Journal of Refractive Surgery

Wavefront Analyzers Induce Instrument Myopia

Alejandro Cervino, OD(EC), MCOptom; Sarah L Hosking, BSc, PhD, MCOptom, FAAO, DBO; Gurjeet K Rai, BSc, MCOptom; Shezhad A Naroo, BSc, MSc, PhD, MCOptom, FIACLE; Bernard Gilmartin, BSc, PhD, FCOptom, FAAO

Abstract

ABSTRACT

PURPOSE: To assess the accuracy of three wavefront analyzers versus a validated binocular open-view autorefractor in determining refractive error in non-cycloplegic eyes.

METHODS: Eighty eyes were examined using the SRW-5000 open-view infrared autorefractor and, in randomized sequence, three wavefront analyzers: 1) OPD-Scan (NIDEK, Gamagori, Japan), 2) WASCA (Zeiss/Med itec, Jena, Germany), and 3) Allegretto (WaveLight Laser Technologies AG, Erlangen, Germany). Subjects were healthy adults (19 men and 21 women; mean age: 20.8±2.5 years). Refractive errors ranged from +1.5 to -9.75 diopters (D) (mean: +1.83±2.74 D) with up to 1.75 D cylinder (mean: 0.58±0.53 D). Three readings were collected per instrument by one examiner without anticholinergic agents. Refraction values were decomposed into vector components for analysis, resulting in mean spherical equivalent refraction (M) and J0 and J45 being vectors of cylindrical power at 0° and 45°, respectively.

RESULTS: Positive correlation was observed between wavefront analyzers and the SRW-5000 for spherical equivalent refraction (OPD-Scan, r=0.959, P<.001; WASCA, r=0.981, P<.001; Allegretto, r=0.942, P<.001). Mean differences and limits of agreement showed more negative spherical equivalent refraction with wavefront analyzers (OPD-Scan, 0.406+0.768 D [range: 0.235 to 0.580 D] [P<.001]; WASCA, 0.511±0.550 D [range: 0.390 to 0.634 D] [P<.001]; and Allegretto, 0.434±0.904 D [range: 0.233 to 0.635 D] [P<.001]). A second analysis eliminating outliers showed the same trend but lower differences: OPD-Scan (n=75), 0.24±0.41 D (range: 0.15 to 0.34 D) (P<.001); WASCA (n=78), 0.46+0.47 D (range: 0.36 to 0.57 D) (P<.001); and Allegretto (n=77), 0.30+0.62 D (range: 0.16 to 0.44 D) (P<.001). No statistically significant differences were noted for Jp 0 and J45.

CONCLUSIONS: Wavefront analyzer refraction resulted in 0.30 D more myopia compared to SRW-5000 refraction in eyes without cycloplegia. This is the result of the accommodation excess attributable to instrument myopia. For the relatively low degrees of astigmatism in this study (<2.0 D), good agreement was noted between wavefront analyzers and the SRW-5000. [J Refract Surg. 2006;22:795-803.]

Abstract

ABSTRACT

PURPOSE: To assess the accuracy of three wavefront analyzers versus a validated binocular open-view autorefractor in determining refractive error in non-cycloplegic eyes.

METHODS: Eighty eyes were examined using the SRW-5000 open-view infrared autorefractor and, in randomized sequence, three wavefront analyzers: 1) OPD-Scan (NIDEK, Gamagori, Japan), 2) WASCA (Zeiss/Med itec, Jena, Germany), and 3) Allegretto (WaveLight Laser Technologies AG, Erlangen, Germany). Subjects were healthy adults (19 men and 21 women; mean age: 20.8±2.5 years). Refractive errors ranged from +1.5 to -9.75 diopters (D) (mean: +1.83±2.74 D) with up to 1.75 D cylinder (mean: 0.58±0.53 D). Three readings were collected per instrument by one examiner without anticholinergic agents. Refraction values were decomposed into vector components for analysis, resulting in mean spherical equivalent refraction (M) and J0 and J45 being vectors of cylindrical power at 0° and 45°, respectively.

RESULTS: Positive correlation was observed between wavefront analyzers and the SRW-5000 for spherical equivalent refraction (OPD-Scan, r=0.959, P<.001; WASCA, r=0.981, P<.001; Allegretto, r=0.942, P<.001). Mean differences and limits of agreement showed more negative spherical equivalent refraction with wavefront analyzers (OPD-Scan, 0.406+0.768 D [range: 0.235 to 0.580 D] [P<.001]; WASCA, 0.511±0.550 D [range: 0.390 to 0.634 D] [P<.001]; and Allegretto, 0.434±0.904 D [range: 0.233 to 0.635 D] [P<.001]). A second analysis eliminating outliers showed the same trend but lower differences: OPD-Scan (n=75), 0.24±0.41 D (range: 0.15 to 0.34 D) (P<.001); WASCA (n=78), 0.46+0.47 D (range: 0.36 to 0.57 D) (P<.001); and Allegretto (n=77), 0.30+0.62 D (range: 0.16 to 0.44 D) (P<.001). No statistically significant differences were noted for Jp 0 and J45.

CONCLUSIONS: Wavefront analyzer refraction resulted in 0.30 D more myopia compared to SRW-5000 refraction in eyes without cycloplegia. This is the result of the accommodation excess attributable to instrument myopia. For the relatively low degrees of astigmatism in this study (<2.0 D), good agreement was noted between wavefront analyzers and the SRW-5000. [J Refract Surg. 2006;22:795-803.]

Ocular wavefront analysis systems are widely considered state-of-the-art autorefractors, which go beyond the assessment of spherocylindrical error.1-3 Although autorefraction per se is commonly used as a starting point for subjective refraction, or for refractive screening purposes,45 wavefront analyzers may be used to establish higher and lower order aberrations prior to refractive surgical techniques, and appropriate interpretation of their output is therefore critical to a successful surgical outcome.

In customized refractive surgery procedures, greater accuracy of wavefront analysis preoperatively should translate into better results of the surgery itself, as well as the assessment of these results in subsequent follow-up. This is particularly important when measuring the aberrations of the eye in normal viewing conditions. Currently, the assessment of wavefront aberrations with clinical wavefront analyzers is commonly performed after instillation of dilating drops to ensure a pupil size sufficient to facilitate collection of wavefront readings. Pharmacologically induced dilation of the pupil prior to wavefront aberration assessment is currently a subject of debate, as it would offer an aberration profile that is less relevant to the natural pupil view normally experienced by the patient.6 A balance exists between the need for a sufficiently large pupil to provide reliable aberrometry data and the desire to establish the impact of aberrations as they affect the patient in the natural viewing state. It therefore follows that the acquisition of clinically relevant, stable refractive/ aberrometry data without the use of dilating agents would provide an ideal baseline for treatment.

Shack-Hartmann sensors exhibit good agreement with open field autorefractors, both centrally and peripherally in cycloplegic eyes.7 Because cycloplegic refraction invariably differs from manifest refraction, it follows that the corresponding wavefront error will differ from that found in the non-cycloplegic state due to the effect on the tone of the ciliary muscle. Pupil dilation with anticholinergic agents typically has a modest effect in reducing distance refraction, particularly in adults; however, it can reduce the range of near accommodation by up to 6.0 diopters (D), which in turn reduces the impact of instrument myopia.8 Instrument myopia is attributed to a relatively low level surfeit of accommodation, and variations in accommodation have been widely reported to induce variations in higher order aberrations.9-11 Therefore, when patients are not dilated pharmacologically, it is imperative to know that excessive instrument myopia is not being induced.

The OPD-Scan/ARK-10000 (NIDEK, Gamagori, Japan) is a device that determines wavefront aberration using the principle of dynamic skiascopy. With this device, an infrared slit scans across the pupil and measures the time difference of the light reflected by the retina between the center and several photo detectors on each side from the center.2,12 The slit rotates through 180°, covering all 360° semi-meridians. The system then generates autorefraction data as well as wavefront aberrations from the difference in refractive power across the pupil area. Little has been reported on the performance of clinical devices using this principle, showing poor repeatability for higher order aberration values in non-cycloplegic eyes13 and close agreement with Shack-Hartmann wavefront sensors in determining root-mean-square (RMS) values in model eyes and human eyes with induced mydriasis14; however, it has been successfully coupled to laser systems for customized refractive surgery procedures15-19 and for the assessment of the optical quality of pathological and postoperative eyes.20

The Wavefront Analysis Supported Customized Ablation system (WASCA; Zeiss/Meditec, Jena, Germany) is a Shack-Hartmann wavefront analyzer. Most current clinical wavefront analyzers use this principle. If a wavefront is divided spatially, then each subdivision can be approximated to a tilted flat wave. The reconstruction of the tilts of every division will result in an approximation to the original wavefront,21 a process that forms the basis of all Shack-Hartmann wavefront analyzers. The sensor used by the WASCA gives highly accurate and reproducible measures of both low and higher order aberrations up to the fourth order in human and model eyes22 with low variability.23

The Allegretto Wave analyzer (WaveLight Laser Technologies AG, Erlangen, Germany), developed by Mierdel et al,24,25 is an objective version of the Tscherning aberroscope. This method uses a dot pattern mask, which allows several laser rays from the original beam to be projected on the retina. Five to ten frames of the image formed on the retina are first captured and defined through a small aperture and then deformed to greater and lesser extents depending on the aberrations produced by the eye.26'27 The deviation of each retinal spot from its equivalent in the ideal reference image is measured and processed to give local slopes and, therefore, the corresponding wavefront aberrations. Good reproducibility has been demonstrated for this technique for measures of spherocylindrical refraction and total RMS values for higher order aberrations in human eyes.26

All three wavefront analyzers use a closed field internal fixation target, imaged at infinity to minimize accommodation during measurements. In the OPD, the internal target consists of a road receding into the distance. In contrast, a high contrast circular web-like grid and an orange light emitting diode with a central star are the fixation targets for the WASCA and Allegretto, respectively.

Closed field autorefractors are sometimes considered less suitable as a starting point for subjective refraction than other procedures, such as retinoscopy.28'29 The Shin-Nippon SRW-5000, also marketed as Grand Seiko WV500 (Shin-Nippon Ltd, Tokyo, Japan), is an infrared, open-view, objective autorefractor. Its open-view arrangement allows an unimpeded 63.4° horizontal (33.7° either side of fixation) by 31.5° vertical (14.8° above fixation and 16.7° below fixation) binocular view and thus minimizes instrument myopia. It has been shown to compare favorably with subjective refraction, being reliable in both adults (mean difference for the spherical equivalent refraction 0.16 ±0.44 D)30 and children (mean difference for the spherical equivalent refraction 0.24±0.34 D [cycloplegic]),31 and can be converted to measure continuously the accommodative response with high precision.32'33 For these reasons it will be used in this report as the gold standard against which to assess the performance of the OPD-Scan, WASCA, and Allegretto in the determination of spherocylindrical refractive error without cycloplegia.

The clinical application of wavefront analysis is generally to measure ocular aberrations with the eye in its natural state, often in the pre- and postoperative setting. The efficacy of the closed field targets used in wavefront analyzers to relax a patient's accommodation during the measurement process in eyes without cycloplegia is unknown. By comparison to a validated objective refractive technique, this study aims to show whether wavefront analyzer readings taken with physiological mydriasis only is a safe method by which to perform wavefront analysis. The performance of the OPD-Scan, WASCA, and Allegretto wavefront analyzers as clinical autorefractors for the determination of non-cycloplegic refractive error is examined in a group of young adults.

Table

TABLE 1Descriptive Statistics for the Different Components of the Power Vectors Corresponding to the Measurements Obtained With the SRW-5000 and Three Wavefront Analyzers

TABLE 1

Descriptive Statistics for the Different Components of the Power Vectors Corresponding to the Measurements Obtained With the SRW-5000 and Three Wavefront Analyzers

PATIENTS AND METHODS

The refractive errors of 80 eyes of 40 young adults (19 men and 21 women; mean age 20.8±2.5 years) with refractive errors ranging from +1.5 to -9.75 D sphere (mean: -1.83 ±2. 74 D) and up to 1.75 D cylinder (mean: 0.58±0.53 D) were examined with the SRW-5000 autorefractor, OPD-Scan, WASCA, and Allegretto wavefront analyzers under similar low illumination conditions. Initially, right and left eyes were plotted separately (as Bland-Altman difference versus mean plots), but because no differences were found, both eyes were included in the analysis.

This study was approved by the University Ethical Committee of Aston University. All procedures follow the agreement of the Declaration of Helsinki. Measurements were performed after informed consent was obtained from all volunteers.

The examinations were performed by a single examiner (A.C.). Three readings of spherocylindrical refractive error were obtained for the central 5 mm from each of the wavefront analyzers.

Refraction data in conventional clinical notation present a problem for analysis and comparison due to the variance in astigmatic power and axis and consequently need to be transformed to dioptric power space for ready comparison.34,35 For this reason, data were converted into power vector representation as a spherical component M, equal to the mean spherical equivalent of the refraction (sphere +Vfe cylinder) and a cylindrical component being represented by a Jackson cross cylinder at 0°, with a power J0 [1A cylinderXcos(2axis)), and a Jackson cross cylinder at 45°, with power J45 (% cylinder X sin(2axis)).36

Wavefront and refraction data were obtained for the central 5 mm with all three wavefront analyzers, as it would be the closest diameter to the 3 mm analyzed by the SRW-5000 but still offering clinically useful wavefront aberration data.

The bias was assessed statistically (SPSS 12.0; SPSS Inc, Chicago, Ill) as the mean of the differences compared to zero. The hypothesis of zero bias was assessed by two-tailed paired t tests with the Bonferroni adjustment for repeated comparisons, so that P<.012 was the criterion for statistical significance.

Plots of difference against mean were used to represent the agreement between methods.37

RESULTS

The descriptive statistics for the vectorial components M, J0, and J45 given by the four instruments, as shown in Table 1, indicate slightly higher negative values for the three wavefront analyzers for the M, J0, and J45 components compared to the autorefractor.

Table

TABLE 2Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the Components M, J0, and J45

TABLE 2

Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the Components M, J0, and J45

Good correlation between the readings obtained for the spherical equivalent M with the three wavefront analyzers and the autorefractor was evident (Pearson's correlation coefficients were 0.959, 0.981, and 0.942 for the OPD-Scan, WASCA, and Allegretto, respectively). However, as shown in Table 2, the mean differences, level of significance, and limits of agreement for the three components yield, for each of the wavefront analyzers, more negative power for the M component OPD-Scan, 0.41±0.79 D (range: 0.23 to 0.58 D), P<.001 WASCA, 0.51 ±0.55 D (range: 0.39 to 0.63 D), jP<.001 and Allegretto, 0.43 ±0.77 D (range: 0.23 to 0.63 D), jP<.001; however, no statistically significant differences were noted for the astigmatic components J0 and J45 (-0.01±0.29, P=.832 and -0.03 ±0.32, P=.474, respectively) for OPD-Scan, WASCA, and Allegretto.

Bland-Altman dispersion plots show that each of the three wavefront analyzers induce approximately 0.50 D more myopia than the SRW-5000 (P<.001) and a positive bias towards smaller refractive errors, showing less differences between the wavefront analyzers and the SRW-5000 for more myopic eyes. The WASCA shows the smallest confidence intervals and fewer outliers, whereas the OPD-Scan shows the lowest mean difference compared to the SRW-5000 (Figs 1-3).

When the outliers observed in Figures 1-3 are removed, stating a criteria of differences =52.0 D, and a new analysis is done (Table 3), the mean differences and confidence intervals for the M component are reduced to: OPD-Scan (n=75), 0.24±0.41 D (range: 0.15 to 0.34 D) (P<.001); WASCA (n=78), 0.46±0.47 D (range: 0.36 to 0.57 D) (P<.001); and Allegretto (n=77), 0.30±0.62 D (range: 0.16 to 0.44 D) (P<.001), although the same trend can be observed in the corresponding Bland-Altman dispersion plots (Figs 4-6).

The astigmatic components J0 and J45 show mean differences close to zero, which are not statistically significant, compared to the SRW-5000. Confidence intervals are smaller for the WASCA and OPD-Scan, and considerably wider for the Allegretto (Fig 7).

DISCUSSION

The results of this study show statistically significant differences between the wavefront analyzers and the SRW-5000 autorefractor for the spherical component M of the power vector. Specifically, the mean value of M is approximately 0.30 D more myopic when measured using wavefront analyzer technology. We suggest that this finding is the result, at least in part, of instrument myopia induced using these technologies. This finding is consistent with the differences obtained by Jorge et al29 in comparing closed-field autorefraction, retinoscopy, and subjective refraction, also in eyes without cycloplegia. Retinoscopy was found to be a better starting point for subjective refraction than close-field autorefraction as the latter was approximately 0.50 D more myopic, possibly due to instrument myopia. In a study comparing refractions in young myopic eyes with and without cycloplegia as measured using the Complete Ophthalmic Analysis System (COAS) aberrometer (a Shack-Hartmann aberrometer with the same sensor as the WASCA), the NIDEK ARK-2000 autorefractor, and subjective refraction, the wavefront analyzer and autorefractor each showed a vector error of 0.3 to 0.4 D without cycloplegia; under cycloplegia, the vector error for the COAS remained unchanged.1 The lower values of instrument myopia found in this study could possibly be due to the range of refractive errors sampled, as the authors suggest in a more recent study with results in close agreement with those reported here.38 Zadok et al13 reported low repeatability for the OPD-Scan in non-cycloplegic eyes, and suggest disruptions in the tear film and accommodative microfluctuations as a possible cause.

Figure 1. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the OPD-Scan for the spherical component M in diopters.Figure 2. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the WASCA for the spherical component M in diopters.Figure 3. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the Allegretto for the spherical component M in diopters.

Figure 1. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the OPD-Scan for the spherical component M in diopters.

Figure 2. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the WASCA for the spherical component M in diopters.

Figure 3. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the Allegretto for the spherical component M in diopters.

The confidence intervals from the Bland-Altman plots indicate the variability of the data with respect to the SRW-5000, such that a narrow interval indicates better agreement. The confidence intervals in this sample were ±0.171 for OPD-Scan, ±0.201 for Allegretto, and ±0.121 for WASCA, suggesting that the agreement was slightly better for WASCA and slightly worse for Allegretto. After removing the outliers from the analysis and plots, the confidence intervals were ±0.096 for OPD-Scan, ±0.140 for Allegretto, and ±0.106 for WASCA, making it slightly better for OPD-Scan and slightly worse for Allegretto.

Compared to the SRW-5000 there was a trend for the myopic overcorrection measured using the wavefront analyzers to be greater in low degrees of myopia or hypermetropia, as demonstrated by the positive regression line on each graph. This regression line was shallow for the OPD-Scan, suggesting only a mild effect of absolute power, whereas the Allegretto exhibited a steeper regression line indicating that the effect was greater with this device. In addition, careful inspection of the Bland-Altman plots shows a number of data points falling outside of the limits of agreement. This suggests that in each case, some patients differ in measurement from the SRW-5000 by an amount that is outside the normal variability of the device. Typically, these outliers arise when the actual refraction is <- 2.00 D, and the wavefront analyzer errors in these cases can be as high as -3.50 D (OPD-Scan) or even -4.50 D (Allegretto). It is important to note that comparison of the data shows that the outliers obtained in the three data sets can be attributed to both eyes of the same few patients in each case, suggesting that this is a finding that arises in certain patients and will affect all devices to some lesser or greater extent. These outliers account for the positive regression line and suggest that the interpretation of myopia <-2.00 D when measured with all three devices, but particularly with the OPD-Scan and Allegretto devices, can not be relied upon on a case-wise basis when cycloplegia is not used.

Table

TABLE 3Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the IVI Component After Removing OutliersFigure 4. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the OPD-Scan for the spherical component M in diopters after removing the outliers.Figure 5. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the WASCA for the spherical component M in diopters after removing the outliers.

TABLE 3

Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the IVI Component After Removing Outliers

Figure 4. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the OPD-Scan for the spherical component M in diopters after removing the outliers.

Figure 5. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the WASCA for the spherical component M in diopters after removing the outliers.

Differences in the cylindrical components were not statistically significant. This suggests that for the mild to moderate degrees of astigmatism exhibited within the study sample all three methods provided reliable cylindrical data. Conclusions should not be drawn regarding the determination of astigmatism of values greater than those included in this study.

Overall, the three wavefront analyzers demonstrate similar behaviors regarding accuracy and repeatability of measurement although particular differences arose in relation to the effects of absolute refraction, limits of agreement, and the presence of outliers. The different fixation targets used by the three devices may play some role in causing these differences. For example, the OPD-Scan uses a natural scene similar to that used in closed-field autorefractors, whereas the WASCA uses a high contrast web-like pattern and the Allegretto uses an orange light emitting diode with a central star pattern. Equally, the differences in the wavelengths used by the different fixation targets may result in a different response by the accommodation system due to the presence of chromatic aberration.39 The relatively narrower limits of agreement and more moderate outliers observed in the WASCA device would suggest that, if the target is a factor, it may have the better target for the purpose, particularly in eyes with lower degrees of myopia.

It is possible that the mathematical derivations of refractive error impact the agreement with the autorefractor findings. For small pupil sizes, second order coefficients may be sufficient for calculating the spherocylindrical refraction. However, as the pupil size increases, higher order aberrations (mainly spherical aberration) also increase their contribution to these refraction values and should therefore be considered in the calculation.40 In this study, natural pupils were examined with all instruments using similar illumination levels, and the wavefront and refraction data were obtained for the central 5 mm with all three wavefront analyzers, as it would be the closest diameter to the 3 mm analyzed by the SRW-5000 but still offering clinically useful wavefront aberrations data.

Figure 6. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the Allegretto for the spherical component M in diopters after removing the outliers.

Figure 6. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the Allegretto for the spherical component M in diopters after removing the outliers.

Figure 7. Bland-Altman plots for differences against mean (and 95% confidence intervals) between the SRW-5000 and the three wavefront analyzers for the astigmatic components J0 (o) and J45 (Δ) in diopters.

Figure 7. Bland-Altman plots for differences against mean (and 95% confidence intervals) between the SRW-5000 and the three wavefront analyzers for the astigmatic components J0 (o) and J45 (Δ) in diopters.

All values of refraction were obtained from the default settings of the wavefront analyzers. According to the manufacturers, both the OPD-Scan and Allegretto provide an integral "best fit" réfraction, which includes higher order aberrations. The WASCA, however, gives the default refraction values from the low order aberrations only, having been found to give better repeatability,38 with the option of using a Seidel sphere fit that includes spherical aberration in the computation.

In this respect, the acquisition protocols were consistent, suggesting that the contribution of spherical aberration to the incident image would be similar for the three devices; any variability would therefore be due to differences in the optical setup and image analysis.

The determination of mean spherical power obtained with the three clinical wavefront analyzers used in this study, without the instillation of anticholinergic drops, resulted in a myopic overestimation of approximately 0.30 D, which varied to some extent with the degree of myopia (being less with higher degrees of myopia), and in some individuals was seen to be excessive. The relatively frequent finding of outliers in the data suggest that on a case-wise basis preoperative data must be interpreted with caution if optimal postoperative results are to be achieved. With respect to wavefront aberrations per se, it has previously been shown that myopic shifts associated with relatively small amounts of excess accommodation have little impact on the RMS value9'11; however, the implications for those patients exhibiting large amounts of over-accommodation (>3.00 D) maybe profound.

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TABLE 1

Descriptive Statistics for the Different Components of the Power Vectors Corresponding to the Measurements Obtained With the SRW-5000 and Three Wavefront Analyzers

TABLE 2

Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the Components M, J0, and J45

TABLE 3

Mean Difference, Significance Level, and 95% Limits of Agreement Between the SRW-5000 and Wavefront Analyzers for the IVI Component After Removing Outliers

Figure 4. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the OPD-Scan for the spherical component M in diopters after removing the outliers.

Figure 5. Bland-Altman plot for differences against mean (and 95% confidence intervals) and regression line between the SRW-5000 and the WASCA for the spherical component M in diopters after removing the outliers.

10.3928/1081-597X-20061001-10

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