The "floating" meniscus: MRI in knee trauma and implications for surgery.

OBJECTIVE: We describe 21 cases involving meniscal injury in which the meniscus appears free-floating on MRI of the knee. In these cases, the meniscus is completely surrounded by fluid. Correlation with surgical reports shows that the "floating" meniscus corresponds to a meniscal avulsion or detachment from the tibial plateau with an associated disruption of the meniscotibial coronary ligaments, which attach the meniscus to the tibia, allowing fluid to encompass the meniscus. A floating meniscus on MRI may represent a new specific finding for an uncommon form of meniscal injury known as meniscal avulsion.

CONCLUSION: The presence of a floating meniscus on MRI is a result of significant trauma to the knee leading to meniscal avulsion and is often associated with significant ligamentous injury. Alerting the surgeon to the presence of a meniscal avulsion facilitates appropriate surgical planning with meniscal reattachment to the tibial plateau.