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Karsten Sperlich, Stephan Reiss, Heinrich Stolz, Rudolf Guthoff, Anselm Jünemann, Oliver Stachs; Influence of age depending elastic properties of the cornea on applanation tonometry. Invest. Ophthalmol. Vis. Sci. 2016;57(12):6467.
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© ARVO (1962-2015); The Authors (2016-present)
Elevated intraocular pressure (IOP) is accepted to be one important criterion for glaucoma and is usually measured by applanation or rebound tonometry. The individual uncertainty due to central cornea thickness (CCT) is thereby corrected, while the error induced by age related elastic modulus (EM) change of the cornea  is ignored. To investigate its influence on IOP measurement we derive an advanced model including the elastic modulus.
Our approach is based on known equations from experimental physics and the following assumption: Applanation due to the tonometer forms a part of the cornea to a flat disc, resulting in an in-plane strain. Assuming the border of the disc is fixed, this strain can be calculated for the posterior and anterior part of the cornea, whereas this mean value can be used for further calculation. Combining the Young-Laplace equation and Hook’s law with the expression for the mean strain gives the absolute IOP based on corneal curvature, CCT and EM of the cornea. Difference between the IOPs of a standard eye and investigated eye gives the individual correction value.
Our correction values are in good agreement with the Dresden correction table  for CCT values up to 650 µm using a mean EM of 0.29 MPa . Considering an EM variation from 0.2 to 0.5 MPa, which relates to ages from infancy to 90 years , results in an IOP error of up to 10 mmHg. A variation of the cornea curvature from 7.4 mm to 8.0 mm results in a total IOP change of about 3 mmHg, which is usually neglected.
The derived model shows that established correction formulas can be insufficient for a reliable IOP determination. In many cases, the conventionally measured IOP may be precise enough, but the uncertainty in IOP determination due to CCT and EM influence are almost in the same range. Measuring the IOP using applanation methods with established correction formulas should not be overestimated without to respect the EM of the cornea. Knox Cartwright NE, Tyrer JR, Marshall J; Invest Ophthalmol Vis Sci. 2011 Jun; 52(7): 4324-9 Kohlhaas M, Boehm AG, Spoerl E, Pürsten A, Grein HJ, Pillunat LE; Arch Ophthalmol. 2006 Apr; 124(4): 471-6 Hamilton, KE; Pye, DC; Optom Vis Sci. 2008 Jun; 85(6): 445-6
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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