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Research Papers

Goldmann Tonometry Correction Factors Based on Numerical Analysis

[+] Author and Article Information
Ahmed Elsheikh

Division of Civil Engineering, University of Dundee, Dundee DD1 4HN, UKa.i.h.elsheikh@dundee.ac.uk

Daad Alhasso

Division of Civil Engineering, University of Dundee, Dundee DD1 4HN, UKd.alhasso@dundee.ac.uk

David Pye

School of Optometry and Vision Science, University of New South Wales, Sydney 2052, Australiad.pye@unsw.edu.au

J Biomech Eng 131(11), 111013 (Oct 26, 2009) (9 pages) doi:10.1115/1.4000112 History: Received November 26, 2008; Revised August 12, 2009; Posted September 01, 2009; Published October 26, 2009

With the world’s aging population, it is expected that the number of people affected by glaucoma, the second most common cause of irreversible blindness, will increase considerably. Current knowledge on glaucoma progression relates elevation of the intraocular pressure (IOP) to optic nerve damage and hence visual impairment. For this reason, IOP measurement in tonometry has become an essential part of routine eye examinations needed for the diagnosis and management of the disease. The accuracy of the current reference standard in tonometry, the Goldmann applanation tonometer, is known to be affected by the natural variations in corneal thickness, curvature, and material properties. Earlier studies attempted to quantify these effects and produced correction factors that considered the variations in each one of these parameters separately, and no guidance was given as to how to combine the effects of variations in more than one parameter. The present research attempted to address this gap by conducting a multidimensional numerical study that considered variations in thickness, curvature, material properties, and IOP, and used the results to develop a single correction equation that considered these parameters simultaneously. The results of the analysis and the correction equation were validated successfully against the outcome of earlier clinical and mathematical studies on the effect of individual parameters, and the correction equation was presented in a simple form suitable for clinical application.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Discretization method of numerical model. The model shown has 2 layers of solid elements arranged in 6 segments and 12 rings. The total number of elements is 144 elements per segment per layer×6 segments×2 layers=1728.

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Figure 7

Effect of R variation on IOP measurement as predicted by the present numerical study, the mathematical modeling carried out by Liu and Roberts (14) and Orssengo and Pye (7), and the clinical data for eyes with CCT between 480 μm and 560 μm. In their analysis, Liu and Roberts (14) used CCT=526 μm and E=0.19 MPa. Orssengo and Pye (7) used CCT=520 μm and E=0.0229 IOP. All analyses considered IOPT=15 mm Hg. (a) Comparison against earlier mathematical results and (b) comparison against trend lines in clinical data.

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Figure 8

Variation of the correction factor over a range of CCT, R, and age values. The correction factors presented have the average values obtained within the range of IOPG considered (10–30 mm Hg).

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Figure 2

Stress progress under IOP and tonometry pressure. (a) Typical stress-strain behavior of corneal tissue under loading and unloading conditions, (b) cornea under IOP with all areas following loading behavior, and (c) cornea under both IOP and tonometry pressure with area under tonometer experiencing unloading.

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Figure 3

Numerical estimation of the influence of CCT on IOP measurement using GAT: (a) IOP=15 mm Hg, (b) IOP=20 mm Hg, and (c) IOP=25 mm Hg

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Figure 4

Numerical estimation of the influence of R on IOP measurement using GAT: (a) IOP=15 mm Hg, (b) IOP=20 mm Hg, and (c) IOP=25 mm Hg

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Figure 5

Numerical estimation of the influence of age on IOP measurement using GAT: (a) IOP=15 mm Hg, (b) IOP=20 mm Hg, and (c) IOP=25 mm Hg

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Figure 6

Effect of CCT variation on IOP measurement as predicted by the present numerical study, the mathematical modeling carried out by Liu and Roberts (14) and by Orssengo and Pye (7), and the clinical data for eyes with R between 7.6 mm and 8.0 mm. In their analysis, Liu and Roberts (14) used R=7.8 mm and E=0.19 MPa. Orssengo and Pye (7) used R=7.8 mm and E=0.0229 IOP. All analyses considered IOPT=15 mm Hg. (a) Comparison against earlier mathematical results and (b) comparison against trend lines in clinical data.

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Figure 9

Association between IOP measurements and corneal thickness, CCT, radius, R, and age; ((a), (c), and (e)) raw IOPG measurements, ((b), (d), and (f)) corrected IOPG measurements, ((a) and (b)) association between IOPG and CCT, ((c) and (d)) association between IOPG and R, and ((e) and (f)) association between IOPG and age

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