The causes of pain underneath the patient’s right big toe are as follows: First, bunion tenosynovitis: the flexor tendon and tendon sheath of the big toe are located underneath the big toe, if you often carry out weight-bearing, activities and sports, it is easy to cause friction between the tendon sheath and the tendon and tendon sheath, tendon sheath inflammation will occur, which mainly manifests itself as pain in the sacral part of the bunion, and even a popping sound of the bunion will occur in the case of tendon sheath hyperplasia. Second, interphalangeal arthritis of the bunion: mostly due to old age, degeneration and wear and tear of articular cartilage, narrowing of the joint space, periarticular osteophytes will cause the concentration of force on the joints, which will cause chronic pain in the joints when walking. Third, rheumatic or rheumatoid arthritis: especially rheumatoid arthritis will cause pain in small joints all over the body, and joint pain is often symmetrical.