The document discusses corneal tomography parameters and their significance in evaluating post-refractive surgery eyes and detecting ectatic disorders. It provides normal values and characteristics for pachymetry, keratometry, sagittal and elevation maps, and other Pentacam parameters. Abnormal findings that may indicate conditions like keratoconus and pellucid marginal degeneration are also described.

2. Mohamed Zaky MSc.
2017

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

7. N. 530 um ± 30 at
thinnest location TL
normal pachymetry map has a concentric shape

8.  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

11.  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

12. 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 .

15. 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

17. Power = n2 – n1 / r

18. Sim K :
like keratometer readings
K1 & K2 of the central 3 mm of anterior corneal
surface.
Normal : flat>34 & steep<48
Sigificance :
• 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

19.  Average K (Km) :
Average of K1 and K2
 Significance :
if < 40 : Free cap
if > 46 : Button hole

20.  K max :
The highest K in the cornea
Normal :
= K2 (or difference < 1D)
= Kmax of the other eye (or difference <
2D)
Significance :
Suggest more corneal irregularities / should be
considered in hyperopic LASIK

21. 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.

23.  More details , more sensitive to ectasia
 More noise

24.  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 ?)

25. 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°

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

28. 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)

29.  use the MAN setting with the reference
surface set to SPHERE and the DIAMETER set
to 8.0 mm, float
Which is flat ?

30.  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

31.  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 .

35. 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.

37. 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

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

40.  Measurment of corneal asphericty :
• Spheric Q = 0
• Prolate Q < 0
• Oblate Q > 0
Most corneas Q is between 0 to -0.5

42.  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
No optimum Q value

45. 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 .

48. Pupil diameter:
optical zone (OZ) should be 0.5 mm larger
than the scotopic pupil

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

51.  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 .

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

56.  The difference between PMD and KC is only
from pacymetry
 InPMD : bell shape

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

73. 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