What causes meniscal avulsion?
A meniscal avulsion may be the result of a twisting injury or of degeneration from an earlier problem within the body of the meniscus.
In this condition the menisco-tibial ligament - which usually anchors the meniscus (left image) - tears away from the tibia bone on its lower surface. This allows it to 'float up' (right image) if there is excess joint fluid or the knee is irrigated with fluid under pressure. This is most obvious on an X-ray.
The meniscus itself is not torn, only its root attachment
The MRI scan shows a fluid gap between the bottom of the meniscus and the tibia bone. Generally, the meniscus itself remains intact, but without this anchorage the meniscus eventually becomes incompetent as a shock absorber and arthritis may ensue if the situation is not surgically remedied.
"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."
"The medial meniscus has a stable fixation to the tibia due to the anterior and posterior root attachments, the meniscotibial (coronary) ligament around the entire perimeter of the meniscus and the deep medial collateral ligament."