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Estimation of corneal rigidity factor and the true intraocular pressure using principles of corneal applanation and shell theory

16 May 1986


A laboratory procedu re for quantification of corneal rigidity factor (l) and the true intraocular pressure ( IOP) has been developed in this study and is named Lamometry. It utilizes corneal applanation principles, requiring at least two central corneal applanations of different diameters (between 2 and 3 mm) . The forces in g rams (accurate to second decimal p lace) needed to applanate the determined areas , together with data on the corneal geometric characteristics ( e.g .: horizontal limbal diameter, radius of curvature of the front surface and central thickness of the co rnea) are used in the calculation of L (in Lams o r gm/mm2) and I O P ( i n mm Hg ) of t h e eye. The IOP th us measured is, in theory, independent of the geometric characteristics and rigidity of the cornea because the mathematical equations allocate a certain fraction of the applied applanating force to the resistance due to corneal rigidity, and the rest of the force to the resistance offered by the IOP. Certain assumptions are made to simp lify the calcu lations. The i n itial attempt to verify the p rocedu re was by use of a corneal analogue of realistic d imensi ons, mounted on a water chamber manometer to s imu late the real eye conditions. Unfortunately, the material (Hefi lco n B, 45% water content) of the corneal analogue was not stable enough for the test, and suitable materials h ave not been found to date. The procedure was then applied to 40 human eyes (20 subjects) and the calculated IOP compared to the IOP measured by Goldmann tonometry. The differences between IOP's measured by the two methods were not statistically significant (0.05 level) and were uncorrelated with 1 the amount of corneal toricity and cylinder axis orientation. The major problem with existing apparatus appears to be inadequate resolution when measuring the force required to applanate the cornea. However, the procedure should ultimately be capable of verification o n a corneal analog u e of suitable mate rial that is subjected to a known simulated IOP.


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