The popularity of refractive surgery has increased the importance of accurate corneal thickness measurements. Preoperative assessment of central corneal thickness (CCT) is a requisite to ensure patient eligibility in terms of a sufficient corneal thickness for a safe correction and an acceptable residual thickness to reduce the risk for iatrogenic ectasia.1,2 Pachymeters are generally based on ultrasound (US) or optical technologies.3 The most commonly used devices are US pachymeters, which are also recognized as the gold standard for performing CCT measurements.4–6 However, in this technique, a probe must come in contact with the corneal surface. This requires local anesthesia and is associated with the risk for infection.7,8 Other disadvantages of the technique are the probability of misalignment, variations in placing the probe in repeated measurements, and corneal indentation by the probe.9,10 Also, US measurements are affected by the fluctuations in corneal tissue hydration. For these reasons, use of noncontact methods with an acceptable accuracy has been suggested.11
The Orbscan topography system (Bausch & Lomb) uses a computerized slit-scanning method to image the anterior and posterior corneal surfaces and a full corneal pachymetry map in a noncontact acquisition that takes fewer than 2 seconds.12–14 Although highly repeatable,2,15–17 the validity of its corneal thickness readings compared with US measurements is a matter of debate.
Results in initial comparative studies showed that the Orbscan overestimated CCT up to approximately 30 μm5,11,14,18; thus, the use of acoustic equivalent correction factors was suggested to make the Orbscan readings more comparable to US readings. The Orbscan II software allows users to apply such an acoustic factor to corneal thickness measurements displayed by the system, and the value recommended by the manufacturer is 0.92.
González-Méijome et al.19 measured and compared the CCT and peripheral corneal thickness at equal distances from the center. They found that a different equation was required for each measured point. Other studies suggest using a different correction factor for corneas that are thin postoperatively8,20 and performing further studies of the validity of the Orbscan corneal thickness measurements in thin and thick corneas.21
Our experience and published reports hint that the Orbscan II tends to underestimate the CCT in thinner corneas, whether normal, keratoconic,22,23 or after laser refractive surgery,24 and generates overestimated readings in thicker corneas. This study was designed to determine the effect of corneal thickness on the agreement between the Orbscan II and US measurements in unoperated eyes. We also tried to determine whether any correction factor could provide acceptable validity for Orbscan II measurements, especially in corneas thinner or thicker than normal.
PATIENTS AND METHODS
Prospectively, in a stratified sampling approach, 177 patients admitted to Noor Vision Correction Center were enrolled in 1 of 3 groups based on corneal thickness: less than 500 μm (thin corneas), 500 to 600 μm (medium corneas), and more than 600 μm (thick corneas). Exclusion criteria were a history of ocular surgery, history of ocular trauma causing apparent damage to the corneal surface, very poor cooperation, corneal haze, keratoconus, and use of a soft contact lens in the past 3 days or a hard contact lens in the past 3 weeks.
Central Corneal Thickness Measurement
The central thickness was measured in all corneas at least 3 hours after waking, first with the noncontact Orbscan II and then with the US method. The same technician performed measurements in each patient on the same day, and readings in the right eyes were recorded.
With the Orbscan II, after proper positioning, patients were instructed to focus on the red blinking light in front of them. The slits were adjusted, and an acquisition was made. During acquisition, 40 slits were projected and scanned over the cornea, each providing 240 data points from the anterior and posterior corneal surfaces. The corneal thickness was computed for every corneal point from the elevation difference between the 2 surfaces. In this study, only central readings were collected without applying an acoustic equivalent correction factor.
For US measurements, the cornea was anesthetized with tetracaine 0.5% eyedrops, and the sterilized probe was placed on the cornea as perpendicularly as possible. Five consecutive measurements were generated, the mean of which was used in the analysis.
The Kolmogorov-Smirnov test was used to confirm the normal distribution of data sets (P>.05). Mean interdevice differences were determined through paired t tests in each group and in all eyes. The Pearson correlation coefficient and regression coefficient between the mean Orbscan II and US readings and their differences were calculated. The interdevice agreement was determined by the method described by Bland and Altman,25 differences between paired measurements were plotted against their means, and 95% limits of agreement (LoA) were calculated. Intraclass correlation coefficients (ICC) were used to assess the level of agreement as described by Fleiss and Cohen.26 To interpret the degree of agreement, the guidelines provided by Landis and Koch27 (Table 1) were used.
Regression analysis was used to determine regression equations: Ultrasound CCT = (Orbscan II CCT × slope) + intercept. The equation obtained in each group was applied to that group's Orbscan II data as the acoustic equivalent correction factor to derive corrected readings, after which the interdevice difference and agreement analyses were repeated. Results are expressed as mean ± standard deviation (SD) based on a significance level of <.05.
One hundred seventy-seven right eyes were examined in 66 men (37%) and 111 women (63%). The mean age of the patients was 29.7 ± 8.2 years (range 18 to 54 years). Table 2 shows the number of eyes, male-to-female ratio, and age range of patients in each corneal thickness group. The Pearson correlation coefficient between mean Orbscan II and US readings and their differences was 0.414 (P<.001) (Figure 1).
Table 3 shows the mean CCT readings and the interdevice differences. In the less than 500 μm group, the Orbscan II underestimated the CCT in 19 of 31 eyes but the mean difference (−1.6 ± 15.8 μm) was not statistically significant (P = .580). The Orbscan II overestimated the CCT in 105 of 115 eyes in the 500 to 600 μm group, and the mean difference (21.3 ± 20.2 μm) was statistically significant (P = .0001). In the more than 600 μm group, the Orbscan II overestimated the readings in 28 of 31 eyes (P<.0001), with a mean difference of 27.2 ± 20.9 μm. Overall, there was a mean overestimation of 18.0 ± 21.7 μm by the Orbscan II, a difference that was statistically significant (P<.0001).
Linear Correlation Between Data Sets
Table 4 shows the equations derived from the regression analyses. After these equations were applied, the mean difference between the data sets was near zero in all groups (P>.05). These differences are shown in Table 4.
Table 5 shows the ICC values in each group and in all eyes. There was a substantial interdevice agreement in the thin and medium cornea groups. Measurements in the thick cornea group, however, showed moderate agreement between the 2 systems.
Bland-Altman plots for each group before and after applying each correction equation (Table 4) are shown in Figures 2 and 3, respectively. Identical scales are used for easier comparisons. Correcting the Orbscan II readings with these equations reduced data distribution and the width of 95% LoA. This change was more apparent in the less than 500 μm group. The upper and lower 95% LoA before and after correcting the Orbscan II readings are shown in Table 6.
Studies of the earlier versions of the Orbscan report overestimation of corneal thickness compared with US measurements,5,11,14,18 just as we found a mean overestimation of 18 μm in the total group, for which the following equation was computed: (0.77 × Orbscan II) + 110.70. Based on such findings, the Orbscan II and acoustic equivalent correction factor were introduced and the value recommended by the manufacturer was 0.92. Most subsequent studies found underestimations by this system when the correction factor was used4,21,28,29 and concluded that the interdevice agreement is not acceptable and the devices should not be used interchangeably.17,19,28,30–34 However, some based their judgment on mean differences and correlation coefficients,14,35,36 reported no significant interdevice differences,6,31,35 and accepted the validity of the Orbscan corneal thickness measurements.14,21,35,36 In addition, several acoustic equivalent correction factors for improving the validity of the Orbscan pachymetry readings have been suggested.9,14,31,32
In this study, we attempted to investigate this matter further and assess the effect of corneal thickness on interdevice differences. Although it has been mentioned19,28 (H. Hashemi, MD, S. Mehravaran, MD, “Central Corneal Thickness Measurement with Pentacam, Orbscan II, and Ultrasound Before and After Laser Refractive Surgery for Myopia,” presented at the XXIV Congress of the European Society of Cataract & Refractive Surgeons, London, United Kingdom, September 2006), to our knowledge, there are no peer-reviewed published reports concerning such an effect. For this purpose, we divided our sample into groups of different CCT ranges and found that the mean interdevice difference was not significant in the thin group but increased in thicker corneas. Naturally, correcting measurements in the thin group with an equation proper for the thick group would lead to great underestimations. This may, to some extent, explain the Orbscan II underestimations after ablative procedures, as reported in the above-mentioned presentation.
In addition, we determined the interdevice agreement in each group by computing the ICC and 95% LoA; results were indicative of lower agreement in measurements of thicker corneas. However, the range of data can affect the ICC. In this study, the ICC for the total may have been overrated because of the wide range in the total sample population, while in the group of thick corneas, the low ICC may be attributable to the small range of values.
To compare levels of agreement expressed in 95% LoA, it would be easier to consider the width of these limits. For example, according to the information in Table 6, the width of the 95% LoA was 62.2 μm (−32.7 to 29.5) in thin corneas, 79.1 μm (−18.2 to 60.9) in medium corneas, and 82.1 μm (−13.8 to 68.3) in thick corneas. This means the agreement is worst with thick corneas and the difference between the Orbscan II CCT measurements and their US pairs may range from an underestimation of 13.8 μm to an overestimation of 68.3 μm in 95% of cases. The remaining 5% may even have differences beyond this range. Such differences are not acceptable because a difference as little as 10 μm can lead to a different treatment plan.
In the next step, the correction factor for each range of thickness was calculated and applied to the Orbscan data and the analyses were repeated with corrected data. As expected, the mean difference in all 3 groups changed to near zero and the levels of agreement improved. To be more precise, the width of the 95% LoA decreased as much as 25 μm in thin corneas, 9 μm in medium corneas, and 17 μm in thick corneas, leaving widths of 37 μm, 70 μm, and 65 μm, respectively. One could reason that the agreement did not acceptably improve. However, a practical advantage of using correction factors is that the ranges of expected differences lie symmetrically along the zero axis. For instance, if the CCT reading is 550 μm and we use the bottom equation in Table 4, there is a 95% chance that the acoustic equivalent is 550 ± 35 μm because the 95% LoA for this correction factor are −35 μm and 35 μm.
Overestimations and underestimations were seen not only in the total sample but also in each separate group. However, rates varied in different groups; the Orbscan II underestimated the CCT in 19.8% of the total 177 corneas, 61.0% in the thin group, 11.0% in the medium group, and 9.7% in the thick group. Applying a correction factor evens out the distribution (ie, evens rates of overestimation and underestimation) rather than reduces the variability. This means a simple linear transformation is not sufficient to achieve interchangeable data.
Further studies with more samples in the higher and lower ranges of corneal thickness are needed to investigate this issue in more detail, as are studies of the accuracy of US pachymeters. In the meantime, these 2 devices should not be used interchangeably and when the accuracy of the measurement is vital or a reading is borderline for proceeding with a particular procedure, it is advisable to recheck the corneal thickness with a US pachymeter.
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