There are two general causes of intra-articular free bodies: 1. Damage to the meniscus or cartilage. When the meniscus is shattered there are small fragments of the meniscus that become free in the joint cavity and a free body can appear. When a joint is traumatized, cartilage is damaged, and the cartilage becomes free in the joint cavity, also called a free body. 2. Synovial chondromatosis is usually secondary to osteoarthritis of the knee, especially in the elderly, when the joint degenerates, the cartilage will wear away, and if the worn cartilage continues to accumulate in the joint cavity, the free body will appear. If the synovial membrane becomes cartilaginous, i.e., hardens, then intra-articular free bodies may also appear. When the free body is in the joint cavity, the joint becomes stuck, which means that the knee joint cannot move when the free body is stuck in the joint. If the free body moves to another place after moving the leg, the joint can be moved again.