A knee mouse is the same thing as a 'loose body' - a fragment of cartilage floating freely in the knee joint cavity.
Symptoms of knee mice
A knee mouse may be unsymptomatic most of the time, hiding in the folds of tissue around the edges of the joint cavity and around the back, but every now and again one bit can float out and catch between the bones, causing pain and giving way.
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Efficacy of arthroscopic loose body removal for knee osteoarthritis. Zhao B, Yu Y, Liu W and Du J. Exp Ther Med. 2018 Feb; 15(2): 1666–1671.