Transverse fingernail curvature in adults: a quantitative evaluation and the influence of gender, age, and hand size and dominance

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Abstract
The nail plate's convex shape in both the longitudinal and transverse directions is thought to contribute to its mechanical rigidity, and overcurvature can be a symptom of local and/or systemic disorders. Although a number of methods to measure the longitudinal nail curvature have been proposed, evaluation of the transverse nail curvature has been largely limited to visual estimation of overcurved nail plates. The aim of this study was therefore to measure the transverse curvature of healthy adult fingernail plates and thereby provide a baseline range for 'normal curved' nail plates. In addition, the influence of gender, age, hand size and hand dominance on the transverse fingernail curvature was investigated. The transverse fingernail curvature in 92 men and 90 women (aged 21-90 years) was measured using a set of radius gauges, and the nail plate curvature was expressed as the radius of a circle whose curve most closely approximated that of the nail plate. ANOVA was used to investigate the influence of digit nature on curvature, whereas general linear model was used to assess the influence of gender, age, and hand length, breadth and dominance on transverse nail curvature. Subsequently, the influence of hand dominance and age was further examined by paired t-test and ANOVA, respectively. In this study, baseline values for the transverse nailplate curvature of the thumb, index, middle, ring and little fingers are presented. The thumbnail is the flattest, followed jointly by the index and middle fingernails, then the ring fingernail and finally the little fingernail. Transverse nail curvature is influenced by a person's gender, age, hand dominance and hand width, but not by hand length. Thus, nails are flatter in the dominant hand, in men, in older individuals and in those with wider hands.

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