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A. Elsheikh; Future of Numerical Simulation in Ocular Biomechanics: Applications in Tonometry. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4627.
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Long-term Research has been progressing to advance computer-based numerical simulations to the point where they could be reliably used to improve the accuracy of tonometry, guide the design of refractive surgery and assist in the management of diseases such as keratoconus.
Numerical simulations based on the nonlinear finite element method have been adapted to consider the main ocular topographic and material properties including variable thickness, elliptical profile, hyperelasticity, hysteresis and anisotropy. These developments allowed the study of two of the main methods of tonometry, using the Goldmann Applanation Tonometer (GAT) and Ocular Response Analyzer (ORA), to improve the accuracy of their intraocular pressure (IOP) measurements. Simulations of the two methods were analysed within wide multi-dimensional studies covering the natural ranges of variation in ocular dimensions and material behaviour, and leading to techniques to eliminate the effect of ocular rigidity on IOP measurements. The outcome of the study was assessed against IOP measurements taken within a Moorfields Eye Hospital clinical dataset involving 520 participants.
In the study of cases with different values of IOP and corneal and scleral topography and material parameters, the simulations predicted the pressure measurements to be made by both GAT and ORA. A statistical analysis of the results led to a correction equation for GAT readings of the form:True IOP = IOPG/[A(CCT).A(K).A(age).A(IOPG)],where the A parameters were correction factors related to central corneal thickness, CCT, central corneal radius, K, age and the GAT measurement of IOP, IOPG.The study also produced predictions of the ORA’s two applanation pressures, P1 and P2, and correlated them to the true IOP (such that the P1 and P2 dependence on ocular parameters was minimised) in an equation of the form:True IOP = 1.85 P2 - P1 + [0.11 x (87 - age) - 1]Both equations have been applied to the results of the Moorfields data to obtain a corrected value of IOP based on measured IOPG and ocular parameters, and also on the measured values of P1 and P2. Both exercises resulted in IOP values that were considerably less dependent on corneal rigidity parameters than actual measurements.
Numerical simulation has undergone significant recent developments in recent years, and their applications in tonometry demonstrate their potential in ophthalmology. Further applications to guide the planning of refractive surgery and assist in the management of keratoconus are underway to exploit this potential.
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