Flexion and extension pain of the right thumb may be caused by joint trauma, rheumatoid arthritis, tenosynovitis and other factors. 1. Joint trauma: when the right thumb is squeezed by the outside world or suddenly exerted too much force, it will easily cause minor fracture, leading to the phenomenon of right thumb joint flexion and extension pain. 2. Osteoarthritis of the thumb: with age, osteoarthritis of the thumb joint is formed, and when inflammation occurs, pain in flexion and extension of the right thumb joint may occur. 3. Tenosynovitis of the thumb: when the thumb is repeatedly fatigued, it may cause repeated friction between the tendon and the tendon sheath, resulting in aseptic inflammation, causing tenosynovitis of the thumb, and the symptom of pain in flexion and extension of the right thumb joint. Flexion and extension pain in the right thumb may also be due to other reasons such as gout, and the pain should be promptly consulted in the hospital for appropriate treatment and therapy under the guidance of the doctor if the pain is not relieved.