The changes in brittle, friable nails are often confined to the surface of the nail plate; this occurs in superficial white onychomycosis and may be seen after the application of nail
polish or base coat which causes ‘granulations’ in the nail keratin. In advanced psoriasis and fungal infection the friability may extend throughout the entire nail. The changes in nail consistency may be due to impairment of one or more of the factors on which the health of the nail depends, for example variations in the water content or keratin structure and corneocyte adhesion. In addition, changes in the intercellular structures and cell membranes, and intracellular changes in the arrangement of keratin fibrils, have been revealed by electron microscopy. Normal nails contain approximately 18% water. After prolonged immersion in water this percentage is increased and the nail becomes soft; this makes toe-nail trimming and nail biopsies much easier. A low lipid content may decrease the nail’s ability to retain water. If the water content is considerably reduced, the nail becomes brittle. Splitting, which results from this brittle quality, is probably partly due to repeated uptake and rapid drying out. The keratin content may be modified by chemical and physical insults, especially in occupational nail disorders. Amino acid chains may be broken or distorted by alkalis, oxidizing agents and thioglycollates (used in the permanent waving of hair). These break or distort the multiple disulphide bond linkages which join