The use of the Ocular Response Analyser to determine corneal hysteresisin eyes before and after excimer laser refractive surgery

Sunil Shah a,b,c, Mohammad Laiquzzaman a,*, Ian Yeung c, Xueliang Pan d, Cynthia Roberts e

a Heart of England Foundation Trust, Solihull, UKb Ophthalmic Research Group, Aston University, Birmingham, UKc Midland Eye Institute, Birmingham, UKd Department of Statistics, The Ohio State University, Columbus, OH, USAe Department of Ophthalmology and Department of Biomedical Engineering, The Ohio State University, Columbus, OH, USA

Contact Lens & Anterior Eye 32 (2009) 123–128

A R T I C L E I N F O

Keywords:

CCT

CH

CRF

IOPg

IOPcc

A B S T R A C T

Purpose: To compare corneal biomechanical parameters and two measures of intraocular pressure (IOP)

in eyes before and after excimer laser refractive surgery, with the Ocular Response Analyser (ORA).

Materials and methods: Eighty normal eyes of 41 patients undergoing excimer laser refractive surgery in

Birmingham, U.K. were recruited into three groups: Laser Assisted-Epithelial Keratomileusis (LASEK)

(Myopes), Laser Assisted in Situ Keratomileusis (LASIK) (myopes) and LASIK (hyperopes). The preop and

3 months postop Goldmann correlated IOP (IOPg), corneal compensated IOP (IOPcc), corneal hysteresis

(CH), and corneal resistance factor (CRF) were measured by the ORA. Central corneal thickness (CCT) was

measured using ultrasonic pachymeter. The differences of the changes in IOPg, IOPcc, CH, CRF and CCT

between the three groups were estimated. A General Linear Model was selected to investigate the

influence of gender, age, initial conditions (CH, CRF, CCT, IOPcc and IOPg) and changes in CCT on the

measured IOP.

Results: The differences between the mean IOPg, CH and CRF after refractive surgery were statistically

significant for all three groups. The hyperopic LASIK group had a significantly smaller change compared

to the other groups (which had no statistical significance). The preop IOPg, preop CH and gender were

significant predictors of the changes in measured pressure and biomechanical parameters after surgery

in the myopic groups only.

Conclusion: CH and CRF were found to decrease after both myopic and hyperopic refractive surgery. CH

and CRF measurement may prove important tools to clarify the role of corneal biomechanics for

refractive surgery.

� 2009 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Contact Lens & Anterior Eye

journa l homepage: www.e lsev ier .com/ locate /c lae

1. Introduction

The cornea is a largely transparent tissue that also serves as avery powerful optical lens [1] accounting for 70% of the totalrefractive power of the eye [2]. It forms a mechanically toughmembrane. It is because of its mechanical strength and transpar-ency that the cornea serves both as an efficient protective barrierand as a refractive lens.

Various surgical procedures used for corneal refractive surgeryresult in substantial changes in corneal tissue structure that affectthe central corneal thickness (CCT) and curvature [3–7]. Cornealrefractive surgery can thus alter corneal biomechanical properties.

* Corresponding author at: Midland Eye Institute, 50 Lode Lane, Solihull, West

Midlands B91 2AW, UK. Tel.: +44 121 711 2020; fax: +44 121 711 4040.

E-mail address: mohammadlaiquzzaman@hotmail.com (M. Laiquzzaman).

1367-0484/$ – see front matter � 2009 British Contact Lens Association. Published by

doi:10.1016/j.clae.2009.02.005

There is evidence that the biomechanical properties of the corneacan influence the diagnosis and hence management of oculardisease(s), but the factors which influence these properties are notwell understood and hence cannot be controlled without theknowledge and ability to measure the biomechanical properties in

vivo [8]. An example is the validity of intraocular pressure (IOP)after refractive surgery [9]. With the ever increasing popularity ofrefractive surgery, the knowledge of the factors that determine thebiomechanical properties of the eye and their importance in themanagement of disease process has gained importance.

So far there has been no easy method reported to determinebio-mechanical corneal properties in vivo other than indirectlythrough CCT measurements. A recent addition to the armamen-tarium for assessing biomechanics has been the Ocular ResponseAnalyser (ORA) [Reichert Ophthalmic Instruments, Buffalo, USA]which in addition to being a non-contact tonometer, measures newmetrics referred to as ‘corneal hysteresis’ (CH) which is said to be a

Elsevier Ltd. All rights reserved.

S. Shah et al. / Contact Lens & Anterior Eye 32 (2009) 123–128124

measure of the visco-elastic properties of the cornea and cornealresistance factor (CRF) which is said to be a measure of elasticity.

CH is the difference between the inward and outwardapplanation pressure. The air pressure pulse which impinges onthe corneal surface is a precisely, metered ramp of air. If purelyelastic, it would be expected that the cornea would applanate atthe same pressure in both directions i.e. inward and outward;however, due to the visco-elastic properties of the cornea twodifferent applanation pressures are noted. The CH is described asthe loss or delay of energy due to resistance of the cornea to theair puff. Reichert believes that CRF derived in this process isdominated by the elastic properties of the cornea and appears to bean indicator of the overall ‘‘resistance’’ of the cornea [10].

This study focuses on the change in three corneal parameters,CH, CRF, CCT, as well as two measures of IOP, corneal compensatedIOP (IOPcc) and Goldmann correlated IOP (IOPg), 3 monthsfollowing corneal refractive surgery. This is the first study toaddress the differences in biomechanical properties in hyperopiceyes compared to myopic eyes following refractive surgery.

2. Materials and methods

Eighty eyes of 41 patients (25 females and 16 males) wererecruited for this study from the Midland Eye Institute, Solihull,U.K. The mean age was 41.2 � 10.8 SD (age range 21.0–62.0 years).All had normal eyes and no history of ocular disease, surgery ortrauma. IRB approval was obtained from the Local Research EthicsCommittee. These patients undergoing refractive surgery weredivided into three groups: Laser Assisted-Epithelial Keratomileusis(LASEK) (Myopes), Laser Assisted in Situ Keratomileusis (LASIK)(myopes) and LASIK (hyperopes) based on refractive error, cornealthickness and patient and surgeon preference i.e. these were notrandomised groups. A full assessment for refractive surgery includingrefraction, full ocular examination, topography, abberometry andpachymetry was performed.

The ORA measurements were taken while the patient wasseated in a standard fashion. The IOPg, IOPcc, CH and CRF weredetermined by the ORA. The CCT was measured using a hand heldultrasonic pachymeter (SP-2000, Tomey Corp, Japan) after instil-ling a drop of topical anaesthetic Proxymethacaine (Bausch &Lomb, Rochester, New York, USA) in the eye prior to performingpachymetry. The patient was asked to fixate at a target in order tominimise any eye movement, and to avoid damage to the cornealepithelium. The pachymeter probe was gently placed onto themid-pupillary axis in a perpendicular orientation. Upon contactwith the corneal surface, the CCT value was displayed on themonitor attached to the probe. Six readings were taken and themean value was used as the CCT. These measurements were takenbefore and after refractive surgery in the same order to avoid anybias in the data collection.

All the patients underwent LASEK or LASIK by one surgeon (SS).All surgery was uncomplicated. For LASEK, the cornea wasanaesthetised by topical proxymethacaine. The patient was made

Table 1Initial characteristics of patients.

Group n Age** M/F Preop CCT**

(mm)

1. LASEK (myopes) 35 38.3 � 11.3 8/27 532.1 � 37.3

2. LASIK (myopes) 26 42.6 � 6.9 6/20 557.4 � 36.3

3. LASIK (hyperopes) 19 45.3 � 11.4 15/4 557.5 � 23.6

All 80 41.1 � 10.5 29/51 546.4 � 36.1

Means � SDs; IOPg = (IOP1 + IOP2)/2.** The initial value of CCT, IOPg and IOPcc are significantly different between three gro

value of CH are marginally different (p = 0.06 for both). The preop differences between t

preop differences between the LASIK myopes and LASIK hyperopes are not statistically

to lie on the couch and asked to focus on a flashing light. A lidspeculum was inserted to open the lids. A 9.0 mm alcohol well wasapplied and was filled with 18% ethanol and left for 30 s. An 8 mmepithelial flap was fashioned, and then laser was applied to thebare corneal stroma. For LASIK, the cornea was anaesthetised and alid speculum inserted. The cornea was marked with gentian violetto help realign the flap. A suction ring was applied to the limbusand IOP increased to ensure a smooth cut. A NIDEK MK2000automated microkeratome (Gamagori, Japan) was used to create acorneal stromal flap, and all flaps were cut to a nominal depth plateof 160 mm. The hinge was made nasally. Laser was applied toablate the corneal stroma. After laser ablation, the stromal flap wasreplaced.

Data were collected preoperatively and 3 months post-operatively following refractive surgery.

For the data analysis, several computer packages were used,including Excel (Microsoft1 inc.) and MINITAB1 (Minitab Inc.). Thechanges of IOPg, IOPcc, CH, and CRF after surgery were comparedbetween different groups of patients. ANOVA with post hoc testwas performed for statistical analysis. The level of statisticalsignificance level was chosen at 0.05. In addition, the influentialpredictive variables were identified based on the best subsetregression and general linear models from a group of predictorvariables, such as age, gender, the initial conditions (preop CCT,preop IOPg and preop CH), and ablation (CCT difference), on the IOPand CH change (IOPg difference, IOPcc difference, CH difference,and CRF difference).

To count for dependence between the two eyes of the sameperson in this study, we analysed the data based on two models;the simple model assuming the independence of eyes in that samepatient and the nested model that counted for the dependence.While the p-values for the tests are slightly different, theconclusions of the significance are the same. For simplicity, onlythe results based on the simple models are reported.

3. Results

The preop conditions of the patients from three different groupsare summarised in Table 1. It is important to note that the threegroups are not comparable (except for CRF) but this is to beexpected as the corneal parameters were taken into account whenchoosing the appropriate procedure for the patients. There was nosignificant difference between the LASIK myopes and LASIKhyperopes (Fig. 1a and b). On average, the myopic LASIK eyeshad 25 mm greater preop CCT, 0.9 mmHg higher preop CH and0.5 mmHg higher preop CRF than the myopic LASEK eyes, as well as0.9 mmHg lower preop IOPg, and 1.7 mmHg lower preop IOPcc.

Fig. 2 shows the CCT before and after the surgery for three groups.The IOPg and CH changes after surgery for three groups of patientsare shown in Fig. 3a and b. The changes of the CCT, IOPg, IOPcc, CHand CRF after refractive surgery are summarised in Table 2. Themean IOPg, CH and CRF after refractive surgery were statisticallylower than their initial values for all three groups of patients. The

Preop IOPg**

(mmHg)

Preop CH**

(mmHg)

Preop IOPcc**

(mmHg)

Preop CRF

(mmHg)

13.3 � 1.4 11.0 � 1.6 14.0 � 1.8 10.4 � 1.5

12.4 � 1.3 11.9 � 2.3 12.3 � 1.9 10.9 � 2.2

12.6 � 1.7 12.1 � 1.5 12.2 � 2.4 11.1 � 1.2

12.9 � 1.5 11.5 � 1.9 13.0 � 2.1 10.7 � 1.7

ups of patients (p-values are 0.006, 0.029, 0.001 respectively), while age and initial

he LASEK and the LASIK patients are statistically significant for all parameters. The

significant for any parameter.

Fig. 1. (a) Boxplot showing the IOPg before surgery for the three groups of patients.

(b) Boxplot showing the IOPcc before surgery for the three groups of patients.

Fig. 2. Boxplot showing the CCT before and after the surgery for three groups.

Fig. 3. (a) Boxplot showing the IOPg changes after surgery for three groups of

patients. (b) Boxplot showing CH changes after surgery for three groups of patients.

S. Shah et al. / Contact Lens & Anterior Eye 32 (2009) 123–128 125

mean IOPcc, however, increased after surgery. While the myopicLASEK eyes and the myopic LASIK eyes have similar values in IOPgdifference (�1.4 � 1.1 and �1.7 � 1.8 respectively), CH difference(2.7� 1.4 mmHg and 2.5 � 1.7 mmHg) and CRF difference (2.7�1.4 mmHg and 2.6 � 1.3 respectively) after surgery, the hyperopicLASIK eyes have statistically significantly smaller values of IOPgdifference (0.6� 1.2 mmHg), CH difference (1.1� 1.7 mmHg) and CRFdifference (1.2� 1.3) than the previous two myopic groups with p-values of 0.016, 0.002 and 0.001 respectively.

There are many potential factors that may influence the IOPgdifference, CH difference and CRF difference, such as surgery type(LASEK/LASIK), diagnosis (myopes/hyperopes) the initial conditions(preop IOPg, preop CCT, preop CH and preop CRF), ablation depth(CCT difference), age, and gender. Since the three groups are notcomparable in terms of preop CCT, preop IOPg, preop CH and CCTdifference (but were comparable for CRF), it is important to evaluatethe influence of these four factors. Table 3 shows the correlationbetween these initial conditions that were statistically significantly

Table 2Changes in CCT, IOPg, hysteresis, IOPcc and CRF after surgery.

Group n CCT difference** IOP difference** CH difference** IOPcc difference CRF difference**

1. LASEK (myopes) 35 63 � 27 �1.7 � 1.4* �2.5 � 1.7* 1.2 � 2.7* �2.6 � 1.3*

2. LASIK (myopes) 26 86 � 22 �1.4 � 1.1* �2.6 � 1.4* 1.5 � 1.5* �2.7 � 1.4*

3. LASIK (hyperopes) 19 48 � 33 �0.6 � 1.3 �1.1 � 1.7* 0.7 � 2.5 �1.2 � 1.3*

All 80 67 � 311 �1.3 � 1.5 �2.2 � 1.7* 1.2 � 2.3 �2.3 � 1.4*

Means � SDs.

For IOPg, CH, IOPcc and CRF, the differences are calculated as postop preop, for example, IOPg difference = postop IOP � preop IOPg; for CCT, CCT difference = preop CCT � postop

CCT.* Denotes the value is statistically different from zero at 0.05 level.** CCT difference, IOPg difference, CH difference, and CRF difference are significantly different among three groups (p = 0.001, 0.045, 0.006, and 0.001 respectively); the

differences were significant between myopes and hyperopes and insignificant between LASIK myopes and LASEK myopes, with the exception of CCT difference.

Fig. 4. (a) Scatterplot showing the influence of initial IOPg on the IOPg difference in

three groups of patients. (b) Scatterplot showing the influence of initial CH on the

CH difference in three groups of patients.Fig. 5. (a) Scatterplot showing the influence of initial CH on the CRF difference in

three groups of patients. (b) Scatterplot showing the influence of initial IOPg on the

IOPcc difference in three groups of patients.

S. Shah et al. / Contact Lens & Anterior Eye 32 (2009) 123–128126

different and the CCT difference across all three groups. While thepreop CH correlated to the preop CCT (r = 0.489), no other pair wiselinear relationship was statistically significant among them.

Figs. 4a, b and 5a, b show the influence of initial IOPg and CH onthe IOPg difference, CH difference, IOPcc difference and CRFdifference. For all three groups of patients, the IOPg difference isproportional to the preop IOPg value. The higher the preop IOPg,the larger the IOPg difference (i.e. IOPg decreased more aftersurgery). The preop CH had a weak influence on the CH change asone can see from Fig. 4a and b. Similar relationships can also beobserved between IOPcc difference and preop IOPg and betweenCRF difference and preop CH (Fig. 5a and b) and CH difference andCCT difference (Fig. 6).

Comparing the two groups of myopic patients, according to themodels selected by the best subset regression, the influences of thetype of surgery (LASEK and LASIK procedures) on IOPg difference,IOPcc difference, CH difference and CRF difference at the 3 monthspost-op time point are not statistically significant when otherfactors were considered. The factors influencing the IOPgdifference are preop IOPg and preop CH. The IOPg decreases themost for those with higher preop IOPg and higher preop CH. Thefactors influencing the IOPcc difference are preop IOPg, preop CH

Table 3The Pearson correlation between preop CCT, preop IOPg, preop CH and CCT

difference. .

Preop IOPg Preop CH Preop CCT

Preop CH 0.125

Preop CCT 0.189 0.489a

CCT difference �0.167 �0.06 0.193

a Denotes the value is statistically different from zero at 0.05 level.

and gender. The IOPcc decreases the most for females with a higherpreop IOPg. The factors influencing the CH difference were preopCH, preop IOPg and gender. The CH decreased the most for maleswith a higher preop CH and lower preop IOPg. The factorsinfluencing the CRF difference were preop CH and gender. The CRFdecreased the most for males with a higher preop CH. Age, ablation(CCT difference) and preop CCT were not influential on IOPgdifference, IOPcc difference, CH difference and CRF difference.

Comparing the two groups of patients who had LASIK surgery,the type of diagnosis (myopia/hyperopia) has a significantinfluence on the IOPg difference, IOPcc difference, CH differenceand CRF difference. For IOPg difference, the additional influential

Fig. 6. Scatterplot showing the change in CCT versus change in CH in three groups of

patients.

S. Shah et al. / Contact Lens & Anterior Eye 32 (2009) 123–128 127

factors are gender and preop IOPg. For IOPcc difference, theadditional influential factors are preop CH and preop IOPg. For CHdifference, the additional influential factors are preop CH andpreop IOPg. For CRF difference, the additional influential factor ispreop CH. Age, ablation (CCT difference), and preop CCT are notinfluential on IOPg difference, IOPcc difference, CH difference andCRF difference.

4. Discussion

The corneal stroma constitutes 90% of the corneal thickness andis a highly specialised tissue which is responsible for themechanical and refractive properties of cornea [11]. The specificarchitecture of the most anterior part of the corneal stroma (100–120 mm) has been suggested to be responsible for the stability ofthe corneal shape [12,13]. The exact mechanism which maintainsthe corneal shape is not known but this may be due to a passivedistension of corneal tissue by IOP. This is maintained due tocorneal mass, the elastic properties of the corneal tissue and themechanical force acting on this tissue [14].

Corneal properties are different in different individuals and thisis related to the structure of the corneal tissue. CCT is only one ofthe factors governing the corneal biomechanics [15]. To performrefractive surgery more accurately, it has been proposed that thebiomechanical properties of the cornea should be taken intoaccount [4].

Several studies have reported that when the structure ofcorneal tissues is altered, for example either by excimer laserablation [3] or due to disease process such as keratoconus [16–18]corneal tissue rigidity (or elasticity) decreases. Reduced postrefractive surgery IOP readings (measured IOP rather than trueIOP) have also been cited as indirect evidence of a change in thebiomechanical properties of the cornea [4].

Various investigators have conducted studies to measure ocularrigidity [14,16–19] but these studies used techniques that are notpractical for ophthalmologists in busy clinical settings.

The ORA is a new device developed by Reichert OphthalmicInstruments which is a non-contact tonometer that measures theIOP as well as new metrics: CH (which is said to represent thevisco-elastic response of the cornea) [10] and CRF represents theelastic properties of the cornea and appears to be an indicator ofthe overall ‘‘resistance’’ of the cornea. The ORA been reported toprovide reproducible corneal biomechanical and IOP measure-ments in non-operated eyes [20].

This study was performed to assess CH and CRF pre and postrefractive surgery to see if there was a change in these properties.The results show that both CH and CRF values were significantlylower after refractive surgery (p < 0.0001, paired t-test). This isconsistent with previous reports in the literature following myopicrefractive surgery [9,21]. Table 2 shows the distribution of CH andCRF and indicates these values to be lower after both myopic andhyperopic refractive surgery. This is the first paper to report on acomparison of hyperopes and myopes.

The difference between the CCT values before and afterrefractive surgery was also statistically significant (p < 0.0001,paired t-test). Fig. 2 shows the distribution of CCT indicating lowerCCT readings after the refractive surgery for all three groups as onewould expect. The scatter plot (Fig. 6) shows the change in CCTversus change in CH. It shows a small, but insignificant correlationbetween these two (r = 0.155, p = 0.170). This indicates changingthe CCT does not have a predictable effect on the change in CH.

Although no differences were found in the change of biomecha-nical properties between the myopic LASIK and myopic LASEKgroups in the current study, it is important to recognize that the twogroups were distinct preoperatively from a biomechanical perspec-tive. The LASEK eyes were thinner, had higher IOP, and lower CH and

CRF than the LASIK eyes, confirming a preoperative bias of thesurgeon.

The significant predictors of post-operative changes in mea-sured IOP and biomechanical parameters were found to be thepreoperative magnitudes of CH and IOPg, as well as gender.Therefore, any differences that might exist in the response basedon type of surgery, whether LASIK or LASEK, may be hidden by thepreoperative biomechanical distinctions between the populations.In fact, it is interesting that the myopic LASIK group hadsignificantly larger ablations than the myopic LASEK group, butexperienced similar changes in properties and measured IOP. Thehyperopic group experienced the smallest changes in biomecha-nical properties and IOP post-operatively, demonstrating that thisprocedure is bio-mechanically distinct from a myopic procedure.The caveat in the current study is that the ablation depths werealso much lower in the hyperopic group.

The mean measured IOPg in this study was significantly lower(p < 0.0001, paired t-test) after refractive surgery. The meandifference between the IOPg pre and post refractive surgery was1.3 mmHg. This result is in agreement with earlier studies who alsoreported mean IOP readings to be lower after refractive surgery[3,4,7,22–25].

IOPcc is a corneal compensated IOP value, where the differencein the two pressure readings is calculated which is termed CH andis used to calculate the IOPcc. IOPcc is an IOP that has been claimedto be less affected by corneal biomechanical properties than othermethods of measurements. It is believed to compensate for thebiomechanical properties of the cornea by adjusting for hysteresisand not just the corneal thickness. Correction of IOP measurementsusing CCT only may result in significant errors in the adjusted IOPvalues if the other biomechanical properties are ignored.

The results of this study revealed that the mean IOPcc was higherin post-op eyes in all the three groups. If one suggests that the trueIOP should be the same following refractive surgery, then it wouldappear that the difference in IOPcc calculation should be zero.

Refractive surgery has been shown to influence cornealbiomechanics in a manner that differs from the change in CCT,however, the magnitude of effect is subject to much individualvariation. Theoretically a LASIK cut of collagen fibres would inducebiomechanical changes and weaken the cornea. However, sub-sequent healing may modify this effect. There are no availablepublications that the authors are aware of that have measuredcorneal biomechanics after a lamellar cut with no other loss oftissue. Chihara et al. [7] suggested that ablation of the collagenfibres of the cornea may lead to reduced corneal rigidity as the cutcollagen fibres do have tensile force hence ablation depth per semay not be principal cause of underestimation of IOP post excimerlaser refractive surgery. The biomechanical properties of thecornea are variable in different individuals and may have aninfluence on the treatment outcome [8]. It was shown in thecurrent study that the preoperative properties and measuredpressure are the strongest predictors of post-operative change inbiomechanical properties and measured IOP. Thus, greater under-standing of the biomechanical properties of cornea in terms of CHand CRF may assist in improving outcomes. Future studies of bio-mechanically matched populations with matched ablation depthswill be important to investigate the biomechanical differences ininduced response between surface ablation and LASIK.

Each author states that he or his family members have noproprietary interest in the development or marketing of anyinstruments used.

References

[1] Holly FJ, Lemp MA. Tear physiology and dry eyes. Surv Ophthalmol1977;22:69–87.

S. Shah et al. / Contact Lens & Anterior Eye 32 (2009) 123–128128

[2] Hjortdal JO, Jensen PK. In vitro measurement of corneal strain, thicknessand curvature using digital imaging processing. Acta Ophthalmol 1995;73:5–113.

[3] Kaufmann C, Bachmann LM, Thiel MA. Intraocular pressure measurementusing dynamic contour tonometry after laser in situ keratomileusis. InvestOphthalmol Vis Sci 2003;44:3790–4.

[4] Munger R, Dohadwala AA, Hodge WG, Jackson WB, Mintsioulis G, Damji KF.Changes in measured intraocular pressure after hyperopic photorefractivekeratectomy. J Cataract Refract Surg 2001;27:1254–62.

[5] Mardelli PG, Piebenga LW, Whitacre MM, Siegmund KD. The effect of excimerlaser photorefractive keratectomy on intraocular pressure measurementsusing the Goldmann applanation tonometer. Ophthalmology 1997;104:945–9.

[6] Yamaguchi T, Kaufman HE, Fukushima A, Safir A, Asbell PA. Histologic andelectron microscopic assessment of endothelial damage in the monkey cornea.Am J Ophthalmol 1981;92:313–27.

[7] Chihara E, Takahashi H, Okazaki K, Park M, Tanito M. The preopera-tive intraocular pressure level predicts the amount of underestimatedintraocular pressure after LASIK for myopia. Br J Ophthalmol 2005;89:160–4.

[8] Roberts C. Biomechanical customization: the next generation of laser refrac-tive surgery. J Cataract Refract Surg 2005;31:2–5.

[9] Liu J, Roberts C. Influence of corneal biomechanical properties on intraocularpressure measurement: quantitative analysis. J Cataract Refract Sur 2005;31:146–55.

[10] Luce DA. Determining in-vivo biomechanical properties of the cornea with anocular response analyzer. J Cataract Ref Surg 2005;31:156–62.

[11] Tripathi RC, Tripathi BJ. Anatomy of the human eye and adenexa. In: Davson H,editor. The eye. Vegetative physiology and biochemistry. Florida, USA: Aca-demic Press; 1984. p. 1–102.

[12] Muller LJ, Pels E, Vrensen GF. The specific architecture of the anterior stromaaccounts for maintenance of corneal curvature. Br J Ophthalmol 2001;85:437–43.

[13] Boote C, Dennis S, Newton RH, Puri H, Meek KM. Collagen fibrils appear moreclosely packed in the prepupillary cornea: optical and biomechanical implica-tions. Invest Ophthalmol Vis Sci 2003;44:2941–8.

[14] Edmund C. Assessment of an elastic model in the pathogenesis of keratoconus.Acta Ophthalmol 1987;65:545–50.

[15] Brubaker RF. Tonometry and corneal thickness. Arch Ophthalmol1999;117:104–5.

[16] Hartstein J, Becker B. Research into the pathogenesis of keratoconus. A newsyndrome: low ocular rigidity, contact lenses and keratoconus. Arch Ophthal-mol 1970;84:728–9.

[17] Edmund C. Corneal elasticity and ocular rigidity in normal and keratoconiceyes. Acta Ophthalmol 1988;66:134–40.

[18] Foster CS, Yamamoto GK. Ocular rigidity in keratoconus. Am J Ophthalmol1978;86:802–6.

[19] Pallikaris IG, Kymionis GD, Ginis HS, Kounis GA, Tsilimbaris MK. Ocular rigidityin living human eyes. Invest Ophthalmol Vis Sci 2005;46:409–14.

[20] Moreno-Montanes J, Maldonado MJ, Gracia N, Mendiluce L, Gracia-Gomez PJ,Sequi-Gomez. Reproducibility and clinical relevance of the ocular responseanalyzer in non-operated eyes: corneal biomechanical and tonometric impli-cations. Invest Ophthalmol Vis Sci 2008;49:968–74.

[21] Pepose JS, Geigenbaum SK, Qazi MA, Sanderson JP, Roberts CJ. Changes in cornealbiomechancis and intraocular pressure pre- and post-LASIK using static,dynamic and non-contact tonometry. Am J Ophthalmol 2007;143:39–47.

[22] Montes-Mico R, Charman WN. Intraocular pressure after excimer laser myopicrefractive surgery. Ophthalmic Physiol Opt 2001:21;228–35.

[23] Chatterjee A, Shah S, Bessant DA, Naroo SA, Doyle SJ. Reduction in intraocularpressure after excimer laser photorefractive keratectomy; correlation withpre-treatment myopia. Ophthalmology 1997;104:355–9.

[24] Faucher A, Gregoire J, Blondeau P. Accuracy of Goldmann tonometry afterrefractive surgery. J Cataract Refract Surg 1997;23:832–8.

[25] Lee GA, Khaw PT, Ficker LA, Shah P. The corneal thickness and intraocularpressure story: where are we now? Clin Exp Ophthalmol 2002;30:334–7.