The common causes of itching inside the knee are calcium deficiency, localized eczema, synovitis and early arthritis. 1. Calcium deficiency: when the body lacks calcium, the symptoms of itchy knees will appear. It usually occurs in adolescents at the developmental stage. Middle-aged and elderly people usually do not have itchy knees due to calcium deficiency, but only have the feeling of sore legs. 2. Localized eczema: due to dry weather and allergic factors lead to eczema on the outside of the knee, mainly manifested as itching, redness and swelling, etc., and when itching is serious, there will also be a feeling of itching inside the knee. 3. Synovitis: when the synovium inside the knee is stimulated by the outside world, it will cause synovial edema and hyperplasia, which will result in an increase in the exudate, stimulating the cartilage in the knee joint and the surrounding soft tissues, which will make the inside of the knee itchy. 4. Early arthritis: when patients appear in the knee itch is mainly the early manifestation of arthritis, joint inflammation inside the joint, the stimulation of the articular cartilage. The surface of the articular cartilage does not have nerve endings distribution, the distribution of nerve endings is in the subchondral bone. When there is inflammation to stimulate the cartilage, each person expression is different, some say it is cold, some say it is itchy, some say it is heavy. When the patient appeared inside the knee itchy, should promptly consult a professional doctor under the guidance of a targeted treatment according to the cause.