The elbow has hard skin blackened usually associated with neurodermatitis, repeated skin friction, post-traumatic pigmentation. 1. Neurodermatitis: If the patient’s elbow lesions manifested as multiple flat papules fused into a mossy skin, accompanied by a burst of itching, can be considered neurodermatitis. There is no clear cause of this disease, neuropsychiatric factors, scratching and chronic friction, excessive sun exposure, excessive cleaning and so on can induce this disease. This disease occurs in the neck, back, elbow and other parts of the body, usually at the beginning of itching or friction and other stimuli, after scratching and gradually appeared deep grooves and elevated typical moss-like changes. 2. Repeated skin friction: the elbow is in a special position, often as a medium of contact between the body and the outside world, there are repeated contact with external objects and friction (such as hard desktop, wall, clothes, etc.), and long-term repeated friction will make the local skin thickening, pigmentation, which is manifested in the elbow with a hard skin and black. 3. Trauma: After elbow trauma, affected by the environment or other factors (such as wound cleaning and disinfection is not complete, with secondary bacterial infections, etc.), resulting in slow healing of the wound, there will also be affected areas of hyperpigmentation, hard skin and black. It is recommended that patients go to a regular medical institution, under the guidance of the doctor, combined with their own condition, to choose the appropriate treatment plan.