- Email: [email protected]
Comparison of central corneal thickness measurements with a new optical device and a standard ultrasonic pachymeter
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
Comparison of central corneal thickness measurements with a new optical device and a standard ultrasonic pachymeter Gabor Nemeth, MD, Alexis Tsorbatzoglou, MD, Katalin Kertesz, MD, Attila Vajas, MD, Andras Berta, MD, PhD, DSci, Laszlo Mo´dis Jr, MD, PhD
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 G6.0 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 mm G 3.9 (SD) with PCI and 547.8 G 36.0 mm with the ultrasonic device. The difference between groups was significant (P Z .001). There was no difference between CCT values measured in right and left eyes (P Z .55) with ultrasonography and PCI (P Z .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 Z .91, P Z .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. J Cataract Refract Surg 2006; 32:460–463 Q 2006 ASCRS and ESCRS
Corneal thickness measurement has become an important factor in most types of anterior segment surgery in the evaluation of corneal dehydrating function and for determining intraocular pressure. To avoid complications in refractive surgery such as keratectasia after laser in situ keratomileusis or perforation in photorefractive keratotomy, accurate corneal thickness measurements are also important.1–5
Accepted for publication August 5, 2005. From the Department of Ophthalmology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary. No author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Gabor Nemeth, MD, Department of Ophthalmology, University of Debrecen, Nagyerdei Blvd. 98, H-4012 Debrecen, Hungary. E-mail: [email protected]. Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
460
For decades, the standard technique of central corneal thickness (CCT) measurement was the contact ultrasonographic method; however, differences in corneal thickness measurements have been reported between ultrasonic pachymeter devices.6,7 The aim of this study was to evaluate CCT by 2 different methods: a new device using noncontact partial coherence interferometry (PCI) and the standard contact ultrasound (US) method. PATIENTS AND METHODS The study comprised 136 eyes of 70 patients (70 right and 66 left eyes). The mean age of the patients was 66.2 years G 11.3 (SD) (range 36 to 86 years) with a male-to-female ratio of 36:34. Central corneal thickness was measured with a new device that used the PCI method (ACMaster, Zeiss) and with an ultrasonic pachymeter (AL-2000, Tomey). Patients with a history of wearing contact lenses, a spherical refractive error greater than G6.0 diopters (D), keratometric astigmatism greater than 2.0 D, 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2005.12.138
CCT MEASUREMENTS BY PCI AND US
Table 1. Mean central corneal thickness values with PCI and with ultrasonic pachymeter.
Eye Right Left
PCI
Ultrasonography
533.6 G 4.3 mm 528.9 G 3.8 mm
545.9 G 35.9 mm 549.5 G 36.2 mm
or any anterior segment abnormalities were excluded from the study. Five separate, sequential measurements were performed by 1 examiner, first with the noncontact PCI device and then with the US device. The mechanism of the PCI method has been described.8–10 For the PCI method, the patient assumed a sitting position, placing the chin on a chin rest and the forehead into a headband. The patient was instructed to keep both eyes open and to focus the eye being tested on the built-in measurement target of the device. For the US method, the corneas were anesthetized with topical tetracaine hydrochloride. Patients were again asked to sit and look straight at a target on the wall. The probe tip of the ultrasonic pachymeter was applied to the central cornea perpendicularly and the surface of the cornea was slightly touched. The probe tip was sterilized with alcohol after use on each patient. Statistical analysis was performed and CCT data were described in terms of means and standard deviations. The intraobserver variability was described with a coefficient of variation (CV). Differences in CCT values between the devices were recorded with the paired test of Wilcoxon, and a P value of .01 was considered the level of significance. The association between groups was described with the Spearman correlation.
RESULTS
The mean CCT was 531.2 G 3.9 mm when measured by the PCI device and 547.8 G 36.0 mm when measured by US pachymetry. The difference between the CCT values was strongly significant (P Z.001). There was no significant difference between the right and left eyes (PCI, P Z.67; US, P Z.55) (Table 1). Figure 1 shows the measured CCTs. Intraobserver variability was lower with the PCI method than with the US technique (CV 0.73% and 6.5%, respectively). There were no differences in intraobserver variability between right and left eyes with PCI (CV 0.8% versus 0.72%) or US (CV 6.5% versus 6.58%). The correlation between pachymetric devices was strong, as shown in Figure 2 (r Z .91; P Z.001).
DISCUSSION
Ophthalmic biometry should provide rapid, objective, and accurate measurements of different ocular parameters. Change in corneal thickness is an important sign of worsening function of the corneal endothelium. In corneal refractive surgery, a high degree of accuracy in corneal thickness measurement is required.3–5
Figure 1. The CCT values measured with the ACMaster and AL-2000 ultrasonic pachymeters. The mean CCTs are presented with continuous (US) and dotted lines (PCI).
Ultrasound pachymetry has been the standard method of evaluating corneal thickness for the past few decades. Numerous new pachymetric techniques are now available, although ultrasonic pachymetry is still the common standard. Many studies have compared CCT measurement using different methods and devices (eg, specular microscope, optical coherence tomography, ultrasound biomicroscope, Orbscan (Bausch & Lomb), and devices based on PCI).7,11–16 In this study, CCT was measured with a new noncontact ophthalmic device developed for the anterior segment of the eye using the PCI method, and the results were compared with measurements obtained using the standard ultrasonic pachymetry technique. The mean CCT values obtained with the noncontact optical method were smaller than those derived from the ultrasonic method. The differences in CCT values were statistically significant (P Z.001). The unquestionable advantage of the PCI technique is the noncontact method. The noncontact (ie, noninvasive) pachymetry method has important advantages besides its short measuring time. There is no need for anesthesia and no risk for corneal infection, which ensure a high degree of patient comfort. Giasson and Forthomme17 report significant left–right differences in ultrasonic CCT measurements due to the handedness of the examiner; ie, left corneal thickness was measured significantly larger by a right-handed examiner. This phenomenon has been reported in several studies and has been attributed to measurements not being performed perpendicularly to the corneal surface.11 In our
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
461
CCT MEASUREMENTS BY PCI AND US
Figure 2. Mean CCT values obtained with PCI and ultrasonic pachymetry.
study, the investigator sat on the left side of the patient while measuring CCT with the ultrasonic device. Therefore, measuring CCTof the right eye was somewhat complicated, but this factor did not generate large statistical differences between right and left CCT values (P Z .55). One possible explanation for this finding is that our ultrasonic device measures the CCT in an automated mode only when the probe tip is perpendicular to the corneal surface. Some authors postulate additional reasons for the larger corneal thickness values measured with ultrasonic pachymeters, including ultrasonic probe tip tilting, epithelial edema from local anesthesia, repeated pressure of the cornea,18,19 and inadequate calibration of the devices. Others suspect that the speed of sound varies between different layers of the cornea and between normal and pathologic corneal tissues.6,20,21 Some authors recommend using a modified refractive index for optical coherence
tomography (OCT) measurements (n Z 1.365) and a modified velocity of sound (1664 m/s) for ultrasonic CCT measurements.11 We measured the CCT with ultrasonography at the sound velocity of 1640 m/s. Mean corneal thickness values obtained with optical pachymeters (PCI method), OCT, and Orbscan were significantly less than values obtained with US pachymetry, suggesting that US-based devices slightly overestimate corneal thickness.11–14,16,22 These differences might be clinically relevant. However, the reason for the significantly smaller CCT values measured with the PCI method, OCT, and Orbscan remains unknown. Other studies mention larger CCTs obtained with specular microscopy, US biomicroscopy, and findings opposite the above-mentioned Orbscan findings.13 Moreover, it is impossible to state whether the optical or the US measurements are closer to the actual corneal thickness. The measured CCTs depend on the
Table 2. Differences between CCT values measured with different methods and devices and recorded as extracted value of the 2. method by the 1. method.
Author*
1. Method
2. Method
Micrometer Difference (2.-1. method)
Bechmann Mo´dis15 Rainer18 Rainer18 Wheeler7 Fishman12 Wirbelauer24 Mo´dis15 Fishman12 Tam14 Tam14
OCT Noncontact specular PCI Orbscan Specular OCT OCT Ultrasonography Ultrasonography Ultrasonography Ultrasonography
Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Specular Orbscan Specular Ultrasound biomicroscopy
50 28 21.5 20 20 12 8 68 40 22 5
Present study
PCI
Ultrasonography
16.6
22
OCT Z optical coherence tomography; PCI Z partial coherence interferometry *First author
462
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
CCT MEASUREMENTS BY PCI AND US
refractive index of the cornea and the US velocity in the cornea. In our study, we compared the CCT values obtained using a new optical device and an ultrasonic device. The mean CCT was 16.6 mm greater when measured with ultrasonography than when measured with the PCI method. This is consistent with data derived from previous studies (Table 2).11–16,22 The intraobserver reproducibility represents the variability of values in a series of measurements (all of our measurements were taken by the same investigator). The results of our study show that the intraobserver variability with the PCI method is almost one-tenth smaller than with the US method (CV 0.73% versus 6.5%). This contradicts the literature in which these data are comparable. However, some data obtained with the Orbscan system show even greater variability. This fact confirms the accuracy and reproducibility of CCT values observed with the PCI method.13,23 A high positive correlation coefficient was obtained between the 2 methods of central corneal pachymetry (r Z 0.91; P Z .001), suggesting that the CCT values of these methods are count over to each other, but are not interchangeable. In conclusion, we have shown that the CCT values measured by the ultrasonic pachymeter are higher than those obtained by the PCI method. The differences between optical and ultrasonic pachymetry measurements were statistically significant. The accuracy of these pachymetric devices is acceptable in measuring CCT. Further studies are required to confirm these findings and to evaluate the accuracy, reproducibility, and independence of corneal pachymetry research using the PCI method. REFERENCES 1. McLeod SD, Kisla TA, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. Arch Ophthalmol 2000; 118:282–284 2. Joo C-K, Kim-G. Corneal ectasia detected after laser in situ keratomileusis for correction of less than 12 diopters of myopia. J Cataract Refract Surg 2000; 26:292–295 3. Vinciguerra P, Camesasca FI. Prevention of corneal ectasia in laser in situ keratomileusis. J Refract Surg 2001; 17:S187–S189 4. Argento C, Cosentino MJ, Tytiun A. Corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg 2001; 27:1440–1448 5. Rowsey JJ, Balyeat HD. Radial keratotomy: preliminary report of complications. Ophthalmic Surg 1982; 13:27–35
6. Reader AL III, Salz JJ. Differences among ultrasonic pachymeters in measuring corneal thickness. J Refract Surg 1987; 3:7–11 7. Wheeler NC, Morantes CM, Kristensen RM, et al. Reliability coefficients of three corneal pachymeters. Am J Ophthalmol 1992; 113:645–651 8. Fercher AF, Mengedoht, Werner W. Eye-length measurement by interferometer with partially coherent light. Opt Lett 1988; 13:186–188 9. Hitzenberger CK, Baumgartner A, Drexler W, Fercher AF. Interferometric measurement of corneal thickness with micrometer precision. Am J Ophthalmol 1994; 118:468–476 10. Drexler W, Baumgartner A, Findl O, et al. Submicrometer precision biometry of the anterior segment of the human eye. Invest Ophthalmol Vis Sci 1997; 38:1304–1313 11. Marsich MM, Bullimore MA. The repeatability of corneal thickness measures. Cornea 2000; 19:792–795 12. Fishman GR, Pons ME, Seedor JA, et al. Assessment of central corneal thickness using optical coherence tomography. J Cataract Refract Surg 2005; 31:707–711 13. Salz JJ, Azen SP, Bernstein J, et al. Evaluation and comparison of sources of variability in the measurement of corneal thickness with ultrasonic and optical pachymeters. Ophthalmic Surg 1983; 14:750–754 14. Tam S, Rootman DS. Comparison of central corneal thickness measurements by specular microscopy, ultrasound pachymetry, and ultrasound biomicroscopy. J Cataract Refract Surg 2003; 29: 1179–1184 15. Mo´dis L Jr, Langenbucher A, Seitz B. Corneal thickness measurements with contact and noncontact specular microscopic and ultrasonic pachymetry. Am J Ophthalmol 2001; 132:517–521 16. Rainer G, Findl O, Petternel V, et al. Central corneal thickness measurements with partial coherence interferometry, ultrasound, and the Orbscan system. Ophthalmology 2004; 111:875–879 17. Giasson C, Forthomme D. Comparison of central corneal thickness measurements between optical and ultrasound pachometers. Optom Vis Sci 1992; 69:236–241 18. Rainer G, Petternel V, Findl O, et al. Comparison of ultrasound pachymetry and partial coherence interferometry in the measurement of central corneal thickness. J Cataract Refract Surg 2002; 28:2142– 2145 19. Herse P, Siu A. Short-term effects of proparacaine on human corneal thickness. Acta Ophthalmol (Copenh) 1992; 70:740–744 20. Coleman DJ, Lizzi FL, Jack RL. Ultrasonography of the Eye and Orbit. Philadelphia, Lea & Febiger, 1977; 113–114 21. Chivers RC, Round WH, Zieniuk JK. Investigation of ultrasound axially traversing the human eye. Ultrasound Med Biol 1984; 10:173–188 22. Bechmann M, Thiel MJ, Neubauer AS, et al. Central corneal thickness measurement with a retinal optical coherence tomography device versus standard ultrasonic pachymetry. Cornea 2001; 20:50–54 23. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol 2000; 44:367–408 24. Wirbelauer C, Scholz C, Hoerauf H, et al. Noncontact corneal pachymetry with slit lamp-adapted optical coherence tomography. Am J Ophthalmol 2002; 133:444–450
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
463
Comparison of central corneal thickness measurements with a new optical device and a standard ultrasonic pachymeter Gabor Nemeth, MD, Alexis Tsorbatzoglou, MD, Katalin Kertesz, MD, Attila Vajas, MD, Andras Berta, MD, PhD, DSci, Laszlo Mo´dis Jr, MD, PhD
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 G6.0 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 mm G 3.9 (SD) with PCI and 547.8 G 36.0 mm with the ultrasonic device. The difference between groups was significant (P Z .001). There was no difference between CCT values measured in right and left eyes (P Z .55) with ultrasonography and PCI (P Z .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 Z .91, P Z .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. J Cataract Refract Surg 2006; 32:460–463 Q 2006 ASCRS and ESCRS
Corneal thickness measurement has become an important factor in most types of anterior segment surgery in the evaluation of corneal dehydrating function and for determining intraocular pressure. To avoid complications in refractive surgery such as keratectasia after laser in situ keratomileusis or perforation in photorefractive keratotomy, accurate corneal thickness measurements are also important.1–5
Accepted for publication August 5, 2005. From the Department of Ophthalmology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary. No author has a financial or proprietary interest in any material or method mentioned. Reprint requests to Gabor Nemeth, MD, Department of Ophthalmology, University of Debrecen, Nagyerdei Blvd. 98, H-4012 Debrecen, Hungary. E-mail: [email protected]. Q 2006 ASCRS and ESCRS Published by Elsevier Inc.
460
For decades, the standard technique of central corneal thickness (CCT) measurement was the contact ultrasonographic method; however, differences in corneal thickness measurements have been reported between ultrasonic pachymeter devices.6,7 The aim of this study was to evaluate CCT by 2 different methods: a new device using noncontact partial coherence interferometry (PCI) and the standard contact ultrasound (US) method. PATIENTS AND METHODS The study comprised 136 eyes of 70 patients (70 right and 66 left eyes). The mean age of the patients was 66.2 years G 11.3 (SD) (range 36 to 86 years) with a male-to-female ratio of 36:34. Central corneal thickness was measured with a new device that used the PCI method (ACMaster, Zeiss) and with an ultrasonic pachymeter (AL-2000, Tomey). Patients with a history of wearing contact lenses, a spherical refractive error greater than G6.0 diopters (D), keratometric astigmatism greater than 2.0 D, 0886-3350/06/$-see front matter doi:10.1016/j.jcrs.2005.12.138
CCT MEASUREMENTS BY PCI AND US
Table 1. Mean central corneal thickness values with PCI and with ultrasonic pachymeter.
Eye Right Left
PCI
Ultrasonography
533.6 G 4.3 mm 528.9 G 3.8 mm
545.9 G 35.9 mm 549.5 G 36.2 mm
or any anterior segment abnormalities were excluded from the study. Five separate, sequential measurements were performed by 1 examiner, first with the noncontact PCI device and then with the US device. The mechanism of the PCI method has been described.8–10 For the PCI method, the patient assumed a sitting position, placing the chin on a chin rest and the forehead into a headband. The patient was instructed to keep both eyes open and to focus the eye being tested on the built-in measurement target of the device. For the US method, the corneas were anesthetized with topical tetracaine hydrochloride. Patients were again asked to sit and look straight at a target on the wall. The probe tip of the ultrasonic pachymeter was applied to the central cornea perpendicularly and the surface of the cornea was slightly touched. The probe tip was sterilized with alcohol after use on each patient. Statistical analysis was performed and CCT data were described in terms of means and standard deviations. The intraobserver variability was described with a coefficient of variation (CV). Differences in CCT values between the devices were recorded with the paired test of Wilcoxon, and a P value of .01 was considered the level of significance. The association between groups was described with the Spearman correlation.
RESULTS
The mean CCT was 531.2 G 3.9 mm when measured by the PCI device and 547.8 G 36.0 mm when measured by US pachymetry. The difference between the CCT values was strongly significant (P Z.001). There was no significant difference between the right and left eyes (PCI, P Z.67; US, P Z.55) (Table 1). Figure 1 shows the measured CCTs. Intraobserver variability was lower with the PCI method than with the US technique (CV 0.73% and 6.5%, respectively). There were no differences in intraobserver variability between right and left eyes with PCI (CV 0.8% versus 0.72%) or US (CV 6.5% versus 6.58%). The correlation between pachymetric devices was strong, as shown in Figure 2 (r Z .91; P Z.001).
DISCUSSION
Ophthalmic biometry should provide rapid, objective, and accurate measurements of different ocular parameters. Change in corneal thickness is an important sign of worsening function of the corneal endothelium. In corneal refractive surgery, a high degree of accuracy in corneal thickness measurement is required.3–5
Figure 1. The CCT values measured with the ACMaster and AL-2000 ultrasonic pachymeters. The mean CCTs are presented with continuous (US) and dotted lines (PCI).
Ultrasound pachymetry has been the standard method of evaluating corneal thickness for the past few decades. Numerous new pachymetric techniques are now available, although ultrasonic pachymetry is still the common standard. Many studies have compared CCT measurement using different methods and devices (eg, specular microscope, optical coherence tomography, ultrasound biomicroscope, Orbscan (Bausch & Lomb), and devices based on PCI).7,11–16 In this study, CCT was measured with a new noncontact ophthalmic device developed for the anterior segment of the eye using the PCI method, and the results were compared with measurements obtained using the standard ultrasonic pachymetry technique. The mean CCT values obtained with the noncontact optical method were smaller than those derived from the ultrasonic method. The differences in CCT values were statistically significant (P Z.001). The unquestionable advantage of the PCI technique is the noncontact method. The noncontact (ie, noninvasive) pachymetry method has important advantages besides its short measuring time. There is no need for anesthesia and no risk for corneal infection, which ensure a high degree of patient comfort. Giasson and Forthomme17 report significant left–right differences in ultrasonic CCT measurements due to the handedness of the examiner; ie, left corneal thickness was measured significantly larger by a right-handed examiner. This phenomenon has been reported in several studies and has been attributed to measurements not being performed perpendicularly to the corneal surface.11 In our
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
461
CCT MEASUREMENTS BY PCI AND US
Figure 2. Mean CCT values obtained with PCI and ultrasonic pachymetry.
study, the investigator sat on the left side of the patient while measuring CCT with the ultrasonic device. Therefore, measuring CCTof the right eye was somewhat complicated, but this factor did not generate large statistical differences between right and left CCT values (P Z .55). One possible explanation for this finding is that our ultrasonic device measures the CCT in an automated mode only when the probe tip is perpendicular to the corneal surface. Some authors postulate additional reasons for the larger corneal thickness values measured with ultrasonic pachymeters, including ultrasonic probe tip tilting, epithelial edema from local anesthesia, repeated pressure of the cornea,18,19 and inadequate calibration of the devices. Others suspect that the speed of sound varies between different layers of the cornea and between normal and pathologic corneal tissues.6,20,21 Some authors recommend using a modified refractive index for optical coherence
tomography (OCT) measurements (n Z 1.365) and a modified velocity of sound (1664 m/s) for ultrasonic CCT measurements.11 We measured the CCT with ultrasonography at the sound velocity of 1640 m/s. Mean corneal thickness values obtained with optical pachymeters (PCI method), OCT, and Orbscan were significantly less than values obtained with US pachymetry, suggesting that US-based devices slightly overestimate corneal thickness.11–14,16,22 These differences might be clinically relevant. However, the reason for the significantly smaller CCT values measured with the PCI method, OCT, and Orbscan remains unknown. Other studies mention larger CCTs obtained with specular microscopy, US biomicroscopy, and findings opposite the above-mentioned Orbscan findings.13 Moreover, it is impossible to state whether the optical or the US measurements are closer to the actual corneal thickness. The measured CCTs depend on the
Table 2. Differences between CCT values measured with different methods and devices and recorded as extracted value of the 2. method by the 1. method.
Author*
1. Method
2. Method
Micrometer Difference (2.-1. method)
Bechmann Mo´dis15 Rainer18 Rainer18 Wheeler7 Fishman12 Wirbelauer24 Mo´dis15 Fishman12 Tam14 Tam14
OCT Noncontact specular PCI Orbscan Specular OCT OCT Ultrasonography Ultrasonography Ultrasonography Ultrasonography
Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Ultrasonography Specular Orbscan Specular Ultrasound biomicroscopy
50 28 21.5 20 20 12 8 68 40 22 5
Present study
PCI
Ultrasonography
16.6
22
OCT Z optical coherence tomography; PCI Z partial coherence interferometry *First author
462
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
CCT MEASUREMENTS BY PCI AND US
refractive index of the cornea and the US velocity in the cornea. In our study, we compared the CCT values obtained using a new optical device and an ultrasonic device. The mean CCT was 16.6 mm greater when measured with ultrasonography than when measured with the PCI method. This is consistent with data derived from previous studies (Table 2).11–16,22 The intraobserver reproducibility represents the variability of values in a series of measurements (all of our measurements were taken by the same investigator). The results of our study show that the intraobserver variability with the PCI method is almost one-tenth smaller than with the US method (CV 0.73% versus 6.5%). This contradicts the literature in which these data are comparable. However, some data obtained with the Orbscan system show even greater variability. This fact confirms the accuracy and reproducibility of CCT values observed with the PCI method.13,23 A high positive correlation coefficient was obtained between the 2 methods of central corneal pachymetry (r Z 0.91; P Z .001), suggesting that the CCT values of these methods are count over to each other, but are not interchangeable. In conclusion, we have shown that the CCT values measured by the ultrasonic pachymeter are higher than those obtained by the PCI method. The differences between optical and ultrasonic pachymetry measurements were statistically significant. The accuracy of these pachymetric devices is acceptable in measuring CCT. Further studies are required to confirm these findings and to evaluate the accuracy, reproducibility, and independence of corneal pachymetry research using the PCI method. REFERENCES 1. McLeod SD, Kisla TA, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. Arch Ophthalmol 2000; 118:282–284 2. Joo C-K, Kim-G. Corneal ectasia detected after laser in situ keratomileusis for correction of less than 12 diopters of myopia. J Cataract Refract Surg 2000; 26:292–295 3. Vinciguerra P, Camesasca FI. Prevention of corneal ectasia in laser in situ keratomileusis. J Refract Surg 2001; 17:S187–S189 4. Argento C, Cosentino MJ, Tytiun A. Corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg 2001; 27:1440–1448 5. Rowsey JJ, Balyeat HD. Radial keratotomy: preliminary report of complications. Ophthalmic Surg 1982; 13:27–35
6. Reader AL III, Salz JJ. Differences among ultrasonic pachymeters in measuring corneal thickness. J Refract Surg 1987; 3:7–11 7. Wheeler NC, Morantes CM, Kristensen RM, et al. Reliability coefficients of three corneal pachymeters. Am J Ophthalmol 1992; 113:645–651 8. Fercher AF, Mengedoht, Werner W. Eye-length measurement by interferometer with partially coherent light. Opt Lett 1988; 13:186–188 9. Hitzenberger CK, Baumgartner A, Drexler W, Fercher AF. Interferometric measurement of corneal thickness with micrometer precision. Am J Ophthalmol 1994; 118:468–476 10. Drexler W, Baumgartner A, Findl O, et al. Submicrometer precision biometry of the anterior segment of the human eye. Invest Ophthalmol Vis Sci 1997; 38:1304–1313 11. Marsich MM, Bullimore MA. The repeatability of corneal thickness measures. Cornea 2000; 19:792–795 12. Fishman GR, Pons ME, Seedor JA, et al. Assessment of central corneal thickness using optical coherence tomography. J Cataract Refract Surg 2005; 31:707–711 13. Salz JJ, Azen SP, Bernstein J, et al. Evaluation and comparison of sources of variability in the measurement of corneal thickness with ultrasonic and optical pachymeters. Ophthalmic Surg 1983; 14:750–754 14. Tam S, Rootman DS. Comparison of central corneal thickness measurements by specular microscopy, ultrasound pachymetry, and ultrasound biomicroscopy. J Cataract Refract Surg 2003; 29: 1179–1184 15. Mo´dis L Jr, Langenbucher A, Seitz B. Corneal thickness measurements with contact and noncontact specular microscopic and ultrasonic pachymetry. Am J Ophthalmol 2001; 132:517–521 16. Rainer G, Findl O, Petternel V, et al. Central corneal thickness measurements with partial coherence interferometry, ultrasound, and the Orbscan system. Ophthalmology 2004; 111:875–879 17. Giasson C, Forthomme D. Comparison of central corneal thickness measurements between optical and ultrasound pachometers. Optom Vis Sci 1992; 69:236–241 18. Rainer G, Petternel V, Findl O, et al. Comparison of ultrasound pachymetry and partial coherence interferometry in the measurement of central corneal thickness. J Cataract Refract Surg 2002; 28:2142– 2145 19. Herse P, Siu A. Short-term effects of proparacaine on human corneal thickness. Acta Ophthalmol (Copenh) 1992; 70:740–744 20. Coleman DJ, Lizzi FL, Jack RL. Ultrasonography of the Eye and Orbit. Philadelphia, Lea & Febiger, 1977; 113–114 21. Chivers RC, Round WH, Zieniuk JK. Investigation of ultrasound axially traversing the human eye. Ultrasound Med Biol 1984; 10:173–188 22. Bechmann M, Thiel MJ, Neubauer AS, et al. Central corneal thickness measurement with a retinal optical coherence tomography device versus standard ultrasonic pachymetry. Cornea 2001; 20:50–54 23. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol 2000; 44:367–408 24. Wirbelauer C, Scholz C, Hoerauf H, et al. Noncontact corneal pachymetry with slit lamp-adapted optical coherence tomography. Am J Ophthalmol 2002; 133:444–450
J CATARACT REFRACT SURG - VOL 32, MARCH 2006
463