Reliability of Orbscan II topography measurements in relation to refractive status

Repeatability of corneal pachymetry and epithelial thickness measurements with spectral-domain optical coherence tomography (SD-OCT) and correlation to ocular surface parameters

PURPOSE

To assess the repeatability of corneal pachymetry and epithelial thickness measurements with spectral-domain optical coherence tomography (SD-OCT) and identify correlations between epithelial thickness and ocular surface parameters.

METHODS

Adults who happened to have prolonged computer use were recruited, excluding those with conditions interfering with corneal measurements or tear production. All subjects filled in the ocular surface disease index (OSDI) questionnaire. Three consecutive measurements of central and peripheral corneal and epithelial thickness were performed with SD-OCT (RTVue XR). Schirmer test I and tear film break-up time (TBUT) were performed. Repeatability was evaluated with intraclass correlation coefficient (ICC), coefficient of variation and repeatability limit. Spearman correlation was used for non-parametric variables.

RESULTS

113 eyes of 63 subjects were included in the study. ICC was ≥ 0.989 for all corneal and ≥ 0.944 for all epithelial pachymetry segments. The best repeatability was found centrally and the worst superiorly both for corneal and epithelial measurements. Central epithelial thickness was weakly correlated with Schirmer test I (rho = 0.21), TBUT (rho = 0.02), OSDI symptoms and OSDI score (rho <|0.32|). OSDI symptoms and OSDI score were weakly correlated with Schirmer test I (rho <|0.3|) and TBUT (rho <|0.34|).

CONCLUSION

RTVue XR measurements of corneal and epithelial thickness are highly repeatable in all segments. The lack of correlation between epithelial thickness and ocular surface parameters could suggest the assessment of epithelial integrity with reliable methods such as SD-OCT.

Calculation of Toric Intraocular Lens Power with the Barrett Calculator and Data from Three Keratometers

Aim

To investigate the interdevice agreement for differences in toric power calculated using data on anterior corneal astigmatism obtained with corneal topography/ray-tracing aberrometry (iTrace), partial coherence interferometry (IOLMaster 500), and Scheimpflug imaging (Pentacam).

Methods

The analysis included 101 eyes (101 subjects) with regular astigmatism. The main outcome measures were corneal cylinder power, axis of astigmatism, and keratometry values. Toricity and toric IOL power were calculated using the online Barrett toric calculator. Interdevice agreement for measurement and calculation was assessed using a paired sample t-test and a nonparametric test.

Results

Significant interdevice differences were noted in the magnitude of astigmatism and flat, steep, and mean keratometry values between iTrace and IOLMaster (all P < 0.01); in flat, steep, and mean keratometry values (all P < 0.001) but not in the magnitude of astigmatism (P=0.325) between iTrace and Pentacam; and in the magnitude of astigmatism and steep and mean keratometry values (all P < 0.01) but not in flat keratometry values (P=0.310) between IOLMaster and Pentacam. The toric IOL power calculated using data from the three devices showed the following trend: iTrace > IOLMaster (0.49 ± 0.36, P < 0.001) and Pentacam (0.39 ± 0.42, P < 0.001) and Pentacam was <IOLMaster (-0.10 ± 0.39, P=0.009). There were differences in toricity calculated using data from the three devices (P=0.004).

Conclusions

Differences in toric IOL power and toricity calculated using anterior keratometry data from iTrace, IOLMaster 500, and Pentacam should be noted in clinical practice.

Agreement between Pentacam and handheld Auto-Refractor/Keratometer for keratometry measurement

OBJECTIVE

This study was conducted to evaluate the level of agreement in keratometry measurements between a rotating Scheimpflug imaging-based system (Pentacam) and a handheld auto-refractokeratometer (handheld NIDEK ARK-30).

METHOD

This analytical cross-sectional study was conducted in the right eyes of 579 subjects. Keratometry measurements were conducted with the Pentacam and the handheld NIDEK ARK-30 systems. The SPSS Software version 22 and MedCalc V3 were applied to estimate descriptive statistics using paired t-test, Pearson correlation coefficient, 95% limits of agreement (LoA), and Bland-Altman plot.

RESULTS

In the total sample, the inter-device difference in the mean flat and steep keratometry values was -0.266 diopter (D) (P-value<0.001) and 0.052D (P-value=0.093), respectively. There was a significant difference in mean flat keratometry between the two devices in all groups of refractive errors (paired difference <0.5D and P-value<0.001). The difference in mean steep keratometry was significant only in myopic subjects (P-value=0.046). The 95% LoA between the two devices measurements was 2.51D, 3.98D, and 6.37D for flat keratometry and 2.6D, 3.2D, and 3.9D for steep keratometry in emmetropic, myopic, and hyperopic subjects, respectively.

CONCLUSION

Our study showed relatively wide limits of agreement between handheld NIDEK ARK-30 and Pentacam; therefore, these devices cannot be used interchangeably for measuring corneal curvature.

Evaluation of the anterior chamber angle in keratoconus and normal subjects

PURPOSE

To evaluate the anterior chamber angle in keratoconus eyes by use of the Visante™ OCT and Orbscan™ II.

METHODS

Anterior chamber angle was measured with the Visante™ OCT and Orbscan™ II in 52 subjects, 26 KC subjects and 26 age and control subjects.

RESULTS

When comparing the nasal and temporal angles obtained with the two techniques no correlation was found (R(2) always below 0.01) in either the control subjects or in the KC subjects. Despite this, there was an overall statistically significant difference in mean anterior chamber angles (p<0.001) between Visante™ OCT and Orbscan™ II. There was no statistical difference (p>0.05) between nasal and temporal anterior chamber angles when comparing controls and KC subjects with either of the two instruments. In general, the Visante™ OCT gave a smaller estimate of the anterior chamber angle.

CONCLUSION

The values from the Visante™ OCT and Orbscan™ II cannot be interchanged since the difference in measurement of the anterior chamber angle was significantly different between the two instruments.

Evaluation of pachymetric measurements with scheimpflug photography-based system and optical coherence tomography pachymetry at different stages of keratoconus

The aim of this study was to compare the central and peripheral pachymetric measurements determined with Sirius system and Visante OCT and evaluate the agreement between them at different stages of keratoconus. Measurements were not significantly different in all patients and subgroups and showed high correlation for the corneal thicknesses of the entire cornea in different stages of keratoconus.

Central and midperipheral corneal thickness measured with Scheimpflug imaging and optical coherence tomography

Purpose

To compare corneal thickness measurements using Pentacam (Oculus, Germany), Sirius (CSO, Italy), Galilei (Ziemer, Switzerland), and RTVue-100 OCT (Optovue Inc., USA).

Methods

Sixty-six eyes of 66 healthy volunteers were enrolled. Three consecutive measurements were performed with each device. The mean value of the three measurements was used for subsequent analysis. Central corneal thickness (CCT), thinnest corneal thickness (TCT), and midperipheral corneal thickness (MPCT; measured at superior, inferior, nasal, and temporal locations with a distance of 1 mm (CT_2mm) or 2.5 mm (CT_5mm) from the corneal apex) were analyzed. Differences and agreement between measurements were assessed using the repeated-measures analysis of variance (ANOVA) and Bland-Altman analyses, respectively.

Results

Statistically significant differences (p<0.001) among the four devices were revealed in CCT, TCT and CT_2mmmeasurements. The CCT, TCT, and CT_2mm values were ranked from the thickest to the thinnest as follows: Galilei>Sirius>Pentacam>RTVue OCT. For these measurements, agreement between measurements by Sirius and Pentacam was good, whereas Galilei overestimated and RTVue underestimated corneal thickness compared to Sirius and Pentacam. As regards CT_5mm measurements, Pentacam provided the largest values, whereas RTVue OCT yielded the smallest values. Agreement of the CT_5mm measurements was good between the Pentacam, Sirius, moderate between Galilei and the other two Scheimpflug systems, and poor between the RTVue OCT and the remaining devices.

Conclusions

The Pentacam and Sirius can be used interchangeably for CCT measurements, while the Galilei and RTVue systematically over- and underestimate CCT, respectively. The three Scheimpflug cameras, but not the RTVue, may be used interchangeably for MPCT measurements.

Corneal topography and corneal thickness in children

PURPOSE

To evaluate the corneal topography, thickness, and elevation (anterior and posterior corneal surface) in children.

METHODS

One hundred eyes of 100 patients with a mean ± standard deviation age of 10.6 ± 2.7 years (range: 5 to 15 years) were imaged. The corneal elevation maps were classified into regular ridge, irregular ridge, incomplete ridge, island, and unclassified patterns.

RESULTS

Mean simulated keratometry (Sim K) astigmatism was -0.69 ± 0.35 diopters (D) with mean maximum and minimum keratometry (K) of 44.26 ± 1.55 and 43.56 ± 1.57 D, respectively. Mean astigmatism and refractive power in the 3- and 5-mm zones was 0.64 ± 0.36, 43.85 ± 1.53 and 0.78 ± 0.5, 43.41 ± 1.48, respectively. The thinnest site on the cornea had an average thickness of 540 ± 34.03 μm. This site was most commonly located in the inferotemporal quadrant in 61% of eyes, followed by the superotemporal quadrant in 31%. Among the nine regions of the cornea evaluated, the central cornea had the lowest average thickness of 540 ± 34.03 μm and the superonasal cornea had the greatest average thickness of 628 ± 38.94 μm. The most common anterior corneal elevation pattern was the incomplete ridge (52%), followed by the island pattern. The incomplete ridge was most commonly observed on the posterior corneal elevation map (35%).

CONCLUSIONS

These results provide normal standards for elevation and curvature topography and corneal thickness in children. These data may prove useful as a reference for future comparative studies of different corneal diseases in children.

Univariate and bivariate polar value analysis of corneal astigmatism measurements obtained with 6 instruments

PURPOSE

To compare the corneal astigmatism measurements from 6 instruments in preoperative assessment for toric intraocular lens (IOL) implantation.

SETTING

Institute of Vision Research, Department of Ophthalmology, Yonsei University College of Medicine, Seoul, South Korea.

DESIGN

Prospective comparative observational study.

METHODS

This study included patients with cataract and more than 1.00 diopter (D) of corneal astigmatism. For preoperative evaluation of toric IOL implantation, the net astigmatism was evaluated using manual keratometry, autokeratometry, partial coherence interferometry (PCI) (IOLMaster), corneal topography/ray-tracing aberrometry (iTrace), scanning-slit topography (Orbscan), and Scheimpflug imaging (Pentacam). All net astigmatisms were converted to polar values. Using the astigmatism measurements from manual keratometry as a standard, Bland-Altman analysis, linear mixed-model, and bivariate graphic analysis were performed.

RESULTS

The study group comprised 257 eyes of 141 patients. Bland-Altman plots showed good agreement between manual keratometry and each instrument for polar values. There was no significant between-instrument difference in KP(90) and KP(135) in the linear mixed model analysis or in bivariate polar values in bivariate confidence ellipses.

CONCLUSION

The corneal astigmatism measurements from autokeratometry, PCI, corneal topography/ray-tracing aberrometry, scanning-slit topography, and Scheimpflug imaging were comparable to those from manual keratometry and can be used interchangeably with manual keratometry to measure corneal astigmatism.

Corneal astigmatism in unilateral Fuchs heterochromic iridocyclitis

PURPOSE

To analyze the results of scanning slit topography in both eyes of patients with unilateral Fuchs heterochromic iridocyclitis (FHI).

METHODS

A noninterventional, case-controlled study was conducted with 30 patients with unilateral FHI. Corneal topographic parameters, obtained with the Orbscan II (Bausch & Lomb), were compared between the two eyes and analyzed statistically. The same number of age-matched normals was enrolled as a control group.

RESULTS

Thirty patients were enrolled in this study. The mean age was 33.3 years ± 10.6 (SD). The mean Sim K astigmatism was 1.65 ± 1.27 diopter in the FHI eyes and 0.88 ± 0.52 diopter in the normal eyes (p = .001).

CONCLUSION

Corneal astigmatism is a common finding in patients with FHI and disparity of corneal astigmatism between the two eyes can be considered a sign of the unilateral form of this disease.

Evaluation of central corneal thickness measurement with RTVue spectral domain optical coherence tomography in normal subjects

PURPOSE

To determine (1) repeatability of central corneal thickness (CCT) measurements by spectral domain optical coherence tomography (RTVue; Optovue, Inc, Fremont, CA) and (2) agreement between CCT measurements by RTVue and those by ultrasonic pachymetry, Orbscan, and anterior segment optical coherence tomography (ASOCT).

METHODS

In a prospective observational study, 2 cohorts of normal subjects were recruited. In the first cohort (51 subjects), 5 measurements of CCT were obtained by RTVue during the same visit to determine the repeatability. In the second cohort (65 subjects), CCT measurements were obtained by RTVue, ultrasonic pachymetry, Orbscan, and ASOCT during the same visit to determine the agreement among these instruments. Repeatability was assessed by intraclass correlation coefficient (ICC), within-subject standard deviation, coefficient of repeatability, and within-subject coefficient of variation. Agreement was assessed by ICC and Bland and Altman plots.

RESULTS

Repeatability of CCT measurements by RTVue as assessed by ICC, within-subject standard deviation, coefficient of repeatability, and within-subject coefficient of variation was 0.99 (0.99-0.99), 2.2 (1.9-2.5), 4.2 μm (3.6-4.8), and 0.4% (0.3-0.5), respectively. The average CCT by RTVue (529 μm) was comparable to that by ultrasonic pachymetry (539 μm; P = 0.15), Orbscan (536 μm; P = 0.54), and ASOCT (526 μm; P = 0.77). The 95% limits of agreement on Bland and Altman plots ranged from 20 μm (between RTVue and ASOCT) to 33 μm (between RTVue and Orbscan).

CONCLUSIONS

CCT measurements by RTVue have an excellent repeatability. Although CCT measurements by RTVue are comparable to that by ultrasonic pachymetry, Orbscan, and ASOCT, the difference between instruments can be significant depending on the clinical situation considered.

Investigation of corneal topography after deep lamellar endothelial keratoplasty

PURPOSE

To analyze the 6-month anterior and posterior topographic changes of the cornea after deep lamellar endothelial keratoplasty.

METHODS

Orbscan topographies of 22 eyes from 21 patients with corneal decompensation were retrospectively analyzed: 11 eyes received a 9-mm scleral access incision (large-incision group) and 11 eyes received a 5-mm scleral access incision (small-incision group). All the preoperative and postoperative corneal raw data were imported into custom software which computed the average composite corneal maps and difference maps for both study groups in order to evaluate the corneal response to the surgery. The software delineated 2 concentric zones of the cornea to characterize the regional response following the surgery: the central and peripheral regions.

RESULTS

There were no significant differences (analysis of variance, <0.35 D, p>0.05) between 6-month postoperative and preoperative average anterior central corneal topographies in either group. At the end of follow-up, the average posterior curvature tangential map did not significantly differ from before surgery in either group (<0.35 D, p>0.05). However, a significant increase (>0.60 D, p<0.01) in the average posterior central astigmatic power of the cornea was found in both groups.

CONCLUSIONS

Deep lamellar endothelial keratoplasty effectively preserves the preoperative keratometric corneal topography, minimizing changes in curvature and astigmatism of the cornea. The great predictability of corneal topography following deep lamellar endothelial keratoplasty is likely to be attributed to the minimal changes that occur in the anterior stroma, the portion of the cornea that appears to be mainly responsible for maintenance of corneal shape.

Multizone model for postsurgical corneas: analysis of standard and custom LASIK outcomes

A multizone model for postsurgical corneal topography is presented and applied to a comparative analysis of the outcome of standard and customized myopic LASIK. The different zones are segmented automatically by a clustering algorithm. The algorithm uses a set of three local descriptors, which correspond to normalized physical magnitudes computed for each point of the corneal topography map: Gauss curvature, root-mean-square (RMS) fit error to an ellipsoid surface model, and distance to the center of the topographic map. Both presurgical and post-LASIK corneal topographies of 31 eyes were analyzed using monozone and multizone models. The patients were classified into three groups according to the different LASIK treatments applied: Allegretto, Zyoptix, and PlanoScan. For post-LASIK corneas, the multizone model provided a lower fit error, an average of 1.2+/-0.4 microm versus 2.4+/-0.7 microm (monozone). The comparative analysis of the three different LASIK treatments showed no improvement of custom over standard treatments. The outcomes of Zyoptix and PlanoScan were basically equivalent and consistent with previous findings: The higher-order aberration (HOA) increased by a factor of two. The increase in HOA was higher, by a factor of three, after the Allegretto treatment. The multizone model shows a higher-fidelity representation and permits a deeper understanding of the postsurgical cornea.

Central corneal thickness measurement with Pentacam, Orbscan II, and ultrasound devices before and after laser refractive surgery for myopia

PURPOSE

To determine the agreement in central corneal thickness (CCT) measurements between the gold standard method of ultrasound (US) pachymetry (UP-1000, Nidek) and 2 noncontact systems based on Scheimpflug imaging (Pentacam, Oculus) and scanning-slit topography (Orbscan II, Bausch & Lomb) in myopic eyes before and after laser refractive surgery.

SETTING

Noor Vision Correction Center, Tehran, Iran.

METHODS

In this prospective study, 30 consecutive patients having refractive surgery for myopia were enrolled. All 60 eyes were examined with the 3 devices preoperatively and 6 weeks after surgery; the US measurements were performed last. The Pentacam and Orbscan II CCT readings were compared with the US readings. Both the original and corrected Orbscan II readings were used in the analyses.

RESULTS

The mean CCT readings with US, Pentacam, and Orbscan II were, respectively, 555 microm, 548 microm, and 580 microm before surgery and 478 microm, 468 microm, and 474 microm after surgery. Preoperatively, the 95% limits of agreement (LoA) with US were -31 microm and +19 microm for the Pentacam device and -5 microm and +57 microm for the Orbscan II device. Postoperatively, the LoA were -40 microm and +19 microm and -51 microm and +50 microm, respectively. Corrected Orbscan II measurements gave 95% LoA of -48 microm and +6 microm before surgery and -85 microm and +5 microm after surgery.

CONCLUSIONS

Refractive surgery had a modest effect on the agreement between Pentacam readings and US measurements. With Orbscan II, the 95% LoA width nearly doubled after surgery. Although the Pentacam seems to show better agreement than Orbscan II, especially after refractive surgery, it is not advisable to use the 3 devices interchangeably in every clinical situation.

Factors influencing central corneal thickness in a Turkish population

PURPOSE

The aim of this study is to find out the central corneal thickness (CCT) values for a Turkish patient group and to investigate the possible influences of age, sex, IOP, refractive status, keratometry readings, systemic disorders (hypertension, diabetes mellitus, hyperlipidemia, heart disease and asthma) on CCT values.

MATERIALS AND METHODS

Six hundred twenty five subjects (276 male (44%), 349 female (56%)) of ages 6 to 88 years were recruited. Subjects who had corneal diseases, purulent conjunctivitis or blepharitis were excluded. Refraction and keratometry readings were made by MRK-3100 premium auto-ref/keratometer, IOP was measured by Reichert AT-555 auto noncontact pneumotonometer.

RESULTS

Mean age was 44.1 +/- 16.6 years +/- SD for male subjects, 41.0 +/- 16.9 for females. Mean CCT +/- SD values for male was 552.2 +/- 35.9 microm, for female was 552.3 +/- 35.4 microm, respectively. There was no significant difference between right and left eye CCT values for both genders. Age and CCT was not correlated for the whole study group but there was a slight negative correlation in male subjects. IOP and CCT had moderately significant correlation for males and females. There was a slight significant correlation between Kh-Kv and CCT values for the whole group. In a multivariate regression model only Kv values seemed to affect CCT values.

DISCUSSION

There are studies showing the variation of CCT values among different nations and ethnicities. There is no agreement about the relationship between age, IOP, Kh-Kv, spherical equivalence of refractive error, systemic disorders, menopause and CCT. In our study CCT was correlated with Kh-Kv and IOP in correlation analysis but in multivariate regression analysis only Kv appeared to influence corneal thickness.

Central and peripheral pachymetry measurements according to age using the Pentacam rotating Scheimpflug camera

PURPOSE

To investigate the mean values and standard deviations as well as the reliability of consecutive examinations of central and peripheral corneal thickness measurements according to age using the Pentacam rotating Scheimpflug camera (Oculus, Inc.).

SETTING

Department of Ophthalmology, University of Heidelberg, Heidelberg, Germany.

METHODS

Seventy-six healthy volunteers were enrolled in a clinical prospective study. Three consecutive Pentacam measurements of 1 eye per subject were taken. Evaluated were the central corneal thickness (CCT), corneal thickness at 4 peripheral points (3.0 mm superior, inferior, nasal, and temporal), and the thinnest point of the cornea. The volunteers were then assigned to 3 groups to assess the influence of increasing age on the study parameters.

RESULTS

The mean age of the subjects was 46.6 years +/- 16.8 (SD). The mean CCT was 539.62 +/- 31.87 microm. Peripherally, the corneal thickness was between 11% and 19% higher than centrally, with the superior cornea being the thickest followed by the nasal, the inferior, and the temporal cornea. The thinnest point was located in the inferotemporal quadrant in 92% of eyes and in the superotemporal quadrant in 8%. There was no correlation between age and corneal thickness. Minor mean standard deviations of consecutive measurements were noted in the corneal center (4.33 microm), increasing significantly toward the periphery (mean 8.31 microm). Increasing age was not associated with decreasing reliability.

CONCLUSIONS

Using the Pentacam, it was possible to acquire information about corneal thickness across the entire cornea. No correlation was found between increasing age and change in peripheral or CCT. Good reliability was noted for pachymetry measurements decreasing slightly toward the periphery, which was independent of age.

Validity of the keratometric index: Large population-based study

PURPOSE

To determine the accuracy of the keratometric index of 1.3315 based on the Gullstrand model eye in predicting the power of the posterior cornea, Gullstrand's model was compared to a calculated keratometric index derived from actual measurements of the cornea.

SETTING

Eye Institute, Tan Tock Seng Hospital, Singapore.

METHODS

One eye of 2429 subjects with a mean spherical equivalent of -5.32 diopters (D) +/- 2.88 (SD) was measured with the Orbscan II (Bausch & Lomb). The following variables were analyzed: anterior radius of curvature (r(anterior)), posterior radius of curvature (r(posterior)), radius of keratometry (r(simK)), and central pachymetry.

RESULTS

The r(anterior), r(posterior), and r(simK) were normally distributed, with a mean of 7.87 +/- 0.25 mm (95% confidence interval [CI], 7.38-8.36), 6.46 +/- 0.26 mm (95% CI, 5.95-6.97), and 7.71 +/- 0.27 mm (95% CI, 7.18-8.24), respectively. The mean ratio between the anterior corneal curvature and posterior corneal curvature was 1.22 +/- 0.03 (95% CI, 1.16-1.28). Based on the measurements of each eye, the mean calculated keratometric index, N(calc), was 1.3273 +/- 0.0013 (95% CI, 1.3248-1.3298). Using N(calc), the posterior corneal power was predicted to within +/-0.50 D of the actual posterior power in 98.3% of eyes. The mean absolute error between the actual and calculated posterior power was 0.157 +/- 0.123 D using N(calc) and 0.326 +/- 0.133 D using the Gullstrand model.

CONCLUSION

Modifying the keratometric index increased the accuracy of predicting the posterior corneal power.

Regional Repeatability Measures of Corneal Thickness: Orbscan II and Ultrasound

PURPOSE

To compare repeatability of the measures of corneal thickness obtained by slit-scanning light method (Orbscan II) with those obtained by an ultrasound pachymeter, with special interest in the peripheral region of the cornea.

METHODS

On 24 normal adults, aged 20 to 58 years (average 36 years) with up to -8.5 DS refractive error, three measures of corneal thickness were taken using Orbscan II and then by ultrasound pachymetry (under topical anesthesia with benoxinate 0.4%). The Orbscan central sample zone of 1 mm was selected, or the numerical maps were used to extract single point data along the horizontal corneal meridian to the nasal and temporal sides out to 4.5 mm. Ultrasound readings were taken from the central cornea and at the periphery just inside the limbus (4.5 mm from center) with a 2.4-mm diameter probe.

RESULTS

For a central 1-mm diameter zone, the coefficient of variation (CV) for three consecutive corneal thickness measures was 0.81%+/-0.44%, but was marginally higher (p=0.004), if just the central single point data was taken with Orbscan (0.86%+/-0.45%). Similar repeatability was noted for the numerical output across the temporal side along the horizontal meridian out to 2.5 mm from the center, but farther out to 4 mm and on the nasal side the repeatability was slightly less and around 1.0% (p<0.001). Orbscan point readings of thickness could only sometimes be obtained at 4.5 mm temporally (with a poorer CV of 1.32%) and very rarely at 4.5 mm on the nasal side. No absolute differences in Orbscan repeatability were noted when comparing emmetropic with myopic subjects (p>or=0.5). Ultrasound pachymetry readings across the central zone were repeatable to 0.82%+/-0.67%. When measured with the edge of the ultrasound probe just touching the limbus, the repeatability of ultrasound readings was 1.37%+/-1.10% temporally and 1.49%+/-1.02% nasally, but neither was statistically worse that the most peripheral readings for Orbscan (p>or=0.210). However, it was also noted that the absolute values of corneal thickness, even with the application of the 0.92 acoustic factor, were 0.010 mm greater for Orbscan at the center and 0.040 mm in the corneal periphery.

CONCLUSIONS

Orbscan II provides the clinician with a repeatable noninvasive method of measuring corneal thickness that is not necessarily any better than ultrasound pachymetry, and should not be considered as interchangeable with that for ultrasound.

Corneal Radii and Anterior Chamber Depth Measurements Using the IOLMaster Versus the Pentacam

PURPOSE

Corneal radii (R1, R2) and anterior chamber depth are important parameters for biometry and refractive surgery. This study aimed to investigate whether the IOLMaster, a biometric device, and the Pentacam, a rotating Scheimpflug camera, provide comparable results.

METHODS

In this prospective study, corneal radii and anterior chamber depth were analyzed in 82 eyes of 41 phakic patients (median age 72 years) using the IOLMaster and Pentacam. Normal distribution was confirmed using the Kolmogorov-Smirnov test. A paired t test was used for statistical analysis.

RESULTS

No statistically significant difference was noted for R2 using the Pentacam versus the IOLMaster (P = .40). There was a small statistical difference of 0.03 mm (P < .01) for R1 (corresponding to 0.08 diopters [D]). The IOLMaster measured a mean R1 of 7.87 mm and R2 of 7.67 mm. The Pentacam measured mean R1 of 7.90 mm and R2 of 7.69 mm. For anterior chamber depth values, a small statistically significant difference of 0.05 mm (P < .001) was found (corresponding to 0.01 D). Mean anterior chamber depth measured from the epithelium was 3.20 mm for the IOLMaster and 3.25 mm for the Pentacam. The Bland-Altman plot showed no distorting trends for either variable.

CONCLUSIONS

Keratometry and anterior chamber depth were comparable for the two devices. If the axial length is known, the Pentacam can be used as a biometric device.

Reliability of Corneal and Total Wavefront Aberration Measurements With the SCHWIND Corneal and Ocular Wavefront Analyzers

PURPOSE

To evaluate the reliability of the SCHWIND Corneal and Ocular Wavefront Analyzers.

METHODS

This study comprised 115 eyes of 58 healthy volunteers (26 [44.8%] women and 32 [55.2%] men) with no corneal or lenticular pathologies and normal visual acuity. All eyes underwent three consecutive measurements by one examiner with the Corneal Wavefront Analyzer and the Ocular Wavefront Analyzer (Hartmann-Shack principle). Corneal wavefront errors were calculated using the topography data, a standard eye model, and ray tracing. The reliability was tested for 2nd, 3rd, and 4th order aberrations as mean values of the standard deviations for all measurements.

RESULTS

Mean patient age was 23 +/- 2.1 years. The mean refraction was -0.77 +/- 1.56 diopters (D) (range: +3.33 to -5.28 D). The repeatability test revealed a good reliability for both machines with a slightly better value for the Ocular Wavefront Analyzer for 3rd and 4th order higher order aberrations (P < .05, Wilcoxon test).

CONCLUSIONS

Wavefront measurements of corneal and total optical aberrations performed with the Corneal Wavefront Analyzer and the Ocular Wavefront Analyzer have good reliability. Both measurements are recommended prior to any refractive surgery treatment.

Anterior chamber measurements using Pentacam rotating Scheimpflug camera

PURPOSE

To investigate the mean values and standard deviations according to age, reliability, and correlation between different parameters of anterior chamber measurements using the Pentacam rotating Scheimpflug camera.

SETTING

Heidelberg IOL and Refractive Surgery Research Group, Department of Ophthalmology, University of Heidelberg, Heidelberg, Germany.

METHODS

This prospective clinical study comprised 76 healthy volunteers (mean age 46.6 years +/- 16.8 [SD]). Three consecutive measurements were made of only 1 eye and anterior chamber depth (ACD), mean and minimum anterior chamber angle (ACA), and anterior chamber volume (ACV) were evaluated.

RESULTS

Mean ACD was 2.93 +/- 0.36 mm, mean ACA was 34.81 +/- 5.05 degrees, minimum ACA was 29.99 +/- 5.53 degrees, and mean ACV was 160.3 +/- 36.81 mm3. Increasing age was associated with reduced ACD and ACV; however, mean and minimum ACAs were lowest in patients aged 40 to 59 years. Excellent correlation was found between ACD and ACV (R = 0.92). Anterior chamber depth and mean ACA correlated only moderately (R = 0.65). The correlation coefficient between ACD and minimum ACA was smaller (R = 0.58). There was no correlation between ACV and ACA (R = 0.37). Minor standard deviations were noted (ACD 0.02 +/- 0.02 mm, mean ACA 1.12 +/- 0.94 degrees, minimum ACA 2.04 +/- 2.67 degrees, and ACV 2.48 +/- 1.65 mm3).

CONCLUSIONS

Using the Pentacam, it was possible to examine different parameters of the anterior chamber within a short period and with good reliability. The evaluation of the ACA in different positions can help to classify the potential risk for angle-closure glaucoma.

Precision of NIDEK OPD-Scan Measurements

PURPOSE

To evaluate the repeatability of wavefront measurements using the NIDEK OPD-Scan.

METHODS

A total of 179 eyes from 90 healthy volunteers (57 women and 33 men) with no corneal or lenticular pathology and normal visual acuity were enrolled in this study. Mean patient age was 39 years (range: 17 to 85 years). All patients underwent four consecutive measurements by one examiner with the NIDEK OPD-Scan. Total, corneal, and internal wavefront errors were measured and calculated with the device, using slit retinoscopy. Repeatability of the measurements was evaluated for spherical aberration, coma, and trefoil.

RESULTS

The repeatability test revealed a good result for all three higher order aberrations evaluated. The best repeatability values were found for total aberrations, followed by internal and corneal aberrations.

CONCLUSIONS

The NIDEK OPD-Scan has good precision in the wavefront measurement of total, corneal, and internal optical aberrations.