Corneal thickness measurement by confocal microscopy, ultrasound, and scanning slit methods

PURPOSE: To measure corneal thickness by using a calibrated confocal microscope and to compare this measurement to thickness determined by ultrasonic and noncontact scanning slit pachymetry. DESIGN: Comparison of corneal thickness measured by using four instruments in normal subjects. METHODS: Thickness measured by a clinical confocal microscope (Tandem Scanning) was calibrated from measurements of polymethylmethacrylate contact lenses with known thickness. Corneal thickness was measured in one eye of 24 normal subjects by using this instrument, two ultrasonic pachymeters (DHG-1000 and Sonogage), and a noncontact optical scanning slit pachymeter (Orbscan II). RESULTS: Mean corneal thickness measured by confocal microscopy was 516 +/- 30 microm (+/-SD). This was less than the mean thickness measured by both ultrasonic pachymeters, 554 +/- 28 microm by the DGH, and 555 +/- 28 microm by the Sonogage (P <.001). Thickness measured by the Orbscan II pachymeter was 540 +/- 35 microm (P <.001, compared with either confocal or ultrasound) after applying an "acoustic factor" of 0.92, a default correction of the software. CONCLUSION: Corneal thickness measured by calibrated confocal microscopy is approximately 39 microm (7.0%) less than thickness measured by two commonly used ultrasonic pachymeters and approximately 24 microm (4.4%) less than thickness measured by the corrected Orbscan II pachymeter. These differences are important for planning and measuring the effects of refractive and other surgical procedures. The precision of confocal microscopy is limited by corneal motion in an anterior-posterior direction. The difference between instruments suggests that verification of clinical ultrasonic pachymeters should be revisited.     

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.     

In vivo pachymetry in normal eyes of rats,mice and rabbits with the optical low coherence reflectometer

The aim of this study was to determine the central corneal thickness (CCT) in living rats, mice and rabbits using a non-contact, high-speed optical low coherence reflectometer (OLCR) mounted on a regular slit lamp. Both eyes of eight male Wistar rats, eight male balb-c mice and eight male Japanese rabbits were measured. Each eye was measured twice (one measurement consists of 20 scans), the average calculated. Additionally, CCT was measured in rabbits using an ultrasound pachymeter. The mean CCT was: RATS: 159.08 microm (SD+/-14.99 microm), MICE: 106.0 microm (SD+/-3.45 microm) and RABBITS: 356.11 microm (SD+/-14.34 microm). With the use of OLCR we were able to accurately measure the CCT of rats, mice and rabbits in vivo. This technique may prove useful in further refractive, pharmacological and glaucoma studies.     

Quantitative anatomical differences in central corneal thickness values determined with scanning-slit corneal topography and noncontact specular microscopy

PURPOSE: This study was designed to analyze the differences in central corneal thickness values determined with noncontact specular microscopy and scanning-slit corneal topography. The measurements were performed on the same eye. METHODS: We analyzed the central corneal thickness values of 93 patients (n = 93) by means of noncontact specular microscopy (Topcon SP-2000P noncontact specular microscope, Topcon Corp., Tokyo, Japan) and scanning-slit corneal topography (Orbscan Topography System II, Orbscan Inc., Salt Lake City, UT). One experienced physician performed 3 consecutive central corneal thickness measurements with both devices. RESULTS: The central corneal thickness values obtained by means of Orbscan pachymetry were 17 +/- 2.7 (range, 12-24) microm greater. A significant correlation was observed between scanning-slit corneal topography and noncontact specular microscopy (Pearson correlation coefficient, r = 0.976; P < 0.001). CONCLUSIONS: Researchers should know of the existence of this difference between noncontact specular microscopy and Orbscan pachymetry when interpreting central corneal thickness values.     

Comparison of corneal thickness measurements using ultrasound and Orbscan slit-scanning topography in normal and post-LASIK eyes.

PURPOSE: To compare corneal thickness measurements made by ultrasonic and slit-scanning techniques in normal eyes and in eyes after laser in situ keratomileusis (LASIK). SETTING: Corneal Diseases and Excimer Laser Research Unit, University of Dundee, Dundee, Scotland. METHODS: Central corneal thickness (CCT) was measured in 101 eyes of 59 normal subjects and in 30 eyes of 21 post-LASIK patients. Measurements were made with an Orbscan slit-scanning elevation topographer and immediately afterward with an ultrasound pachymeter. RESULTS: The difference in mean CCT between ultrasound (538.0 microm +/- 36.7 [SD]) and Orbscan (566.6 +/- 40.7 microm) pachymetry was statistically significant (P <.001) in the normal eyes; the Orbscan measurement was approximately 28 microm higher than that of the ultrasound pachymeter. The difference in mean CCT between the ultrasound and the slit-scanning techniques was also statistically significant in the post-LASIK eyes (mean values 475.3 +/- 50.3 microm and 461.9 +/- 74.2 microm, respectively; P <.0001). Differences in CCT in individual subjects were much more variable in the post-LASIK eyes than in the normal eyes. The Bland and Altman method for assessing clinical agreement between 2 instruments showed that in 95% of cases, the CCT measurements with both instruments would be within 65 microm in normal eyes and 150 microm in post-LASIK eyes. CONCLUSION: Central corneal thickness measurements were, on average, 28 microm higher with the Orbscan than with the ultrasound pachymeter in normal eyes and 13 microm lower in post-LASIK eyes. The degree of variability within each group indicated that these 2 techniques are not clinically comparable, precluding interchangeable use of their data in planning or assessing corneal surgery.     

Scanning-slit and specular microscopic pachymetry in comparison with ultrasonic determination of corneal thickness

PURPOSE: To determine the central corneal thickness values in healthy eyes with the recently developed Orbscan scanning-slit system, contact and noncontact specular microscopic pachymetry and compare the results to conventional ultrasonic pachymetry. METHODS: In the following sequence, Orbscan, Topcon SP-2000P noncontact specular microscope, AL-1000 ultrasound, and Tomey contact specular microscope were used to record thickness values. Thirty-four healthy right corneas of 34 healthy subjects were investigated. RESULTS: Orbscan pachymetry correlated significantly with ultrasound (r = 0.64, p < 0.001), contact (r = 0.45, p < 0.001), and noncontact specular microscopy (r = 0.72, p < 0.001). Likewise, the Topcon SP-2000P noncontact specular microscopy pachymetry disclosed similar statistical results compared with ultrasound (r = 0.88, p < 0.001), and contact specular microscopy pachymetry (r = 0.76, p < 0.001). The mean central corneal thickness results were significantly higher ( p < or = 0.01) than ultrasonic values (580 +/- 43 microm) using the contact specular microscope (640 +/- 43 microm) or Orbscan system (602 +/- 59 microm) but were significantly lower ( p < 0.001) using the noncontact specular microscope (547 +/- 49 microm). CONCLUSIONS: The results indicate that the devices tested cannot be simply used interchangeably. For long-term patient follow-up, one specific instrument is recommended. Recently developed pachymetry machines are especially helpful when additional corneal data such as thickness profile, elevation maps, anterior chamber depth, and endothelial morphology are required.     

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.     

Corneal thickness measurements with contact and noncontact specular microscopic and ultrasonic pachymetry

PURPOSE: To evaluate the central corneal thickness values in normal and postkeratoplasty corneas with the new Topcon SP-2000P noncontact specular microscopic, contact specular microscopic, and the "common standard" ultrasonic pachymetry. METHODS: Central corneal thickness was determined in 119 eyes of 81 patients (73 normal eyes of 44 patients and 46 eyes after penetrating keratoplasty) first with a noncontact specular microscopic (Topcon SP-2000P; Topcon Corporation, Tokyo, Japan), then an ultrasonic (AL-1000; Tomey, Erlangen, Germany), and finally with a contact specular microscopic (EM-1000; Tomey, Erlangen, Germany) pachymetry two times each by the same investigator. RESULTS: Reliability of the central corneal measurements was equally high both in normal and in postkeratoplasty corneas with all of the instruments (Cronbach alpha = 0.99). Noncontact specular microscopic corneal thickness determination correlated significantly both with ultrasonic (r =.86, P <.0001) and contact specular microscopic pachymetry (r =.62, P <.0001). The ultrasonic pachymetry correlated well with the Tomey pachymetry (r =.69, P <.0001). The Topcon normal mean central corneal thickness value (542 +/- 46 microm) was 28 +/- 4 microm lower (P <.0001) compared with the ultrasonic data (570 +/- 42 microm), which was 68 +/- 1 microm lower (P <.0001) compared with Tomey thickness (638 +/- 43 microm). CONCLUSIONS: Central corneal thickness measurements with noncontact specular microscopic, contact specular microscopic, and ultrasonic pachymetry demonstrate that each of the instruments is reliable but cannot be simply used interchangeably.     

Corneal thickness and endothelial cell density measured by non-contact specular microscopy and pachymetry in Rhesus macaques (Macaca mulatta) with laser-induced ocular hypertension

PURPOSE: Sustained increase in intraocular pressure (IOP) in humans results in a loss of corneal endothelial cells and an increase of corneal thickness. The effects of chronically elevated IOP on the corneal endothelium of monkeys with laser-induced ocular hypertension, a commonly used animal model of human glaucoma have not been documented. This study examined the central corneal thickness (CCT), the corneal endothelial cell density (ECD), and the corneal endothelial cell size (ACS) in Rhesus monkeys with experimental ocular hypertension.Materials and methods. Ten male monkeys with argon laser-induced ocular hypertension in one eye for an average duration of 2.4+/-0.7 years, were sedated with ketamine hydrochloride, and the CCT, ECD, and ACS measured at the center of the cornea of both eyes with a Topcon SP-2000P non-contact specular microscope (Topcon America Corporation((R)), Paramus, NJ, USA). CCT was also measured using a DHG-500 Pachette ultrasonic pachymeter (DHG Technology Inc., Exton, PA, USA). Mean and standard deviation (S.D.) of CCT, ECD and ACS for each eye was calculated and statistically compared.Results. Mean CCT in the hypertensive and normal eyes measured by specular microscopy was 0.477+/-0.023mm and 0.468+/-0.020 mm, respectively. Mean ECD in the hypertensive and normal eyes was 2601.7+/-631.8 and 3990.2+/-402.9 cells mm(-2), respectively. The mean size of the endothelial cells was 252.4+/-23.9 micro m(2) in the normal eye and 408.7+/-115.0 microm m(2) in the hypertensive eye. No significant difference in the measurement of CCT was observed between the specular microscope and the pachymeter (p=0.46).No significant difference in the mean CCT was observed between the two eyes (p=0.4820), whereas the mean ECD was significantly lower in the hypertensive eye than in the normal eye (p<0.001). The ECD was inversely related to the length of IOP elevation (p<0.001). CONCLUSIONS: No difference in the corneal thickness measurement was observed between the specular microscopy and the pachymetry techniques. Chronic ocular hypertension did not significantly affect the CCT, but caused a significant loss of endothelial cells in the center of the cornea of the laser treated eyes compared to the normotensive eyes. The duration of elevated IOP was the most important factor affecting the ECD.     

Central corneal thickness in individuals with intellectual disabilities

PURPOSE: To assess central corneal thickness (CCT) values in individuals with intellectual disabilities (ID). METHODS: The study group was made up of 25 participants with ID (mean age, 36.9 +/- 8.7 years). The control group was made up of 25 healthy individuals (mean age, 37.1 +/- 10.1 years) with normal intellectual capacity and without any systemic or intraocular pathology. CCT value was measured by ultrasound pachymetry. Ten consecutive measurements were made at the center of the cornea of each eye. RESULTS: In the ID group, mean CCT value was 554.0 +/- 39.7 microm in the right eye and 556.8 +/- 38.7 microm in the left eye. In the control group, mean CCT value was 535.7 +/- 24.2 microm in the right eye and 536.5 +/- 24.8 microm in the left eye. CCT value in the ID group was significantly greater than in the control group for both right (P < 0.05) and left eyes (P < 0.02). CONCLUSIONS: CCT should be kept in mind during measurements of intraocular pressure (IOP) in individuals with ID because their CCTs may be greater than those in the general population.     

Comparison of optical coherence reflectometry and ultrasound central corneal pachymetry

In 50 eyes of 25 patients we prospectively measured the central corneal thickness (CCT) comparing the OLCR (Optical Low Coherence Reflectometry) pachymeter with the contact ultrasound pachymeter. The OLCR system was mounted on to a Haag-Streit slit lamp. Every single measurement was the result of 5 scans. With the contact ultrasound Sonomed pachymeter we performed 5 separate measurements and calculated the mean. The correlation between the two measurements was excellent (r = 0.99). The mean standard deviation (SD) of the measurements taken with the non-contact OLCR pachymeter was significantly lower than with the contact ultrasound pachymeter, 0.49 microm and 4.71 microm respectively (p < 0.01). The variability of the CCT measurements taken with the non-contact OLCR pachymeter is significantly lower than the variability of the CCT measurements taken with the contact ultrasound pachymetry.     

Evaluating central corneal thickness measurements with noncontact optical low-coherence reflectometry and contact ultrasound pachymetry

PURPOSE: To compare the central corneal thickness (CCT) measurements obtained with noncontact optical low-coherence reflectometry (OLCR) and ultrasound (US) pachymetry. DESIGN: Prospective, comparative observational study. METHODS: Three sequential ultrasonic measurements and a set of five OLCR scans of 52 eyes of 26 healthy subjects were recorded. Noncontact measurement was repeated five minutes after anesthetic drop instillation. RESULTS: Mean CCT values for noncontact OLCR and US pachymetry were 544.03 microm and 548.66 microm, respectively, with mean SDs of 0.97 microm and 4.63 microm, respectively. Noncontact OLCR measured on average 4.64 microm less than US pachymetry (95% confidence interval -7.56 to -1.72; P = .003). The OLCR measured 1.68 microm less than US pachymetry in the thinner cornea group (< or =548.7 microm, n = 24) and 7.48 mum less in the thicker group (n = 25). This difference was statistically significant (P = .04). CONCLUSIONS: There was agreement between the two pachymetric measurements. Noncontact OLCR appeared to measure slightly smaller than US pachymetry.     

Corneal endothelial cell density and pachymetry measured by contact and noncontact specular microscopy

PURPOSE: To determine the endothelial cell density and thickness of normal human and postkeratoplasty corneas with contact specular microscopy and to compare these measurements with those obtained by noncontact specular microscopy. SETTING: Department of Ophthalmology, University of Erlangen-Nürnberg, Erlangen, Germany.METHODS: The central corneal endothelial cell density and thickness were determined in 65 healthy eyes of 39 patients with a mean age of 71 years +/- 12 (SD) and in 50 corneal grafts of 41 patients with a mean age 53 +/- 17 years using noncontact (Topcon SP-2000P, Topcon Corp.) and contact (EM-1000, Tomey) specular microscopes. Appropriate conversion factors were used for accurate cell count comparison. RESULTS: The mean cell count of the normal corneas was 2445 +/- 425 cells/mm(2) measured by noncontact specular microscopy and 2471 +/- 393 cells/mm(2) measured by contact specular microscopy (P =.70). After penetrating keratoplasty, the mean cell density was 1610 +/- 499 cells/mm(2) and 1584 +/- 469 cells/mm(2), respectively (P =.88). Significantly lower thickness was measured with the noncontact specular microscope than by contact pachymetry in normal eyes (543 +/- 46 micro m and 642 +/- 42 micro m, respectively) and postkeratoplasty eyes (538 +/- 61 micro m and 627 +/- 48 micro m, respectively) (P <.0001). CONCLUSION: To determine endothelial cell density, contact and noncontact specular microscopy may be used interchangeably. However, for the combined measurement of endothelial cell density and pachymetry, the use of the same specular microscope is recommended for long-term patient follow-up.     

Comparison of central corneal thickness measurements by specular microscopy,ultrasound pachymetry,and ultrasound biomicroscopy

PURPOSE: To compare the reproducibility and mean values of central corneal thickness (CCT) obtained by specular microscopy, ultrasound pachymetry, and ultrasound biomicroscopy (UBM). SETTING: Department of Ophthalmology, University of Toronto, Toronto, Ontario, Canada. METHODS: Thirty-one healthy volunteers were recruited for a sample size of 62 eyes. All subjects had pachymetric measurements by specular microscopy, ultrasound pachymetry, and UBM. Three separate measurements meeting criteria for centrality and perpendicularity were recorded for each eye. RESULTS: The mean CCT by specular microscopy was 572 microm (95% confidence interval (CI), 566-578 microm), which was significantly greater than 550 microm (95% CI, 545-556 microm) (P<.001) and 555 microm (95% CI, 550-560 microm) (P<.001) by ultrasound pachymetry and UBM, respectively. The mean standard deviation (SD) of repeated measurements by specular microscopy was 7.82 microm, which was significantly greater than the mean SDs of 4.14 microm (P<.001) and 3.90 microm (P<.001) by ultrasound pachymetry and UBM, respectively. There was no statistically significant difference between the mean SDs by ultrasound pachymetry and UBM (P=.156). CONCLUSIONS: Although the CCT measurements by specular microscopy were significantly less reproducible than those by ultrasound pachymetry and UBM, the error levels were clinically acceptable. Both ultrasound pachymetry and UBM produced similar CCT measurements, which were significantly less than those generated by specular microscopy. One should be aware of the advantages and limitations of each machine and of possible differences in the CCT measurements by optical and ultrasound pachymetry.     

Comparison of central corneal thickness measurements with a new optical device and a standard ultrasonic pachymeter

PURPOSE: To compare central corneal thickness (CCT) values obtained with ultrasonic pachymetry and a new optical method using partial coherence interferometry (PCI). SETTING: Department of Ophthalmology, Medical Health and Science Center, University of Debrecen, Debrecen, Hungary. METHODS: The study comprised 136 eyes of 70 patients whose spherical refractive error was not greater than +/-6.0 diopters (D) and whose keratometric astigmatism was not greater than 2.0 D. Central corneal thickness was measured 5 times with a new optical device (ACMaster, Zeiss) and with an ultrasonic pachymeter (AL-2000, Tomey). All measurements were obtained by the same investigator. RESULTS: Mean CCT was 531.2 microm +/- 3.9 (SD) with PCI and 547.8 +/- 36.0 microm with the ultrasonic device. The difference between groups was significant (P = .001). There was no difference between CCT values measured in right and left eyes (P = .55) with ultrasonography and PCI (P = .67). The coefficient variation was 0.73% for PCI and 6.5% for ultrasonography. Correlation between the CCT measurements with both devices was strong and statistically significant (Spearman correlation = .91, P = .001). CONCLUSIONS: Mean CCT values measured by the PCI method were significantly smaller than those measured by the ultrasonic device. Central corneal thickness measured with PCI is more reproducible and seems to be more reliable than that measured by ultrasonography.     

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.     

Repeatability of central corneal thickness measurements measured with the Topcon SP2000P specular microscope

Background The non-contact specular microscope has become the method of choice for a quick, accurate and non-invasive assessment of central corneal thickness (CCT), which is an important variable to monitor before and after refractive surgery. The consistency of the results produced by such widely used methods/equipment must be assessed to determine their reliability. The purpose of this study was to assess within- and between-observer repeatability of, and to determine if a systematic bias exists in the measurements made by, the Topcon SP2000P specular microscope.Methods The CCT of the right eyes of 70 adult subjects, divided equally between men and women, was assessed on two separate occasions (4–7 days apart) by each of two examiners using the low-intensity auto mode of the SP2000P specular microscope.Results The average CCT values for men and women, measured by one observer, were 0.52±0.03 mm (mean ± SD) and 0.52±0.04 mm, respectively. The average for the entire sample was 0.52±0.04 mm. Within- and between-observer repeatability were assessed by plotting the mean difference (for each subject) between two readings made by the same observer or one each by both observers against the combined average CCT reading of both sessions; the mean difference between two sets of observations was not significantly different from zero (P<0.05). For the first observer, the 95% limits of repeatability were between –0.015 and 0.017 mm. For the second observer, the 95% limits of repeatability were between –0.018 and 0.018 mm. For the between-observer repeatability, the 95% limits of agreement were between –0.016 and 0.016 mm. For both within- and between-observer repeatability, we found no systematic bias of the mean difference with the average CCT reading.Conclusion The within- and between-observer limits of agreement we found were similar to those previously reported for the Topcon SP2000P specular microscope; the range of the 95% limits of repeatability were within ±1 SD of the average CCT reading for both sessions. We suggest that a technique be considered reliable if: (1) the mean difference between two measurements does not vary significantly from zero, (2) there is no systematic bias of the mean difference with the magnitude of the measured quantity and (3) the error inherent in a measurement technique is within ±1 SD of the average measurement of the two sessions.     

Comparison of corneal thickness measurements using Orbscan II, non-contact specular microscopy, and ultrasonic pachymetry in eyes after laser in situ keratomileusis

AIMS: To compare central corneal thickness measurements of three pachymetry devices in eyes after laser in situ keratomileusis (LASIK). METHODS: Central corneal thickness was measured in 203 eyes after myopic LASIK. Orbscan II scanning slit topography (Bausch & Lomb), SP-2000P non-contact specular microscopy (Topcon), and ultrasonic pachymetry (Tomey) were used in this sequence. RESULTS: Three devices gave significantly different corneal thickness readings (p<0.0001, repeated measure analysis of variance). The measurements of Orbscan II (445.6 (SD 60.0) microm) were significantly smaller than those of noncontact specular microscopy (467.9 (SD 40.2) micro m; p<0.0001, Tukey multiple comparison) and ultrasonic pachymetry (478.8 (SD 41.9) microm; p<0.0001). The value obtained with SP-2000P non-contact specular microscopy was significantly smaller than that taken with ultrasonic pachymetry (p<0.001). There were significant linear correlations between scanning slit topography and non-contact specular microscopy (Pearson's correlation coefficient r = 0.912, p<0.0001), non-contact specular microscopy and ultrasonic pachymetry (r = 0.968, p<0.0001), and ultrasonic pachymetry and scanning slit topography (r = 0.933, p<0.0001). CONCLUSION: In post-LASIK eyes, Orbscan II scanning slit topography significantly underestimated corneal thickness. Non-contact specular microscopy gave smaller thickness readings than ultrasonic pachymetry, but these two units showed an excellent linear correlation.     

Comparison of ultrasound pachymetry and partial coherence interferometry in the measurement of central corneal thickness

PURPOSE: To compare central corneal thickness (CCT) measurements obtained with 3 ultrasound pachymeters and with partial coherence interferometry (PCI) and evaluate the effect of repeated contact by a pachymetry probe on corneal thickness. SETTING: Department of Ophthalmology, University of Vienna, Vienna, Austria. METHODS: Central corneal thickness was measured in 20 eyes of 20 healthy volunteers with 3 different ultrasound pachymeters (DGH 500, DGH Technology Inc.; SP 2000, Tomey Inc.; Paxis, Biovision Inc.) and with PCI. In each eye, 5 measurements with PCI were followed by 5 measurements with each ultrasound pachymetry device and another 5 measurements with PCI. RESULTS: The mean CCT measured with the DGH 500, SP 2000, and Paxis was 541.0 microm, 539.2 microm, and 545.1 microm, respectively. Although the differences among the 3 ultrasound pachymetry devices were within 6.0 microm, they were statistically significant (P<.05). The PCI measurements were significantly smaller (P <.05) than the ultrasound measurements: 518.8 microm before and 517.5 microm after repeated contact with the ultrasound pachymetry probe (P <.05). The mean precision (standard deviation) was 0.77 microm for PCI and between 2.40 microm and 3.58 microm for ultrasound pachymetry measurements. The correlation coefficients for the intraobserver variability were 0.999 for PCI and between 0.987 and 0.995 for ultrasound pachymetry measurements. CONCLUSIONS: Partial coherence interferometry was the more precise method of measuring CCT and had better intraobserver variability. Repeated contact by a pachymetry probe reduced corneal thickness by 1.3 microm. However, the reason for the smaller measurements with PCI than with ultrasound pachymetry remains unclear.     

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.