Comparison of central corneal thickness as measured by non-contact specular microscopy and ultrasound pachymetry before and post LASIK

Agreement Between Predicted and Actual Measured Ablation Depth After FS-LASIK Using Different Rotating Scheimpflug Cameras and OCT

Purpose

To compare the predicted ablation depth (AD) with the postoperatively measured corneal ablation depth (postop-AD) at central, paracentral, and midperipheral locations using two rotating Scheimpflug analyzers and a Fourier-domain optical coherence tomographer in eyes that underwent femtosecond laser-assisted LASIK (FS-LASIK).

Methods

The values of corneal thickness were measured preoperatively and postoperatively at one and three months. The difference between preoperative and postoperative was defined as postop-AD. Measurements were performed at the corneal vertex and mid-peripheral area. The mid-peripheral corneal thickness was measured at the superior, inferior, nasal, and temporal locations at a distance of 1.0 or 2.5 mm from the corneal vertex. The predicted AD was calculated by ORK-CAM software (Schwind eye tech-solutions GmbH, Kleinostheim, Germany), and the difference between the predicted AD and postop-AD was defined as Δ-AD. Paired t-test analysis was employed to evaluate the differences, agreement was assessed by the Bland-Altman method.

Results

Forty-two eyes of 42 patients were investigated. At one month, the predicted AD in the central and paracentral areas was underestimated by the Pentacam HR (Oculus, Wetzlar, Germany), Sirius (Costruzione Strumenti Oftalmici, Florence, Italy) and RTVue OCT (Optovue Inc., Freemont, CA, United States), whereas Δ-AD was negative as established by all devices and predominantly statistically significant. The Δ-AD values approximated zero at three months. The mean difference of Δ-AD at three months at the corneal vertex was 0.67 ± 9.39 mm, −7.92 ± 9.05 mm and −1.36 ± 8.31 mm, respectively. The mid-peripheral measurements had positive values at one month and even more highly positive at three months (with statistically significant differences in most of the cases). The agreement between the predicted and postop-AD was moderate with all devices, but slightly better with RTVue.

Conclusion

The predicted AD seems to be underestimated in the central and paracentral corneal area and overestimated in the mid-periphery.

Translational Relevance

The study could help to partly explain and prevent the refractive errors after FS-LASIK.

Non-contact specular microscopy with Topcon instruments to assess central corneal thickness of healthy human eyes - A 20 year review

BACKGROUND

The purpose of this review was to evaluate the consistency of central corneal thickness (CCT) values reported with use of Topcon SP-2000 P and SP-3000 P non-contact specular microscopes since their introduction in 1999 with the two microscopes having been commonly used in a wide range of studies.

METHODS

As a primary resource, PubMed was used to search for peer-reviewed articles in any language that included CCT values obtained with non-contact specular microscopy reported for humans with nominally healthy corneas. Relevant articles were obtained and any cited publications also checked.

RESULTS

A total of 76 articles were identified which reported CCT on different small-to-moderate sized groups of individuals, published between 1999 and 2019. From these, an overall group mean CCT value of 0.525 ± 0.013 mm (median 0.525 mm) can be calculated. An estimated 95 % confidence interval (CI, based on 1.96 SD) would be between 0.500 and 0.550 mm. For the two Topcon models, the group mean ± SD values were 0.529 ± 0.013 mm and 0.517 ± 010 mm respectively. An assessment of the CCT data sets in relation to the reported average age indicated no statistically significant effect (p = 0.289, r = -0.129). Very similar average CCT values were also encountered in 4 other reports where these microscopes were used in large-scale population studies as well as in 2 other reports using the newer Topcon SP-1 P model.

CONCLUSIONS

The Topcon stand-alone non-contact specular microscopes have yielded consistent and predictable corneal thickness measures over many years.

Comparison of a new anterior segment optical coherence tomography and Oculus Pentacam for measurement of anterior chamber depth and corneal thickness

Background

Accurate measurements of anterior chamber depth (ACD) and regional corneal thickness are especially important for the diagnosis and treatment of many ocular disease. This study aimed to evaluate the repeatability and reliability of a new swept source anterior segment optical coherence tomography (AS-OCT) and its agreement with Oculus Pentacam for measurements of ACD and corneal thickness (CT).

Methods

The central corneal thickness (CCT), superior corneal thickness (SCT), inferior corneal thickness (ICT), nasal corneal thickness (NCT), temporal corneal thickness (TCT) and ACD of the right eye from forty-nine adults aged 18 to 36 years (24.78±4.36 years old) were measured with Pentacam and AS-OCT (CASIA2). All subjects were measured twice with each device. The repeatability was determined using the coefficient of repeatability (COR), the relative COR and the limits of agreement (LOA). Bland-Altman plot was also used for evaluating the agreement between parameters from the two devices.

Results

For the repeatability of CASIA2, the COR of the two measurements of ACD, CCT, SCT, ICT, NCT and TCT was 0.31 mm, 18.58, 25.83, 28.32, 26.71 and 22.09 µm respectively. There were no statistically significant differences between the CT and ACD measurements by CASIA2 (P>0.05). For measurements with Pentacam and CASIA2, the COR of ACD, CCT, SCT, NCT, ICT and TCT was 0.294 mm, 13.10, 51.57, 48.06, 56.21 and 47.69 µm respectively. No significant differences were found between the values measured by CAISA2 and Pentacam for CT and ACD (P>0.05). The Bland-Altman analysis also suggested high consistency between measurements obtained by Pentacam and CASIA2.

Conclusions

Our results suggest that Pentacam and CASIA2 have good agreement in CT and ACD measurements. The two devices can be considered interchangeable for these parameters' measurements in healthy subjects when monitoring corneal conditions or planning ocular surgery. However, subtle differences between CASIA2 and Pentacam should also be kept in mind for certain specific clinical or research purposes.

Evaluation of Corneal Oedema – Tools we Have and Those Under Investigation

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Corneal Thickness Response after Anesthetic Eye Drops: Our Own Results and Meta-Analysis

We aimed to test if there are different patterns in the central corneal thickness (CCT) response after instilling oxybuprocaine anesthetic eye drops and also to determine whether there is a significant change in the CCT. CCT was measured in 60 eyes of 60 healthy subjects before and during the hour after oxybuprocaine 0.4% eye drops were instilled. In addition, a systematic review and meta-analysis were carried out in order to answer the following PICO (patient, intervention, comparison, and outcome) question: What effect do anesthetic eye drops have on CCT values? We found no significant changes in the mean CCT values during the hour's observation (ANOVA, p = 0.209), and the meta-analysis revealed no statistically significant changes in the CCT after anesthesia (Q-Value = 1.111; p value = 1.000; I2 = 0.000; Tau2 = 0.000; Stderr = 0.020). However, we found three CCT response patterns 5 minutes after anesthesia: Pattern 1, subjects with no significant changes in their CCT values (n = 14, 46.7%); Pattern 2, subjects with significant CCT increases (n = 11, 36.7%); and Pattern 3, subjects with significant CCT decreases (n = 5, 16.7%). In sum, there are no significant changes in the CCT after anesthesia, but there are three different CCT response patterns 5 minutes after anesthesia.

Comparison of specular microscopy and ultrasound pachymetry before and after cataract surgery

PURPOSE

To compare specular microscopy (Topcon SP-3000P, Topcon Corporation, Tokyo, Japan) and ultrasound (US) technology when evaluating central corneal thickness (CCT) prior to and after phacoemulsification.

METHODS

Corneal edema was assessed in phacoemulsification patients due to senile cataract by measuring CCT preoperatively and 1 day, 1 month, and 3 months postoperatively. Bland-Altman analysis was performed to assess interchangeability between pachymetry techniques for each visit. Repeated measures analysis of variance was performed to evaluate variation in CCT agreement depending on the degree of corneal edema.

RESULTS

One hundred and eighteen patients aged 73.9 ± 10.1 years were recruited. Topcon SP-3000P provided significant (p < 0.0001) lower CCT values than US with and without induced corneal edema. Mean differences between CCT techniques for baseline, 1 day, 1 month and 3 months after cataract surgery were -28.9 ± 22.6, -30.5 ± 41.4, -32.3 ± 16.2 and -33.0 ± 16.9 μm, respectively. The systematic bias observed was not significantly different among the 4 visits (p = 0.59). The estimated limits of agreement (based on 1.96 standard deviation) were substantial, being 90.5, 165.8, 64.9 and 67.5 um at baseline, 1 day and 1 and 3 months.

CONCLUSIONS

Topcon SP-3000P provides similar systematic bias in comparison with US technique for CCT measurements regardless of the degree of corneal edema after phacoemulsification, which should be reduced after applying a constant calibration adjustment of ∼30 μm. However, interchangeability between techniques may be still limited by the notable random measurement error.

Changes in central corneal thickness and refractive error after thin-flap laser in situ keratomileusis in Chinese eyes

BACKGROUND

Refractive stability is influenced by alterations in corneal curvature and corneal thickness after laser in situ keratomileusis (LASIK). The aim of this study was to analyze the changes of central corneal thickness (CCT) and refractive error following thin-flap LASIK surgery in Chinese eyes.

METHODS

One hundred and fifty-eight myopic patients (302 eyes) who underwent thin-flap LASIK surgery were prospectively evaluated. CCT was measured by non-contact specular microscopy before, and 1 day, 1 week, and 1, 3, and 6 months following surgery. Age, refractive error, and optic zone diameter were also recorded.

RESULTS

Preoperatively, the mean CCT was 531.6 ± 24.3 μm. At 1 day, 1 week, and 1, 3, and 6 months after surgery, mean CCTs were 431.4 ± 38.4 μm, 422.6 ± 3 7.8 μm, 427.2 ± 38.0 μm, 434.4 ± 38.2 μm, and 435.6 ± 38.0 μm, respectively. Significant changes were detected in CCT values at each time point after thin-flap LASIK treatment (P < 0.05). The mean preoperative spherical equivalent (SE) was -5.73 ± 2.30 diopters (D). At 1 day, 1 week, and 1, 3, and 6 months after surgery, it was 0.26 ± 0.58 D, 0.54 ± 0.52 D, 0.49 ± 0.53 D, 0.45 ± 0.49 D, and 0.37 ± 0.42 D, respectively. The spherical equivalent refraction at 6 months postsurgery was close to the predicted value (0.34 ± 0.30 D). The changes in CCT within 6 months (4.06 ± 9.99 μm) were negatively correlated with age, preoperative refractive error, and optical zone diameter, respectively (r = -0.180, P < 0.05; r = -0.187, P < 0.001; r = -0.171, P < 0.05, respectively). No significant correlation was found between CCT changes and SE changes at different time points, postoperatively.

CONCLUSIONS

CCTs decreased significantly at 1 day after surgery, and continued to decline at 1 week after surgery, then increased over time. From postoperative 1 week, SE over time continually shifted to the myopic side.

Age-related changes in central corneal thickness in normal eyes among the adult Lithuanian population

Background

The purpose of this study was to estimate mean central corneal thickness (CCT) and determine whether there are any correlations between CCT, age, and sex in the adult Lithuanian population.

Methods

A total of 1,650 Caucasians of Lithuanian origin (aged 18–89 years) comprising 688 (41.7%) men and 962 (58.3%) women were examined. Subjects were stratified by age into seven groups. CCT was measured using ultrasonic pachymetry. Correlations between CCT, age, and sex were sought.

Results

Mean (± standard deviation) CCT for both eyes was 544.6±30.5 μm. Mean CCT was 545.2±30.5 μm in the left eye and 544.6±30.5 μm in the right eye, and was 545.0±25.6 μm in men and 544.4±33.5 μm in women. Mean CCT was 550.8±35.7 μm in subjects aged 18–29 years, 557.5±27.6 μm in those aged 30–39 years, 551.3±31.4 μm in those aged 50–59 years, 544.0±31.4 μm in those aged 50–59 years, 544.2±31.6 μm in those aged 60–79 years, 535.1±27.8 μm in those aged 70–79 years, and 530.1±16.8 μm in those aged 80–89 years. No statistically significant difference in CCT was found between the sexes (P>0.05). However, there was a significant difference in subjects aged 18–29 years; men had higher CCT than women (P<0.05). A statistically significant negative correlation was found between CCT and age (r=-0.263, P<0.05) that was stronger in men (r=-0.406, P<0.05) than in women (r=-0.118, P<0.05).

Conclusion

The mean CCT in adult Lithuanians was 544.6±30.5 μm, of the left eye 545.2±30.5 μm and of the right – 544.6±30.5 μm. CCT of the right eye was equal to the CCT of both eyes. Mean CCT was 545.0±25.6 μm in men and 544.4±33.5 μm in women. Young men tended to have higher CCT than women. CCT decreases over the lifetime, meaning that older people have thinner corneas. CCT’s dependence on age is stronger in men.

Effects of inhaled fluticasone on intraocular pressure and central corneal thickness in asthmatic children without a family history of glaucoma

Purpose:

The aim of this study is to report the effects of fluticasone-inhaled corticosteroid on intraocular pressure (IOP) and central corneal thickness (CCT) of asthmatic children without a family history of glaucoma.

Materials and Methods:

In this prospective study, 93 children were divided into two groups: 69 asthmatic children with no family history of glaucoma who were taking inhaled fluticasone propionate 250 μg daily for at least 6 months (Group 1) and 24 age-matched control subjects without asthma (Group 2). Three measurements each, of IOP and CCT, were performed with a hand-held noncontact tonometer and a noncontact specular microscope, respectively, over a 12-week period. The order of IOP and CCT measured were randomized at each visit. Between-group comparison and the relationship between CCT and IOP measurements were investigated. P < 0.05 was statistically significant.

Results:

The mean age was 8 ± 2.4 years (range, 5–15 years) and 9 ± 2.9 years (range, 5–15 years) for Groups 1 and 2, respectively (P = 0.1337). The mean IOP was 14 ± 3.3 mmHg (range, 10–24 mmHg) and 14 ± 2.9 mmHg (range, 11–22 mmHg) for Groups 1 and 2, respectively (P = 0.3626). The mean CCT was 531 ± 30.1 μm (range, 467–601 μm) and 519 ± 47.0 μm (range, 415589 μm) for Groups 1 and 2, respectively (P = 0.1625). There was a weak but statistically significant correlation between IOP and CCT in Group 1 (Pearson's R = 0.3580, P = 0.0025).

Conclusions:

Inhaled fluticasone at the regular dose used in this study over a short period (6–24 months) was not associated with a significant effect on CCT and IOP measured with noncontact devices in asthmatic children between 5 and 15 years, without a family history of glaucoma. A weak correlation between IOP and CCT values in asthmatic children did exist.

Repeatability and interobserver reproducibility of Artemis-2 high-frequency ultrasound in determination of human corneal thickness

Background

The purpose of this study was to assess the repeatability and limits of agreement of corneal thickness values measured by a high-frequency ultrasound (Artemis-2), hand-held ultrasound pachymeter (DGH-500) and a specular microscope (SP-3000P).

Methods

Central corneal thickness (CCT) was analyzed in this prospective randomized study that included 32 patients (18 men and 14 women) aged 21–24 years. Measurements were obtained in two sessions, one week apart, by two examiners with three devices in a randomized order. Nine measurements were taken (three with each device) on one randomly selected eye of each patient in each measurement session. The coefficient of repeatability and interobserver reproducibility for the values of each method were calculated. The limits of agreement between techniques were also evaluated.

Results

There were no significant differences in CCT values between sessions for each of the three devices (P > 0.05). The repeatability coefficients for the Artemis-2 (±8 μm/±9 μm) were superior to those of the SP-3000P (±9 μm/±11 μm) and DGH 500 (±12 μm/±12 μm) in session 1/session 2 respectively, while the interobserver reproducibility index (differences between session 1 and session 2) was superior for the SP-3000P (±17 μm) with respect to DHG-500 (±29 μm) and the Artemis-2 (±31 μm). In session 1 and session 2, the limits of agreement between the techniques were 35 μm to −31 μm and 34 to −20 μm, respectively, for DGH-500 versus Artemis-2, 73 μm to 3 μm and 60 μm to 9 μm for Artemis-2 versus SP-3000P, and 58 μm to 22 μm and 72 μm to 10 μm for DGH-500 versus SP-3000P comparisons. The DGH-500 and Artemis-2 gave similar values (P > 0.05) in both sessions, but both (Artemis-2 and DGH-500) values were significantly greater than that of the SP-3000P (P < 0.05) in both sessions.

Conclusion

Repeatability was comparably good for the three techniques. However, interobserver reproducibility was approximately twice as good with the SP-3000P compared with the other two devices. The Artemis-2 CCT values consistently agreed with the DGH-500 and less so with the SP-3000P. The Artemis-2 provided CCT values that were, on average, 38 μm and 34 μm greater than that of the SP-3000P in session 1 and session 2, respectively.

Repeatability of intraoperative central corneal and residual stromal thickness measurement using a handheld ultrasound pachymeter

PURPOSE

To determine and compare the repeatability of intraoperative central corneal thickness (CCT) and residual stromal bed thickness measurements using a handheld ultrasound (US) pachymeter.

SETTING

London Vision Clinic, London, United Kingdom.

DESIGN

Comparative evaluation of a diagnostic test or technology.

METHODS

This study comprised eyes that had laser in situ keratomileusis retreatment by flap lift in which handheld US pachymetry (Corneo-Gage Plus 50 MHz) had been performed intraoperatively. In each case, 5 consecutive measurements were obtained centrally immediately before and after the flap was lifted. The within-eye repeatability was calculated as the standard deviation of the 5 repeated measurements for the CCT measurements and the central residual stromal thickness (RST) measurements.

RESULTS

The study evaluated 134 eyes (79 patients). The mean CCT was 467 μm ± 40 (SD) (range 393 to 577 μm). The repeatability of CCT measurements was 6.83 μm, the coefficient of repeatability was 13.40 μm, and the coefficient of variation (CoV) was 1.46%. The mean central RST was 335 ± 46 μm (range 259 to 465 μm). The repeatability of central RST measurements was 4.91 μm, the coefficient of repeatability was 9.62 μm, and the CoV was 1.46%.

CONCLUSION

The repeatability of intraoperative handheld US pachymetry was similar between measurements of CCT and measurements of central RST; the CoV was 1.46% in both cases.

Evaluation of a recently developed noncontact specular microscope in comparison with conventional pachymetry devices

PURPOSE

The study was conducted to assess the central corneal thickness (CCT) of the healthy cornea with a recently developed noncontact specular microscope (EM-3000; Tomey) and compare the results with those measured with a contact specular microscope and an ultrasound pachymeter. Agreement between measurements taken by 2 investigators was also studied.

METHODS

The right eyes of 41 healthy individuals who had negative history of contact lens wear, ophthalmic disease, or ocular surgery were examined. The CCT was determined sequentially with a noncontact specular microscope, a contact specular microscope (EM-1000; Tomey), and an ultrasound pachymeter (AL-2000; Tomey). Each evaluation with the specular microscopes was performed by 2 independent operators.

RESULTS

A significant difference was detected in pachymetry measurements among the 3 instruments (p=0.01; analysis of variance). The mean CCT values were lower measured with the ultrasound pachymeter (537+/-30 microm) than the contact endothelial microscope (543+/-37 microm, p=0.17, Student t-test) and the noncontact microscope (549+/-33 microm, p<0.0001) (operator 1). There was no statistically significant difference in CCT measurements between the 2 endothelial microscopes (p=0.19). We found significant correlations (p<0.0001) in thickness measurements between each pair of instruments (r=0.91, noncontact microscopy and ultrasound pachymetry; r=0.74, noncontact and contact microscopy; r=0.72, contact microscopy and ultrasound pachymetry; Spearman rank correlation).

CONCLUSIONS

The strong correlations among the 3 pachymetry devices suggest that the tested instruments provide reliable measurements; however, they cannot be used interchangeably.

Repeatability of layered corneal pachymetry with the artemis very high-frequency digital ultrasound arc-scanner

PURPOSE

To assess the three-dimensional repeatability of thickness measurements for epithelium, stroma, cornea, flap, and residual stromal bed using the Artemis very high-frequency (VHF) digital ultrasound arc-scanner (ArcScan Inc).

METHODS

Five consecutive measurements were obtained for 10 eyes of 10 patients 1 year after LASIK using the Artemis VHF digital ultrasound arc-scanner across the central 10-mm diameter of the cornea. Repeatability analysis was performed for thickness measurements for each corneal layer-epithelium, stroma, cornea, flap, and residual stromal bed. The standard deviation of repeated measurements (point-repeatability) was calculated for each measurement location in 0.1-mm steps for the 10×10-mm matrix. The pooled standard deviation of the point-repeatability for each measurement location within the central 1-, 2-, and 3-mm radius was calculated (region-repeatability). The corneal thickness of the baseline scan set was compared to that of subsequent scan sets within the same session and plotted over time to assess any possible hydration effects of the immersion technique.

RESULTS

The repeatability at the corneal vertex was 0.58 μm for epithelium, 1.78 μm for stroma, 1.68 μm for cornea, 1.68 μm for flap, and 2.27 μm for residual stromal bed. The region-repeatability within the central 1-mm radius was 1.01 μm for epithelium, 3.44 μm for stroma, 3.35 μm for cornea, 2.81 μm for flap, and 3.97 μm for residual stromal bed. The mean difference in corneal thickness from the baseline value was within 1.25 μm for each of the subsequent four scan sets over a 5-minute immersion period.

CONCLUSIONS

Layered pachymetry of the epithelium, stroma, cornea, flap, and residual stromal bed showed high repeatability with the Artemis VHF digital ultrasound arc-scanner. The high repeatability validates the use of the Artemis for in vivo layered pachymetry.

Corneal ablation depth readout of the MEL 80 excimer laser compared to Artemis three-dimensional very high-frequency digital ultrasound stromal measurements

PURPOSE

To evaluate the accuracy of the ablation depth readout for the MEL 80 excimer laser (Carl Zeiss Meditec).

METHODS

Artemis 1 very high-frequency digital ultrasound measurements were obtained before and at least 3 months after LASIK in 121 eyes (65 patients). The Artemis-measured ablation depth was calculated as the maximum difference in stromal thickness before and after treatment. Laser in situ keratomileusis was performed using the MEL 80 excimer laser and the Hansatome microkeratome (Bausch & Lomb). The Aberration Smart Ablation profile was used in 56 eyes and the Tissue Saving Ablation profile was used in 65 eyes. All ablations were centered on the corneal vertex. Comparative statistics and linear regression analysis were performed between the laser readout ablation depth and Artemis-measured ablation depth.

RESULTS

The mean maximum myopic meridian was -6.66±2.40 diopters (D) (range: -1.50 to -10.00 D) for Aberration Smart Ablation-treated eyes and -6.50±2.56 D (range: -1.34 to -11.50 D) for Tissue Saving Ablation-treated eyes. The MEL 80 readout was found to overestimate the Artemis-measured ablation depth by 20±12 μm for Aberration Smart Ablation and by 21±12 μm for Tissue Saving Ablation profiles.

CONCLUSIONS

The accuracy of ablation depth measurement was improved by using the Artemis stromal thickness profile measurements before and after surgery to exclude epithelial changes. The MEL 80 readout was found to overestimate the achieved ablation depth. The linear regression equations could be used by MEL 80 users to adjust the ablation depth for predicted residual stromal thickness calculations without increasing the risk of ectasia due to excessive keratectomy depth as long as a suitable flap thickness bias is included.

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.

Agreement of non-contact pachymetry after LASIK: comparison of combined scanning-slit/Placido disc topography and specular microscopy

PURPOSE

To assess the interchangeability of central corneal thickness (CCT) measurements between combined scanning-slit/Placido disc topography (Orbscan-II) and specular microscopy (Topcon SP-2000P) in patients who underwent laser in situ keratomileusis (LASIK) for myopia.

PATIENTS AND METHODS

We recruited 118 consecutive patients who underwent LASIK for myopia. The CCT was measured using Orbscan-II and Topcon SP-2000P randomly. Orbscan-II and Topcon SP-2000P CCT data were analysed using the paired-sample t-test and the limits of agreement (LoA) were calculated with the method described by Bland-Altman.

RESULTS

The average CCT measurements by Orbscan-II and Topcon SP-2000P were 447.55+/-49.78 microm and 461.38+/-35.35 microm, respectively (P<0.0001; mean difference, 13.83+/-22.31 microm; 95% confidence interval, 9.77-17.09 microm). The Bland-Altman plot showed an inverse association between the average and the difference between the devices: Topcon-Orbscan-II=174.23 -0.353xAverage (P<0.01). The widths of the crude and regression-based 95% limits of agreement were 87.45 and 63.72 microm, respectively.

CONCLUSIONS

Orbscan-II measurements of CCT after myopic LASIK were significantly lower than those obtained using Topcon SP-2000P. The limits of agreement between the two devices were too broad and, therefore, both pachymetric values cannot be used interchangeably. Further, the tendency towards comparably Orbscan-II readings in thinner corneas precludes that one technique can directly replace the other. This is important for the adequate medium- and long-term follow-up of the growing LASIK patient population.