To estimate the agreement of anterior segment parameters between a swept-source optical biometry (IOLMaster 700; Carl Zeiss Meditec AG, Jena, Germany) and a Scheimpflug-based topography with high resolution (Pentacam HR; OCULUS, Wetzlar, Germany).
Interinstrument reliability analysis.
A total of 62 eyes from 62 young adults were included in the study. Average keratometry (AveK) and simulated keratometry (SimK) along 2.0-mm-ring measurements provided by Pentacam HR, keratometry readings provided by IOLMaster 700, and central corneal thickness (CCT) and anterior chamber depth (ACD) values obtained from both devices were recorded. J0 and J45 vectoral components of astigmatism were obtained using power vector analysis. Mean keratometry (Km) values of IOLMaster 700 were compared for each type of Km value from Pentacam HR, while other parameters were compared between devices. To assess the agreement between measurements of the devices, Bland-Altman analysis was performed.
The Pentacam HR exhibited significantly lower Km and CCT measurements ( P < .001, for all); however, no significant difference emerged in J0, J45, and ACD measurements ( P = .057, P = .574, and P = .64, respectively). The mean difference between AveK, SimK 2.0 mm, and the IOLMaster 700 Km was −0.20 diopter (D) and −0.14 D, respectively, while the mean difference between J0, J45, CCT, and ACD measurements was 0.07 D, −0.016 D, −5.05 μm, and 0.004 mm, respectively.
In clinical practice, Pentacam HR and IOLMaster 700 can be used interchangeably to measure J0 and J45 vectoral components of astigmatism for SimK 2.0 mm and IOLMaster keratometry values, as well as ACD and CCT measurements. However, SimK 2.0 mm and AveK values can be not interchangeable, as the devices have clinical and statistical differences in measurements.
For clinical applications such as refractive and cataract surgery, accurate anterior segment measurements are critical for enhancing the success of vision correction. Currently, several devices are available for measuring anterior segment parameters, including those that perform Scheimpflug topography, optical coherence tomography (OCT), swept-source (SS) optical biometry, optical low-coherence reflectometry, and partial coherence interferometry (PCI), as well as slit-scanning topography and pachymetry systems. When using those devices, however, clinicians need to consider interdevice differences in measurement.
Among these devices, with the help of a rotating Scheimpflug camera, the Pentacam HR (OCULUS, Wetzlar, Germany) is designed to analyze anterior ocular segments. The device has a special 3-dimensional, high-resolution scanning mode, with which the camera captures 138 000 data points in fewer than 2 seconds. As a result, a single scan can produce topographic maps of the anterior and posterior corneal surfaces, anterior chamber analysis, and complete corneal pachymetry.
At the same time, optical biometry has become the gold standard for determining biometric measurements and intraocular lens (IOL) power calculations. Among devices that can achieve those ends, a newly available SS-OCT-based optical biometry device, named the IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany), also enables OCT imaging and visualization across the entire length of the eye. In so doing, it provides corneal keratometry, central corneal thickness (CCT), anterior chamber depth (ACD), white-to-white distance, pupil diameter, axial length, and lens thickness measurements.
Advances in anterior segment imaging have allowed clinicians to objectively evaluate and measure parameters characterizing the eye’s anterior segment. In fact, parameters generated by both Pentacam and IOLMaster 700 have demonstrated excellent repeatability. However, it remains unknown whether the measurements obtained with those devices are interchangeable or even comparable. In response, the present study was conducted to estimate the agreement of anterior segment parameters’ keratometry, CCT, and ACD in normal eyes between an SS-OCT biometry device and a Scheimpflug-based topography device with high resolution.
Participants and Protocol
This study, designed as an interinstrument reliability analysis, was approved by the ethics committee of Mugla University and conducted according to the Declaration of Helsinki. Each participant was informed of the purpose of the study and signed a written consent form.
All participants received a standard examination during a single visit to the clinic that included general anamnesis to gather data regarding age, sex, and medical history. Each participant was also subjected to spherical refractive error and intraocular pressure measurements using the TRK-2P automated Kerato-Refractometer tono-pachymeter (TOPCON Corp, Tokyo, Japan), anterior slit-lamp biomicroscopy, Scheimpflug-based corneal topography using the Pentacam HR (version 1.20r76), and SS optical biometry using the IOLMaster 700 (software version 1.5). Measurements were performed prior to pupil dilation. After Pentacam HR and IOLMaster 700 measurements, the pupil was dilated for posterior segment examination.
Participants with poor fixation, corneal disease, cataract, glaucoma, or dry eye; those who wore contact lenses; and those who had undergone previous ocular surgery were excluded.
Devices and Measurements
Using a rotating Scheimpflug camera (180 degrees) and monochromatic slit-light source (ie, blue LED lights at 470 nm) combined with a static camera, the Pentacam HR can provide a 3-dimensional model of the anterior segment, as well as elevation maps of the anterior and posterior corneal surfaces, pachymetry maps, biometric measurements of the anterior segment, and anterior and posterior corneal power calculations. The average keratometry (AveK) is calculated as the arithmetic means of the pair of meridians 90 degrees apart (K1 and K2) within the central 3-mm zone. Power Distribution Display permits evaluation of the simulated keratometry (SimK) values in preferred zone or ring.
The IOLMaster 700 uses SS-OCT technology (laser with variable wavelength) to generate optical B-scans, or optical cross-sections, to determine biometric eye data. The device can obtain multiple measurements for each of the various parameters in a single capturing process and presents their average value. More specifically, the SS-OCT technology acquires the CCT, ACD, anterior aqueous depth, lens thickness, and axial length measurements from the single OCT image aligned with the eye’s visual axis. Meanwhile, white-to-white distance is measured using the light-emitting diode light source according to iris configuration, whereas the SS-OCT optical biometer measures keratometry using telecentric keratometry. The IOLMaster 700 software provides keratometry measurements in the 2.5-mm zone. To calculate corneal power, the device uses the anterior corneal radius and standardized keratometric index of 1.3375.
Pentacam HR and IOLMaster 700 measurements were taken in random order in the same dimly lit room with a 10-minute rest period from 9:00 AM to 12:00 PM in order to minimize variation in the results.
SimK 2.0 mm values (flat K, steep K, their corresponding axes, and mean SimK) were obtained from Power Distribution Display by centering x and y axes at 0.0 mm and selecting the 2.0 mm ring diameter option. AveK, CCT, and ACD values automatically provided by the Pentacam HR software were also recorded. Using the IOLMaster 700, mean keratometry (Km), flat K, steep K; and their corresponding axes values; CCT; and ACD measurements were taken.
Power vector analysis was conducted using the method proposed by Thibos for obtaining vectors along the 0-degree and 45-degree meridians according to the following equations: (1) vector along the 0-degree meridian (J0) = [−(Ksteep − Kflat)/2 × Cos2α]; (2) vector along the 45-degree meridian (J45) = [−(Ksteep − Kflat)/2 × Sin2α]. SimKflat, SimKsteep, and axes values in 2.0 mm ring for Pentacam HR and Kflat, Ksteep, and axes values automatically provided by IOLMaster 700 software were used for the above-mentioned calculations.
Using both devices, ACD was measured from the corneal epithelium to the anterior lens surface; only scans with an examination quality specification of “OK” using the Pentacam HR were retained for analysis. Quality control criteria were used with the IOLMaster 700 in accordance with manufacturer recommendations. For each device, 3 measurements obtained from the same eye were recorded, and their means were used in statistical analysis. The Km values of IOLMaster 700 were compared for each type of Km value from the Pentacam HR, while other anterior segment parameters were compared between measurements from both devices.
The Kolmogorov-Smirnov test was used to confirm the normal distribution of data. A paired t test was applied to compare the mean values of parameters provided by the Pentacam HR and IOLMaster 700. To assess the agreement between the measurements of the devices, Bland-Altman analysis was performed. All statistical tests were performed using the Statistical Package for Social Sciences, version 18.0 (SPSS Inc, Chicago, Illinois, USA). Significance was set at P < .05.
This prospective study recruited 62 adult participants (34 men and 28 women) with a mean manifest spherical equivalent refraction of −0.37 ± 0.75 diopters (D) (range +1.0 to −1.0 D). The mean age of participants was 35.3 ± 4.3 (range 18–40) years.
The Table demonstrates Km measurements, J0 and J45 vector components of astigmatism, CCT, and ACD values for both the Pentacam HR and IOLMaster 700. The Pentacam HR exhibited significantly lower keratometry and CCT values than the IOLMaster 700 ( P < .001, for all parameters) However, no significant difference emerged in J0, J45, and ACD measurements between the devices ( P = .057, P = .574, and P = .64, respectively).
|Pentacam HR||IOLMaster 700||Difference||P Value|
|AveK/Km (D)||43.0 ± 1.3||43.2 ± 1.3||−0.20 ± 0.09||<.001|
|SimK 2.0 mm/Km (D)||43.06 ± 1.5||43.2 ± 1.3||−0.14 ± 0.16||<.001|
|J 0 (D)||0.35 ± 0.23||0.28 ± 0.22||0.07 ± 0.09||.057|
|J 45 (D)||−0.018 ± 0.18||−0.002 ± 0.16||−0.016 ± 0.15||.574|
|CCT (μm)||538.3 ± 45||543.35 ± 48.8||−5.05 ± 7.67||<.001|
|ACD (mm)||3.48 ± 0.38||3.476 ± 0.36||0.004 ± 0.04||.64|
Figure 1 shows the Bland-Altman plot for the Pentacam HR AveK and IOLMaster 700 Km. The mean difference was −0.20 D, at 95% limits of agreement (LoA) (−0.02 and −0.38). Figure 2 shows the Bland-Altman plot for Pentacam HR SimK 2.0 mm and the IOLMaster 700 Km. The mean difference of keratometry measurements was −0.14 D (95% LoA, 0.17 and −0.45). Meanwhile, Figures 3 and 4 show the Bland-Altman plots for J0 and J45 vector components of astigmatism between the devices, and the mean difference was 0.07 D (95% LoA, 0.24 and −0.10) and −0.016 D (95% LoA; 0.27 and −0.31), respectively.
Figures 5 and 6 display the Bland-Altman plots for CCT and ACD values between the Pentacam HR and IOLMaster 700; the mean difference was −5.05 μm (95% LoA, 9.8 and −19.9) and 0.004 mm (95% LoA, 0.09 and −0.08), respectively.