Intramuscular dissection of Baker's cysts: report on three cases.

Baker's cysts are fluid distensions of the gastrocnemius-semimembranosus bursa and are the most common cystic lesion around the knee. Typically cysts enlarge along intermuscular planes around the knee. We report three cases in which the expanding cyst did not respect these planes and dissected along an intramuscular route as confirmed by MR imaging. Such behaviour by Baker's cysts is hitherto unreported in the literature. Possible mechanisms to account for this phenomenon are discussed.