of keratorefractive patients. J Refract Surg. 2003;19:(1):24-29. 6. Maguire LJ, Bourne WM. Corneal topography of early keratoconus. Am J Ophthalmol. 1989;108(2):107-112. 7. Maeda N, Klyce SD, Tano Y. Detection and classification of mild irregular astigmatism in patients with good visual acuity. Surv Ophthalmol. 1998;43(1):53-58. 8. Belin MW, Litoff D, Strods SJ, Winn SS, Smith RS. The PAR Technology Corneal Topography System. Refract Corneal Surg. 1992;8(1):88-96.
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9. Wilson SE. Cautions regarding measurements of the posterior corneal curvature. Ophthalmology. 2000;107(7):1223. 10. Huang D. A reliable corneal tomography system is still needed. Ophthalmology. 2003;110(3):455-456. 11. 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(2):406-409. 12. Twa MD, Roberts C, Mahmoud AM, Chang JS Jr. Response of the posterior corneal surface to laser in situ keratomileusis for myopia. J Cataract Refract Surg. 2005;31(1):61-71. 13. Dupps WJ Jr, Roberts C. Effect of acute biomechanical changes on corneal curvature after photokeratectomy. J Refract Surg. 2001;17(6):658-669. 14. Ciolino JB, Khachikian SS, Cortese MJ, Belin MW. Long-term stability of the posterior cornea after laser in situ keratomileusis. J Cataract Refract Surg. 2007;33(8):1366-1370. 15. Kim SW, Sun HJ, Chang JH, Kim EK. Anterior segment measurements using Pentacam and Orbscan II 1 to 5 years after refractive surgery. J Refract Surg. 2009;25(12):1091-1097. 16. Ambrósio R Jr, Alonso RS, Luz A, Coca Velarde LG. Cornealthickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg. 2006;32(11):1851-1859. 17. Belin MW, Khachikian SS. An introduction to understanding elevation-based topography: how elevation data are displayed– a review. Clin Experiment Ophthalmol. 2009;37(1):14-29. 18. Binder PS, Lindstrom RL, Stulting RD, Donnenfeld E, Wu H, McDonnell P, Rabinowitz Y. Keratoconus and corneal ectasia after LASIK. J Refract Surg. 2005;21(6):749-752. 19. Binder PS, Trattler WB. Evaluation of a risk factor scoring system for corneal ectasia after LASIK in eyes with normal topography. J Refract Surg. 2010;26(4):241-250. 20. Klein SR, Epstein RJ, Randleman JB, Stulting RD. Corneal ectasia after laser in situ keratomileusis in patients without apparent preoperative risk factors. Cornea. 2006;25(4):388-403. 21. Ambrósio R Jr, Dawson DG, Salomão M, Guerra FP, Caiado AL, Belin MW. Corneal ectasia after LASIK despite low preoperative risk: tomographic and biomechanical findings in the unoperated, stable, fellow eye. J Refract Surg. 2010;26(11):906-911. 22. Ambrósio R Jr, Tervo T, Wilson SE. LASIK-associated dry eye and neurotrophic epitheliopathy: pathophysiology and strategies for prevention and treatment. J Refract Surg. 2008;24(4):396407.
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