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HE Kanngiesser, YC A Robert; Dynamic Contour Tonometry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):301.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We present the theoretical basis for Dynamic Contour Tonometry, a new principle for measuring intraocular pressure (IOP) directly and dynamically. The method relies on an electronic pressure sensor, built into the contoured surface of a Goldmann like tip, touching the cornea in a controlled fashion. Methods: Using shell theory, we modeled the mechanical behavior of the cornea during IOP measurement, and derived the ideal shape of the pressure-sensing surface. We calculated the capillary forces in the tear film and the rigidity forces in the cornea, and derived the complete force relationships between the pressure-sensing surface and the eye, as well as the volume displacement inside the eye caused by the measurement. To compare our approach to other directly measuring tonometers which employ flat pressure sensing surfaces (such as those described by Perkins or Draeger), measurements and calculations of the behavior of a flat pressure-sensing surface under point load were performed. To validate the concept, in-vitro measurements on enucleated, cannulated human globes and in-vivo measurements on healthy volunteer's eyes were performed, using manometric pressure and Goldmann Applanation Tonometry (GAT), respectively, as a reference. Results: A pressure-sensing surface with a matched shape produces a constant pressure distribution over the surface. Without this contour match, direct pressure measurements on the human cornea lead to an overestimation of IOP due to discontinuously distributed loads on the sensing surface. Determination of the forces between the device and the eye, and of the volume displacement inside the eye, shows that eyes with corneal radii in the range of 7 to 9 mm may be measured precisely and independently from corneal thickness using a single standardized contour match. Pressure provocation due to volume displacement is <1%, which is less than the provocation induced by GAT. While GAT measures statically the force applied, which relates in a simple fashion to IOP if the prescribed geometrical conditions are met, the Contour Tonometry procedure is largely independent from the force applied and measures dynamically and directly the pressure. Conclusion: Contour Tonometry devices yield IOP measurements which are independent from mechanical properties and shape of the cornea over a wide range of physiologically significant values (particularly radii and thicknesses). The method allows for continuous recording of dynamic IOP changes. Contour Tonometry furnishes IOP measurements that agree well with the Goldmann "Gold standard" for "normal" corneas. Further clinical studies to corroborate the concept are underway.
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