Ординатура / Офтальмология / Английские материалы / Step by Step Reading Pentacam Topography (Basics and Case Study Series)_Sinjab_2010

Case Study 83

Fig. 5.2H: The ablation profile. It is peripheral and on axis 180° to steepen this axis and to correct the +2.5 dpt astigmatism in this case.

84 Step by Step Reading Pentacam Topography

The Quantifying Step

To correct the refractive error of the right eye, we have to look at four important figures: the K-readings, the thickness at the thinnest location, Q-value and pupil center coordinates because there is a plus component, i.e. mixed astigmatism.

1.K-readings: The patient has mixed astigmatism. In laser treatment profile, the treatment is designed to be +2.5 cylindrical diopters and -2 spherical diopters. The periphery of the cornea is ablated on axis 180° in order to steepen this axis and correct the +2.5 dpt (Fig. 5.2H). This increases the flat K-readings on axis 180° by 2.5 × 1 = 2.5 dpt, therefore, the flat K will be increased up to 48 dpt. As we see, the cylindrical correction eliminates the difference between the two major axes of corneal astigmatism. Correcting the -2 dpt sphere will reduce the overall K-readings in the OZ by 2 × 0.75 = 1.5 dpt to become finally 48 – 1.5 = 46.5, which is still within the accepted range.

2.Thickness: Correcting plus astigmatism ablates the periphery, while correcting minus sphere ablates the center of the cornea. In our case, the central ablation depth is almost 2 × 15.7 = 31.4 µ. Considering the two rules of thickness limits, we are still on the safe side even when creating thick flap (160 µ) as follows: 524 – 160 – 31.4 = 332.6 µ, which is the residual stromal bed (RSB) thickness.

3.Q-value and pupil center coordinates: There are two reasons for choosing the topography-guided treatment modality in general: first, a difference of >0.3 between vertical and horizontal Q-values, second, large angle

Case Study 85

kappa. If we look at Fig. 5.2F, we find that there is no significant difference between vertical and horizontal Q-values. Also, the pupil center coordinates (and therefore angle kappa) are not significant. In this case we can use the standard treatment.

The Scoring Step

1.Corneal topography is normal with symmetrical bow tie pattern, giving the score 0.

2.RSB: in our case, even if we choose the thick flap (160 µ) the RSB will be >300 µ, giving the score 0.

3.Age: The patient is 21-year old, giving the score 3.

4.CT is 524 giving the score 0.

5.MRSE is less than -8, giving the score 0.

The final score is almost 0 to 1, therefore, the relative

risk is low and we can proceed with Lasik.

86 Step by Step Reading Pentacam Topography

CASE 3: HYPEROPIA

A 50-year-old female came with stable refractive error. Her MR was:

Fig. 5.3A shows her right eye corneal topography (4 refractive maps).

The Qualifying Step

1.In a general look, the sagittal curvature map has a slightly irregular pattern. There is an obvious tonguelike pattern on both elevation BFS maps (white arrows). The thickness map shows very clear pattern of thinnest location displacement (red arrows).

2.Main page analysis reveals that the quality of the capture (QS) is OK. The K-readings in the central 3 mm are normal (45.1, 45.4) and corneal astigmatism is almost insignificant. Q-value at the 6 mm circle of the cornea front is normal (-0.31). Corneal thickness at the thinnest location is below 500 µ (496 µ), and has abnormal coordinates with pachy apex especially on “y” axis (>-1000 µ). There is no difference in thickness between pachy apex and thinnest location.

3.Studying each map:

•Corneal thickness map (Fig. 5.3B): The shape of this map is abnormal; there is a significant displacement of the thinnest location, which gives the map the

Case Study 87

shape of a cone. The difference between the lower and upper points of the central 4 mm circle is 31 µ (red circles).

•The sagittal curvature front (Fig. 5.3C): Although this map is slightly irregular, the two major axes of cornea front astigmatism are perpendicular and show no skew or angulation between the lower and upper axes of the pattern. The difference between the upper and lower points (the superior-inferior difference) of the 4 mm circle is less than 1.5 dpt (white arrows).

•The elevation front map (Fig. 5.3D): There is an irregular shape with tongue-like extension, but still the values are normal within the central 4 mm circle.

•The elevation back map (Fig. 5.3E): It is more irregular with tongue-like extension. The central 4 mm circle values are abnormal giving the impression of cone like shape (white arrow). There are also some points where the difference between anterior and posterior values is >+5 µ.

P.S. In case of any irregularity or suspicious values on elevation maps with the BFS, it is recommended to see the elevation maps with Toric Ellipsoid float reference body; when the irregularities are due to corneal astigmatism, they will vanish, otherwise they stay when they are due to real corneal surface irregularity. The toric ellipsoid option eliminates the effect of corneal astigmatic slope on the elevation maps. This is important especially for the back surface where the subclinical keratoconus begins.

Case Study 89

Fig. 5.3B: The corneal thickness map. Abnormal shape because of the significant thinnest location displacement. No important superiorinferior difference (red circles).