Ahmed Elsheikh1,2, Akram Joda1, Ahmed Abass3, David Garway-Heath2. 1. a School of Engineering, University of Liverpool , Brownlow Hill , Liverpool , UK . 2. b National Institute for Health Research (NIHR) Biomedical Research Centre, Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology , London , UK and. 3. c Structural Biophysics Group, School of Optometry and Vision Sciences, Cardiff University , Cardiff , UK.
Abstract
PURPOSE: The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP. MATERIALS AND METHODS: A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r(2) between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA's measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results. RESULTS: The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = -2) to (k1 = -2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets. CONCLUSIONS: The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea's stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.
PURPOSE: The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP. MATERIALS AND METHODS: A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r(2) between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA's measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results. RESULTS: The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = -2) to (k1 = -2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets. CONCLUSIONS: The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea's stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.