May 2007
Volume 48, Issue 13
ARVO Annual Meeting Abstract  |   May 2007
Computer Simulation of the Holden-Mertz (H-M) Threshold for Daily Wear
Author Affiliations & Notes
  • N. A. Brennan
    Brennan Consultants, Auburn Village, Australia
  • Footnotes
    Commercial Relationships N.A. Brennan, Bausch & Lomb, CooperVision, Johnson & Johnson, Clearlab, C; Bausch & Lomb, CooperVision, Johnson & Johnson, Clearlab, R.
  • Footnotes
    Support None.
Investigative Ophthalmology & Visual Science May 2007, Vol.48, 5403. doi:
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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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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose:: 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.

Methods:: 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.

Results:: 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.

Conclusions:: The common perception that the H-M threshold value of 24 represents the 'median' cornea is incorrect.

Keywords: contact lens • cornea: clinical science 

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