Secondary Logo

Journal Logo

Article

Corneal changes with accommodation using dual Scheimpflug photography

Sisó-Fuertes, Irene MSc*; Domínguez-Vicent, Alberto MSc; del Águila-Carrasco, Antonio MSc; Ferrer-Blasco, Teresa PhD; Montés-Micó, Robert PhD

Author Information
Journal of Cataract & Refractive Surgery: May 2015 - Volume 41 - Issue 5 - p 981-989
doi: 10.1016/j.jcrs.2014.08.038
  • Free

Abstract

In 1795, Home1 attributed accommodative capacity to 3 changes in the eye as follows: “an increase of curvature in the cornea, an elongation of the axis of vision, and a motion of the crystalline lens.” This was considered but finally rejected by Young,2 who stated that the mechanism of accommodation mostly accounts for changes in the crystalline lens. Subsequently, and according to the widely accepted Helmholtz3 theory of accommodation, it is known that the rise in optical power during accommodation is a consequence of a contraction of the annular ciliary muscle, which increases the thickness and curvature of the crystalline lens. This muscle pulls the ciliary body forward and reduces tension in the zonular fibers attached on either side of the lens equator.3 However, even though today it is known that the lens is the ocular structure that undergoes the principal anatomic changes during accommodation, the cornea is known to be a very malleable entity, and the corneal changes that occur during accommodating remain uncertain.

Farmaid4 and Löpping and Weale5 were first to study whether corneal changes occur during accommodation using an ophthalmometer and photokeratometer, respectively. Since then, and as a result of technological evolution, there have been controversy and a diversity of measurement methods reported in the published literature. The possible corneal changes occurring during accommodation have been mostly assessed using corneal topography, including keratometers,6 videokeratoscopes,7,8 and Placido disk–based topographers.9,10 Recently, new instruments based on Scheimpflug photography have been used to obtain images not only of corneal topography but also of other parameters and structures in the anterior eye segment. The Pentacam HR (Oculus) and Galilei (Ziemer) are 2 devices that use Scheimpflug technology. The former has been used by Read et al.11 and Ni et al.12 to study the influence of accommodation in the cornea. The Pentacam HR system uses a rotating Scheimpflug camera (180 degrees), while the Galilei system uses a dual rotating Scheimpflug camera integrated with a Placido disk. The Galilei G4 is the latest version of this system, which enables fast acquisition of thousands of data points per scan. This allows one to calculate a 3-dimensional model of the anterior segment. To our knowledge, this system has never been used to assess corneal changes during accommodation.

The idea that the cornea plays a role in accommodation comes from the assumption that the ciliary muscle affects the cornea. It has been suggested that this effect occurs mainly in the corneal periphery because of the anatomic proximity of the ciliary muscle to the limbus.7,10,12 Therefore, assessments of the corneal periphery (≥7.0 mm) have shown steepening of the corneal topography in the maximum and minimum keratometry (K) values,7,10 an increase in refraction,10 a rise in corneal volume,12 and a change in corneal aberrations with accommodation.8,12 Other studies11,13 suggest that the origin of corneal changes with accommodation is the significant cyclotorsion produced in the corneal topography when changing focus. When this rotation is corrected, the corneal changes decrease considerably and are not statistically significant.11,13 On the other hand, using corneal topographic images and analyzing the mean K and astigmatic readings, Bayramlar et al.9 found neither corneal changes nor corneal cyclotorsion during accommodation. This is in agreement with previous assumptions of Schachar et al.,14 who used the cornea and sclera as invariant positional references to align anterior segment ultrasound biomicroscopy images in different accommodated states. These findings were in agreement with the findings of Drexler et al.,15 who addressed corneal changes during accommodation using central pachymetry only.

Thus, this study sought to clarify how accommodation affects the cornea and its aberrations, assessing it in a more peripheral corneal area (7.0 to 10.0 mm) than in previous studies using dual rotating Scheimpflug–Placido disk technology.

Subjects and methods

All subjects were healthy emmetropic volunteers from the University of Valencia staff who were not using topical or systemic medication that could affect accommodation. In addition, none of them had corneal refractive surgery or any other surgery that could distort the measurements in both eyes. All patients were informed about the details of this study and provided written informed consent in accordance with the tenets of the Declaration of Helsinki.

Measurement System

The Galilei G4 was used for the measurements in all cases. This noninvasive noncontact optical diagnostic system is based on processed optical images from an integrated rotating dual-Scheimpflug and a 20-ring Placido disk capable of measuring up to 100 000 points. It incorporates a patented iris-based eye-motion compensation feature that monitors corneal changes. The system has a red light–emitting diode that serves as a fixation target and can be moved in 0.25 diopter (D) steps from −20.0 to +20.0 D.

Measurement Procedure

Data were obtained from several reports from the dual rotating Scheimpflug–Placido disk system to obtain a comprehensive overview of the cornea. Different corneal zone data with various diameters, which in the system are called central (0.0 up to 4.0 mm), paracentral or mid (4.0 up to 7.0 mm), and peripheral (7.0 up to 10.0 mm) were collected for the anterior and posterior axial curvatures, total corneal power (TCP), and corneal pachymetry. The latter is the corneal thickness calculated across the 3 zones. The TCP is the power of the cornea in diopters; it is calculated by ray tracing using Snell’s law and pachymetry data with the reference plane in the posterior corneal surface. The dual rotating Scheimpflug–Placido disk system also displays wavefront maps of the total cornea (front and back surfaces) in microns for a region of interest 6.0 mm in diameter and provides a pyramid of Zernike polynomials from which the factors of the 2nd-order, 3rd-order, and 4th-order polynomials are taken.

All measurements were taken during the same session. To facilitate natural pupil dilation, measurements were taken in a dark room. Before each measurement, central Placido rings were focused, after which the instrument was aligned. Next, the subject was asked to blink and look at the fixation target, ensuring he or she could clearly see the accommodation stimulus for all conditions. The subjects were asked to stare at it for 2 seconds to obtain an appropriate accommodation response16 and a homogeneous tear film along the cornea. This allowed the person taking the measurements to obtain good-quality images and to avoid changes in corneal aberrations with time after blink.17 After, the subject was asked not to blink during the measurement. Every measurement was taken monocularly. The examined eye was fixated on the optical target and the contralateral one covered with a patch to restrict the examined eye from adduction in convergence. The possible corneal changes were measured at different accommodation states, from unaccommodated up to 4.0 D. To make sure the subjects were not accommodating, the different parameters were measured at +1.0 D. Then, the stimulus was progressively changed up to −4.0 D in 1.0 D steps. To ensure that all the subjects were accommodating, the aqueous humor depth and central corneal thickness (CCT) data were recorded in all cases. Both parameters are related to the anterior chamber depth (ACD) through this expression: ACD = aqueous humor depth + CCT.

Statistical Analysis

Because of the small sample in this study and the likely effect of random errors, all findings were first analyzed in graph form and then confirmed statistically. Statistical analysis was performed using SPSS for Windows software (version 20, SPSS, Inc.). Linearity of corneal parameters was tested using graphs, and 2-way repeated-measures analysis of variance (ANOVA) was performed to determine whether corneal parameters changed under different accommodative states and corneal zones and to determine whether there was an interaction between these 2 factors on the various dependent variables.

Regarding corneal aberrations, 2nd-, 3rd-, and 4th-order Zernike polynomials and the root mean square (RMS) for every participant were assessed using graphs.

To statistically test whether significant changes in corneal aberrations existed with accommodation, repeated-measures ANOVA was used. All these tests were calculated for a significance level of 0.05.

Results

This study comprised 12 eyes of 7 subjects. The mean age was 30 years ± 5.83 (SD) (range 23 to 37 years).

There were no significant changes in anterior or posterior corneal keratometry, TCP, or pachymetry during accommodation for the mean of the sample (Figure 1). The best-fit linear trend line shows minimal slope, which means that the variation of corneal parameters with different accommodative demands was almost negligible because it was smaller than the possible measurement error. This observation was true for all 3 zones (central, mid, and peripheral), indicating a significant linear trend for all the parameters and different levels of accommodation (P < .01). Thus, accommodation and anterior and posterior corneal keratometry, TCP, and pachymetry were independent variables.

Figure 1
Figure 1:
The mean anterior and posterior corneal curvature keratometry, mean TCP, and mean pachymetry at different accommodative demands for the 3 corneal zones (central, mid, and peripheral).

Table 1 shows the mean anterior and posterior corneal surface keratometry, TCP, and pachymetry for different accommodative demands. There were no statistically significant differences in any of measured parameters during accommodation with different stimuli (P > .05).

Table 1
Table 1:
Mean anterior and posterior corneal surface keratometry, TCP, and pachymetry for the different accommodative demands. Two-way repeated ANOVA results and significance.

Based on the above, it can be assumed that the different corneal parameters were constant with accommodation. Figure 2 shows the significant differences between the corneal zones for the 4 corneal parameters evaluated. These findings were confirmed on statistical analysis. When accommodation was ignored, there were statistically significant differences between corneal zones for anterior (F[2.22] = 104.232; P = .000) and posterior (F[2.22] = 76.604; P = .000) corneal surface keratometry, TCP (F[2.22] = 82.299; P = .000), and pachymetry (F[2.22] = 402.265, P = .000). In addition, the Bonferroni post hoc test showed that differences between each zone were significantly significant for all parameters (P < .05).

Figure 2
Figure 2:
Anterior and posterior corneal keratometry, TCP, and pachymetry for the 3 corneal zones (central, mid, and peripheral) with the assumption that they are constant with accommodation. Boxplots with medians (lines), 25% to 75% quartiles (boxes), ranges (whiskers), and outliers (circles).

Two-way repeated-measures ANOVA of the effect of accommodation and the 3 corneal zones on anterior and posterior corneal surface keratometry, TCP, and pachymetry showed no statistically significant interaction between accommodation and corneal zones for any parameter as follows: anterior keratometry, F(8.88) = 1.285 and P = .262; posterior keratometry, F(8.88) = 1.882 and P = .073; TCP, F(8.88) = 0.569 and P = 0.801; pachymetry, F(8.88) = 0.272 and P = .974. Figure 3 shows this lack of interaction.

Figure 3
Figure 3:
The mean anterior and posterior corneal keratometry, TCP, and pachymetry for the 3 corneal zones (central, mid, and peripheral) with accommodation.

Repeated-measures ANOVA of the ACD showed a statistically significant difference between different accommodated states (F[4.44] = 24.603; P = .000). However, CCT differences were not statistically significant (F[4.44] = 1.384; P = .255).

Figure 4 shows mean defocus Z(2,0) and spherical aberration Z(4,0) of the sample. These 2 coefficients were more stable between individuals at different accommodative demands. On the graph, both show a linear plain trend line, indicating that both Zernike coefficients were independent of accommodation. This behavior was the same for all aberrations in this study. Repeated-measures ANOVA showed no statistically significant differences in any Zernike polynomial between the accommodative states (P > .05). Despite this, high standard deviation (SD) values showed great variability between subjects. Table 2 shows the mean coefficient for each Zernike and for the different accommodative demands.

Figure 4
Figure 4:
Mean defocus Z(2,0) and spherical aberration Z(4,0) at different accommodative demands.
Table 2
Table 2:
Mean of Zernike polynomials for the different accommodative states. Repeated ANOVA results and significance.

Discussion

To our knowledge, the only studies in the literature that assessed corneal changes at different accommodation ranges were by Buehren et al.13 and Yasuda et al.10 All other studies8–12 assessed the changes for the unaccommodated state and for 5.0 D of accommodation. Hence, our study provides new information by assessing the changes at different accommodative states from unaccommodated up to 4.0 D, which is the most common practical near working distance.

The differences we found between corneal zones were not a surprising finding because of the generally accepted knowledge that the anterior and posterior corneal shapes can be described as aspheric surfaces whose asphericity increases toward the periphery. Both the anterior and posterior corneal surfaces are prolate ellipses whose conic constants are different, with the posterior being more negative than the anterior.18,19 This means that the cornea is thicker in the periphery. This agrees with the differences we found in each parameter between corneal zones.

Moreover, effective accommodation has been proven objectively with ACD measurement. Our results agree with those in many studies20–23 that found an increase in lens thickness that led to a decrease in the ACD with accommodation. It also objectively proves that all the subjects were correctly accommodating during the data collection and that the central cornea did not undergo changes. Therefore, the global description provided by the axial curvature data as well as the TCP and pachymetry data show a constant linear trend with accommodation, indicating that no changes were produced in the cornea during accommodation.

Because some studies attribute the origin of corneal changes with accommodation to significant cyclotorsion produced in the corneal topography when changing focus,11,13 we avoided any eye movement during the acquisition process to obtain motion-corrected data. In addition, with the Galilei G4 system, eye motion during a measurement can affect the “apparent image” or elevation measured from the posterior surface (which also affects pachymetry). However, this is overcome by combining the 2 camera views using the patented dual-Scheimpflug feature. The systematic error is automatically corrected, and it removes decentration error caused by eye motion or misalignment. This prevents the acquired images from possible cyclotorsion, and this movement is also avoided because of the measurement procedure. While the measurements were being taken, convergence eye movements and misalignments between the eye and the measurement system were prevented by covering the contralateral eye.24 Hence, our results were not affected by significant torsional movement and thus agree with the findings of Read et al.11 and Buehren et al. Both studies found no consistent significant changes when data were recentered and cyclotorsion was corrected.

Furthermore, it has been suggested that because the ciliary muscle is close to the cornea, it is the peripheral cornea that might change more when accommodating.6,11 This is the reason we took data from 3 corneal zones. However, we found the expected differences between zones because of the aspheric configuration of the cornea; however, we did not find a significant change in any of them during accommodation. This is in agreement results of Yasuda et al.10 and Read et al.11 who, despite having found contradictory results regarding corneal changes with accommodation, stated that these changes with accommodation were equal at all measured points. Nevertheless 1 subject in the study by Read et al.11 had changes in the posterior peripheral corneal surface. Conversely, Ni et al.12 reported differences in corneal zones in terms of the magnitude of change and corneal volume with accommodation; however, they did not statistically analyze the differences.

On the other hand, the invariability in total corneal aberrations with accommodation in our study is in agreement with that in the study by Li et al.,25 who assessed the corneal, internal, and total aberrations in accommodated human eyes. They found no difference in corneal aberrations between accommodated eyes and unaccommodated eyes. Thus, the crystalline lens is thought to be the main source of changes in aberrations with accommodation, even though the cornea is the major refracting surface of the eye. This confirms the suggestions of Dubbelman et al.23 and Díaz et al.,26 who stated that the accommodation-related changes in aberrations are explained by lens surface asphericities as well as the gradient refractive index structure. He et al.8 and Ni et al.12 also studied corneal aberrations during accommodation. Our results agree with those of He et al.,8 who found no general changes in RMS or 2nd-order astigmatism Z(2,−2) and Z(2,2) in the anterior corneal surface with accommodation. This was the general trend, although the authors did find changes in some individuals. Our high SD values also indicate that great variations exist between subjects. However, He at al.8 found changes in horizontal coma Z(3,1) and spherical aberration Z(4,0), while we did not. Ni et al.12 measured corneal aberrations from the anterior, posterior, and entire cornea with a single Scheimpflug device. They found changes in vertical coma Z(3,−1) and spherical aberration Z(4,0) and a decrease in higher-order aberrations (3rd to 8th order) for the anterior and entire cornea. These outcomes do not match our findings and might be the result of ethnic differences between the samples. The study by Ni et al.12 was of Asian subjects, while our study evaluated a sample of white patients. Demographic data on refractive error show higher average levels of astigmatism in Asian eyes than in non-Asians eyes, a difference that has been attributed to greater tightness of Asian eyelids and narrower palpebral apertures, which cause pressure that produces changes in corneal topography.27–29 Given that corneal aberrations are directly extracted from topography, this could be the reason for the difference in findings between the study of Ni et al.12 and our study.

This study has limitations that led to our results being different from those in previous studies. A possible source of disagreement in results between studies is the difference in the devices used. The devices used to assess corneal changes during accommodation before Scheimpflug photography was available could not monitor accommodation during data collection and required visual or manual fixation control. In addition, keratometers, videokeratoscopes, and Placido disk–based topographers consider the anterior surface of the cornea as a convex mirror. They obtain the curvature by deriving the slope data from the reflection of the concentric rings of light rather than by reconstructing the corneal surface by splines (piecewise defined curve by polynomials), which is what the Galilei G4 system does. Moreover, depending on the size and curvature of the Placido disk or the device’s constraints, corneal topography cannot be acquired in the total cornea while the Galilei G4 system obtains data from a 10.0 mm diameter area. Thus, the equipment used in this study provides trustworthy results because of the duality; dual Scheimpflug systems are reported to have good repeatability and more reliable and accurate detection of pachymetry data and corneal posterior surface data than single Scheimpflug devices.30–32

The main limitation of this study is the small sample. Although the results allow us to accurately describe our sample, extrapolation to the rest of the population should be carefully considered. Moreover, similar to other studies,9,11,12 we assessed corneal changes with accommodation in emmetropic subjects only. However, studies of corneal biomechanical properties33 have found that lower corneal hysteresis values indicate a soft and flexible cornea and are associated with high myopia. Thus, this might be why the results of Yasuda et al.,10 who included patients with refraction ranging from −8.50 to +0.50 D, showed the greatest significant corneal changes between the unaccommodated state and the accommodated state. Nevertheless, based on the same proven assumption that ocular rigidity is lower in keratoconic eyes34 and therefore these eyes are more susceptible to changes, Buehren et al.13 did not find greater corneal changes with accommodation in keratoconic corneas than in normal corneas. Hence, studies with a larger sample that includes ametropic subjects should be performed to clarify this.

A decline in the corneal resistance factor has also been found with increasing age. However, when changes in corneal parameters with accommodation were assessed in elderly people,10,12 significant differences were not found between young people and presbyopic people. Similarly, total corneal aberrations increase with age, with spherical aberration being the main contributor.19 As a result of the misalignment between surfaces, coma-like aberrations also increase with age.19 This encourages a reduction in accommodation capacity with age and thus accounts for the accommodation that is not explainable by lens changes, which is called pseudoaccommodation. It is key to understand pseudoaccommodation to advance our knowledge and achieve the goal of restoring accommodation. Hence, aging affects corneal biomechanical properties and aberrations and the baseline for a study like this one would be different. Thus, a study using the latest technology of the Galilei G4 system in a larger cohort including subjects with different refraction conditions and in different age groups would provide more robust results and would answer unresolved questions.

In summary, central, paracentral, and peripheral anterior and posterior corneal keratometry, TCP, and pachymetry unaffected by cyclotorsional effects were stable during accommodation. Similarly, total corneal aberrations were constant at different accommodated states. This reaffirms the classic statements by Young.2 However, a more robust study with a more diverse and larger sample is needed to improve our understanding and contribute to the development and improvement of therapeutic and surgical solutions.

What Was Known

  • There is no consensus about corneal changes with accommodation.

What This Paper Adds

  • Corneal parameters in various zones were uniform during different accommodative demands, thus showing a linear trend line that indicates each parameter does not depend on accommodation.

References

1. Home E. The Croonian lecture on muscular motion. Phil Trans R Soc Lond. 85, 1795, p. 202-220, Available at: http://rstl.royalsocietypublishing.org/content/85/202.full.pdf. Accessed January 11, 2015.
2. Young T. The Bakerian lecture: on the mechanism of the eye. Phil Trans R Soc Lond. 91, 1801, p. 23-88, Available at: http://rstl.royalsocietypublishing.org/content/91/23.full.pdf. Accessed January 11, 2015.
3. Helmholtz H. Ueber die Accommodation des Auges. About accommodation of the eyes, Albrecht von Graefes Arch Ophthalmol 1855;1(2):1-74.
4. Fairmaid JA. The constancy of corneal curvature; an examination of corneal response to changes in accommodation and convergence. Br J Physiol Opt. 1959;16:2-23.
5. Löpping B, Weale RA. Changes in corneal curvature following ocular convergence. Vision Res. 1965;5:207-215.
6. Pierścionek BK, Popiołek-Masajada A, Kasprzak H. Corneal shape change during accommodation. Eye. 15, 2001, p. 766-769, Available at: http://www.nature.com/eye/journal/v15/n6/pdf/eye2001246a.pdf. Accessed January 11, 2015.
7. Yasuda A, Yamaguchi T. Steepening of corneal curvature with contraction of the ciliary muscle. J Cataract Refract Surg. 2005;31:1177-1181.
8. He JC, Gwiazda J, Thorn F, Held R, Huang W. Change in corneal shape and corneal wave-front aberrations with accommodation. J Vis. 3, 2003, p. 456-463, Available at: http://www.journalofvision.org/content/3/7/1.full.pdf. Accessed January 11, 2015.
9. Bayramlar H, Sadigov F, Yildirim A. Effect of accommodation on corneal topography. Cornea. 2013;32:1251-1254.
10. Yasuda A, Yamaguchi T, Ohkoshi K. Changes in corneal curvature in accommodation. J Cataract Refract Surg. 2003;29:1297-1301.
11. Read SA, Buehren T, Collins MJ. Influence of accommodation on the anterior and posterior cornea. J Cataract Refract Surg. 2007;33:1877-1885.
12. Ni Y, Liu X, Lin Y, Guo X, Wang X, Liu Y. Evaluation of corneal changes with accommodation in young and presbyopic populations using Pentacam high resolution Scheimpflug system. Clin Exp Ophthalmol. 2013;41:244-250.
13. Buehren T, Collins MJ, Loughridge J, Carney LG, Iskander DR. Corneal topography and accommodation. Cornea. 2003;22:311-316.
14. Schachar RA, Tello C, Cudmore DP, Liebmann JM, Black TD, Ritch R. In vivo increase of the human lens equatorial diameter during accommodation. Am J Physiol. 1996;271:R670-R676.
15. Drexler W, Baumgartner A, Findl O, Hitzenberger CK, Fercher AF. Biometric investigation of changes in the anterior eye segment during accommodation. Vision Res. 1997;37:2789-2800.
16. López-Gil N, Fernández-Sánchez V, Legras R, Montés-Micó R, Lara F, Nguyen-Khoa JL. Accommodation-related changes in monochromatic aberrations of the human eye as a function of age. Invest Ophthalmol Vis Sci. 49, 2008, p. 1736-1743, Available at: http://www.iovs.org/cgi/reprint/49/4/1736. Accessed January 12, 2015.
17. Montés-Micó R. Role of the tear film in the optical quality of the human eye. J Cataract Refract Surg. 2007;33:1631-1635.
18. Kiely PM, Smith G, Carney LG. The mean shape of the human cornea. Optica Acta. 1982;29:1027-1040.
19. Navarro R, Rozema JJ, Tassignon M-J. Optical changes of the human cornea as a function of age. Optom Vis Sci. 90, 2013, p. 587-598, Available at: http://journals.lww.com/optvissci/Fulltext/2013/06000/Optical_Changes_of_the_Human_Cornea_as_a_Function.10.aspx. Accessed January 12, 2015.
20. Richdale K, Sinnott LT, Bullimore MA, Wassenaar PA, Schmalbrock P, Kao C-Y, Patz S, Mutti DO, Glasser A, Zadnik K. Quantification of age-related and per diopter accommodative changes of the lens and ciliary muscle in the emmetropic human eye. Invest Ophthalmol Vis Sci. 54, 2013, p. 1095-1105, Available at: http://www.iovs.org/content/54/2/1095.full.pdf. Accessed January 12, 2015.
21. Kasthurirangan S, Markwell EL, Atchison DA, Pope JM. MRI study of the changes in crystalline lens shape with accommodation and aging in humans. J Vis. 11(3): 2011, 19, 1–16. Available at: http://www.journalofvision.org/content/11/3/19.full.pdf. Accessed January 12, 2015.
22. Ostrin L, Kasthurirangan S, Win-Hall D, Glasser A. Simultaneous measurements of refraction and A-scan biometry during accommodation in humans. Optom Vis Sci. 83, 2006, p. 657-665, Available at: http://journals.lww.com/optvissci/Fulltext/2006/09000/Simultaneous_Measurements_of_Refraction_and_A_Scan.10.aspx. Accessed January 12, 2015.
23. Dubbelman M, van der Heijde GL, Weeber HA. Change in shape of the aging human crystalline lens with accommodation. Vision Res. 2005;45:117-132.
24. Bolz M, Prinz A, Drexler W, Findl O. Linear relationship of refractive and biometric lenticular changes during accommodation in emmetropic and myopic eyes. Br J Ophthalmol. 91, 2007, p. 360-365, Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1857649/pdf/360.pdf. Accessed January 12, 2015.
25. Li Y-J, Choi JA, Kim H, Yu S-Y, Joo C-K. Changes in ocular wavefront aberrations and retinal image quality with objective accommodation. J Cataract Refract Surg. 2011;37:835-841.
26. Díaz JA, Fernández-Dorado J, Sorroche F. Role of the human lens gradient-index profile in the compensation of third-order ocular aberrations. J Biomed Opt. 2012;17:075003.
27. Kame RT, Jue TS, Shigekuni DM. A longitudinal study of corneal astigmatism changes in Asian eyes. J Am Optom Assoc. 1993;64:215-219.
28. Shaw AJ, Collins MJ, Davis BA, Carney LG. Eyelid pressure: inferences from corneal topographic changes. Cornea. 2009;28:181-188.
29. Read SA, Collins MJ, Carney LG. A review of astigmatism and its possible genesis. Clin Exp Optom. 90, 2007, p. 5-19, Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1444-0938.2007.00112.x/pdf. Accessed January 12, 2015.
30. Crawford AZ, Patel DV, McGhee CNJ. Comparison and repeatability of keratometric and corneal power measurements obtained by Orbscan II, Pentacam, and Galilei corneal tomography systems. Am J Ophthalmol. 2013;156:53-60.
31. Aramberri J, Araiz L, Garcia A, Illarramendi I, Olmos J, Oyanarte I, Romay A, Vigara I. Dual versus single Scheimpflug camera for anterior segment analysis: precision and agreement. J Cataract Refract Surg. 2012;38:1934-1949.
32. Salouti R, Nowroozzadeh MH, Zamani M, Fard AH, Niknam S. Comparison of anterior and posterior elevation map measurements between 2 Scheimpflug imaging systems. J Cataract Refract Surg. 2009;35:856-862.
33. del Buey MA, Lavilla L, Ascaso FJ, Lanchares E, Huerva V, Cristóbal JA. Assessment of corneal biomechanical properties and intraocular pressure in myopic Spanish healthy population. J Ophthalmol. 2014, 2014, 905129, Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3955599/pdf/JOPH2014-905129.pdf. Accessed January 12, 2015.
34. Shah S, Laiquzzaman M, Bhojwani R, Mantry S, Cunliffe I. Assessment of the biomechanical properties of the cornea with the Ocular Response Analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci. 48, 2007, p. 3026-3031, Available at: http://www.iovs.org/cgi/reprint/48/7/3026. Accessed January 12, 2015.
© 2015 by Lippincott Williams & Wilkins, Inc.