The condition is called a floating meniscus where the MRI scan shows a fluid gap between the bottom of the meniscus and the tibia bone, due to high-force traumatic detachment of the menisco-tibial (or coronary) ligaments that are a normal part of the anchorage system for the meniscus. Generally, the meniscus itself remains intact, but without this anchorage the meniscus becomes incompetent as a shock absorber.
Meniscus incompetence after meniscal root avulsion
Avulsion of the meniscal root can lead to immediate incompetence, and the meniscus may extrude over the edge of the tibia.
The “Floating” Meniscus: MRI in Knee Trauma and Implications for Surgery Bikkina RS, Tujo CA, Schraner AB and Nancy M. Major NM. American Journal of Roentgenology. 2005;184: 200-204.
Technique for Arthroscopically Assisted Superficial and Deep Medial Collateral Ligament–Meniscotibial Ligament Repair With Internal Brace Augmentation. Aaron K. Black AK, Schlepp C, Zapf M and Reid JB III. Arthrosc Tech. 2018 Nov; 7(11): e1215–e1219.