purpose. To investigate the topography of the central and peripheral cornea in a group of young adult subjects with a range of normal refractive errors.

methods. Corneal topography data were acquired for 100 young adult subjects by a method that allows central and peripheral maps to be combined to produce one large, extended corneal topography map. This computer-based method involves matching the common topographical features in the overlapping maps. Corneal height, axial radius of curvature, and axial power data were analyzed. The corneal height data were also fit with Zernike polynomials.

results. Conic fitting to the corneal height data revealed the average apical radius (*R*o) was 7.77 ± 0.2-mm and asphericity (*Q*) was −0.19 ± 0.1 for a 6-mm corneal diameter. The conic fit parameters were both found to change significantly for increasing corneal diameters. For a 10-mm corneal diameter, *R*o was 7.72 ± 0.2 mm and *Q* was −0.36 ± 0.1. A slight but significant meridional variation was found in *Q*, with the steepest principal corneal meridian found to flatten at a slightly greater rate than the flattest meridian. The RMS fit error for the conic section was found to increase markedly for larger corneal diameters. Higher-order polynomial fits were needed to fit the peripheral corneal data adequately. Analysis of the axial power data revealed highly significant changes occurring in the corneal best-fit spherocylinder with increasing distance from the corneal center. The peripheral cornea was found to become significantly flatter and to decrease slightly in its toricity. Individual subjects exhibited a range of different patterns of central and peripheral corneal topography. Several of the higher order corneal surface Zernike coefficients were found to change significantly with increasing corneal diameter.

conclusions. Highly significant changes occur in the shape of the cornea in the periphery. On average, the peripheral cornea becomes significantly flatter and slightly less astigmatic than the central cornea. A conic section is a poor estimator of the peripheral cornea.

^{ 1 }in rigid and soft contact lens fitting,

^{ 2 }

^{ 3 }

^{ 4 }in the screening of refractive surgery candidates,

^{ 5 }

^{ 6 }and for customized refractive surgery corrections.

^{ 7 }

^{ 8 }

^{ 9 }

^{ 10 }

*R*o) and the asphericity parameter (

*Q*), which describes the type of conicoid that best fits the corneal shape (i.e., the degree to which the surface departs from a sphere) where

*Q*= 0 describes a sphere,

*Q*> 0 describes an oblate or steepening ellipse, and −1 <

*Q*< 0 describes a prolate, or flattening, ellipse.

*R*o and

*Q*for a normal adult population.

^{ 11 }

^{ 12 }

^{ 13 }

^{ 14 }

^{ 15 }These studies have produced average

*R*o ranging from 7.68 to 7.85 and average

*Q*ranging from −0.33 to −0.18 (i.e., nearly all subjects exhibit corneas with a prolate elliptical shape). Although the average results across these studies have been relatively consistent, most investigators note that large variations in corneal shape exist between normal subjects. Recently, more complex quantitative descriptors of shape such as Fourier series analysis

^{ 16 }and Zernike polynomials

^{ 17 }

^{ 18 }have been used to describe the shape of the cornea.

^{ 19 }Corneal coverage with Placido-based videokeratoscopes is improved by using a small measurement cone, as obscuration by the nose and brow is avoided. Videokeratoscopes based on other principles (such as slit scanning and raster stereography techniques) have the potential to measure larger corneal areas than Placido-based systems,

^{ 20 }although they are still limited by interference from the eyelids and eyelashes. However, these other techniques have generally not proved to be as precise or accurate in their measurements as small cone Placido-based systems.

^{ 21 }

^{ 22 }

^{ 23 }used offset fixation points in a videokeratoscope to measure peripheral corneal topography in 11 subjects and found that for peripheral measurements, an ellipse was a poor descriptor of cornea shape, due to increased flattening of the corneal surface in the periphery. An average tangential radius of 11.29 ±1.82 mm was found at 4.5 mm from corneal center for the 11 subjects tested. Reddy et al.

^{ 3 }classified subjects’ corneal astigmatism based on central and peripheral corneal topography data.

^{ 24 }Using this technique, we measured the total corneal topography of the right eyes of 100 subjects. The purpose of this study was to provide normative information regarding the shape of the total cornea for a large population of young, healthy, adult subjects with a range of normal refractive errors.

^{ 21 }Tang et al.

^{ 21 }found the E300 to exhibit a mean height error of 2 μm for measuring spherical and aspheric test surfaces. This instrument has also exhibited highly repeatable results for measurements on corneas in vivo.

^{ 22 }Cho et al.

^{ 22 }found that two repeated measurements were required with the instrument to ensure a precision in corneal elevation data of 2 μm. The videokeratoscope has a sophisticated range-finding device that determines the distance from the corneal apex to the instrument’s camera and automatically captures the videokeratoscopic image only when good focus and alignment of the eye are attained. To reduce the effects of any diurnal variation in corneal topography, all measurements were taken in the morning.

^{ 25 }As prior visual tasks may also affect corneal topography, subjects were also asked to refrain from performing significant close work immediately before testing.

^{ 25 }

^{ 26 }

^{ 24 }The corneal topography of one eye is measured while the subject views an external-fixation target positioned 1.5 m away from the videokeratoscope with the fellow eye (through the use of a mirror). The external-fixation target has a central target and six peripheral targets in different peripheral angles of gaze (at 0°, 60°, 120°, 180°, 240°, and 300° and approximately 30 cm from the central target). Three videokeratoscope images are captured with the subject fixating on the instrument’s internal central fixation target. The subject is then instructed to fixate on the external target with the fellow eye, and videokeratoscope images are captured with the subject looking at each of the six peripheral fixation targets in turn. A total of four videokeratoscope images are captured for each of the six peripheral directions of gaze (i.e., 3 central images and 24 peripheral images are captured).

^{ 24 }that examines the correlation between central and peripheral corneal topography data. The purpose of this process is to find the point in the peripheral map corresponding to the center of the central topography map (the vertex normal). Essentially, it locates the point on the peripheral map (defined by the radial distance, azimuthal angle, and degree of cyclorotation from the center of the peripheral map) where the sum of squares of differences in the overlapping portions of the central and peripheral corneal topography maps is minimized.

^{ 24 }compared the original central map data with the rotated peripheral map data. Data from a conic test surface showed a less than 0.5 μm difference between central and rotated peripheral data, and data from a real cornea showed errors between the central and rotated peripheral maps of less than 1 μm across the central 7 mm of data.

^{ 27 }

^{ 28 }

^{ 29 }A control experiment was performed to investigate whether the eye movements (and subsequent changes in extraocular muscle tension) used in our protocol to capture the peripheral corneal topography maps had an effect on the peripheral corneal topography data. Topography maps of the temporal corneal periphery were taken for two subjects, first by moving the videokeratoscope camera (i.e., no eye movements used) and second by changing fixation (i.e., eye movements used). The peripheral corneal topography data from the two conditions was then compared. No significant difference was found between the peripheral corneal data with or without eye movement. We concluded from this, that the alteration of extraocular muscle tension associated with the change in fixation in our peripheral map capturing protocol (approximately 11°) was not enough to cause significant changes in the peripheral corneal topography data.

^{ 30 }The position of the vertex normal relative to the geometric center of the cornea is known to differ from person to person.

^{ 31 }Therefore to have each subject’s corneal topography map centered to a common reference point, we rotated each combined map to the corneal geometric center. The best central videokeratoscope image (as determined through the map correlation process) for each subject was analyzed to determine the position of the corneal geometric center using customized computer software that locates the corneal limbus in the videokeratoscope image.

^{ 32 }

*R*o and

*Q*were calculated based on Baker’s equation for conic sections:

*y*

^{2}= 2

*R*o

*x*−

*px*

^{2}, where

*y*is the distance from corneal center and

*x*is the corneal height.

^{ 33 }A linear in parameters least squares fitting was performed to calculate the

*R*o and

*p*for the averaged semimeridian data. The asphericity parameter

*Q*is related to

*p*by the equation

*Q*=

*p*− 1.

*R*o and

*Q*were calculated for 6-, 8-, 9-, and 10-mm corneal diameters. For this average across all semimeridians, only subjects with at least 200 semimeridians of complete data were included in the analysis. Any missing data (even in the combined maps) tend to be in the superior semimeridians due to interruptions from the brow and eyelashes; therefore, the larger-diameter analyses will be slightly biased toward the horizontal regions. We would expect this bias to have only a slight effect and only on the 9- and 10-mm diameter data. The root mean square (RMS) fit error was also calculated for each corneal diameter. A repeated-measures analysis of variance (ANOVA) with one within-subject factor (corneal diameter) was used to investigate whether

*R*o and

*Q*changed significantly for the different corneal diameters tested.

*R*o and

*Q*were then calculated for each meridian for each subject. To investigate meridional variations in

*R*o and Q, repeated-measures ANOVA was performed with two within-subject factors (corneal diameter and corneal meridian).

^{ 24 }showed that for larger corneal diameters, a polynomial function fit corneal height data better than a conic fit. They found that a fourth-order polynomial was needed to fit 7-mm diameter data and that for a 10.7-mm diameter a ninth-order polynomial fit was necessary. The average corneal height data were also therefore fit with a polynomial function of the form

*x*=

*Ay*+

*By*

^{2}+

*Cy*

^{3}+

*Dy*

^{4}… and so on (where

*x*is the corneal height and

*y*is the distance from corneal center). For each subject, the third- through to the ninth-order polynomial functions were all fit to the corneal height data for 6-, 8-, 9-, and 10-mm diameters. The RMS fit error was also calculated for each of the polynomial orders and for each corneal diameter.

^{ 34 }We found these fitting routines to be highly sensitive to any missing data points, therefore only subjects with 300 complete semimeridians of axial power data to the edge of the outer diameter were included in each of the analyses. The best fit spherocylinder data for each subject was converted into the power vectors

**M**(best sphere),

**J0**(astigmatism 90°/180°) and

**J45**(astigmatism 45°/135°),

^{ 35 }to allow the group mean and SD to be calculated. The best fit corneal spherocylinder was calculated for corneal diameters of 6, 7, 8, and 9 mm. To investigate for significant changes in the corneal spherocylinder with increasing diameter, a repeated-measures ANOVA was used with one within-subject factor (corneal diameter).

**M**,

**J0**, and

**J45**. For this analysis, only subjects with complete semimeridians of axial power data to 8-mm diameter were included.

^{ 3 }

^{ 4 }

^{ 36 }Zernike polynomials up to and including the sixth radial order were fit to the corneal height data, for 6-, 8-, and 9-mm corneal diameters. The polynomials were expressed using the double indexed Optical Society of America (OSA) convention.

^{ 37 }This fitting routine is also sensitive to any missing or invalid data. We therefore included only subjects with complete corneal height data to the 9-mm diameter in the analysis. A repeated-measures ANOVA was used with one within-subject factor (corneal diameter) to investigate changes in each of the higher-order Zernike polynomials (third-order and above) with increasing corneal diameter.

*R*o and

*Q*), polynomial function, and RMS fit errors for the average corneal height data across all meridians are presented in Table 1 . The average

*R*o for a 6-mm corneal diameter was found to be 7.77 ± 0.2 mm and the average

*Q*value was −0.19 ± 0.1. For a 10-mm corneal diameter, the average

*R*o was 7.72 ± 0.2 and

*Q*was −0.36 ± 0.2. This indicates an increase in the rate of corneal flattening for the peripheral cornea. Repeated-measures ANOVA revealed that both

*R*o and

*Q*changed significantly with increasing corneal diameter (

*P*< 0.0001 for both

*R*o and

*Q*). The RMS fit error was found to increase dramatically for larger corneal diameters. For the conic fitting of the corneal data, the RMS fit error increased from a mean of 0.79 ± 0.4 μm for the 6-mm diameter to 21.18 ± 11.1 μm for the 10-mm corneal diameter. These fit errors highlight the inadequacy of the conic section to describe the peripheral cornea. To reduce the RMS fit error for larger corneal diameters, polynomial fitting to the data was required. For increasing corneal diameters, progressively higher-order fits were needed to fit the data reasonably (i.e., to reduce the RMS fit error). For a 6-mm corneal diameter, a fourth-order polynomial had an average RMS fit error of 0.03 ± 0.01 μm. For the 10-mm corneal diameter, a ninth-order polynomial fit gave an average RMS fit error of 0.28 ± 0.4 μm.

*R*o and

*Q*for the different diameters tested and illustrates the relatively wide range of

*R*o and

*Q*in the population. The shift in

*Q*to more a negative value for the larger corneal diameters is also highlighted in the frequency distribution plots.

*R*o and

*Q*for the steepest and flattest corneal meridians are displayed in Table 2 . The average

*R*o along the steepest corneal meridian was found to be 7.69 ± 0.2 mm and was 7.83 ± 0.2 mm for the flattest meridian for a 6-mm diameter. The mean

*Q*was −0.21 ± 0.1 along the steepest and −0.17 ± 0.1 along the flattest meridian for the 6-mm diameter. Both

*R*o and

*Q*were found to change significantly with increasing corneal diameter (

*P*< 0.001 for

*R*o and Q). This was a change similar to that found for the data averaged across all meridians. The repeated-measures ANOVA also revealed significant meridional variation in

*R*o and

*Q*. As would be expected, the

*R*o was significantly different between the two meridians (

*P*< 0.0001). The

*Q*was also significantly different between the steepest and flattest meridians, with

*Q*along the steepest meridian being significantly more negative (

*P*< 0.01). This indicates that the steepest corneal meridian has a slightly greater rate of peripheral flattening. Both

*R*o and

*Q*showed significant diameter and meridian interactions (

*P*< 0.05 for

*R*o and

*Q*).

**M**and scatter plots of astigmatism 90°/180° (

**J0**) and astigmatism 45°/135° (

**J45**) for the 6-, 7-, and 8-mm diameters. The average

**M**was found to be 48.2 ± 1.5 D,

**J0**was 0.32 ± 0.4 D, and

**J45**was −0.05 ± 0.2 D for a 6-mm corneal diameter (this equates to an average corneal spherocylinder of 48.5/ −0.64 × 176). It is evident from the scatterplots in Figure 4that most of the subjects exhibited positive

**J0**and relatively small

**J45**. In other words, most subjects exhibited a corneal cylinder axis relatively close to horizontal (i.e., with-the-rule [WTR] corneal astigmatism). With increasing corneal diameter the average

**M**,

**J0**, and

**J45**all were reduced slightly in magnitude. These changes with increasing corneal diameter were found to be significant (

*P*< 0.0001 for

**M**and

**J45**and

*P*< 0.01 for

**J0**). This indicates that the cornea flattens significantly in the periphery and exhibits a slight reduction in its toricity.

**M**was 48.3 ± 1.5 D,

**J0**was 0.32 ± 0.4 D, and

**J45**was −0.07 ± 0.2 D (48.6/−0.7 × 174). For the peripheral annulus (4- to 8-mm diameter annulus) the group mean

**M**was 47.6 ± 1.4 D,

**J0**was 0.27 ± 0.3 D, and

**J45**was −0.001 ± 0.2 D (47.9/−0.5 × 180).

**M**,

**J0**, and

**J45**all decreased in magnitude with increasing annulus diameter. This manifests itself as a general flattening, a slight change in corneal cylinder axis and a slight reduction in corneal astigmatic power in the more peripheral cornea. Repeated-measures ANOVA revealed the changes occurring in

**M**,

**J0**, and

**J45**with increasing annulus diameter to be highly significant (

*P*< 0.0001 for

**M**and

**J45**and

*P*< 0.001 for

**J0**). Based on the classification according to central corneal type there were 42 subjects with a spherical central cornea (<0.75 D astigmatism) and 36 subjects with an astigmatic central cornea (>0.75 D astigmatism). A significant interaction was found to occur between corneal annulus diameter and central corneal type for the change in

**J0**(

*P*= 0.0002). This indicates that the reduction occurring in

**J0**in the peripheral cornea was greater in subjects with greater central corneal astigmatism. Figure 5shows the group mean corneal cylinder power and axis as a function of distance from corneal center for the 0.5-mm annulus analysis (data for both the central spherical and central astigmatic corneas are shown). It can be seen in this figure that the change in astigmatic power in the peripheral cornea was much greater in the subjects with astigmatic central corneas (astigmatic eyes showed an average reduction in cylinder from center to periphery of 0.43 D, whereas subjects with spherical central corneas showed only a 0.09-D reduction). The average change in cylinder axis in the peripheral cornea is similar between the two groups with both the astigmatic and spherical central cornea groups showing a slight anticlockwise shift in cylinder axis (right eye) with increasing distance from corneal center.

*n*= 30), type 2, c (central astigmatic, peripheral astigmatism decreasing,

*n*= 17), type 2, a (central astigmatic, peripheral astigmatism stable,

*n*= 16), type 1, c (central spherical, peripheral astigmatism decreasing,

*n*= 7), type 1, b (central spherical, peripheral astigmatism increasing

*n*= 5), and type 2, b (central astigmatic, peripheral astigmatism increasing

*n*= 3). The most common peripheral corneal types were peripheral astigmatism stable (

*n*= 40) and peripheral astigmatism decreasing (

*n*= 24). Only 8 of the 78 subjects exhibited an increase in astigmatism in the peripheral cornea. Figure 6shows examples of axial power maps and corneal cylinder power annulus maps for subjects with the different patterns of corneal astigmatism.

**M**(

*r*= −0.266,

*P*= 0.019), indicating a tendency for the more myopic subjects to exhibit slightly steeper corneas. The best sphere refraction also showed a significant correlation with corneal

**J0**(

*r*= −0.385,

*P*= 0.001) and corneal

**J45**(

*r*= −0.284,

*P*= 0.012), indicating that the more myopic subjects also exhibit more astigmatic corneas (particularly with the rule astigmatism). The correlation coefficients and significances were also found to be similar for the 4-mm, 6-mm, and peripheral annulus corneal analysis diameters. No significant correlation was found between the asphericity parameter

*Q*and best sphere refractive error in our population for any of the corneal analysis diameters tested (

*r*= 0.102,

*P*= 0.335 for the 8-mm corneal diameter).

_{3}

^{−1}(

*P*= 0.001), Z

_{3}

^{1}(

*P*= 0.002), Z

_{4}

^{−2}(

*P*= 0.009), Z

_{4}

^{0}(

*P*< 0.0001), Z

_{4}

^{4}(

*P*= 0.006), and Z

_{6}

^{0}(

*P*< 0.0001) all showed highly significant change (

*P*< 0.01) with increasing corneal diameter. Figure 7shows the group mean third- and fourth-order corneal surface Zernike polynomial coefficient values (and Zernike term Z

_{6}

^{0}, as this was the only fifth- or sixth-order term exhibiting highly significant change) for the 6-, 8-, and 9-mm corneal diameters. It is evident in Figure 7that the fourth-order term Z

_{4}

^{0}was the higher-order coefficient of the largest magnitude and exhibited the largest change with increasing corneal diameter.

*R*o and

*Q*is presented in Table 5 . It is clear from Table 5that several different techniques have been used to measure the cornea in these studies. Despite this, our results (for the 6-mm corneal diameter) compare closely to these previous studies and correlate particularly closely with the two more recent studies that have also used a videokeratoscope for corneal measurements. Corneal topography measures have been shown to exhibit several changes with subject age (with a shift toward a predominance of against-the-rule corneal astigmatism and slightly steeper, more irregular corneas found after the age of 50).

^{ 38 }

^{ 39 }Differences in the age of subjects may therefore be a reason for some of the slight differences in results between our study, and previous studies. Our subjects were all young adults (age range, 18–35), whereas most previous studies have investigated a wider range of ages, with many including subjects over the age of 50.

*Q*for the population in our study (and from previous studies) indicates that the average cornea has a prolate elliptical shape (i.e., steeper centrally and flattened in the periphery). Some investigators have stated that a small subset of normal subjects exhibit an oblate corneal shape (i.e., a cornea that is steeper in the periphery).

^{ 11 }

^{ 12 }

^{ 13 }Eghbali et al.

^{ 13 }found that 8 of their 41 subjects exhibited an oblate corneal profile. In our data, only one of our subjects exhibited a positive

*Q*(oblate cornea) for the 6-mm measurement zone. For corneal diameters larger than 8 mm, all subjects were found to exhibit a prolate elliptical shape.

^{ 11 }also investigated meridional variation in the asphericity of the cornea and could find no specific trend for

*Q*to differ from one meridian to the other. Other investigators have noted some small meridional variations to exist in corneal asphericity.

^{ 13 }

^{ 15 }The difference that we found between the two principal corneal meridians was small in magnitude, but was highly statistically significant.

*R*o and

*Q*are highly dependent on the diameter of the cornea that is measured. With increasing corneal diameter

*R*o reduces and

*Q*becomes more negative (indicating an increased rate of flattening in the peripheral cornea). With increasing corneal diameter, the RMS fit error for the conic section also increases markedly. This indicates that although it is convenient and simple to understand, the conic section is a poor estimator of the peripheral cornea. To accurately specify the contour of the peripheral cornea, more complex fitting is required. We found that the use of a ninth-order polynomial function provided an excellent fit to the average corneal contour over a 10-mm diameter, producing an RMS fit error 75 times smaller than that given by the simple conic fitting.

^{ 40 }

^{ 41 }This marked change in corneal stromal collagen orientation is the possible anatomic reason for the flattening and reduction in astigmatism found in the peripheral cornea.

^{ 11 }

^{ 38 }

^{ 42 }

^{ 43 }

^{ 44 }The best-fit corneal spherocylinder was also found to change significantly with increasing distance from corneal center. On average, the best fit sphere becomes flatter and the amount of corneal astigmatism reduces slightly in the peripheral cornea. This reduction in toricity of the peripheral cornea is consistent with the meridional variation found in the asphericity parameter

*Q*. That is, as the steeper central meridian flattens at a faster rate in the periphery, then the degree of corneal astigmatism decreases in the corneal periphery.

^{ 12 }noted that most of their subjects displayed similar central and peripheral levels of astigmatism, but also noted some individual variations, with some subjects exhibiting a reduction or an increase in peripheral astigmatism. Reddy et al.

^{ 3 }also classified their subjects’ corneal topography based on central and peripheral corneal astigmatism. In contrast to our study, they found the most common form of astigmatism to be an increased or irregular astigmatism in the periphery, with stable or reducing corneal astigmatism found to be less common. This difference may be due to the subject selection procedures. In our study, we sought to examine the average corneal topography of normal healthy subjects, whereas the study by Reddy et al. involved subjects who were fitted with soft toric contact lenses. Thus, their population would be expected to have a much larger proportion of subjects with high degrees of corneal astigmatism (in fact, only 6% of their subjects had spherical central corneas). Their method for calculating peripheral astigmatism was also different to ours and was based on four corneal topography data points 3.5 mm from the center of the cornea.

_{3}

^{−3}, Z

_{3}

^{−1}, Z

_{3}

^{1}) and the fourth order term Z

_{4}

^{0}. The two terms displaying the most significant change with change in corneal diameter were the terms Z

_{4}

^{0}and Z

_{6}

^{0}(these corneal surface Zernike terms are analogous to the spherical aberration corneal wavefront error terms). The highly significant changes in these terms are due to the significant flattening occurring in the corneal periphery. Studies into corneal aberrations in normal healthy corneas have also found that the third-order terms and fourth-order spherical aberration term Z

_{4}

^{0}to be the dominant higher-order aberration terms.

^{ 45 }

^{ 46 }

^{ 47 }Increasing the corneal diameter over which the corneal aberrations are measured has also been found to cause a general increase in the higher order corneal aberrations, particularly for the spherical aberration and coma terms.

^{ 48 }

^{ 49 }

^{ 50 }

**Figure 1.**

**Figure 1.**

**Figure 2.**

**Figure 2.**

Corneal Diameter | |||||||
---|---|---|---|---|---|---|---|

6 mm (n = 92) | 8 mm (n = 92) | 9 mm (n = 92) | 10 mm (n = 84) | ||||

Conic fit | |||||||

Ro | 7.77 ± 0.2 | 7.76 ± 0.2 | 7.73 ± 0.2 | 7.72 ± 0.2 | |||

Q | −0.19 ± 0.1 | −0.23 ± 0.1 | −0.30 ± 0.1 | −0.36 ± 0.1 | |||

RMS fit error (μm) | 0.79 ± 0.4 | 4.19 ± 4.4 | 11.66 ± 7.9 | 21.18 ± 11.1 | |||

Polynomial fit (x = Ay + By ^{2} + Cy ^{3}…) | |||||||

A | −8.815E−05 | −2.598E−04 | −1.266E−04 | −7.276E−06 | |||

B | 6.467E−02 | 6.518E−02 | 6.469E−02 | 6.381E−02 | |||

C | −1.982E−04 | −6.910E−04 | −8.062E−05 | 1.812E−03 | |||

D | 2.760E−04 | 4.659E−04 | 1.280E−04 | −2.235E−03 | |||

E | −2.549E−05 | 6.001E−05 | 1.704E−03 | ||||

F | −8.064E−06 | −6.692E−04 | |||||

G | 1.520E−04 | ||||||

H | −1.843E−05 | ||||||

I | 9.114E−07 | ||||||

RMS fit error (μm) | 0.03 ± 0.01 | 0.10 ± 0.2 | 0.26 ± 0.4 | 0.28 ± 0.4 |

**Figure 3.**

**Figure 3.**

Corneal Diameter | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|

6 mm (n = 92) | 8 mm (n = 86) | 9 mm (n = 64) | |||||||||

Steep | Flat | Steep | Flat | Steep | Flat | ||||||

Conic fit | |||||||||||

Ro | 7.69 ± 0.2 | 7.83 ± 0.2 | 7.69 ± 0.3 | 7.83 ± 0.2 | 7.68 ± 0.3 | 7.81 ± 0.2 | |||||

Q | −0.21 ± 0.1 | −0.17 ± 0.1 | −0.27 ± 0.1 | −0.23 ± 0.1 | −0.32 ± 0.1 | −0.30 ± 0.1 | |||||

RMS fit error (μm) | 1.99 ± 1.0 | 1.40 ± 0.7 | 5.39 ± 7.2 | 3.60 ± 1.8 | 15.59 ± 16.9 | 10.62 ± 5.8 |

Corneal Diameter | |||||||
---|---|---|---|---|---|---|---|

6 mm (n = 78) | 7 mm (n = 78) | 8 mm (n = 78) | 9 mm (n = 38) | ||||

M (D) | 48.2 ± 1.5 | 48.1 ± 1.5 | 47.96 ± 1.5 | 47.28 ± 1.5 | |||

J0 (D) | 0.32 ± 0.4 | 0.31 ± 0.4 | 0.30 ± 0.4 | 0.26 ± 0.3 | |||

J45 (D) | −0.05 ± 0.2 | −0.04 ± 0.2 | −0.03 ± 0.2 | −0.05 ± 0.2 |

**Figure 4.**

**Figure 4.**

Outer Annulus Diameter (n = 78) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|

2 mm | 3 mm | 4 mm | 5 mm | 6 mm | 7 mm | 8 mm | |||||||

M (D) | 48.4 ± 1.5 | 48.3 ± 1.5 | 48.2 ± 1.5 | 48.0 ± 1.5 | 47.8 ± 1.5 | 47.6 ± 1.4 | 47.1 ± 1.4 | ||||||

J0 (D) | 0.34 ± 0.4 | 0.32 ± 0.4 | 0.31 ± 0.4 | 0.31 ± 0.4 | 0.28 ± 0.4 | 0.24 ± 0.3 | 0.23 ± 0.3 | ||||||

J45 (D) | −0.09 ± 0.2 | −0.06 ± 0.2 | −0.05 ± 0.2 | −0.03 ± 0.2 | −0.001 ± 0.2 | −0.003 ± 0.2 | 0.03 ± 0.2 |

**Figure 5.**

**Figure 5.**

**Figure 6.**

**Figure 6.**

**Figure 7.**

**Figure 7.**

Author | n | Age (y) | Method of Measurement | Corneal Diameter | Mean Ro | Mean Q |
---|---|---|---|---|---|---|

Kiely et al.^{ 11 } | 88 | 16–80 | Photokeratoscope | 6 mm | 7.72 ± 0.3 | −0.26 ± 0.2 |

Guillon et al.^{ 12 } | 110 | 17–60 | Photokeratoscope + keratometer | 9 mm | 7.78 ± 0.3 | −0.15 ± 0.2 |

Eghbali et al.^{ 13 } | 41 | 23–61 | Videokeratoscope | 6 mm | 7.67 ± 0.2 | −0.18 ± 0.2 |

Douthwaite et al.^{ 15 } | 98 | 20–59 | Videokeratoscope | 6 mm | 7.86 ± 0.2 | −0.21 ± 0.1 |

Current study | 92 | 18–35 | Videokeratoscope | 6 mm | 7.77 ± 0.2 | −0.19 ± 0.1 |