Investigation of a real-time location system of corneal astigmatic axis
Zhao et al. Eye and Vision
Investigation of a real-time location system of corneal astigmatic axis
Jian-Guo Zhao 0
An-Peng Pan 0
Ke Zheng 0
A-Yong Yu 0
0 The Eye Hospital of Wenzhou Medical University , 270 Xueyuan West Road, Wenzhou (325000), Zhejiang , People's Republic of China
Background: To construct a real-time computerized location system (RCLS) to analyze and display the axis of corneal astigmatism and to compare its accuracy with the Scheimpflug method. Methods: Fifty-seven eyes of 39 volunteers with corneal astigmatism more than 1.00 diopter (D) were recruited. The RCLS was composed of a circular light-emitting diode (LED) light source, surgical microscope, surgical video system, computer and self-programming image analysis software. Scheimpflug imaging measurements (Pentacam HR, Oculus, Wetzlar, Germany) were performed on all subjects to determine the axis and power of corneal astigmatism. Thereafter, the axis of corneal astigmatism was analyzed in real-time and displayed by the RCLS on supine position, and videos were recorded. The MB-Ruler 4.0 software was used to measure the astigmatic axis. The accuracy of the RCLS was compared with the Scheimpflug method. Results: The RCLS was able to display the axis of corneal astigmatism in real-time. The axial deviation of corneal astigmatism between the two methods was 0.63 ± 3.78° when astigmatism was 1.00 to 2.00 D and decreased to 0.06 ± 1.38° when astigmatism was greater than 2.00 D. A linear correlation of astigmatic axis was noted between the two methods: AxisRCLS = 1.01 × AxisScheimpflug − 1.02 (R2 = 0.998, P < 0.001). The Bland-Altman analysis revealed that the RCLS agreed sufficiently well with the Scheimpflug method. Conclusions: The RCLS can accurately analyze and display the axis for corneal astigmatism greater than 1.00 D in real-time. The RCLS simplifies marking procedures and may have potential clinical application to improve the postoperative visual outcomes in surgical correction of corneal astigmatism.
Corneal astigmatism; Location; Real time; Mark
Correction of pre-existing corneal astigmatism during
cataract surgery is increasingly accepted in clinical
practice because it can improve visual quality and minimize
postoperative spectacles dependence. It had been
reported that 30% or more cataract patients have corneal
astigmatism greater than 1.00 diopter (D) [
ocular residual astigmatism may influence postoperative
]. Clinically, several methods are available to
correct corneal astigmatism during cataract surgery: (1)
clear corneal incision at the steep corneal meridian; (2)
astigmatic keratotomy at the steep corneal meridian; [
(3) limbal or peripheral corneal relaxing incision at the
steep corneal meridian; [
] and (4) implantation of a
toric intraocular lens (IOL) [
For all the surgical methods, accurately locating the
axis of pre-existing corneal astigmatism plays an
important role. In the clinic, there are four methods commonly
used to measure the axis of corneal astigmatism
preoperatively: manual-keratometry, optical biometry,
autokeratometry, and the Scheimpflug method. A study
carried out by Minwook Chang et al. showed that there
were no significant differences among these four
methods in measuring the axis of pre-existing corneal
astigmatism when correcting astigmatism with toric
The change between upright and supine position will
cause eye cyclotorsion [
], which may bring in possible
misalignment between the axis of toric IOLs and corneal
astigmatism. To eliminate the off-axis deviation when
implanted toric IOLs, the traditional method was to
mark reference points at 3- and 9-o’clock on the limbus
or conjunctiva under the slit lamp microscope before
surgery. Then, based on the two reference points, a
protractor was used to mark the desired axis of placement
of the IOL. The traditional method, depending on
patient cooperation and subjective judgment of different
surgeons, was tedious and time-consuming, which
increased the probability of off-axis error in surgical
correction of astigmatism.
With the high demands on refractive cataract surgery,
there is increasing concern about the real-time location
of corneal astigmatism, simplifying the marking
procedures and improving the accuracy of astigmatism
correction. In this study, we built a real-time
computerized location system (RCLS) to display the axis of
corneal astigmatism, and compared its accuracy with
the Scheimpflug method.
The volunteers with regular corneal astigmatism of 1.00
D or greater were enrolled in this prospective controlled
trial at the Eye Hospital of Wenzhou Medical University,
China. Exclusion criteria included any corneal diseases,
dry eye whose breakup time of tear film was less than
5 s, active ocular inflammation, contact lens wear within
three months, or history of ocular surgery or trauma.
This study followed the tenets of the Declaration of
Helsinki. All subjects provided informed consent and
approval was obtained from the Institutional Review Board
of the Eye Hospital of Wenzhou Medical University.
The construction of the system has been described in
detail in the patent RCLS [
]. The RCLS was mainly
composed of a circular light-emitting diode (LED) light
source, computer and self-programming image analysis
software (Fig. 1a, b). Although the entire cornea was not
a sphere, its central 3 mm area could be regarded as a
convex mirror [
]. If corneal astigmatism existed, the
reflex image of the LED ring on the cornea would be an
ellipse, and the major axis of the ellipse could be turned
into the axis of corneal astigmatism when analyzed by
the software. A perfect circle was used to calibrate the
image recording and analysis system before the test. The
software analyzed the reflex image of LED circle from
the cornea in real-time, and displayed a line on the
cornea, which represented the axis of corneal astigmatism.
The axis of surgical incision was determined by the surgeon.
The pre-determined axis of a toric IOL was calculated by
AcrySof Toric Calculators (http://www.acrysoftoriccalcula
tor.com/aspheric/Calculator.aspx). According to the
relative angles to the axis of corneal astigmatism input
preoperatively, the pre-determined axis of a toric IOL,
the axis of the surgical incision, or any other axes
could be entered into the software and displayed
simultaneously or separately. Figure 1c shows the
realtime RCLS simultaneously displaying the flat axis of
corneal astigmatism, the pre-determined axis of a
toric IOL, and the axis of the surgical incision.
All procedures were conducted by the same skilled
ophthalmologist. Scheimpflug imaging measurements
(Pentacam HR, Oculus, Wetzlar, Germany) were
performed on all subjects to measure the axis and power of
anterior corneal astigmatism. After topical anesthesia
with one drop of 0.5% hydrochloric acid proparacaine
(Alcaine, Alcon, USA), subjects were seated in front of a
slit lamp microscope and they gazed at a distant target.
A narrow slit beam was oriented horizontally and
centered on the pupil. The corneal limbus was marked at
3and 9- o’clock using a disposable medical sterile pen. The
MB-Ruler 4.0 software was used to measure the astigmatic
axis relative to the 3- and 9- o’clock reference points.
The subjects were asked to lay down on the surgical
bed told to gaze directly at the center of the light source
of the surgical microscope that was coaxial with the
RCLS, which was used to analyze and display the desired
axis. The RCLS was performed on each subject and
three fragments of the video were recorded. In each
fragment, one frame in which the light of microscope
was at the center of the cornea was chosen to be
analyzed. The MB-Ruler 4.0 software was used to measure
the astigmatic axis in degrees in the chosen frames.
When the MB-Ruler 4.0 was used to align the 0 scale
line to the 3- and 9- o’clock direction, another scale line
was the value of astigmatic axis. The average result of
three RCLS measurements was used to compare with
the Scheimpflug method to analyze the accuracy of axis
Statistical analysis was performed using MedCalc for
Windows (Version 15.11.0; Ostend, Belgium). The
relationship of the astigmatic axis between the RCLS and
Scheimpflug method was analyzed using the Spearman
correlation test. Independent sample t test was
performed for comparing average deviation between the
low and high astigmatism groups. The Bland-Altman
method was used to analyze the agreement between the
two methods. In order to assess the correlation and
agreement between the two methods, six astigmatic axes
were converted accordingly by adding 180° e.g., the
correlation and agreement were analyzed between 185°
(RCLS) and 180° (Scheimpflug method), rather than 5°
(RCLS) and 180° (Scheimpflug method). The level of
significance was set at P < 0.05.
This study included 57 eyes of 39 subjects, 18 males
(25 eyes) and 21 females (32 eyes). The average age
was 37.2 ± 18.8 years old, ranging from 20 to
78 years old. Corneal astigmatism was between 1.00
D to 3.68 D.
Table 1 and Fig. 2 show the deviation of the astigmatic
axis between the RCLS and Scheimpflug method. The
deviation decreased significantly when cylindrical power
There was a linear correlation of corneal astigmatic
axis between the RCLS and Scheimpflug method:
AxisRCLS = 1.01 × AxisScheimpflug − 1.02 (R2 = 0.998,
P < 0.001, Fig. 3).
According to the Bland-Altman analysis (Fig. 4), when
astigmatism was 1.00 to 2.00 D, there were 96.9% of
points within the Confidence Interval (CI) of Limits of
Agreement (LoA) (P = 0.35), in which the maximum
absolute axial deviation was 8.91°. The axial deviation
outside the CI of LoA was −9.47°. When astigmatism was
greater than 2.00 D, there were 96.0% of points within
the CI of LoA (P = 0.82), in which the maximum
absolute axial deviation was 3.19°. The axial deviation outside
the CI of LoA was −3.83°.
Accurate localization of corneal astigmatic axis plays a
significant role in the successful correction of
preexisting astigmatism. This study constructed the RCLS
to display the corneal astigmatic axis in real-time, and
the results were found to agree sufficiently well with the
Scheimpflug method when corneal astigmatism is
greater than 1.00 D. The Bland-Altman analysis revealed
that when astigmatism was 1.00 to 2.00 D, the maximum
and average absolute axial deviation in the CI of LoA
between the RCLS and Scheimpflug method were 8.91°
and 0.63°. There was only one point outside the CI of
Fig. 2 The relationship between the deviation of the astigmatic axis
and cylindrical power. The deviation of the astigmatic axis between
the Real-time Computerized Location System and Scheimpflug imaging
method decreased when cylindrical power increased (P = 0.004)
LoA. When astigmatism was greater than 2.00 D, the
maximum and average absolute axial deviation in the CI
of LoA between the RCLS and Scheimpflug method was
3.19° and 0.06°. There was only one point (2.20 D,
−3.83°) outside the CI of LoA. Thus, we concluded that
when astigmatism is greater than 1.00 D, the RCLS
and Scheimpflug method could be interchangeable in
the clinic in terms of locating corneal astigmatic axis.
In fact, the RCLS became more accurate with
The RCLS can detect and display the axis of corneal
astigmatism in real-time by analyzing the corneal reflex
image. Location of the axis of corneal astigmatism is one
of the hot topics in the field of toric IOL implantation.
A new location method has been introduced by Cha et
] for toric IOL implantation. In Cha’s method, an
anterior segment photograph was taken to identify
reference vessels and marking points. Actual distance was
calculated from reference vessel points to axis marking
points using an expression derived from the sizes of the
photographed image and the actual cornea. Finally, the
axis marking points were marked on the limbus during
surgery. Cha’s axis-marking method simplified the steps
and was a good alternative for those with sunken eyes or
with poor cooperation. However, there were several
factors that should be taken into consideration: 1) the
reference vessel points must not be far away from the
corneal limbus; 2) the method was unable to identify the
axis for those lacking limbal vessels or vessels in color
that were too slight to be recognized; 3) it may take the
surgeon some time to familiarize himself with the
photoshop program used preoperatively; 4) minimal scale
calipers used may produce errors.
In traditional axis-marking methods, preoperative
marking of reference points and the stability of toric
IOLs were the most significant surgical factors to
determine the postoperative visual outcomes in high
astigmatism. With the development of material and design of
toric IOLs, significant rotation of IOLs rarely occurred
postoperatively. Mencucci et al. reported that the mean
IOL rotation was 3.00 ± 1.69° postoperatively [
Chassain C provided a similar result that the mean IOL
rotation was 2.16 ± 1.95° [
]. According to Visser N et
al., a commonly used 3-step ink-marker procedure
before implanting toric IOLs produced a mean off-axis
error of approximately 5° [
]. If the error of
preoperative marking and the rotation of an implanted IOL both
rotated toward the same clockwise, a total off-axis error
of a toric IOL to the pre-determined axis will be up to
approximately 10° and may lead to a significant loss of
astigmatic correction. The RCLS simplified the marking
procedures and did not need any additional steps before
the patient laid down on the surgical bed. What a
surgeon only needed to do was to check the parameters
entered in the software, thus eliminating the off-axis error
arising from preoperative horizontal marking as used in
traditional marking method.
The RCLS can also display in real-time the axis of
surgical incision based on the relative angle to the axis of
corneal astigmatism if it was calculated and input at the
beginning of surgery. In the implantation of toric IOLs,
precise corneal incision positioning is another important
factor to guarantee the efficacy of surgical correction of
corneal astigmatism. Studies had shown that a 2.2 mm
incision produced 0.31 ± 0.54 D surgically induced
astigmatism (SIA), and a 2.75 mm incision produced
0.56 ± 0.42 D SIA [
]. Thus, the surgical incision
should be calculated and placed at the certain
predetermined axis, otherwise, it will lead to new
astigmatism. Traditional marking methods took several steps to
mark the pre-operative reference points on the corneal
limbus, and the incision location was determined based
on its relative position to the reference marking, which,
again, was time-consuming and increased the probability
of off-axis error. In addition, the minimum scale of the
axis marker was 5° to 10°, which further highlighted its
inability to precisely mark the axis. Therefore, the
simultaneous display of the axis of the surgical incision in the
RCLS can be a potential application to improving the
efficacy of surgical correction of corneal astigmatism.
Currently, there are several real-time imaging
technologies available for intraoperative toric IOL alignment.
These include the VERION Digital Marker (Alcon
Laboratories, Ft. Worth, TX), Callisto Eye with Z-Align
(Carl Zeiss Meditec AG, Jena, Germany), the iTrace with
Zaldivar Toric Caliper (Tracey Technologies, Houston,
TX). The key steps for the operation of these systems
are: 1) obtaining a high-resolution preoperative image
that contains the limbal vessels, iris feature and/or
scleral vessels; 2) intraoperative registration of the
surgical eye based on iris landmarks, scleral and limbal
vessels; and 3) digitally display the intended toric IOL axis
intraoperatively for toric IOL alignment. These imaging
technologies are real-time ‘display’ systems, rather than a
real-time ‘measure’ system, where its purpose is to
improve the accuracy of toric IOL alignment. Moreover,
in the femtosecond laser–assisted cataract surgery, the
vacuum rise during the suction may induce a
circumferential subconjunctival hemorrhage, which can interfere
with image recognition and eye registration
intraoperatively. In addition, the area outside the central cornea,
especially the part of the limbus, can be blocked by
surgical instruments during surgery, which, again, will
interfere with eye registration. In contrast, the RCLS is a
real-time ‘measure’ and ‘display’ system as it can analyze
and display the astigmatic axis intraoperatively. The
RCLS also shows great compatibility even with the
femtosecond laser–assisted cataract surgery because it only
analyzes the central cornea (unaffected by
subconjunctival hemorrhage and/or surgical instruments), and there
is no need for image recognition and eye registration.
Another intraoperative ‘measure’ system is the Optiwave
Refractive Analysis (ORA) System (WaveTec Vision
Systems Inc., Aliso Viejo, CA) wavefront aberrometer,
which measures aphakic refractive status
intraoperatively after lens removal and then calculates the IOL
power along with the desired toric IOL axis. However,
the astigmatism axis measured after lens removal may
be affected by surgical incision and corneal edema,
therefore, will be different with the postoperative
corneal astigmatism axis after corneal edema subsides
and corneal incision healing. Most formulas use the
preoperative astigmatism measurement rather than
the intraoperative astigmatism measurement after lens
removal, to design the intended toric IOL axis (also
considered as SIA).
However, the RCLS still has several additional factors
to be considered. The accuracy of the RCLS will still be
questioned despite the sufficient agreement between
RCLS and Scheimpflug method because the test-to-test
variability of Scheimpflug method has been a point of
] Furthermore, the repeatability of the
RCLS has not been assessed in this study, further
investigation to fully assess the repeatability of the
RCLS and comparing its accuracy with a more
reliable device is needed. The system can only detect
corneal astigmatism of an area within the central
3 mm area. The low resolution of the CCD receiving
the corneal reflex image decreased pixel points of the
output image for precise analysis. The width of the
LED circle, which resulted in a certain width of
reflection image, increased the difficulty of image
recognition. In future, higher accuracy may be expected
with the following improvements: 1) the use of
infrared light to reduce a patient’s discomfort; 2)
increasing the resolution of the surgical video system.
Furthermore, the RCLS will be compared with
commercial devices such as the Alcon Wavetec ORA, Verion, and
Zeiss Callisto eye.
In conclusion, the RCLS can accurately analyze and
display the astigmatic axis when corneal astigmatism is
1.00 D or greater, and the results agree sufficiently well
with the Scheimpflug method. The RCLS analyzes
corneal astigmatic axis directly based on the corneal optical
feature, rather than the anatomic feature of the
noncorneal ocular surface to indirectly locate the axis as
used in other methods. Therefore, the RCLS can
eliminate the effects of eye cyclotorsion and simplify marking
procedures i.e., by using the RCLS, tedious preoperative
data storage, intraoperative data transmission and
intraoperative eye registration based on the preoperative
image can be effectively avoided. The RCLS may be a
potential clinical application for improving the
postoperative visual outcomes in the surgical correction of
This material is based upon work funded by the Nature and Science Foundation
of China (Grant No. 81570869), Nature and Science Foundation of Zhejiang
Province, China (Grant No. Y2110784), Zhejiang Provincial Foundation
of China for Distinguished Young Talents in Medicine and Health (Grant
No. 2010QNA018), Foundation of Wenzhou City Science & Technology
Bureau (Grant No. Y20140705), and Engineering Development Project
of Ophthalmology and Optometry (Grant No. GCKF201601). The funding
sources were not involved in study design; in the collection, analysis
and interpretation of data; in the writing of the report; or in the decision to
submit the article for publication.
Design and conduct of study (JZ, ZK, AY); Collection (JZ, AY), Management
(JZ, AY), Analysis (JZ, AP, AY), Interpretation of data (JZ, AP, AY); Preparation
(JZ, AP, AY), Review or approval of manuscript (AY); Responsibility for the
integrity of the entire study and manuscript (AY). All authors read and
approved the final manuscript.
The authors declare that they have no competing interests.
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