Introduction

The measurement of central corneal thickness (CCT) has become increasingly important in the pre- and postopera-tive management of corneal surgical procedures, such as laser in situ keratomileusis (LASIK) and Epipolis LASIK (Epi-LASIK). Ultrasonic pachymetry is currently the most commonly used technique to evaluate CCT. However, that technique entails direct contact of the probe with the cornea,

which may increase the risk of infection and corneal epithe-lium damage. Moreover, its accuracy depends on the place-ment of the probe on the cornea; off-center placement may yield thicker measurements than the true CCT. Thus, the examiner’s level of experience can infl uence the reliability of the measurements.

Recently, several optical techniques have been intro-duced that offer the advantages of noncontact technology and objective determination of the center of the cornea. Orbscan II (Bausch & Lomb, Rochester, NY, USA) scan-ning-slit corneal topography/pachymetry and the Pentacam (Oculus, Wetzlar, Germany) rotating Scheimpfl ug camera have multiple functions that help to assess the cornea, including its thickness profi le, anterior and posterior topog-raphy, elevation, and anterior chamber depth.1–7 The

Jpn J Ophthalmol 2008;52:245–249 DOI 10.1007/s10384-008-0550-x© Japanese Ophthalmological Society 2008

CLINICAL INVESTIGATION

Comparison of Central Corneal Thickness Measurements by Orbscan II and Pentacam After

Corneal Refractive Surgery

Jumpei Matsuda1,2, Osamu Hieda1,3, and Shigeru Kinoshita1

1 Department of Ophthalmology, Kyoto Prefectural University of Medicine, Kyoto, Japan; 2 Department of Ophthalmology, National Center for Geriatrics and Gerontology, Obu, Japan;

3 Baptist Eye Clinic, Kyoto, Japan

Abstract

Purpose: To compare the accuracy of postoperative pachymetry between Orbscan II (Bausch & Lomb) scanning-slit corneal topography/pachymetry and the Pentacam (Oculus) rotating Scheimpfl ug camera.

Methods: Central corneal thickness (CCT) was determined in 24 patients (48 eyes) before and after laser in situ keratomileusis (LASIK) or Epipolis LASIK (Epi-LASIK) procedures. All eyes were examined by Orbscan II and Pentacam prior to refractive surgery and at the fi rst, fourth, and twelfth week post-operatively. The residual CCT (RCCT) measured by each instrument was compared to the theoretical RCCT.

Results: On the fi rst, fourth, and twelfth week after the refractive surgery, the mean RCCT measure-ments by Orbscan II were 413 ± 72, 435 ± 65, and 440 ± 69 μm, respectively. Those of Pentacam were 434 ± 51, 436 ± 53, and 438 ± 50 μm, respectively. Orbscan II measurements at the postoperative fi rst and fourth week were signifi cantly smaller than the theoretical RCCT (P < 0.01, P < 0.01, paired t test). There was no statistical difference between the theoretical RCCT and the Pentacam measurements at any stage.

Conclusion: The Orbscan II measurement values of postoperative corneas tended to be thinner than the theoretical values, but not those of the Pentacam. Jpn J Ophthalmol 2008;52:245–249 © Japanese Ophthalmological Society 2008

Key Words: central corneal thickness, corneal opacity, laser in situ keratomileusis, Orbscan, Pentacam

Received: August 29, 2007 / Accepted: January 24, 2008Correspondence and reprint requests to: Jumpei Matsuda, Depart-

ment of Ophthalmology, Kyoto Prefectural University of Medicine, 465 Kajii-cho, Hirokoji-agaru, Kawaramachi-dori, Kamigyo-ku, Kyoto 602-8566, Japane-mail: jmatsuda@ophth.kpu-m.ac.jp

246 Jpn J Ophthalmol Vol 52: 245–249, 2008

usefulness of pachymetry obtained with these systems has been reported.1,4,8–10 Furthermore, Pentacam has a program that can evaluate the opacity of the lens by Scheimpfl ug densitometry.11 We previously applied this program to the cornea and defi ned an index of corneal opacity that we named central corneal densitometry (CCD), and found that CCD is useful for evaluating corneal clarity after refractive surgery.12 At the same time, several studies have reported no difference, or only a small systematic (statistical) differ-ence, between ultrasonic pachymetry and noncontact techniques, and a signifi cant linear correlation between noncontact technology pachymetry and ultrasonic pachym-etry measurements of postoperative CCT.1,4,8–10,13 Although the use of noncontact optical technologies is increasing, only a few studies have compared postoperative CCT between Orbscan II and Pentacam.14–17 To the best of our knowledge, only one comparison of the measurements of postoperative CCT by these two devices has been pub-lished.18 Although reports show that scanning-slit corneal topography/pachymetry underestimates CCT in eyes after excimer laser keratorefractive surgery,14–17 the accuracy of CCT measurements using the rotating Scheimpfl ug camera is still unclear. Since biomechanical stability of the cornea after excimer laser surgery depends on residual corneal thickness, postoperative pachymetry is very important, especially in candidates for enhancement surgery.19,20 Underestimation of CCT may result in some patients not obtaining the enhancement procedure, while overestima-tion may increase the risk of corneal ectasia in ineligible patients.19,21–23

This study investigated the accuracy of postoperative pachymetry by Orbscan II and Pentacam. As scanning-slit corneal topography/pachymetry underestimates the CCT in corneas with haze,16,24 the relation between corneal opacity and measurement error was investigated with CCT data.

Patients and Methods

CCT was determined in 48 post-LASIK/Epi-LASIK eyes of 24 patients with a mean age of 34.0 ± 10.3 (mean ± SD) years. We examined 24 eyes of 12 patients aged 36.4 ± 10.3 (range, 22–57) years undergoing LASIK and 24 eyes of 12 patients aged 31.6 ± 9.8 (range, 20–50) years undergoing Epi-LASIK for myopia. The manifest spherical equivalent of refraction before LASIK and Epi-LASIK was −6.58 ± 2.50 (range, −2.00 to −11.75) and −6.66 ± 2.19 (range, −2.13 to −10.25) diopters (D). The ablation depth of LASIK and Epi-LASIK was 97 ± 29 (range, 40–142) and 108 ± 30 (range, 56–148) μm. In these examinations, there were no statistical differences between patients undergoing LASIK and those undergoing Epi-LASIK. Informed consent was obtained from all participants. All measurements were taken at the same time of day, and all eyes were examined by Orbscan II and Pentacam prior to refractive surgery and at the fi rst, fourth, and twelfth week postoperatively. We used the mea-surement value at the thinnest point, which was automati-cally displayed on Orbscan II and Pentacam as the central

corneal thickness. All thinnest points were located within an area 1 mm in diameter. For Orbscan II measurements, the patients were seated and placed their chins on the chin rest with their forehead pressed against the forehead strap. Patients were instructed not to move and to keep their eyes open, and then the scan was initiated. An acoustic equiva-lent correction factor (0.92) was used to achieve equiva-lence with the ultrasonic evaluation, as recommended by the manufacturer. After the Orbscan II measurements were taken, the Pentacam examination was performed. As with Orbscan II, patients placed their chins on the chin rest and their foreheads against its headband. The patients were asked to look at a blinking fi xation target, and the images were captured. With Pentacam, the central corneal thick-ness and densitometry readings were used.

The study consisted of three parts. In part 1, we com-pared the accuracy of Orbscan II and Pentacam in measur-ing the cornea thickness in eyes after refractive surgery by comparing the residual CCT measured by each instrument to the theoretical residual CCT. The theoretical residual CCT for each instrument was calculated by subtracting the depth of ablation from the preoperative CCT measure-ments of each instrument.

In part 2, we compared the CCD prior to refractive surgery and at the fi rst, fourth, and twelfth weeks post-operatively. Twenty-fi ve Scheimpfl ug images were obtained by the 3D Scan measurement mode for 1 s, and the gray scale at the center of the cornea was automatically changed into a numeric value between 0 and 100. We then calculated the CCD as the average of these numeric values of 25 images, substituting the value of the next image when the eyelash was present in the image.12

In part 3, the relationship between corneal opacity and measurement error was investigated by using the CCD results. Corneal opacity was evaluated by the change in the CCD calculated by subtracting the preoperative value from the postoperative value at the fi rst, fourth, and twelfth weeks. Measurement errors were determined by measuring the difference between the theoretical corneal thickness and the measured values. We thus investigated the correla-tion between the change of CCD and the differences in corneal thickness shown to be signifi cant in part 1 of this study.

The statistical analysis was performed with Student’s t test, paired t test, Wilcoxon signed-ranks test, and Spear-man’s correlation. P values at the 5% confi dence interval were considered statistically signifi cant.

Results

Part 1: Pre- and Postoperative CCT

The mean preoperative CCT values measured by Orbscan II and Pentacam were 548 ± 31 and 539 ± 29 μm, respec-tively. There was no signifi cant difference between the mea-surements of these two instruments (Student t test; Fig. 1). The postoperative theoretical residual CCT of Orbscan II

J. MATSUDA ET AL. 247COMPARISON OF POSTOPERATIVE PACHYMETRY BY ORBSCAN II AND PENTACAM

and Pentacam were 445 ± 49 and 436 ± 47 μm, respectively. On the fi rst, fourth, and twelfth week after refractive surgery, the mean Orbscan II CCT measurements were 413 ± 72, 435 ± 65, and 440 ± 69 μm, respectively. The respective Pentacam measurements were 434 ± 51, 436 ± 53, and 438 ± 50 μm (Table 1). Orbscan II measurements on the post-operative fi rst and fourth week were signifi cantly smaller than the theoretical residual CCT (P < 0.01, P < 0.01, respec-tively, paired t test), but no difference was observed between the postoperative twelfth-week measurement and the theo-retical residual CCT. There was no statistical difference between the theoretical residual CCT and the Pentacam measurements at any stage.

Part 2: Pre- and Postoperative CCD

At the pre- and postoperative fi rst, fourth, and twelfth weeks, the CCD levels were 30.9 ± 2.3, 36.4 ± 7.6, 33.3 ± 5.8, and 33.0 ± 10.6, respectively (Fig. 2). The fi rst- and fourth-week CCD levels were signifi cantly higher statistically than the preoperative level (P < 0.001, P < 0.01, Wilcoxon signed-ranks test), whereas there was no difference between the twelfth-week CCD level and the preoperative level.

Part 3: Correlation of CCD and CCT

We investigated the correlation between the change in CCD and the difference in CCT measured by Orbscan II at the fi rst and fourth postoperative weeks. Orbscan II showed signifi cant linear correlations between the change in CCD and the measurement error at the fi rst and fourth post-operative weeks (Spearman’s correlation coeffi cient r = 0.676, P < 0.0001; r = 0.621, P < 0.001, respectively; Figs. 3 and 4).

Figure 1. Mean measurements of preoperative central corneal thick-ness using Orbscan II and Pentacam. There was no signifi cant differ-ence between the mean measurements of preoperative central corneal thickness using Orbscan II and Pentacam (Student t test). N.S., not signifi cant.

Table 1. Theoretical residual corneal thickness and mean central corneal thickness measurements

Orbscan II Pentacam

Theoretical thickness 445 ± 49 436 ± 47Post 1 week measurement 413 ± 73 * * 434 ± 51Post 4 weeks measurement 435 ± 66 436 ± 53Post 12 weeks measurement 440 ± 69 438 ± 50

Values are means ± SD (μm).*P < 0.01, paired t test.

Figure 2. Pre- and postoperative fi rst-, fourth-, and twelfth-week central corneal densitometry (CCD). The postoperative fi rst- and fourth-week CCD were at a statistically signifi cantly higher level than the preoperative level (P < 0.001, P < 0.01, respectively, Wilcoxon signed-ranks test), whereas the postoperative twelfth-week CCD showed no difference from the preoperative level (*P < 0.001, **P < 0.01). One women showed a CCD of 100 because her cornea had haze.

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Figure 3. Correlations between the change in CCD and the measure-ment error at the fi rst postoperative week. There was a signifi cant linear correlation between the change of CCD and the measurement error at the fi rst postoperative week with Orbscan II (Spearman’s cor-relation coeffi cient: r = 0.676, P < 0.0001).

248 Jpn J Ophthalmol Vol 52: 245–249, 2008

Discussion

Amano et al.1 reported no statistically signifi cant difference between measurements of CCT by Orbscan II and Pen-tacam in normal eyes, which was confi rmed by the results of part 1 of our study. The postoperative results at the fi rst and fourth weeks in part 1 of this study are also consistent with those of other studies14–17 showing that scanning-slit corneal topography/pachymetry underestimates pachyme-try in eyes after refractive surgery, but the twelfth postop-erative week results do not support the results of other studies. The results of recent studies show that scanning-slit corneal topography/pachymetry measurements tend to be thinner in postoperative corneas and corneas with haze,16,24 and this is confi rmed by data from part 3 of the present study, which showed a signifi cant linear correlation between CCD changes and differences in the CCT measurement error. The results of part 2 of the present study suggest that improvements in the corneal opacity by the time of the twelfth postoperative week measurements might have eliminated the measurement error. At the same time, Pen-tacam measurements in part 1 of this study were statisti-cally stable in both postoperative corneas and corneas with haze.

We hypothesize that the light rays may be deviated as they cross the cornea, thus inducing a displacement of the posterior profi le of the slit. Since both Orbscan II and Pen-tacam rely on measurements of scattered refl ected light beams through the corneal tissues, when the corneal medium is not clear or has optical interference, the pathways of the light rays will necessarily be interrupted. In addition, the refractive index of a hazy cornea is not stable, and refractive index changes affect the corneal thickness even when mea-sured with optical methodology. Both Orbscan II and

Pentcam are devices in which a slit illuminates the cornea, and a camera watches that slit at an angle. Although theo-retically the results should therefore be similar, the amount of optical error differed greatly between Orbscan II and Pentacam. We suggest possible reasons for this difference as follows.

With Orbscan II, the camera, which faces the cornea, catches scattered refl ected light beams as it moves to the right and left. As the angle of illumination changes, the distance at which the instrument captures the images changes. As a result, the perfect focal distance cannot be achieved for both the anterior and posterior cornea. Corneal haze causes additional distortion and lowers the brightness of the posterior edge of the cornea, resulting in a lower quality posterior surface image. On the other hand, the Scheimpfl ug camera of the Pentacam catches rotating, scat-tered refl ected light beams which are set at the front of the cornea. The camera itself rotates about an imaginary line through the center of the cornea, keeping a stable angle with the central corneal axis, thereby fulfi lling the Scheimp-fl ug principle. This principle is a geometric rule that states that if a planar subject is also parallel to the image plane, it can coincide with the plane of focus, and the entire subject can be rendered sharply. If the subject plane is not parallel to the image plane, it will be in focus only along the line of intersection with the plane of focus. The Scheimpfl ug camera system provides the advantages of a higher depth of focus and a sharp, undistorted picture. Thus, Pentacam is more accurate at measuring CCT than Orbscan II.

Both instruments calculate the distance between the anterior and posterior corneal surfaces and transform them into corneal pachymetry measurements by computer analy-sis. Since the accuracy of corneal pachymetry measurements is directly related to the accuracy of the analysis of the posterior corneal surface, the differences between the two systems may also be due to better programming of the Pentacam.

Several studies have reported that preoperative analysis of the posterior corneal surface is important when screening of patients for refractive surgery in order to avoid surgery on patients with early keratoconus.25,26 Postoperative analy-sis is also important in the observation of patients prior to LASIK.19,21,23,27–29 If the observed position of the posterior slit profi le differs from the true position of the posterior corneal surface, artifactual pseudoectasia images and a decrease in pachymetric measurements may result.15

Hashemi and Mehravaran30 report signifi cant differences in the measured postoperative changes of the posterior corneal surface between Pentacam and Orbscan II. They show that Orbscan II yields larger posterior corneal eleva-tion values after surgery than Pentacam. Nawa et al.31 report that the apparent ectasia detected by Orbscan may be an artifact. Our results regarding the superior accuracy of Pen-tacam in the measurement of postoperative pachymetry may indicate that Pentacam is also superior in the analysis of the postoperative posterior corneal surface.

In conclusion, Orbscan II CCT measurements tended to be thinner in postoperative corneas and corneas with haze,

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Figure 4. Correlations between the change of CCD and the measure-ment error at the fourth postoperative week. There was a signifi cant linear correlation between the change of CCD and the measurement error at the fourth postoperative week with Orbscan II (Spearman’s correlation coeffi cient: r = 0.621, P < 0.001).

J. MATSUDA ET AL. 249COMPARISON OF POSTOPERATIVE PACHYMETRY BY ORBSCAN II AND PENTACAM

while those of Pentacam were statistically stable in postop-erative corneas and corneas with haze.

References

1. Amano S, Honda N, Amano Y, et al. Comparison of central corneal thickness measurements by rotating Scheimpfl ug camera, ultrasonic pachymetry, and scanning-slit corneal topography. Oph-thalmology 2006;113:937–941.

2. Wei RH, Lim L, Chan WK, Tan DT. Evaluation of Orbscan II corneal topography in individuals with myopia. Ophthalmology 2006;113:177–183.

3. Lee JY, Kim JH, Kim HM, Song JS. Comparison of anterior chamber depth measurement between Orbscan IIz and ultrasound biomicroscopy. J Refract Surg 2007;23:487–491.

4. O’Donnell C, Maldonado-Codina C. Agreement and repeatability of central thickness measurement in normal corneas using ultra-sound pachymetry and the OCULUS Pentacam. Cornea 2005;24:920–924.

5. Nemeth G, Vajas A, Kolozsvari B, Berta A, Modis L Jr. Anterior chamber depth measurements in phakic and pseudophakic eyes: Pentacam versus ultrasound device. J Cataract Refract Surg 2006;32:1331–1335.

6. Buehl W, Stojanac D, Sacu S, Drexler W, Findl O. Comparison of three methods of measuring corneal thickness and anterior chamber depth. Am J Ophthalmol 2006;141:7–12.

7. Konstantopoulos A, Hossain P, Anderson DF. Recent advances in ophthalmic anterior segment imaging: a new era for ophthalmic diagnosis? Br J Ophthalmol 2007;91:551–557.

8. Barkana Y, Gerber Y, Elbaz U, et al. Central corneal thickness measurement with the Pentacam Scheimpfl ug system, optical low-coherence refl ectometry pachymeter, and ultrasound pachymetry. J Cataract Refract Surg 2005;31:1729–1735.

9. Lackner B, Schmidinger G, Pieh S, Funovics MA, Skorpik C. Repeatability and reproducibility of central corneal thickness mea-surement with Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005;82:892–899.

10. Suzuki S, Oshika T, Oki K, et al. Corneal thickness measurements: scanning-slit corneal topography and noncontact specular micros-copy versus ultrasonic pachymetry. J Cataract Refract Surg 2003;29:1313–1318.

11. Tkachov SI, Lautenschlager C, Ehrich D, Struck HG. Changes in the lens epithelium with respect to cataractogenesis: light micro-scopic and Scheimpfl ug densitometric analysis of the cataractous and the clear lens of diabetics and non-diabetics. Graefes Arch Clin Exp Ophthalmol 2006;244:596–602.

12. Matsuda J, Hieda O, Kinoshita S. Quantifi cation of corneal opacity after refractive corneal surgery using the anterior segment analyzer (in Japanese). Nippon Ganka Gakkai Zasshi (J Jpn Ophthalmol Soc) 2007;111:447–453.

13. Fujioka M, Nakamura M, Tatsumi Y, Kusuhara A, Maeda H, Negi A. Comparison of Pentacam Scheimpfl ug camera with ultrasound pachymetry and noncontact specular microscopy in measuring central corneal thickness. Curr Eye Res 2007;32:89–94.

14. Kawana K, Tokunaga T, Miyata K, Okamoto F, Kiuchi T, Oshika T. Comparison of corneal thickness measurements using Orbscan

II, non-contact specular microscopy, and ultrasonic pachymetry in eyes after laser in situ keratomileusis. Br J Ophthalmol 2004;88:466–468.

15. Prisant O, Calderon N, Chastang P, Gatinel D, Hoang-Xuan T. Reliability of pachymetric measurements using Orbscan after excimer refractive surgery. Ophthalmology 2003;110:511–515.

16. Boscia F, La Tegola MG, Alessio G, Sborgia C. Accuracy of Orbscan optical pachymetry in corneas with haze. J Cataract Refract Surg 2002;28:253–258.

17. Iskander NG, Anderson Penno E, Peters NT, Gimbel HV, Feren-sowicz M. Accuracy of Orbscan pachymetry measurements and DHG ultrasound pachymetry in primary laser in situ keratomileu-sis and LASIK enhancement procedures. J Cataract Refract Surg 2001;27:681–685.

18. Ho T, Cheng AC, Rao SK, Lau S, Leung CK, Lam DS. Central corneal thickness measurements using Orbscan II, Visante, ultra-sound, and Pentacam pachymetry after laser in situ keratomileusis for myopia. J Cataract Refract Surg 2007;33:1177–1182.

19. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999;106:406–409; discus-sion 409–410.

20. Liu Z, Huang AJ, Pfl ugfelder SC. Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topog-raphy system. Br J Ophthalmol 1999;83:774–778.

21. Seitz B, Torres F, Langenbucher A, Behrens A, Suarez E. Poste-rior corneal curvature changes after myopic laser in situ keratomi-leusis. Ophthalmology 2001;108:666–672; Discussion 673.

22. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998;14:312–317.

23. Kamiya K, Miyata K, Tokunaga T, Kiuchi T, Hiraoka T, Oshika T. Structural analysis of the cornea using scanning-slit corneal topography in eyes undergoing excimer laser refractive surgery. Cornea 2004;23:S59–64.

24. Fakhry MA, Artola A, Belda JI, Ayala MJ, Alio JL. Comparison of corneal pachymetry using ultrasound and Orbscan II. J Cataract Refract Surg 2002;28:248–252.

25. Tomidokoro A, Oshika T, Amano S, Higaki S, Maeda N, Miyata K. Changes in anterior and posterior corneal curvatures in kerato-conus. Ophthalmology 2000;107:1328–1332.

26. Tanabe T, Oshika T, Tomidokoro A, et al. Standardized color-coded scales for anterior and posterior elevation maps of scanning slit corneal topography. Ophthalmology 2002;109:1298–1302.

27. Miyata K, Tokunaga T, Nakahara M, et al. Residual bed thickness and corneal forward shift after laser in situ keratomileusis. J Cata-ract Refract Surg 2004;30:1067–1072.

28. Baek T, Lee K, Kagaya F, Tomidokoro A, Amano S, Oshika T. Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology 2001;108:317–320.

29. Yoshida T, Miyata K, Tokunaga T, Tanabe T, Oshika T. Differ-ence map or single elevation map in the evaluation of corneal forward shift after LASIK. Ophthalmology 2003;110:1926–1930.

30. Hashemi H, Mehravaran S. Corneal changes after laser refractive surgery for myopia: comparison of Orbscan II and Pentacam fi nd-ings. J Cataract Refract Surg 2007;33:841–847.

31. Nawa Y, Masuda K, Ueda T, Hara Y, Uozato H. Evaluation of apparent ectasia of the posterior surface of the cornea after kera-torefractive surgery. J Cataract Refract Surg 2005;31:571–573.