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Keywords:

  • acoustic factor;
  • continuous wear;
  • corneal swelling;
  • hydrogel contact lenses;
  • Orbscan pachymetry;
  • ultrasonic perimetry

Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

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.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

The study was a one-week, prospective, double-masked, randomised, controlled trial.

Subjects

Twenty subjects were enrolled in the study (14 women and six men). The mean age was 20.3 ± 2.2 years (range 18 to 25). The spherical equivalent refractive error ranged from +2.00 D to -4.75 D with a mean of -1.60 D ± 1.75 D and none of the patients exhibited anisometropia greater than 1.00 D. Eleven subjects were previous contact lens wearers (two used rigid gas permeable and nine used soft contact lenses). Subjects were excluded if they had active ocular-surface disease, such as significant dry eye, papillary conjunctivitis, corneal opacities, current medication that could affect ocular physiology, astigmatism (greater than 2.00 D) or if they had previously worn continuous wear (CW) lenses. Subjects had vision correctable to 6/6 in each eye. Informed consent was obtained from each subject after approval was granted by the Human Sciences Ethics Committee of the University of Valladolid. All subjects were treated in accordance with the Declaration of Helsinki.

Lenses

Subjects were randomly assigned to one week of continuous wear with a high-Dk contact lens (lotrafilcon A, Focus Night & Day, CIBA Vision, Duluth, GA, USA; Dk = 175) in one eye and a low-Dk lens (etafilcon A, Acuvue 2, Johnson & Johnson Vision Care, Jacksonville, FL, USA; Dk = 33) in the other eye. The characteristics of the lenses used in this study are listed in Table 1. It was predicted that the difference in oxygen permeability (Dk) of these lenses would produce various levels of hypoxia and various degrees of corneal swelling.6,7 Lotrafilcon A is a silicone hydrogel contact lens that has been approved for up to 30 nights of continuous wear. Etafilcon A is a hydrogel lens with much lower oxygen permeability and has been approved (Food and Drug Administration) for up to six nights of overnight wear.

Table 1. Nominal parameters for contact lenses.*Central thickness quoted for -3.00 D.
Lens typeBase curveLens diameterCentral thickness*Water contentDkPower (D)
Lotrafilcon A8.60 mm13.80 mm0.080 mm24%175+2.00 to -4.75
Etafilcon A8.40–8.80 mm14.00 mm0.084 mm58%33+2.00 to -4.75

An independent investigator generated the randomisation schedule. Each subject wore both types of lenses at the same time, with one lens in one eye and the other lens in the second eye.

Instrumentation

Corneal thickness was measured with Orbscan II (Bausch & Lomb, Rochester, NY, USA, version 3.12) and an ultrasonic pachymeter (Sonogage Inc, Cleveland, OH, USA; calibrated by the manufacturer). Ultrasound was always performed after Orbscan to avoid corneal changes resulting from contact with the probe or topical anaesthesia.

Three sets of measurements were performed on each cornea with the Orbscan before the fitting of the contact lens (baseline visit) and immediately after contact lens removal (post-lens wear visit). The procedure involving the use of Orbscan has been described previously.6,7,16,24 Corneal thickness was measured in five different corneal locations, namely, at the centre and at the superior, inferior, nasal and temporal (2.5 mm from the closest limbus) corneal positions. The mean of three measurements of each corneal position was used as the final value. The same experimenter and masked operator took all Orbscan measurements during all visits.

Ultrasonic measurements were obtained before the fitting of the contact lens (baseline visit) and immediately after contact lens removal (post-lens wear visit). Five measurements were obtained at each corneal position, central, superior, inferior, nasal and temporal (2.5 mm from the closest limbus). The cornea was anaesthetised with a drop of proparacaine hydrochloride (0.5%). The probe was applied perpendicular to the corneal surface for each measurement at the apex for the central measurement and approximately 2.5 mm from the closest limbus for the peripheral measurements (standard clinical conditions). The same experimenter and masked operator performed the ultrasonic pachymetry at all visits. The mean of five measurements of each corneal area was used as the final value.

Procedures

Three visits were scheduled for each subject. Participants were screened according to the inclusion and exclusion criteria. The procedures governing the study were explained and informed consent was obtained. Visual acuity, keratometry and biomicroscopic and baseline pachymetry were conducted. One week of without contact lens wear was prescribed to avoid corneal thickness changes related to previous contact lens wear.

Baseline visits were conducted to register the Orbscan and ultrasonic pachymetric measurements, after which both high- and low-Dk contact lenses were fitted randomly in a contralateral eye design. An independent investigator fitted the lenses.

After seven days of continuous wear of the contact lenses, Orbscan and ultrasonic pachymetric measurements were taken immediately after lens removal. Ultrasonic pachymetry (without contact lens) was performed immediately after the Orbscan procedures in central and peripheral corneal locations. To ensure that corneal oedema induced by overnight wear had dissipated, all study-related visits occurred between 4 and 8 p.m. As a result, the observed differences were mainly related to the effect of contact lens wear in prolonging the effects of overnight wear. During this time, the eye was most physiologically stable.25,26

Data analysis

Statistical analysis was performed using SPSS 14.0 (SPSS Inc., Chicago, IL, USA) statistical package for Windows.

The Orbscan measurements (with and without the acoustic factor) of the central and peripheral cornea (superior, inferior, nasal and temporal locations) were compared with the measurements obtained by ultrasonic pachymetry before and after the subject used contact lenses. Linear regression27 was used to quantify the correlation between both thickness measurements (Orbscan and ultrasound) of the central and peripheral cornea. A 95% interval of confidence (95% IC) was calculated for the slope and constant of the linear regression equations obtained from the data from each corneal location. Also, a single equation was calculated based on the data from all corneal locations. Finally, the Pearson correlation coefficient was determined. A p-value of less than 0.05 was considered statistically significant.

The degree of agreement between Orbscan and ultrasonic pachymetry in all corneal locations (before and after contact lens wear) was evaluated using Bland–Altman analysis.28 Limits of agreement were calculated (mean ± two standard deviations).

Central and peripheral corneal swelling was determined based on the difference (in percentage) between the last and baseline visit corneal thickness measurements obtained with both instruments (Orbscan, with and without acoustic factor and ultrasound). A paired t-test was used to detect differences in the thickness of each corneal position attributable to each type of lens. A p-value of less than 0.05 was considered statistically significant.

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

No subjects had significant biomicroscopic signs (greater than grade 1 on the Efron grading scale)29 of contact lens-induced complications (corneal staining, limbal injection, striae, folds et cetera). Two subjects left the study, one because of front surface deposits (lotrafilcon A lens) and the other lost a contact lens (an etafilcon A lens).

Central and peripheral pachymetry in corneas without swelling

Table 2 shows the means, standard deviations and limits of agreement for central and peripheral corneal pachymetric measurements in corneas without corneal swelling (baseline) obtained with ultrasound and Orbscan (with and without the 0.92 acoustic factor). The mean differences between ultrasound and Orbscan, before and after the application of the 0.92 acoustic factor are also shown.

Table 2. Agreement between Orbscan (with and without the 0.92 acoustic factor) and ultrasonic pachymetry in corneas without swelling. Diff = mean difference with standard deviation (SD); IC = interval of confidence; LoA = limits of agreement (mean x 2 SD); Orbscan = Orbscan pachymetry; Pearson CC = Pearson correlation coefficient; US = ultrasonic pachymetry. Number of eyes = 36.
 UltrasonicOrbscanDiff (95% IC)LoAPearson CC
Mean corneal thickness (µm) without the acoustic factor
Central558 ± 40573 ± 45-15 ± 22 (-7 to -22)-59 to 290.869 (p < 0.01)
Superior668 ± 42683 ± 42-15 ± 44 (0 to -30)-103 to 730.445 (p < 0.01)
Inferior697 ± 47658 ± 3839 ± 33 (50 to 28)-26 to 1050.720 (p < 0.01)
Nasal675 ± 52690 ± 48-15 ± 42 (-1 to -29)-99 to 690.654 (p < 0.01)
Temporal688 ± 64639 ± 4350 ± 54 (68 to 31)-59 to 1580.537 (p < 0.01)
Mean corneal thickness (µm) with the (0.92) acoustic factor
Central558 ± 40527 ± 4131 ± 21 (38 to 24)-10 to 720.869 (p < 0.01)
Superior668 ± 42628 ± 3940 ± 42 (54 to 26)-45 to 1240.445 (p < 0.01)
Inferior697 ± 47605 ± 3592 ± 32 (103 to 81)27 to 1570.720 (p < 0.01)
Nasal675 ± 52635 ± 4440 ± 41 (54 to 27)-41 to 1220.654 (p < 0.01)
Temporal688 ± 64587 ± 39101 ± 54 (119 to 83)-7 to 2080.537 (p < 0.01)

Without applying the acoustic factor correction, the Orbscan measurements were higher than the ultrasonic measurements in the centre, superior and nasal corneal positions. After applying the 0.92 acoustic factor correction, Orbscan pachymetry was lower than ultrasonic pachymetry in all corneal locations. Because the acoustic factor has a constant value (0.92) the correlation is the same between Orbscan, with and without acoustic correction and ultrasonic pachymetry (Table 2).

Central and peripheral pachymetry in corneas with swelling

Table 3 shows the means and standard deviations for central and peripheral corneal pachymetry in corneas with corneal swelling (post-lens wear visit) obtained with ultrasound and Orbscan (with and without 0.92 acoustic factor). The mean differences between ultrasound and Orbscan, before and after the application of the 0.92 acoustic factor, are also shown.

Table 3. Agreement between Orbscan (with and without the 0.92 acoustic factor) and ultrasonic pachymetry in corneas with swelling. Diff = mean difference with standard deviation (SD); IC = interval of confidence; LoA = limits of agreement (mean x 2 SD); Orbscan = Orbscan pachymetry; Pearson CC = Pearson correlation coefficient; US = ultrasonic pachymetry. Number of eyes = 36.
 UltrasonicOrbscanDiff (95% IC)LoAPearson CC
Mean corneal thickness (µm) without the acoustic factor
Central562 ± 40594 ± 52-32 ± 17 (-26 to -38)(-66 to 2)0.910 (p < 0.06)
Superior682 ± 40705 ± 46-23 ± 30 (-12 to -34)(-83 to 37)0.634 (p < 0.07)
Inferior711 ± 50683 ± 4528 ± 34 (40 to 16)(-40 to 96)0.612 (p < 0.08)
Nasal693 ± 50710 ± 51-17 ± 44 (-1 to -33)(-105 to 71)0.494 (p < 0.09)
Temporal706 ± 63660 ± 4946 ± 44 (62 to 30)(-42 to 134)0.663 (p < 0.10)
Mean corneal thickness (µm) with the (0.92) acoustic factor
Central562 ± 40546 ± 4816 ± 15 (21 to 11)(-14 to 46)0.910 (p < 0.06)
Superior682 ± 40649 ± 4333 ± 29 (65 to 43)(-25 to 91)0.634 (p < 0.07)
Inferior711 ± 50628 ± 4283 ± 37 (95 to 70)(9 to 157)0.612 (p < 0.08)
Nasal693 ± 50653 ± 4840 ± 42 (63 to 34)(-44 to 124)0.494 (p < 0.09)
Temporal706 ± 63607 ± 4599 ± 44 (114 to 84)(11 to 187)0.663 (p < 0.10)

Without applying the acoustic factor correction, the Orbscan measurements were higher than the ultrasonic pachymetric measurements in the centre superior and nasal corneal positions but, with application of the 0.92 acoustic factor correction, the Orbscan pachymetric measurements were lower than ultrasonic measurements in all corneal locations.

Relationship between Orbscan and ultrasonic pachymetry (central and peripheral cornea)

Correlations between Orbscan and ultrasonic pachymetry were poorer for measurements taken at peripheral corneal locations than in those obtained centrally (r2 value) in corneas without (baseline) and with contact lens-induced swelling (Table 4).

Table 4. Regression equations to obtain the ultrasonic equivalent of Orbscan pachymetry from the original Orbscan measurements without the acoustic equivalent. Slope and constant with 95% confidence interval of the regression equations to obtain the ultasonic equivalent pachymetry from the Orbscan measurements without the acoustic factor are shown. Orbscan = Orbscan without acoustic factor
Corneal locationWithout corneal swelling (baseline)With corneal swelling (post-lens wear visit)
Equation (slope x Orbscan + constant)r2pEquation (slope x Orbscan + constant)r2p
Slope (95% IC)Constant (95% IC)Slope (95% IC)Constant (95% IC)
Central0.73 (0.57 to 0.89)134.45 (41.82 to 227.05)0.88<0.010.73 (0.65 to 0.81)129.66 (80.87 to 178.46)0.92<0.01
Superior0.41 (0.10 to 0.72)381.63 (166.95 to 596.34)0.23<0.010.65 (0.45 to 0.85)224.31 (79.61 to 369.00)0.58<0.01
Inferior0.85 (0.56 to 1.15)135.56 (-60.08 to 331.19)0.50<0.010.77 (0.48 to 1.06)185.90 (-13.07 to 384.88)0.49<0.01
Nasal0.68 (0.37 to 0.98)204.60 (-4.80 to 413.99)0.42<0.010.61 (0.33 to 0.89)260.30 (58.56 to 462.04)0.40<0.01
Temporal0.72 (0.30 to 1.14)232.02 (-36.64 to 500.68)0.26<0.010.94 (0.60 to 1.27)87.04 (-135.18 to 309.25)0.52<0.01

A point-to-point analysis of regression was performed to obtain expressions that would make the data between the Orbscan II and ultrasonic pachymetry more comparable by providing the specific equations shown in Table 4. The 95% confidence intervals of the regression equations (slope and constant) were also calculated for all corneal positions in corneas with and without swelling (Table 4). The magnitude of these differences was not constant across the peripheral cornea; thus, the differences cannot be rectified by the use of a single acoustic correction factor. A similar relationship was found both before and after contact lens wear (corneas with and without swelling) (Figure 1).

image

Figure 1. Regression lines between ultrasonic and Orbscan pachymetry in all corneal locations (centre, superior, inferior, temporal and nasal) in corneas without (black) and with swelling (grey). 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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From the data, an equation (0.71 x Orb + 197.55) with mean slope and constant was calculated to provide a single equation applicable to all corneal positions. Using the mean equation, linear regression in terms of the r2 coefficient was numerically equivalent to that calculated from the specific equations in corneas with swelling (r2 of 0.92 in central, 0.58 in superior, 0.49 in inferior, 0.40 in nasal and 0.52 in temporal corneal locations) and in corneas without swelling (r2 of 0.88 in central, 0.23 in superior, 0.50 in inferior, 0.42 in nasal and 0.26 in temporal corneal locations).

Corneal swelling

The etafilcon A contact lens induced greater corneal swelling than the lotrafilcon A contact lens, as measured by ultrasonic pachymetry and Orbscan (with and without the 0.92 acoustic factor) after one week of continuous wear.

Table 5 shows corneal swelling with lotrafilcon A and etafilcon A contact lenses in all corneal positions. All differences in the extent of central and peripheral corneal swelling induced by the two contact lens types were statistically significant when Orbscan pachymetry was used (p < 0.05 paired t-test), whether or not the acoustic factor was applied. The Orbscan corneal swelling percentage was the same with pachymetry values obtained with and without applying the 0.92 acoustic factor.

Table 5. Corneal swelling induced by lotrafilcon A and etafilcon A contact lenses after one week of continuous wear measured with ultrasonic and Orbscan (with and without the acoustic factor) pachymetry in the centre and the periphery (superior, inferior, nasal and temporal) of the cornea.*Student paired t-test.
% Corneal swellingLotrafilcon A (%)Etafilcon A (%)p-value*
Ultrasonic
Centre-0.89 ± 1.951.89 ± 3.200.003
Superior2.34 ± 5.202.63 ± 7.360.840
Inferior1.69 ± 6.552.91 ± 8.410.404
Nasal1.90 ± 8.235.24 ± 5.540.079
Temporal1.62 ± 9.763.18 ± 9.080.503
Orbscan (with and without the acoustic factor)
Centre1.67 ± 2.224.98 ± 3.070.008
Superior2.04 ± 3.256.62 ± 8.390.034
Inferior1.38 ± 1.745.24 ± 5.540.009
Nasal1.50 ± 2.003.41 ± 3.140.009
Temporal1.27 ± 1.474.46 ± 3.320.003

DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

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.

image

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

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES

We compared 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 acoustic factor in the reliability to detect corneal swelling. The acoustic factor was irrelevant in the assessment of contact lens-induced corneal swelling and cannot be applied constantly across the cornea.

Overall, the present study confirms that Orbscan, a non-invasive, rapid scanning system that evaluates corneal thickness across the entire corneal surface, is useful for the study of central and peripheral contact lens-induced corneal swelling in continuous wear. In prospective studies, it could be recommended that Orbscan readings be used without any form of acoustic factor.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. CONCLUSIONS
  7. REFERENCES
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