Abstract
Purpose: To evaluate the changes incorneal biomechanical properties and their effect on pre and postoperative differences in IOP measurement by each tonometer
 Design: Observational study.
 Methods: The study was done in subjects who underwent phacoemulsification with intraocular lens (IOL) implantation (phaco-IOL) and combined phacoemulsification with IOL implantation and trabeculectomy (phaco-trab). IOP was measured by a single trained examiner using rebound tonometer (RBT), Ocular Response Analyzer (ORA), Goldmann applanation tonometer (GAT), dynamic contour tonometer (DCT), and Tono-Pen. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured using ORA, central corneal thickness (CCT) using ultrasonic pachymeter, and corneal curvature (CR) with manual keratometry. All measurements were done one week prior to surgery and after four weeks and six weeks of the two surgeries, respectively. Only the operated eye was included for analysis.
 Results: Twenty-nine eyes of 29 normal subjects who underwent phaco-IOL and 23 eyes of 23 glaucoma subjects who underwent phaco-trab were studied. Increase in CCT [10.2 ± 14.86 microns, p = 0.001], decrease in CH [0.82 ± 1.38 mmHg, p = 0.003] and CRF [0.97 ± 1.0 mmHg, p < 0.001] were found post-phaco-IOL, whereas post-phaco-trab decrease in CCT [16.61 ± 15.22 microns, p < 0.001], CRF [2.28 ± 1.93 mmHg, p < 0.001] with increase in CH [0.95 ± 1.89 mmHg, p = 0.03] were noted. Multiple linear regression analysis showed significant associations for change in CH and CRF with change in IOP and not with CCT and CR postoperatively.
 Conclusion: Alterations in CH and CRF were associated with changes in IOP measured postoperatively by different tonometers. CH and CRF changes contribute to postoperative changes in measured IOP.
Highlights
Corneal viscoelastic properties depend on the corneal collagen fibrils
There was no significant difference between pre- and postoperative intraocular pressure (IOP) measured by different tonometers except dynamic contour tonometer (DCT) measurements [1.15 mmHg (± 2.87), p = 0.039] (Table 1)
Phaco-intraocular lens (IOL) group In the phaco-IOL group, we found significant increase in central corneal thickness (CCT) and decrease in corneal hysteresis (CH) and corneal resistance factor (CRF) after four weeks after the surgery
Summary
Corneal viscoelastic properties depend on the corneal collagen fibrils. The arrangement of collagen fibrils varies directionally and regionally, which results in regionalChanges in corneal properties and its effect on intraocular pressure measurement following phacoemulsification with intraocular lens implantation with or without trabeculectomy differences in elasticity.[1,2] Hjortdal showed that for a given intraocular pressure (IOP) radially the cornea is stronger at the center and circumferentially at the limbus. Corneal viscoelastic properties depend on the corneal collagen fibrils. It is known that both corneal and scleral fibrils are fused at the limbus.[3] Woo et al found that the corneal biomechanical characteristics are approximately similar to that of the sclera.[4] The Ocular Response Analyzer (ORA; Reichert Inc., Depew, New York, USA), measures in vivo corneal biomechanical properties such as corneal hysteresis (CH) and corneal resistance factor (CRF) by analyzing corneal responses to indentation by a rapid air pulse. CRF represents overall corneal resistance and the ability to withstand the applanation force.[5,6] Eyes with a higher CH tend to have increased damping capacity, which buffers the harmful effect of IOP fluctuation on the optic nerve head. Studies have found that eyes with glaucoma have lower CH compared to normal eyes; this increases the risk for developing glaucomatous optic neuropathy and progression of the disease.[7,8,9,10,11]
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