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 Table of Contents  
Year : 2014  |  Volume : 107  |  Issue : 1  |  Page : 16-19

Comparative measurement of central corneal thickness using Pentacam, ocular response analyzer, and ultrasound pachymetry devices for normal, glaucomatous, and keratoconic eyes

1 Department of Ophthalmology, Al-Azhar University, Cairo, Egypt
2 Department of Kasr El-Eini, Cairo University, Cairo, Egypt

Date of Submission15-Feb-2013
Date of Acceptance15-Mar-2014
Date of Web Publication21-Jun-2014

Correspondence Address:
Mohamed A El-Malah
MD, 252 Terrat El-Gabal street, Hadayk El-Zaitoun, B.O. Box: 11321, Cairo
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2090-0686.134936

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The aim of the study was to compare central corneal thickness (CCT) measurements using Scheimpflug imaging (Pentacam), ocular response analyzer (ORA), and ultrasound pachymetry (USP) devices for normal, glaucomatous, and keratoconic eyes.
Patients and methods
This was a prospective study that enrolled 170 eyes of 85 patients: 86 normal eyes, 50 eyes diagnosed as primary open angle glaucoma (POAG), and 34 eyes with keratoconus (KC). The CCT was measured using three machines - USP, Pentacam, and ORA - at the same time by two expert operators. Microsoft Excel, the Student t-test, and analysis of variance test were used for statistical analyses.
The mean age of the three groups, normal, POAG and KC patients were 45 ± 17.7, 48 ± 7.8 and 26 ± 15.6 years respectively. CCT measurements using USP, Pentacam, ORA revealed: 552 ± 26.2 μ, 562 ± 23.3 μ and 569 ± 29.7 μ for normal eyes, 559 ± 13.4 μ, 570 ± 22.6 μ, and 587 ± 27.6 μ for POAG eyes, and 482 ± 17.7 μ, 505 ± 7.8 μ and 516 ± 8.5 μ for KC eyes respectively. It was noticed that, readings using USP shows reduced values in comparison to two other machines. USP and Pentacam were relatively comparable machines.
The measurements of CCT using ultrasound and Pentacam instruments had significant linear correlations with one another. Both methods had highly satisfactory measurement repeatability. Reading using ORA gives higher values.

Keywords: Ocular response analyzer, central corneal thickness, pentacam, ultrasound pachymetry, primary open angle glaucoma, keratoconus

How to cite this article:
El-Malah MA, Abo-Hussein N. Comparative measurement of central corneal thickness using Pentacam, ocular response analyzer, and ultrasound pachymetry devices for normal, glaucomatous, and keratoconic eyes. J Egypt Ophthalmol Soc 2014;107:16-9

How to cite this URL:
El-Malah MA, Abo-Hussein N. Comparative measurement of central corneal thickness using Pentacam, ocular response analyzer, and ultrasound pachymetry devices for normal, glaucomatous, and keratoconic eyes. J Egypt Ophthalmol Soc [serial online] 2014 [cited 2020 Jul 15];107:16-9. Available from: http://www.jeos.eg.net/text.asp?2014/107/1/16/134936

  Introduction Top

The well-recognized impact of central corneal thickness (CCT) and Goldmann applanation tonometry led to the development of new tonometric methods designed to minimize the errors in intraocular pressure (IOP) measurements induced by corneal thickness [1],[2],[3].

Accurate measurements of CCT have assumed significance in ophthalmic practice. It is important in corneal refractive surgery as well as in glaucoma practice to modify the IOP readings for accuracy [4],[5],[6].

Nowadays, CCT is considered a possible explanation for glaucoma cases where clinical findings do not match [7]. A positive correlation between IOP readings and corneal thickness was found in most of the glaucoma patients [8],[9],[10],[11].

CCT has been recognized as a significant risk factor for progression of ocular hypertension to primary open angle glaucoma (POAG). The ocular hypertension study group was the first to prospectively demonstrate that a thinner CCT predicts the development of POAG in ocular hypertension patients [12],[13].

Scheimpflug Pentacam imaging device uses a rotating camera to image the anterior segment of the eye. It is a noncontact instrument that provides, in a single scan, anterior segment imaging, anterior and posterior corneal topography, and corneal pachymetry. Pentacam can obtain corneal thickness from the central 8-10 mm of the cornea using rotational slit-scanning principle [14],[15].

Ultrasound pachymetry (USP) is currently the most often used method for the measurement of CCT. This method has been reported to have a high degree of intraoperator reproducibility [16],[17].

Although hand-held ultrasound-based systems offer the advantages of high probability and relative ease of use, they experience relatively higher interobserver variability, possibly as a result of difficulties in centration and alignment [18].

A commercially available clinical instrument called ocular response analyzer (ORA) has been proposed to characterize corneal biomechanical responses and CCT using the noncontact tonometry process [19],[20].

The aim of this study was to evaluate the CCT measurement using three different instruments in normal, POAG, and keratoconic eyes and to assess the correlation between them.

  Patients and methods Top

This prospective study was carried out on 170 eyes of 127 patients, in the period from Nov. 2011 to Jan. 2013. CCT was measured for all eyes using three different methods, Pentacam (Oculus Inc.,Wetzlar, Germany), ORA (ORA; Reichert Ophthalmic instruments, Buffalo, NY, USA) and USP (Tomey SP-3000, Tomey Ltd, Nagoya, Japan).

Any patient with an ocular disease other than POAG (in its group) or keratoconus (in its group) was excluded from the study in addition to the patients with previous ocular surgery, such as cataract surgery, LASIK, or other surgeries, or patients wearing contact lens.

Patients included in this study were classified into three groups: the normal eyes group included 86 eyes of 43 patients; the POAG eyes group included 50 eyes of 50 patients; and the keratoconic eyes group included 34 eyes of 34 patients.

Complete eye examination was performed for all eyes, including visual acuity assessment, refraction, slit-lamp examination including gonioscopy, IOP measurement with applanation tonometer, and fundus examination. Thereafter, measurement of CCT was carried out by the three different above-mentioned methods, starting with the noncontact techniques.

When using Pentacam to reduce operator-dependent variables, Pentacam's automatic release mode was used. As CCT is measured repeatedly during the rotational imaging process (in each of the images), very precise detection of CCT can be achieved.

Corneal assessment including corneal biomechanics and CCT was performed using ORA, At least four readings with acceptable signal waveforms were taken; wave score not less than 6.5 was considered acceptable.

The final assessment of CCT was performed using USP. The readings were taken by aligning the probe as perpendicularly as possible at the center of the cornea. Five readings were taken, and then the mean values were used for assessment. Statistical analyses were performed using the SPSS computer program for Windows (SPSS Inc., Chicago, Illinois, USA). All patients were informed about the purpose of the study.

  Results Top

In this prospective study, 170 eyes of 127 patients were enrolled, including 61 male patients and 66 female patients. The mean age was 45 ± 17.7 years (range 21-62 years) in the normal eyes group, 48 ± 7.8 years (range 34-61 years) in the POAG group, and 26 ± 15.6 years (range 17-41 years) in the keratoconic group.

We examined both eyes of the normal group - 86 eyes of 43 patients, one eye of 50 patients in the POAG group, and one eye of 34 patients of the keratoconic eyes group. [Figure 1] shows the CCT distribution for all eyes in the three groups.
Figure 1:

Click here to view

As demonstrated in [Table 1], it was noticed that SD of all measurements was higher in the ORA group than in the USP and Pentacam groups. ORA measurements were in an average of 17 μm higher than USP and 7 μm higher than Pentacam in the normal eyes group. In addition, ORA measurements were in an average of 28 μm higher than USP and 17 μm higher than Pentacam in the POAG eyes group. In the keratoconic eyes group, ORA measurements were 34 μm higher than USP and 11 μm higher than Pentacam.
Table 1: Demographic data and results of mean ± SD of central corneal thickness in the three groups

Click here to view

Hence, in all eyes groups, especially in diseased eyes, we always found that the readings were higher with ORA than with the other two machines by about a mean of 31 μm (average 28-34 μm). However, in normal eyes it was nearly half of this value - mean of 17 μm.

  Discussion Top

Corneal thickness is useful in diagnosing and monitoring corneal diseases such as corneal edema or ectasia. In addition, CCT plays a major role in refractive surgery as well as in IOP evaluation and glaucoma progression in patients with ocular hypertension [5],[6],[7],[8],[9].

USP has been considered since many years as the gold standard machine for detection of CCT [14],[15],[16],[17]. However, new methods such as Pentacam and ORA have the great advantages of noncontact.

To the best of our knowledge, this is the first study on the evaluation of ORA with respect to CCT measurement and to compare it with the other standard machines including USP and Pentacam.

As demonstrated in [Table 1] and [Figure 1], we found that ORA always gives higher readings than USP and Pentacam. Readings of CCT with Pentacam revealed higher values than those with USP in all three groups of eyes. These results were comparable with other results of many literatures [21],[22],[23],[24].

CCT measurement obtained with standard USP varies from 542-550 μm. In this study, CCT measured with USP nearly equal this average in normal eyes (552 μm) as concluded by Piotrowiak et al. [25]. By using Pentacam, in our study CCT was 562 μm in normal eyes, which is little higher in comparison to other studies [26],[27].

No comparable results can be used for ORA results, but on analysis of our data it was found that most of the readings of this machine were higher, especially in diseased eyes such as POAG and keratoconic eyes.

Piotrowiak et al. [25] in their study concluded that USP causes tear film dislocation and epithelium compression, resulting in CCT measurements that are lower by 7-30 μm. The results of our study were comparable with respect to CCT measurements using USP and Pentacam.

With respect to the two standard methods of CCT measurements, the USP and Pentacam, the difference was about 10 μm in the normal eyes group. These results were also comparable with nearly same differences obtained by other authors [28],[29],[30].

  Conclusion Top

The most important finding of this study is that, CCT measurements were comparable between USP and Pentacam in normal and diseased eyes. But, ORA machine always gives higher readings than these two standard machines. It can be documented that ORA is more specific in corneal biomechanical evaluation that CCT measurements.

  Acknowledgements Top

Conflicts of interest

There are no conflicts of interest.

  References Top

1.Ehlers N, Bramsen T, Sperling S. Applanation tonometry and CCT. Acta Ophthalmol (Copenh) 1975; 53:34-43.  Back to cited text no. 1
2. Whitcare MM, Stein RA, Hassanein K. The effects of corneal thickness on applanation tonometry. Am J Ophthalmol 1993; 115:592-596.  Back to cited text no. 2
3. Kohlhaas M, Boehm AG, Spoerl E, Pursten A, Dipl-Ing FH, Grein HG, Pillunat LE. Effects of CCT, corneal curvature and axial length on applanation tonometry. Arch Ophthalmol 2006; 124:471-476.  Back to cited text no. 3
4. Price FW, Koller DL, Price MO. Corneal thickness pachymetry in patients undergoing LASIK. Ophthalmology 1999; 106:2216-2220.  Back to cited text no. 4
5. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular measures: a review and meta-analysis approach. Surv Ophthalmol 2000; 44:367-408.  Back to cited text no. 5
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7. European Glaucoma Society. Terminology and guidelines for glaucoma. European Glaucoma Society. 2nd ed. 2-2:Savona: Dogma; 2003. 2-5.  Back to cited text no. 7
8. Shah S, Chatterjee A, Mathai M, Kelly SP, Kwartz J, Henson D, Mclead D. Relationship between corneal thickness and measured IOP in a general ophthalmology clinic. Ophthalmology 1999; 106: 2154-2160.  Back to cited text no. 8
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10.1Hansen FK. A clinical study of the normal human CCT. Ophthalmologica (Copenh) 1971; 49:82-89.  Back to cited text no. 10
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20.2Luce DA. Determining in vivo biomechanical properties of the cornea with ORA. J Cataract Refract Surg 2005; 31:156-162.  Back to cited text no. 20
21.2Ou T-H, Lai I-C, Teng M-C. Comparison of CCT measured by USP, Orbscan II and SP3000P in eyes with glaucoma or glaucoma suspect. Chang Gung Med J 2012; 35:255-262.  Back to cited text no. 21
22.2Liu Z, Huang AJ, Pflugfelder SC. Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system. Br J Ophthalmol 1999; 83:774-778.  Back to cited text no. 22
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24.2Piotrowiak I, Soldanska B, Burdukt M, Kaluzny BJ, Kaluzny J. Measuring corneal thickness with slit-OCT, the Scheimpflug system and USP. ISRN Ophthalmol 2012; 1:1-5.  Back to cited text no. 24
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26.2Yekta AA, Hashemi H, Khbazkhoob M, Dostdar A, Mehrovaran J, Fotouhi A. Comparison of CCT measurement with Pentacam, Orbscan II and USP. Iranian J Ophthalmol 2009; 21:51-57.  Back to cited text no. 26
27.2Ucakhan OO, Ozkan M, Kanplot A. Corneal thickness measurements in normal and keratokonic eyes: Pentacam comprehensive eye scanner versus non-contact specular microscopy and USP. J Cataract Refract Surg 2006; 32:970-977.  Back to cited text no. 27
28.2Fujioka M, Nakamura M, Tatsumi Y, Kusuhara A, Maeda H, Negi A. Comparison of Pentacam Scheimpflug camera with USP and non-contact specular microscope in measuring CCT. Curr Eye Res 2007; 32:89-93.  Back to cited text no. 28
29.2Lam AK, Chen D. Pentacam pachymetry: comparison with non-contact specular microscope on the central cornea and inter-session repeatability on the peripheral cornea. Clin Exp Optom 2007; 90:108-114.  Back to cited text no. 29
30.3Kotecha A, Elsheikh A, Roberts CR, et al. Corneal thickness and age related biomechanical properties of the cornea measured with ORA. Invest Ophthalmol Vis Sci 2006; 47:2337-2347.  Back to cited text no. 30


  [Figure 1]

  [Table 1]


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