Effect of biometric
characteristics on biomechanical properties of the cornea in cataract patient
Xue-Fei Song1,2, Achim Langenbucher3,
Zisis Gatzioufas1, Berthold Seitz1, Moatasem El-Husseiny1
1Department of Ophthalmology, Saarland University Medical Center, Homburg
Saar 66424, Germany
2Department of
Ophthalmology, Ninth People's Hospital, Shanghai Jiao Tong University, School
of Medicine, Shanghai 200011, China
3Experimental Ophthalmology, Saarland
University, Homburg Saar 66424, Germany
Correspondence
to:
Xue-Fei Song. Department of Ophthalmology, Saarland University Medical Center,
Kirrberger Str 100, Homburg Saar 66424, Germany. 89215667@qq.com
Received: 2015-01-19
Accepted: 2015-03-11
Abstract
AIM: To determine the impact of biometric
characteristics on the biomechanical properties of the human cornea using the ocular response analyzer (ORA) and standard comprehensive
ophthalmic examinations before and after standard phacoemulsification.
METHODS: This study comprised 54 eyes
with cataract with significant lens opacification in stages I or II that underwent phacoemulsification (2.8
mm incision). Corneal hysteresis (CH),
corneal resistance factor (CRF),
Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated
intraocular pressure (IOPcc) were measured by ORA preoperatively and at 1mo postoperatively. Biometric
characteristics were derived from corneal topography [TMS-5, anterior
equivalent (EQTMS) and cylindric (CYLTMS) power], corneal
tomography [Casia, anterior and posterior equivalent (EQaCASIC, EQpCASIA)
and cylindric (CYLaCASIA, CYLpCASIA) power], keratometry
[IOLMaster, anterior equivalent (EQIOL) and cylindric (CYLIOL)
power] and autorefractor [anterior equivalent (EQAR)]. Results from ORA were analyzed and correlated with those from all other examinations taken at the
same time point.
RESULTS: Preoperatively,
CH correlated with EQpCASIA and CYLpCASIA only (P=0.001,
P=0.002). Postoperatively, IOPg and
IOPcc correlated with all equivalent powers (EQTMS, EQIOL,
EQAR, EQaCASIA and EQpCASIA) (P=0.001, P=0.007, P=0.001, P=0.015, P=0.03 for IOPg and P<0.001,
P=0.003, P<0.001, P=0.009, P=0.014 for IOPcc). CH correlated
postoperatively with EQaCASIA and EQpCASIC only (P=0.021, P=0.022).
CONCLUSION: Biometric characteristics may significantly
affect
biomechanical properties of the cornea in terms of CH, IOPcc and IOPg before,
but even more after cataract surgery.
KEYWORDS: cataract surgery; biometric
characteristics; biomechanical properties; ocular response analyzer; corneal tomography; correlation analysis
Citation: Song
XF, Langenbucher A, Gatzioufas Z, Seitz B, El-Husseiny M. Effect of biometric
characteristics on biomechanical properties of the cornea in cataract patient. Int J Ophthalmol 2016;9(6):854-857
INTRODUCTION
In addition to intraocular lens (IOL) implantation
following cataract surgery, the alteration of corneal biomechanical
characteristics was also believed to contribute to refractive changes[1]. Corneal hysteresis
(CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure
(IOPcc), and Goldmann-correlated intraocular pressure (IOPg) are variables
derived from ocular response analyzer (ORA, Reichert, NY, USA, software version
3) to assess corneal biomechanical characteristics[2].
In ORA examination, metamorphosis in human
cornea resulting from energy accumulation, which postpone the applanation
signal peaks in opposite directions, leads to an applanation pressure variance.
Such difference in applanation pressures is defined as CH, and the mean value
of both pressures was named IOPg[2].
From the manufacturer's instruction, CRF is an indicator of the unitary corneal
resistance which could only be influenced by corneal elastic properties, and
IOPcc is less influenced by corneal biomechanical properties by utilizing CH
information[3].
Variables provided by ORA were accounted as
reliable indices, though with some query and contradictions in intraocular
pressure (IOP) examinations[1,3-8].
However, studies on biomechanical
properties and biometric characteristics are mostly focusing on central corneal
thickness (CCT)[3,8].
According to our knowledge, there are no studies in the scientific literature
about a potential relationship between those biomechanical properties of the
cornea and biometric characteristics obtained from standard comprehensive
ophthalmic examinations before and after standard phacoemulsification.
The purpose of this study was to assess the
potential impact of characteristics as determinant by corneal topography,
tomography and keratometry analysis on biomechanical properties before and
after standard cataract surgery.
SUBJECTS AND METHODS
Study Group and Protocol Fifty-four eyes were studied
in the cross-sectional study, which underwent phacoemulsification (2.8 mm
incision) from May 2012 to January 2013. All eyes studied were in stages I or
II with visually significant lens opacification.
Ethics committee of Saarland University approved
this study protocol, following the tenets of the Declaration of Helsiniki. All
participants signed written informed consent forms with absolute comprehension
of the study.
Inclusion Criteria Cataract eye aged between 40
and 80 with normal fundus, without corneal pathologies and oclular sugeries
history.
Exclusion Criteria Astigmatism of more than
3.00 diopters (D). Any other ocular surgery needed beside phacoemulsification.
Surgical Technique All surgeries were performed
by El-Husseiny M, using a retrobulbar anesthesia. Corneal
incision of 2.8 mm was made by a corneal keratome. Ninety degree away from the
main incision, two paracenteses less than 0.9 mm were performed with the
corneal keratome. Healon ophthalmic viscosurgical device (OVD, Abbott Medical
Optics, Illinois, USA) was injected in the anterior chamber. With a
capsulorhexis forceps, capsulorhexis was performed. With balanced saline
solution, hydrodissection and hydrodelineation were performed. By using the
stop-and-chop technique from Alcon Infiniti (Alcon, Texas, USA),
phacoemulsification was achieved. Healon was used again for IOL implantation,
and then aspirated. After hydrating the incision, the wounds were tightly
sealed. Topical ofloxacin 0.3% and prednisolone acetate 1% were routinely used
after surgery.
Patient Examination Song XF
performed ophthalmic examination on all patients, including slit lamp
evaluation, posterior segment inspection, Snellen charts for visual acuity, ORA
(Reichert, Inc., software version 3) for biomechanical characteristics,
IOLMaster (Carl Zeiss Meditec) for biometry, EM-3000 (TOMEY corp.) for
endothelium imaging, Topographic Modeling System 5 (TMS-5, TOMEY corp.) and
CASIA (TOMEY corp.) for tomography.
Examinations were taken between 7 a.m. to 10 a.m. to
minimize the diurnal fluctuation reported before[2].
All examinations were performed before surgery and
4wk after that, which would minimize biomechanical alteration caused by wound
healing procedure.
Main Outcome Measures CH,
CRF,
IOPg, and IOPcc were measured with the ORA
preoperatively and at 1mo postoperatively. Biometric characteristics
were derived from corneal topography included: anterior equivalent (EQTMS)
and cylindric (CYLTMS) power. Those derived from corneal tomography
included: anterior and posterior equivalent (EQaCASIC, EQpCASIA)
and cylindric [anterior and posterior cylindric (CYLaCASIA, CYLpCASIA)]
power. Those derived from keratometry included: anterior equivalent (EQIOL)
and cylindric (CYLIOL) power. Anterior equivalent (EQAR)
was derived from the autorefractor. Results from ORA were
analyzed and correlated with those from all other examinations taken at same
time point in a cross-sectional manner.
Statistical Analysis Statistical analysis was
performed with SPSS (version 19.0 for Windows, SPSS, Inc.). Mean±standard
deviation (SD), range and median were calculated and expressed. To identify the
significant biometric characteristics which could potentially affect the
biomechanical properties before and after surgery, Pearson rank correlation
coefficients r was used in a
correlation analysis. Statistical significance was considered as a P-value less than 0.05.
RESULTS
At the preoperative examination stage, CH
showed a moderate negative correlation with EQpCASIA and, a moderate
positive correlation with CYLpCASIA. That means, that a flatter
corneal back surface is associated with a higher CH. All other preoperatively
measured biometric characteristics which were not significantly correlated with
biomechanical properties are presented in Table 1.
The impact of biometric characteristics on
biomechanical properties (postoperative) is shown in Table 2. At the postoperative examination stage, CH showed
a mild positive correlation with EQaCASIA and a mild negative
correlation with EQpCASIA. That means, that a steeper corneal front
surface and a flatter corneal back surface are associated with a higher CH.
Both IOPcc and IOPg showed a moderate negative correlation with EQTMS,
EQIOL, EQAR, EQaCASIA, and a moderate positive
correlation with EQpCASIA. That means, that steeper corneal front
and back surfaces are associated with a higher IOP. All other postoperatively
measured biometric characteristics which were not significantly correlated with
biomechanical properties are presented in Table 2.
Table 1 Impact of biometric characteristics
on the preoperatively measured biomechanical properties
Biometrical
impact
(D) |
|
CH |
CRF |
IOPcc |
IOPg |
EQTMS |
r |
0.033 |
0.088 |
0.103 |
0.065 |
P |
0.814 |
0.529 |
0.461 |
0.641 |
|
CYLTMS |
r |
0.293 |
0.209 |
-0.099 |
-0.051 |
P |
0.031 |
0.129 |
0.477 |
0.716 |
|
EQIOL |
r |
0.215 |
0.052 |
-0.149 |
-0.155 |
P |
0.118 |
0.709 |
0.282 |
0.263 |
|
CYLIOL |
r |
0.203 |
0.007 |
-0.243 |
-0.187 |
P |
0.140 |
0.962 |
0.077 |
0.177 |
|
EQAR |
r |
0.231 |
0.080 |
-0.140 |
-0.139 |
P |
0.092 |
0.567 |
0.312 |
0.317 |
|
EQaCASIA |
r |
0.201 |
0.114 |
-0.132 |
-0.064 |
P |
0.033 |
0.368 |
0.422 |
0.237 |
|
CYLaCASIA |
r |
0.28 |
-0.128 |
-0.106 |
-0.157 |
P |
0.04 |
0.358 |
0.444 |
0.255 |
|
EQpCASIA |
r |
1-0.434 |
-0.213 |
0.181 |
0.189 |
P |
0.001 |
0.122 |
0.190 |
0.172 |
|
CYLpCASIA |
r |
10.419 |
0.248 |
-0.229 |
-0.128 |
P |
0.002 |
0.071 |
0.096 |
0.357 |
CH: Corneal hysteresis; CRF: Corneal resistance
factor; IOPcc: Corneal compensated intraocular pressure; IOPg: Goldmann-correlated
intraocular pressure; EQTMS:
Average corneal power of the anterior surface (TMS-5); CYLTMS: Astigmatism of the anterior surface (TMS-5); EQIOL: Average corneal power of the
anterior surface (IOL Master); CYLIOL:
Astigmatism of the anterior
surface (IOL Master); EQAR: Average
corneal power of the anterior surface (Autoref K readings); EQaCASIA: Average corneal power of the
anterior surface (CASIA); CYLaCASIA: Astigmatism of the anterior surface
(CASIA);
EQpCASIA: Average
corneal power of the posterior surface (CASIA); CYLpCASIA: Astigmatism of the posterior surface (CASIA). 1Significant
values.
aModerate or strong
statistically significant correlation.
Table 2 Impact of biometric characteristics
on the 1mo postoperatively measured biomechanical properties
Biometrical impact (D) |
|
CH |
CRF |
IOPcc |
IOPg |
EQTMS |
r |
-0.281 |
0.180 |
1-0.457b |
1-0.447b |
P |
0.038 |
0.188 |
<0.001 |
0.001 |
|
CYLTMS |
r |
0.116 |
0.052 |
-0.086 |
-0.055 |
P |
0.397 |
0.708 |
0.532 |
0.687 |
|
EQIOL |
r |
0.248 |
-0.090 |
1-0.348b |
1-0.326a |
P |
0.068 |
0.514 |
0.009 |
0.015 |
|
CYLIOL |
r |
0.179 |
0.120 |
-0.099 |
-0.043 |
P |
0.190 |
0.381 |
0.472 |
0.753 |
|
EQAR |
r |
0.273 |
-0.035 |
1-0.329a |
1-0.311 |
P |
0.043 |
0.801 |
0.014 |
0.030 |
|
EQaCASIA |
r |
0.043 |
0.010 |
-0.042 |
-0.031 |
P |
0.021 |
0.624 |
0.003 |
0.007 |
|
CYLaCASIA |
r |
0.055 |
0.058 |
-0.013 |
0.009 |
P |
0.690 |
0.673 |
0.926 |
0.948 |
|
EQpCASIA |
r |
1-0.307a |
0.154 |
10.454b |
10.436b |
P |
0.022 |
0.262 |
<0.001 |
0.001 |
|
CYLpCASIA |
r |
0.129 |
-0.100 |
-0.215 |
-0.213 |
P |
0.348 |
0.468 |
0.114 |
0.119 |
CH: Corneal
hysteresis; CRF: Corneal resistance factor; IOPcc: Corneal compensated
intraocular pressure; IOPg: Goldmann-correlated intraocular pressure; EQTMS: Average corneal power of
the anterior surface (TMS-5); CYLTMS: Astigmatism of the anterior surface (TMS-5); EQIOL:
Average corneal power of the anterior surface (IOL Master); CYLIOL: Astigmatism of the anterior surface
(IOL Master); EQAR: Average corneal power of the anterior surface
(Autoref K readings); EQaCASIA: Average corneal power of the
anterior surface (CASIA); CYLaCASIA: Astigmatism of the anterior surface (CASIA); EQpCASIA:
Average corneal power of the posterior surface (CASIA); CYLpCASIA: Astigmatism of the posterior surface
(CASIA). 1Significant values. a,bMild/ moderate or strong statistically significant correlation.
DISCUSSION
In this study, we assessed biomechanical
properties before and after standard cataract surgery and correlated these
values with biometric characteristics such as equivalent power and cylindric
power of the cornea obtained by corneal topography, tomography, and
keratometry.
The biomechanical properties are shown
alongside with the biometric data in Table
1 for the preoperative and Table
2 for the postoperative situation.
According to the results of EQpCASIA,
a flatter corneal back surface is associated with a higher CH preoperatively
and, a steeper corneal back surface is associated with a higher IOP. Although a
high correlation between curvatures of anterior and posterior corneal surface
was found, calculation of the corneal power ignoring the posterior surface
would underestimate the power reduction affect corneal refractive surgery[9-12]. For that reason,
Langenbucher et al[13] evaluated the effect of
a separate measurement of the anterior and posterior corneal surface to
calculate the total refractive power of the cornea after myopic laser in situ keratomileusis. Eom et al[14] attempted to improve the Sanders-Retzlaff-Kraff
(SRK)/T formula which is the most commonly used formula for IOL power
calculation in the US, with corneal power specific constants depending on both,
values of anterior and posterior surface data of the cornea. In addition,
corneal power of the posterior surface was also reported to change in corneas
of patients with type I and II diabetes mellitus[15]. Considering the data of previous studies and our
result on EQpCASIA and biomechanical values, we conclude that, in a
cataract surgery, information about the corneal power of the posterior surface
may help to understand the unique indices provided by ORA (CH and IOPcc), which
are related to the viscoelastic properties of the corneal tissue that can be
attributed to the damping effects of the cornea[16].
From the results shown both in Tables 1 and 2, CH is the only ORA index correlated to biometric
characteristics, both in the preoperative and postoperative situation. As an
indication for viscous damping in the cornea, CH is related to the ability of
the cornea to absorb and dissipate energy[2].
CH was only correlated with the corneal power of the anterior surface EQaCASIA
among the four values from different devices that had been approved for
interchangeable use in Wang et al’s[17] work. Results derived via CASIA, IOLMaster and TMS-5 also
suggest that the interchangeable use of those data is possible (data not
shown). But intercorrelation between CH and anterior corneal power was only
moderate.
Theoretically, IOPcc is a pressure
measurement that utilizes information considering CH providing an IOP value
that is less affected by corneal properties[3],
which may offer an attractive alternative to traditional IOP measurements
including Goldmann applanation tonometry (GAT). Under normal IOP condition,
corneal biomechanics play a major role for the stability of the globe, which
were supported by the ORA values[18].
However, such difference did not appear in our present research. According to
our results, IOPcc and IOPg showed a moderate negative correlation with corneal
power values (EQTMS, EQIOL, EQAR, EQaCASIA
and EQpCASIA), which are indices derived
from different devices (TMS-5, IOL Master, Autoref K readings and CASIA). That
means, that steeper corneal front and back surfaces are associated with a
higher IOP. Unlike the mild intercorrelation of CH with anterior corneal power,
both IOPcc and IOPg showed a moderate correlation with the biometric
characteristics. In vitro studies on
radial keratotomy focusing on corneal power and IOP obtained by GAT showed
that, the effect of IOP on corneal power was weak and did not significantly
influence the corneal shape flattening[19-21].
This is in contrast to our findings, but might be explained due to the
differences in the measurement setup. We believe, that cataract surgeons should
- from a biomechanical point of view - not restrict to tomography examinations,
but also consider corneal compensated IOP especially in follow-up studies after
cataract surgery.
Up to our knowledge, this is the first
clinical study providing the impact of biometric characteristics on
biomechanical properties before and after cataract surgery. The roles of
topography and tomography values as well as biometric data on the biomechanical
properties are discussed. In conclusion, in a patient for cataract surgery,
information about the corneal power of the posterior surface may help to
understand the unique indices provided by ORA (CH and IOPcc). Cataract
surhageons should from a biomechanical point of view not restrict to tomography
examinations, but also consider corneal compensated IOP especially in follow-up
studies after cataract surgery. Further research work should be done with a
larger clinical study population to further investigate correlations among biometric
characteristics and biomechanical properties in cataract surgery more in
detail.
ACKNOWLEDGEMENTS
We thank the supports of the China Scholarship Council (CSC) for the
author’s study (Xue-Fei Song).
Conflicts of Interest: Song XF, None;
Langenbucher A, None; Gatzioufas Z, None;
Seitz B, None; El-Husseiny M, None.
REFERENCES
1 Kucumen RB, Yenerel NM, Gorgun E,
Kulacoglu DN, Oncel B, Kohen MC, Alimgil ML. Corneal biomechanical properties
and intraocular pressure changes after phacoemulsification and intraocular lens
implantation. J Cataract Refract Surg 2008;34(12):2096-2098.
[CrossRef] [PubMed]
2
Luce DA. Determining in vivo biomechanical properties of the cornea with an
ocular response analyzer. J Cataract
Refract Surg 2005;31(1):156-162. [CrossRef] [PubMed]
3
Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular
response analyser to establish the relationship between ocular hysteresis,
corneal resistance factor and central corneal thickness in normal eyes. Cont Lens Anterior Eye 2006;29(5):257-262. [CrossRef] [PubMed]
4
Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF. Corneal thickness-
and age-related biomechanical properties of the cornea measured with the ocular
response analyzer. Invest Ophthalmol Vis
Sci 2006;47(12):5337-5347. [CrossRef] [PubMed]
5
Ortiz D, Piñero D, Shabayek MH, Arnalich-Montiel F, Alió JL. Corneal
biomechanical properties in normal, post-laser in situ keratomileusis, and
keratoconic eyes. J Cataract Refract Surg
2007;33(8):1371-1375. [CrossRef] [PubMed]
6 Labiris G, Gatzioufas Z, Sideroudi H, Giarmoukakis A,
Kozobolis V, Seitz B. Biomechanical diagnosis of keratoconus: evaluation of the
keratoconus match index and the keratoconus match probability. Acta Ophthalmol 2013;91(4):e258-e262.
7
Elsheikh A, Joda A, Abass A, Garway-Heath D. Assessment of the Ocular Response
Analyzer as an Instrument for Measurement of Intraocular Pressure and Corneal
Biomechanics. Curr Eye Res 2015;40(11):1111-1119. [CrossRef] [PubMed]
8
Martinez-de-la-Casa JM, Garcia-Feijoo J, Fernandez-Vidal A, Mendez-Hernandez C,
Garcia-Sanchez J. Ocular response analyzer versus Goldmann applanation
tonometry for intraocular pressure measurements. Invest Ophthalmol Vis Sci 2006;47(10):4410-4414. [CrossRef] [PubMed]
9
Chan TC, Liu D, Yu M, Jhanji V. Longitudinal evaluation of posterior corneal
elevation after laser refractive surgery using swept-source optical coherence
tomography. Ophthalmology 2015;122(4):687-692.
[CrossRef] [PubMed]
10
Preussner PR, Hoffmann P, Wahl J. Impact of posterior corneal surface on toric
intraocular lens (IOL) calculation. Curr
Eye Res 2015;40(8):809-814. [CrossRef] [PubMed]
11
Nemeth G, Berta A, Lipecz A, Hassan Z, Szalai E, Modis L Jr. Evaluation of
posterior astigmatism measured with Scheimpflug imaging. Cornea 2014;33(11):1214-1218. [CrossRef] [PubMed]
12
Leyland M. Validation of Orbscan II posterior corneal curvature measurement for
intraocular lens power calculation. Eye
(Lond) 2004;18(4):357-360. [CrossRef] [PubMed]
13
Langenbucher A, Torres F, Behrens A, Suarez E, Haigis W, Seitz B. Consideration
of the posterior corneal curvature for assessment of corneal power after myopic
LASIK. Acta Ophthalmol Scand 2004;82(3):264-269. [CrossRef] [PubMed]
14
Eom Y, Kang SY, Song JS, Kim HM. Use of corneal power-specific constants to
improve the accuracy of the SRK/T formula. Ophthalmology
2013;120(3):477-481. [CrossRef] [PubMed]
15
Wiemer NG, Dubbelman M, Kostense PJ, Ringens PJ, Polak BC. The influence of
chronic diabetes mellitus on the thickness and the shape of the anterior and
posterior surface of the cornea. Cornea 2007;26(10):1165-1170.
[CrossRef] [PubMed]
16
Alió JL, Agdeppa MC, Rodríguez-Prats JL, Amparo F, Piñero DP. Factors
influencing corneal biomechanical changes after microincision cataract surgery
and standard coaxial phacoemulsification. J
Cataract Refract Surg 2010;36(6):890-897. [CrossRef] [PubMed]
17
Wang Q, Savini G, Hoffer KJ, Xu Z, Feng Y, Wen D, Hua Y, Yang F, Pan C, Huang
J. A comprehensive assessment of the precision and agreement of anterior
corneal power measurements obtained using 8 different devices. PLoS One 2012;7(9):e45607. [CrossRef] [PubMed] [PMC free article]
18
Ehrlich JR, Radcliffe NM, Shimmyo M. Goldmann applanation tonometry compared
with corneal-compensated intraocular pressure in the evaluation of primary
open-angle Glaucoma. BMC Ophthalmol 2012;12:52. [CrossRef] [PubMed]
19
Lombardo G, Serrao S, Rosati M, Lombardo M. Analysis of the viscoelastic
properties of the human cornea using Scheimpflug imaging in inflation
experiment of eye globes. PLoS One 2014;9(11):e112169. [CrossRef] [PubMed]
20 Ozcura
F, Aydin S, Uzgoren N. Effects of central corneal thickness, central corneal
power, and axial length on intraocular pressure measurement assessed with
goldmann applanation tonometry. Jpn J Ophthalmol 2008;52(5):353-356.
21 Xu L,
Xu X, Chen H, Li X. Ocular biocompatibility and tolerance study of
biodegradable polymeric micelles in the rabbit eye. Colloids Surf B
Biointerfaces 2013;112:30-34.
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