correlation between corneal biomechanical properties, applanation tonometry and direct intracameral...

6
Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry A-Yong Yu, 1 Su-Fang Duan, 1 Yun-E Zhao, 1 Xing-Yu Li, 1 Fan Lu, 1 Jianhua Wang, 2 Qin-Mei Wang 1 ABSTRACT Aim To investigate the correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry. Methods Patients scheduled for phacoemulsification were enrolled in this prospective observer-masked study. Central corneal thickness (CCT) was obtained with ultrasound pachymetry. Corneal biomechanical properties including corneal hysteresis (CH) and corneal resistance factor (CRF) were measured with the ocular response analyser. Applanation intraocular pressure (IOP) (IOPappla) was measured in the supine position. Intracameral IOP (IOPintra) was measured with a pressure transducer connected to a cannula inserted into the anterior chamber. Results Fifty-eight eyes of 58 patients were included. There was a significant difference between IOPappla and IOPintra (17.364.1 vs 13.864.7 mm Hg, p<0.001). CCT was significantly correlated with CH and CRF (p<0.01). Univariate regression analysis revealed IOPappla was significantly correlated with CCT and CRF (p<0.05). IOPintra had no significant association with the corneal properties (p>0.05). Multivariate regression analysis revealed IOPappla¼10.43+2.69CRFe1.99CH (p<0.001) and the difference between IOPappla and IOPintra (DIOP)¼1.57+0.55CRF (p¼0.032). Conclusion The mean difference between IOPappla and IOPintra was 3.5 mm Hg. While IOPappla was correlated with the corneal biomechanical properties of CH and CRF, IOPintra was not. IOPappla was correlated more with the corneal biomechanical properties assessed by the ocular response analyser than with CCT alone. Intraocular pressure (IOP) is not only an important factor for the diagnosis and monitoring of the progression of glaucoma, 1e3 but is also one of the key indicators to evaluate the clinical outcome of many IOP-related conditions other than glaucoma. 45 Although the IOP determined by applanation tonometry is regarded as a clinical gold standard, the accuracy of the results is still disputable. Various corneal properties such as central corneal thickness (CCT) have been suggested to be responsible for inaccurate applanation IOP (IOPappla) readings. 67 Recent research with the ocular response analyser (ORA) revealed that IOP measurements are inu- enced by corneal biomechanical properties such as hysteresis. 5 8 In contrast, IOP can be measured directly by an intracameral method that reects the true IOP. 9 Because it is exceedingly important to measure IOP accurately, we compared measurements of IOPappla and intracameral IOP (IOPintra) and determined whether differences were correlated with the corneal biomechanical properties measured by the ORA. MATERIALS AND METHODS Patients scheduled for phacoemulsication were enrolled in this prospective study. One eye was randomly selected in every patient. All patients underwent a complete ophthalmological examina- tion including slit-lamp biomicroscopy, corneal topography and dilated funduscopy. The enrolment criteria included: (1) indication for phacoemulsica- tion; (2) ability to xate a target during examina- tion; (3) corneal curvature between 40.00 and 50.00 dioptres (D); and (4) corneal astigmatism less than 1.50 D. None of the subjects had corneal patholog- ical changes, glaucoma, ocular inammation, or previous ocular trauma or surgery. The research protocol adhered to the tenets of the Helsinki Declaration and was approved by the local ethics committee. All patients were fully informed about the details and possible risks inherent in this study. Written informed consent was obtained from each patient. Examinations Measurements were performed in the following order: CCT, ORA, IOPappla and IOPintra. Every investigator was masked to the results of the other tests. CCT was measured by one investigator 1 day before surgery with an ultrasound pachymeter (Tomey SP-3000; PT Hatiko International Co Ltd, Inage ku, Chiba, Japan) while the patient was supine. One drop of topical tetracaine hydrochlo- ride 0.5% was instilled into the conjunctive sac. The patient was asked to xate on a target, and the pachymeter probe was gently placed on the nudpupillary axis in a perpendicular orientation. Nine readings were taken, and the smallest was considered for analysis. Corneal biomechanical properties were measured with an ORA (Reichert Ophthalmic Instruments, Depew, New York, USA) by a second investigator in the waiting room before surgery. Topical anaes- thetic drops were applied so that examination conditions were equivalent to other measurements in this study. We measured corneal hysteresis (CH) and the corneal resistance factor (CRF) as two indications of corneal biomechanical properties. CH results from the dynamic nature of the air pulse and the viscous damping inherent in the cornea. It 1 School of Ophthalmology and Optometry, Wenzhou Medical College, Wenzhou, Zhejiang, China 2 Bascom Palmer Eye Institute, University of Miami, Miami, Florida, USA Correspondence to Dr Qin-Mei Wang, 270 Xueyuan West Road, Wenzhou (325000), Zhejiang, PR China; [email protected] Accepted 10 November 2011 Published Online First 20 December 2011 640 Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124 Clinical science group.bmj.com on April 2, 2014 - Published by bjo.bmj.com Downloaded from

Upload: q-m

Post on 21-Dec-2016

222 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

Correlation between corneal biomechanicalproperties, applanation tonometry and directintracameral tonometry

A-Yong Yu,1 Su-Fang Duan,1 Yun-E Zhao,1 Xing-Yu Li,1 Fan Lu,1 Jianhua Wang,2

Qin-Mei Wang1

ABSTRACTAim To investigate the correlation between cornealbiomechanical properties, applanation tonometry anddirect intracameral tonometry.Methods Patients scheduled for phacoemulsificationwere enrolled in this prospective observer-masked study.Central corneal thickness (CCT) was obtained withultrasound pachymetry. Corneal biomechanical propertiesincluding corneal hysteresis (CH) and corneal resistancefactor (CRF) were measured with the ocular responseanalyser. Applanation intraocular pressure (IOP)(IOPappla) was measured in the supine position.Intracameral IOP (IOPintra) was measured witha pressure transducer connected to a cannula insertedinto the anterior chamber.Results Fifty-eight eyes of 58 patients were included.There was a significant difference between IOPappla andIOPintra (17.364.1 vs 13.864.7 mm Hg, p<0.001).CCT was significantly correlated with CH and CRF(p<0.01). Univariate regression analysis revealedIOPappla was significantly correlated with CCT and CRF(p<0.05). IOPintra had no significant association withthe corneal properties (p>0.05). Multivariate regressionanalysis revealed IOPappla¼10.43+2.69CRFe1.99CH(p<0.001) and the difference between IOPappla andIOPintra (DIOP)¼�1.57+0.55CRF (p¼0.032).Conclusion The mean difference between IOPappla andIOPintra was 3.5 mm Hg. While IOPappla was correlatedwith the corneal biomechanical properties of CH andCRF, IOPintra was not. IOPappla was correlated morewith the corneal biomechanical properties assessed bythe ocular response analyser than with CCT alone.

Intraocular pressure (IOP) is not only an importantfactor for the diagnosis and monitoring of theprogression of glaucoma,1e3 but is also one of thekey indicators to evaluate the clinical outcome ofmany IOP-related conditions other than glaucoma.4 5

Although the IOP determined by applanationtonometry is regarded as a clinical gold standard, theaccuracy of the results is still disputable. Variouscorneal properties such as central corneal thickness(CCT) have been suggested to be responsible forinaccurate applanation IOP (IOPappla) readings.6 7

Recent research with the ocular response analyser(ORA) revealed that IOP measurements are influ-enced by corneal biomechanical properties such ashysteresis.5 8 In contrast, IOP can be measureddirectly by an intracameral method that reflects thetrue IOP.9 Because it is exceedingly important tomeasure IOP accurately, we compared measurementsof IOPappla and intracameral IOP (IOPintra) and

determined whether differences were correlated withthe corneal biomechanical properties measured bythe ORA.

MATERIALS AND METHODSPatients scheduled for phacoemulsification wereenrolled in this prospective study. One eye wasrandomly selected in every patient. All patientsunderwent a complete ophthalmological examina-tion including slit-lamp biomicroscopy, cornealtopography and dilated funduscopy. The enrolmentcriteria included: (1) indication for phacoemulsifica-tion; (2) ability to fixate a target during examina-tion; (3) corneal curvature between 40.00 and 50.00dioptres (D); and (4) corneal astigmatism less than1.50 D. None of the subjects had corneal patholog-ical changes, glaucoma, ocular inflammation, orprevious ocular trauma or surgery.The research protocol adhered to the tenets of

the Helsinki Declaration and was approved by thelocal ethics committee. All patients were fullyinformed about the details and possible risksinherent in this study. Written informed consentwas obtained from each patient.

ExaminationsMeasurements were performed in the followingorder: CCT, ORA, IOPappla and IOPintra. Everyinvestigator was masked to the results of the othertests.CCT was measured by one investigator 1 day

before surgery with an ultrasound pachymeter(Tomey SP-3000; PT Hatiko International Co Ltd,Inage ku, Chiba, Japan) while the patient wassupine. One drop of topical tetracaine hydrochlo-ride 0.5% was instilled into the conjunctive sac.The patient was asked to fixate on a target, and thepachymeter probe was gently placed on thenudpupillary axis in a perpendicular orientation.Nine readings were taken, and the smallest wasconsidered for analysis.Corneal biomechanical properties were measured

with an ORA (Reichert Ophthalmic Instruments,Depew, New York, USA) by a second investigatorin the waiting room before surgery. Topical anaes-thetic drops were applied so that examinationconditions were equivalent to other measurementsin this study. We measured corneal hysteresis (CH)and the corneal resistance factor (CRF) as twoindications of corneal biomechanical properties. CHresults from the dynamic nature of the air pulseand the viscous damping inherent in the cornea. It

1School of Ophthalmology andOptometry, Wenzhou MedicalCollege, Wenzhou, Zhejiang,China2Bascom Palmer Eye Institute,University of Miami, Miami,Florida, USA

Correspondence toDr Qin-Mei Wang, 270 XueyuanWest Road, Wenzhou (325000),Zhejiang, PR China;[email protected]

Accepted 10 November 2011Published Online First20 December 2011

640 Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124

Clinical science

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from

Page 2: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

was measured as the difference between the inward (P1) and theoutward (P2) applanation pressures. CRF is a measurement ofthe cumulative effects of both the viscous and elastic resistanceencountered by the air jet while deforming the corneal surface. Itis equal to (P1�0.68P2).10 ORA measurements were taken intriplicate, and the average value was taken for statistical anal-ysis. Off-scale values were discarded, as were measurements thatcould not be repeated three times. Patients were then guided tothe surgery room.

IOPappla measurements were performed by a third investi-gator with an applanation tonometer (Tonopen XL manufac-tured in 2005; Medtronic Solan, Jacksonville, FL, USA) while thepatient was in the supine position. This position was chosenbecause the IOPintra was measured in the same position. In thisway, positional effects on IOP measurement were ruled out. Thedevice was calibrated daily. The operator touched the corneawith the pen tip several times until a reading was displayed.Only measurements with a SE smaller than 5% were accepted.The procedure was repeated if successive measurements differedby more than 5 mm Hg. Three readings were taken, and theaverage value was calculated.

Then, a fourth investigator, the surgeon, performed IOPintrameasurements. A cannula connected to a pressure transducer(Düsseldorf model G-19235; Geuder AG; Heidelberg, Germany)was introduced through the temporal limbus into the anteriorchamber. IOPintra was taken for approximately 10 s, until thereadings on the monitor were stable. The cannula was thenremoved, and the patient was prepared for surgery.

Statistical analysisData were collected on standardised case report forms, thenentered into a central database for analysis. There were nomissing data in the analysis. Statistical analysis was performedwith commercial software (SPSS V.13.0). Descriptive statisticsfor continuous variables were calculated as means, standarddeviations (except when noted) and 95% CI. Measurementswere compared by paired t tests. Agreements between methodswere assessed by BlandeAltman analysis.11 Regression analysiswas used to evaluate the associations between IOPappla,IOPintra and the difference between them (DIOP) with thecorneal factors of curvature, CCT, CH and CRF. The associationswere initially investigated using univariate analysis. Then, allindependent variables were entered in multivariate linearregression models to assess their relationship with IOP measuredby the different methods. The level of significance was p<0.05.

RESULTSFifty-eight patients (38 women and 20 men, 29 right and 29 lefteyes) with a mean age of 63614 years (range 32e88 years) wereenrolled in this study between July 2007 and June 2008. Nocomplications arose from intracameral measurements. For thesepatients, IOPappla, 17.3 mm Hg, was significantly greater thanIOPintra, 13.8 mm Hg (paired t test, p<0.001, table 1). Therewas a significant correlation between IOPappla and IOPintrawhere

IOPappla ¼ 8:91 þ 0:61IOPintra�r2¼ 0:48; p<0:001Þ:

The BlandeAltman analysis showed that the mean DIOP was3.563.5 mm Hg and the 95% limits of agreement were �3.4and 10.4 mm Hg (figure 1).

Univariate regression showed that IOPappla was significantlycorrelated with CCT and CRF (table 2), but not with cornealcurvature or CH. Each 100 mm increase in CCT resulted ina 3.8 mm Hg increase in IOPappla. IOPintra was not signifi-

cantly associated with any of these variables (table 2, figure 2A).The DIOP was significantly associated only with CRF (table 2).In the univariate analysis, IOPappla was significantly corre-

lated with CCT. In the multivariate analysis, IOPappla wasinfluenced by CRF and CH, but not by CCT. To determine if theinfluence of CCT was lost because it correlated with CRF andCH, we performed univariate analysis for these variables. CCTwas significantly correlated with (figure 2B) CH (r2¼0.19,p¼0.001) and CRF (r2¼0.35, p<0.001), but not with cornealcurvature.Multiple regression incorporating corneal curvature, CCT, CH

and CRF revealed that IOPintra had no significant associationwith these corneal properties. In contrast, IOPappla wassignificantly correlated with CRF and CH:

IOPappla ¼ 10:43 þ 2:69CRF� 1:99CH ðp< 0:001Þ:DIOP was significantly correlated with CRF:

DIOP ¼ �1:57 þ 0:55CRF ðp¼0:032Þ:

DISCUSSIONIOP measurement is important in the clinical practice ofophthalmology. Errors in IOP measurement associated withdifferent methods can lead to substantial misclassification of

Table 1 IOP and corneal properties

Mean±SD Maximum Minimum 95% CI

IOPappla (mm Hg) 17.364.1 30.0 9.0 16.2 to 18.4

IOPintra (mm Hg) 13.864.7 28.0 4.0 12.6 to 15.1

DIOP (mm Hg) 3.563.5 13.0 �1.0 2.6 to 4.4

Corneal curvature (D) 44.6262.03 49.49 40.04 44.06 to 45.18

CCT (mm) 526.7633.8 593.0 455.0 517.8 to 535.5

CH (mm Hg) 9.361.8 12.9 4.0 8.8 to 9.8

CRF (mm Hg) 9.461.9 14.0 4.0 8.9 to 9.9

n¼58 eyes.CCT, central corneal thickness; CH, corneal hysteresis; CRF, corneal resistance factor; IOP,intraocular pressure; IOPappla, applanation IOP; IOPintra, intracameral IOP; DIOP,IOPapplaeIOPintra.

Figure 1 BlandeAltman plot of the agreement between applanationintraocular pressure (IOP) (IOPappla) and intracameral IOP (IOPintra). Thedifference between the measurements was plotted against the averageof the measurements. Solid lines, the 95% limits of agreement; dashedline, mean difference between IOPappla and IOPintra.

Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124 641

Clinical science

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from

Page 3: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

patients, with great impact on the management of certain IOP-related conditions. Therefore, it is important to determine if theIOP measurement is sufficiently accurate and precise. True IOPcan only be obtained with intracameral methods. However,ocular trauma and the risk of infectious disease are concernswith using an invasive approach. Various investigators have triedto assess the corneal properties that affect the accuracy of IOPmeasurements and to account for those properties when calcu-lating the IOP.12e14 However, none of these studies employedthe intracameral methods and the overall corneal biomechanicalproperties at the same time (table 3); rather, they concentratedon CCT and corneal curvature. In the present study, we soughtto understand better the impact of corneal properties on IOPmeasurements. Therefore, we included intracameral tonometryalong with the measurement of overall corneal biomechanicalcharacteristics. These were employed to compare and explainthe difference in IOP measurements obtained by the applanationand intracameral methods.

Applanation tonometry measures IOP with instruments thatindent or flatten the corneal apex. When Goldmann andSchmidt introduced applanation tonometry, they acknowledgedthat the physical dimensions of the cornea might affect the

measurements.27 In the current study, the mean IOPappla was3.5 mm Hg higher than the IOPintra, which is consideredclinically significant. This is in agreement with comparisons ofIOPappla and IOPintra reported in previous research.15 16 In ourstudy, overestimation by IOPappla for individual patients wasup to 9 mm Hg, within the range of overestimation,6.8e29 mm Hg, reported in the literature.17 18 We found thatIOPappla was highly variable, with some cases slightly lowerthan IOPintra and many in which there were wide disparities.Theoretically, IOPintra accurately reflects true IOP, and this issupported by our data showing the independence of IOPintrafrom corneal biomechanical properties. However, the indepen-dence of material properties does not demonstrate that IOPintrais identical to true IOP. They could differ if IOPintra is in facta complex function of the true IOP.We did find a significant univariate correlation between

IOPappla and CCT. Each 100 mm increase in CCT resulted in3.8 mm Hg increase in IOPappla, which is consistent withTonnu and coauthors.19 Various investigators have reportedIOPappla could be influenced by CCT.20e23 Some suggested thatadjustment of the IOPappla measurement might be neededwhen the CCT deviates far from normal, although correction

Figure 2 (A) Scatterplot of intracameral intraocular pressure (IOPintra) versus corneal hysteresis (CH) and corneal resistance factor (CRF). (B)Scatterplot of central corneal thickness (CCT) versus CH and CRF.

Table 2 Univariate regression analysis of the association between IOP measurements and cornealproperties*

IOPappla (mm Hg) IOPintra (mm Hg) DIOP (mm Hg)

Coefficient (SE) p Value Coefficient (SE) p Value Coefficient (SE) p Value

Corneal curvature (D) 0.241 (0.285) 0.402 0.074 (0.321) 0.819 0.168 (0.244) 0.496

CCT (mm) 0.038 (0.016) 0.017 0.025 (0.018) 0.183 0.014 (0.014) 0.329

CH (mm Hg) 0.217 (0.300) 0.474 �0.192 (0.342) 0.577 0.409 (0.250) 0.107

CRF (mm Hg) 1.083 (0.254) 0.001 0.550 (0.325) 0.096 0.533 (0.237) 0.029

CCT, CH and CRF as independent variables (one at a time, in univariate regression).*IOP measurements were entered as dependent variables and corneal curvature.CCT, central corneal thickness; CH, corneal hysteresis; CRF, corneal resistance factor; IOP, intraocular pressure; IOPappla, applanationIOP; IOPintra, intracameral IOP; DIOP, IOPapplaeIOPintra.

642 Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124

Clinical science

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from

Page 4: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

might be unnecessary or insignificant when the CCT is withinthe normal range.

However, the multivariate regression analysis in the currentstudy showed that the CCT did not significantly correlate withthe IOPappla when CH and CRF were considered at the sametime. This suggests that IOPappla is affected more by cornealbiomechanical characteristics than by CCT alone. Possibly thisparticular population was insensitive to CCT. More likely,however, is that CH and CRF are not independent of CCT asshown in this study. Research using ORA found that overallcorneal biomechanical characteristics are positively correlatedwith CCT, and are significantly associated with IOPapplavalues.20 28 29 Mollan and coauthors24 performed a multivariateregression analysis using IOPappla as the dependent variable andCCT, CH and CRF as the independent variables. They foundthat IOPappla¼19.15+1.35CRFe1.38CH. Many studies20 24e26

demonstrated that applanation tonometry was affected more bythe overall corneal biomechanical characteristics than by CCT,and Liu and Roberts25 reported that the effect of CCT on IOPwas non-linear.

In our study, multivariate regression analysis showed that thedifference between IOPappla and IOPintra, DIOP, had a clearrelationship with CRF. Each 1.0 mm Hg increase in the CRFresulted in a 0.55 mm Hg difference in IOP measured byapplanation and intracameral methods. This suggests that the

metrics of corneal rigidity might not correlate with CCT alone,and that the corneal biomechanics measured by ORA reflectmore than CCT. The CH indicates the viscoelastic properties ofthe cornea.23 CRF is an optimised corneal biomechanicalparameter derived using proprietary algorithms that utilisespecific combinations of the inward and outward applanationvalues. It is a measurement of the total viscoelastic response ofthe cornea during the dynamic bidirectional applanation processemployed in the ORA. It appears to be an indicator of the overallcorneal ‘resistance’ to applanation. These suggest that CH andCRF are parameters that reflect the cumulative effects of CCT,corneal curvature and tissue material properties.Because the exact meaning of the corneal biomechanics

measured by ORA remains unclear, there are no validatedformulas to calculate the impact of these corneal biomechanicalcharacteristics on IOP measurements. Considering that CCT iscorrelated with CH and CRF, this might leave open the possi-bility that correcting IOPappla to approximate IOPintra basedon CCT may be adequate, even without knowledge of CH andCRF. However, given that the IOPappla is affected more bycorneal biomechanical characteristics than by CCT alone, anyattempts to adjust the IOPappla readings based on CCT or anyclinically available correction tables suggested by previousstudies should be treated with caution, although it is difficult tostate clearly what this caution should be. Further studies

Table 3 Previously published studies on IOP measurements and corneal properties

Authors Corneal properties IOPappla IOPintra Findings

Feltgen et al9 CCT Yes Yes IOPappla is not affected by CCT. CorrectingIOPappla based on CCT is inappropriate.

Elsheikh et al12 CCT, CC Yes No IOPappla is affected by CCT and CC. Each100 mm increase in CCT results in 1.7 mmHg increase in IOPappla. Correcting IOPapplabased on CCT and CC is necessary.

Saleh et al13 CCT, CC Yes No IOPappla is affected by CCT. Each 100 mmincrease in CCT results in 4.0 mm Hg increasein IOPappla.

Kohlhaas et al14 CCT, CC Yes Yes IOPappla is affected by CCT. Each 25 mmdeviation from a CCT of 550 mm results in1 mm Hg correction for IOPappla. CorrectingIOPappla based on CCT is necessary.

Eisenberg et al15 No Yes Yes IOPappla underestimates IOPintra in anIOP-dependent manner.

Lim et al16 No Yes Yes IOPappla underestimates IOPintra in anIOP-dependent manner.

Whitacre et al17 CCT Yes Yes IOPappla is affected by CCT. Thin corneasproduce underestimations up to 4.9 mm Hg,whereas thick corneas produce overestimationsup to 6.8 mm Hg.

Johnson et al18 CCT Yes Yes IOPappla is affected by CCT. Overestimations ofIOPappla is up to 29 mm Hg.

Tonnu et al19 CCT Yes No IOPappla is affected by CCT. Each 100 mm increasein CCT results in 3.1 mm Hg increase in IOPappla.

Medeiros and Weinreb20 CCT, CC, CRF Yes No IOPappla is affected more by the overall cornealbiomechanical characteristics than by CCT.

Doughty and Zaman21 CCT Yes No IOPappla is affected by CCT.

Lleo et al22 CCT Yes No IOPappla is affected by CCT.

Ehlers et al23 CCT, CC Yes Yes IOPappla is affected by CCT. Correcting IOPapplabased on CCT is necessary.

Mollan et al24 CCT, CH, CRF Yes No IOPappla is affected more by the overall cornealbiomechanical characteristics than by CCT alone.

Liu and Roberts25 CCT, CH, CRF Yes No IOPappla is affected more by the overall cornealbiomechanical characteristics than by CCT alone,and the effect of CCT on IOPappla is nonlinear.

Sahin et al26 CCT, CC, CH, CRF Yes No IOPappla is affected more by the overall cornealbiomechanical characteristics than by CCT alone.

CC, corneal curvature; CCT, central corneal thickness; CH, corneal hysteresis; CRF, corneal resistance factor; IOP, intraocular pressure;IOPappla, applanation IOP; IOPintra, intracameral IOP.

Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124 643

Clinical science

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from

Page 5: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

analysing the relationship between the fundamental principlesof mechanics and assessments with the ORA may providea better understanding of the relationship between cornealproperties and IOPappla.

Differences in the measurement of CCT might influence theresults. Because the human cornea is thin at the centre and thickat the periphery, ultrasonic methods have limitations. Theaccuracy of CCT is influenced by the location where the probe isplaced. If the probe is not perpendicular to the corneal surface, itwill overestimate CCT. So we used the lowest, rather than themean, of nine readings. In addition, the Tonopen may not bea good choice of a tonometer because the IOPappla readings notonly had a larger IOP error than IOPintra, but also greatervariability as demonstrated by BlandeAltman plots in thepresent study. This choice may have decreased the study power.However, Tonopen applanation is a dynamic form of applana-tion, unlike the Goldman style, which is static. The CH and CRFare dynamic properties of the cornea and may relate much moreto Tonopen readings than to Goldman readings.

In conclusion, we found that the mean IOPappla was3.5 mm Hg higher than the mean IOPintra. While IOPappla wascorrelated with the corneal biomechanical properties of CH andCRF, IOPintra was not. IOPappla was correlated more with thecorneal biomechanical properties assessed by the ORA than withCCTalone. These findings may have significant implications forthe understanding of IOP measurement in clinical practice. In thefuture, it would be worthwhile to repeat this study in patientswith glaucoma, as their biomechanics may differ.

Acknowledgements The authors would like to thank Britt Bromberg of XenofileEditing, for providing editing services for the manuscript.

Funding This study was supported by research grants from the Chinese National KeyTechnologies R&D Program, Beijing, China (2007BAI18B09 to FL) and the ZhejiangProvincial Program for the Cultivation of High-level Innovative Health Talents (to FL).

Competing interests None.

Patient consent Obtained.

Ethics approval The research protocol adhered to the tenets of the HelsinkiDeclaration and was approved by the local ethics committee (ethics committee at theEye Hospital, Wenzhou Medical College, 270 Xueyuan West Road, Wenzhou, Zhejiang,China).

Contributors Design of the study: A-YY, Q-MW, FL; conduct of the study: S-FD, Y-EZ,X-YL; review of the manuscript: JW; analysis and interpretation of the data, andpreparation of the manuscript: A-YY.

Provenance and peer review Not commissioned; externally peer reviewed.

REFERENCES1. Congdon NG, Broman AT, Bandeen-Roche K, et al. Central corneal thickness and

corneal hysteresis associated with glaucoma damage. Am J Ophthalmol2006;141:868e75.

2. Kass MA, Heuer DK, Higginbotham EJ, et al. The Ocular Hypertension TreatmentStudy: a randomized trial determines that topical ocular hypotensive medicationdelays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol2002;120:701e13; discussion 829e30.

3. Leske MC, Heijl A, Hussein M, et al. Factors for glaucoma progression and theeffect of treatment: the early manifest glaucoma trial. Arch Ophthalmol2003;121:48e56.

4. Kaufmann C, Bachmann LM, Thiel MA. Intraocular pressure measurements usingdynamic contour tonometry after laser in situ keratomileusis. Invest Ophthalmol VisSci 2003;44:3790e4.

5. Pepose JS, Feigenbaum SK, Qazi MA, et al. Changes in corneal biomechanics andintraocular pressure following LASIK using static, dynamic, and noncontacttonometry. Am J Ophthalmol 2007;143:39e47.

6. Hager A, Wiegand W. [Methods of measuring intraocular pressure independently ofcentral corneal thickness] (In German). Ophthalmologe 2008;105:840e4.

7. Chihara E. Assessment of true intraocular pressure: the gap between theory andpractical data. Surv Ophthalmol 2008;53:203e18.

8. Hager A, Loge K, Fullhas MO, et al. Changes in corneal hysteresis after clear cornealcataract surgery. Am J Ophthalmol 2007;144:341e6.

9. Feltgen N, Leifert D, Funk J. Correlation between central corneal thickess,applanation tonometry, and direct intracameral IOP readings. Br J Ophthalmol2001;85:85e7.

10. Kotecha A, Elsheikh A, Roberts CR, et al. Corneal thickness and age-relatedbiomechanical properties of the corneal measured with the ocular response analyzer.Invest Ophthalmol Vis Sci 2006;47:5337e47.

11. Bland JM, Altman DG. Statistical methods for assessing agreement between twomethods of clinical measurement. Lancet 1986;1:307e10.

12. Elsheikh A, Alhasso D, Gunvant P, et al. Multiparameter correction equation forGoldmann applanation tonometry. Optom Vis Sci 2011;88:E102e12.

13. Saleh TA, Adams M, McDermott B, et al. Effects of central corneal thickness andcorneal curvature on the intraocular pressure measurement by Goldmann applanationtonometer and ocular blood flow pneumatonometer. Clin Experiment Ophthalmol2006;34:516e20.

14. Kohlhaas M, Boehm AG, Spoerl E, et al. Effect of central corneal thickness, cornealcurvature, and axial length on applanation tonometry. Arch Ophthalmol2006;124:471e6.

15. Eisenberg DL, Sherman BG, McKeown CA, et al. Tonometry in adults and children.A manometric evaluation of pneumatonometry, applanation, and TonoPen in vitro andin vivo. Ophthalmology 1998;105:1173e81.

16. Lim JI, Blair NP, Higginbotham EJ, et al. Assessment of intraocular pressure invitrectomized gas-containing eyes. A clinical and manometric comparison of theTono-Pen to the pneumotonometer. Arch Ophthalmol 1990;108:684e8.

17. Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness onapplanation tonometry. Am J Ophthalmol 1993;115:592e6.

18. Johnson M, Kass MA, Moses RA, et al. Increased corneal thickness simulatingelevated intraocular pressure. Arch Ophthalmol 1978;96:664e5.

19. Tonnu PA, Ho T, Newson T, et al. The influence of central corneal thickness and ageon intraocular pressure measured by pneumotonometry, noncontact tonometry, theTono-Pen XL, and Goldmann applanation tonometry. Br J Ophthalmol2005;89:851e4.

20. Medeiros FA, Weinreb RN. Evaluation of the influence of corneal biomechanicalproperties on intraocular pressure measurements using the ocular response analyzer.J Glaucoma 2006;15:364e70.

21. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocularpressure measures: a review and meta-analysis approach. Surv Ophthalmol2000;44:367e408.

22. Lleo A, Marcos A, Calatayud M, et al. The relationship between central cornealthickness and Goldmann applanation tonometry. Clin Exp Optom 2003;86:104e8.

23. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central cornealthickness. Acta Ophthalmol (Copenh) 1975;53:34e43.

24. Mollan SP, Wolffsohn JS, Nessim M, et al. Accuracy of Goldmann, ocular responseanalyser, Pascal and TonoPen XL tonometry in keratoconic and normal eyes.Br J Ophthalmol 2008;92:1661e5.

25. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocularpressure measurement: quantitative analysis. J Cataract Refract Surg2005;31:146e55.

26. Sahin A, Bayer A, Ozge G, et al. Corneal biomechanical changes in diabetes mellitusand their influence on intraocular pressure measurements. Invest Ophthalmol Vis Sci2009;50:4597e604.

27. Goldmann H, Schmidt T. On applanation tonography. Ophthalmologica1965;150:65e75.

28. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocularresponse analyzer. J Cataract Refract Surg 2005;31:156e62.

29. Martinez-de-la-Casa JM, Garcia-Feijoo J, Fernandez-Vidal A, et al. Ocular responseanalyzer versus Goldmann applanation tonometry for intraocular pressuremeasurements. Invest Ophthalmol Vis Sci 2006;47:4410e14.

PAGE fraction trail=5

644 Br J Ophthalmol 2012;96:640e644. doi:10.1136/bjophthalmol-2011-300124

Clinical science

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from

Page 6: Correlation between corneal biomechanical properties, applanation tonometry and direct intracameral tonometry

doi: 10.1136/bjophthalmol-2011-300124December 20, 2011

2012 96: 640-644 originally published onlineBr J Ophthalmol A-Yong Yu, Su-Fang Duan, Yun-E Zhao, et al. direct intracameral tonometryproperties, applanation tonometry and Correlation between corneal biomechanical

http://bjo.bmj.com/content/96/5/640.full.htmlUpdated information and services can be found at:

These include:

References

http://bjo.bmj.com/content/96/5/640.full.html#related-urlsArticle cited in:  

http://bjo.bmj.com/content/96/5/640.full.html#ref-list-1This article cites 29 articles, 7 of which can be accessed free at:

serviceEmail alerting

the box at the top right corner of the online article.Receive free email alerts when new articles cite this article. Sign up in

CollectionsTopic

(1064 articles)Ophthalmologic surgical procedures   � Articles on similar topics can be found in the following collections

Notes

http://group.bmj.com/group/rights-licensing/permissionsTo request permissions go to:

http://journals.bmj.com/cgi/reprintformTo order reprints go to:

http://group.bmj.com/subscribe/To subscribe to BMJ go to:

group.bmj.com on April 2, 2014 - Published by bjo.bmj.comDownloaded from