This was a cross-sectional study conducted at The Chinese University of Hong Kong, Department of Ophthalmology and Visual Science between June to September 2019. Eyes that had previous ocular surgery (including cornea refractive surgery and lens extraction) were excluded. Apart from cataract, eyes with ocular disease including corneal pathologies (e.g. cornea ectasia, pterygium) infectious disease (e.g. infective keratitis, viral conjunctivitis), and /or problem with dry eyes were excluded. Patients with myopia of more than − 6.0 D and visual acuity of worse than Snellen 6/12 were also excluded. Written informed consents were obtained from all subjects. The study was conducted in accordance with the ethical standards stated in the 2013 Declaration of Helsinki and approved by Hong Kong Kowloon Central Research Ethics Committee with written informed consent obtained.
The IOLMaster500 (Carl Zeiss Meditec, Jena, Germany) utilizes the principle of PCI to measure the AL. It evaluates the keratometry with a six-point telecentric technique and an image-based slit lamp system for ACD measurements. It does not provide lens thickness nor CCT measurement. It measures AL from the anterior corneal surface to the retinal pigmented epithelium, along the line of sight.
CASIAII swept-source OCT
The CASIAII swept-source OCT (Tomey, Nagoya, Japan) is a form of Fourier-domain OCT that utilizes a swept-source wavelength of 1310 nm as the light source and a photodetector to detect wavelength-resolved interference signal9, with improved in image resolution, scan speed, width, and depth8, 9. At a scan speed of 50,000 A scan/s and an axial resolution of < 10 µm, it allows multiple high-resolution, up to 256 cross-sectional images of the entire anterior segment and angle to be captured within 3 seconds. The maximum scan width is 16 mm with a scan depth of 13 mm10. It also provides placido corneal topography and automatic measurement software. With the measuring mode of corneal map and lens biometry, 16 radial cross-sectional images, with 800 A-scan per line sampling, a scan width of 16 mm and a scan depth of 11 mm can be delivered with a scan speed of 0.3 second. It only images the anterior segment with no AL measurement.
ANTERION swept-source OCT
The ANTERION swept-source OCT (Heidelberg Engineering, Heidelberg, Germany) is another form of Fourier-domain OCT that offers a fast scan-speed of 50,000 A-sans/second. It utilizes a 1300 nm light source to offer an axial resolution of < 10 µm. Compared with the CASIAII, it provides a wider scan width (up to 16.5 mm wide) and a deeper scan depth range (14 ± 0.5 mm)11. This allows visualization of detail corneal, anterior chamber, angle and lens (both anterior and posterior surfaces). The four different in-built imaging Apps – Cornea App, Cataract App, Metrics App and Imaging App – allows a comprehensive examination of the anterior segment imaging, corneal topography and tomography, anterior segment biometry, IOL calculation and AL measurement in a single scan. With the ANTERION Cornea App, 65 radial B-scan images (256 A-scans per B-Scan) are acquired in less than 1 second and the data can generate corneal maps of 8 mm in diameter12. It measures the distance between the anterior corneal surface and the retinal pigment epithelium, along the line of sight, as the AL12.
Imaging and measurements
One randomly selected eye of each subject was imaged by all three instruments. Each eye was scanned 3 times for each instrument to obtain clear images of the anterior and posterior corneal surfaces. The sequence of measurement recording between the ANTERION, CASIAII and IOLMaster500 was not fixed. The time elapsed between measurement devices included a short break for the patient to relax for tear film recovery and to avoid fatigue. All measurements were performed by a single experienced technician (G.L.) and were taken under dim room illumination. Patients were asked to blink in between consecutive scans to produce an optically smooth tear film, thereby improving the reflectivity of the cornea. During the imaging, the subjects were asked to fixate at an internal fixation target. To avoid lid artifact, the technician would retract the upper and lower lids of the participant while taking the imaging. To ensure stable corneal conditions, patients were asked to withhold soft contact lens wear for 2 weeks before the evaluation; none of our patients worn hard contact lens.
Power vector analysis was conducted for obtaining vectors along the 0o and 45o meridians according to the following equation: J0=(–[Ksteep – Kflat]/2 cos 2α), and J45=(–[Ksteep-Kflat]/2 sin 2 α), for comparison in a Cartesian coordinate system.13 Ksteep, Kflat, and α represent the steep keratometry, flat keratometry, and axes values, respectively. J0 represents astigmatic component along the vertical meridian (with-the-rule or against-the-rule astigmatism), while J45 represents oblique astigmatism.
R 3.2.5 (R Foundation, Vienna, Austria) was used for statistical analysis. Repeatability coefficients (RCs) was used to evaluate the repeatability of measurements obtained by the IOLMaster500, CASIAII and ANTERION. RC is defined as the 95% confidence limit of the difference of measurement between examinations, which is equal to
A high RC value represents a low test-retest variability, and vice versa14.
Comparison of RCs for parameters between the CASIAII vs ANTERION, ANTERION vs IOLMaster500 and CASIAII vs IOLMaster500 were evaluated by empirical bootstrap resampling with 2000 replicates. The first attempted measure of each subject is evaluated. Bland-Altman plots were used to assess the agreement between measurements of the two devices. Differences between the measurement values were plotted against the mean values of the measurements, and the 95% limits of agreement (LoA), which is equal to the mean difference ± 1.96 × SD, were evaluated. Systematic differences of each parameter between the two devices were compared using the t-test. Proportional bias was investigated by linear regression of the difference in values measured by the two devices. A P value less than 0.05 was considered statistically significant. False discovery rate, which measures the percentage of false discovery due to random error, was evaluated for multiple statistical tests with the threshold of P-value < 0.05 15.