Refining the Estimate of the Critical Period for Susceptibility to Enamel Fluorosis in Human Maxillary Central Incisors

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Abstract
The purpose of this study was to determine an improved estimate of the critical period for susceptibility to fluorosis in human maxillary central incisors. The fluorosis score, S, of the incisal (I), middle (M), and cervical (C) third divisions on the labial surface of right maxillary central incisors of subjects (a representative sample of 1,085 Hong Kong Chinese children aged 7 to 12 years surveyed in 1986) was determined according to the Chronological Fluorosis Assessment (CFA) Index. Subject data were grouped by month of birth relative to June 1978, when the designated concentration for waterborne fluoride in the community water supply was reduced from 1.0 to 0.7 mg/L. The analytical task was one of finding the correlation maximum in a system of 12 variates and adjustable parameters including the waterborne fluoride concentration, [F-], and the fluorosis score. The main findings were: (1) the peak correlation of S vs [F-] for the male incisal third centered at 17.5 months after birth; (2) and six months later for females; (3) the correlation of S with [F-] is maximized for S(M) following S(I) by two months, and S(C) following S(M) by a further two months for both males and females; and (4) the critical period for exposure to fluoride is of about four months' duration for each third. The maxillary central incisor, as a whole, appears most at risk to fluorosis from dietary fluoride between age 15 and 24 months for males and between 21 and 30 months for females.