Corneal Tomography in Clinical Practice (Pentacam System): Basics and Clinical Interpretation Sinjab Mazen M
INDEX
Page numbers followed by f refer to figure, and t refer to table
A
Aberrations
classification of 105
measurement of 105
root mean square measurement of 105f
third-order 118
Zernike description of 111t
Aberrometers 106
Aberrometry
ingoing
feedback 106
reflective 106
Ablation zone 195
Abnormal anterior difference map 197f
Accommodation 106
Achromatic axis 3
Airy pattern 107f
Alio-Shabayek
classification 181t
modification 180
Amsler-Krumeich
classification 180, 180t, 181
standards 181t
Angle 31
alpha 4
kappa 3, 24f, 222
estimation of 4f, 5f
lambda 4
Anisometropia 94, 94f
Anterior chamber depth 16, 222
Anterior corneal surface 32, 158f
elevation maps of 45f
radius of curvature of 5
Anterior elevation map 154f, 189, 191, 193, 195f, 198f, 199f
Anterior refractive power map 35f
Anterior sagittal curvature map 33, 17f
patterns of 39f
Anterior sagittal map 35f, 206
Anterior segment optical coherence tomographers-based tomography 12f
Anterior tangential map 189, 190f, 195f, 198f, 206
Apex position 17
Aqueous humor 30
Arcuate incision 220f
Aspheric lenses 169
Asphericity 127
patterns of 76
Astigmatic aberration 117
Astigmatic disparity 99, 99f, 146
management of 100
probabilities of 100t
types of 100
Astigmatic lower-order aberration 83, 117f
Astigmatism 83, 99, 101
axis of 16
classification of 83
compound 84
hypermetropic 84
myopic 84
hyperopic 4
mixed 4, 84
irregular 85
nonperiodic irregular 85
oblique 38, 39f, 53f, 85
origin of 83
periodic irregular 85, 85f
secondary 119
simple 84
tomographic 24, 26f, 99, 100, 101
treatment of 219
types of 83
Asymmetric bowtie
inferior steep 40f
superior steep 40f
B
Back vertex distance 95
Belin ABCD grading system 180
Belin ABCD keratoconus staging 182f, 182t, 183f
comparative display of 183f
Belin/Ambrósio display 178f, 196f, 197f
Belin/Ambrósio ectasia display 54, 56f, 58f, 189
abnormal 59f
false-negative 62f
false-positive 63f
normal 57f
Belin/Ambrósio enhanced ectasia 54, 54f
clinical application of 60
display, principle of 55f
Bell and Globus patterns, high-risk 165f
Bell pattern 64, 65f
Best-corrected visual acuity 88
Best-fit-sphere 19, 47, 48, 75
reference surface 49f, 50f, 51f, 54, 162f
Best-fit-Toric ellipsoid reference surface 50f, 163f
Bowman's layer 89
C
Capture, validating quality of 22
Cataract 99
incision, characteristics of 219
surgery 148, 218
Central corneal thickness 6, 30, 143, 180
Central flat island 88f
Central island 51, 51f, 87, 88f
Chamber volume 31
Clown face 43f
Color map appearance 16
Coma 118, 120f
Cone location and magnitude index 185
Contact lenses 143
use of 99
Conventional camera 11f
Cornea 6f, 9f, 21, 173, 177
abnormal 173t, 177
back, posterior surface 30
front, anterior surface 29
guttata 64, 219
irregular 61f
meridional power of 96f, 97f
normal high-astigmatic 209
normal low-astigmatic 210, 210f, 211f
oblate 76, 130f
parabolic 135f
positive-prolate 130f
volume 31
Corneal and ocular spherical aberration, range of 133f
Corneal apex power 185
Corneal asphericity 6, 75, 76, 127, 139
effect of 137, 139
principle of 75f
Corneal astigmatism 40f, 47f, 51f, 161, 220, 220f, 221f
total 25f
Corneal asymmetry 76, 77, 79
Corneal changes with age 109
Corneal crosslinking 64, 184, 218
Corneal curvature map, patterns of 37
Corneal dimensions 5
Corneal epithelium, effect of 6f
Corneal geometry 3, 5, 8
Corneal graft 148
after removal of 91f
before suture removal 90f
Corneal incisions 220f
Corneal irregularities 218
Corneal landmarks 30f
Corneal levels 106
Corneal opacities 99, 148
Corneal optics 3
Corneal parameters 29, 29f
Corneal pathologies 148
Corneal periphery 18f, 195
Corneal power 6
maps 32
measurement 32, 32t
Corneal refraction 81
Corneal scar 66f69f
tomographic features of 89
Corneal shape 6, 75, 127
Corneal spherical aberration 127, 128
after hyperopic ablation 134f
after myopic ablation 133f
Corneal surgeries 148, 219
previous 99, 148
Corneal thickness 6, 32, 158f, 186
map 20f, 64f, 164f, 165f, 206
overlay for 22f
spatial profile 64, 66, 70f, 219
thinnest 6, 158, 186, 187f
Corneal tomography 94, 106, 171, 218
systematic interpretation of 141
Corneal topography 94
Corneal topometry 75
anterior 190, 191f, 195f, 199f
Corneal toricity 75
Corneal wavefront, fourier analysis of 123f, 124
Corneal zones 6
Corrected distance visual acuity 180
Crab claw 43f
Crystalline lens changes with age 109
Curvature
anterior radius of 187
color scale for 17f
map 16f, 17, 38, 75, 76, 144
anterior 144f
posterior radius of 187
radius of 5, 14
Cycloplegic refraction 94
D
Disciform pattern 64f, 191f
Discrete pattern 201, 202f
Display 55
Droplet pattern 65, 66f
Dry eye 92f, 149f, 150f
disease 146, 218
E
Ectasia 60, 173, 189, 194f
after laser vision correction 176f
Ectatic corneal diseases 31, 48, 54, 78f, 79f, 95, 110, 143, 173, 171, 173t, 180, 184, 185t, 189, 206, 218
diagnosis of 64
early 110
evaluation of 21
grading systems of 180
tomographic characteristics of 173
Efficient optical zone 91, 195
Elevation maps 17f, 19, 45, 45f, 46f, 50f, 53f, 75, 76, 79, 145, 158, 186, 207, 210
abnormal 49f, 50f
color scale of 45f
high-risk 162f, 163f
overlay for 19f
patterns of 49
principle of 45, 45f
Emmetropia 139
Enantiomorphism 152, 152f156f
Epithelium, remodeling characteristic of 7f
Equivalent K-reading power map 35
Excess tear 151f
film 146
Eye, optical system of 3f
F
Focus, depth of 127, 132, 134f, 135f, 139f
Forme-Fruste keratoconus 173, 178f, 205
Four-composite
refractive map 67f, 93f, 98f, 147f, 156f
selective map 68, 194f, 198f, 200f
Fourier analysis 109, 123, 123f, 124, 126f
Fourth order aberrations 118
Foveola 3
Fuchs’ endothelial dystrophy 64, 219
G
Geometric tomography 75
Globus pattern 65, 66f
H
Height asymmetry, index of 185
Higher-order aberrations 48, 99, 105, 111, 114f, 117, 118f
Holladay report 22f, 24f, 205, 205f, 207, 208f, 210f212f, 214, 214f, 215f
specific settings for 20
Horizontal displacement pattern 64, 65f
Horizontal white-to-white diameter 222
Hot spot pattern 144f, 200
Human eye, optical system of 3
Hybrid devices 13
Hyperopia 4, 139
Hyperprolate 128
cornea 128f, 131f, 135f
curvature pattern 77f, 78f
I
Incision
clock-hour extension of 221f
length 220f
location 220f
orientation 221f
planes 221f
shape 220f
Inferior meniscus pattern 201f
Inferior steep 40f
Intereye corneal asymmetry score 155t, 162, 167t
Internal aberration compensation 109
Intraocular lens 12, 205
selection 110
Intraocular pressure 31
Irregular astigmatism 85, 99, 223
evaluation of 90
objective evaluation of 94
subjective evaluation of 91
suspicion of 91
treatment of 219
types 112
K
Keratoconic cornea 183f
Keratoconic eye 145f
Keratoconus 98f, 123f, 143, 173, 174f, 179f, 185, 211, 212f
Alio-Shabayek classification of 181t
Amsler-Krumeich classification of 180t
classification of 180, 181t
pellucid-like 173, 175f, 213f, 214f
peripheral 10
posterior 177, 179f
progression 187f
Keratoglobus 173, 176f
Keratometric dioptric power 95
Keratometry 8, 87
Keratoplasty, penetrating 87
Keratotomy, astigmatic 148, 221
K-readings after corneal crosslinking, changes of 188f
L
Lamellar ablation 87
Lamellar keratoplasty 89
Large angle
kappa 24f, 99, 63f, 146, 156f
lambda 146
Laser vision correction 64, 88, 99, 148, 173, 219
Lazy S-shape pattern 72f
Left eye
corneal shape of 5f
four-composite refractive map of 156f
Lens
subluxation 99
thickness 31
Lenticular astigmatism 99
Light-emitting diodes 10
Limbal relaxing incisions 148, 221
Limbal zone 6
Lower-order aberrations 105, 115
Low-risk elevation maps 162f, 163f
Low-risk irregular curvature map 168
M
Macro-irregular pattern 87
Maps 206
Maximum K-reading 184, 187f
Mean anterior keratometry 167
Mean posterior keratometry 167
Mercedes-Benz image 119f
Meridians 5
Micro-irregular pattern 87
Mild peripheral extrapolation 19f
Minimum radius position front 17
Modulation transfer function 107, 109f
principle of 108f
Myopia 139
N
Negative prolate 128
corneal shape 128f
Negative spherical aberration 131f
Neural adaptation 135
range 169
Nodular point 3
Nonoptimum spectacle-corrected distance visual acuity 91
Nontoxic spherical aberration 138f
Normal cornea 6f, 182f
Fourier analysis of 126f
Normal elevation maps 49f, 50f
O
Objective corneal
dioptric power 95
refraction 95
Objective dioptric power, meridional power of 96f98f
Objective spherocylindric dioptric power 95
Oblate curvature pattern 77f
Oblate elevation pattern 78f
Oblique curvature asymmetry 78f
Ocular pathologies 106
Ocular spherical aberration 128, 135
Ocular surface disease 10, 12, 218
Ocular surgeries, previous 106
Optical axis 3, 38
Optical coherence tomographers 8
Optical transfer function 108
Optical zones 190f, 195f
P
Pachy apex 30
Pachymetric data 54, 55
Pachymetric progression index 67, 70f
Pachymetry 17f
map 64, 69f, 167f, 195f, 201, 211
low-risk relative 167f
relative 160, 189, 191, 191f, 193, 199f, 207, 209, 213
profiles 160, 165f
thinnest 167
Para ectasia 173, 177f, 178f
Parabola 127
Paracentral zone 6
Parameters 46
Pellucid marginal degeneration 10, 64, 173, 175f, 214f, 215f
Peripheral corneal scar 92f
Peripheral scars 10
Peripheral zone 6
Photokeratoscopy 8
Pinhole test 91
Piston 116f
Placido cone 9
Placido disk 8f
Placido mires, reflection of 9f
Point spread function 107
Postastigmatic ablation pattern 198f, 199f
Postcorneal-graft pattern 199, 200f, 201f
Posterior corneal surface 32
radius of curvature of 5
Posterior elevation map 21f, 50f, 89, 154f
Posterior tangential curvature map 33
Posthyperopic ablation pattern 194f197f
Posthyperopic astigmatic ablation pattern 199f
Posthyperopic laser ablations 7f
Postkeratoplasty 206
Postkeratorefractive surgeries 206
Postmydriatic test 94
Postmyopic ablation pattern 190f, 191f, 198f
Postoperative corneal
flattening 223
steepening 222
Power distribution display 98f
Practical subjective
optical coherence tomographers selection 169, 170
scoring system 30, 157, 157t, 178
Premium lenses 169
Previous corneal refractive surgeries, diagnosis of 64
Progression, parameters of 184
Pterygium, bilateral 93f
Pupil 3
center 30
coordinates 146
intereye symmetric coordinates of 24f
size 222
Pupillary axis 3
Pupillary levels 106
Purkinje images 3
Pyramid display 113f
Q
Quality specification 22, 145
Quick slope 71f
Q-value and corneal asphericity 128
R
Rabinowitz method 41f
Radial keratotomy 148
Reference surface 45
parameters of 47f
Reflective aberrometry 106
Reflex, irregular 91
Refractive effect 32
Refractive errors 117
Refractive index 5, 30, 32
Refractive power map 33
Refractive surgery
candidate, right eye of 152f
laser-based 162, 168t
Regular astigmatism 83, 84f, 124, 125f
treatment of 219
Retina 3
Retinal level 107
Retinoscopy 91
Right eye
corneal geometry of 5f
four-composite refractive map of 156f
superior view of 3f
Rigid gas permeable 91
contact lenses 143
Root mean square 88, 105f, 106
S
Sagittal curvature map 42, 161f
Sagittal map 209211
Scheimpflug camera 11f
principle of 10f
Scheimpflug image 99f
Scheimpflug-based technology, principle of 11f
Scleral incisions 220f
Secondary astigmatism 119
higher-order aberration 122f
Segmental elevation asymmetry 80f
Sight, principle line of 3
Sim-K measurement 8f
Skewed radial axis index 42, 161f, 221
high-risk 160f
Slit-scanning technology, principle of 10f
Soft contact lens 93f, 143, 144f
Spatial thickness profile 145
Spheric lenses 169
Spherical aberration 32, 35f, 121f, 128, 129f, 131f, 132, 132f, 134f, 136f, 139f
classification of 136f
positive 130f
range of 132f
types of 129f
Spherical component 124, 124f
Spherical cornea 76, 130f
depth of focus in 134f
Spherical curvature pattern 77f
Spherical elevation pattern 77f
S-shape pattern 71f
Standard deviation 49
Steep and flat semimeridians, significant segmentation of 38f
Sturm interval 84f
Subjective refraction, prediction of 109
Surface ablation 87
T
Tangential map 209211
Tear film disturbance 99, 146
Tetrafoil 119, 122f
Thickness map 74f, 145, 155
abnormal relative 74f
overlay 19
Thin cornea, normal 174f
Tolerable spherical aberration 137f
Tomographers
elevation-based 10, 13
optical coherence tomographers-based 11
types of 11
Tomography 8, 11, 12t
elevation-based 12
optical coherence tomographers-based 12
skillful interpretation of 157
Tongue-like extension pattern 80f
Topographers 8, 13
Topography 8, 11, 12, 12t
Topometric display 160f
Topometric indices 185
Topometry display 159f
Total corneal refractive
power 25f, 95, 99
map 37, 37f
principle 37f
Toxic spherical aberration 138f
Transitional zone 191, 195
Trefoil 118, 119f
True corneal astigmatism 219
True net power map 34
True refractive index 32
U
Uncorrected distance visual acuity 143
V
Vertex keratoscope normal 3
Vertical asymmetry, index of 185
Vertical displacement pattern 64, 65f
Videokeratoscope 3, 37, 38
Videokeratoscopy, computerized 9
Visual acuity and manifest refraction 186
Visual axis 3, 38
Visual function and spherical aberration 132
Vortex pattern 44f
W
Wavefront analysis 94
and measurements, basics of 105
principles of 105
Wavefront principle 105f
Wavefront science 103
Wavefront technology, clinical application of 109
Z
Zernike analysis 111, 168f
Zernike coefficient 109, 111, 112
Zernike polynomials 111, 112, 115
display, standard setting of 113f
Zernike pyramid 106
×
Chapter Notes

Save Clear


1Introduction
  • Corneal Optics and Geometry
  • Measuring Corneal Geometry
  • Screening Guidelines
2

Corneal Optics and GeometryCHAPTER 1

 
THE OPTICAL SYSTEM OF THE HUMAN EYE
The optical system of the human eye is composed of:
  • Four main noncoaxial optical elements: The anterior and posterior corneal and lens surfaces.
  • The pupil.
  • The retina. It is aplanatic to compensate for the native spherical aberration (SA) and coma through its nonplanar geometry.
The optical surfaces are aligned almost coaxially, but the deviations from a perfect optical alignment result in a range of axes and their inter-relationships (Fig. 1). That guides us to the following definitions:
  • The visual axis (VA): It is the line connecting the fixation point with the foveola, passing through the two nodal points of the eye, but not necessarily through the pupil center.
  • The optical axis (OA): It is the axis connecting the center of curvatures of the optical surfaces of the eye. It can be recognized by the Purkinje images I, II, III, and IV, namely of the outer corneal surface (I), inner corneal surface (II), anterior surface of the lens (III), and the posterior surface of the lens (IV). If the ocular optical surfaces were perfectly coaxial, these four images would be coaxial, but seldom observed.
  • The principle line of sight (LOS): It is the ray from the fixation point reaching the foveola via the center of the entrance pupil (EP).
  • The pupillary axis (PA): It is the normal line to the corneal surface that passes through the center of the EP and the center of curvature of the anterior corneal surface.
  • The achromatic axis: It is defined as the axis connecting the EP center with the nodal points.
  • The vertex keratoscope (VK) normal: It is the axis that is perpendicular to the plane of the capturing machine (original the keratoscope) and intersecting with the anterior corneal surface at the corneal apex (corneal vertex). Therefore, corneal apex is not necessarily the highest point of anterior corneal slope and not necessarily the anatomical center of the cornea.
  • Angle kappa (measured in degrees): It is the angle between PA and VA. Angle kappa may be a source of false findings and should be differentiated from misalignment (Chapter 17).
    zoom view
    Fig. 1: Optical system of the eye (superior view of the right eye): Surfaces, angles, and axes.
    (EP: entrance pupil [the opening within the dotted line]; F: foveola; FP: focal point; LOS: line of sight; N: nodular point; OA: optical axis; PA: pupillary axis; VK: video keratoscope axis; VA: visual axis).
    4
    It also affects the decision and the plan for laser-based and lens-based refractive surgery. In laser-based refractive surgery, angle kappa should be compensated for by recentration of the laser profile and the flap cut, particularly in hyperopic treatment (hyperopia, hyperopic astigmatism, and mixed astigmatism), and in myopic astigmatic treatment when the myopic astigmatic magnitude is ≥1.5 diopters (D). In lens-based refractive surgery, premium IOL implantation is contraindicated when angle kappa is >400 µm to avoid the risk of postoperative intractable dysphotopsia.
    Normal distribution of angle kappa was studied by using Orbscan II (Placido-based) and the Synoptophore. It was found that values of angle kappa measured by the Orbscan II were almost as twice as when measured by the Synoptophore. Based on Orbscan II, Hashemi and associates determined an average value of angle kappa of 5.46 ± 1.33° in Iranian adults with an insignificant intergender difference. In another study, Gharaee H and associates determined average value of 4.96 ± 1.38° in total, average horizontal angle kappa of −0.02 ± 0.49 mm, and average vertical angle kappa of −0.09 ± 0.32 mm.
    In addition, studies reporting normative angle kappa values in different conditions found that angle kappa was significantly higher in exotropes than in esotropes or controls, and tended to be larger in the left eye than in the right eye. Moreover, there was a positive correlation between angle kappa and positive refractive errors, which can be explained by the negative correlation with the axial length of the globe.
    Unlike Placido-based topographers, Scheimpflug-based tomographers cannot measure angle kappa directly. Therefore, the Pentacam cannot measure angle kappa directly. There are two methods to estimate the angle by the Pentacam, chord μ in the Holladay report (Fig. 2), and considering half the values of X and Y coordinates of pupil center if Holladay report is not available. Chord μ is the chord distance from vertex normal (assumed to be the visual axis) and the EP. On the Pentacam, the normal value is 0.20 ± 0.11 mm, so values above 0.42 mm (highlighted in yellow) would be highly unusual. Figure 3 shows the X and Y coordinates of the pupil center. In this example, angle kappa is estimated to be (−0.10, +0.02) in OD and (−0.02, −0.05) in OS.
  • Angle alpha (measured in degrees): The angle formed at the first nodal point by OA and VA.
  • Angle lambda (measured in degrees): The angle between PA and LOS.
  • Chord μ (measured in mm): As mentioned above, it is the chord distance from vertex normal (assumed to be the visual axis) and the EP.
The refractive power of the human eye comes mainly from the cornea and the crystal lens. In emmetropia, corneal power ranges from 39 to 48 D (average 43.05 D), while the power of the crystalline lens ranges from 15 to 24 D (average 19.11 D). The refractive media in the human eye is: tear film (n = 1.336), cornea (n = 1.376), aqueous humor (n = 1.336), crystalline lens (n = 1.406), and vitreous humor (1.336); where “n” is the refractive index (RI) of the media measured relatively to air (n = 1.000). The dioptric power of the media is determined by the radius of curvature, the RI, and the distance amongst various interfaces.
zoom view
Fig. 2: Estimation of angle kappa in the Pentacam by the Holladay report.
5
zoom view
Fig. 3: Estimation of angle kappa in the Pentacam by the pupil center coordinates.
 
CORNEAL GEOMETRY
The cornea has two surfaces separated by corneal substance. The anterior surface is coated with the tear film, and they form one refractive surface separating air from the corneal substance. The posterior surface separates corneal substance from aqueous humor. The cornea is not a part of a perfect sphere. The shape of both surfaces is defined as an aspheric prolate, toric, asymmetric conoidal shape (Figs. 4 and 5). Each of the previous expressions is explained in detail in the following paragraphs.
 
Corneal Dimensions
Corneal dimensions include diameters, meridians, radii of curvature, corneal zones, corneal thickness, corneal shape, and corneal power.
 
Diameters
The sclerocorneal junction (base of the cornea) is an ellipse. The vertical corneal diameter is 10.6 mm on average, whereas the average horizontal corneal diameter is 11.7 mm.
 
Meridians
The normal cornea in the adults has two meridians that are 90° apart. Due to the elliptical base of the cornea at the sclerocorneal junction, the vertical diameter is generally shorter than the horizontal one, meaning that the vertical meridian is steeper (smaller radius of curvature) than the horizontal one (greater radius of curvature). Due to this difference, corneal shape is considered as toric. This toricity is responsible for corneal astigmatism. In younger eyes, this toricity is represented as with-the-rule (WTR) astigmatism, where the vertical meridian is steeper than the horizontal one. This steepness reverses with age, leading to against-the-rule (ATR) astigmatism.
zoom view
Fig. 4: Corneal geometry of the right eye (OD).
(T: temporal; N: nasal; Ra: radius of curvature of the anterior corneal surface; Rp: radius of curvature of the posterior corneal surface; n: refractive index).
zoom view
Fig. 5: Corneal shape of the left eye. The normal human cornea has a conoidal shape.
 
Radius of Curvature
The cornea has two surfaces, anterior with an approximate radius of 7.8 mm and posterior with an approximate radius of 6.5 mm. These two radii are for the central (axial) zone of the cornea. The radii increase while moving to the periphery, indicating a flatter corneal periphery. The normal cornea flattens progressively from center to periphery by 2–4 D, with the nasal area flattening more than the temporal area; this is shown on the curvature map as the nasal side becoming blue (flat)6 more quickly (Fig. 6). The normal average anterior/posterior radii ratio is 1.21 in virgin nonoperated corneas. This ratio is altered by keratorefractive surgeries, which is a leading source of wrong IOL measurements.
 
Corneal Thickness
Due to the difference in radius between the two corneal surfaces, the cornea is thinner in its central zone than at its periphery. There are two important values in corneal thickness, the central corneal thickness (CCT) and thinnest corneal thickness (TCT). Both are discussed later in this chapter.
 
Corneal Zones
Clinically, the cornea is divided into zones that surround fixation (corneal vertex or apex) and blend into one another:
  • The central zone (central 3 mm): It overlies the pupil and is responsible for high definition vision. The central part is almost spherical and is also called the apical or axial zone.
  • The paracentral zone (3–6 mm): It has a doughnut shape with an outer diameter of 6 mm. It represents an area of progressive flattening toward the third zone.
  • The peripheral zone (6–9 mm): It is also known as the transitional zone. This zone is asymmetrically flatter than the central zone. The nasal and superior segments are flatter than the temporal and inferior ones.
  • The limbal zone (>9 mm): It is adjacent to the sclera and is the area where the cornea steepens before merging with the sclera at the limbal sulcus.
    The central and paracentral zones are responsible for the refractive power of the cornea, and they are in charge of contact lens fitting. Being steeper in the center and flatter at the periphery gives the cornea what is known as a “prolate” aspheric shape.
zoom view
Fig. 6: Nasal, temporal asymmetry in a normal cornea.
 
Corneal Shape
The corneal shape is “Conoidal” (Fig. 5). It is a composition of toricity, asphericity, and asymmetry. From a meridional viewpoint, the cornea is “Toric,” which is the source of corneal astigmatism. From the zonal viewpoint, the cornea is “aspheric” because the radius of curvature differs from the center toward the periphery. From a sectorial viewpoint, the cornea is asymmetric because the nasal sector is usually flatter than the temporal sector as shown in Figure 6.
Corneal asphericity is represented by some of values, such as Q-value, p-value, E-value, and eccentricity. The most popular one is the Q-value, which represents the ratio between the central and the peripheral radii of curvature. The relationship between corneal shape, Q-value, corneal SA, depth of focus, and contrast sensitivity is discussed in detail in Chapter 16.
 
Corneal Power
The anterior corneal surface with its associated tear film layer plays a role of a convex refractive surface. Due to both its convexity and separation between two different media: air (smaller RI; n = 1.000) and corneal substance (larger RI; n = 1.376), it is the most powerful refractive surface in the optical system of the eye. The refractive power of the central (apical or axial) zone of the anterior corneal surface is approximately 49 D.
On the other hand, although the posterior surface of the cornea is convex, it acts as a negative concave surface because it separates corneal substance (larger RI; n = 1.376) from aqueous humor (smaller RI; n = 1.336). The refractive power of the posterior corneal surface is approximately −6 D.
Moreover, corneal epithelium has an impact on corneal power. The shape of the epithelial layer is responsible for about 0.40 D of astigmatism. The mean Q-value is −0.20 ± 0.13 (0.06 to −0.60) with the epithelium and −0.26 ± 0.23 (0.07 to −1.51) without the epithelium. In other words, the cornea is more prolate without the epithelium, which means that the epithelial layer forms a negative lens (thinner in the center) as shown in Figure 7.
zoom view
Fig. 7: The effect of corneal epithelium on the corneal shape. The cornea is more prolate without the epithelium.
7
zoom view
Fig. 8: The remodeling characteristic of the epithelium. It reduces corneal irregularities.
This fact has a clinical impact on laser-based procedures, especially in surface ablation techniques. This fact is more important in the case of irregular corneal surface because the epithelium has a remodeling (filling) feature, which masks the real corneal power and a significant portion of the underlying corneal irregularities as shown in Figure 8. Moreover, the remodeling feature of the epithelium affects the outcomes of laser-based procedures, characterized by a partial loss of effect after both myopic and hyperopic corrections. The epithelium forms a positive convex lens after myopic ablation and a negative concave lens after hyperopic correction (Fig. 9).
zoom view
Fig. 9: The remodeling effect in postmyopic and posthyperopic laser ablations.
Corneal power methods of measurements and their clinical applications are discussed in Chapter 5.