A 52-year-old woman complains of cosmetic defects in her teeth that she noticed several years ago. Objectively, on the vestibular surfaces in the paracervical areas of her premolars and incisors, there are fissure-like defects within the mantle dentine, formed by smooth glossy surfaces. The necks of the teeth are exposed. The gums are firm and pale pink. The defects are painless when probed. Make the diagnosis.