Simple Remedies details that dark knuckles can be caused by friction, dry skin, eczema, psoriasis and dermatitis. Acanthosis nigricans, a condition that causes thicker and darker patches to form on the skin, can also cause dark knuckles, according to Kids Health. Other areas affected by this condition include armpits, elbows, knees and neck. Interestingly, those with darker skin tones tend to develop dark patches more readily than light-skinned individuals.
WebMD explains that dark, thick skin often points to diseases and disorders such as diabetes and Cushing's syndrome. Both of these conditions can lead to acanthosis nigricans, which affects many parts of the body but particularly the tops of the knuckles. In addition to changing the appearance of the knuckles, the condition causes small patches to form on the skin. According to WebMD, most people who develop the condition are overweight.
Simple Remedies advises that dark knuckles can be treated by gently exfoliating the affected area and applying a liberal amount of certain topical treatments, including lime juice, buttermilk and sesame oil. Supplements of vitamin A and vitamin C may also help as the former helps improve skin tone and texture while the latter protects skin against infection, diseases and disorders.