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N. A. Brennan; Computer Simulation of the Holden-Mertz (H-M) Threshold for Daily Wear. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5403.
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The H-M threshold of 24 is assumed to represent the 'median' cornea in that 50% of the population will demonstrate positive corneal swelling during open eye wear of a contact lens with Dk/t above this value. But if 50% of the population show no swelling at a Dk/t of 24 and 50% show positive swelling, then the net mean value must be positive, not zero as H-M found. The meaning of the Dk/t threshold is challenged here by computer simulation to identify the underlying parameters necessary to reconstruct the H-M curve.
The original H-M Dk/t values were amended for edge effect correction. The H-M plot was reconstructed on the following principals; swelling of a population of simulated corneas was generated by inverse normal distribution of random numbers; swelling versus Dk/t for each cornea was biphasic and described by a 2-parameter curve below a Dk/t threshold unique to that cornea and a constant small deswelling value above this Dk/t; the co-efficient of the curved section of the plot and the deswelling value were constant for all corneas; and the departure of a given cornea from the group mean swelling was constant for Dk/t values below their unique threshold. Input values were the two parameters of the curve for the median cornea below its threshold Dk/t, a deswell amount, and inter and intra-cornea variance in swelling. The output value was the sum-of-squares difference between the simulated and amended H-M curve. The Solver function in Microsoft Excel was then used to optimize input parameters that minimized the output value.
Incorporation of the edge effect correction alone led to an estimate of 20.7 as the amended H-M threshold. The simulation predicts that the true Dk/t threshold for the 'median' cornea is actually 17.6, some 73% of the population will have zero swelling at the amended H-M criterion of 20.7, and over 85% show zero swelling at a Dk/t value of 24.
The common perception that the H-M threshold value of 24 represents the 'median' cornea is incorrect.
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