PENTACAM

pentacam has been in wide use to get elevation tomography done

Placido-based (or curvature-based) systems rely on the data collected from the anterior surface of the cornea,without the information about the posterior surface, complete
pachymetric evaluation of the cornea is not possible

the posterior surface of the cornea is being more appreciated as a sensitive indicator of corneal ectasia and can often be abnormal in spite of a normal
anterior corneal surface. It is now recognized that while the refractive power of the cornea is mostly determined by the anterior
surface, the biomechanical behavior of the cornea is at least equally determined by both surfaces.

cornea is being more appreciated as a sensitive indicator of corneal ectasia and can often be abnormal in spite of a normal
anterior corneal surface. It is now recognized that while the refractive power of the cornea is mostly determined by the anterior
surface, the biomechanical behavior of the cornea is at least equally determined by both surfaces.

The Enhanced Spherical Reference Body
Michel Belin has developed this shape for early detection of KC. Figure 4.12 shows an illustration of a KC. With the BFS
float mode, the computer adjusts a spherical body (yellow line) in a position that matches the total values of elevations and
depressions in the examined surface, depending on the mean central radii as mentioned before. If the computer is asked to
delete an area of 4 mm in diameter, where the center of this area is the center of the cone itself (Figure 4.13), the computer will
adjust another spherical body (Figure 4.14: the red line). Notice the appearance of the cone when elevations are related to the
modified red body rather than the yellow one. Obviously, the cone will be more clearly displayed

Corneal Thickness Map
Principle
The computer measures the thickness of the cornea at all points depending on the elevation maps; the difference between the
front and back surface elevations indicates corneal thickness.
Clinical Use
This map is necessary for:
• Diagnosing ectatic diseases of the cornea, such as KC, PMD and iatrogenic ectasia.
• Confirming the diagnosis of diseases like Fuchs dystrophy and cornea guttata.
• Observing the progression of the previous diseases.
• Taking right decisions in refractive surgery whether it was photorefractive surgery, intracorneal ring implantation or corneal
crosslinking (CxL).
• Deciding the exact amount of correction by LASIK taking into consideration that there are certain standards to be followed:
i. The first standard: The remaining thickness of stromal bed should not be less than 55 percent of the original corneal
thickness at the thinnest location.
ii. The second standard: The ablated amount should not exceed 18 to 20 percent of the original corneal thickness at the
thinnest location.
Example 1: A patient with a 600 μ thickness at the thinnest location. The remaining bed thickness should not be less than
600 × 55% = 330 μ. This means: if we use a 100 μ flap, the amount allowed to be ablated is 600 – 330 – 100 = 170 μ. But,
according to the second standard, this is not recommended. It is strongly recommended to ablate no more than 600 × 18
= 108 μ.
Example 2: A cornea with a thickness of 490 μ at the thinnest location and the patient is myopic –8 D. These –8 spherical
diopters need 8 × 14 = 112 μ to be ablated, that means we exceed the allowed limit, which is about 88 μ (490 × 18 = 88.2).
There are three methods to solve the problem:
1. Reducing the diameter of the ablation zone to reduce the amount of ablation and gain some more correction, but
this may produce spherical aberrations and night glare particularly in eyes with light colored iris.
2. Reducing the amount of correction, which means that the patient will have residual refractive error.
3. The treatment modalities such as intraocular refractive lenses (phakic IOLs).
• Planning for intracorneal rings in the management of KC and PMD. Very briefly, it is essential to study the thickness in the
whole resumed passage of the ring in order to plan for the plane of insertion and to avoid intraoperative complications.
Main Elements
The computer displays the thickness map in two patterns:
1. Five values: A central value representing the central thickness, and four values around at the 5 mm central circle
(Figure 5.1A).
2. Distributed values all over the cornea (Figure 5.1B): The distributed pattern is more important and valuable.
However, those are not the main elements of the corneal thickness map. The main elements are three locations that appear
on the main page (Figure 5.2): the thinnest location, the corneal apex and the pupil center. These three locations are displayed
with their coordinates, where the corneal apex is the origin point (zero point). The direction of axis X is from the patient’s right
to his/her left when the patient is seated opposite to the physician. The direction of axis Y is from the bottom up. Example: A
point “e” in the left cornea is located at “+0.3, -0.5” position, i.e. this point is located 0.3 mm temporal to and 0.5 mm inferior
to corneal apex. The relationship between the coordinates of the three main elements is of great importance as will be discussed
in the next paragraph.

Main Page
Analysis

The main page in Pentacam consists of figures on the left and four maps on the right. The figures are for cornea front (anterior
corneal surface), cornea back (posterior corneal surface), parameters of thickness and miscellaneous. The main four maps are:
anterior sagittal curvature map, anterior and posterior elevation maps and pachymetry (thickness) map (Figure 8.1). The main
four maps should be studied all together

Cornea Front Surface (Figure 8.3)
Meanings of Abbreviations
K1: Curvature power of the flat meridian of the cornea measured within the central 3 mm circle and expressed in diopters.
K2: Curvature power of the steep meridian of the cornea measured within the central 3 mm circle and expressed in diopters.
Km: Mean curvature power of the cornea within the central 3 mm expressed in diopters.
Rh: Horizontal curvature radius of the central 3 mm expressed in millimeters.
Rv: Vertical curvature radius of the central 3 mm expressed in millimeters.

Rm: Mean curvature radius of the central 3 mm radius expressed in mm.
Qs: Quality specification. It specifies the quality of the topographic capture and should be displayed “OK.” Otherwise, there is
some missed information which was virtually produced (extrapolated) by the computer and the capture should preferably be
repeated.
Q-val: Value of Q within the central 6 mm as shown between two brackets. Any other central circle can be chosen through the
program settings.
Astig: Amount of corneal astigmatism on the front cornea surface, i.e. the amount of differentiation between the two curvature
radii (K2 – K1) within the central 3 mm.
Axis: The axis of corneal astigmatism within the central 3 mm.
Rmin: Minimum radius of curvature expressed in millimeters. It is shown as a symbol as shown in Figure 8.4. It is not necessarily
central. The importance of the Rmin will be discussed as in the next paragraph.
Rper: Radius of corneal curvature in the peripheral 9 mm of the cornea expressed in millimeters.

Cornea Back Surface (Figure 8.11)
The cornea back surface plays the role of a concave lens; it separates between two refractive media of different refractive power,
and the incident light comes from the higher refractive medium (the cornea: index 1.376) to the lower refractive medium (the
aqueous humor: index 1.336). Therefore, the curvature power is displayed on the back surface as negative values (see Figure 3.5).
Values displayed in this field are not clinically important since there are other more important figures and maps that estimate
the posterior corneal surface such as the keratometric power deviation map as mentioned

Coordinates in Corneal Thickness (Figure 8.12)
Pachy Apex
It is corneal thickness at the apex. The computer considers the apex as the origin of the coordinates, X for the horizontal and
Y for the vertical. Therefore, zero is displayed in both squares of pachy apex coordinates. The direction of axis X is from the
patient’s right to his/her left, when the patient is seated opposite to the physician. The direction of axis Y is from the bottom up.
Example: a point “e” in the left cornea is located at “+0.3, –0.5” position, i.e. this point is located 0.3 mm temporal to and 0.5
mm inferior to corneal apex

Pupil Center
It is corneal thickness at the point corresponding to pupil center. The X- and Y-coordinates show the position of the pupil center
according to the apex. The two coordinates differ according to pupil medriasis or miosis; the pupil center is usually shifted
superior-temporally when dilated. This is very important in the process of decentration, or what is known as offset pupil; it will
be discussed in Chapter 14 in details.
Thinnest Location
It is the thinnest point in the cornea. It is of most important in the process of taking the decision in refractive surgery. The
X- and Y-coordinates indicate the position of the thinnest location according to the apex. This has been mentioned i

Keratoconus
Indices

Keratoconus (KC) indices page consists of two diagrams, one table, progression index, indices of corneal irregularity and KC
staging (Figure 9.1).
Thickness/Location Diagram (Figure 9.2)
This diagram describes the change of corneal thickness in relation to location. The horizontal axis represents the location as
circles centered on the thinnest location. These circles have diameters of 2, 4, 6, 8 and 10 mm. The vertical axis represents corneal
thickness.
The computer measures the average corneal thickness of all points distributed at each circle. A diagram is then drawn
showing the relation between corneal thickness and the location. The computer gives red color to the curve of the examined
cornea. The black-dashed lines show the result of the standard value study (the study carried out on normal untreated corneas
to adopt the average values as standard). The central line represents the average value of the corneal thickness of all corneas
analyzed. The upper or lower line represents the double standard deviation (CI: 95%) of the corneal thickness.
What is important for the red curve is: First, the red curve should be within the normal range. Second, the course of the red
curve should follow the course of the normal range. If the red curve deviates at the 6 mm circle or after, it is normal. Otherwise, it
is a risk factor because the quick downward deviation means that the corneal center is relatively thin in relation to the periphery
which has been proven to be risky either with LASIK, or to some extent with PRK, whether the patient is hyperopic or myopic

Occasionally, the curve may take an S-shape as shown in Figure 9.4. Therefore, whenever this sign is seen, corneal topography
should be studied carefully in both eyes, family history of KC should be taken carefully, and whenever possible, corneal
topography to first degree relatives should be studied.
Thickness/Location Percentage Diagram (Figure 9.5)
The horizontal axis represents the location as circles centered on the thinnest location. These circles have diameters of 2, 4, 6, 8
and 10 mm. The vertical axis represents the percentage of increments in thickness.
If corneal thickness at the thinnest location is, for example, 500 μ and the average corneal thickness at the 2 mm and 4 mm
circles is 550 μ and 600 μ respectively, it means that the percentage of increments in thickness at the 2 mm circle is 10 percent,
and at the 4 mm circle is 20 percent, and so on for the other circles.
The computer gives red color to the curve of the examined cornea. Here also, the black-dashed lines represent the result
of the standard value study (the study carried out on normal untreated corneas to adopt the average values as standard). The
central line represents the average value of the corneal thickness of all corneas analyzed. The upper or lower line represents the
double standard deviation (CI: 95%) of the corneal thickness.
It is important for the red curve to be within the normal range and the course of the red curve to be parallel to the normal
range. The red curve is normal when it deviates at the 6 mm circle or after. Otherwise, it is a risk factor because the quick

Progression Index
Thickness/location relationship can be introduced as an index called “the progression index”. The normal value of the progression
index is ≤1.1. The progression index reflects the rapid change of thickness from the thinnest location to corneal periphery; the
bigger the progression, the higher the index and vice versa. High index (>1.1) is usually encountered in KC and ectatic corneal
disorders.
Figure 9.10 shows:
Avg: Average value of the progression index of the whole cornea (red).
Min: Average value of the progression index on the meridian representing the smallest change in corneal thickness (green) as
shown in Figure 9.11.
Max: Average value of the progression index on the meridian representing the largest change in corneal thickness (blue) as

Indices of Corneal Irregularity
There are 8 indices representing corneal irregularity within the central 8 mm of the cornea. They are displayed within squares.
White squares indicate normal values, yellow squares indicate border line values, and red squares indicate abnormal values. In
case of KC, many red squares can be seen (Figure 9.12