Journal of Refractive Surgery

The articles prior to January 2013 are part of the back file collection and are not available with a current paid subscription. To access the article, you may purchase it or purchase the complete back file collection here

Central Corneal Thickness Measurements in Unoperated Eyes and Eyes After PRK For Myopia Using Pentacam, Orbscan II, and Ultrasonic Pachymetry

Sun Woong Kim, MD; Yeo Jue Byun, MD; Eung Kweon Kim, MD, PhD; Tae-im Kim, MD

Abstract

ABSTRACT

PURPOSE: To compare central corneal thickness measurements obtained in unoperated eyes and eyes after myopic photorefractive keratectomy (PRK) using a rotating Scheimpflug camera (Pentacam), a scanning slit corneal topography system (Orbscan II), and ultrasonic pachymetry.

METHODS: Corneal thickness was measured using Pentacam, Orbscan II, and ultrasonic pachymetry in 25 unoperated eyes (unoperated group), 24 eyes 1 to 3 months after myopic PRK (early postoperative PRK group), and 21 eyes 4 months or more after myopic PRK (late postoperative PRK group).

RESULTS: In the unoperated group, corneal thickness measurements were similar for all three methods (P =.12 5). In the early postoperative PRK group, Orbscan measurements were thinner than Pentacam and ultrasonic measurements by a mean of 69.4 µm and 63.4 µm (P<.001 and P=.002, respectively). In the late postoperative PRK group, Orbscan measurements were thinner than Pentacam measurements by a mean of 36.0 pm (P= .017). Pentacam and ultrasonic pachymetry measurements were similar for all three groups with a mean difference of approximately 10 ???.

CONCLUSIONS: Following myopic PRK, Pentacam was comparable to ultrasonic pachymetry in measuring corneal thickness, whereas Orbscan measurements were thinner. [J Refract Surg. 2007;23:888-894.]

Abstract

ABSTRACT

PURPOSE: To compare central corneal thickness measurements obtained in unoperated eyes and eyes after myopic photorefractive keratectomy (PRK) using a rotating Scheimpflug camera (Pentacam), a scanning slit corneal topography system (Orbscan II), and ultrasonic pachymetry.

METHODS: Corneal thickness was measured using Pentacam, Orbscan II, and ultrasonic pachymetry in 25 unoperated eyes (unoperated group), 24 eyes 1 to 3 months after myopic PRK (early postoperative PRK group), and 21 eyes 4 months or more after myopic PRK (late postoperative PRK group).

RESULTS: In the unoperated group, corneal thickness measurements were similar for all three methods (P =.12 5). In the early postoperative PRK group, Orbscan measurements were thinner than Pentacam and ultrasonic measurements by a mean of 69.4 µm and 63.4 µm (P<.001 and P=.002, respectively). In the late postoperative PRK group, Orbscan measurements were thinner than Pentacam measurements by a mean of 36.0 pm (P= .017). Pentacam and ultrasonic pachymetry measurements were similar for all three groups with a mean difference of approximately 10 ???.

CONCLUSIONS: Following myopic PRK, Pentacam was comparable to ultrasonic pachymetry in measuring corneal thickness, whereas Orbscan measurements were thinner. [J Refract Surg. 2007;23:888-894.]

Measuring corneal thickness is crucial for evaluating patient eligibility for refractive surgery. Overestimation of thickness in preoperative evaluations can increase the risk of keratectasia, whereas underestimation can result in exclusion of patients who may be candidates for safe refractive surgery.1,2 Ultrasonic pachymetry has been considered the gold standard for measuring corneal thickness. However, relatively higher interoperator variability, the need for topical anesthesia, and direct contact of the probe with the cornea have resulted in a search for noninvasive methods.34

Various preoperative assessment methods have been developed for measuring the corneal surface in refractive surgery patients.510 Previous studies in normal eyes showed central corneal thickness measurements using a rotating Scheimpflug camera (Pentacam; Oculus Ine, Wetzlar, Germany) correlated well with ultrasonic pachymetry and scanning slit corneal topography (Orbscan; Bausch & Lomb, Salt Lake City, Utah) measurements.68 In contrast, several studies reported discrepancies between ultrasonic pachymetry and Orbscan measurements after LASIK or after photorefractive keratectomy (PRK).913 Stromal haze was considered one of the causes of the discrepancies because underestimation of corneal thickness occurred more obviously in eyes after PRK than in eyes after LASIK.1112 Moreover, it was reported thickness underestimation in Orbscan measurements following PRK was not observed after stromal haze resolved.13

This study compared corneal thickness measurement in unoperated and postoperative PRK eyes using Pentacam, Orbscan II, and ultrasonic pachymetry. This is the first study to evaluate corneal thickness following PRK using Pentacam. The study examined eyes in both the early (1 to 3 months) and the late (>4 months) postoperative period following myopic PRK to assess the accuracy of each device in terms of a wound healing time course. The study also examined whether Pentacam and Orbscan demonstrated similar problems in terms of corneal thickness underestimation in eyes after PRK for myopia.

Table

TABLE 1Patient Characteristics for the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups

TABLE 1

Patient Characteristics for the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups

PATIENTS AND METHODS

Seventy eyes of 47 patients were included in the study. Patients were divided into three groups. The unoperated group comprised 25 unoperated eyes of 18 refractive surgery candidates, the early postoperative PRK group comprised 24 eyes of 15 patients who had undergone PRK for correction of myopia 1 to 3 months previously, and the late postoperative PRK group comprised 21 eyes of 14 patients who had undergone PRK for correction of myopia more than 4 months previously. Written informed consent was obtained from all patients prior to study enrollment.

Central corneal thickness was measured using the Pentacam rotating Scheimpflug camera, Orbscan II scanning slit topography system, and ultrasonic pachymetry (UP-1000; NIDEK Co Ltd, Gamagori, Japan). Measurements were obtained sequentially in the order of Pentacam, Orbscan II, and ultrasonic pachymetry. For Orbscan II measurements, an acoustic equivalent factor of 0.94 was used, which had been determined previously in our clinic to provide equivalence with ultrasonic pachymetry. For ultrasonic pachymetry, the cornea was anesthetized using proparacaine 0.5% (Alcaine; Alcon Laboratories Ine, Ft Worth, Tex), and an average of five consecutive measurements was obtained.

Measurements obtained with the three different methods were compared using repeated-measures analysis of variance (ANOVA) and Scheff? multiple comparison test. Pearson's correlation and Bland-Altman plots were used to assess the difference between individual measurements for each patient. A P value <.05 was considered statistically significant. Statistical analysis was performed using Statistical Package for Social Sciences software version 12.0 (SPSS Ine, Chicago, Ill).

RESULTS

Mean patient age, preoperative spherical equivalent refraction, and preoperative corneal thickness were similar for the three groups (Table 1). Table 2 lists the corneal thickness measurements for Pentacam, Orbscan, and ultrasonic pachymetry for the three groups. In the unoperated group, the three measurements were similar with a mean difference within 20 µm (P=. 125). In contrast, in the early postoperative PRK group, Orbscan measurements were thinner than both the Pentacam by a mean of 69.4 pm (P<.001) and ultrasonic pachymetry measurements by a mean of 63.4 pm (P=. 002). In the late postoperative PRK group, the Orbscan measurement was thinner than the Pentacam measurement by a mean of 36.0 µm (P=. 017), whereas the ultrasonic pachymetry measurement was similar to both the Pentacam (P=.589) and Orbscan (P=.169) measurements.

Table 3 shows the mean difference, standard deviation, 95% limits of agreement, and Pearson coefficients between any two of the three modalities. For all three patient groups, there were linear correlations between the Pentacam and ultrasonic pachymetry measurements, Orbscan and ultrasonic pachymetry measurements, and Pentacam and Orbscan measurements (P<.001). However, the Orbscan measurements had a relatively lower correlation with the other two methods, especially in the early postoperative PRK group.

Bland-Altman plots were used to assess differences between individual measurements as a function of the mean of two methods. Pentacam measurements showed good agreement with ultrasonic pachymetry measurements, with a mean difference of approximately 10 pm for all three groups (Figs 1-3). In the early and late postoperative PRK groups, Orbscan measurements tended to underestimate corneal thickness compared to ultrasonic pachymetry and Pentacam measurements (Figs 2 and 3). This underestimation was more prominent in corneal thickness measurements <450 µm. In the unoperated group, all three methods showed good agreement, with the mean difference being within ±20 µm.

Table

TABLE 2Corneal Thickness Measurements in the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups Using Pentacam, Orbscan H9 and Ultrasonic PachymetryTABLE 3Mean Difference, Standard Deviation, Limits of Agreement, and Pearson Correlation in Corneal Thickness Measurements for Pentacam, Orbscan II, and Ultrasonic Pachymetry in the Unoperated, Early Postoperative PRK, and Late Postoperative PRK Groups

TABLE 2

Corneal Thickness Measurements in the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups Using Pentacam, Orbscan H9 and Ultrasonic Pachymetry

TABLE 3

Mean Difference, Standard Deviation, Limits of Agreement, and Pearson Correlation in Corneal Thickness Measurements for Pentacam, Orbscan II, and Ultrasonic Pachymetry in the Unoperated, Early Postoperative PRK, and Late Postoperative PRK Groups

DISCUSSION

Accurate assessment of central corneal thickness is an integral part of the preoperative evaluation of patients undergoing refractive surgery and has become more important with the rapidly increasing popularity of refractive surgery. In refractive surgery, there appears to be a consensus that at least 250 µm should remain in the stromal bed after laser ablation.1,2 This is especially important when treating patients with high myopia or borderline corneal thickness. Furthermore, highly accurate corneal thickness measurements are critical for patients who want enhancement procedures.910 In addition to refractive surgical procedures, central corneal thickness measurements are important in assessing intraocular pressure (IOP). Monitoring corneal thickness after refractive surgery is important because postoperative IOP becomes lower than preoperative IOP as a result of reduced central corneal thickness.14 Assessment of corneal thickness is helpful in diagnosing corneal disorders such as keratoconus or other ectatic corneal disorders.15

Figure 1. Difference between modalities over the mean of two corneal thickness measurements for the nono pera ted group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 X standard deviation).

Figure 1. Difference between modalities over the mean of two corneal thickness measurements for the nono pera ted group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 X standard deviation).

Ultrasonic pachymetry has been considered the gold standard for measuring corneal thickness. However, errors can arise from problems with the corneal-probe contact, incorrect probe placement, lack of a fixation light for gaze control, variability of sound speed in wet and dry tissues, and the possible compression effect, although no correlation has been shown between compression and applied force.3-10 Therefore, reproducibility of measurements using this system is largely dependent on the experience of the examiner.

The introduction of the Orbscan system has resulted in a growth in popularity of non-contact pachymetry. The Orbscan system can provide refractive surgery preoperative assessments such as anterior and posterior corneal topography measurements, elevation maps, and corneal thickness. It also provides anterior chamber depth and white-to-white diameter for phakic intraocular lens surgery. As a result, some refractive surgeons rely solely on Orbscan measurements to determine patient eligibility for LASIK.510 The Pentacam is a relatively new instrument that uses a rotating Scheimpflug camera, which provides threedimensional scanning of the whole anterior segment of the eye. The images can be used to quickly and noninvasively determine anterior and posterior corneal topography, corneal thickness, anterior chamber depth and angle, and lens density. In addition, Orbscan and Pentacam offer more comfortable settings for patients because the center of the cornea is determined automatically and probe-cornea contact is not required.6-8

Figure 2. Difference between modalities over the mean of two corneal thickness measurements for the early postoperative PRK group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 × standard deviation).

Figure 2. Difference between modalities over the mean of two corneal thickness measurements for the early postoperative PRK group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 × standard deviation).

Despite the inherent advantages of non-contact pachymetry, various studies have reported discrepancies between Orbscan and ultrasonic pachymetry.11,13,15,17 An acoustic equivalent correction factor (0.92) was incorporated into the Orbscan system to overcome problems of overestimation of corneal thickness compared to ultrasonic pachymetry, and several studies have reported good agreement between these two such measurements in normal eyes.5-8

However, subsequentreports have demonstrated discrepancies between Orbscan and ultrasonic pachymetry, particularly in eyes that had undergone LASIK or PRK.11-13 Iskander et al9 reported Orbscan measurements were lower than DGH ultrasound pachymetry (DGH Technology Ine, Exton, Pa) measurements by 18.4 µm in preoperative eyes and by 50.1 µm in eyes after refractive surgery. Prisant et al11 reported similar results, demonstrating the Orbscan measurements to be 13 µm lower in preoperative eyes, 29 µm lower in postoperative LASIK eyes, and 79 pm lower in postoperative PRK eyes compared to DGH ultrasonic measurements. They considered the Orbscan underestimation as an inherent limitation of optical pachymetry. Because optical pachymetry is dependent on measuring scattered reflected light beams through the corneal tissues, the pathways of the light rays may be interrupted when the cornea is not clear or has optical interfaces, and this can lead to erroneous measurements. Boscia et al13 showed Orbscan underestimations that occurred in corneas with residual haze after PRK were not observed after treatment of haze.

Figure 3. Difference between modalities over mean of two corneal thickness measurements for the late postoperative PRK group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 × standard deviation).

Figure 3. Difference between modalities over mean of two corneal thickness measurements for the late postoperative PRK group for A) Pentacam and ultrasound, B) ultrasound and Orbscan, and C) Pentacam and Orbscan. The solid line represents the mean difference, and the dotted lines represent the upper and lower borders of the 95% limits of agreement (mean±1.96 × standard deviation).

Like Orbscan, the Pentacam is also an optical device, and as such, a correction factor may be required to correlate its measurements to those obtained using ultrasonic pachymetry. Several studies have reported central corneal thickness measurements were comparable using the Pentacam, Orbscan, and ultrasonic pachymetry in normal eyes.6"8 However, a correction factor for the Pentacam and its reliability for corneal thickness measurement in postoperative eyes has not been addressed.

Our study examined whether the Pentacam had similar problems to the Orbscan in measuring corneal thickness after myopic PRK. The results demonstrated Pentacam measurements were comparable to those of ultrasonic pachymetry in the cornea both preoperatively and after laser surface ablation. In contrast, in treated eyes, the Orbscan values were lower than those of the other two methods, and this was more obvious in early postoperative eyes. These findings are consistent with others showing that Orbscan corneal thickness measurements were less accurate in eyes with haze and that such inaccuracy diminished over the course of wound healing.11,13

The Pentacam uses a Scheimpflug camera and a monochromatic slit light source that together rotate around an optical axis and acquire 25 slit images that contain 500 measurement points on the front and back corneal surfaces. In contrast, the Orbscan uses a horizontally moving slit beam to produce multiple slit images of the cornea. Therefore, a rotating camera system possibly acquires more uninterrupted images, and the thickness measurements may be less influenced by corneal haze.

We found linear correlations between measurements of the Pentacam and ultrasound pachymetry, Orbscan and ultrasound pachymetry, and the Pentacam and Orbscan. However, the correlation coefficients are not relevant when comparing two different methods. A better approach is to demonstrate how much one instrument is likely to differ from another, and this can be demonstrated using a special method for assessing clinical agreement.18,19 Pentacam and ultrasonic pachymetry were found to be in good agreement, with a mean difference of approximately 10 µm and an upper limit of agreement of approximately 30 µm for all three groups (see Figs 1-3). The Orbscan underestimated corneal thickness compared to ultrasound pachymetry and Pentacam, and this was more obvious for measurements <450 µm. Although the reason is unclear, it supports a previous opinion that use of the same Orbscan correction factor may not be appropriate over a range of corneal thicknesses.9-11

Although the present study does not identify which device measures corneal thickness more accurately, the Pentacam measurements were more comparable with ultrasonic pachymetry in both unoperated eyes and eyes after PRK for myopia, whereas there was a greater difference with the Orbscan measurements in ablated corneas. The data suggest the Pentacam is clinically acceptable for measuring corneal thickness in both normal and laser- ablated eyes.

REFERENCES

1. Iskander NG, Peters NT, Penno EA, Gimbel HV. Postoperative complications in laser in situ keratomileusis. Curr Opin Ophthalmol. 2000;11:273-279.

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

3. Miglior S, Albe E, Gu?rese hi M, Mandelli G, Gomarasca S, Orzalesi N. Intraobserver and inter observer reproducibility in the evaluation of ultrasonic pachymetry measurements of central corneal thickness. BrJ Ophthalmol. 2004;88:174-177.

4. Solomon OD. Corneal indentation during ultrasonic pachometry. Cornea. 1999;18:214-215.

5. Rainer G, Findl O, Petternel V, Kiss B, Drexler W, Skorpik C, Georgopoulos M, Schmetterer L. Central corneal thickness measurements with partial coherence interferometry, ultrasound, and the Orbscan system. Ophthalmology. 2004;111:875-879.

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. Amano S, Honda N, Amano Y, Yamagami S, Miyai T, Samejima T, Ogata M, Miyata K. Comparison of central corneal thickness measurements by rotating Scheimpflug camera, ultrasonic pachymetry, and scanning slit corneal topography. Ophthalmology. 2006;113:937-941.

8. Lackner B, Schmidinger G, Pieh S, Funovics MA, Skorpik C. Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orb sc an, and ultrasound. Optom Vis Sci. 2005;82:892-899.

9. Iskander NG, Anderson Penno E, Peters NT, Gimbel HV, Ferens o wiez M. Accuracy of Orb sc an pachymetry measurements and DHG ultrasound pachymetry in primary laser in situ keratomileusis and LASIK enhancement procedures. J Cataract Refract Surg. 2001;27:681-685.

10. Kawana K, Tokunaga T, Miyata K, Okamoto F, Kiuchi T, Oshika T. Comparison of corneal thickness measurements using Orb sc an II, non-contact specular microscopy, and ultrasonic pachymetry in eyes after laser in situ keratomileusis. BrJ Ophthalmol. 2004;88:466-468.

11. Prisant O, Calderon N, Chastang P, Gatinel D, Hoang-Xuan T. Reliability of pachy metric measurements using Orb sc an after excimer refractive surgery. Ophthalmology. 2003;110:511-515.

12. 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.

13. B o scia F, La Tegola MG, Alessio G, Sb orgia C. Accuracy of Orbscan optical pachymetry in corneas with haze. J Cataract Refract Surg. 2002;28:253-258.

14. Patel S, Asiani de s IM. Main causes of reduced intraocular pressure after excimer laser photorefractive keratectomy. J Refract Surg. 1996;12:673-674.

15. Gherghel D, Hosking SL, Mantry S, Banerjee S, Naroo SA, Shah S. Corneal pachymetry in normal and keratoconic eyes: Orbscan II versus ultrasound. J Cataract Refract Surg. 2004;30:1272-1277.

16. Suzuki S, Oshika T, Oki K, Sakabe I, Iwase A, Amano S, Araie M. Corneal thickness measurements: scanning- slit corneal topography and noncontact specular microscopy versus ultrasonic pachymetry. J Cataract Refract Surg. 2003;29:1313-1318.

17. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness measurements with the Orb sc an topography system and ultrasonic pachymetry. J Cataract Refract Surg. 1997;23:1345-1350.

18. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1:307-310.

19. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8:135-160.

TABLE 1

Patient Characteristics for the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups

TABLE 2

Corneal Thickness Measurements in the Unoperated, Early Postoperative PRK9 and Late Postoperative PRK Groups Using Pentacam, Orbscan H9 and Ultrasonic Pachymetry

TABLE 3

Mean Difference, Standard Deviation, Limits of Agreement, and Pearson Correlation in Corneal Thickness Measurements for Pentacam, Orbscan II, and Ultrasonic Pachymetry in the Unoperated, Early Postoperative PRK, and Late Postoperative PRK Groups

10.3928/1081-597X-20071101-04

Sign up to receive

Journal E-contents