Inflexible thumb pain may be caused by tenosynovitis, osteoarthritis, rheumatoid arthritis and so on. 1. Tenosynovitis: If the hand suffers from tenosynovitis, the thumb’s flexion and extension movements will be affected, and may be due to the tendon sheath cysts compression of the surrounding nerves, resulting in edema of the nerves, which will lead to stiffness and inflexibility of the thumb, and pain symptoms, and abnormal numbness when the thumb is moving. 2. Osteoarthritis: may occur in the thumb due to osteoarthritis, this disease is degenerative lesions, prevalent in the elderly, due to bone degeneration, resulting in reduced joint integrity, the normal function of the joints decreased, which may lead to finger inflexibility, due to long-term strain, which may lead to pain symptoms. 3. Rheumatoid arthritis: Rheumatoid arthritis is a synovial autoimmune disease that often accumulates in multiple joints of the hand, leading to stiffness and pain, and the disease can also lead to morning stiffness of the fingers. Thumb inflexibility and pain may also have other causes, such as fractures, carpal tunnel syndrome, etc. It is recommended to seek early medical attention to clarify the diagnosis and target treatment.