Fluid buildup inside the foot is usually a buildup of fluid in the ankle joint. The possible causes of fluid accumulation in the foot include osteoarthritis of the ankle joint, acute synovitis, trauma, etc. 1. Osteoarthritis of the ankle joint: fluid accumulation in the foot is mostly related to ankle osteoarthritis caused by prolonged excessive weight-bearing activities and proliferation and degeneration of the joints, and mostly occurs in the elderly. Early to joint soreness and pain mainly, later can appear persistent pain, accompanied by joint swelling, limited activities and other manifestations. 2. Acute synovitis: Acute synovitis may be caused by overstrain or cold. Due to the synovial membrane lesions and lead to excessive production of fluid, and in a short period of time can not be timely absorbed and caused by the foot inside the fluid. Redness, swelling, heat and pain are more obvious in the acute stage. 3. Trauma: It may be caused by soft tissue injury around the ankle joint due to trauma, which causes fluid retention in the foot. There may also be other reasons for the accumulation of water in the feet, it is recommended to go to the regular hospital in time to improve the relevant examination, to clarify the cause of the disease, under the guidance of the doctor to carry out targeted treatment or treatment.