Correlation between corneal and total wavefront aberrations in myopic eyes

PURPOSE: Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality depends on all optical elements of the eye, including the human lens. We investigated correlations between corneal and total wavefront aberrations and the relevance of corneal aberrations for representing the optical quality of the total eye. METHODS: Thirty-three eyes of 22 myopic patients were measured using a corneal topography system and a Tscherning-type wavefront analyzer. Pupils were dilated to at least 6 mm in diameter. All measurements were centered with respect to the line of sight. Corneal and total wavefront aberrations were calculated up to the 6th Zernike order in the same reference plane. RESULTS: Statistically significant correlations (P<.05) between corneal and total wavefront aberrations were found for astigmatism (C3,C5) and all 3rd Zernike order coefficients such as coma (C7,C8). No statistically significant correlations were found for 4th, 5th, or 6th order Zernike coefficients. On average, all Zernike coefficients for corneal aberrations were larger than the Zernike coefficients for total wavefront aberrations. CONCLUSIONS: Due to the lack of correlation between corneal and total wavefront aberrations in most of the higher order aberrations, measurement of corneal aberrations are of limited use for representation of the optical quality of the human eye, especially after corneal laser surgery. Corneal aberrations and optical elements within the eye are optically balanced. As a consequence, ideal customized ablations must take both corneal and total wavefront aberrations into consideration.     

Effect of higher-order wavefront aberrations on binocular summation

PURPOSE: To determine the effect of individual Zernike wavefront aberrations on binocular summation and binocular visual acuity. METHODS: A 0.25-microm wavefront aberration of second, third and fourth order Zernike modes were introduced into a set of log minimum angle of resolution unit (logMAR) visual acuity charts convolved by CTView. Subjects were dilated and fitted with an artificial pupil of 3 mm. For each set of charts, right eye, left eye, and binocular acuity was measured. The gain in binocular visual acuity over monocular visual acuity was defined as binocular summation. The visual acuity lost binocularly reading aberrated charts was normalized for each subject and defined as the aberration induced loss in acuity. RESULTS: Binocular summation was 10.0% (95% limits of agreement 8.8 to 11.1%) in the unaberrated state and ranged from 17.3 to 3.4% in the Zernike modes studied. Binocular summation was greatest in defocus followed by coma and astigmatism. The aberration induced loss in monocular and binocular acuity was higher for Zernike modes with low angular frequency compared to those with high angular frequency. Linear regression showed a significant relationship between aberration-induced loss of visual acuity and binocular summation. CONCLUSIONS: A fixed amount of root mean square (RMS) aberration has a varied effect on binocular vision depending on the angular frequency and radial order of Zernike mode. Binocular vision has a positive effect in reducing the visual impact of aberrations as Zernike modes that suffer from the most loss of visual acuity also experience the greatest amounts of binocular summation.     

Dynamics of ocular surface topography

PURPOSE: To investigate fluctuations in the ocular surface, we used high-speed videokeratoscopy (50 Hz) to measure the dynamics of the ocular surface topography. METHODS: Ocular surface height difference maps were computed to illustrate the changes in the tear film in the inter-blink interval. Topography data were used to derive the ocular surface wavefront aberrations up to the fourth radial order of the Zernike polynomial expansion. We examined the ocular surface dynamics and temporal changes in the ocular surface wavefront aberrations in the inter-blink interval. RESULTS: During the first 0.5 s following a blink, the ocular surface height at the upper edge of the topography map increased by about 2 mum. Temporal changes occurred for some ocular surface wavefront aberrations and appeared to be related to changes in the distribution of tear film. CONCLUSION: In the clinical measurement of ocular surface topography using videokeratoscopy or optics of the eye using wavefront sensors, care should be taken to avoid the initial tear film build-up phase following a blink to achieve more consistent results.     

Relationship between ocular wavefront aberrations and refractive error in Chinese school children

Background: The relationship between ocular wavefront aberrations and refractive error in children's eyes remains controversial. The purpose of this study is to re‐examine this relationship in Chinese school children under natural distance accommodation. Methods: Ocular wavefront aberrations were measured in 86 Chinese children with spherical equivalent refraction (SER) between +0.5 D and ‐6.0 D and astigmatism less than ‐1.00 D. Wavefront aberrations were calculated using an objective method based on the Hartmann‐Shack principle. Refractive error was obtained using a phoropter after cycloplegia. Subjects were categorised into three groups based on the mean SER: emmetropia (SER from ‐0.50 D to +0.50 D), mild myopia (SER greater than ‐0.50 D to ‐3.00 D) and moderate myopia (SER greater than ‐3.00 D to ‐6.00 D). Of the 86 participants, 22 were emmetropic, 43 were mildly myopic and 21 were moderately myopic. The root mean square (RMS) values of higher‐order aberrations, Zernike coefficients (third‐, fourth‐ and fifth‐order aberrations) and R_j (the ratio of third‐, fourth‐ or fifth‐order aberrations to total higher‐order aberrations) were compared across the three refractive groups. Results: No significant correlations were found between the RMS values of total higher‐order aberrations, third‐order aberrations, fourth‐order aberrations, fifth‐order aberrations, spherical aberration or coma and SER. No significant differences in the RMS values of total higher‐order aberrations or R_j were observed among the groups. The difference in fifth‐order aberrations was statistically significant among the groups (p = 0.022); no other differences in higher‐order aberration were found. Aside from C (3,1), no other differences were observed for Zernike coefficients. Conclusion: Ocular wavefront aberrations are similar among Chinese school children with different refractive errors under natural accommodation for a distance target. There is no evidence that myopes have a different amount of ocular higher‐order aberrations than emmetropes.     

Advantages and disadvantages of the Zernike expansion for representing wave aberration of the normal and aberrated eye

PURPOSE: Zernike expansion has been selected for use in describing wavefront aberrations in the human eye. The advantages and limitations of this approach are assessed for eyes with varying degrees of aberration. METHODS: Corneal topography examinations were taken with the Nidek OPD-Scan topographer/aberrometer. These higher data density corneal topography examinations were converted to height data and subsequently to wavefront representations. System noise was evaluated with a 2D frequency analysis of 43-D test balls. Both Zernike polynomials and 2D Fourier transforms were used to evaluate fidelity in the presentation of the point spread function. A display format for potential clinical use was developed based upon Zernike decomposition. RESULTS: Systematic noise from the corneal topographer was found to be minimal and, when eliminated, produced small changes in the point spread function. Using Zernike decomposition up to the 30th order failed to preserve the higher frequency aberrations present in aberrated eyes. Use of a Zernike decomposition display with a fixed micron scale presented only clinically significant details of spherical aberration, coma, trefoil, irregular components above third order and total higher-order aberrations (above second order). CONCLUSIONS: Zernike polynomials excel in extracting the low-order optical characteristics of visual optics. Zernikes accurately represent both low- and high-order aberrations in normal eyes where high-order aberrations are clinically insignificant. For eyes after corneal surgery or eyes with corneal pathology such as keratoconus that have significant higher-order aberrations, the Zernike method fails to capture all clinically significant higher-order aberrations.     

角膜塑型术后眼波前像差变化

目的 探讨角膜塑型术后眼波前像差变化及其影响因素，了解第3代角膜塑型术对视觉质量的影响。方法 对进行角膜塑型术的21例（42眼）进行了前瞻性临床研究，将角膜塑型术术前及术后达最佳裸眼视力时的眼波前像差情况作测量与比较。检查结果通过Matlab软件分析．直接获取35项Zemike系数及每阶Zemike系数的RMS值。结果 角膜矫型术后总像差，第3、第4、第5、第7阶RMS值较术前增加，且差异有显著性（P〈0．05）。第2、第6阶RMS值较术前增加，但差异无显著性（P〉0．05）。结论 角膜塑型术后眼的像差增加，降低了视觉质量。角膜光学区直径的缩小以及偏离、角膜表面的非规则性增加，角膜非生理形态的改变等是导致像差增加的原因。     

Spot size and quality of scanning laser correction of higher order wavefront aberrations

PURPOSE: To investigate the effect of laser spot size on the outcome of aberration correction with scanning laser corneal ablation. METHODS: Numerical simulation of ablation outcome. RESULTS: Correction of wavefront aberrations of Zernike modes from second to eighth order were simulated. Gaussian and top-hat beams of 0.6 to 2.0-mm full-width-half-maximum diameters were modeled. The fractional correction and secondary aberration (distortion) were evaluated. Using a distortion/correction ratio of less than 0.5 as a cutoff for adequate performance, we found that a 2 mm or smaller beam is adequate for spherocylindrical correction (Zernike second order), a 1 mm or smaller beam is adequate for correction of up to fourth order Zernike modes, and a 0.6 mm or smaller beam is adequate for correction of up to sixth order Zernike modes. CONCLUSIONS: Since ocular aberrations above Zernike fourth order are relatively insignificant, current scanning lasers with a beam diameter of 1 mm or less are theoretically capable of eliminating most of the higher order aberrations of the eye.     

A new method for describing the aberrations of the eye using Zernike polynomials.

The standard Zernike polynomial functions are reformulated in a way so that the number of functions (or terms) needed to describe an arbitrary wavefront surface to a given Zernike radial order is reduced by a factor of approximately two, and the terms are described in a fashion quite similar to that used to describe common sphero-cylindrical errors of the eye. A wavefront is represented using these terms by assigning a pair of values, a magnitude and an axis, to all terms that are radially symmetric so that the individual aberrations are presented in a way similar to the way common astigmatism is currently given in terms of cylinder power and axis. The root mean square of these magnitudes gives the root mean square wavefront error just as does the root mean square of the standard Zernike coefficients. Formulas are given to convert standard Zernike coefficients to the magnitude and axis values.     

Spot size and quality of scanning laser correction of higher-order wavefront aberrations

PURPOSE: To investigate the effect of laser spot size on the outcome of aberration correction with scanning laser corneal ablation. SETTING: Cleveland Clinic Foundation, Cleveland, Ohio, USA. METHODS: Corrections of wavefront aberrations of Zernike modes from the second to eighth order were simulated. Gaussian and top-hat beams of 0.6 to 2.0 mm full-width-half-maximum diameters were modeled. The fractional correction and secondary aberration (distortion) were evaluated. RESULTS: Using a distortion/correction ratio of less than 0.5 as a cutoff for adequate performance, a 2.0 mm or smaller beam was adequate for spherocylindrical correction (Zernike second order), a 1.0 mm or smaller beam was adequate for correction of up to fourth-order Zernike modes, and a 0.6 mm or smaller beam was adequate for correction of up to sixth-order Zernike modes. CONCLUSIONS: Since ocular aberrations above the Zernike fourth order are relatively insignificant in normal eyes, current scanning lasers with a beam diameter of 1.0 mm or less are theoretically capable of eliminating most higher-order aberrations.     

Wavefront aberrations in eyes of emmetropic and moderately myopic school children and young adults

Wavefront aberrations were measured using a psychophysical ray-tracing technique in both eyes of 316 emmetropic and moderately myopic school children and young adults. Myopic subjects were found to have greater mean root mean square (RMS) value of wavefront aberrations than emmetropic subjects. Emmetropic adults had the smallest mean RMS, which remained smaller than the values for myopic adults and children and for emmetropic children both when second order Zernike aberrations (astigmatism) and third order Zernike aberrations were removed. Twenty percent of myopic adults had RMS values greater than values for all of the emmetropic adults, with significantly greater values for Zernike aberrations from second to seventh orders. High amounts of wavefront aberrations, which degrade the retinal image, may play a role in the development of myopia.     

Corneal and total wavefront aberrations in phakic and pseudophakic eyes after implantation of monofocal foldable intraocular lenses

PURPOSE: To compare the correlation between corneal and total wavefront aberrations in normal phakic and pseudophakic eyes after implantation of foldable monofocal intraocular lenses (IOLs). SETTING: University Hospital, Eye Clinic, Zurich, Switzerland. METHODS: Wavefront aberrations and corneal topography of 29 eyes that had cataract surgery with implantation of hydrophobic monofocal foldable IOL (AcrySof, Alcon Labs) were measured at least 2 months postoperatively and compared with wavefront measurements performed in 33 normal young phakic eyes. The total wavefront aberrations were measured by means of a Tscherning wavefront sensor at a wavelength of 660 nm (Allegro Wave Analyzer, WaveLight Laser Technology). The corneal aberrations were derived from corneal topography measurements ascertained with a Placido-based topography system (Keratograph 70600, Oculus). The correlations between corneal and total wavefront aberrations were calculated for all Zernike coefficients from 2nd up to 6th order. RESULTS: There was a significant correlation between corneal and total wavefront aberrations in astigmatism C3 and C5 as well as for all 3rd-order Zernike coefficient in both groups (except C8 in the pseudophakic group). The correlation between corneal and total astigmatism (C3 and C5) was higher in the pseudophakic than in the phakic eyes. In contrast, the correlation for the coma-like aberrations was weaker in the pseudophakic eyes (R>0.18) than in the group of phakic eyes (R>0.58). In both groups, there was no significant correlation between spherical aberration C12 of the cornea and the C12 of the total eye. CONCLUSION: After cataract surgery with an IOL implantation, both vertical and horizontal coma, as well as spherical aberration, were of higher value than in normal eyes. The compensation effect for corneal aberrations of the natural lens is absent in the IOL and explains these findings. The corneal aberrations in pseudophakic eyes reflect better the optical quality of the total eye than the phakic eyes. Nevertheless, the missing correlation in some specific aberrations, such as C8 and C10, shows the inability of corneal topography to provide suitable information on the optical quality of the total eye after cataract surgery. Thus, both corneal and total wavefront measurements are relevant for the assessment of outcomes after cataract surgery.     

Corneal optics after reading, microscopy and computer work

PURPOSE: To compare lid-induced changes in corneal optics following reading, microscopy and computer work. METHODS: Nine subjects with normal ocular health were recruited for the study. Five subjects were myopic, two were emmetropic, one was astigmatic and one was hyperopic. Corneal topography was measured before and after 60 mins of reading a novel, performing a blood cell counting task on a microscope and Internet searching. Corneal topography data were used to derive the corneal wavefront Zernike coefficients up to the fourth order. A meridian analysis of instantaneous corneal power along the upper 90-degree semi-meridian was performed to examine local changes caused by eyelid pressure. Digital photography was used to capture body posture and eyelid position during the tasks. RESULTS: Each of the three tasks showed systematically different effects on both the characteristics and location of corneal topography changes. Reading and microscopy generally exhibited larger and more centrally located changes compared with the computer task. Differences in wavefront aberration characteristics between the three tasks were apparent in both lower and higher order aberrations. The location of corneal distortions differed significantly between microscopy and computer work, with microscopy causing distortions to occur closer to the videokeratoscope measurement axis compared with computer work (p = 0.015). CONCLUSIONS: Reading, microscopy and computer work have different effects on corneal aberrations. The results are in agreement with the hypothesis that lid-induced corneal aberrations may play a role in myopia development.     

Influence of age on ocular wavefront aberration changes with accommodation

PURPOSE: To perform a quantitative analysis of high order wavefront aberrations of the entire optical system of the eye that accompanies accommodation demands in the healthy and emmetropic eye. METHODS: Fifty-six healthy emmetropic eyes of 30 patients were separated into three groups according to age: group 1, 20 to 29 years; group 2, 30 to 39 years; and group 3, 40 to 49 years. Using iTrace ray-tracing technology, pupil diameter, objective refraction, and wavefront aberrations at accommodation demands of 0 to 4.00 diopters (D) in five 1.00-D increments were determined. In analyzing higher order aberrations in the central 4-mm diameter zone of the pupil, Zernike polynomials were used to study the changes associated with accommodation in the magnitude of total higher order aberrations, third order aberrations (coma-like aberrations), fourth order aberrations (spherical-like aberrations), and the spherical aberration component (C4(0)) of the fourth order aberrations. RESULTS: Pupil diameter decreased significantly (P < .05) with accommodation demand in each group. Total higher order aberrations in group 1 increased significantly (P < .05). No significant change was seen in third order aberrations in any group, but fourth order aberrations increased significantly (P < .05) in group 1. Spherical aberration (C4(0)) showed phase shifts from positive to negative in all groups, but the shift in group 3 was minimal, remaining positive even for a 4.00-D accommodation demand. CONCLUSIONS: Among higher order aberrations accompanying accommodation demands, spherical aberration (C4(0)) in particular showed characteristic changes; however, these changes showed age-related disparities. The changes were probably due to the effects of reduction in accommodation resulting from age-related changes in the crystalline lens.     

Corneal first surface wavefront aberrations before and after pterygium surgery

PURPOSE: To determine the higher order aberrations at the corneal first surface before and after surgery for pterygium. METHODS: Data were drawn from a longitudinal study of patients undergoing pterygium excision at Royal Victoria Infirmary, Newcastle upon Tyne, England between September 1998 and May 2004. Corneal topography was taken with the TMS-2 Topographic Modeling System (Computed Anatomy Corp) prior to and 6 months after surgery, exported to VOLPro software v7.08 (Sarver & Associates), and wavefront aberrations were derived for a 5.0-mm pupil using a 10th order Zernike polynomial expansion. Pre- to postoperative changes were assessed for significance using analyses of variance, and the relative risk of significant postoperative aberrations by pterygium size was determined. RESULTS: Satisfactory corneal topography was available on 67 eyes (mean age 53.8 +/- 16.7 years [range: 25-86 years]). The root-mean-square (RMS) fit error in preoperative eyes was 0.15 +/- 0.10 microm. Preoperatively, the total higher order RMS wavefront aberration was 0.94 +/- 0.83 microm. All Zernike modes were elevated, with trefoil being the major contributor 0.52 +/- 0.50 microm. Pterygium excision significantly reduced wavefront aberrations across all modes and orders (F(1, 129) = 6.7 to 22.6, P < .01): total higher order RMSpostop 0.45 +/- 0.35 microm. Cases with visually significant postoperative aberrations occurred and were more likely with larger pterygia: relative risk compared to pterygia 1.0 to 1.9 mm was 1.3 for 2.0 to 2.9 mm, 8.5 for 3.0 to 3.9 mm, 13.3 for 4.0 to 4.9 mm, and 10.2 for 5.0 to 5.9 mm. CONCLUSIONS: Zernike polynomial fitting well describes wavefront aberrations in eyes with pterygia. Pterygia are associated with wavefront aberrations, especially trefoil, but these were largely eliminated by surgery. Earlier excision of pterygia reduces the likelihood of significant residual aberrations.     

Monochromatic wavefront aberrations in the human eye with contact lenses.

PURPOSE: The aim of this study was to investigate the effect of contact lenses on the optical performance of the eye by measuring wavefront aberrations for the eyes with or without contact lenses. METHOD: A sensitive aberrometer was used to measure wavefront aberrations for 54 eyes in 27 subjects for three conditions: with no contact lens (non-CL), with soft-contact lenses (soft-CL) and with rigid gas permeable contact lenses (RGP-CL). The root mean square (RMS) value of the wavefront aberrations and Zernike aberrations were calculated. RESULTS: A change in the RMS values of wavefront aberrations with CL wear was observed for every eye. The change in wavefront aberrations with CL wearing was found to vary substantially from individual to individual. Relative to the mean RMS value of the group for the non-CL condition, the mean RMS value was increased for the soft-CL condition and was significantly reduced for the RGP-CL condition. A significant increase in mean RMS for the soft-CL condition was found when astigmatisms were removed. Although soft-CL wearing resulted in significant increases in higher orders of Zernike aberrations (fourth, fifth, and higher), the RGP-CL condition led to a significant decrease in second-order Zernike aberrations. For the eyes with low wavefront aberrations in the non-CL condition, either soft-CL wearing or RGP-CL wearing results in increases in the RMS values. CONCLUSION: Contact lens wearing, either with soft lenses or the RGP lenses, causes changes in the wavefront aberrations of the eye. The changes in wavefront aberrations vary substantially from eye to eye. Although soft-CL wearing tends to induce more higher-order aberrations, RGP-CL effectively reduces the astigmatisms. Both soft-CL and RGP-CL induce more aberrations for the eyes that have low wavefront aberrations. The change in wavefront aberrations due to contact lens wearing may explain the changes in visual performance for contact lens wearers reported previously.     

The contribution of accommodation and the ocular surface to the microfluctuations of wavefront aberrations of the eye

We have used videokeratoscopy and wavefront sensing to investigate the contribution of the ocular surface and the effect of stimulus vergence on the microfluctuations of the wavefront aberrations of the eye. The fluctuations of the wavefront aberrations were quantified by their variations around the mean and by using power spectrum analysis. Integrated power was determined in two regions: 0.1-0.7 Hz (low frequencies) and 0.8-1.8 Hz (high frequencies). Changes in the ocular surface topography were measured using high-speed videokeratoscopy and variations in the ocular wavefront aberrations were measured with a wavefront sensor. The microfluctuations of wavefront aberrations of the ocular surface were found to be considerably smaller than the microfluctuations of the wavefront aberrations of the total eye. The fluctuations in defocus while viewing a closer target at 2 or 4 D were found to be significantly greater than fluctuations in defocus when viewing a far target. This increase in defocus fluctuations (p < or = 0.001) occurred in both the low- and high-frequency regions of the power spectra.     

A comparison of wavefront aberrations in eyes wearing different types of soft contact lenses.

PURPOSE: The purpose of this study is to investigate the optical quality of eyes wearing different types of soft contact lenses (sCLs) using wavefront aberrometry. METHOD: A psychophysical aberrometer (WFA1000B; BriteEye, SuZhou, China) was used to measure wavefront aberrations for 56 eyes of 28 subjects under four conditions: 1) without contact lenses (non-CL), 2) with lathe-cut contact lenses (lathe-cut CL; Shuliang, Weicon Optics, Shanghai, China), 3) with cast-molded contact lenses (cast-molded CL; Vistavue, J&J Vision Care, U.S.), and 4) with spun-cast contact lenses (spun-cast CL; Lanfenzhiji, Justcome Optics, Beijing, China). The three types of sCLs were made from different materials and lens design. The root mean square (RMS) values of the wavefront aberrations and individual Zernike aberrations were calculated and compared among the CL-wearing conditions. RESULTS: Astigmatism was not significantly changed by any of the three soft contact lenses when compared with the non-CL condition. However, a significant difference in the RMS values of higher-order aberrations was observed between the lens types (F=8.29, p<0.0001). Induced higher-order aberrations were found for both the cast-molded CL and the spun-cast CL conditions. There were significant differences in the aberrations in each of the higher orders between the CL conditions (F=3.77, p=0.011 for third order; F=5.31, p=0.002 for fourth order; F=13.86, p<0.0001 for fifth and higher orders). More higher-order Zernike aberrations such as coma and spherical aberration were induced by cast-molded CLs and/or the spun-cast CLs. CONCLUSION: Wavefront aberrations in the eyes with soft contact lenses vary from one individual lens type to another. Although the variation in wavefront aberrations could be attributed to the differences in the methods of manufacture, influences from other factors such as the lens material and lens design could not be excluded and deserve further investigation.     

Diurnal fluctuation of higher order ocular aberrations: correlation with intraocular pressure and corneal thickness

PURPOSE: Optimal wavefront-guided refractive corneal laser surgery requires sufficiently exact data of optical higher order aberrations. We investigated whether these aberrations had a systematic during-the-day variation, studied the range of variation, and changes in intraocular pressure and central corneal thickness. METHODS: In 22 eyes of 22 young volunteers the optical aberrations of higher order were measured by means of a Tscherning-type ocular aberrometer three times during one day (7 AM, 12 noon, 4 PM). In addition, in 12 of these eyes the intraocular pressure and central corneal thickness were measured. The intraocular wavefront aberration was computed using Zernike polynomials up to the sixth order, and Zernike coefficients of third and fourth order were analyzed. RESULTS: Only the coefficient Z 2/4 (C13) showed a significant increase during the day by a mean 0.016 microm. A significant regression could be detected between changes of coefficients Z3/3, Z-2/4, Z0/4, Z4/4, and changes of intraocular pressure or central corneal thickness during the day. CONCLUSIONS: Due to the small values, the measured during-the-day changes of higher order aberrations had no direct practical consequences for the aberrometry-guided corneal laser surgery. Alterations of some Zernike coefficients during the day may be explained by the biomechanical behavior of the cornea.     

Prospective randomized study of corneal aberrations 1 year after radial keratotomy or photorefractive keratectomy

PURPOSE: To evaluate the optical properties of the cornea 1 year after either radial keratotomy (RK) or photorefractive keratectomy (PRK) in a randomized group of patients with low myopia. METHODS: Ninety-six patients with myopia between -0.75 and -5.00 D were randomized to either radial keratotomy (n = 46) or photorefractive keratectomy (n = 50). Topography maps were obtained 1 year after surgery and analyzed by computation of total corneal wavefront aberration and Zernike polynomial coefficients for pupil sizes of 2, 4, and 6 mm. The 4-mm pupil size was used for optimization of the model. RESULTS: The total corneal wavefront aberrations after RK and PRK were similar and not statistically different. Wavefront aberrations arising from astigmatism or defocus accounted for approximately 70% of the total wavefront error at all pupil sizes in both groups. All types of aberrations, and in particular spherical aberration, increased significantly with increasing pupil size. Higher-order wavefront aberrations were almost twice as high after RK than after PRK at pupil sizes of 4 and 6 mm. Spherical aberration and coma were slightly higher after PRK than after RK. CONCLUSIONS: Pupil size had a major effect on corneal aberrations after RK and PRK. The most important aberrations were sphero-cylindrical, in which eyes became significantly more myopic with increasing pupil size. The image forming properties of the cornea are better after PRK compared with RK due to the lesser amount of higher-order aberrations.     

Measurement repeatability of corneal aberrations

PURPOSE: Customized procedures that aim to improve visual performance beyond traditional sole correction of sphere and cylinder require the reliable determination of higher order corneal and ocular aberrations for individual eyes. The aim of the current investigation was to determine the repeatability of corneal aberration measurement. METHODS: Ten subjects were measured on 10 different days using a videokeratoscope. The Zernike coefficients and the wavefront error were calculated from the height data files using CTView. Repeatability was calculated for apertures of 3, 4.5, and 6 mm at the 95% confidence level. RESULTS: The wavefront error absolute repeatability was similar for the 4th and 10th order analysis. The absolute repeatability for individual Zernike coefficients was coefficient dependent; the poorest repeatability was recorded for coma-like aberrations (uncertainty 3 mm = 0.060 microm; 4.5 mm = 0.083 microm; 6 mm = 0.109 microm). The relative repeatability compared to the population means was good for wavefront error (5% to 20%) but poor for some individual Zernike coefficients. CONCLUSION: Videokeratoscope clinical repeatability was sufficient to reliably determine wavefront error at a single session, but may require several examinations and/or repeated independent measurements at the same examination to determine reliably some Zernike coefficients. This has significant practical implications for accurate customized correction of aberrations.