| Currently, viscoelastic properties of the cornea have a major influence in measuring intraocular pressure. Until recent times, other than pachymetry to obtain central corneal thickness (CCT), there was no reliable instrument to measure the biomechanical properties of the cornea in-vivo. Thus CCT was used as a proxy measurement of corneal rigidity. Corneal hysteresis (CH) is a recently developed biomechanical indicator of the result of viscous damping in the corneal tissue that is measured by the Ocular Response Analyzer(ORA; Reichert OphthalmicInstruments, Buffalo, New York, USA). The ORA obtains the corneal measurements by utilizing a puff of air aimed at the cornea and measuring the deformation caused, and there is no actual physical contact with the cornea. Literature on the value of corneal hysteresis measurements has been steadily increasing. Several studies have reported that CH is a more useful parameter than CCT in analyzing the structural differences and changes in the viscoelastic properties of the cornea.The ORA uses a patented dynamic bidirectional applanation process. It utilizes the principle that a precise strength of air-pulse applied on the cornea will deform the corneal surface based on the visco-elastic properties of the specific cornea,then get the opposite pressure. An electro-optical collimation detector system monitors the corneal deformation in the central 3.0mm2 throughout the 20 milliseconds measurement period. The cornea is initially applanated and then goes into slight concavity due to the air pressure. As the pressure decreases, the cornea begins to return to its normal configuration, the cornea throughout the applanation once again. Theoretically, the absolute pressures obtained from the inward and outward corneal displacement should be identical. However, the viscoelastic properties of the cornea causes some damping of the pressures, resulting in a lag in values between the applanations. The difference between the two pressure values is termed"hysteresis"(CH).There have been several recent studies that have employed the measurements obtained by the ORA, namely CH, corneal resistance factor (CRF), corneal compensated intraocular pressure (IOPcc), to better understand corneal pathology. CH diseases that have been investigated include keratoconus, Fuchs'corneal dystrophy,pre and post--LASIK operation,normal pressure glaucoma and primary open angle glaucoma. The ORA provides important information for understanding corneal disease.At the present time, cataract phacoemulsification is the best technique for treating cararacts, and it has been applied extensively. However, the emulsification head generates heat that may cause injury to the cornea. Prior to the ORA, the only measured corneal outcome post-surgery was a raised CCT. With the advent of the ORA, we can study in detail the changes that occur in corneal biomechanics pre- and post- cataract phaco-emulsification.We chose 22 eyes of 22 patients (8 males,14 females)randomly who have senile cataract in mature period (Group 1). The age ranged from 51 to 84 years old (average 65.5±8.48). All of the patients did not have any other ocular pathology. As a control group,we selected 15 patients randomly who with slight cataract or no cataracts (Group 2) (age range 60 - 75 years old, average 66±5.43 years old). Group 1 patients underwent detailed in-hospital examinations, including visual acuity measurement, IOP, B-scan ultrasound, A-scanultrasound, and corneal endothelial microscope measurement,ect. The 22 Group 1 patients all underwent the cataract phacoemulsification by the same cataract surgeon. The CH, CRF, IOPcc, Goldmann correlated intraocular pressure(IOPg)and Non-Contact Ophthalmotonometer IOP (NCT-IOP) was measured on Day 0 (pre-op) and Day 1 (post-op).All data was statistically analyzed using SPSS v13.0.Results: The value of CH prior to operation and after operation on the first day are 10.64±1.76mmHg, 9.41±1.99mmHg respectively. CH(10.64±1.76 vs 10.92±1.96 mmHg,P=0.704) and CRF(10.32±1.88 vs 11.25±2.30 mmHg,P=0.188) are not significant between experimental group and control group. On the first day after operation,CH(9.41±1.99 vs 10.64±1.76 mmHg,P=0.008) and CRF(8.87±2.04 vs 10.32±1.88 mmHg,P=0.007) decreased compared with preoperation, but IOPcc(19.9±5.21 vs 14.77±3.17 mmHg,P<0.01),IOPg(20.03±6.28 vs 14.48±3.29 mmHg,P=0.001) increase.NCT-IOP(14.2±4.75 vs 14.13±3.23 mmHg,P=0.888) has on difference.Preoperation(Group1):The correlation between CH and CRF of cataract patients was 0.850(P<0.01).The correlation between CH and IOPcc was -0.462(P<0.05).There has no correlation between IOPg and NCT-IOP(P﹥0.05).The correlation between CRF and IOPg was 0.629(P<0.01).The correlation between CRF and NCT-IOP was 0.554(P<0.01).The correlation between IOPcc and IOPg was 0.808(P<0.01).The correlation between IOPg and NCT-IOP was 0.508(P<0.05). Between other each index have no correlation.After operation on the first day(Group1):The correlation between CH and CRF was 0.461(P<0.05); between CH and IOPcc,IOPg have no correlation; between CH and NCT-IOP was 0.725(P<0.01).The correlation between CRF and IOPcc was -0.462(P<0.05);between CRF and IOPgNCT-IOP have no correlation.The correlation between IOPcc and IOPg was 0.878(P<0.01);between IOPcc and NCT-IOP was 0.535(P<0.01).The correlation between IOPg and NCT-IOP was 0.639(P<0.01).Discussion: CH value of senile cataract patients in our study was 10.64±1.76mmHg, this value was lower than the reported average(12.5±1.35 mmHg,12.2～12.7mmHg) for western general population(European) who have no ocular pathology. CH and CRF were decreased post-operatively compared with pre-operative values. The major reason for the rise of CH and CRF post-operatively was some degree of corneal edema and raised IOP. Moreover,Our findings suggest that cataract probably not influence the bio-mechanical properties of the cornea as measured by corneal hysteresis and corneal resistance factor.
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