Ординатура / Офтальмология / Английские материалы / Step by Step Reading Pentacam Topography (Basics and Case Study Series)_Sinjab_2010
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Case Study 71 
Fig. 5.1H: The keratoconus indices page. The red curves are within normal ranges with no rapid deviation. The average is 0.8, which is normal.
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•The keratoconus indices page (Fig. 5.1H): the curve lines are within the normal range, no deviation before the 6 mm circle. The average is normal (0.8<1.1). The indices of irregularity are displayed with white, which is normal.
The Quantifying Step
If we want to correct the refractive error of the right eye of the patient in the given example, we have to look at three important figures: the K-readings, the thickness at the thinnest location and the Q-value.
1.K-readings: Because the patient is myopic, we have to consider the flattest K, here is 43.3 dpt. The spherical refractive error is -8.00 dpt, this means that after correction, the flat K will be reduced roughly by: -8.00 × 0.75 = -6 dpt, therefore, the post flat K will be = 43.3 – 6 = 37.3 dpt, which is normal (> 34D).
2.Thickness: Correcting each -1 dpt causes ablation of 12-16 µ depending on the diameter of the Optical Zone (OZ), for example 13.4 µ for 6.0 mm OZ and 15.7 µ for 6.5mm OZ. This means ablation of 7(not 8) × 15.7 µ = 110 µ (this is according to Wellington Clinic Nomogram used with Allegretto) to correct the -8 dpt. We have to follow 2 rules in this field: the first rule, we must not ablate more than 20% of the original corneal thickness, the second rule is, we must keep at least 55% of the original corneal thickness as a residual stromal bed (RSB), putting in mind that the RSB still is >250 µ. So, in our case, we must not ablate more than 20% × 535 = 107 µ, and must keep 55% × 535 = 294 µ. If you add the thickness of the thin flap (100 µ), you will have the
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allowed ablation depth according to the second rule 294 + 100 = 394, 535 – 394 = 141 µ. We always take the smaller calculated value to be on the safe side, so we should consider the 107 µ. In this case we can correct all the refractive errors.
3.Q-value: correcting ≥ -6 dpt leads to positive Q-value, which means oblate cornea. In such cases, it is recommended to use the Q-guided profile in order to conserve the normal Q-value of the patient, otherwise we will end up with spherical aberrations.
The Scoring Step
Scoring the right eye:
1.Corneal topography is normal with symmetrical bow tie pattern, giving the score 0.
2.RSB: This depends in our case on the thickness of the flap. If we chose the 100 µ flap, the RSB will be 535-107- 100 = 328 µ, giving the score 0.
3.Age: The patient is 21-year old, giving the score 3.
4.CT is 535 giving the score 0.
5.MRSE is about -9, giving the score 1.
The final scoring is 1, therefore, the relative risk is low
and we can proceed with Lasik.
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CASE 2: MIXED ASTIGMATISM
A 21-year-old female came with stable refractive error. Her MR was:
Eye |
SPH |
CYL |
AXIS |
|
OD |
+0.5 |
-2.5 |
180 |
|
OS |
+0.5 |
-2 |
180 |
Fig. 5-2A shows her right eye corneal topography (4 refractive maps).
The Qualifying Step
1.In a general look, the sagittal curvature map has a symmetrical bow tie pattern oriented as with-the-rule astigmatism. The elevation maps—by BFS reference body—seem to be symmetric with no isolated islands.
2.Main page analysis reveals that the quality of the capture (QS) is OK. The K-readings in the central 3 mm are normal (45.5, 47.9) and corneal astigmatism (the algebraic sum of anterior and posterior astigmatism) is also normal (-1.6 <-6). Q-value at the 6 mm circle of the cornea front is normal (-0.34). Corneal thickness at the thinnest location is normal (524) and has normal coordinates with pachy apex. There is also no significant difference in thickness between pachy apex and thinnest location (<5 µ).
3.Studying each map:
•Corneal thickness map (Fig. 5.2B): the shape of this map is normal shape, i.e. no significant displacement of the thinnest location (< -500 µ on ‘y’ axis). But the
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difference between the lower and upper points of the central 4 mm circle is >30 µ (36 µ, which is not significant) (Fig. 5.2B red circles).
•The sagittal curvature front map (Fig. 5.2C): Symmetrical bow tie, no skew or angulation between the lower and upper axes of the pattern, the difference between the upper and lower points of the 4 mm circle is less than 1.5 dpt.
•The elevation BFS front map (Fig. 5.2D): Regular shape with no isolated island or tongue-like extension, normal values within the central 4 mm circle.
•The elevation BFS back map (Fig. 5.2E): Regular shape with no isolated island or tongue-like extension. There are some suspected values within the central 4 mm circle (arrows) and there are some points where the difference between anterior and posterior values is >+5 µ.
•The Topometric map (Fig. 5.2F red arrows): Both the average vertical value and the lower value are normal (<-0.55).
•The keratoconus indices page (Fig. 5.2G): The curve lines are within the normal range, no deviation before the 6 mm circle. The average is normal (1). All irregularity indices are displayed in white except one, which is the corneal aberration coefficient factor (ABR), and there is no risk in treating the cornea with
ABR less than 2.5.
The mentioned abnormal findings are not significant since all other parameters are normal.
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Fig. 5.2A: The four refractive maps. Quality of the capture (QS) is OK. Symmetric bow tie pattern oriented as with-the-rule astigmatism. The elevation maps seem to be symmetric with no isolated islands. All other features are normal, e.g. K readings, corneal astigmatism, Q-value,...etc.
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Fig.5.2B:The corneal thickness map. Normal shape with no significant displacement of the thinnest location (white arrows), but the difference between the upper and lower values on the 4 mm circle is >30 µ (red circles).
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Fig.5.2C:The sagittal curvature front map. Symmetric bow tie oriented as with the rule astigmatism. Normal superior-inferior values on the 4 mm circle.
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Fig. 5.2D: The elevation front map. Regular shape with normal central values.
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Fig. 5.2E: The elevation back map. Regular shape with some suspicious values (arrows).