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- MATERIALS AND METHODS
Purpose: The aim of the present study was to compare central and peripheral corneal thickness measurements in corneas with and without contact lens (CL)-induced corneal swelling, in order to establish the constancy of the Orbscan acoustic factor (AF) for reliability in detection of CL-induced corneal swelling.
Methods: Corneal thickness was measured in five corneal locations (central, superior, inferior, nasal and temporal) using both ultrasonic and Orbscan pachymetry before and after one week of high Dk (lotrafilcon A) and low Dk (etafilcon A) contact lens continuous wear (CW). Lenses were randomly fitted for CW in the right and left eyes of 20 healthy subjects to induce various amounts of corneal swelling.
Results: Etafilcon A contact lenses induced greater corneal swelling than the lotrafilcon A contact lenses (p < 0.05, paired Student t-test). Equal corneal swelling percentages (with and without the application of the acoustic factor) were determined from Orbscan measurements. Application of the 0.92 AF did not permit adjustment to all corneal locations. The correlations between Orbscan with and without the 0.92 AF and ultrasonic pachymetry were higher for central pachymetry (r2 > 0.88; p < 0.01) and lower for peripheral pachymetry (r2 < 0.58; p < 0.01) with similar trends in corneas with and without CL-induced swelling.
Conclusions: The acoustic factor proposed by the manufacturer to obtain corneal thickness with the Orbscan compared with those from ultrasonic pachymetry was not valid for all corneal topographic positions. The AF is unnecessary when determining CL-induced corneal swelling.
Corneal thickness measurement is a reliable technique for determination of corneal health and is useful in the study of corneal physiology.1 Corneal thickness measurement is considered essential in the diagnosis of multiple corneal diseases2 and in the study of the effects of contact lens (CL) wear.3 Hypoxia is the primary cause of corneal swelling in contact lens wearers4,5 and contact lenses with low oxygen permeability (Dk) induce greater corneal swelling as determined by central6 and peripheral7 pachymetry.
Techniques for measuring corneal thickness include optical pachymetry (Haag–Streit pachymetry), ultrasonic pachymetry, confocal microscopy, ultrasonic biomicroscopy, optical scanning slit analysis (Orbscan II, Bausch & Lomb Inc, Rochester, NY, USA), the Scheimpflug camera (Pentacam, Oculus Inc, Lynnwood, WA, USA) and optical coherence tomography (OCT) with well-correlated measurements of central corneal thickness.8–14
Orbscan is a scanning slit (light)-based optical reflectance non-contact method that measures central and peripheral corneal thickness.11–14 Because, in general, Orbscan central corneal thickness measurements are rather higher than those obtained using ultrasound pachymetry,15,16 the manufacturer recommends using an algorithm (the so-called acoustic factor[AF]) that involves multiplying the corneal thickness by 0.92;17 however, numerous studies18–23 have found that, for clinical purposes, the same correction factor (0.92) cannot be applied accurately to all corneal points.18 As a result, the AF must be specified for each corneal location and for each range of measurement.18,20 Other authors conclude that Orbscan and ultrasound are not interchangeable and that Orbscan should be used without any form of AF.19,23
The purpose of this study was to compare central and peripheral corneal thickness measurements using ultrasonic and Orbscan pachymetry in corneas with and without contact lens-induced corneal swelling to establish the constancy of the AF for reliability in detection of contact lens-induced corneal swelling.
- Top of page
- MATERIALS AND METHODS
The precision and repeatability of optical and ultrasonic pachymeters, slit-scanning topography (Orbscan) and other techniques for measurement of corneal thickness and swelling have been determined previously for central corneal thickness in healthy subjects,14,30,31 corneal refractive surgery procedures32,33 and central corneal swelling induced by contact lens hypoxia.6,20,34–36 Because the accuracy of any pachymeter cannot be directly assessed in vivo in humans, it is evaluated indirectly.20 We used different contact lenses (with low and high Dk) to induce various levels of hypoxia and various degrees of corneal swelling.
The corneal swelling percentage in central and peripheral corneal locations was determined (Table 5) for each type of contact lens using measurements obtained with ultrasound and the Orbscan with and without the acoustic factor correction. Differences were statistically significant in the central cornea (p < 0.05) with both ultrasonic and Orbscan pachymetry; however, in the corneal periphery, differences were significant only with Orbscan pachymetry (p < 0.05), whether or not the 0.92 acoustic factor was applied.7 This result could be related to better precision or repeatability of the Orbscan peripheral corneal thickness measurements7,37 and to problems associated with performing the ultrasonic peripheral measurement (for example, repeatability of the placement, perpendicularity of the probe with the cornea, indentation of the corneal surface with the probe, problems with the topical anaesthetic and patient co-operation). Etafilcon A induced more corneal swelling than lotrafilcon A. This is due to the fact that the degree of contact lens-induced hypoxia is directly related to the oxygen transmissibility of the contact lens.4–6,38–41
The use of a correction (acoustic) factor to obtain central and peripheral Orbscan pachymetric measurements comparable with those of ultrasonic pachymetry is controversial. Some authors recommend that a specific acoustic factor be calculated for each corneal location and ideally for each range of measurement,18,20 while others found that the acoustic factor is affected by corneal swelling and conclude that the use of a single acoustic factor is untenable.20 Finally, others recommend that no acoustic factor be used.19,23 The results of the present study confirm that Orbscan (with and without an acoustic factor) is useful in studies of the effects of the contact lens on central and peripheral corneal thickness and that it permits monitoring of corneal thickness variation due to contact lens-induced corneal swelling.
Ultrasonic pachymetry has been the standard for estimating corneal thickness for the past few decades18 but this technique shows lower repeatability than Orbscan, especially in peripheral pachymetry,19,42 and is also more variable in corneas with oedema.3,42 Therefore, in clinical practice, it could be desirable to combine both procedures, ultrasound and Orbscan, because the results are not interchangeable,19 especially in peripheral measurements.
Our results are in agreement with conclusions from several previous reports.18,21,22 For example, Gonzalez–Meijome and colleagues18 compared superior and inferior corneal profiles obtained by Orbscan and by ultrasonic pachymetry in 24 healthy subjects (no contact lens wearers) and concluded that the acoustic factor proposed by the manufacturer was not valid for all corneal topographic positions. Doughty and Jonuscheit21 and Jonuscheit and colleagues22 compared Orbscan (without an acoustic factor) with ultrasound in central, mid-peripheral (2.50 to 2.75 mm from the centre) and peripheral locations (4.0 to 4.5 mm from the centre) in a nasal to temporal corneal profile in healthy subjects (no contact lens wearers) and concluded that corneal thickness at different locations cannot be aligned by the use of a single acoustic factor.21,22,37
The main difference between our study and previous studies is that we compared Orbscan and ultrasonic pachymetry centrally and peripherally in horizontal and vertical corneal profiles at four peripheral locations (2.5 mm from the closest limbus). Additionally, while previous studies compared measurements from a single visit,18,19,22 we compared measurements taken at two different visits, including a first visit in corneas without swelling (baseline) and a second visit in corneas with swelling (after one week of continuous wear of contact lenses), to determine the constancy of the acoustic factor for detection of contact lens-induced corneal swelling. The use of this experimental design showed that use of the acoustic factor in prospective studies is irrelevant because the Orbscan measurement of the corneal swelling percentage was the same with and without applying the 0.92 acoustic factor (Table 5).
Lu and associates20 studied the constancy of the acoustic factor for adjustment of Orbscan central pachymetry after wearing corneal refractive therapy lenses overnight and control high Dk (Dk = 100) RGP contact lenses in 44 healthy subjects. They found that the acoustic factor depended on the corneal thickness and suggested that adjustment of the pachymetric measurements using a single acoustic factor would be unjustifiable.20 They concluded that there is a positive correlation between the average corneal swelling and the percentage change of the adjusted acoustic factor over time. They explained the differences in the acoustic factor by suggesting that the refractive index in the posterior stroma of the swollen corneas may be even closer to that of the aqueous, which may lead to increased difficulty in Orbscan detection of the posterior corneal edge.20
We believe that this hypothesis is unlikely because Lu and associates20 did not describe the percentage corneal swelling induced by RGP contact lenses. Moreover, we found the same corneal swelling percentage with and without the application of the 0.92 acoustic factor. An accurate assessment of corneal thickness is necessary to determine the level of corneal oedema, and Orbscan measurements have detected the expected difference between low and high Dk contact lenses in the centre6 and in the periphery of the cornea (Table 5).7,42
Another difference from previous studies is that the present work uses data obtained to calculate specific linear regression equations to improve the agreement between ultrasound and Orbscan for pachymetry of central and four peripheral corneal locations (Table 4). The 95% CI of the slope and constant were also calculated (Table 4) and show a wide range for clinical and research use. Using the average slope and average constant of the five corneal locations, we calculated a single equation (0.71 x Orb + 197.55). The specific slopes and constants of the regression lines obtained at each corneal location (Figure 2) had similar correlation coefficients (r2, Table 4). This result suggests that the use of specific linear regression equations to adjust the Orbscan pachymetry could have a limited use in clinical application and scientific research.
Figure 2. Regression lines between ultrasonic pachymetry and equivalent pachymetry calculated with Orbscan measurements with specific equations (Table 2) to each corneal location (black) and with mean equation (0.71 x Orb + 197.55) to all corneal locations (grey). Solid regression lines represent the regression to baseline values (corneas without swelling) and discontinued regression lines represent the regression to final values (corneas with swelling). Circles represent data to baseline (corneas without swelling) and triangles represent data to final visit (post-lens wear visit; corneas with swelling).
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A definitive reason for the discrepancy between corneal thicknesses measured with Orbscan has not been found.21,19,43 The following hypotheses have been proposed: the thickness of the tear film, changes in the stromal refractive index, different degrees of corneal hydration, change in corneal shape, corneal density and corneal structure, different corneal locations of the ultrasonic probe, misalignment of the probe or the Orbscan, the optical acquisition process of the Orbscan, and others.18,19,21,44–47 The cause of the error lies in the optical acquisition process because the repeatability of the measurements is good.19,37,48
The percentages for the corneal swelling suggest that Orbscan pachymetry (with and without the 0.92 acoustic factor) could be useful in prospective studies of central and peripheral contact lens-induced corneal swelling. Our results suggest that use of an acoustic factor is unnecessary in determining corneal contact lens-induced swelling (Table 5). This is in agreement with previous results,6,7,16,38,42 which demonstrated that the Orbscan can be used in corneal research and in clinical settings to study contact lens-induced corneal swelling. Therefore, Orbscan readings may be used without any form of acoustic factor19,23 to determine corneal thickness changes in prospective studies.
Nevertheless, specific clinical recommendations may be necessary to determine a ‘cut value’ for Orbscan pachymetry because an accurate assessment of corneal thickness is important in many clinical situations. Examples include glaucoma evaluation because of the effect of corneal thickness on the measurement of intraocular pressure24,49 in the differential diagnosis of corneal ectatic conditions50 or in the assessment of patient eligibility for refractive surgery to avoid post-operative complications, such as keratectasia.32,33,51,52 A combination of both methods, Orbscan and ultrasound, could be useful in diagnosis and evaluation.19