Tendon sheath cysts present as a smooth, rounded mass on the ankle. In clinical practice, it is common to encounter patients who present with abnormal masses that are palpated or touched by the physician. Some are normal abdominal structures, while others suggest abnormalities or disease, and care should be taken to discern whether the problem is pathological or physiological. Many diseases can also cause smooth round lumps in the foot and ankle, for example, the following diseases may also be the cause of smooth round lumps in the foot and ankle: 1. osteoarthritis Osteoarthritis (osteoarthritis) is a non-inflammatory degenerative degeneration of joint cartilage and bone superfluous formation at the edge of the joint due to old age or other causes such as trauma, congenital abnormalities of the joint, joint deformities, etc., which can clinically produce joint The clinical symptoms may include joint pain, limitation of movement and joint deformity. There are many synonyms, such as osteoarthrosis, degenerative joint disease, age-related arthritis, hypertrophic arthritis, etc. Degenerative changes in cartilage may begin in the late 20’s, and in people over 50 years of age, most radiographs show signs of osteoarthritis. The lesions tend to be more prominent in women than in men and involve the finger joints, knees, hips, spine, etc. It is the most common cause of the elderly activities. 2, acute septic tenosynovitis The tendon sheath of the permanent surface of the hand is mostly caused by deep stabbing infection, but can also occur from the spread of infection in nearby tissues. The causative organism is mostly Staphylococcus aureus. Infection of the tendon sheath of the dorsal extensor muscle of the hand is rare. Bursitis is an acute or chronic inflammation of the bursa. The bursa is a sac-like space in the connective tissue, a closed capsule composed of endothelial cells with a synovial membrane lining and a small amount of synovial fluid. A few are connected to the joints and are located between the bony prominence near the joint and the tendon or muscle or skin. The bursa can be present wherever there is friction or pressure, and its role is mainly to facilitate sliding, thus reducing or avoiding friction and pressure between the bony prominences and soft tissues near the joint. Many joint lesions can cause this disease. 4. Tendonitis and tenosynovitis Inflammation of the tendon (tendonitis) and inflammation of the lining of the tendon sheath (tenosynovitis) usually occur together. The tendon sheath lined with synovial membrane is usually the most inflamed site, but the inflammatory response can involve the tendon it encases (e.g., as a result of calcium deposition). 5.superior oblique tendon sheath syndrome The superior oblique tendon sheath syndrome refers to the excessive thickening or adhesion of the tendon and sheath of the superior oblique tendon due to congenital anatomical abnormalities or secondary to trauma or surgery, which restricts the upward rotation of the inferior oblique tendon, resulting in the fixation of the eye in a downward gaze. Giant cell tumor of tendon sheath The lesions of giant cell tumor of tendon sheath are firm and painless masses that occur on the fingers and hands, but rarely on the toes. The tumor may erode the adjacent bones.