Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009

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pattern and the lower right map shows the pachymetry (thickness of the cornea). The Orbscan is a threedimensional slit scan topographic machine. If we were doing topography with a machine, which does not have slit scan imaging facility, we would not be able to see the topography of the posterior surface of the cornea. Now, if the patient had an abnormality in the posterior surface of the cornea, for example as in primary posterior corneal elevation this would not be diagnosed. Then if we perform LASIK on such a patient we would create an iatrogenic keratectasia. The Orbscan helps us to detect the abnormalities on the posterior surface of the cornea.

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Figure 1.4: General quad map of a primary posterior corneal elevation. Notice the upper right map has an abnormality, whereas the upper left map is normal. This shows the anterior surface of the cornea is normal and the problem is in the posterior surface of the cornea

Let us now understand this better in a case of a primary posterior corneal elevation. If we see the General quad map of a primary posterior corneal elevation (Figure 1.4) we will see the upper left map is normal. The upper right map shows abnormality highlighted in red. This indicates the abnormality in the posterior surface of the cornea.

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Figure 1.5: Quad map of a primary posterior corneal elevation with the normal band scale filter on. This shows the abnormal areas in red and the normal areas are all green. Notice the abnormality in the upper right map

Now, we can put on the normal band scale filter on (Figure 1.5) and this will highlight the abnormal areas in red. Notice in Figure 1.5 the upper right map shows a lot of abnormality denoting the primary posterior corneal elevation. One can also take the three-dimensional map of the posterior surface of the cornea and notice the amount of elevation in respect to the normal reference

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displaying 2-D data in corneal and anterior segment topography. Although there are no universally standardized colors, the spectral direction (from blue to red) is always organized in definite and intuitive way.

Blue = low, level, flat, deep, thick, or aberrated. Red = high, steep, sharp, shallow, thin, or focused.

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Keratoconus is characterized by non-inflammatory stromal thinning and anterior protrusion of the cornea. Keratoconus is a slowly progressive condition often presenting in the teen or early twenties with decreased vision or visual distortion. Family history of keratoconus is seen occasionally. Patients with this disorder are poor candidates for refractive surgery because of the possibility of exacerbating keratectasia. The development of corneal ectasia is a well recognized complication of LASIK and attributed to unrecognized preoperative forme fruste keratoconus.

All eyes to undergo LASIK are examined by Orbscan. Eyes are screened using quad maps (Figure 1.6A ) with the normal band (NB) filter turned on. Figure 1.6B shows quad map with normal band scale filter on in the same eye as in Figure 1.6A.

Four maps included (a) anterior corneal elevation: NB = ± 25 µ of best-fit sphere. (b) Posterior corneal elevation: NB = ± 25 µ of best-fit sphere. (c) Keratometric mean curvature: NB = 40 to 48 D, K. (d) Corneal thickness (pachymetry): NB = 500 to 600 µ. Map features within normal band are colored green. This effectively filters out variation falling within normal band. When abnormalities are seen on the normal band quad map screening, a standard scale quad map is examined.