Damage to the joint cartilage may result in bits breaking off and floating free in the joint.
The joint fluid may nourish the cells of the loose body, and the fragment may actually grow.
Loose bodies are often referred to as 'knee mice' or 'joint mice'. The bits may start off quite small, but may grow, nourished by the joint fluid (synovial fluid). They may be washed into the gutters at either side of the joint (parapatellar gutters) or behind the joint in the popliteal space, but sometimes one sweeps out and gets caught between the bones, causing sudden pain and sometimes locking or giving way.
A rare condition - synovial chondromatosis - may result in the formation of multiple loose bodies that arise from abnormal cells in the joint lining.
Removal of loose bodies is surgical.
The Knee Joint Loose Body as a Source of Viable Autologous Human Chondrocytes. Melrose J. Eur J Histochem. 2016 Apr 11; 60(2): 2645.
Arthroscopic removal of loose bodies using the accessory portals in the difficult locations of the knee: a case series and technical note. Chen B, Chen L, Chen H, Yang X, Tie K and Wang H. J Orthop Surg Res. 2018; 13: 258..