What are the reasons for pigmentation changes on the joints of my hand?

Often hormonal: Pigmentation occurs from the substance melanin, which with light activation also makes people tan. Knuckle hyper pigmentation can occur with physiological aging; inflammation from burns, acne; hormonal changes such as pregnancy or menopause. When generalized it can be of concern to the patient but is not a functional problem that leads to any concern.

Hard to tell without: In-person examination. Pigmentation changes can be a result of inflammation coupled with ultraviolet exposure, but this is such a broad question that specific answers will require a physician examination. Start with your dermatologist or general physician.