ARTICLE
Myopic Laser Corneal Refractive Surgery Reduces Interdevice
Agreement in the Measurement of Anterior Corneal Curvature
Haiying Jin,
M.D.,
Zhongmin Ou,
M.D.,
Objectives: To investigate interdevice differences and agreement in the
measurement of anterior corneal curvature obtained by different technologies after laser corneal refractive surgery.
Methods: The prospective study comprised 109 eyes of 109 consecutive
patients who had undergone laser-assisted in situ keratomileusis (LASIK).
Preoperative and postoperative corneal parameters were measured by
Scheimpflug imaging (Pentacam), Placido-slit-scanning (Orbscan) and
auto-keratometry (IOLMaster). Preoperative and postoperative anterior
corneal curvatures (K readings) were compared between devices. Interdevice agreement was evaluated by Bland–Altman analysis.
Results: Preoperatively, the difference of K reading for Pentacam–IOLMaster
(0.0460.20 D) was not statistically significant (P¼0.059). The differences
between Pentacam–Orbscan and Orbscan–IOLMaster were 0.2060.34 D
(P,0.001) and 20.1760.29 D (P,0.001), respectively. After surgery, no
difference was found for Pentacam–Orbscan (20.0560.38, P¼0.136). The
differences between Pentacam–IOLMaster and Orbscan–IOLMaster were
0.1360.29 D (P,0.001) and 0.1960.34 D (P,0.001). Preoperative interdevice agreement (95% limit of agreement [LOA]) between Pentacam and
Orbscan, Pentacam and IOLMaster, and Orbscan and IOLMaster were 1.31 D,
0.79 D and 1.14 D, respectively. The 95% LOAs decreased to 1.47 D, 1.14 D,
and 1.34 D after refractive surgery.
Conclusion: Corneal refractive surgery changed the preoperative and postoperative interdevice differences in corneal curvature measurements and reduced
interdevice agreement, indicating that the devices are not interchangeable.
Key Words: Laser corneal refractive surgery—Anterior corneal curvature
measurement—Interdevice agreement—Corneal power.
(Eye & Contact Lens 2017;0: 1–7)
From the Department of Ophthalmology (H.J., P.Z.), Xinhua Hospital
Affiliated to Medical College of Shanghai Jiaotong University, Shanghai,
China; Department of Physics (Z.O.), Shanghai University, Shanghai, China;
and Department of Ophthalmology (H.G.), Aier Eye Hospital, Zhengzhou,
China.
The authors have no conflict of interests to disclose.
Supported in part by National Natural Science Foundation of China
(81100655). This research was not supported by the funding from any of
the following organizations: National Institutes of Health (NIH); Wellcome
Trust; Howard Hughes Medical Institute (HHMI); and other(s).
Address correspondence to Peiquan Zhao, M.D., Department of Ophthalmology, Xinhua Hospital Affiliated to Medical College of Shanghai
Jiaotong University, Kongjiang Road, Shanghai 200092, China; e-mail:
13916917206@163.com
Accepted December 12, 2016.
Copyright 2017 The Author(s). Published by Wolters Kluwer Health,
Inc. on behalf of the Contact Lens Association of Opthalmologists. This is
an open-access article distributed under the terms of the Creative Commons
Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND),
where it is permissible to download and share the work provided it is
properly cited. The work cannot be changed in any way or used commercially without permission from the journal.
DOI: 10.1097/ICL.0000000000000364
Eye & Contact Lens Volume 0, Number 0, Month 2017
Haike Guo,
M.D.,
and Peiquan Zhao,
M.D.
M
isevaluation of corneal power is one of the principal sources of error in intraocular lens (IOL) power miscalculation
after corneal laser refractive surgery.1–15 Methods for estimating
actual corneal power after refractive surgery fall into two categories, the historical method1 and the nonhistorical methods.2–15 The
latter includes evaluation of corneal power using devices that can
measure both anterior and posterior corneal surfaces,4–6 the
ray-tracing method,6–8 and corrective algorithms calculating the
corneal power from measured anterior corneal curvature (or keratometric power), including the Shammas method,9,10 modified
Maloney method,11–14 and Haigis-L formula.15 Because anterior
corneal radius (corneal curvature) is an essential parameter in the
nonhistorical methods, measurement precision of the anterior corneal curvature after refractive surgery is paramount in corneal
power estimation. With alterations to the corneal anterior curvature
and asphericity induced by laser ablation,16 it is necessary to evaluate the comparability and agreement of corneal curvature measured by different devices after corneal refractive surgery.
A number of publications have compared corneal curvature
measured by different instruments and technologies such as autokeratometry, Placido-based computerized topography, Placido-slitscanning, Scheimpflug cameras, dual Scheimpflug devices and
point-source color light-emitting diode-based (LED) topography,
and swept-source optical coherence tomography in patients before
or after cataract surgery, in candidates before refractive surgery and
in postrefractive cases.17–29 For postrefractive cases, comparisons
of refractive changes and keratometric changes (with a keratometric
index of 1.3375) measured by keratometry or computerized topography after refractive surgery have also been studied.30,31 However,
to the best of our knowledge, very few manuscripts have reported
the influence of refractive surgery on the measurement of anterior
corneal curvature concerning interdevice differences and agreement based on different measuring technologies using the same
group of patients by a prospective study.17
This study had two aims. First, to compare different devices and
technologies, including a Scheimpflug camera, Placido-slit-scanning
computerized topography and auto-keratometry, with respect to
anterior corneal curvature measurements before and after corneal
refractive surgery and to evaluate the variations in interdevice
differences induced by refractive surgery. Second, to evaluate the
influence of refractive surgery on interdevice agreement.
PATIENTS AND METHODS
Consecutive patients who were scheduled for laser-assisted In situ
Keratomileusis (LASIK) at Xinhua Hospital Affiliated to Medical
College, Shanghai Jiaotong University, China, were invited to
participate. The study followed the tenets of the Declaration of
1
H. Jin et al.
Eye & Contact Lens Volume 0, Number 0, Month 2017
Helsinki was approved by the ethics committee of Xinhua Hospital
Affiliated to Medical College, Shanghai Jiaotong University.
Informed consent was obtained from the subjects after the nature of
procedure(s) had been explained. Inclusion criteria were healthy
individuals aged 18 to 40 years with a spherical equivalent ranging
from 21.00 to 29.00 diopters (D). The corrected visual acuities were
20/20 in all eyes. Exclusion criteria included corneal diseases, previous corneal surgery, dry eye with poor tear film, and retinal diseases. One eye of each patient was randomly selected using
a predetermined computer-generated randomization schedule generated by SPSS software (version 15.0, SPSS Inc., Chicago, IL). A
single experienced examiner performed three sets of measurements
per device for each eye in a random order. The three devices were
rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH,
Wetzlar, Germany), Placido-slit-scanning (Orbscan IIz, Bausch &
Lomb-Obtek, Inc., Salt Lake City, UT) and auto-keratometry
(IOLMaster, Carl Zeiss Meditec AG, Jena, Germany). To avoid
misevaluations because of poor quality of examinations induced by
different devices, both preoperative and postoperative examinations
were performed immediately after blinking and were carefully
inspected before being included in the study. In the event that any
device was unable to obtain a scan of acceptable quality after three
attempts, the fellow eye of the patient was used instead.
For Pentacam and Orbscan, simulated K values (K readings) at
the 3 mm ring were obtained. K readings at the 2.3 mm ring and
axial length were measured by IOLMaster. Tropicamide Phenylephrine Eye Drops (Santen, Osaka, Japan) was administrated for
cycloplegic refraction determination. After examinations of autorefraction and retinoscopy, manifest refraction was obtained by
refinements of spherical power, cylinder power and axis. Laserassisted in situ keratomileusis (LASIK) was performed by one of
us (J.H.). Corneal parameters at postoperative 12 months were
measured using the three devices. Manifest refraction 12 months
after surgery was obtained.
Theoretically, anterior corneal power or anterior corneal radius
should be compared for corneal curvature measurement. However,
keratometric power calculated using a corneal radius with a keratometric index of 1.3375 has been widely used in previous
publications and is adopted in the Shammas method and modified
Maloney method for corneal power estimation after refractive
surgery.9–15 K readings calculated with an index of 1.3375 were
therefore used for corneal curvature comparisons in our research.
To analyze the accuracy of the postoperative corneal curvature
measured by different devices, a theoretically calculated postoperative K reading was determined based on preoperative
measured corneal power and the refractive change at the corneal
plane induced by refractive surgery by the following equations:
Calculate the Sphero-Equivalent Refraction
(SEQ) at the Corneal Plane From SEQ at
Vertex Plane
postop SEQcorneal
plane 5
postop SEQspectacle plane
1 2 0:012 · postop SEQspectacle
plane
Calculate refractive change at the corneal plane (DSEQ):
DSEQ 5 postop SEQcorneal
plane 2 preop
SEQcorneal
plane
Calculate Postoperative Anterior Corneal Power
As has been proven in previous publications, the change in
posterior corneal power induced by laser refractive surgery can be
overlooked clinically, and the change in refraction is primarily
induced by altering the anterior corneal power.12–14 The calculated
postoperative anterior corneal power is calculated from preoperative anterior corneal power (376/337.5·preoperative K) and DSEQ
according to the following equation:
Calculated postoperative anterior corneal power
376
· preoperative K 2 DSEQ
5
337:5
The calculated postoperative K value is then determined from
the calculated anterior corneal power by using an index of 337.5/
376 according to the following equation: Calculated postoperative
K¼337.5/376·calculated anterior corneal power.
Sample Size Estimation for the Comparison of
Anterior Corneal Curvature
A difference of more than 0.20 D for a K reading measurement
was considered clinically relevant.32 In agreement with previously
published studies, the standard deviation of the interdevice difference was 0.5 D. Assuming an alpha (two-sided)¼0.05 and a power
of 0.9, the required sample size was determined to be 66 (Stata
10.0, Stata Corp., College Station, TX). To increase the power of
the study, additional patients were enrolled in this research.
Statistical Analysis
Statistical analysis was performed using SPSS for Windows
software. The distributions of the dataset (anterior corneal curvature)
were checked for normality using Kolmogorov–Smirnov tests. The
results indicated that the data were normally distributed (P.0.05). K
readings obtained by different methods were compared by paired
sample t tests. P values of 0.05 or less were considered statistically
significant. Correlation between different methods was determined
by linear regression. The Bland–Altman method33 performed by
MedCalc (version 15, MedCalc Software bvba, Ostend, Belgium)
was used to determine variability in the differences between K values
obtained by different methods. Differences between methods (y-axis)
were plotted against their mean (x-axis), and 95% limits of agreement (LOA) were determined. The preoperative and postoperative
Bland–Altman analyses were plotted in the same coordinate system
to identify the change of LOA induced by refractive surgery.
Calculate preoperative spherical equivalent (SEQ¼S+1/2 C) at
the corneal plane:
preop SEQcorneal
plane
5
preop SEQspectacle plane
1 2 0:012 · preop SEQspectacle
0.012 is the vertex distance in meters (m)
Calculate postoperative SEQ at the corneal plane:
2
RESULTS
plane
Study Population for Anterior Corneal
Curvature Measurement
Overall, 109 eyes of 109 patients were enrolled in this study.
The demographics of the study population are presented in Table 1.
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Eye & Contact Lens Volume 0, Number 0, Month 2017
TABLE 1.
Postrefractive Corneal Curvature Measurement
Patient Demographic and Clinical Information
Mean6SD/
Number (%)
Parameter
Range
P
Eyes (patients)
109 (109)
—
Male gender
52 (47.7%)
—
Right eye
51 (53%)
—
Age, yrs
25.564.7
19 to 39
Preoperative axial length, mm
25.6461.01
24.01 to 28.46 ,0.001
Postoperative axial length, mm
25.5260.99
23.78 to 28.28
Preoperative refraction
24.5661.76 D 21.0 to 29.0 D ,0.001
Postoperative refraction
0.2160.46 D 20.88 to 1.25 D
D, diopters; mm, millimeters, P value, P value of paired sample
t test; SD, standard deviation.
No subject with the fellow eye was used instead because of acceptable quality scanning obtained by the three devices in the
measurement.
Preoperative K Readings Measured by
Different Devices
The mean preoperative K values measured by Pentacam,
Orbscan and IOLMaster are presented in Table 2. The difference
(0.0460.20 D) between Pentacam and IOLMaster was not statistically significant (P¼0.056, independent t test). The difference
between Pentacam and Orbscan was 0.2060.34 D, which was
significantly different from zero (P,0.001, independent t test).
The difference between Orbscan and IOLMaster was 0.1760.29
D (P,0.001, independent t test) (Table 3).
Relatively high correlations between K reading measurements
were found for different devices. Correlation coefficients (r)
between Pentacam and Orbscan, Pentacam and IOLMaster, and
Orbscan and IOLMaster were 0.963 (P,0.001), 0.986
(P,0.001) and 0.973 (P,0.001), respectively (Table 3).
Bland–Altman analysis of the 95% LOA showed that Pentacam
and IOLMaster exhibited the highest agreement (20.36 D to 0.43
D, size 0.79 D). The 95% LOA between Pentacam and Orbscan
ranged from 20.45 D to 0.86 D, size 1.31 D. The 95% LOA
between IOLMaster and Orbscan ranged from 20.40 D to 0.74 D,
size 1.14 D (Fig. 1).
Postoperative K Readings Measured by
Different Devices
The mean postoperative K readings measured by Pentacam, Orbscan
and IOLMaster were 38.6961.99 D (range 33.83–42.95 D),
TABLE 2.
38.7561.91 D (33.55–43.00 D) and 38.5662.03 D (range
34.17–43.32 D), respectively (Table 2). IOLMaster provided flatter
K values than Pentacam and Orbscan. The difference between
Pentacam and IOLMaster was 0.1360.29 D (P,0.001, independent t test). The difference between Orbscan and IOLMaster was
0.1960.34 (P,0.001, independent t test). The difference between
Pentacam and Orbscan was 0.0560.38 D, which was not significantly different (P¼0.136, paired samples t test) (Table 3).
Changes of interdevice differences were observed for PentacamOrbscan (0.25 D), Pentacam-IOLMaster (0.09 D), and OrbscanIOLMaster (0.36 D) (Fig. 1).
Relatively high correlations were found between different
technologies. The correlation coefficients (r) between Pentacam
and Orbscan, Pentacam and IOLMaster, and Orbscan and
IOLMaster were 0.981 (P,0.001), 0.989 (P,0.001) and 0.987
(P,0.001), respectively (Table 3). However, interdevice agreement was reduced compared with preoperative measurements.
Bland–Altman analysis showed that agreement between Pentacam and Orbscan was moderate (95% LOA 20.79 to 0.68 D, size
1.47 D), Pentacam and IOLMaster generated higher agreement
(95% LOA 20.44 to 0.79 D, size 1.14 D), and the 95% LOA
between Orbscan and IOLMaster ranged from 20.48 to 0.86 D
(size, 1.34 D) (Fig. 1).
Comparison of Measured K and Calculated K
After Refractive Surgery
To evaluate the reliability of postoperative K readings measured
by different devices, the theoretically calculated postoperative K
value was determined by using preoperative corneal data and
refractive change at the corneal plane. The calculated K values for
Pentacam, Orbscan and IOLMaster were 38.7361.95 D (range
34.14–43.54 D), 38.5361.95 D (33.84–43.31 D) and
38.6961.94 D (range 34.17–43.32 D), respectively. Paired t test
showed that no difference was found between the measured and
calculated Pentacam K values (P¼0.355). Differences were found
between the measured and calculated K values for Orbscan
(P,0.001, paired t test) and IOLMaster (P¼0.001, paired samples
t test) (Table 3). Relatively high correlations were revealed
between the calculated and measured K values in different devices.
Bland–Altman analysis showed that the 95% LOA between the
measured and calculated Pentacam K values ranged from 20.88 to
0.81 D (size 1.69 D), and the 95% LOA between the measured and
calculated IOLMaster K values ranged from 20.95 to 0.68 D (size
1.63 D). The agreement between the measured and calculated
Orbscan K values (20.79 to 1.22 D size 2.01 D) were lower than
those of the other two devices.
K Values Obtained by Different Methods
K Value
Preoperative measured K value
Pentacam
Orbscan
IOLMaster
Postoperative measured K value
Pentacam
Orbscan
IOLMaster
Postoperative calculated K value
Pentacam
Orbscan
IOLMaster
Mean6SD (D)
Range (D)
43.2561.20
43.0461.25
43.2161.21
39.98–46.02
39.65–45.80
39.95–45.81
38.6961.99
38.7461.91
38.5662.03
33.83–42.95
33.55–43.00
33.37–42.67
38.7361.95
38.5361.95
38.6961.94
34.14–43.54
33.84–43.31
34.17–43.32
D, diopters; SD, standard deviation.
DISCUSSION
Precise measurements of corneal parameters are of vital clinical
importance in both refractive and cataract surgeries. Several studies
have compared different devices in the measurement of corneal
parameters in preoperative and postoperative cataract patients,
healthy participants and candidates before and after refractive
surgery17–29 However, very few studies have evaluated the impact
of refractive surgery on interdevice differences and agreement
based on different measuring technologies.17 A summary of previous studies and the present research comparing K reading measurements with enrolled sample size is provided in Table 4. In the
Copyright 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the Contact Lens Association of Opthalmologists.3
H. Jin et al.
Eye & Contact Lens Volume 0, Number 0, Month 2017
TABLE 3.
Comparisons and Agreements of K Values Obtained by Different Methods
Difference Between Devices
K Value
Preoperative measured K
Pentacam-Orbscan
Pentacam-IOLMaster
Orbscan-IOLMaster
Postoperative measured K
Pentacam-Orbscan
Pentacam-IOLMaser
Orbscan-IOLMaster
Postoperative calculated K-measured K
Calculated K-measured K (Pentacam)
Calculated K-measured K (Orbscan)
Calculated K-measured K (IOLMaster)
Agreement Between Devices (95% LOA)
Correlation Between Devices
Mean6SD
Range
P
Range
Size
r
P
0.2060.34
0.0460.20
20.1760.29
20.81 to 1.18
20.54 to 0.52
20.81 to 0.61
,0.001
0.059
,0.001
20.45 to 0.86
20.36 to 0.43
20.74 to 0.40
1.31
0.79
1.14
0.963
0.986
0.973
,0.001
,0.001
,0.001
20.0560.38
0.1360.29
0.1960.34
21.13 to 1.04
20.58 to 1.13
20.61 to 1.24
0.136
,0.001
,0.001
20.79 to 0.68
20.44 to 0.70
20.48 to 0.86
1.47
1.14
1.34
0.981
0.989
0.987
,0.001
,0.001
,0.001
20.0460.43
0.2260.51
20.1360.42
20.90 to 0.99
20.88 to 1.47
21.42 to 0.91
0.35
,0.001
0.001
20.88 to 0.81
20.79 to 1.22
20.95 to 0.68
1.69
2.01
1.63
0.975
0.965
0.979
,0.001
,0.001
,0.001
D, diopters; LOA, limit of agreement; SD, standard deviation.
present study, the required sample size (66 eyes) was calculated.
Moreover, one eye of each patient should be used in a study to
avoid bias induced by the symmetry of both eyes. Our research
strictly adhered to the study protocol and had a relatively larger
sample size that met the required sample size (Table 4).
The present study found that before refractive surgery, Pentacam
and IOLMaster provided similar corneal curvature measurements.
The difference between the two devices was 0.0460.20 D
(P¼0.059). The difference has no clinical significance. These
two devices also had the highest agreement with a 95% LOA of
0.79 D. The observations in our study resonate with those of most
previous studies. Several studies have compared K values measured by Pentacam and IOLMaster and have found similar mean
K values and relatively good agreement between the technologies.20–22 Lee et al.23 found that the mean difference (Pentacam-IOLMaster) in K values was 0.05 D, with 95% of
measurement differences falling within roughly 61 D (95%
LOA from 21.02 to +1.13 D). Other studies have found that the
difference between Pentacam and IOLMaster is not statistically
significant.20–22 Savini et al.24 found that IOLMaster provided
steeper K readings than Pentacam, and the difference (0.3 D)
was statistically significant. Several factors may account for the
discrepancies between that study and our research and other studies. First, the sample size was 41 eyes, which was smaller than the
required sample size. Second, the enrolled patients were elderly
cataract patients (Mean age 76.568.4 years), which may have
caused the difference in imaging quality in the measurement owing
to poor fixation because of an existing cataract and poor tear film
because of elder age.34–36
The present study also showed that K readings measured by
Orbscan were flatter than Pentacam and IOLMaster before
refractive surgery. The difference between Pentacam and Orbscan
was 0.2060.34 D (P,0.001) with a 95% LOA ranging from
20.45 to 0.86 (size 1.31 D). The difference between IOLMaster
and Orbscan was 0.1760.29 (P,0.001) with a 95% LOA ranging
from 20.40 to 0.74 (size 1.14 D). These results are in agreement
with a study performed by Tajbakhsh et al.,18 in which a relatively
larger sample size (115 eyes of 115 patients) showed that K readings measured by Orbscan were 0.37 D flatter than those measured
by Pentacam. The difference was statically significant, and the 95%
LOA value was 1.046 D. Crawford et al.19 compared K readings of
steep and flat meridians measured by Orbscan and Pentacam sep4
arately with a sample size of 30 eyes (30 patients). K readings
measured by Orbscan were flatter than those measured by Pentacam with values of 0.260.5 D (steep axis) and 0.160.5 D (flat
axis). The difference in K readings for the steep axis approached
statistical significance (0.078). Two other researchers showed dissimilar results. Hashemi and Mehravaran17 found that K readings
by Orbscan were steeper than those of Pentacam. Whang et al.25
compared the K values measured by Orbscan and IOLMaster and
found that Orbscan provided steeper K readings than IOLMaster
(0.6360.78, P¼0.000). The disparities between the two studies
and our results and those of other studies may be due to the sample
size and characteristics of the investigated populations. Both eyes
of 23 patients were enrolled in Hashemi and Mehravaran’s
research.17 The sample size was low, and biases may have been
introduced with both eyes enrolled. The sample size of Whang
et al.’s25 study (69 eyes of 69 patients) was also smaller than in
our research. Moreover, the research was a retrospective study and
enrolled eyes of patients undergoing cataract surgery.
The importance of this study was to compare anterior corneal
curvature measurements obtained by different devices and to
evaluate the agreement between devices after refractive surgery
based on the evaluation of corneal power for IOL calculation after
refractive surgery. It is well accepted that misevaluation of corneal
power is one of the principal sources of error in IOLpower
miscalculation after corneal laser refractive surgery.1–15 The precision of anterior corneal curvature measurement is paramount in
the estimation of actual corneal power after refractive surgery in
the nonhistorical methods. As there are various instruments based
on different technologies for measuring the anterior corneal curvature, it is not clear whether these devices are interchangeable or
whether interdevice differences and agreement are similar to preoperative parameters. The present study showed that interdevice
differences and agreement in anterior corneal measurement were
influenced by refractive surgery. Reduced interdevice agreement
was observed after surgery, indicating that corneal measurement
precision might decrease because of alteration of the corneal curvature and asphericity induced by refractive surgery. This might be
one source of error in IOL power calculations after refractive surgery. Pentacam and IOLMaster showed greater agreement both
before and after refractive surgery. The 95% LOA value was
0.79 D before surgery and was 1.14 D after surgery. Although
relatively higher agreement between Pentacam and IOLMaster
Eye & Contact Lens Volume 0, Number 0, Month 2017
Eye & Contact Lens Volume 0, Number 0, Month 2017
Postrefractive Corneal Curvature Measurement
FIG. 1. Bland–Altman analysis evaluating
agreement of measured K values between
different devices before and corneal
refractive surgery. (A) Agreement between
Pentacam and Orbscan; (B) Agreement
between Pentacam and IOLMaster; (C)
Agreement between Orbscan and IOLMaster. Dots: preoperative interdevice
agreement; triangles: Postperative interdevice agreement. The value between
mean values shows the change of preoperative and postoperative interdevice
difference. The difference between preoperative and postoperative LOAs shows
the reduced interdevice LOA induced by
refractive surgery.
Copyright 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the Contact Lens Association of Opthalmologists.5
H. Jin et al.
Eye & Contact Lens Volume 0, Number 0, Month 2017
TABLE 4.
First Author
Hashemi and
Mehravaran17
Tajbakhsh
et al.18
Crawford
et al.19
No. of
Eyes (No.
Patients)
Summary of Researches Comparing K Readings Measured by Different Technologies
Instruments
46 (23)
Orbscan vs Pentacam (axial
power, 3 mm zone)
Orbscan vs Pentacam,
(tangential power, 3 mm
zone)
Orbscan vs Pentacam (after
refractive surgery, axial
power, 3 mm zone)
Orbscan vs Pentacam (after
refractive surgery,
tangential power, 3 mm
zone)
115 (115) Orbscan vs Pentacam
K Value
Measured by
Device 2 (D)
Mean6SD
(D)
Difference
Between
Technologies
(D)
P
Range (D)
Size
(D)
Correlation
(r)
44.5261.67
43.8461.60
0.68
,0.01
20.08 to 1.45
1.53
0.972
44.4361.73
43.7761.68
0.66
,0.01
20.93 to 2.24
3.17
0.888
40.6961.86
40.0561.78
0.63
,0.01
20.23 to 1.5
1.73
0.972
41.3461.69
40.8061.84
0.55
0.02
21.64 to 2.73
4.37
0.804
43.5261.48
43.7961.50
20.37
,0.001
1.046
NA
44.361.9
44.561.8
20.260.5
0.078
20.790 to
0.256
21 to 0.7
1.7
NA
43.361.8
43.461.6
20.160.5
No statistical
significance
0.118
21 to 0.9
1.9
NA
20.15 to 0.92
1.07
0.931
NA
NA
NA
0.946
0.180
NA
NA
NA
0.102
NA
NA
NA
0.49
,0.001
21.02 to 1.13
20.59 to 1.18
2.15
1.77
0.94
20.6870
0.000
,0.001
NA
20.45 to 0.86
NA
1.31
NA
0.963
Symes and
Ursell.20
63 (49)
Orbscan vs Pentacam (Kvalue of steep meridian)
Orbscan vs Pentacam (Kvalue of flat meridian)
Pentacam vs IOLMaster
Symes et al.21
29 (29)
Pentacam vs IOLMaster
43.5361.736
Saad et al.22
50 (50)
IOLMaster vs Pentacam
3 mm ring (simulated K)
IOLMaster vs Pentacam
3 mm zone
Pentacam vs IOLMaster
IOLMaster vs Pentacam
43.6861.28
43.9361.415 20.11
calculated
by author
43.4761.564 0.06
calculated
by author
43.7661.31 20.0860.27
43.6861.28
43.7761.33
20.0860.35
43.3161.62
43.9761.44
43.2661.59
43.6761.49
44.6761.53
43.2561.20
44.0361.41
43.0461.25
0.0560.55
0.30
calculated
by author
0.6360.78
0.2060.34
43.2561.20
43.2161.21
0.0460.20
0.06
20.36 to 0.43
0.79
0.986
43.0461.25
43.2161.21
20.1760.29
,0.001
20.74 to 0.40
1.14
0.973
38.6961.99
38.7561.91
20.0560.38
0.136
20.79 to 0.68
1.47
0.981
38.6961.99
38.5662.03
0.1360.29
,0.001
20.44 to 0.70
1.14
0.989
38.7561.91
38.5662.03
0.1960.34
,0.001
20.48 to 0.86
1.34
0.987
Lee et al.23
Savini et al.24
Whang et al.25
The present
study
30 (30)
95% LOA
K Value
Measured by
Device 1
Mean6SD (D)
49 (41)
41 (41)
69 (69) Orbscan vs IOLMaster
109 (109) Pentacam vs Orbscan
(before refractive
surgery)
Pentacam vs IOLMaster
(before refractive
surgery)
Orbscan vs IOLMaster
(before refractive
surgery)
Pentacam vs Orbscan (after
refractive surgery)
Pentacam vs IOLMaster
(after refractive surgery)
Orbscan vs IOLMaster
(after refractive surgery)
43.8261.439
D, diopters; SD, standard deviation.
might indicate the superiority of the two devices in the measurement of anterior corneal curvature after refractive surgery, the
change in interdevice difference and reduced agreement indicated
that the three devices were not interchangeable. Before surgery,
Pentacam and IOLMaster showed similar results, and Orbscan provided flatter K readings than Pentacam and IOLMaster. After
refractive surgery, the difference between Pentacam and Orbscan
was not statistically significant, and IOLMaster provided slightly
steeper K readings than Pentacam and Orbscan. To further investigate the reliability of different devices in corneal curvature measurement after surgery, comparisons between measured and
calculated K values after refractive surgery were performed for
each device. No difference was found between the measured and
calculated K value for Pentacam, whereas the differences between
the measured and calculated K values were significantly different
6
for IOLMaster and Orbscan. The differences in K value measurements obtained by different devices may originate from the different measuring principles of the three devices.
Several studies have compared K readings measured by different
technologies after corneal refractive surgery.26,27 Ventura et al.26
compared K readings measured by a novel point-source color
LED-based topographer, a Placido-disk topographer, and a combined Placido-based and dual Scheimpflug device in normal eyes
and in postrefractive eyes. No interdevice difference was found
between the devices. In another study performed by Lee et al.,27
dual rotating Scheimpflug–Placido, swept-source optical coherence
tomography, and Placido-scanning-slit systems were used to compare corneal parameters in normal eyes and in postrefractive cases.
Interdevice differences were revealed in both groups. In contrast to
our study, the normal and postrefractive eyes were enrolled from
Eye & Contact Lens Volume 0, Number 0, Month 2017
Eye & Contact Lens Volume 0, Number 0, Month 2017
two groups of populations in the two studies. Therefore, the influence of refractive surgery on interdevice agreement could not be
evaluated. To the best of our knowledge, only one study has compared parameters measured by Pentacam and Orbscan before and
after refractive surgery using the same group of patients.17 Changes
in both anterior and posterior corneal powers were studied. The
research revealed that Orbscan yielded steeper anterior corneal
curvature measurements than Pentacam both before and after
refractive surgery, which is not in agreement with our result. As
has been discussed, the possible reasons for the disparity may be
due to the study design. In that study, both eyes of 23 patients were
enrolled. Moreover, although preoperative and postoperative interdevice agreements were analyzed respectively, a reduced agreement between them was not observed after surgery. The present
research suggests for the first time that myopic laser corneal refractive surgery reduces interdevice agreement in the measurement of
anterior corneal curvature.
In conclusion, to the best of our knowledge, we believe that this
study is the first to observe reduced interdevice agreement after
corneal refractive surgery, indicating that devices for anterior
corneal curvature measurement are not interchangeable. The
reduced agreement might be one source of error in IOL power
calculation after corneal refractive surgery. Further studies with
more corneal curvature measurement devices and comparisons of
their roles in IOL power calculation accuracy after refractive
surgery are necessary in our future studies.
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Copyright 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the Contact Lens Association of Opthalmologists.7