Pentacam and topography

الرحيم الرحمن الله بسم

PentacamMohamed Zaky MSc.

2017

topography

Orbscan II

Disadvantage of Orbscan is that it over estimates the posterior corneal surface in post refractive surgery eyes.

N. 530 um ± 30 at thinnest location TL

normal pachymetry map has a concentric shape

1) Pachymetry

Cornea apex - TL = < 10 um with Y coordinate of TL < 0.5 mm

Cornea periphery : CTS profile

I – S difference at 5 mm circle < 30Um

TL of the other eye = < 30 Um

comparisons

No LASIK if TL < 500 No PRK if TL < 470 ( ? KC ) RSB should be > 300 in LASIK and > 400 in

PRK Amount of correction should be < 20% of TL Amount of correction (um) = 1/3 (error of refraction in diopters) x (zone in mm)2

Significance

a. Horizontal displacement of the TL .b. Dome shape. The TL is vertically

displaced .c. Bell shape. There is a thin band in the

inferior part of the cornea . It is a hallmark for Pellucid Marginal Degeneration (PMD).

d. Keratoglobus. A generalized thinning reaching the limbus .

abnormal

CTSP

a. Quick Slope . The red curve leaves its course before 6-mm zone. It is encountered in FFKC & ectatic disorders. The average is usually high > 1.1 .

b. S-shape . The red curve has a shape of an “S”. It is encountered in FFKC and ectatic disorders. The average is usually high > 1.1 .

c. Flat shape . The red curve takes a straight course. It is encountered in diseased thickened (oedematous) corneas such as Fuch’s dystrophy & cornea Guttata. The average is low < 0.8 .

d. Inverted . The red curve follows an upward course. It is encountered in some cases of PMD. The average is very low < 0.8 and may take a minus value

CSTP

2) Power (K)

Power = n2 – n1 / r

Sim K :like keratometer readingsK1 & K2 of the central 3 mm of anterior corneal surface.

Normal : flat>34 & steep<48Sigificance : • 1 D myopic correction will decrease k by 0.8• 1 D hyperopic will increase k by 1.2• K post operative should be flat>34 & steep<48• Can be used for IOL biometry in virgin cornea

Average K (Km) :Average of K1 and K2

Significance :if < 40 : Free capif > 46 : Button hole

K max :

The highest K in the corneaNormal : = K2 (or difference < 1D) = Kmax of the other eye (or difference <

2D) Significance :Suggest more corneal irregularities / should

be considered in hyperopic LASIK

We have to calculate algebraic sum of astigmatism of the anterior and posterior corneal surfaces, then we should compare with the manifest refraction to exclude causes of incongruence, such as

lenticular astigmatism, subtle posterior subcapsular cataract, tear film disturbance,…etc.

Topographic astigmatism

More details , more sensitive to ectasia

More noise

Tangential map

Normal :* Symmetric Bow tie (SB)* I-S difference at 5 mm circle should be < 1.5 D ( or 2.5 D if superior is more steep)

The SB pattern can be encountered in KC when K readings are very high

(symmetric ? / asymmetric ? / skewed ? / another shape ?)

Anterior sagittal (Axial) power map

1. Round (R) .2. Oval (O) .3. Superior Steep (SS) .4. Inferior Steep (IS) .5. Irregular (Irr) .6. Abnormal Symmetric Bowtie (SB) . K READING IS HIGH .7. Symmetric Bowtie with Skewed Radial Axis (SB/SRAX). The

angle between the axes of the two lobes is >22° .8. Asymmetric Bowtie/Inferior Steep (AB/IS); the I-S difference is

>1.5 D .9. Asymmetric Bowtie/Superior Steep (AB/SS); the S-I difference

is >2.5 D .10. Asymmetric Bowtie with Skewed Radial Axis (AB/SRAX). The

angle between the axes of the two lobes is >22°

Abnormal :

Elevation maps are more accurate in depicting the morphology of the cone than sagittal curvature displays and should be used to classify keratoconus

Displaced apex $

True Net Power: Corneal power calculation accounting for both anterior and posterior corneal surfaces and their respective optical performance. (depend on sagital maps)

Total Cornea Refractive Power: Ray tracing calculation of the corneal vergence power, considering the front and back elevation data along with corneal thickness (uses n=1.376 / the most accurate / used for toric IOL)

Equivalent Keratometer Readings (EKR): Used for post refractive IOL calculations, this reading utilizes both the anterior and posterior corneal surfaces to produce a graphical and tabular representation of the “adjusted” post surgical “K” readings at varying pupil sizes. (n = 1.3375 (the same as IOL formula) (best to use 4.5 mm EKR)

use the MAN setting with the reference surface set to SPHERE and the DIAMETER set to 8.0 mm, float

3) Elevation maps

Which is flat ?

Central “High” point – Steeper than the sphere • Peripheral “High” point – Flatter than the sphere • Central “low” point (negative number) – Flatter than the sphere -- NOT A

CONCAVITY still highest spot on cornea • Peripheral “low” point – Steeper than the sphere

In general, we have to use both the Best Fit Sphere (BFS) and the Best Fit Toric Ellipsoid (BFTE).

The BFS is important for three reasons: (1) To see the shape of the cornea, (2) To search for an important risk factor, that is the isolated island or the tongue like extension, (3) To locate the cone in KC .

On the other hand, the BFTE is important for two reasons: (1) To evaluate the details of corneal surface , (2) To evaluate the severity of the cone in KC .

Refrence surface

a. Skewed hourglass . Normally, it can be seen with large angle Kappa and misalignment during taking the capture, otherwise it indicates an abnormal distorted cornea.

b. Tongue-like extension and irregular hourglass . They are seen in abnormal distorted corneas.

c. Isolated island . It is encountered in abnormal distorted corneas with central or paracentral protrusion.

abnormal

1. The elevation values on the front surface map should not exceed +12 μ..

2. The elevation values on the back surface map should not exceed +17 μ.

3. The difference between the back and front surfaces (back-front) should not exceed +5 μ at the same point.

4. If there is any isolated island on either front or back surfaces, it would be suspected, even with values within the normal limits

BFTE central 5 mm

Thinnest point elevation values are the most reproducible and most suitable for general screening purposes

Measurment of corneal asphericty :

• Spheric Q = 0• Prolate Q < 0• Oblate Q > 0

Most corneas Q is between 0 to -0.5

4) Q-value

US FDA clinical trial data show no correlation between corneal shape and visual acuity or contrast sensitivity outcomes

Corneal asphericity does not appear to be a determining factor in post-op quality of vision

Better visual outcomes are more likely with a customized shape than a standard aspheric shape

Comparison between customized and optimized

No optimum Q value

Angle Kappa :

The angle between visual Axis and central pupillary line .

In pentacam : Its estimated by pupil coordinate in

relation to cornea apex Normal : < 200 Um Significance :

ablation should be centered on visual axis not pupil .

5) Angles , AC and pupil

Pupil diameter:

optical zone (OZ) should be 0.5 mm larger than the scotopic pupil

ACA , ACV , ACD :

safe parameters for phakic IOL (PIOL) implantation are ACD ≥ 3.0 mm, ACA > 30° and ACV ≥ 100 mm3

6) Keratoconus screening

In ectatic corneal disorders, the cone can be localized by the BFS float mode , and can be quantified by the BFTE float mode . On the BFS, the cone can be central, paracentral or peripheral when its apex is located within the central 3-mm zone, between 3-mm and 5-mm, or outside the central 5-mm zone, respectively .

the cone is peripheral, the elevation map takes “kissing birds” sign

BAD

The difference between PMD and KC is only from pacymetry

InPMD : bell shape

PMD

Saggital : central flat Pachy : central thining Ant : step at edge of OZ Post : un touched

Quality of the image (QS) is OK for both surfaces. K-readings are within the normal range; both K2 and K-max are < 49 D and (K-max—K2) is < 1 D. The amount and axis of TA should be compared with MA. Q-value of both surfaces is within the normal range [–1 , 0]. TL thickness is > 500 μm. Difference in thickness between the TL and pachy apex is < 10 μm. There is no vertical displacement of the TL . Angle kappa is not significant ; x-coordinate is < 200 μm in absolute value. ACV is > 100 mm3, ACA is normal and > 30°, ACD is normal (> 2.1 mm) but < 3.0 mm

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