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D. Luce, B. Grolman; Operation Theory of the Reichert NCTCR, a New Tonometer that Measures Excess Ocular Pressure (EOP) and Corneal Response . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4350.
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© ARVO (1962-2015); The Authors (2016-present)
Much recent literature has created serious doubts about the validity of applanation derived IOP. The single parameter provided by current IOP measurement tools is inadequate to describe the effects of two independent variables, cornea and IOP. Correcting IOP measurements using corneal thickness alone has significant limitations. Basic concepts of a new non-contact tonometer , the NCTCR, that uses the two applanation events of a single highly dynamic air pulse measurement, will be discussed. The procedure which converts the events data to measures of IOP, corneal resistance and a new parameter, EOP (Excess Ocular Pressure) will be described. The concept of "corneal hysteresis" (CH) will be introduced and data presented to show that CH is a measure of the corneal resistance, the result of visco-elastic properties, thickness, hydration and curvature and factors not yet identified. Relationships between applanation related measurement velocity, CH and air impulse pressure will be presented. Measurements of a population at baseline (normal) and at two elevated pressures (using a special phthalmodynamometer ) will be presented that show the averaged population CH is essentially independent of pressure. The strong correlation of the CH and IOP for two normal populations (N = 144, r = 0.81, p < 0.001; N = 201, r = 0.83, p < 0.0007) will be discussed with a view to determining the role of the cornea in current IOP measurement processes. Diurnal measurements of CH and IOP of one subject over a two week period are presented and comparative population diurnal results discussed. The linear regression slopes (CH vs. IOP)of the two normal populations and the diurnal data are virtually identical. The concept of EOP (Excess Ocular Pressure) will be introduced and discussed. EOP, a parameter that incorporates population correlation of the CH and IOP, effectively "removes" corneal effects from the pressure measurement.
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