Subjects above 18 years old were recruited from the student body and staff of Hadassah Academic College and Dept. of Optometry clinics from 2014–2015. All examinations took place in the refraction clinics at Hadassah Academic College. Due to compliance issues, not all subjects participated in all exams, yielding a different cohort for each validation procedure (Table 1). However, the same cohort was used in each type of experiment. The methods and the nature of the experiment were orally explained to the participants prior to their participation and they signed a statement of informed consent. This project was approved by the Ethics committee of Hadassah Academic College and followed the Declaration of Helsinki. Subjects were compensated for their time and travel to the clinic.

Subjects with systematic pathology, pregnant women, subjects with nystagmus, and subjects with any other corneal or conjunctival pathology or infection were excluded from the study. Subjects with contact lenses removed the lenses at least 12h before the study for hard lenses and 30min for soft lenses or until there are no signs of corneal warpage. Subjects with known epileptic history were also excluded.

All instrument measurements were performed per the manufacturer's guidelines. Instrument measurements were taken by qualified optometrists or optometry students trained in the use of each instrument. An examination including autorefraction, autokeratometry, central corneal thickness (CCT) and iridiocorneal angle (IR) measurements was performed using the VX120 system. The VX120 uses a very short flash of blue LED light with wavelength 450nm and with a constant power of approximately 50μW. It combines a Hartmann–Shack aberrometer, a Placido disk corneal topographer, a Scheimpflug imaging-based system and an air tonometer. The Placido disk system projects 24 rings on the corneal surface, measuring more than 100,000 points, and this information is used to provide all corneal topographic information. The Scheimpflug imaging-based system uses monochromatic blue light of 450nm to obtain pachymetric measurements with a resolution of ±10μm, and iridocorneal angle measurements with a resolution of ±1°. The Hartmann–Shack aberrometer of the VX120 system measures 1500 points in 0.2s in an area ranging from 2.0 to 7.0mm2 of diameter.

Besides measurements with the VX120 system, an examination was also performed with the Sirius system (Costruzioni Strumenti Oftalmici, CSO, Firenze, Italy). Subjects with abnormal findings were referred for further clinical evaluation and were excluded from the study.

Manifest refraction was performed by a single qualified optometrist (CK) who was masked to the results of autorefraction. In the agreement studies, exams were performed in a random order. Three measurements were obtained with the VX120 and Sirius systems, and the average was used in further analysis and for intra-test assessment of repeatability.

Only the right eye of each subject was used in all analyses.8 Inter-test (within 10 days after the initial objective measures) repeatability for each instrument (and for subjective refraction) was assessed on a subset of the original subjects, who were selected based on their willingness to comply with further testing. To avoid the effects of diurnal corneal thickness variations, these measurements were carried out at the same time of day, between 10am and 4pm.9

The different studies that were performed are summarized in Table 1. In each study, the same cohort of subjects was used.

The statistical analysis was performed using excel. Normality of all data distributions was confirmed by means of the Anderson–Darling test. Parametric statistics were used. Agreement and precision analyses were done using the Bland and Altman method10,11 which included calculation of the mean difference between the instruments, the standard deviation (SD) and the 95% limits of agreement (LoA=mean difference±1.96×standard deviation of the difference). Intra-test repeatability for different parameters was assessed by means of the following statistical variables: the within-subject standard deviation (Sw) of the 3 consecutive measurements, coefficient of repeatability (1.96×Sw×the square root of the number of measurements), and the intraclass correlation coefficient (ICC).

Sample size was calculated per the formulas recommended by McAlinden et al.6 For three repeated measurements (as in this study), sample sizes of 96, 43 and 24 subjects were required for a confidence in the estimate of 10%, 15% and 20%, respectively.

Student's t-test for paired data was used to assess the difference between variables and p-values of 0.05 or lower were considered as statistically significant.

The spherocylindrical refractions obtained with the device as well as subjectively were converted to vectorial notation using the power vector method described by Thibos and Horner.12

A total of 61 subjects (19 men, mean age 24.4±7.3 years, range 18–50) participated in the refraction agreement study (VX120 autorefraction vs. subjective refraction). Their refractive error varied between −8.00D and +1.00D of sphere, and −1.75D and 0.00D of cylinder (mean spherical equivalent, −1.25±1.85D). Results from the VX120 for 3-mm pupils were used.

The mean difference (value±std dev) and correlation between the VX120 autorefraction and subjective refraction for sphere, spherical equivalent (SE), cylinder and astigmatic vectors J0 and J45 described in Table 2 and Fig. 1. The 95% LoA between the two methods of obtaining the refraction were +1.06 to −0.77D (or ±0.92D), +0.86 to −0.83D (or ±0.85D) and +0.32 to −0.84D (or ±0.58D) for the spherical, spherical equivalent and cylinder component, and +0.46 to −0.26D (or ±0.36) and +0.38 to −0.29D (or ±0.34) for J0 and J45, respectively (Fig. 1).

Figure 1.

Difference in (A) spherical component, (B) spherical equivalent, (C) cylindrical component and (D) J0 and J45 vectors between the VX120 and subjective refraction/the Sirius, plotted against the mean refractive error/mean cylindrical Component/mean of the J0 and J45 components respectively. Mean bias is indicated by the solid lines and the 95% limits of agreement by the dashed lines.

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Approximately, 38% of the VX120 results were within ±0.25D and 69% within ±0.50D of the spherical components of the prescription found subjectively (Fig. 2A). For the spherical equivalent, 54% of the autorefraction readings were within ±0.25D and 77% within ±0.50D of the subjective refraction (Fig. 2B). For the VX120 cylinder components, 43% were within ±0.25D and 80% within ±0.50D of the subjective results.

Figure 2.

Comparison of the frequency of differences between the objective and subjective refractive techniques for (A) spherical component and (B) spherical equivalent.

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The agreement between the axes of the cylindrical components is shown in Table 3. For all cylindrical powers, 74.4% of VX120 axes were within ±20° of those found subjectively, and for cylindrical powers of 0.75D or greater, 78.6% of VX120 axes were within ±20° of those found subjectively.

Intra-test repeatability for the VX120 system in terms of refraction showed small within-subjects standard deviation (see Table 4).

Inter-test repeatability for the refraction parameters were assessed for the VX120 system and subjective refraction on a subset of the original cohort of 37 and 41 patients, respectively (Table 5). No statistically significant difference was observed between the first and the second session for sphere, cylinder and spherical equivalent parameters for the VX120 system and for all subjective refraction parameters (Table 5).

A total of 103 subjects (20 men, mean age 23.2±5.0 years, range 18–47) participated in the validation study comparing CCT, IR and keratometry between the VX120 and the Sirius systems (Table 6). Their refractive error varied between −9.13D and 4.51D of sphere, and −2.56D and −0.09D of cylinder (mean spherical equivalent, −1.44±2.20D). The results (Table 6, Fig. 3) show no statistically significant difference between the instruments for keratometry measurements.

Figure 3.

Difference in (A) Central Corneal Thickness (CCT), (B) Iridocorneal angle (IR) and (C) K readings between the VX120 and the Sirus, plotted against the mean CCT/IR/K readings. Mean bias is indicated by the solid lines and the 95% limits of agreement by the dashed lines.

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The average thickness was 547.5±6.5μm and 543.9±2.7μm for VX120 and Sirius, respectively. The average difference between the two instruments (3.51±8.64) was statistically, but not clinically significant (p<0.01, see Table 6). For IR, the average difference for IR was both clinically and statistically significant (7.6±4.2, p<0.01). Fig. 3 shows the Bland–Altman comparative analysis of VX120 and Sirius measurements.

Intra-test repeatability for the VX120 system (n=103) showed relatively low within-subjects standard deviations (Sw) for all parameter measured (Table 7).

A subset of the original cohort, 31 subjects (31 eyes, 9 men, 22 women) repeated measurements of both the VX120 and Sirius. Subjects’ mean age (±SD) was 23.6±5.9 years (range 18–46). Refractive error for these normal subjects varied between −9.13D and 3.10D of sphere, and between −2.07D and −0.14D of cylinder (mean spherical equivalent, −1.52±2.70D). High inter-test repeatability was demonstrated for most parameters measured (see Table 5).