A bulge on the back of the foot that is hard can be a traumatic injury to the periosteum, or it can be a tendon sheath cyst. A tendon sheath cyst is a small, round, smooth, elastic, possibly painful to the touch, and can change in size over time. When this occurs, there is concern that it is a tumor, but in fact the condition is most likely a benign condition of a tendon sheath cyst. Tendon sheath cysts are cystic masses that occur in the joint or tendon sheath and contain fluid. Tendon sheath cysts are common human soft tissue masses that can occur in all age groups, most commonly in 1-40 years old and slightly more in women. Tendon sheath cysts can occur anywhere on the body, most commonly on the dorsal side of the wrist, the palmar side of the wrist, and may also grow on the dorsum of the foot, and less commonly on the knee, shoulder, spine, or other intra- or extra-articular soft tissues, periosteum, or periosteum. The pathogenesis is unknown, and repetitive motion is a contributing factor to tendon sheath cysts. The tendon sheath is a layer of soft tissue structure that wraps around the outside of the tendon and has a good relationship with the tendon, such as the inner shaft of the brake and the outer protective sleeve. Tendon sheath cysts are localized cystic swellings produced by the protrusion of tough dense tissues, such as tendon sheaths, ligaments, joint capsules, and bursae, etc. Cysts are benign lesions, inside which is cellular mucus. Some people try to squeeze the cysts by hand, and the cysts disappear after the fluid is dispersed, but there is also a possibility of recurrence. In Chinese medicine, there are acupuncture and acupuncture treatment, and in Western medicine, cyst excision can be done, and the whole cyst and cyst wall must be removed to prevent recurrence.