Journal of Refractive Surgery

Original Article Supplemental Data

Repeatability of Keratometry Measurements Obtained With Three Topographers in Keratoconic and Normal Corneas

Emmanuel Guilbert, MD; Alain Saad, MD; Maud Elluard, MD; Alice Grise-Dulac, MD; Hélène Rouger, OD; Damien Gatinel, MD

Abstract

PURPOSE:

To assess the repeatability of the corneal topography functions of Orbscan II (Bausch & Lomb, Rochester, NY), OPD-Scan III (Nidek, Gamagori, Japan), and iTrace (Tracey Technologies, Houston, TX) in keratoconic eyes and in a control group of normal patients.

METHODS:

In this prospective cohort study, patients were recruited between November 2011 and May 2012. Measurements were performed with a combined Placido-scanning slit system (Orbscan II) and two combined Placido-aberrometer systems (OPD-Scan III and iTrace). Repeatability limit and intraclass correlation coefficients (ICCs) of keratometric readings were calculated.

RESULTS:

Fifty-nine keratoconic eyes of 34 patients and 54 normal eyes of 27 patients were included. Three groups were evaluated: all stage I–IV keratoconic eyes (59 eyes), a subgroup consisting of stage I–II keratoconic eyes (41 eyes), and normal eyes (54 eyes). For almost all parameters studied, the repeatability limit was higher in the two groups of keratoconic eyes compared to normal eyes with all three topographers, indicating lower repeatability. For the maximum keratometry measurement, repeatability limit was 1.73, 1.49, and 1.41 diopters (D) in the stage I–IV keratoconic eyes group, 1.11, 1.02, and 0.98 D in the stage I–II keratoconic eyes group, and 0.61, 0.37, and 1.02 D in the normal eyes group with Orbscan II, OPD-Scan III, and iTrace, respectively.

CONCLUSIONS:

Topographies performed in keratoconic eyes are less repeatable than those performed in normal eyes. Threshold values of keratometric changes used to ascertain keratoconus progression should be carefully considered. Caution should be taken when interpreting the topographies of such patients. The higher variability should be taken into account before performing any treatment.

[J Refract Surg. 2016;32(3):187–192.]

Abstract

PURPOSE:

To assess the repeatability of the corneal topography functions of Orbscan II (Bausch & Lomb, Rochester, NY), OPD-Scan III (Nidek, Gamagori, Japan), and iTrace (Tracey Technologies, Houston, TX) in keratoconic eyes and in a control group of normal patients.

METHODS:

In this prospective cohort study, patients were recruited between November 2011 and May 2012. Measurements were performed with a combined Placido-scanning slit system (Orbscan II) and two combined Placido-aberrometer systems (OPD-Scan III and iTrace). Repeatability limit and intraclass correlation coefficients (ICCs) of keratometric readings were calculated.

RESULTS:

Fifty-nine keratoconic eyes of 34 patients and 54 normal eyes of 27 patients were included. Three groups were evaluated: all stage I–IV keratoconic eyes (59 eyes), a subgroup consisting of stage I–II keratoconic eyes (41 eyes), and normal eyes (54 eyes). For almost all parameters studied, the repeatability limit was higher in the two groups of keratoconic eyes compared to normal eyes with all three topographers, indicating lower repeatability. For the maximum keratometry measurement, repeatability limit was 1.73, 1.49, and 1.41 diopters (D) in the stage I–IV keratoconic eyes group, 1.11, 1.02, and 0.98 D in the stage I–II keratoconic eyes group, and 0.61, 0.37, and 1.02 D in the normal eyes group with Orbscan II, OPD-Scan III, and iTrace, respectively.

CONCLUSIONS:

Topographies performed in keratoconic eyes are less repeatable than those performed in normal eyes. Threshold values of keratometric changes used to ascertain keratoconus progression should be carefully considered. Caution should be taken when interpreting the topographies of such patients. The higher variability should be taken into account before performing any treatment.

[J Refract Surg. 2016;32(3):187–192.]

Corneal topography has revolutionized the diagnosis and follow-up of keratoconus, making it possible to make accurate measurements of keratometry and corneal thickness at each point of the corneal surface.1–3 Corneal topography is also used in refractive surgery because it helps to identify patients at risk for ectasia after LASIK before surgery.4–7

Collagen cross-linking is considered to be the only therapeutic procedure capable of slowing or halting the natural history of keratoconus and is indicated for progressive keratoconus.8 The definition of progressive keratoconus varies in the literature. In most studies, an increase of the steepest keratometry value of 1.00 diopter (D) or more or an increase in the mean keratometry value of 0.75 D or more is considered as a sign of progression of the ectatic disease.9 It is thus essential to assess the repeatability of topographic measurements because the decision to perform cross-linking treatment depends on topography results. Indeed, a worsening of the disease can only be confirmed when the evolution of topographic parameters is greater than the repeatability limit (a value less than or equal to the absolute difference between two test results obtained under repeatability conditions can be expected to lie within a probability of 95%).

The Orbscan II system (Bausch & Lomb, Rochester, NY) combines a Placido disk with scanning-slit topography. It is still widely used, although newer technologies have emerged in recent years, such as the Scheimpflug camera. Recent articles in refereed journals have indeed used this device to perform topographic measurements.10 Previous studies showed that it may be less accurate for pachymetry measurements for keratoconus.11 The OPD-Scan III (Nidek, Gamagori, Japan) and iTrace (Tracey Technologies, Houston, TX) are two devices combining a Placido topographer and aberrometer. To our knowledge, no previous study has investigated their repeatability regarding the corneal topography of patients with keratoconus.

The purpose of this study was to assess the repeatability of the corneal topography functions of these three devices in keratoconus and to compare it to the repeatability of measurements obtained in a control group of normal patients.

Patients and Methods

Patients

Patients were prospectively recruited from the Department of Refractive Surgery at the Rothschild Ophthalmic Foundation, Paris, France, between November 2011 and May 2012. The local ethics committee approved this study, which followed the tenets of the Declaration of Helsinki. The study and data accumulation were achieved with approval from the Rothschild Foundation Institutional Review Board.

Three groups of patients were constituted. The first group was the group of all eyes with keratoconus (stage I to IV according to the Amsler–Krumeich classification). The second group was a subgroup of the first group, excluding the eyes with stage III and IV keratoconus (only eyes with stage I or II keratoconus were included in this group). The third group was the control group, with candidates for refractive surgery who had corneal topographies considered as normal. All topographies were reviewed by an expert in corneal topography analysis (DG).

Rigid contact lens wearers and patients with other ocular pathology (elevated intraocular pressure, glaucoma, previous ocular surgery, ocular trauma, corneal scars, and inability to maintain fixation) and other mental or physical disease preventing topographies from being properly recorded were excluded from the study. Soft contact lens wearers had to remove them at least 72 hours before the examination was performed.

Measurements were taken at the same time of day (between 10 AM and 6 PM). Both eyes of each patient were used for statistical analysis because eyes were not compared to one another.

Corneal Topography

All enrolled patients underwent corneal topography with three corneal topographers: a Placido-scanning-slit system (Orbscan II) and two combined Placidoaberrometer systems (OPD-Scan III and iTrace). Three successive measurements were performed with each device in random order. If an examination was considered unsatisfactory because of excessive eye movement or eyelid blinking, it was performed again.

Repeatability of the following parameters was analyzed for the three devices: keratometry in the steepest (Kmax) and flattest (Kmin) axis, keratometric astigmatism (SimK), and maximum and minimum keratometry at 3 mm (K13mm and K23mm, respectively) and 5 mm (K15mm and K25mm, respectively). Repeatability of anterior and posterior best-fit sphere (BFSant and BFSpost, respectively) and thinnest point and irregularity index at 3 mm and 5 mm (Irreg3mm and Irreg5mm, respectively) measurements were also studied for Orbscan II. The maximum and minimum keratometric values were recorded over the axial map.

Repeatability Calculation

Repeatability is the closeness of agreement between the results of successive measurements of an identical test material performed under defined conditions. Conditions include the same operator, same apparatus, and a short time between analyses. The conditions under which these measurements were performed are known as the repeatability conditions. The results of the repeatability experiments can be used to calculate a standard deviation, called the repeatability standard deviation. This value is useful in determining a repeatability limit; a value less than or equal to the absolute difference between two test results obtained under repeatability conditions can be expected to lie within a probability of 95%.

The repeatability limit was calculated from the individual standard deviations as follows:

R=SD×t0.05,n
where R is the repeatability limit, SD is the standard deviation, and t is the critical value of the Student's t distribution at the 95% confidence level (t = 4.3 for 3 measurements).12

The mean repeatability limit in the population was calculated by adding the square of individual repeatability of each individual eye and calculating the root mean square of the mean value as follows:

R=(R2+R2……+R2+R2/N
where N is the number of patients in the study population. Repeatability is given with 95% confidence. In the remaining sections of the article, the term repeatability is used as equivalent to the confidence interval of repeatability corresponding to the range of random errors determined at the 95% confidence level. Standard deviation and repeatability limit are expressed in absolute values and in percentage of the mean values of each tested parameter in the study population.

Results

Fifty-nine keratoconic eyes of 34 patients and 54 normal eyes of 27 patients were included in this study. Nine eyes of patients with keratoconus were excluded from the study: 2 patients had a corneal opacity in one eye, 2 patients had undergone penetrating keratoplasty in one eye, and 5 patients had a unilateral keratoconus (forme fruste keratoconus in the fellow eye). According to the Amsler–Krumeich classification, 13 eyes were classified as stage I, 28 eyes as stage II, 9 eyes as stage III, and 9 eyes as stage IV.

The mean age was 28.91 ± 10.90 years in the stage I–IV keratoconic eyes group (59 eyes), 30.55 ± 9.02 years in the stage I–II keratoconic eyes group (41 eyes), and 31.89 ± 9.00 years in the control group (54 eyes). Table 1 shows the mean values obtained for the studied parameters in the three groups. Table A (available in the online version of this article) shows the mean values, repeatability limits, and ICCs obtained with Orbscan II for keratometry, corneal astigmatism, irregularity, thinnest point, and BSF measurements in the three groups. Tables BC (available in the online version of this article) show the mean values, repeatability limits, and ICCs obtained with OPD-Scan III and iTrace, respectively, for keratometry and corneal astigmatism in the three groups.

Mean Readings Obtained With the Three Topographers (Mean ± SD)

Table 1:

Mean Readings Obtained With the Three Topographers (Mean ± SD)

Mean Values, Repeatability Limits, and ICCs Obtained With Orbscan II

Table A:

Mean Values, Repeatability Limits, and ICCs Obtained With Orbscan II

Mean Values, Repeatability Limits, and ICCs Obtained With OPD-Scan III

Table B:

Mean Values, Repeatability Limits, and ICCs Obtained With OPD-Scan III

Mean Values, Repeatability Limits, and ICCs Obtained With iTrace

Table C:

Mean Values, Repeatability Limits, and ICCs Obtained With iTrace

In the stage I–IV keratoconus eyes group, repeatability limits for Kmax measurements were 1.73, 1.49, and 1.41 D for Orbscan II, OPD-Scan III, and iTrace, respectively. It was 1.45, 1.62, and 1.33 D for Kmin measurements and 0.98, 1.61, and 1.38 D for SimK measurements. In the stage I–II keratoconus eyes group, repeatability limits for Kmax measurements were 1.11, 1.02, and 0.98 D for Orbscan II, OPD-Scan III, and iTrace, respectively. It was 1.17, 1.14, and 1.35 D for Kmin measurements and 0.72, 1.69, and 1.04 D for SimK measurements. In the control group, repeatability limits for Kmax measurements were 0.61, 0.37, and 1.02 D for Orbscan II, OPD-Scan III, and iTrace, respectively. It was 0.65, 0.32, and 0.91 D for Kmin measurements and 0.36, 0.51, and 0.38 D for SimK measurements.

Figure 1 shows the repeatability limit of measurements obtained with Orbscan II in the three groups. Figure 2 shows the repeatability limit of measurements obtained with OPD-Scan III in the three groups. Figure 3 shows the repeatability limit of measurements obtained with iTrace in the three groups.

Repeatability limit of measurements obtained with Orbscan II (Bausch & Lomb, Rochester, NY) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Figure 1.

Repeatability limit of measurements obtained with Orbscan II (Bausch & Lomb, Rochester, NY) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Repeatability limit of measurements obtained with OPD-Scan III (Nidek, Gamagori, Japan) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Figure 2.

Repeatability limit of measurements obtained with OPD-Scan III (Nidek, Gamagori, Japan) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Repeatability limit of measurements obtained with iTrace (Tracey Technologies, Houston, TX) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Figure 3.

Repeatability limit of measurements obtained with iTrace (Tracey Technologies, Houston, TX) in patients and controls. Kmax = keratometry in the steepest axis; Kmin = keratometry in the flattest axis; SimK = keratometric astigmatism; K13mm = maximum keratometry at 3 mm; K23mm = minimum keratometry at 3 mm; K15mm = maximum keratometry at 5 mm; K25mm = minimum keratometry at 5 mm

Discussion

Measurement validity or accuracy is dependent on two types of measurement uncertainties: systematic errors and random errors. The accuracy (validity) of an instrument indicates the closeness between the mean measured value and the true value of each measurement. The precision (repeatability, reliability) indicates the instrument's ability to repeat its own results.13,14 Assessing the accuracy of the three devices we used was not the goal of this study. However, accuracy testing is dependent on the repeatability of the device, which we did study.

The calibration of an instrument against known standards eliminates systematic errors. The errors associated with routine use of an instrument are random; these can be minimized by a detailed routine procedure and the use of repeated independent measurements. The determination of random errors leads to the identification of instrument measurement repeatability.

To our knowledge, this is the first study evaluating repeatability of keratometric measurements in keratoconus with Orbscan II, OPD-Scan III, and iTrace. Furthermore, no previous study on this subject has used the repeatability limit as a statistical method. Few previous studies have evaluated repeatability of topography measurements in keratoconic eyes. Ortiz-Toquero et al. found that the Oculus Keratograph provided repeatable measurements of corneal topography in healthy and keratoconic eyes.15 For Montalbán et al.16 and Savini et al.,17 the Sirius system provided repeatable measurements in eyes with keratoconus. Conversely, McMahon et al. observed a loss of repeatability in patients with keratoconus with the three Placido ring videokeratography instruments used in this study.18 In a recent study comparing five devices, Hashemi et al. found that repeatability of keratometry readings was acceptable in mild keratoconus, whereas all devices had reduced repeatability in cases with maximum keratometry readings greater than 55.00 D.19

In our study, ICCs obtained for the studied parameters were generally excellent with the three devices, in both the control and keratoconus groups. Concerning Orbscan II, this is in accordance with previous studies that showed that repeatability of measurements was good in normal non-ectatic corneas.20–22 To our knowledge, no previous study investigated the repeatability of the OPD-Scan III and iTrace for keratometry readings. Data currently available only show that repeatability is lower when measuring corneal aberrations with the OPD-Scan III than with the iTrace.23 Our data show that this is also the case for keratometric readings in keratoconic eyes for most of the parameters studied. Conversely, in normal eyes, the repeatability limit is lower (and thus the repeatability is better) for most of the keratometric readings with the OPD-Scan III compared to the iTrace.

Although ICC is generally used to assess the reliability of measurements, a good ICC does not mean that the variation observed between two successive measurements is low enough to be insignificant. If the variation of a measured parameter is low compared to the usual value of this parameter, ICC will be good and the variation between successive measurements can be significant in clinical practice (eg, Kmin and Kmax are usually between 40.00 and 47.00 D for a normal patient, and the range of variation of these parameters when successive measurements are performed is less than 1.00 D). To better assess the potential clinical impact of these variations, we calculated the repeatability limit, which is a value less than or equal to the absolute difference between two test results obtained under repeatability conditions that can be expected to lie within a probability of 95%.

In our series, the repeatability limit was lower (and therefore the repeatability was better) in the control group than in the two keratoconic eyes groups for almost all studied parameters and with the three devices tested. This demonstrates the need for caution with the results provided by these topographers in patients with keratoconus because their readings are less repeatable. For example, the repeatability limit of the steepest keratometry in the stage I–IV keratoconic eyes group was 1.73 D with the Orbscan II, 1.49 D with the OPD-Scan III, and 1.41 D with the iTrace (vs 0.61, 0.37, and 1.02 D, respectively, in the control group). This means that the difference in Kmax between two measurements should be above those values to certify a real progression of the disease. When considering stage I–II keratoconic eyes only, which have higher risk of progression and for which cross-linking is of most interest, repeatability is better but still lower than in normal eyes: the repeatability limit of the Kmax was 1.11 D with the Orbscan, 1.02 D with the OPD-Scan, and 0.98 D with the iTrace. This shows that an increase of the Kmax of 0.75 or even 1.00 D is probably not enough to assert that keratoconus is actually progressive, especially with the Orbscan II. Previous studies showed similar results: repeatability of topographic or aberrometric measurements is lower in ectatic corneas compared to normal corneas.24–27

The lower repeatability observed in keratoconic eyes may have several explanations. Ectatic corneas have a less regular and more complex shape, which makes them more difficult to analyze by topographers. Furthermore, patients with keratoconus have more difficulties to fixate at a target because of blurred vision, which can result in a slight offset of the topography. In addition, because of the irregular surface of the cornea of these patients, spreading of tears between each blink is probably less uniform in patients with keratoconus compared to patients with normal corneas. It has been shown that the condition of the tear film can influence topography readings.28–30 This may in part explain the lower repeatability of topographic measurements in ectatic corneas. Finally, poorer repeatability observed in keratoconic eyes is probably partly due to normal fixation nystagmus. Fixation normally causes the gaze to hunt around 0.1 mm or more while instrument alignment and focus are achieved. This displacement does not cause a significant variation in the normal nearly spherical cornea, but can cause a large variation in corneas with large curvature variations (as with keratoconus).

In most studies, keratoconus is considered progressive if the keratometry values increase by at least 0.75 or 1.00 D between two topographies performed at 6-month intervals.31–34 Our results show that this difference may not be sufficient to conclude that the disease is actually progressive, because the difference between two successive topographies performed on the same eye can be higher than the expected difference by which the keratoconus is considered to be progressive. Ignoring this variability of topography in keratoconic corneas could lead to treatment with collagen cross-linking in some eyes with otherwise stable keratoconus disease. Similarly, it could result in the wrongful interpretation that this treatment was effective in stabilizing a cornea, when in fact the keratoconus disease was actually stable before performing the cross-linking.

Our results show that topographies performed in keratoconic eyes are less reliable than those performed in normal eyes and this difference, also in mild stage I–II keratoconus, increases with the grade of the keratoconus. This suggests that the threshold values of keratometric changes used to ascertain keratoconus progression should be carefully considered, depending on the device on which they were performed. In the meantime, caution should be taken when interpreting the topography results of such patients, and their higher variability should be taken into account before deciding to perform any treatment such as collagen cross-linking.

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Mean Readings Obtained With the Three Topographers (Mean ± SD)

ParameterOrbscan IIOPD-Scan IIIiTrace



Keratoconus Stage I-IVKeratoconus Stage I-IIControlsKeratoconus Stage I–IVKeratoconus Stage I–IIControlsKeratoconus Stage I–IVKeratoconus Stage I–IIControls
Kmax (D)48.19 ± 0.3046.46 ± 0.1944.04 ± 0.1148.16 ± 0.2146.40 ± 0.1643.71 ± 0.0747.32 ± 0.2445.64 ± 0.1743.50 ± 0.19
Kmin (D)44.45 ± 0.2644.03 ± 0.2142.78 ± 0.1244.85 ± 0.2144.08 ± 0.1642.74 ± 0.0644.48 ± 0.2343.46 ± 0.1842.54 ± 0.17
SimK (D)3.71 ± 0.162.43 ± 0.111.27 ± 0.073.11 ± 0.252.28 ± 0.230.95 ± 0.092.84 ± 0.232.19 ± 0.150.96 ± 0.06
K13mm (D)51.07 ± 0.4049.41 ± 0.3244.12 ± 0.1351.37 ± 0.2449.46 ± 0.2343.99 ± 0.1048.14 ± 0.2246.20 ± 0.1643.77 ± 0.19
K23mm (D)44.49 ± 0.3843.54 ± 0.3942.70 ± 0.1245.34 ± 0.2943.95 ± 0.2242.69 ± 0.1144.74 ± 0.1943.44 ± 0.1442.82 ± 0.16
K15mm (D)50.15 ± 0.2949.10 ± 0.2643.95 ± 0.1351.40 ± 0.2650.14 ± 0.1943.84 ± 0.0849.71 ± 0.2349.30 ± 0.1643.79 ± 0.19
K25mm (D)44.27 ± 0.2444.35 ± 0.2542.56 ± 0.1341.66 ± 0.3342.07 ± 0.3542.38 ± 0.1241.40 ± 0.1740.94 ± 0.1042.32 ± 0.18
Irreg3mm (D)4.60 ± 0.233.81 ± 0.171.26 ± 0.12
Irreg5mm (D)4.76 ± 0.223.75 ± 0.171.71 ± 0.13
TP (µm)468.10 ± 5.65467.81 ± 4.58555.66 ± 3.50
FSant (D)43.52 ± 0.1843.38 ± 0.1242.10 ± 0.09
BFSpost (D)53.63 ± 0.3953.20 ± 0.2551.06 ± 0.16

Mean Values, Repeatability Limits, and ICCs Obtained With Orbscan II

ParameterKeratoconus Stage I–IVKeratoconus Stage I–IIControls



Mean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICC
Kmax (D)48.19 ± 0.301.73 (3.59%)0.98446.46 ± 0.191.11 (2.39%)0.98544.04 ± 0.110.61 (1.38%)0.988
Kmin (D)44.45 ± 0.261.45 (3.27%)0.97444.03 ± 0.211.17 (2.65%)0.97342.78 ± 0.120.65 (1.52%)0.985
SimK (D)3.71 ± 0.160.98 (26.36%)0.9052.43 ± 0.110.72 (29.39%)0.8171.27 ± 0.070.36 (28.21%)0.995
K13mm (D)51.07 ± 0.402.20 (4.31%)0.97949.41 ± 0.321.86 (3.77%)0.97844.12 ± 0.130.67 (1.52%)0.989
K23mm (D)44.49 ± 0.382.35 (5.29%)0.92243.54 ± 0.392.35 (5.40%)0.89042.70 ± 0.120.65 (1.52%)0.986
K15mm (D)50.15 ± 0.291.65 (3.30%)0.94349.10 ± 0.261.35 (2.76%)0.92043.95 ± 0.130.68 (1.54%)0.986
K25mm (D)44.27 ± 0.241.25 (2.82%)0.97744.35 ± 0.251.28 (2.89%)0.97242.56 ± 0.130.65 (1.52%)0.987
Irreg3mm (D)4.60 ± 0.231.23 (26.83%)0.9733.81 ± 0.171.08 (28.40%)0.9741.26 ± 0.120.64 (50.67%)0.931
Irreg5mm (D)4.76 ± 0.221.22 (25.52%)0.7723.75 ± 0.171.09 (29.05%)0.7151.71 ± 0.130.62 (36.37%)0.967
TP (µm)468.10 ± 5.6529.24 (6.25%)0.981467.81 ± 4.5824.67 (5.27%)0.980555.66 ± 3.5019.22 (3.46%)0.942
BFSant (D)43.52 ± 0.181.12 (2.57%)0.95243.38 ± 0.120.95 (2.18%)0.96142.10 ± 0.090.51 (1.21%)0.991
BFSpost (D)53.63 ± 0.392.34 (4.37%)0.92053.20 ± 0.251.80 (3.38%)0.92651.06 ± 0.160.90 (1.76%)0.985

Mean Values, Repeatability Limits, and ICCs Obtained With OPD-Scan III

ParameterKeratoconus Stage I–IVKeratoconus Stage I–IIControls



Mean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICC
Kmax (D)48.16 ± 0.211.49 (3.10%)0.96246.40 ± 0.161.02 (2.20%)0.89843.71 ± 0.070.37 (0.85%)0.996
Kmin (D)44.85 ± 0.211.62 (3.62%)0.94044.08 ± 0.161.14 (2.59%)0.84542.74 ± 0.060.32 (0.76%)0.994
SimK (D)3.11 ± 0.25 D1.61 (51.86%)0.7952.28 ± 0.231.69 (74.23%)0.9540.95 ± 0.090.51 (53.73%)0.963
K13mm (D)51.37 ± 0.241.43 (2.77%)0.95949.46 ± 0.231.20 (2.43%)0.95743.99 ± 0.100.53 (1.20%)0.991
K23mm (D)45.34 ± 0.291.94 (4.27%)0.84143.95 ± 0.221.66 (3.78%)0.80342.69 ± 0.110.64 (1.49%)0.987
K15mm (D)51.40 ± 0.261.78 (3.46%)0.95550.14 ± 0.191.24 (2.47%)0.93243.84 ± 0.080.42 (0.96%)0.995
K25mm (D)41.66 ± 0.332.31 (5.54%)0.57342.07 ± 0.352.57 (6.12%)0.56842.38 ± 0.120.80 (1.89%)0.979

Mean Values, Repeatability Limits, and ICCs Obtained With iTrace

ParameterKeratoconus Stage I–IVKeratoconus Stage I–IIControls



Mean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICCMean ± SDRepeatability Limit (%)ICC
Kmax (D)47.32 ± 0.241.41 (2.98%)0.98145.64 ± 0.170.98 (2.15%)0.98443.50 ± 0.191.02 (2.35%)0.967
Kmin (D)44.48 ± 0.231.33 (3.00%)0.96843.46 ± 0.181.35 (3.12%)0.94942.54 ± 0.170.91 (2.14%)0.972
SimK (D)2.84 ± 0.231.38 (48.49%)0.9402.19 ± 0.151.04 (47.56%)0.9570.96 ± 0.060.38 (39.68%)0.975
K13mm (D)48.14 ± 0.221.15 (2.39%)0.99246.20 ± 0.161.02 (2.20%)0.98843.77 ± 0.191.04 (2.38%)0.965
K23mm (D)44.74 ± 0.191.11 (2.48%)0.98743.44 ± 0.141.27 (2.92%)0.96442.82 ± 0.160.86 (2.00%)0.978
K15mm (D)49.71 ± 0.231.25 (2.52%)0.99449.30 ± 0.161.40 (2.84%)0.99143.79 ± 0.191.08 (2.47%)0.968
K25mm (D)41.40 ± 0.171.06 (2.56%)0.98240.94 ± 0.101.10 (2.69%)0.98242.32 ± 0.181.02 (2.41%)0.970
Authors

From the Département of Cataract & Refractive Surgery, Rothschild Foundation, Paris, France (EG, AS, ME, AG-D, HR, DG); and the Center for Expertise and Research in Optics for Clinicians, Paris, France (EG, AS, AG-D, DG).

Drs. Gatinel and Saad are consultants for Technolas Perfect Vision, and Dr. Gatinel is a consultant for Nidek and Physiol Inc. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (EG, AS, DG); data collection (HR); analysis and interpretation of data (EG, AS, ME, AG-D, DG); writing the manuscript (EG); critical revision of the manuscript (AS, ME, AG-D, HR, DG); statistical expertise (EG); supervision (DG)

Correspondence: Emmanuel Guilbert, MD, Fondation Ophtalmologique Adolphe de Rothschild, Service du Dr. Gatinel, 25 rue Manin, 75019 Paris, France. E-mail: emmanuel.guilbert83@gmail.com

Received: March 27, 2015
Accepted: December 03, 2015

10.3928/1081597X-20160113-01

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