Comparison of central corneal thickness and anterior chamber depth measurements using three imaging technologies in normal eyes and after phakic intraocular lens implantation

A quantitative study of the effect of ICL orientation selection on post-operative vault and model-assisted vault prediction

Background

Appropriate vault height of implantable collamer lens (ICL) implantation matters for it has risks of corneal endothelial cell loss, cataract formation and intraocular pressure elevation, which could lead to irreversible damage to optic nerve. Therefore, pre-operative prediction for an ideal vault height is a hotspot. However, few data exist regarding quantitative effect of ICL orientation on vault height. This study is aimed to quantitatively investigate the effect of ICL implantation orientation on vault height, and built a machine-learning (ML)-based vault prediction model taking implantation orientation into account.

Methods

473 consecutive case series treated with ICL implantation were retrospectively analyzed (408 were horizontally implanted, and 65 were vertically implanted). Multivariable logistic regression analysis was performed to determine the association between ICL orientation and achieved vault. ML was performed to develop a new vault height prediction model taking ICL orientation into account. Receiver operating characteristic curve (ROC) and net reclassification index (NRI) were obtained to assess the prediction ability.

Results

95% of all the patients achieved 20/20 uncorrected distance visual acuity (UDVA) or better. No complications including cataract formation, dispersion or optic nerve injury were observed in any cases. Sex, sphere power, cylinder power, axis, ICL size and ICL orientation were all significant risk factors associated to vault height, and age was positively co-related. Of note, ICL size and ICL orientation were the top-ranking risk factors. Comparing to conventional horizontal implantation, vertical implantation could reduce the achieved vault by 81.187 μm (p < 0.001). In regarding to different ICL sizes, vertical implantation had no good to vault reduction when using ICL of 12.1 mm. However, it could reduce the vault by 59.351 μm and 160.992 μm respectively when ICL of 12.6mm and 13.2 mm were implanted (p = 0.0097 and p = 0.0124). For prediction of vault height, ML based model significantly outperformed traditional multivariable regression model.

Conclusion

We provide quantitative evidence that vertical implantation of ICL could effectively reduce the achieved vault height, especially when large size ICL was implanted, comparing to traditional horizontal implantation. ML is extremely applicable in development of vault prediction model.

Pentacam versus anterior segment OCT in measuring intended versus achieved ablation depth post-myopic LASIK correction

Background

Accurate quantitative measurements of central corneal thickness (CCT) provide valuable, clinical information for preoperative assessment, surgical planning, and follow-up in myopic patients who have undergone LASIK correction. Typically, an ultrasound is used to measure such parameters. However, noncontact devices such as the Visante anterior segment optical coherence tomographer [AS-optical coherence tomography (OCT)] and Pentacam are becoming more popular to measure ocular parameters. It is important to assess the level of similarity between these two optical devices to compare between the intended and achieved ablation depth after LASIK procedures in cases of myopia and myopic astigmatism.

Methods

This cross-sectional, prospective study included 80 eyes of 40 patients attending Beni-Suef University Hospital between November 2018 and November 2019. All patients underwent LASIK surgery for the correction of myopia with or without myopic astigmatism with spherical equivalent ranging from − 1.5 to − 12 diopters.

Results

No statistically significant difference was observed between the intended ablation depth and the clinically measured ablation depth calculated by Pentacam and AS-OCT 3 months after surgery (P > 0.05).

Conclusion

When planning corneal refraction surgery, ablation depth readouts calculated by the computer software of the excimer laser used in this study are reliable. Both Pentacam and AS-OCT are accurate, sensitive, and specific in measuring the CCT and calculating ablation depth. Pentacam provides slightly higher accuracy and sensitivity compared with AS-OCT. LASIK is a safe, predictable, and effective procedure in the treatment of simple myopia and myopic astigmatism.

Agreement of implantable collamer lens sizes using parameters from different devices

Objective

To assess agreement and repeatability of white-to-white (WTW) and anterior chamber depth (ACD), and agreement of implantable collamer lens (ICL) size using these measurements from different devices.

Methods and analysis

A retrospective review of 83 eyes with ICL implantation (42 patients) was conducted. The agreement of WTW (measured with WaveLight Topolyzer and Orbscan IIz) and ACD (measured with WaveLight Oculyzer and Orbscan IIz) was analysed. Correlation of ICL sizes and difference of eyes with unacceptable vaults between two data sets (WaveLight platform; Topolyzer and Oculyzer and Orbscan IIz) were assessed.

Results

Average WTW measured by Orbscan IIz and Topolyzer demonstrated good agreement (Ρ 0.884) with low systematic bias (−0.03±0.1 mm) and narrow 95% limits of agreement (LoA) of −0.28 to 0.22. Average ACD measured by Orbscan IIz and Oculyzer also showed good agreement (Ρ 0.903) with low systematic bias (−0.04±0.1 mm) and relatively narrow 95% LoA (0.2 to 0.12). ICL size selected according to two data sets showed moderate to strong level of agreement (Kappa=0.81). There was a statistically significant difference (p<0.001) in the proportion of eyes with unacceptable postoperative vaults when using the Wavelight platform data set (five eyes, 6.02%) and the Orbscan IIz data set (12 eyes, 14.46%).

Conclusion

Although the agreement of WTW and ACD between devices was good, there was a significant difference in proportion of eyes with unacceptable postoperative vaults when using two data sets. Therefore, Topolyzer and Oculyzer might not be suitable for operating interchangeably with Orbscan IIz for ICL size selection.

Comparison of a New Optical Biometer That Combines Scheimpflug Imaging With Partial Coherence Interferometry With That of an Optical Biometer Based on Swept-Source Optical Coherence Tomography and Placido-Disk Topography

PURPOSE

To evaluate measurement precision and to compare the Pentacam AXL (Oculus Optikgeräte, Wetzlar, German), a new optical biometer based on Scheimpflug imaging and partial coherence interferometry (PCI) with that of the OA-2000 biometer (Tomey, Nagoya, Japan), which combines swept-source optical coherence tomography (SS-OCT) and Placido-disk topography.

METHODS

Axial length (AL), central corneal thickness (CCT), anterior chamber depth (ACD), aqueous depth (AQD), mean keratometry (Km), astigmatism vectors J0, J45, and corneal diameter (CD) were measured in triplicate by two technical operators. Within-subject standard deviation (Sw), repeatability and reproducibility (2.77 Sw), coefficient of variation (CoV), and intraclass correlation coefficient (ICC) were used to assess the Pentacam AXL intra-observer repeatability and inter-observer reproducibility. Paired t-test and Bland-Altman plots were used to determine the agreement between the two biometers.

RESULTS

The new optical biometer had high intra-observer repeatability [all parameters evaluated had low CoV (<0.71%) and high ICC (>0.88)]. Inter-observer reproducibility was also excellent, with high ICC (>0.95) and low CoV (<0.52%). The 95% LoA between the new biometer and OA-2000 were insignificant for most of the parameters evaluated, especially for AL. However, the measurement agreement was moderate for CCT.

CONCLUSIONS

Intra-observer repeatability and inter-observer reproducibility were excellent for all parameters evaluated using the new optical biometer based on Scheimpflug imaging and PCI. There was a high agreement between the two devices and hence could be clinically interchangeable for the measurement of most ocular parameters.

Agreement Between Angle-to-Angle Distance and Aqueous Depth Obtained With Two Different Optical Coherence Tomographers and a Scheimpflug Camera

PURPOSE

To compare angle-to-angle (ATA) distance and aqueous depth (AQD) readings produced by two different optical coherence tomography (OCT) devices and a Scheimpflug camera.

METHODS

ATA distance and AQD were measured in 60 eyes using the Visante time-domain OCT (TD-OCT) (Carl Zeiss Meditec AG), the Anterion swept-source OCT (SS-OCT) (Heidelberg Engineering GmbH), and the Pentacam HR Scheimpflug camera (Oculus Optikgeräte GmbH). Moreover, ATA distance was measured along the horizontal and vertical meridians. Bland-Altman analysis was used to assess the agreement between devices.

RESULTS

All three devices did not yield similar horizontal ATA distance: 11.96 ± 0.47 mm (TD-OCT), 11.96 ± 0.42 mm (SS-OCT), and 11.05 ± 0.52 mm (Pentacam). More specifically, the Pentacam yielded significantly smaller values (approximately 0.9 mm, P < .001). Mean vertical ATA distance was 12.61 ± 0.65 and 12 ± 0.65 mm for the TD-OCT and SS-OCT, respectively (P = .983). Vertical ATA distance was significantly larger than horizontal ATA distance for both OCT devices (approximately 0.6 mm, P < .001). The lowest mean difference was found between both OCT devices (0.0068 and -0.0415 mm, for horizontal and vertical meridians, respectively) and the highest between the Pentacam and the two OCT devices (approximately 0.9 mm). As for AQD, inter-device mean values were also statistically significant: 2.89 ± 0.48, 2.82 ± 0.49, and 2.79 ± 0.50 mm, for the TD-OCT, SS-OCT, and Pentacam, respectively (P < .001). Mean differences were similar for the TD-OCT versus SS-OCT and SS-OCT versus Pentacam (approximately 0.02 mm), and larger between the TD-OCT and Pentacam (approximately 0.05 mm).

CONCLUSIONS

The results show that the TD-OCT and SS-OCT are interchangeable instruments for ATA distance measurement but the Pentacam is not with either of the two OCT devices. A clinical criterion should assess whether these three devices could be used interchangeably for AQD measurement. [J Refract Surg. 2021;37(2):133-140.].

Comparison of Anterior Ocular Biometric Measurements Using Swept-Source and Time-Domain Optical Coherence Tomography

Purpose

To compare central corneal thickness (CCT), aqueous depth (AQD), and anterior chamber depth (ACD) measurements using the swept-source (CASIA SS-1000, Tomey, Japan) and time-domain (Visante, Carl Zeiss Meditec, USA) anterior segment optical coherence tomographers (OCT) in normal eyes.

Methods

Sixty-eight eyes of 68 subjects were included. Three consecutive scans of each subject were obtained using both devices in a random order by one experienced operator. Standard deviation (S _w), coefficient of repeatability (CoR), coefficients of variation (CoV), and intraclass correlation coefficients (ICC) were used to evaluate the intraoperator repeatability. Agreement was assessed using the Bland-Altman plots and 95% limits of agreement (LoA).

Results

All measurements of the swept-source OCT (SS-OCT) and time-domain OCT (TD-OCT) showed high repeatability with low CoR (CCT: 2.34 μm and 6.16 μm; AQD: 0.05 mm and 0.09 mm; ACD: 0.06 mm and 0.09 mm), low CoV (CCT: 0.16% and 0.42%; AQD: 0.61% and 0.97%; ACD: 0.53% and 0.83%), and high ICC (>0.98). The mean CCT with SS-OCT was slightly thicker than the results with TD-OCT (difference = 4.55 ± 2.62 μm, P < 0.001). There was no statistically significant difference in AQD or ACD measurements between the two devices (0.01 ± 0.05 mm, P=0.111; 0.02 ± 0.05 mm, P=0.022, respectively). The 95% LoA between the SS-OCT and TD-OCT were -0.59 to 9.69 μm for CCT, -0.10 to 0.12 mm for AQD, and -0.09 to 0.12 mm for ACD.

Conclusions

High levels of repeatability and agreement were found between the two devices for all three parameters, suggesting interchangeability. SS-OCT demonstrated superior repeatability compared with TD-OCT.

Differences between Scheimpflug and optical coherence tomography in determining safety distances in eyes with an iris-fixating phakic intraocular lens

Purpose

To investigate the agreement and reliability of anterior segment optical coherence tomography (AS-OCT) and Scheimpflug imaging in measuring the distance from the anterior edge of an iris-fixated phakic intraocular lens (IF-pIOL) to the corneal endothelium.

Methods

Anterior segment configuration was assessed in a total of 62 eyes of which 25 hyperopic and 37 myopic eyes, all corrected with an IF-pIOL. Measurements were performed by two independent observers using AS-OCT (Visante, Model 1000, Carl Zeiss Meditec Inc.) and Scheimpflug imaging (Pentacam HR, Oculus Optikgerate). The distance from the anterior edge of the pIOL to the endothelium was measured in five different positions using both modalities with their corresponding pIOL software. The measurements as well as the inter- and intra-observer reliability of the two imaging modalities were then compared.

Results

Distance measurements for all positions performed by AS-OCT were found to be significantly larger than those performed by Scheimpflug imaging, with mean differences ranging from 0.11 to 0.22 mm. Both instruments exhibited good inter- and intra-observer reliability.

Conclusion

Anterior pIOL edge to endothelium distance measurements by AS-OCT and Scheimpflug imaging have good intra- and inter-observer reliability. However, as AS-OCT provides larger measurements, these two modalities cannot be used interchangeably. Correction of this difference might be essential for proper decision-making during pre-operative screening for pIOL implantation and post-operative safety monitoring.

Electronic supplementary material

The online version of this article (10.1007/s00417-020-04874-7) contains supplementary material, which is available to authorized users.

Comparison of corneal thickness in patients with dry eye disease using the Pentacam rotating Scheimpflug camera and anterior segment optical coherence tomography

The purpose of this study was to compare central corneal thickness, thinnest corneal thickness, and the thinnest point of the cornea between Pentacam and anterior segment optical coherence tomography (ASOCT) in patients with dry eye disease (DED). This cross-sectional study included 195 participants between November 2015-June 2017. DED was diagnosed using the Asia Dry Eye Society criteria and further divided into mild and severe DED based on kerato-conjunctival vital staining. Central corneal thickness, thinnest corneal thickness, and the thinnest point of the cornea measured by Pentacam and ASOCT were compared, and Pearson's correlation coefficients were estimated. The differences in central corneal thickness and the thinnest corneal thickness between Pentacam and ASOCT were analysed using Bland-Altman and multivariate regression analyses adjusted for age and sex. This study included 70 non-DED subjects and 52 patients with mild and 73 with severe DED. The Pentacam and ASOCT measurements of central corneal thickness and thinnest corneal thickness were strongly correlated, but the respective values were higher when measured with Pentacam. The Bland-Altman analysis revealed differences in central corneal thickness (non DED, 11.8; mild DED, 13.2; severe DED, 19.6) and in thinnest corneal thickness (non DED, 13.1; mild DED, 13.4; severe DED, 20.7). After adjusting for age and sex, the differences in central corneal thickness (β = 7.029 μm, 95%CI 2.528-11.530) and thinnest corneal thickness (β = 6.958 μm, 95%CI 0.037-13.879) were significantly increased in the severe-DED group. The distribution of the thinnest point of the cornea in the cornea's inferior temporal quadrant between Pentacam and ASOCT deviated in severe DED (Pentacam: 90.4% vs. ASOCT: 83.6%). Clinicians should consider that there were significant differences in corneal-morphology assessment between the measurements with Pentacam and ASOCT in severe DED.

Comparative study of anterior segment measurements using 3 different instruments in myopic patients after ICL implantation

Background

To compare and correlate anterior segment measurements of myopic eyes implanted with Implantable Collamer lens (ICL V4c) by using anterior segment optical coherence tomography (AS-OCT), Pentacam and ultrasound biomicroscopy (UBM).

Methods

Anterior chamber depth (ACD), distance between corneal endothelium and anterior surface of ICL(C-ICL) and central vault were measured in 82 phakic myopic eyes of 82 patients who underwent ICL surgery, by using AS-OCT, Pentacam and UBM consecutively at 3 months follow up. The correlation and agreement of instruments were accessed by using Intraclass correlation coefficient (ICC) and the Bland-Altman plot.

Results

AS-OCT showed higher ACD, C -ICL and central vault measurements than both of Pentacam and UBM (P < 0.001), while Pentacam showed lower measurements than UBM (P < 0.05). The Pearson correlation coefficient (r) was 0.91 to 0.96, and ICC was 0.95 to 0.98 for all measurements between difference devices (all P < 0.001). The 95% limits of agreement of ACD, C-ICL, vault measurements were 0.13 to 0.38 mm, − 0.07 to 0.27 mm, 0.08 to 0.34 mm between AS-OCT and Pentacam, − 0.03 to 0.33 mm, − 0.16 to 0.31 mm, − 0.10 to 0.26 mm between AS-OCT and UBM, and − 0.29 to 0.07 mm, − 0.25 to 0.20 mm, − 0.31 to 0.05 mm between Pentacam and UBM, respectively.

Conclusions

AS-OCT demonstrated significantly higher value, while Pentacam demonstrated significantly lower value than UBM for ACD, C-ICL and central vault measurements in myopic eyes after ICL surgery. Measurements with these instruments were highly correlated, but could not replace each other especially for vault. This study provided valuable information about how to judge the results of anterior segment parameters of eyes implanted with ICL V4c from different devices.

Trial registration

Registration number: ChiCTR-OOC-16008987. Retrospectively registered: 08 August 2016.

Reproducibility of Central Corneal Thickness Measurements in Normal Eyes Using the Zeiss Cirrus 5000 HD-OCT and Pentacam HR

Objectives:

To determine the repeatability and reproducibility of Central Corneal Thickness (CCT) measurements using two different anterior segment imaging modalities, including those obtained with the new anterior segment lens attachments for the Cirrus 5000 HD-OCT.

Methods:

A total of 32 eyes from 16 normal volunteers (8 male, 8 female) were enrolled in this prospective study. CCT was measured by the same examiner using the Cirrus 5000 HD-OCT and Pentacam HR. The results of CCT obtained by each method were averaged and compared using t-test analysis. The agreement between the measurement methods was evaluated. Coefficient of Repeatability (CoR) and Intra-Class Correlation Coefficient (ICC) were computed.

Results:

The mean measurements taken with the Cirrus OCT anterior chamber lens (CCT_AC), HD cornea lens (CCT_HDC) and pachymetry scans (CCT_Pach) were 545.35 ± 31.02, 537.87 ± 26.82, and 532.04 ± 29.82 µm, respectively. The mean CCT obtained with the Pentacam (CCT_Pent) was 545.51 ± 30.71 µm. CCT_Pent were significantly higher than CCT_HDC and CCT_Pach (p< 0.0001). In contrast, the CCT_Pent and CCT_AC were similar (p=0.87). CCT, as evaluated by the two different instruments, showed excellent correlation (r > 0.98, p< 0.0001) with an ICC > 0.99 (95% CI, 0.97 – 0.99). CoR was the highest for CCT_Pach (3.7 ± 1.4, 95% CI (3.0- 4.6)).

Conclusion:

CCT measurements from the Cirrus OCT using the new anterior segment lens attachments and the Pentacam HR are highly correlated. This should allow the use of a standardized correction factor if necessary to inter-relate the measurements between the two devices.

Comparison of anterior segment measurements with LenStar and Pentacam in patients with newly diagnosed glaucoma

PURPOSE

To compare anterior segment measurements obtained using the Pentacam; Oculus, HR and the LenStar LS 900 in patients with newly diagnosed glaucoma.

METHODS

Patients with ocular hypertension and primary open-angle glaucoma who had been treated with PGA were included in the study. Anterior segment measurements including central corneal thickness (CCT), keratometry, anterior chamber depth (ACD) and white-to-white (WTW) corneal diameter obtained with the optic low-coherent reflectometer (LenStar LS-900, Haag-Streit AG, Switzerland) and with the Scheimpflug system (Pentacam; Oculus, HR) were compared. In order to compare LenStar and Pentacam measurements, paired sample t test and 'Bland-Altman' plot were used in the statistical analysis.

RESULTS

Twenty-two female (59.5%) and 15 male (40.5%) totally 37 cases with newly diagnosed glaucoma were included in the study. Anterior segment parameter measurements obtained with both the LenStar and the Pentacam were significantly correlated for right and left eyes, so the right eye values were used in statistical analysis. WTW and ACD values measured with LenStar were statistically significantly higher than those measured with Pentacam (p: 0.0001, p: 0.0001, respectively). There was no statistically significant difference between the CCT values measured by the two devices (p: 0.217).

CONCLUSION

There was a statistically significant difference between the mean values of ACD and WTW measured with the LenStar and Pentacam. These biometric devices should not be used interchangeably. It should be appropriate to use the same device on follow-up of glaucoma patients.

Device interchangeability on anterior chamber depth and white-to-white measurements: a thorough literature review

We have reviewed a set of recently published studies that compared the anterior chamber depth (ACD) and/or white-to-white (WTW) distance obtained by means of different measuring devices. Since some of those studies reached contradictory conclusions regarding device interchangeability, this review was carried out in attempting to clarify which clinical devices can or cannot be considered as interchangeable in clinical practice to measure ACD and/or WTW distance, among these devices: A-scan, ultrasound biomicroscopy, Orbscan and Orbscan II (Bausch&Lomb Surgical Inc., San Dimas, California, USA), Pentacam and Pentacam HR (Oculus, Wetzlar, Germany), Galilei (Ziemer, Switzerland), Visante optical coherence tomography (Visante OCT, Carl Zeiss Meditec Inc., Dublin, California, USA), IOLMaster (Carl Zeiss Meditec, Jena, Germany), and Lenstar LS 900/Biograph (Haag-Streit AG, Koeniz, Switzerland/Alcon Laboratories Inc., Ft Worth, Texas, USA).

Comparative analysis of corneal measurements obtained from a Scheimpflug camera and an integrated Placido-optical coherence tomography device in normal and keratoconic eyes

PURPOSE

To assess the agreement between a Scheimpflug camera (Pentacam) and a combined Placido-optical coherence tomography device (Visante OMNI) in measuring corneal curvature, thickness and elevation values in normal and keratoconic eyes.

METHODS

Corneal measurements of 110 normal eyes (one eye per subject) and 70 keratoconic eyes were obtained from both devices and compared. Agreement was determined using the Bland-Altman analysis 95% limits of agreement (LoA).

RESULTS

The Pentacam measured significantly greater keratometry readings in the flattest (K1) and steepest meridians (K2) in normal and keratoconic eyes. The 95% LoA in normal eyes were -0.32 to 0.59 dioptres (D) (K1) and -0.41 to 0.74 D (K2). In keratoconic eyes, the 95% LoA were -1.35 to 1.92 D (K1) and -1.38 to 1.99 D (K2). The Pentacam recorded significantly higher central corneal thickness (CCT) values in both groups of eyes. The 95% LoA were -4.31 to 39.89 microns (μ) and -12.92 to 41.35 μ in normal and keratoconic eyes, respectively. Pentacam anterior and posterior corneal elevations were significantly greater in both groups of eyes. The devices demonstrated excellent repeatability and reproducibility for corneal curvature and thickness but not elevation measurements.

CONCLUSIONS

The Pentacam measured significantly greater corneal curvature, thickness and elevation values compared to the Visante OMNI in normal and keratoconic eyes. The devices agree moderately for anterior corneal elevations in normal eyes and do not appear to be interchangeable for corneal measurements in clinical practice.

Estimation of intraocular lens power calculation after myopic corneal refractive surgery: using corneal height in anterior segment optical coherence tomography

PURPOSE

To investigate the feasibility of estimating effective lens position (ELP) and calculating intraocular lens power using corneal height (CH), as measured using anterior segment optical coherence tomography (AS-OCT), in patients who have undergone corneal refractive surgery.

METHODS

This study included 23 patients (30 eyes) who have undergone myopic corneal refractive surgery and subsequent successful cataract surgery. The CH was measured with AS-OCT, and the measured ELP (ELPm) was calculated. Intraocular lens power, which could achieve actual emmetropia (Preal), was determined with medical records. Estimated ELP (ELPest) was back-calculated using Preal, axial length, and keratometric value through the SRK/T formula. After searching the best-fit regression formula between ELPm and ELPest, converted ELP and intraocular lens power (ELPconv, Pconv) were obtained and then compared to ELPest and Preal, respectively. The proportion of eyes within a defined error was investigated.

RESULTS

Mean CH, ELPest, and ELPm were 3.71 ± 0.23, 7.74 ± 1.09, 5.78 ± 0.26 mm, respectively. The ELPm and ELPest were linearly correlated (ELPest = 1.841 × ELPm - 2.018, p = 0.023, R = 0.410) and ELPconv and Pconv agreed well with ELPest and Preal, respectively. Eyes within ±0.5, ±1.0, ±1.5, and ±2.0 diopters of the calculated Pconv, were 23.3%, 66.6%, 83.3%, and 100.0%, respectively.

CONCLUSIONS

Intraocular lens power calculation using CH measured with AS-OCT shows comparable accuracy to several conventional methods in eyes following corneal refractive surgery.

Interchangeability among five devices that measure anterior eye distances

PURPOSE

The aim was to study the interchangeability of various devices that measure anterior chamber depth (ACD) and white-to-white (WTW) distance.

METHODS

Eighty right eyes of eighty patients aged 20 to 40 years were included. Their spherical equivalent ranged from -4.25 to +1.00 dioptres. Each eye's ACD and the WTW distance were measured using the following devices: a Castroviejo surgical calliper, CA-200F Corneal Analyser, Orbscan II, Pentacam HR and IOLMaster 500.

RESULTS

The Pentacam HR measured significantly greater ACD (p < 0.01) than the other two devices, whereas IOLMaster 500 and Orbscan II yielded comparable (p > 0.01) distances. Nonetheless, in every device comparison, the mean difference was within clinically acceptable limits. The device comparison showed that for WTW distance measurements IOLMaster 500 yields the greatest value (p < 0.01), whereas CA-200F Corneal Analyser yields the smallest one (p < 0.01).

CONCLUSION

Pentacam HR, Orbscan II and IOLMaster 500 can be used interchangeably to measure ACD, whereas Orbscan II, IOLMaster 500, Castroviejo surgical calliper and CA-200F Corneal Analyser cannot be considered interchangeable, when it comes to measuring WTW distance.

Safety and efficacy of epithelium removal and transepithelial corneal collagen crosslinking for keratoconus

This review aims to assess the efficacy and safety of epithelial removal (ER) and transepithelial (TE) corneal collagen crosslinking (CXL) for the treatment of keratoconus. We used MEDLINE to identify all ER and TE CXL studies on keratoconic eyes (n≥20, follow-up ≥12 months). Ex vivo and studies for non-keratoconus indications or in conjunction with other procedures were excluded. Data on uncorrected (UDVA) and corrected (CDVA) distance visual acuity, refractive cylinder, maximum keratometry (Kmax), and adverse events were collected at the latest follow-up and 1 year. Only one randomised controlled trial (RCT) qualified inclusion. Forty-four ER and five TE studies were included. For logMAR UDVA, CDVA, mean spherical equivalent, refractive cylinder and Kmax, at latest follow-up 81, 85, 93, 62, and 93% ER studies vs 66.7, 80, 75, 33, and 40% TE studies reported improvement, respectively. Whereas at 1 year, 90, 59, and 91% ER studies vs 80, 50, and 25% TE studies reported improvement, respectively. The majority of studies showed reduced pachymetry in both groups. Treatment failure, retreatment rates, and conversion to transplantation were reported to be up to 33, 8.6, and 6.25%, respectively, in ER studies only. Stromal oedema, haze, keratitis, and scarring were only reported in ER studies, whereas endothelial cell counts remained variable in both groups. Both ER and TE studies showed improvement in visual acuity, refractive cylinder but Kmax worsened in most TE studies. Adverse events were reported more with ER studies. This review calls for more high quality ER and TE studies with comparable parameters for further assessment of safety and efficacy.

Agreement between Orbscan II, VuMAX UBM and Artemis-2 very-high frequency ultrasound scanner for measurement of anterior chamber depth

Background

The aim was to compare the anterior chamber depth (ACD) measurements taken with Orbscan II, ultrasound biomicroscopy (UBM) and the Artemis-2 VHF (very-high-frequency) ultrasound scanner in normal subjects.

Methods

In this prospective study, one eye from each of 60 normal subjects was randomly selected. Three subjects dropped out of the study because they were apprehensive about the UBM examination; their data were excluded entirely. Measurements of ACD were taken with the Orbscan II, UBM and Artemis-2 VHFUS. Results were obtained for coefficient of variance (CV) and intra-class correlation coefficient (ICC), and statistical analysis was by repeated-measures analysis of variance (ANOVA) for intra-observer repeatability. ANOVA and Bland–Altman analyses were used to determine limits of agreement (LOA) between the three instruments.

Results

The average ACD (± standard deviation) was 3.13 ± 0.34 mm, 2.96 ± 0.27 mm and 2.87 ± 0.31 mm for the Orbscan II, UBM and Artemis-2 VHFUS, respectively. The repeatability scores were 0.015 ± 0.014%, 0.08 ± 0.09% and 0.07 ± 0.06% for the Orbscan II, UBM and Artemis-2 VHFUS, respectively. The ICC for repeatability of Orbscan II, UBM and Artemis-2 VHFUS measurements was high and equal to 0.99%. The intra-observer repeatability scores of the ACD measurement p-values using Orbscan II, UBM and Artemis-2 VHFUS were 0.12, 0.70 and 0.10, respectively. The mean difference and standard deviations for ACD measurements using Orbscan II vs UBM, Orbscan II vs Artemis-2 VHFUS and UBM vs Artemis-2 VHFUS were 0.17 ± 0.31 mm, 0.27 ± 0.34 mm and 0.10 ± 0.18 mm, respectively. LOAs were 0.78 to -0.44 mm, 0.93 to -0.39 mm and 0.45 to -0.26 mm. ANOVA revealed a statistically significant difference between the Orbscan II, UBM and Artemis-2 VHFUS (p < 0.0001).

Conclusions

Measurements by the three instruments show high repeatability. UBM and the Artemis-2 VHFUS can be used interchangeably, but the Orbscan II cannot be used interchangeably with UBM or the Artemis-2 VHFUS.

Anterior segment parameters of rabbits with rotating Scheimpflug camera

BACKGROUND

Rabbit is one of the most commonly used experimental animals for corneal studies due to similarity of size to human cornea and ease of manipulation. In this study, we assessed anterior segment parameters of the healthy rabbit eyes with Pentacam HR (Oculus, Wetzlar, Germany).

METHODS

Six-month-old, approximately 2.5-3 kg weighted, 30 female New Zealand rabbits were used in the study. Right eye of the each rabbit was imaged with Pentacam HR under intramuscular ketamine hydrochloride (Ketalar; Eczacibasi, Turkey) anesthesia (50 mg/kg). After the imaging, the rabbits with blinking errors, which results in low-quality images, were excluded from the study. Keratometric readings, central corneal thickness (CCT), anterior chamber depth (ACD), and anterior and posterior elevation values, and lens density were noted.

RESULTS

In this study, the flattest and the steepest keratometric values were found as 43.34 ± 1.86, 42.7 ± 2.0, and 43.9 ± 1.9 diopters, respectively. The mean CCT and ACD of rabbits were found as 388 ± 39 μm and 2.08 ± 0.16 mm, respectively. Mean of the anterior and posterior elevation at thinnest point was found as 1.29 ± 4.28 and 3.91 ± 6.17 μm, respectively.

CONCLUSIONS

Keratometric readings and anterior and posterior elevation values of rabbits were similar to human; however, corneal thickness and anterior chamber depth (ACD) values were lower than humans.

Comparison of anterior segment optical tomography parameters measured using a semi-automatic software to standard clinical instruments

Objective

To compare anterior segment parameters measured using a semi-automatic software (Zhongshan Angle Assessment Program, ZAP) applied to anterior segment optical coherence tomography (AS-OCT) images, with commonly used instruments.

Methods

Cross-sectional study of a total of 1069 subjects (1069 eyes) from three population-based studies of adults aged 40–80 years. All subjects underwent AS-OCT imaging and ZAP software was applied to determine anterior chamber depth (ACD), central corneal thickness (CCT), anterior and keratometry (K) – readings. These were compared to auto-refraction, keratometry and ocular biometry measured using an IOLMaster, ultrasound pachymeter and auto-refractor respectively. Agreements between AS-OCT (ZAP) and clinical instrument modalities were described using Bland-Altman, 95% limits of agreement (LOA).

Results

The mean age of our subjects was 56.9±9.5 years and 50.9% were male. The mean AS-OCT (ZAP) parameters of our study cohort were: ACD 3.29±0.35 mm, CCT 560.75±35.07 µm; K-reading 46.79±2.72 D. There was good agreement between the measurements from ZAP analysis and each instrument and no violations in the assumptions of the LOA; albeit with a systematic bias for each comparison: AS-OCT consistently measured a deeper ACD compared to IOLMaster (95% LOA −0.24, 0.55); and a thicker CCT for the AS-OCT compared to ultrasound pachymetry (16.8±0.53 µm 95% LOA −17.3, 50.8). AS-OCT had good agreement with auto-refractor with at least 95% of the measurements within the prediction interval (P value <0.001).

Conclusion

This study demonstrates that there is good agreement between the measurements from the AS-OCT (ZAP) and conventional tools. However, small systematic biases remain that suggest that these measurement tools may not be interchanged.

Refractive error assessment: influence of different optical elements and current limits of biometric techniques

PURPOSE

To identify and quantify sources of error on refractive assessment using exact ray tracing.

METHODS

The Liou-Brennan eye model was used as a starting point and its parameters were varied individually within a physiological range. The contribution of each parameter to refractive error was assessed using linear regression curve fits and Gaussian error propagation analysis. A MonteCarlo analysis quantified the limits of refractive assessment given by current biometric measurements.

RESULTS

Vitreous and aqueous refractive indices are the elements that influence refractive error the most, with a 1% change of each parameter contributing to a refractive error variation of +1.60 and -1.30 diopters (D), respectively. In the phakic eye, axial length measurements taken by ultrasound (vitreous chamber depth, lens thickness, and anterior chamber depth [ACD]) were the most sensitive to biometric errors, with a contribution to the refractive error of 62.7%, 14.2%, and 10.7%, respectively. In the pseudophakic eye, vitreous chamber depth showed the highest contribution at 53.7%, followed by postoperative ACD at 35.7%. When optic measurements were considered, postoperative ACD was the most important contributor, followed by anterior corneal surface and its asphericity. A MonteCarlo simulation showed that current limits of refractive assessment are 0.26 and 0.28 D for the phakic and pseudophakic eye, respectively.

CONCLUSIONS

The most relevant optical elements either do not have available measurement instruments or the existing instruments still need to improve their accuracy. Ray tracing can be used as an optical assessment technique, and may be the correct path for future personalized refractive assessment.

Correlation of central corneal thickness measurements using Topcon TRK-1P, Zeiss Visante AS-OCT and DGH Pachmate 55 handheld ultrasonic pachymeter

BACKGROUND

Corneal pachymetry is an important aspect of modern eye care. This study aims to compare the measurement of central corneal thickness (CCT) using automated optical pachymetry, anterior segment optical coherence tomography (AS-OCT) and ultrasound.

METHODS

Fifty volunteers participated in this study. Central corneal thickness was measured using the Topcon TRK-1P (Topcon, Tokyo, Japan) automated optical pachymeter, the Visante AS-OCT (Carl Ziess Meditec, Dublin, CA, USA) and the Pachmate 55 handheld ultrasonic pachymeter (DGH, Exton, PA, USA). Pearson correlation coefficients were calculated to compare automated optical pachymetry and AS-OCT with ultrasound. Bland-Altman plots were generated and limits of agreement (LOA) calculated.

RESULTS

Fifty eyes were measured for comparison. The mean and standard deviation of the CCT measured by optical pachymetry was 523.7 ± 42.3 μm (range: 412 to 594), 536.0 ± 36.9 μm (range: 425 to 607) for AS-OCT and 553.4 ± 40.7 μm (range: 431 to 630) for ultrasound. Comparison of optical pachymetry and AS-OCT with ultrasound, generated Pearson correlation coefficients of 0.96 and 0.98, respectively. Measurements using optical pachymetry and AS-OCT were consistently lower than when measuring with ultrasound. Bland-Altman plots revealed mean differences of 29.7 μm between optical pachymetry and ultrasound (LOA -5.4 to -54.1) and of 17.4 μm (LOA +0.9 to -35.7) between AS-OCT and ultrasound.

CONCLUSION

Due to AS-OCT and automated optical pachymetry consistently measuring lower than ultrasound, it should be noted that these devices are not interchangeable in clinical practice.

Comparison of central corneal thickness and anterior chamber depth measured using LenStar LS900, Pentacam, and Visante AS-OCT

PURPOSE

To compare central corneal thickness (CCT) and anterior chamber depth (ACD) measured using 3 different techniques.

METHODS

CCT and ACD were measured in one eye of 27 healthy subjects (age, mean ± SD: 34 ± 7 years), using the LenStar LS900, Pentacam, and Visante AS-OCT. The agreement between the measurement techniques was assessed using the coefficient of agreement (CoA) and the 95% limits of agreement (LoA).

RESULTS

Mean ± SD values for CCT using the LenStar, Pentacam, and Visante were 550.7 ± 37.4 μm, 542.7 ± 37.9 μm, and 556.7 ± 44.4 μm, respectively (P = 0.005). Plots of differences against means displayed relatively good agreement between the Pentacam and LenStar (CoA, 19.97 μm; LoA, 15.53 μm to -24.40 μm), poorer agreement between the LenStar and Visante (CoA, 31.26 μm; LoA, 40.78 μm to -21.74 μm), and agreement was poor between the Pentacam and Visante (CoA, 37.36 μm; LoA, 25.61 μm to -49.11 μm). Mean ± SD values for ACD using the LenStar, Pentacam, and Visante were 2.93 ± 0.30 mm, 2.96 ± 0.32 mm, and 3.03 ± 0.29 mm, respectively (P < 0.001). Relatively good agreement for ACD measures was observed between the Pentacam and LenStar (CoA, 0.08 mm; LoA, 0.12 to -0.04 mm), poor agreement was obtained between the Visante and LenStar (CoA, 0.13 mm; LoA, 0.21 to -0.06 mm), and agreement was also poor between the Pentacam and Visante (CoA, 0.14 mm; LoA, 0.11 to -0.17 mm).

CONCLUSIONS

Our data show that the LenStar, Pentacam, and Visante provide measurements that are in agreement with published values for CCT and ACD in human subjects. Although reasonable agreement for CCT and ACD was found between the Pentacam and LenStar, agreement was poorer between the Visante and LenStar and between the Visante and Pentacam. Thus, CCT and ACD measures from these instruments should not be used interchangeably.

Cohort profile: Shahroud Eye Cohort Study

The Shahroud Eye Cohort Study was set up to determine the prevalence and incidence of visual impairment and major eye conditions in the 40-64-year-old population of Shahroud as a Middle Eastern population. The first phase of the study was conducted in 2009-10. Using random cluster sampling, 6311 Shahroud inhabitants were invited for ophthalmologic examinations; of these, 5190 participants completed phase 1 (participation rate of 82.2%). All participants were interviewed to collect data on participants' demographics, occupation status, socioeconomic status, history of smoking, and medical and ophthalmic history, as well as history of medication, and the quality and duration of their insurance. DNA and plasma samples, as well as four dots of whole blood were collected from participants. Extensive optometric and ophthalmologic examinations were performed for each participant, including lensometry of current glasses, testing near and far visual acuity; determining objective and subjective refraction; eye motility; cycloplegic refraction; colour vision test; slit-lamp biomicroscopy and intraocular pressure measurement; direct and indirect fundoscopy; perimetry test; ocular biometry; corneal topography; lens and fundus photography; and the Schirmer's (1008 participants) and tear breakup time tests (1013 participants). The study data are available for collaborative research at Noor Ophthalmology Research Center, Tehran, Iran.

Repeatability and agreement of 2 Scheimpflug analyzers in measuring the central corneal thickness and anterior chamber angle, volume, and depth

PURPOSE

To evaluate the repeatability and agreement of 2 Scheimpflug tomographies (Pentacam HR, Oculus GmbH; Sirius, CSO Inc.) in measuring the central corneal thickness (CCT) and depth (ACD), volume (ACV), and angle (ACA) of the anterior chamber.

METHODS

Three consecutive measurements were performed in one eye of each subject, with both instruments. The following were evaluated: CCT, ACD, ACV, and ACA. Repeatability was assessed by the Friedman test and agreement was assessed by 4 different Wilcoxon tests for paired data.

RESULTS

The study enrolled 30 patients. With Sirius tomography the Friedman test p values for testing homogeneity across measurements was with respect to CCT 0.320, ACD 0.315, ACV 0.171, and ACA 0.020; and with Pentacam CCT 0.191, ACD 0.010, ACV 0.704, and ACA 0.150. The Wilcoxon test p values for testing the measurement homogeneity between the corneal tomography Sirius and Pentacam showed the following: ACD 0.000, ACD 0.000, ACV 0.853, and ACA 0.000.

CONCLUSIONS

The measurements may be considered substantially stable for each corneal tomography and each variable, but the 2 corneal tomographies do not produce statistically equivalent measurements.

Corneal biometry from volumetric SDOCT and comparison with existing clinical modalities

We present a comparison of corneal biometric values from dense volumetric spectral domain optical coherence tomography (SDOCT) scans to reference values in both phantoms and clinical subjects. We also present a new optically based "keratometric equivalent power" formula for SDOCT that eliminates previously described discrepancies between corneal power form SDOCT and existing clinical modalities. Phantom objects of varying radii of curvature and corneas of normal subjects were imaged with a clinical SDOCT system. The optically corrected three-dimensional surfaces were used to recover radii of curvature and power as appropriate. These were then compared to the manufacturer's reference values in phantoms and to measurements from topography and Scheimpflug photography in subjects. In phantom objects, paired differences between SDOCT and reference values for radii of curvature were not statistically significant. In subjects, there were no significant paired differences between SDOCT and reference values from the other modalities for anterior radius and corneal keratometric power. In contrast to other studies, we found that dense volumetric scans with available SDOCT can be used to recover corneal biometric values-including power-that correspond well with existing clinical measurements.

Reliability of the corneal thickness measurements with the Pentacam HR imaging system and ultrasound pachymetry

PURPOSE

To obtain the central corneal thickness (CCT) of healthy eyes using the rotating Scheimpflug imaging system and compare these results with conventional ultrasound pachymetry values. Interoperator reliability and repeatability of the different techniques were also determined.

METHODS

CCT was measured on the right eye of 46 healthy subjects with the Pentacam HR (Pentacam High Resolution; Oculus, Wetzlar, Germany) and contact ultrasound pachymetry (AL-2000; Tomey, Tennenlohe, Germany) by 2 independent investigators.

RESULTS

Both observers obtained significantly higher CCT readings with the Pentacam HR imaging system (572 ± 33 and 575 ± 31 μm) than with the ultrasound device (546 ± 27 and 548 ± 28 μm) (P < 0.0001, paired Student t test). Strong and significant correlation was found between the 2 pachymetry techniques regardless of the investigator (r = 0.845 and 0.831, Spearman correlation coefficient) (P < 0.0001). For the Pentacam HR values, statistically significant difference was detected between the observers (P = 0.033). For both pachymetry methods, the degree of agreement between the investigators was reflected by the high interoperator intraclass correlation coefficient value (ICC = 0.96). No significant difference was disclosed between the 3 consecutive readings of either instrument (P > 0.05, analysis of variance).

CONCLUSIONS

Significant difference in CCT values between the high-resolution Pentacam imaging system and the ultrasound pachymeter suggests that the 2 techniques cannot be used interchangeably in clinical practice.

Central corneal thickness measurements with Fourier-domain optical coherence tomography versus ultrasonic pachymetry and rotating Scheimpflug camera

PURPOSE

To compare the accuracy and repeatability of Fourier-domain optical coherence tomography (FD-OCT) with ultrasonic pachymetry (USP) and a rotating Scheimpflug camera for measuring the central corneal thickness (CCT).

METHODS

The CCT was measured in 30 subjects (30 normal corneas) by the same examiner using RTVue-100 FD-OCT with an anterior segment adaptor, Pentacam rotating Scheimpflug camera, and SP-2000 USP. Two examiners obtained one FD-OCT measurement from 10 eyes of 5 subjects to assess interexaminer reproducibility.

RESULTS

The mean CCT (±SD) measured by FD-OCT, USP, and the Pentacam were 530 ± 33, 544 ± 34, and 552 ± 35 μm, respectively. Significant correlations were found between FD-OCT and USP (r = 0.97; P < 0.0001), FD-OCT and Pentacam (r = 0.97; P < 0.0001), and USP and Pentacam (r = 0.96; P < 0.0001). Pairwise comparisons showed that the FD-OCT CCT measurement was significantly thinner than those of the other 2 methods (P < 0.001 for all comparisons). Regarding intraexaminer repeatability, the intraclass correlation coefficients ranged between 0.97 and 0.98. There was high repeatability of the CCT measurements with all methods. FD-OCT also had high interexaminer reproducibility (intraclass correlation coefficient = 0.98).

CONCLUSIONS

RTVue-100 FD-OCT may be a useful alternative for measuring the CCT; however, it significantly underestimates the CCT compared with the USP and the Pentacam with slight differences. Although highly correlated, the measurements are not directly interchangeable in clinical practice.

Anterior chamber depth in normal subjects by rotating scheimpflug imaging

PURPOSE

Anterior chamber depth (ACD) is an important preoperative parameter in anterior segment surgery. Several factors are known to influence ACD, including race and geography. Our purpose was to sample data from various countries to characterize differences in ACD worldwide and, if any, assess their level of clinical significance.

SETTING

International, multicenter.

METHODS

Cross-sectional study. Using the Pentacam Eye Scanner (OCULUS GmbH, Wetzlar, Germany), we analyzed ACD measurements from 1077 eyes of 568 normal adults from nine countries spanning six continents. Differences between countries were assessed by comparison of 95% confidence intervals and by ANOVA. Normative thresholds were constructed at three standard deviations (SD) above and below the mean.

RESULTS

Mean ACD was 3.11 mm overall, ranging from 2.91 mm (New Zealand) to 3.24 mm (United States). The ACD among New Zealanders was significantly shallower (P < .0001) than that among Chinese, Egyptians, Germans, Indians, and Americans. The maximum difference in the mean ACDs was 0.33 mm, between New Zealand and the United States. The shallowest 0.15% of normal ACD values occurred below 2.04 mm overall, ranging from 1.69 mm (New Zealand) to 2.42 mm (United States). The deepest 0.15% of normal ACD values occurred above 4.18 mm overall, ranging from 4.03 mm (Saudi Arabia) to 4.35 mm (Brazil).

CONCLUSIONS

ACD did not vary significantly in the countries studied, with the notable exception of New Zealand. Surgeons should anticipate a greater likelihood of a shallow ACD when evaluating patients from New Zealand. Clinical examination and direct measurement of ACD are recommended. Finally, deep ACD has limited clinical utility in screening for keratoconus.

Objective evaluation of the changes in the crystalline lens during accommodation in young and presbyopic populations using Pentacam HR system

AIM

To quantify the changes in the lens profile with accommodation in different age groups.

METHODS

The Pentacam HR system was used to obtain the images of the anterior eye segment from 23 young and 15 presbyopic emmetropic subjects in unaccommodated (with an accommodation stimulus of 0.0D) and accommodated (with an accommodation stimulus of 5.0D for the young group and 1.0D for the presbyopic group) states. The phakic crystalline lens shape, including curvature of crystalline lens and central lens thickness (CLT), and the measurements of anterior segment length (ASL), central anterior chamber depth (CACD) were investigated. The anterior chamber volume (ACV) was also measured.

RESULTS

The reduction of CACD and ACV were significant in both groups after accommodation stimulus. From the profile of anterior eye segment, a significant decrease in anterior crystalline lens radii of curvature (-2.52mm) and a mean increase in CLT (0.222mm) and ASL (0.108mm) were found in the young group with an accommodation stimulus of 5.0D. However, no statistically significant changes of CLT, ASL, or crystalline lens radii of curvature were found in the presbyopic group.

CONCLUSION

Our data showed that the shallowing of anterior chamber during accommodation was caused by the forward bulging of the anterior lens surface, rather than by anterior shifting of lens position in either young or presbyopic subjects.

Comparison of central corneal thickness measurement using ultrasonic pachymetry, rotating Scheimpflug camera, and scanning-slit topography

PURPOSE

To evaluate and compare central corneal thickness measurements using rotating Scheimpflug camera, scanning-slit topography, and ultrasound pachymetry in virgin, healthy corneas.

DESIGN

Prospective, observational, cross-sectional study.

METHODS

Central corneal thickness in 157 healthy eyes of 157 patients without ocular abnormalities other than refractive errors was measured, in a sequential order, once with rotating Scheimpflug camera and scanning-slit topography and 3 times with ultrasound pachymetry as the last part of examination. All measurements were performed by a single experienced examiner. The results from scanning-slit topography are given with and without correction for "acoustic correction factor" of 0.92.

RESULTS

The average measurements of central corneal thickness by rotating Scheimpflug imaging, scanning-slit pachymetry, and ultrasound were 537.15 ± 32.98 μm, 542.06 ± 39.04 μm, and 544.07 ± 34.75 μm, respectively. The mean differences between modalities were 6.92 μm between rotating Scheimpflug and ultrasound (P < .0001), 2.01 μm between corrected scanning-slit and ultrasound (P = .204), and 4.91 μm between corrected scanning-slit and rotating Scheimpflug imaging (P = .001). According to Bland-Altman analysis, highest agreement was between ultrasonic and rotating Scheimpflug pachymetry.

CONCLUSIONS

In the assessment of normal corneas, rotating Scheimpflug topography measures central corneal thickness values with higher agreement to ultrasound pachymetry.