Sudden pain and limited movement in the knee joint in middle-aged and elderly patients may be due to free bodies. Arthritic bone spurs that break or fall off into the joint can constitute a free body. Most of these free bodies are still attached to the synovial membrane and do not produce symptoms, so they do not need to be surgically removed. It is a rare lesion of the synovial tissue due to the overgrowth of the synovial villus, which later becomes chondrogenic and is shed into the joint cavity, and eventually most of these free bodies calcify. Most of these free bodies eventually calcify. Synovectomy is performed at the same time as removal of the free bodies. If there is a free body in the knee joint cavity, it can be ignored if it does not affect the phenomenon of strangulation, but if it is larger and interlocking of the knee joint occurs, surgery is recommended, and arthroscopic surgery is feasible if economic conditions are available. Clinically, joint interlocking often occurs in knee free bodies. Sudden joint interlocking occurs when a smaller free body is pinched between the joint surfaces. When present, the patient experiences severe pain and the position of the interlock is often variable (sometimes in flexion, not extension; sometimes in extension, not flexion). Patients may experience joint swelling and effusion due to mechanical irritation of the periosteum, weakness in the knee, or palpable movable masses due to free bodies swimming to the superficial part. In addition, during interlocking and unlocking, the patient may hear or feel a rattling sound, a sense of misalignment, and in some cases, may cause a kneeling phenomenon. Treatment of knee free bodies is divided into conservative treatment and surgical treatment. Conservative treatment includes: 1) elimination of pain and restriction of activities: patients can use knee pads for protection, traction, and pain medication; 2) elimination of swelling: local heat, physical therapy, and internal Chinese medicine; 3) manual treatment: release of interlock by flexion and extension repositioning, push and pull repositioning, and other manual treatments; 4) joint puncture, fluid extraction, and injection of sodium vitrate to improve symptoms. Surgical treatment includes arthroscopic surgery to remove the free body, surgical incision to completely remove the free body and broken cartilage in the joint. Since arthroscopic surgery is less invasive and more effective, some patients with better conditions can choose this treatment method.