Comparison of Four Corneal Pachymetry Techniques in Corneal Refractive Surgery

Bibliometric and visualized analysis of myopic corneal refractive surgery research: from 1979 to 2022

Background

Myopic corneal refractive surgery is one of the most prevalent ophthalmic procedures for correcting ametropia. This study aimed to perform a bibliometric analysis of research in the field of corneal refractive surgery over the past 40 years in order to describe the current international status and to identify most influential factors, while highlighting research hotspots.

Methods

A bibliometric analysis based on the Web of Science Core Collection (WoSCC) was used to analyze the publication trends in research related to myopic corneal refractive surgery. VOSviewer v.1.6.10 was used to construct the knowledge map in order to visualize the publications, distribution of countries, international collaborations, author productivity, source journals, cited references, keywords, and research hotspots in this field.

Results

A total of 4,680 publications on myopic corneal refractive surgery published between 1979 and 2022 were retrieved. The United States has published the most papers, with Emory University contributing to the most citations. The Journal of Cataract and Refractive Surgery published the greatest number of articles, and the top 10 cited references mainly focused on outcomes and wound healing in refractive surgery. Previous research emphasized "radial keratotomy (RK)" and excimer laser-associated operation methods. The keywords containing femtosecond (FS) laser associated with "small incision lenticule extraction (SMILE)" and its "safety" had higher burst strength, indicating a shift of operation methods and coinciding with the global trends in refractive surgery. The document citation network was clustered into five groups: (1) outcomes of refractive surgery: (2) preoperative examinations for refractive surgery were as follows: (3) complications of myopic corneal refractive surgery; (4) corneal wound healing and cytobiology research related to photorefractive laser keratotomy; and (5) biomechanics of myopic corneal refractive surgery.

Conclusion

The bibliometric analysis in this study may provide scholars with valuable to information and help them better understand the global trends in myopic corneal refractive surgery research frontiers. Two stages of rapid development occurred around 1991 and 2013, shortly after the innovation of PRK and SMILE surgical techniques. The most cited articles mainly focused on corneal wound healing, clinical outcomes, ocular aberration, corneal ectasia, and corneal topography, representing the safety of the new techniques.

Comparison of central corneal thickness measurements using three different imaging devices

PURPOSE

The purpose of this study was to compare central corneal thickness (CCT) values and evaluate the agreement obtained with three different devices in healthy eyes.

METHODS

A total of 120 eyes of 60 healthy individuals (36 men and 24 women) were enrolled in this retrospective study. CCT measurements were performed using an optical biometer (AL-Scan), spectral-domain optical coherence tomography (SD-OCT) (Topcon 3D) and ultrasonic pachymetry (UP) (Accupach VI), and the results were compared. Bland-Altman analysis was used to quantify the agreement between methods.

MAIN RESULTS

The mean patient age was 28±5.73years (18-40years). The mean CCT values obtained by AL-Scan, UP, and SD-OCT were 532.4μm±29.7, 549μm±30.4, and 547μm±30.6, respectively. The mean differences in CCT were 15.30±9.52μm between AL-Scan and OCT (P<0.01), 17.15±8.42μm between AL-Scan and UP (P<0.01), and 1.85± 8.78μm between UP and OCT (P=0.067). All three methods of CCT measurement were closely correlated with each other.

CONCLUSION

The present study results suggest that, despite good agreement between the three devices, AL-Scan significantly underestimated CCT compared to UP and OCT. Therefore, clinicians should be aware that different results can be obtained using different devices for CCT measurements. It would be a better approach not to use them as interchangeable in clinical practice. CCT examination and follow-up should be performed using the same device, especially for patients who will undergo refractive surgery.

Peripheral corneal thickness and associated factors - results from the population-based German Gutenberg Health Study

PURPOSE

Changes in peripheral corneal thickness are described in various corneal diseases such as corneal ectasia. However, few data exist describing the increase in corneal thickness from central to peripheral and reporting the normal distribution of corneal thickness in rings around the corneal centre. The aim of this study was to report these cornea characteristics and investigate associated factors in a population-based setting.

METHODS

The Gutenberg Health Study is a prospective, population-based study examining participants in a 5-year follow-up (age range 40-80 years) using Scheimpflug imaging. Corneal thickness was assessed in each participant at the apex, as well as in the corneal centre (thinnest corneal thickness) and in rings with 2, 4, 6, 8 and 10 mm diameter around the corneal centre, and the increase in corneal thickness towards the periphery. The relationship between corneal thickness at these locations and possible associated factors was determined using linear regression models. For this purpose, general and ocular parameters were included.

RESULTS

A total of 9729 participants were included in the present analysis (4874 women, age 59.2 ± 10.8 years). Multivariable analysis showed a correlation between the increase in corneal thickness in the circles from 0 to 10 mm (diameter) and the following parameters: age (B = -0.24 µm per year, p < 0.001); body height (B = -0.04 µm, p = 0.005); smoking (B = -0.72 µm, p < 0.001); spherical equivalent (B = -0.70 µm per dioptre, p < 0.001); white-to-white distance (B = -0.75 µm/mm, p < 0.001); mean corneal radius (B = -3.61 µm/mm, p < 0.001); intraocular pressure (B = -0.12 µm/mmHg, p < 0.001); glaucoma (B = -1.94 µm, p < 0.001); and pseudophakia (B = 0.89 µm, p < 0.001).

CONCLUSION

The results of the present study suggest that several general and ocular parameters are associated with peripheral corneal thickness. In the context of diagnosing glaucoma, a smaller increase in corneal thickness towards the periphery might be a new additional marker.

Comparability of corneal thickness and opacity depth assessed by OCT and UBM

OBJECTIVE

To compare the measurement of central thickness and depth of involvement of opacity-bearing corneas at different intensities (mild, moderate, and dense) using different instruments: Visante and Optovue OCTs, ultrasound biomicroscopy (UBM), and ultrasound pachymetry (central thickness).

METHODS

Sample of 102 eyes: 70 eyes (68.63 %) with corneal opacity; 32 eyes (31.37%) with normal corneas. Corneal opacity grading included mild (28 eyes, 40.00 %), moderate (27 eyes, 37.57 %), and dense (15 eyes, 21.43 %). Opacity intensity was graded and documented. Central corneal thickness was determined using Optovue and Visante OCTs, ultrasound pachymetry, and UBM VuMax (50 MHz), and depth of corneal opacity, using Optovue and Visante OCTs and UBM.

RESULTS

Total corneal thickness of the control group showed differences with a small correction factor using Optovue OCT (534.03 ± 39.88 μm), Visante OCT (523.72±38.70 μm), and ultrasound pachymetry (529.84 ± 39.76 μm), and were higher when compared to UBM (492.06 ± 37.93 μm). In mild opacity, depth measurements were the same with OCT Optovue and Visante and higher than those by UBM. In moderate opacities, there were no differences in measurements by different instruments. In dense opacities, OCT Optovue and Visante measurements were the same and higher than those by UBM.

CONCLUSIONS

Central corneal thickness measurements were considered higher in corneas with opacity than in normal corneas. We observed that the greatest thicknesses were measured in corneas with dense opacities. There was a difference between the measurements taken by different instruments, both in central thickness and in opacity depth in cases of mild and dense opacity, and no difference in moderate opacity.

Comparison of Central Corneal Thickness Measurements by Contact and Non-contact Pachymetry Devices

Aim and objective

To compare central corneal thickness (CCT) measurements obtained by handheld contact ultrasound pachymetry (HCUP) and non-contact pachymetry devices.

Materials and methods

Ninety eyes of 90 patients (52 male and 38 female) were included in the study. Measurements from two non-contact devices, -specular microscopy (SM, Konan Medical, CA, USA) and Oculus Pentacam (Oculus Inc., Germany)-were compared against HCUP (Pachmate 2, DGH Technology, Inc, PA, USA). Ultrasound measurements were obtained 3 times by the same user and averaged. The differences were calculated by one-way ANOVA. Agreement between measurements were assessed by Bland-Altman plots and intraclass correlation coefficient tests. Coefficient of repeatability (%, CR) was defined as 1.96*standard deviations of the differences between pairs of measurements divided by the average of the means.

Results

The mean age was 34.31 ± 14.39 (14-74) years, and the mean intraocular pressure was 16.48 ± 2.63 mm Hg (12-21). Mean CCT measured by HCUP, SM, and Pentacam was 557.76 ± 36.76 μm, 550.29 ± 43.74 μm, and 541.41 ± 35.7 μm, respectively (p < 0.05). In the Bland-Altman plot, 95% limit of agreements were 19.5 and 14.18 μm among HCUP measurements, 34.55 μm between HCUP and Pentacam, 41.49 μm between SM and Pentacam, and 46.98 μm between HCUP and SM. CR values (%) were 3.49, 2.54, 6.28, 7.68, and 8.47, respectively.

Conclusion

There were significant differences between the mean CCT values of the measurement devices.

Clinical significance

Contact and non-contact devices may not interchangeable in the clinical assessment of CCT.

How to cite this article

Mayali H, Altinisik M, Diri I, et al. Comparison of Central Corneal Thickness Measurements by Contact and Non-contact Pachymetry Devices. J Curr Glaucoma Pract 2021;15(1):28-31.

Factors affecting central corneal thickness measurement agreement between Scheimpflug imaging and ultrasound pachymetry in keratoconus

PURPOSE

To evaluate the agreement of central corneal thickness (CCT) measurement between Scheimpflug imaging and ultrasound (U/S) pachymetry in keratoconic eyes, and investigate factors that affect the agreement.

METHODS

This post hoc analysis within a prospective, observational non-randomised study preformed at the Kensington Eye Institute, Toronto, Ontario, Canada, included crosslinking candidates with progressive keratoconus (KC). Main outcome measures were the agreement of CCT measurement between Scheimpflug imaging (Pentacam Oculus, Wetzlar, Germany) and U/S pachymetry (PachPen 24-5100, Accutome Inc., Malvern, Pennsylvania, USA), and factors that affect the agreement.

RESULTS

A total of 794 keratoconic eyes of 456 subjects with a mean age of 27.6±8.0 years (69.7% males and 49.6% right eyes) were included. Agreement between devices was moderate (intraclass correlation coefficient: 74.9%, Bland-Altman limits of agreement: -48.5 μm to +62.5 μm). In a multivariable analysis, cone decentration (p<0.001, coefficient +10.13 [+6.73 to +13.53 95% CIs]) and Kmax (p<0.001, coefficient +0.68 [+0.46 to +0.90 95% CIs]) were significantly associated (both clinically and statistically) with the level of agreement between the devices; the discrepancy in CCT between the devices increased on average by 10.13 μm for every mm of cone decentration, and by 6.8 μm for every 10D of Kmax. Age, corneal astigmatism and spherical equivalent were statistically but not clinically significant factors affecting agreement.

CONCLUSION

The agreement of CCT measurement between Scheimpflug imaging and U/S pachymetry in KC was moderate. To ensure the safety of crosslinking in keratoconic corneas, Scheimpflug and U/S CCT measurements should not be used interchangeably, especially in steep corneas and corneas with decentred cones.

Accuracy of Cirrus HD-OCT and Topcon SP-3000P for measuring central corneal thickness

PURPOSE

To compare and analyze the interchangeability of three measuring systems, each based on a different technique, for central corneal thickness (CCT) analysis.

METHODS

CCT measurements were measured using optical coherence tomography (OCT), non-contact specular microscopy (NCSM), and ultrasonic pachymetry (USP) in 60 eyes of 60 healthy patients with a mean age of 66.5±15.0 years and a mean spherical equivalent of 0.43±1.14 D. Analysis of variations in measurement concordance and correlation among the three different methods were performed. Comparison of CCT measurements were done using Bland-Altman plots (with bias and 95% confidence intervals), intraclass correlation coefficient (ICC), and paired t-student analysis.

RESULTS

Mean CCT values were: 549.20±26.91μm for USP (range 503-618μm), 514.20±27.49μm for NCSM (range 456-586μm) and 542.80±25.56μm for OCT (range 486-605μm). CCT values obtained with NCMS were significantly lower than those obtained with OCT and USP methods. NCMS CCT value was 36.08±10.72μm lower than USP value (p<0.05), and NCMS CCT value was 7.88±8.86μm lower than OCT value (p<0.05). ICC between USP-NCSM pair was 0.488 and 0.909 between USP-OCT pair.

CONCLUSION

OCT and UPS offered highly comparable results, whereas NCSM offered lower mean CCT values compared to the other two methods. Therefore, NCSM should not be considered a reliable method for measuring CCT and should rather be considered for assessing longitudinal changes in the same patient.

Distribution of keratometry and its determinants in a general population of 6- to 12-year-old children

PURPOSE:

To determine the distribution of keratometry and its determinants in Iranian school children.

METHODS:

The present cross-sectional study was conducted in 2015 in Shahroud in the north of Iran. The entire rural population of elementary school children was invited to the study. In urban areas, cluster sampling was conducted. Pentacam HR (Oculus Inc., Lynnwood, WA) was used to measure the flat meridian, the steep meridian, and the mean keratometry. Linear regression was used to determine the associated variables with mean keratometry.

RESULTS:

Of 5620 participated in the study, 5559 children were analyzed after applying the exclusion criteria. Mean keratometry was 43.56 ± 1.96 diopters (D) (95% confidence interval = 43.48-43.64) in the total sample, 43.18 ± 2.23 D (95% confidence interval = 43.09-43.26) in boys, and 44.01 ± 1.46 D (95% confidence interval = 43.95-44.07) in girls (p < 0.001). The highest and lowest mean keratometry was 43.28 ± 1.66 D (95% confidence interval = 43.00-43.55) and 42.89 ±2.70 D (95% confidence interval = 42.68-43.11) in 6-year-old and 10-year-old children, respectively (p = 0.031). The results of multiple linear regression showed that mean keratometry in girls was 0.82 D higher than in boys (p < 0.001), and in groups older than 9 years, it was significantly decreased. Mean keratometry in myopic children was 0.62 D higher than emmetropic children (p < 0.001).

CONCLUSION:

This study provided valuable findings from the status of keratometry in Iranian children. In line with other studies, corneal power was higher in girls than in boys, and the cornea becomes flatter with age in children.

Agreement between Corneal Thickness Measurements Using Pentacam Scheimpflug Camera, Noncontact Specular Microscopy, and Ultrasonographic Pachymetry in Diabetic Patients

PURPOSE

To evaluate the agreement of corneal thickness (CT) measurements obtained by the Pentacam Scheimpflug camera, noncontact specular microscopy (SM), and ultrasonographic pachymetry (UP) in diabetic (DM) patients; and whether duration of diabetes and level of Hb A1c affect the agreement.

MATERIALS AND METHODS

The CT was measured in 127 patients with DM, and 137 age and sex-matched healthy controls sequentially by Pentacam, SM, and UP. Also diabetic subjects were subdivided according to duration of diabetes and Hb A1c levels. Pearson correlation analysis, linear regression analysis, and Bland-Altman plots were used for examination of agreement.

RESULTS

We found an excellent and statistically significant correlation of CT measurements between Pentacam-SM (R² = 0.768; R² = 0.855), Pentacam-UP (R² = 0.546; R² = 0.652), and SM-UP (R² = 0.759; R² = 0.797) in diabetic and control groups, respectively. We performed further comparisons of the pair of instruments using the Bland-Altman analysis, and the mean difference between pair of methods was much smaller for SM-UP pair (-4.20 ± 9.79 in diabetic, and -4.58 ± 8.08 in control group). The group having Hb A1c level less than 7.5% showed the best agreement between SM-UP pair as in the control group. Whereas Pentacam-UP showed the best agreement in the group having Hb A1c level ≥7.5%. The best agreement was between Pentacam-UP in 0-4 year group, and between Pentacam-SM in 5-9 year and ≥10 year groups.

CONCLUSION

SM-UP pair showed a higher agreement in diabetic patients. According to subgroup analyses, Hb A1c level, rather than the duration of diabetes, may determine the agreement of these pachymetry devices. But the difference in CT measurements between devices can still influence clinical diagnosis and treatment. Therefore, these methods are not completely interchangeable.

Influences of topical cyclopentolate on anterior chamber parameters with a dual-Scheimpflug analyzer in healthy children

PURPOSE

To evaluate the changes of anterior segment parameters following the topical instillation of cyclopentolate hydrochloride 1% with the Galilei dual-Scheimpflug analyzer (GSA) (Ziemer Ophthalmology Co., Port, Switzerland) in healthy children.

METHODS

Fifty children (29 boys, 21 girls) were evaluated before and 60 minutes after instillation of three drops of 1% cyclopentolate hydrochloride using the GSA. The measurements before and after cycloplegia, including anterior chamber depth (ACD), anterior chamber angle (ACA), anterior chamber volume (ACV), central corneal thickness (CCT), and pupil size, were evaluated using the paired t test.

RESULTS

The mean age of the patients was 10.27 ± 3.32 years (range: 5 to 15 years). Measurements between the two sessions were significantly different for all parameters (P < .05), except for CCT (P > .05).

CONCLUSIONS

The GSA demonstrated a statistically significant increase in ACD, ACV, ACA, and pupil size following the topical application of cyclopentolate hydrochloride 1%. These results should be considered during biometric measurement and refractive surgery planning.

Comparison of central corneal thickness, thinnest corneal thickness, anterior chamber depth, and simulated keratometry using galilei, Pentacam, and Sirius devices

PURPOSE

The aim was to evaluate the agreement in the central corneal thickness (CCT), thinnest corneal thickness (TCT), anterior chamber depth (ACD), and mean simulated keratometry (simK) measurements using Pentacam, Galilei, and Sirius Scheimpflug systems in normal eyes.

METHODS

Anterior segment measurements were performed with Pentacam, Galilei, and Sirius devices in 32 healthy subjects. The right eye of each participant was selected. Measurements obtained with the 3 systems were compared using repeated-measures analysis of variance and Bonferroni multiple comparisons test.

RESULTS

Analysis of variance determined a significant difference in the anterior segment measurements of CCT, TCT, ACD, and simK between the 3 devices (P < 0.001). Pairwise comparisons of CCT and TCT measurements were significantly different except for the comparison between Pentacam and Sirius. All pairwise comparisons for ACD were statistically significant. The pairwise comparison results for simK values showed that the Galilei and Sirius systems demonstrated better agreement with each other than with Pentacam.

CONCLUSIONS

The results of this study suggest that the Pentacam, Galilei, and Sirius Scheimpflug systems should not be accepted as interchangeable for CCT, TCT, ACD, and simK in healthy subjects.

Correlation of central and peripheral corneal thickness in healthy corneas

PURPOSE

To study the thickness profile of the normal cornea in order to establish any correlation between central and peripheral points.

METHODS

Sixty-seven eyes of 40 patients were subjected to central corneal thickness measurement (CCT) with an ultrasound pachymeter (UP) and corneal thickness mapping with the Oculus Pentacam. The corneal apex thickness (CAT), pupil centre thickness (recorded as CCT and corresponded to CCT of UP) and thickness at the thinnest location (CTL) were obtained and compared with each other. Corneal thickness data at 3 mm and 7 mm temporally, nasally, superiorly and inferiorly from the corneal apex were obtained. The mean corneal thickness values along the 2, 4, 6, 8 and 10 mm diameter concentric circles, with the CTL as the centre, were also obtained. The above data at different points were statistically correlated.

RESULTS

There was no significant difference between CCT readings measured by UP and Pentacam (P=0.721). There was high positive correlation between the CAT values and the thickness at 3 mm (R≥0.845, P<0.001) and at 7 mm points (R≥0.654, P<0.001). A gradual increase in thickness was noted from the centre to the periphery with a high positive correlation between the CTL values and the mean thickness at the circles of 2, 4, 6, 8 and 10 mm (R≥0.635, P<0.001).

CONCLUSION

The results suggest that central corneal thickness can serve as a good guide for predicting peripheral thickness. For surgical procedures specifically undertaken at mid-peripheral and peripheral zones, the actual measurements at the site of surgery may confer some advantage.

Comparison of corneal thickness measurement with the Pentacam, the PARK1 and an ultrasonic pachymeter

PURPOSE

The aim was to determine the agreement between PARK1 pachymetric readings and measurements made with the Pentacam and ultrasonic pachymetry.

METHODS

In this prospective study, we enrolled refractive surgery patients free of other ocular conditions and performed Pentacam and PARK1 acquisitions followed by ultrasonic pachymetry. We recorded pachymetric readings on the centre, apex and thinnest point with the Pentacam, corneal thickness values of the apex and minimum readings from the PARK1, and the central corneal thickness with the ultrasonic system. Data were compared using t-tests, and the Pearson correlations and 95% limits of agreement (LoA) of all pairs were determined.

RESULTS

The mean age of the 47 participants was 32 ± 10.4 years and data from their right eyes were used. Overall, ultrasonic measurements were significantly higher than all readings from the PARK1 and the Pentacam. The smallest difference was between the PARK-apex and Pentacam-apex readings (0.2 ± 11 µm); the 95% LoA of this pair gave a width of 43.7 µm (-21.7 to 22.0 µm). The smallest width pertained to the Pentacam-centre and ultrasound-centre paired readings; the width was 32.5 µm (-21.8 to 11.8 µm), while the mean difference was -5.54 ± 8 µm.

CONCLUSION

The PARK1 measurements of the apical corneal thickness can be a substitute for central corneal pachymetry with an ultrasonic device, provided that the range of agreement is taken into consideration in the interpretation of the results. Similar studies on populations with different corneal conditions such as keratoconus are needed.

Comparison of central corneal thickness measurements by Pentacam, noncontact specular microscope, and ultrasound pachymetry in normal and post-LASIK eyes

PURPOSE

To compare central corneal thickness (CCT) measurements taken with Pentacam, noncontact specular microscope (NCSM), and ultrasound pachymetry (US) in normal and post-laser in situ keratomileusis (LASIK) eyes and to assess the agreement between the three devices.

DESIGN

Prospective clinical trial.

PATIENTS AND METHODS

Central corneal thickness (CCT) was measured in 94 eyes of normal volunteer and in 72 eyes of post-LASIK patients. Measurements were made with the three devices.

RESULTS

In normal eyes, the mean (±SD) CCT taken with Pentacam, NCSM, and US was 552.6 ± 36.8 μm, 511.9 ± 38.6 μm, and 533.3 ± 37.9 μm, respectively. The average values of CCT taken with the three instruments were significantly different. In post-LASIK eyes the mean CCT with Pentacam, NCSM, and US was 483.02 ± 6.03 μm, 450.7 ± 5.3 μm, and 469.5 ± 5.8 μm, respectively. The average values of CCT taken were significantly different for Pentacam vs. NCSM (P = 0.046) and Pentacam vs. US (P = 0.02), but not significant for NCSM vs. US (P = 0.352). The Bland and Altman method for assessing clinical agreement between 3 instruments showed that in normal eyes, the mean values and paired differences of the three CCT devices were found to be statistically independent. In post-LASIK eyes, there was significant association between the difference and the mean of the Pentacam and NCSM, and US and NCSM.

CONCLUSION

The three devices cannot be used interchangeably in normal and post-LASIK eyes. Pentacam tends to give significantly thicker reading than ultrasound pachymetry.

Corneal thickness association with ocular and corneal high-order aberrations

PURPOSE

To investigate the association among central corneal thickness, peripheral corneal thickness (PCT), and wavefront aberrations in the anterior cornea, posterior cornea and the whole eye in myopia.

METHODS

Sixty-four eyes of 64 myopic subjects were evaluated with a Pentacam rotating Scheimpflug camera (Oculus, Inc., WA) corneal topographer for: (1) wavefront aberrations from the anterior and posterior corneal surface, (2) corneal thickness (central and peripheral), and (3) with a wavefront aberration-supported cornea ablation wavefront analyzer (Wavescan, Visx, Inc., Santa Clara, CA) for wavefront aberrations generated in the whole eye. Relationships between the wavefront aberrations and corneal thickness were analyzed.

RESULTS

The mean age of subjects was 34.75 +/- 10.08 years. The central corneal thickness was 550.5 +/- 28.459 microm. The mean peripheral thickness was 629.9 +/- 32.1 microm. Central and PCTs were not significantly correlated with corneal or ocular high-order aberrations. Intraocular pressure was significantly correlated with ocular trefoil (r = -0.307, P=0.001).

CONCLUSIONS

Central and PCT were not significantly associated with either anterior or posterior corneal Zernike aberrations; in addition, no association with the whole ocular wavefront aberrations was found.

Comparison of Pentacam Scheimpflug Camera with Ultrasound Pachymetry and Noncontact Specular Microscopy in Measuring Central Corneal Thickness

PURPOSE

Although central corneal thickness (CCT) can be measured by several methods, interchangeability of different modalities has not been fully investigated. CCT is known to correlate with intraocular pressure (IOP). The aim of this study was to evaluate the agreement of Pentacam Scheimpflug system with noncontact specular microscopy (NCSM) and ultrasound (US) pachymetry in measuring CCT and the relation between IOP taken with Goldmann applanation tonometer (GAT) and the CCT measured with these three methods.

METHODS

The right eyes of 135 enrolled persons without antiglaucoma drug use (100 females and 35 males), who comprised 32 patients with primary open-angle glaucoma, 14 with ocular hypertension, 45 with primary angle-closure glaucoma, and 44 controls, were studied. Intermethod comparison of CCT was made by the 95% limits of agreement analysis according to Bland and Altman. Linear regression analysis was used to assess the relationship between IOP and CCT taken with each modality.

RESULTS

The mean CCT (+/-SD) taken with Scheimpflug, US, and NCSM was 559.49 +/- 38.44 microm, 553.01 +/- 39.33 microm, and 552.04 +/- 42.95 microm, respectively. The average values of CCT taken with the three instruments were not significantly different (one-factor ANOVA; p = 0.26), although the marginal mean difference between Scheimpflug and US or NCSM was statistically significant (paired t test; p = 0.0009 and 0.005, respectively). The 95% limits of agreement were 6.47 +/- 43.21 microm between Scheimpflug and US, 7.45 +/- 58.86 microm between Scheimpflug and NCSM, and 0.98 +/- 51.69 microm between US and NCSM. There was a positive association between IOP and CCT measured with US or NCSM, whereas there was no correlation between IOP and CCT measured with Scheimpflug.

CONCLUSIONS

Although CCT values measured with Scheimpflug, US, and NCSM are closely similar, clinicians should keep in mind that these methods are not simply interchangeable.

Thickness mapping of the cornea and epithelium using optical coherence tomography

PURPOSE

To measure corneal and epithelial thickness across four meridians using Optical Coherence Tomography (OCT) and to compare these measurements between normal non-lens wearers (NLW), rigid gas permeable (RGP) lens wearers, and RGP-wearing keratoconics (KC).

METHODS

Both eyes of 60 subjects were measured (20 NLW, nine female:11 male, 27.6 +/- 5.9 years; 20 RGP, 20 female, 23.9 +/- 7.6 years; and 20 KC, seven female:13 male, 32.4 +/- 8.1 years). A customized fixation target employing LEDs in eight directions of gaze was attached to the OCT and corneal images obtained. Raw OCT scans were analyzed to yield values for corneal and epithelial thickness and color-coded maps were compiled.

RESULTS

Central corneal thickness (CCT) was thinnest in KC (447 +/- 68 microm) and similar between RGP (518 +/- 32 microm; pKC < 0.001) and NLW (517 +/- 21 microm) (p(KC) < 0.001 NLW pRGP > 0.05). Peripheral corneal thickness in NLW was thickest in the superior temporal and thinnest in the inferior (I) regions (superior temporal(thickest) vs. I(thinnest) p < 0.001). Central epithelial thickness was thinnest in KC (44 +/- 7 microm), followed by RGP (50 +/- 4 microm), then NLW (54 +/- 2 microm) (pKC < 0.001 NLW p(RGP) < 0.05). Central epithelial thickness in the KC group was significantly thinner than in the RGP group (p < 0.001). In the NLW group, peripheral epithelial thickness was thicker (63 +/- 5 microm) than central (p < 0.001) and was thickest in the superior (S) region and thinnest in the inferior (I) region (S(thickest) vs. I(thinnest) p < 0.001). KC epithelium was thinnest in the inferior temporal meridian (42 +/- 5 microm).

CONCLUSIONS

Thickness of the normal cornea and epithelium was greatest in the superior region. In all groups, the inferior cornea and epithelium was thinnest, and to a greater extent in the KC group.

Contact lens-induced corneal peripheral swelling differences with extended wear

PURPOSE

To compare central and peripheral corneal swelling with high-Dk (lotrafilcon A) and low-Dk (etafilcon A) contact lenses (CLs) during extended wear (EW).

METHODS

Corneal thickness was measured in 5 corneal locations (central, superior, inferior, nasal, and temporal) with Orbscan II pachymetry 1 week before the wearing of CLs and after 1 week of EW. High-Dk (lotrafilcon A) and low-Dk (etafilcon A) soft CLs were randomly fitted for EW in the right and left eyes of 20 subjects with normal ocular health. Orbscan was also performed before CL removal after 3 and 7 days of EW.

RESULTS

Lotrafilcon A induced a lower percentage of corneal swelling in all 5 corneal locations than etafilcon A (P < 0.05, paired t test). Lotrafilcon A swelling percentages were as follows: central (1.67% +/- 2.22%), superior (2.04% +/- 3.25%), inferior (1.38% +/- 1.74%), nasal (1.50% +/- 2.00%), and temporal (1.27% +/- 1.47%). Etafilcon A swelling percentages were as follows: central (4.98% +/- 3.07%), superior (6.62% +/- 8.39%), inferior (5.24% +/- 5.54%), nasal (3.41% +/- 3.14%), and temporal (4.46% +/- 3.32%). Differences between the central and peripheral corneal swelling during EW of both types of lenses were not significant (P > 0.05, repeated-measures analysis of variance).

CONCLUSIONS

Central and peripheral corneal thicknesses increase more with etafilcon A than with lotrafilcon A during EW, as measured by Orbscan pachymetry.

The comparative study of two recently developed A-scan devices: determination of central corneal thickness, anterior chamber depth and axial length

PURPOSE

The clinical use and comparison of two recently developed A-scan ultrasound devices.

METHODS

The same investigator determined central corneal thickness (CCT), anterior chamber depth (ACD) and axial length (AL) with an OcuScan RxP (Alcon, Forth Worth, Texas, USA) and an AL-2000 (Tomey, Erlangen, Germany) ultrasound device in 80 eyes of 40 patients. The mean patient age was 63.72 years [standard deviation (SD) 18.92]. The patients did not suffer from any systemic or eye disease affecting the anterior and posterior segments of the eye, and their refractive error (spherical and astigmatic) did not exceed +/- 3.0 dioptres.

RESULTS

The value of the CCT was 541.55 (SD 34.97) microm with the OcuScan RxP, and 547.46 (SD 35.70) microm with the AL-2000 device (P < 0.001). With respect to the ACD and AL, significantly lower values were obtained with the AL-2000 instrument (P < 0.001). The ACD was 2.92 (SD 0.48) mm with the AL-2000 and 3.07 (SD 0.47) mm with the OcuScan RxP device. The AL was 22.67 (SD 0.84) mm with the AL-2000 and 22.81 (SD 0.87) mm with the OcuScan RxP instrument. However, a positive and significant correlation could be demonstrated between the devices (r = 0.88 CCT, r = 0.86 ACD and r = 0.91 AL; P < 0.001).

CONCLUSION

The instruments are reliable in clinical use; however, statistically significant differences were found between the devices. During patient follow-up, the devices cannot simply be used interchangeably.

Effect of corneal thickness on the agreement between ultrasound and Orbscan II pachymetry

PURPOSE

To study the agreement between ultrasound (US) and Orbscan II (Bausch & Lomb) measurements with different ranges of central thickness in normal corneas.

SETTING

Noor Ophthalmology Research Center, Tehran, Iran.

METHODS

The central corneal thickness was measured first by the Orbscan II and then with a US pachymeter in 177 right eyes. Data were compared in 3 thickness groups: less than 500 microm, 500 to 600 microm, and more than 600 microm. In all cases, uncorrected Orbscan II data were used. For each range, a correction factor was calculated through regression analysis, which was then used in a new set of analyses. To study the agreement between the 2 methods, the 95% limits of agreement (LoA) and intraclass correlations coefficients (ICC) were determined.

RESULTS

The mean interdevice difference was 2.7 +/- 16.9 (SD) in the less than 500 microm group (P = .365), 21.3 +/- 3.1 in the 500 to 600 microm group (P = .0001), and 27.2 +/- 20.9 in the more than 600 microm group (P = .0001). No clinically acceptable LoA were found in any group. The application of regression equations to the Orbscan II readings improved the agreement in the less than 500 microm group better than the other 2 groups. The ICC was 0.76, 0.61, and 0.43 in the less than 500 microm, 500 to 600 microm, and more than 600 microm groups, respectively.

CONCLUSIONS

The Orbscan corneal thickness readings had relatively lower validity than US measurements. The 2 devices should not be used interchangeably, especially when the validity of the measurement is vital. In patient screening for corneal surgery, it is advisable to recheck corneal thickness with a US pachymeter.

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.

Measuring Corneal Thickness With the ConfoScan 4 and Z-Ring Adapter

PURPOSE

To assess the precision and accuracy of the z-ring adapter with the ConfoScan 4 confocal microscope for measuring corneal thickness.

METHODS

Thirty healthy corneas of 15 volunteers were scanned twice with a ConfoScan 4 confocal microscope equipped with a z-ring adapter (Nidek, Inc., Fremont, CA) and with a Tandem Scanning confocal microscope (Tandem Scanning Corporation, Reston, VA). Corneal thickness was determined from the position of the focal plane at the epithelial and endothelial surfaces. Distances measured by both instruments were calibrated from scans of 15 polymethylmethacrylate contact lenses with known thicknesses between 400 and 650 microm. Corneal thickness was also measured by ultrasonic pachymetry (DGH Technologies, Inc., Exton, PA).

RESULTS

Corneal thickness measured with the ConfoScan 4 (mean, 529 +/- 35 microm) was not significantly different from thickness measured with the Tandem Scanning confocal microscope (mean, 531 +/- 30 microm) (P=0.30). The mean difference between the first and second scans was 1.1 +/- 20 microm and -2.6 +/- 17 microm with the ConfoScan 4 and Tandem Scanning microscopes, respectively. Both confocal microscopes indicated thinner corneas than ultrasonic pachymetry did (567 +/- 35 microm) (P<0.0001).

CONCLUSIONS

Mean corneal thickness measured with the z-ring adapter with the ConfoScan 4 agrees with mean corneal thickness measured with the Tandem Scanning confocal microscope when both instruments are correctly calibrated. Ultrasonic pachymetry indicated a mean corneal thickness that was at least 35 microm greater than was indicated by either confocal microscope.

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.

Central Corneal Thickness

PURPOSE

To compare the repeatability and validity of corneal pachymetry by a corneal confocal microscope with a z-axis adapter (Confoscan 4.0 with z-ring adapter: z-CS4) versus ultrasound (US) pachymetry in the measurement of central corneal thickness (CCT).

METHODS

CCT in 44 eyes of 44 subjects was determined with z-CS4. Z-CS4 exams were used to estimate the repeatability of thickness measurement by z-ring adapter for this confocal microscope. Intraclass Correlation Coefficient (ICC) between two different z-CS4 users was also determined. CCT in the same 44 eyes was determined with US pachymetry and measurements were compared with z-CS4 CCT.

RESULTS

Z-CS4 CCT showed high intrainstrument reproducibility (ICC = 0.989; 95%CI 0.982-0.993; P < 0.0001). Mean difference among three CCT consecutive measures, in the same eye, was 0.8 +/- 11.1 microm. High correlation was found between two users (ICC = 0.896; 95%IC 0.830-0.937; P < 0.0001). Z-CS4 CCT showed high correlation with US pachymetry (ICC = 0.921; 95%CI 0.851-0.958; P < 0.0001). Mean corneal thickness determined was statistically different with the two methods (US: 512.6 +/- 65.8 microm; z-CS4: 487.8 +/- 60.1 microm; P < 0.0001).

CONCLUSION

Z-CS4 seems an accurate, noninvasive and reproducible technique for CCT evaluation and confirms that central cornea is thinner when measured with confocal microscopy compared to ultrasounds.

3D average human corneal models

In this paper we propose a method of building an average model or atlas of the cornea based on topographic data. Specific models can be constructed for the left or right eye, gender, age, or ametropia, to assess differences and similarities. An application of this atlas construction methodology to the study of corneal shape evolution with age is presented. Results show significant differences between age groups. This numerical atlas could also be helpful in the design of algorithms targeting the detection of corneal shape abnormalities, such as keratoconus or previous laser surgery.

Comparison of Optical Low-Coherence Reflectometry and Ultrasound Pachymetry in Measuring Corneal Graft Thickness

PURPOSE

To compare optical low-coherence reflectometry (OLCR) and ultrasound pachymetry in measuring corneal graft thickness in patients after keratoplasty.

METHODS

We retrospectively measured the central graft thickness in 41 eyes of 41 patients with the OLCR pachymeter (Haag Streit, Koeniz, Switzerland) and the SP-2000 contact ultrasound pachymeter (Tomey, Nagoya, Japan). Five separate measurements were performed on each eye with both methods. Mean, SD, repeatability, and coefficient of variation of measurements were calculated, and the correlation between the 2 methods was studied with Spearman regression.

RESULTS

Mean central graft thickness was 546 +/- 51 (SD) microm with the contact ultrasound pachymeter and 546 +/- 47 microm with the OLCR pachymeter. The correlation between both methods was strong (rs = 0.96). No significant differences in mean SD of measurements were observed between OLCR pachymetry (mean SD = 4.66 microm) and contact ultrasound pachymetry (mean SD = 4.88 microm). The repeatability of both methods was comparable (P = 0.06) and high (the average coefficient of variation of the central corneal graft thickness was 0.9% with both pachymeters). The postoperative time did not affect the correlation between both pachymeters (P > 0.05).

CONCLUSIONS

Central corneal graft thickness values obtained with the OLCR pachymeter were similar to those obtained with a contact ultrasound pachymeter. In some cases of lamellar keratoplasty, the corneal refractive index could change at the interface level that could affect OLCR measurements.

Confocal Microscopy Comparison of IntraLase Femtosecond Laser and Moria M2 Microkeratome in LASIK

PURPOSE

To analyze the quality of LASIK flaps created using a femtosecond laser or a mechanical microkeratome.

METHODS

A prospective study of consecutive bilateral LASIK flaps (100 patients) created with a 15 kHz femtosecond laser (IntraLase) or with a mechanical microkeratome (Moria M2) was performed. Outcome measures included visual and refractive outcomes, flap dimensions, and intraoperative complications. The first consecutive ten patients (20 eyes) were examined using confocal microscopy 1 and 3 months following surgery.

RESULTS

No significant differences were noted in the visual outcomes between groups (t test, P > .05 for all comparisons). Analysis of superficial measurements revealed more round flaps and greater predictability in surface dimensions for the femtosecond laser group (t test, P = .016). Mean deviation of flap thickness from target was the same for both instruments, 10 microm, with greater variability for the mechanical microkeratome. The incidence of diffuse lamellar keratitis was significantly greater in the femtosecond laser group (17%) than in the mechanical group (0%) (chi-square test, P < .001). Confocal microscopy revealed a higher wound healing opacity index in the femtosecond laser group.

CONCLUSIONS

Although the clinical results are similar, the femtosecond laser produces LASIK flaps with more accurate dimensions compared to the mechanical microkeratome.

Age-Related Changes in Central and Peripheral Corneal Thickness

PURPOSE

The thickness of corneal tissue is an important parameter in refractive surgery, Goldmann applanation tonometry, and corneal diseases. The purpose of the study was to record system-specific normal values with the Orbscan II system and to investigate the influence of sex and age on central and peripheral corneal thickness to characterize more precisely the anatomy of the cornea.

METHODS

The Orbscan II topography system is a computer-assisted slit-beam scanning technology that can map the anterior section of the eye. It was used to take 2 measurements at 10-minute intervals on 777 eyes of 390 normal white subjects ranging in age between 10 and 80 years. Two hundred forty-two participants were men and 148 were women. The central corneal thickness and the nasal, superior, inferior, and temporal peripheral corneal thickness at a distance of 3 mm from the center were analyzed. No correction factor was used. Using nonparametric test methods, we carried out a statistical analysis of the parameters of sex and age.

RESULTS

The mean central corneal thickness of all 777 eyes was 595 +/- 41 microm. No sex-specific central difference was identifiable (P = 0.33), there was no significant difference between the sides (P = 0.23), and no significant difference was found between the first and second measurement. The group of 10- to 39-year-olds had a mean central corneal thickness of 591 +/- 41 microm, whereas that of 40- to 80-year-olds was 600 +/- 39 microm, which was a significant difference (P = 0.03). The mean peripheral corneal thickness was 689 +/- 46 microm nasally, 688 +/- 42 microm superiorly, 667 +/- 40 microm inferiorly, and 655 +/- 42 microm temporally. Nasally and superiorly, we found a significant negative correlation with age (Spearman rank correlation, P = 0.02).

CONCLUSIONS

The normal values presented here are a suitable reference basis for future studies in whites. Peripheral corneal thickness is asymmetric and seems to undergo age-related anatomic changes. This information should be considered when planning penetrating keratoplasty and refractive surgery.

Clinical corneal confocal microscopy

Confocal microscopy allows non-invasive in vivo imaging of the ocular surface. Its unique physical properties enable microscopic examination of all layers of the cornea and have been used to investigate numerous corneal diseases: epithelial changes, numerous stromal degenerative or dystrophic diseases, endothelial pathologies, corneal deposits, infections, and traumatic lesions. It offers a new approach to study the physiological reactions of the cornea to different stimuli and the pathophysiologic events leading to corneal dysfunction in certain diseases. Confocal microscopy proves to be a powerful diagnostic tool and is especially of value in certain corneal diseases by allowing straightforward and non-invasive recognition of the pathologic conditions.

Ultrasound and Partial Coherence Interferometry With Measurement of Central Corneal Thickness

PURPOSE

To compare noncontact pachymeter measurements with ultrasound pachymeter measurements and assess their reproducibility.

METHODS

Central corneal thickness was measured in 104 eyes of 56 patients with three laser interference pachymeters (OLCR [Haag Streit, Könitz, Switzerland], OCP [4optics AG, Lübeck, Germany], and ACMaster [Carl Zeiss Meditec, Jena, Germany]) and an ultrasound pachymeter (Tomey AL2000 [Tomey Corp, Nagoya, Japan]).

RESULTS

Compared to the ultrasound measurements, the mean difference for the laser interference pachymeter measurements were +8.8 microm (standard deviation [SD] 5.68) for the OLCR, -8.0 microm (SD 5.39) for the OCP, and -0.12 microm (SD 5.88) for the ACMaster. Reproducibility could only be estimated as not all of the devices allowed access to individual measurements. For all laser interference devices, reproducibility was estimated to be approximately 2 microm. Ultrasound measurements yielded a reproducibility of approximately 3.4 microm.

CONCLUSIONS

Although ultrasound pachymeter measurements differed significantly from OLCR and OCP measurements, agreement was considered good because the mean differences were <10 microm, and the results can be regarded as clinically interchangeable.

Corneal thickness measurements in normal and keratoconic eyes: Pentacam comprehensive eye scanner versus noncontact specular microscopy and ultrasound pachymetry

PURPOSE

To compare central corneal thickness (CCT) measurements taken with the Pentacam comprehensive eye scanner (CES), noncontact specular microscopy (SM), and ultrasound pachymetry (UP) in normal and keratoconic corneas.

SETTING

Department of Ophthalmology, Ankara University School of Medicine, Ankara, Turkey.

METHODS

In a prospective study, 3 CCT measurements were taken with the Pentacam CES, SM, and UP in that sequence from 1 eye of 45 consecutive patients with myopia (group A) and 62 consecutive patients with keratoconus (group B). Eyes with keratoconus were further divided into 3 subgroups, mild, moderate, and severe, according to the mean keratometry readings.

RESULTS

Pentacam CES (r = 0.994) and UP (r = 0.993) demonstrated very high and comparable reproducibility in group A. In group B, Pentacam CES displayed better reproducibility (r = 0.988) than UP (r = 0.969) and SM (r = 0.901). The mean CCT measurements of Pentacam CES and UP were not significantly different in group A (P = .37) and in eyes with mild keratoconus (P = .29), whereas significant differences between all instrument pairs were evident in group B and in moderate and severely keratoconic eyes (P<.05). Noncontact SM measurements were significantly smaller than those of Pentacam CES and UP in all groups (P<.05) when a measurement could be obtained. There were significant linear correlations between CCT measurements of Pentacam CES, UP, and noncontact SM in groups A, B, and mildly keratoconic eyes (P<.001).

CONCLUSIONS

Results suggest that whereas Pentacam CES and UP may be used interchangeably in normal eyes in the clinical setting for the measurement of CCT, one should be cautious interpreting corneal thickness data from Pentacam CES, UP, and particularly SM in eyes with keratoconus. Whereas, in normal and mildly keratoconic eyes, Pentacam CES and UP demonstrated very high and comparable reproducibility, in moderately keratoconic eyes, Pentacam CES readings showed better reproducibility than UP.

Confocal Microscopy of Disposable and Nondisposable Heads for the Moria M2 Microkeratome

PURPOSE

To study the quality of the flap obtained with two different heads, disposable and nondisposable, and two different speeds, low and high, using the same Moria M2 microkeratome (Moria, Antony, France).

METHODS

In a prospective randomized study, 51 consecutive eyes (28 patients) underwent myopic LASIK (-4.39 +/- 2.15 diopters). The eyes were randomly assigned to four groups related to cutting head model (disposable and nondisposable) and cut speed (low and high). In all cases, the intended cut was 160 microm and the same M2 unit was used. During surgery, superficial measurements of the flap and the surgeon's opinion about the quality of the cut were recorded. One month after LASIK, the cut depth, interface particle density, and transparency of the cornea (confocal wound healing opacity index) were measured with a confocal microscope.

RESULTS

Flaps performed with a disposable cutting head and high speed were statistically significantly larger than the flaps created in the other groups (Kruskal-Wallis test; P = .041). No statistical differences were noted in cut depth, number of particles, and wound healing opacity index. The surgeon's opinion parameters were similar in all groups.

CONCLUSIONS

The Moria M2 microkeratome is safe and provided reproducible results, regardless of cutting head and speed. Disposable and nondisposable head models were similar in their outcome in the quality control model used in this study.

Orbscan Global Pachymetry: Analysis of Repeated Measures

PURPOSE

The Orbscan II is a hybrid slit-scanning and Placido disc corneal topographer capable mapping global pachymetry over a 10-mm corneal diameter. In this study, the repeatability of the Orbscan global pachymetry was determined.

METHODS

Five consecutive Orbscan examinations were performed on one eye of 20 healthy volunteers by one examiner in one session. Mean, standard deviation, range of readings, and coefficient of variance were calculated for the central, thinnest point and global pachymetry.

RESULTS

Orbscan repeatability for the central pachymetry was 3.62 microm (95% confidence interval [CI], 0.57-6.66) with a coefficient of variance of 0.67% (95% CI, 0.09-1.25) and intraclass correlation coefficient (ICC) of 0.978 (95% CI, 0.959-0.990). Thinnest point pachymetry had a repeatability of 4.25 microm (95% CI, 1.19-7.31) with a coefficient of variance of 0.80% (95% CI, 0.19-1.42) and ICC of 0.973 (95% CI, 0.950-0.988). Global pachymetry had a coefficient of variance <1.5% in the central 4 mm and <<2.5% across the entire cornea. Global pachymetry showed an area of greatest repeatability (<< +/- 5 microm) within a central 3.0-mm horizontal and 4.0-mm vertical diameter.

DISCUSSION

Orbscan is capable of giving repeatable pachymetry readings that are comparable to the ultrasound pachymeter within the central and thinnest point of the cornea. There is a gradual loss of repeatability toward the peripheral cornea possibly as a result of a lack of overlapping data points.

Agreement and Repeatability of Central Thickness Measurement in Normal Corneas Using Ultrasound Pachymetry and the OCULUS Pentacam

PURPOSE

To compare the accuracy and repeatability of the OCULUS Pentacam (a new Scheimpflug-based imaging system) with ultrasound pachymetry in the measurement of central corneal thickness (CCT).

METHODS

CCT was measured in 21 subjects (21 normal corneas) on 2 separate occasions by the same examiner, using an Allergan-Humphrey 850 ultrasonic pachymeter and an OCULUS Pentacam instrument.

RESULTS

Mean values of CCT for both visits for each instrument were 534 +/- 47 microm and 528 +/- 45 microm using the ultrasonic pachymeter and the Pentacam, respectively. Plots of differences against means displayed relatively good agreement (limits of agreement were -13.0 to +26.6 microm). The repeatability (limits of agreement) of the ultrasound pachymeter was -18.3 to +17.7 microm, while for the OCULUS Pentacam it was -24.1 to +21.1 microm.

CONCLUSIONS

Our data showed that the Pentacam instrument provided measurements that were slightly but systematically lower than the measurements provided by ultrasonic pachymetry, which is currently the clinical gold standard method. The results, coupled with a unique ability to image and analyze the anterior chamber in vivo, make the OCULUS Pentacam a promising new instrument for anterior eye evaluation.

Central Corneal Thickness Measurements Using Orbscan II Scanning Slit Topography, Noncontact Specular Microscopy, and Ultrasonic Pachymetry in Eyes With Keratoconus

PURPOSE

To compare corneal thickness measurements using Orbscan II scanning slit topography, Topcon SP-2000P noncontact specular microscopy, and ultrasonic pachymetry in eyes with keratoconus.

METHODS

Central corneal thickness was measured in 22 eyes with keratoconus. Eyes with apparent corneal opacity were excluded. Scanning slit topography, noncontact specular microscopy, and ultrasonic pachymetry were used in this sequence. The acoustic equivalent correlation factor (0.92) was used for Orbscan readings.

RESULTS

Three devices gave significantly different corneal thickness readings (P < 0.001, repeated-measure analysis of variance). Measurements with Orbscan scanning slit topography (449.5 +/- 43.2 [SD] mum) were significantly smaller than those of ultrasonic pachymetry (485.0 +/- 29.3 microm; P < 0.001, Tukey multiple comparison) and SP-2000P noncontact specular microscopy (476.7 +/- 28.3 microm; P = 0.002). There were significant linear correlations between ultrasonic pachymetry and scanning slit topography (Pearson correlation coefficient r = 0.741, P < 0.001), between scanning slit topography and noncontact specular microscopy (r = 0.880, P < 0.001), and between noncontact specular microscopy and ultrasonic pachymetry (r = 0.811, P < 0.001).

CONCLUSION

In eyes with keratoconus, Orbscan II scanning slit topography system gave significantly smaller corneal thickness readings than the other 2 devices. Measurements taken by noncontact specular microscopy and ultrasonic pachymetry were comparable. Three devices showed significant linear correlations with one another.

Repeatability and Reproducibility of Central Corneal Thickness Measurement With Pentacam, Orbscan, and Ultrasound

PURPOSE

The purpose of this study was to compare central corneal thickness (CCT) measurements obtained with a novel rotating Scheimpflug camera (Pentacam; Oculus) with scanning slit topography (Orbscan; Bausch & Lomb), and with ultrasound pachymetry (SP-2000; Tomey).

METHODS

CCT in 30 healthy eyes was measured twice with each modality by 2 independent observers in random order. The results from scanning slit topography are given both with and without multiplication with the "acoustic correction factor" of 0.92. In addition, the displayed images from the rotating Scheimpflug camera and scanning slit topography were used to calculate the signal difference-to-noise ratios (SD/N) between cornea and background signal.

RESULTS

The mean CCT values as determined with the different modalities (+/-standard deviation) were: 542+/-29 microm, 576+/-37 microm, 530+/-34 microm, and 552+/-32 microm for rotating Scheimpflug imaging, for uncorrected and for corrected scanning slit pachymetry, and for ultrasound, respectively. The differences between modalities (+/-95% limits of agreement) were -9.8+/-31 microm between rotating Scheimpflug and ultrasound, 24+/-31.2 microm between scanning slit and ultrasound, and 33+/-27 microm between scanning slit and rotating Scheimpflug imaging. The limits of agreement for within and between observer effects were within 4.2% of the absolute CCT values for scanning slit and ultrasound and within 2.2% for the rotating Scheimpflug imaging. The rotating Scheimpflug camera showed similar SD/N ratios but steeper edges of the corneal surfaces in the intensity profile plots.

CONCLUSION

In the assessment of normal corneas, the Pentacam measured CCT values closer to ultrasound pachymetry and with less variability compared with Orbscan. The (interobserver) reproducibility with the Pentacam was highest of all 3 modalities.

Orbscan computerized topography: Attributes, applications, and limitations

An extensive electronic search was undertaken in January 2004 to identify all relevant peer-reviewed publications on Orbscan slit-scanning/Placido computerized topography. Ninety-one publications were identified. These address elevation topography and best-fit sphere, accuracy and repeatability of anterior and posterior corneal elevation and keratometric maps, comparison of Orbscan-acquired data and Placido-based computerized videokeratography instruments, pachymetry measurement and correlation with ultrasound, screening eye-bank corneas, detection of keratoconus, identifying corneal ectasia after refractive surgery, and miscellaneous applications. Studies were analyzed and critically compared in relation to attributes, applications, and limitations of Orbscan corneal topography. The review highlights advantages of this technique in assessing the cornea in health and disease and after surgery and identifies specific aspects that require further investigation and clarification.