A dented thumbnail can be caused by trauma, or by diseases such as onychomycosis or psoriasis. 1. Trauma: If the thumbnail has recently suffered physical injury, causing damage to the structure of the nail, it is easy to appear in the middle of the thumbnail concave in the situation. 2. Nail fungus: Trichophyton rubrum is the main causative agent of nail fungus, followed by Candida and Trichophyton mustelii. This disease may also be caused by direct transmission of tinea pedis, and common susceptibility factors include impaired immune function. Typical symptoms are uneven surface of the nail plate, discoloration, atrophy, etc., which can cause the phenomenon of concave in the middle of the thumbnail. 3. Psoriasis: the etiology of this disease is not clear, and studies have shown that it is the result of a combination of genetic and environmental factors. Patients will have erythematous scaly skin, swollen joints, and pitting of the nails, which can be manifested as a concave thumbnail. In addition to this, the thumbnail is concave in the middle can also be caused by other diseases. Patients need to go to the hospital in time, do the relevant examination, clarify the specific cause, and cooperate with the doctor to actively treat.