ORIGINAL ARTICLE  
Accuracy of the Corvis Biomechanical Index in  
Keratoconus Screening  
Fazle Hanan1, Zulfiqar Ali2, Muhammad Naeem3  
1Amanat Eye Hospital, Peshawar, 2Ayub Medical College, Abbottabad, 3Lady Reading Hospital, Peshawar  
ABSTRACT  
Purpose: To evaluate the accuracy of the Corvis Biomechanical Index (CBI) in screening of patients with  
Keratoconus.  
Study Design: Cross sectional study.  
Place and Duration of Study: Amanat Eye Hospital Peshawar from July 2018 to June 2019.  
Material and Methods: One thousand eyes were included in this study. Patients who came for keratorefractive  
laser procedure or collagen cross linkage were included in the study by convenient sampling technique. Six  
hundred and eleven patients had early Keratoconus and 389 were normal controls. Control group included those  
individuals who had visual acuity of 6/6 with no clinical feature of Keratoconus and normal Tomographic and  
Biomechanical Index (TBI). All individuals included in the study underwent a thorough ocular examination, CBI  
and TBI tests. SPSS version 23 was used for statistical analysis of collected data.  
Results: The mean CBI value was 0.3186 ± 0.407 standard deviation (SD), the standard error (SE) of the mean  
was 0.0129 with a 95% confidence interval (CI) of 0.293 to 0.343. The minimum CBI value was 0.00 and the  
maximum value was 1.00. The mean Tomographic Biomechanical Index (TBI) value was 0.465 ± 0.392 SD, SE  
was 0.124 with a 95% CI of 0.222 to 0.708. The range of TBI values was 0.00 to 1.00. For CBI, sensitivity was  
58.2%, specificity was 100%, positive predictive value was 100%, and negative predictive value was 61.2%.  
Conclusion: The current study showed acceptable accuracy of CBI in terms of specificity and sensitivity.  
However, the result should be interpreted in combination with clinical data and other topographic and tomographic  
parameters.  
Key Words: Corvis Biomechanical index, Tomographic and Biomechanical index, Belin/Ambrosio enhanced  
ectasia index.  
How to Cite this Article: Hanan F, Ali Z, Naeem M. Accuracy of the Corvis Biomechanical Index in Keratoconus  
Screening. Pak J Ophthalmol. 2020; 36 (3): 216-220.  
Doi: 10.36351/pjo.v36i3.1072  
INTRODUCTION  
Identification and exclusion of Forme Fruste  
Keratoconus prior to any photorefractive procedure is  
important because it can develop into established  
Keratoconus after surgery resulting in deterioration of  
vision.1 It is essential that patient should have normal  
corneal parameters without any suspicion or sign of  
Keratoconus. Moreover, if early Keratoconus is  
diagnosed, collagen cross linking can arrest the  
progression of Keratoconus.2,3,4 Keratoconus is a  
bilateral disease in which there is thinning and  
steepening of the central and paracentral cornea,  
appearing around puberty. In early stage, which is also  
Correspondence to: Fazle Hanan  
Amanat Eye Hospital Peshawar  
Email: drfazalhanan@gmail.com  
Received: May 28, 2020  
Accepted: June 25, 2020  
Revised: June 22, 2020  
216  
Pakistan Journal of Ophthalmology, 2020, Vol. 36 (3): 216-220  
Accuracy of the Corvis Biomechanical Index in Keratoconus Screening  
called subclinical, Forme Fruste Keratoconus or  
Keratoconus suspect, the clinical signs are not obvious  
and the diagnosis can be made with the help of a  
screening test, which has high degree of sensitivity and  
specificity5.  
pressure and the IOP is determined at first Applanation  
(A1), which defines the reference position for the  
stiffness parameter (SP-A1) in the form of force  
divided by displacement. Therefore, the SP-A1 is  
defined as resultant pressure (Pr) divided by deflection  
amplitude at A1.  
Different devices are used nowadays for  
Ambrósio's Relational Thickness (ARTh) is  
calculated by first measuring the corneal thickness and  
the percentage thickness increase relative to the  
smallest value at points with 0.2 mm spacing. The  
ratio between the percentage values (percentage  
thickness increase) and the corresponding normative  
values is calculated for each position. The average  
ratio for all positions provides the Pachymetric  
Progression Index (PPI). ARTh is finally calculated by  
dividing corneal thickness at thinnest point with  
pachymetric progression index. The rationale of the  
study was to compare the accuracy of CBI in  
Keratoconus screening with TBI which was taken as  
the gold standard.  
Keratoconus  
screening,  
incorporating  
corneal  
tomography or pachymetry and topography. These  
include; Orbscan II6 (Bausch & Lomb, New York,  
US), Pentacam7 (Oculus Optikgeräte GmbH, Wetzlar  
Germany), Galilei G48 (Ziemer, Port, Switzerland),  
and SIRIUS9 (CSO, Firenze, Italy). These devices  
measure corneal curvature, corneal thickness, and  
elevation of the anterior and posterior corneal surface.  
Recently, it has been found that changes in  
biomechanical stability of the cornea precedes  
topographic  
and  
tomographic  
changes  
in  
Keratoconus.10,11 To study the biomechanical behavior  
of cornea, currently used devices are Ocular Response  
Analyzer R (ORA; Reichert, New York, US) and the  
Corneal Visualization Scheimpflug Technology Corvis  
ST; (Oculus Optikgerate GmbH, Wetzlar, Germany).  
The  
combination  
of  
pachymetric  
and  
biomechanical parameters is referred to as  
tomographic and biomechanical index or TBI12,13,14  
which has proven to be more accurate than other  
diagnostic parameters. The purpose of the current  
study was to evaluate the accuracy of Corvis  
Biomechanical Index (CBI) in Keratoconus screening  
by comparing it with TBI.  
The ocular response analyzer2 determines the  
corneal hysteresis and the corneal resistance factor.  
The Corvis ST is a non-contact tonometer with a dual  
Scheimpflug, high-speed camera that takes more than  
4,300 images per second of the central 8 mm of the  
cornea in horizontal meridian. The Corvis ST  
determine the Dynamic Corneal Response (DCR)  
parameters and the Ambrosio relational thickness  
(ART). The salient DCR parameters include A1 and  
A2 velocities, which are the speeds of corneal apex at  
first and second applanation respectively. The  
deflection and deformation amplitudes; displacement  
of corneal apex with reference to the initial state of  
cornea is the deflection amplitude while the largest  
displacement of corneal apex in the anterior-posterior  
direction at the moment of highest concavity is the  
deformation amplitude which also includes whole eye  
movement. Deflection amplitude ratio describes the  
ratio between the deflection amplitude at the apex and  
the average deflection amplitude measured at 1 or 2  
mm from the center. Similarly, Deformation amplitude  
ratio describes the ratio between the deformation  
amplitude at the apex and the average deformation  
amplitude measured at 1 or 2 mm from the center. The  
Delta Arc length describes the change in Arc length  
during the highest concavity moment from the initial  
state, in a defined 7-mm zone.  
MATERIAL AND METHODS  
This was a cross sectional study of patients who  
visited Amanat eye hospital Peshawar between July,  
2018 to June 2019. Amanat eye hospital Peshawar is  
an eye care center providing Keratoconus screening  
services and laser treatment facilities for patients  
having refractive errors. The data set included two  
types of patients; those who were interested in  
Photorefractive Keratectomy or Femto LASIK  
treatment for their refractive error or those who were  
advised Corvis and TBI tests because of the clinical  
suspicion of Keratoconus in them and hence  
consideration of collagen cross linkage treatment.  
These patients were either seen by the consultant of  
Amanat eye hospital Peshawar, or they were seen and  
referred by other ophthalmologists for screening  
purpose.  
All those patients who had previous  
Keratorefractive procedure and those patients who had  
clinical signs of advanced Keratoconus were excluded.  
The force balance between the external air  
Pakistan Journal of Ophthalmology, 2020, Vol. 36 (3): 216-220  
217  
Fazle Hanan, et al  
Table 2: Tomographic Biomechanical Index.  
The age range was 5 to 50 years and both sexes were  
included. One thousand eyes of 500 patients were  
included in the study, out of whom 611 were patients  
with subclinical or early Keratoconus and 389 were  
normal controls. Control group included those  
individuals who had visual acuity of 6/6 with no  
clinical feature of Keratoconus and normal TBI.  
Frequency  
OD  
Frequency  
OS  
Category  
Total  
%age  
Normal  
Suspicious  
Diseased  
Total  
192  
114  
194  
500  
197  
113  
190  
500  
389  
227  
384  
38.9  
22.7  
38.4  
100  
1000  
All individuals included in the study underwent a  
thorough ocular examination, CBI and TBI tests. The  
devices used for screening were the Oculus Pentacam  
HR Reference 70900 (Oculus Optikgeräte GmbH,  
Wetzlar Germany) and the Oculus Corvis ST  
Reference 72100 (Oculus Optikgerate GmbH, Wetzlar,  
Germany). The study was conducted while strictly  
adhering to the study guidelines of the tenets of  
declaration of Helsinki.  
Table 3: Evaluation of CBI comparing it with TBI.  
CBI Result  
Disease Present Disease Absent  
Total  
Positive test  
(suspicious plus 365 (true positive) 0 (false positive) 365  
diseased)  
246 (false  
negative)  
611  
389 (true  
negative)  
389  
Negative test  
Total  
635  
1000  
Sensitivity = 58.2%  
Specificity = 100%  
Positive predictive value = 100%  
Negative predictive value = 61.2%  
RESULTS  
A total of 1000 eyes of 500 patients were recruited in  
the study. The number of male and female patients  
was 318 and 182 with a ratio of 1.747 to 1  
respectively. The age range of the patients was 5 to 49  
years, with a mean of 21.89 ± 8.434 years. 95%  
confidence interval for age was 21.151-22.628 with a  
standard error of 0.377 years. The cut off ranges for  
CBI and TBI were 0.00 to 0.25 as normal, 0.26 to 0.5  
as suspicious and 0.51 to 1.00 as diseased. The cut off  
points were similar to those considered by Koh S and  
Ambrosio R jr15 in their study which were CBI > 0.5  
and TBI > 0.29.  
Table 4: CBI Vs TBI (p Value Calculation).  
CBI Observed  
(Expected)  
635 (512)  
TBI Observed  
(Expected)  
389(512)  
Total  
1024  
976  
Normal  
Suspicious  
plus diseased  
Total  
365 (488)  
1000  
611(488)  
1000  
2000  
(observed)  
Chi square value = 29.5  
Degree of freedom = 1  
p value = ˂0.001  
The result of CBI in our patients is given in  
table 1. The mean CBI value was 0.3186 ± 0.407, the  
DISCUSSION  
In Keratoconus, cornea is soft and thin. Scarcelli11  
suggested that as the effect of strain is more on the  
softer area of the cornea, intraocular pressure and  
external factors such as eye rubbing cause softer area  
of the cornea to bulge out and become thin to  
redistribute the effect of strain. Corneal bulging results  
in focal reduction in stress as a compensatory  
mechanism resulting in a vicious circle of straining,  
bulging and thinning.  
Table 1: Corvis Biomechanical Index.  
Frequency  
OD  
Frequency  
OS  
Category  
Total  
%age  
Normal  
Suspicious  
Diseased  
Total  
309  
45  
146  
500  
326  
33  
141  
500  
635  
78  
287  
1000  
63.5  
7.8  
28.7  
100  
The data available in literature does not prove to  
be definitive in diagnosing Keratoconus as there is a  
considerable overlap between normal corneas and  
Forme Fruste Keratoconus16,17. In this study, we  
compared the accuracy of CBI in Keratoconus  
screening by calculating their sensitivity and  
specificity against TBI from the collected data and  
their positive and negative predictive value.  
standard error of the mean was 0.0129 with a 95%  
confidence interval of 0.293 to 0.343. The minimum  
CBI value was 0.00 and the maximum value was 1.00.  
The result of TBI is given in table 2. The mean TBI  
value was 0.465 ± 0.392. The standard error was 0.124  
with a 95% confidence interval of 0.222 to 0.708. The  
minimum TBI value was 0.00 and the maximum value  
was 1.00.  
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Pakistan Journal of Ophthalmology, 2020, Vol. 36 (3): 216-220  
Accuracy of the Corvis Biomechanical Index in Keratoconus Screening  
In the current study, sensitivity and specificity of  
CBI was 58.2% and 100% respectively. The positive  
and negative predictive value of CBI was 100% and  
72.3% respectively. In the study of Vinciguerra et al18,  
the sensitivity of CBI was found to be 94.1% and  
specificity of 100%. The reason for the higher  
sensitivity in that study was that they compared the  
results of normal individuals with those having  
established Keratoconus whereas our study included  
Forme Fruste Keratoconus, in which diagnosis is more  
difficult and challenging.  
CONCLUSION  
The result of CBI should be interpreted in combination  
with other topographic, tomographic and topometric  
parameters such as BAD_D, TBI, mean keratometry,  
index of surface variance and index of vertical  
asymmetry.  
Ethical Approval  
The study was approved by the Institutional review  
board/Ethical review board.  
However, Wang et al19 in his study described  
sensitivity and specificity of CBI in Forme Fruste  
Keratoconus as 63.2% and 80.3% respectively. The  
reason for the lower specificity of CBI in that study  
might be the comparison of subclinical Keratoconus  
with normal population whereas we compared both  
subclinical and early established Keratoconus cases  
with normal cases.  
Conflict of Interest  
Authors declared no conflict of interest.  
Authors’ Designation and Contribution  
Fazle Hanan; Associate Professor: Study concept and  
design, data collection, analysis and interpretation of  
data, writing the manuscript, critical revision,  
supervision.  
The difference between the results of different  
studies is due to the observation20 that the  
discriminatory power of these indices decreases in the  
following order a) comparing normal eyes with  
established Keratoconus. b) comparing normal eyes  
with early Keratoconus in which the opposite eye of  
the patient has normal topography. c) comparing  
normal eyes with subclinical Keratoconus in which the  
opposite eye has both normal topography and  
tomography.  
Zulfiqar Ali; Associate Professor: Analysis and  
interpretation of data, critical revision.  
Muhammad Naeem; Assistant Professor: Analysis and  
interpretation of data, critical revision.  
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