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Overlooked features of corneal topographers

2008; Lippincott Williams & Wilkins; Volume: 34; Issue: 5 Linguagem: Inglês

10.1016/j.jcrs.2007.12.042

1873-4502

Carlos G. Arce, Mauro Campos, Paulo Schor,

Intraocular Surgery and Lenses

In the recent article on misalignment during corneal topography, Hick et al.1 did not mention that the Orbscan II and IIZ (Bausch & Lomb) are able to move the achieved data using a few referential marks as map centers. The Orbscan II was the first hybrid combining Placido-disk reflection and elevation-derived analysis of vertical cross-sections of light of the cornea.2,3 While the manufacturer did not explain how data are combined, these 2 technologies seem to be integrated when the surface rotation option of the tools menu is activated and one of the referential marks allowed as a new map center is selected. Consequently, default incomplete Placido-derived axial (Figure 1, A) and tangential-keratometric maps turn into complete images (Figures 1, C to I). 3,4 Default elevation-derived axial (Figure 1, B) and tangential-keratometric maps also change. Anterior and posterior float elevation maps, thickness, power, and all other curvature maps become dislocated (rotated) following the location of the chosen referential mark used as center. Although any point on the maps may be used as center, the Orbscan II surface rotation device allows corneal-dependent centers as the apex (small cross, Figure 1, C) and the thinnest point (small yellow square, Figure 1, E), or noncorneal centers such as the pupil centroid (white spot, Figure 1, D) and the fixation line intersection on the anterior corneal surface (small x). In the example (Figure 1, F to I), the map center was moved 0.6 mm from the Placido-derived map center in 4 directions to simulate the experience of Hick et al.1Figure 1: Examples of Orbscan II surface rotation in a cornea with keratoconus.Another overlay Orbscan II maps have is the center of the system (small white square).3 The distance between the fixation and the system mark theoretically represents the misalignment of exams. Hick et al. did not use this metric even though they showed quantitative changes in keratometric parameters, variation of Sim-K, inferior-superior and temporal-nasal indices, the radius of curvature at the corneal apex, and the asphericity factor (Q). They also found predictable qualitative changes on the maps. On the other hand, the Orbscan II surface rotation device allows centering all maps according to research, diagnostic, or surgical needs. To our knowledge, this is the first time a topographic system is able to present the data in the way the observer wants. With certain limits, this feature seems to compensate qualitative patterns of misaligned maps when curvature, power, or thickness data are moved in a rational way from their original relative location. Furthermore, its use did not alter the Q factor, eccentricity (e), and shape factor (P) shown at the anterior and posterior elevation aconic maps or the Sim-K and the radius of curvature at the corneal apex. However, the relative location and the distance from the map center to the thinnest point, apex, pupil centroid, and kappa intercept on the anterior surface were modified. Additionally, the values of the inferior–superior and temporal–nasal indices vary. Interpretation of these indicators of asymmetry may be improved by choosing a standard common referential mark as a map center instead of a center achieved manually (and therefore influenced by the alignment of examinations, as shown by Hick et al.1) or instead of an automatic forced center achieved during the image acquisition by other equipment. The surface rotation and the statistical analysis device allowed the development of quantitative area topography (Arce CG, et al. IOVS 2006; 47:ARVO E-Abstract 582; Arce CG, Alzamora JB, Schor P, et al., "Quantitative Area Topography with Orbscan II and IIZ" poster presented at the World Ophthalmology Congress, Sâo Paulo, Brazil, February 2006. Educational tutorial software in CD available from Bausch & Lomb do Brasil).3 They are presently used to understand decentered treatments, the spatial variation and asymmetries of thickness and curvature, to center preoperative and postoperative maps so differential maps have the same alignment, and in several other applications.3–8 These features of Orbscan II and IIZ have been underused since their introduction into the market. We regret that Hick et al.1 seemed to miss them. Their details and applications are beyond the purpose of this letter and will appear in a future paper (unpublished data).