The causes of thumb joint flexion include stenosing tenosynovitis, synovial lesion, flexor tenosynovitis and so on. 1. Stenosing tenosynovitis: commonly seen in long-term manual activities, excessive strain, resulting in chronic aseptic inflammation of the finger tendon sheaths, resulting in thickening and narrowing of the tendon sheaths, resulting in tendon compression, which manifests itself as inflexible bending of the thumbs, morning stiffness, and popping sounds during activities. 2. Synovial lesions: such as rheumatoid arthritis, degenerative diseases and other synovial lesions, can lead to joint osteophytes and synovial thickening, thumb can not flex flexion performance, often accompanied by pain symptoms. 3. Flexor tenosynovitis: When you are tired or cold, it leads to aseptic inflammation. The flexor tendon sheaths become swollen and the metacarpophalangeal joints become enlarged, resulting in inflexible thumb bending, which may be accompanied by popping symptoms. It is recommended to limit the activities of the fingers to avoid overstrain. Hot compresses, baking electricity, physiotherapy and other treatments can be used to promote the absorption of inflammation and relieve the local symptoms of tendon compression. If necessary, it is recommended to go to the hospital to avoid delaying the condition.