Medial meniscus posterior root tear with meniscal extrusion

Clinical History

A 55-year-old woman was admitted to our institution following a contusion to her left knee. Clinical examination of the knee did not reveal swelling and the patient complained of postero-medial pain. Plain radiographs and Magnetic Resonance (MR) examination of the knee were performed.

Imaging Findings

The plain radiograph showed severe joint space narrowing in the tibiofemoral joint, due to knee osteoarthritis. There were no sign of traumatic lesions (Fig. 1).
MR evaluation revealed degenerative subarticular cysts on T2-weighted images (WI) in the posterior side of medial tibial plateau (Fig. 2). On T2-WI a large radial tear of medial meniscus posterior root (Fig. 3) with associated meniscal extrusion (Fig. 4) was seen.

Discussion

The meniscus is an important part of the complex bio-mechanical system of the knee, fundamental in increasing the contact surface area and in providing wide and uniform distribution of weight bearing. The posterior horn and the body of the medial meniscus take most of the force applied to the medial compartment of the knee and present less mobility than the lateral meniscus, because of their firm attachment to the medial structures: the posterior horn, in particular, has a strong attachment to the tibia near the tibial spine, known as meniscal root; without firm insertion of the tibia, the medial meniscus tends to subluxate and lose some of its tension, affecting its ability to dissipate loads through hoop stresses.
Damages to the meniscus or alterations of his function can lead to compartmental instability: in particular certain injuries to the meniscus destroy its ability to withstand hoop stress, permitting meniscal extrusion. These include degeneration or tearing of the medial meniscal root as well as radial tears of the medial meniscus [1].
Knees with meniscal extrusion can be considered functionally meniscal-deficient, as well as post-meniscectomy knees: in these situations, the meniscus can no longer distribute load to protect the osteoarticular surfaces or improve femorotibial congruency and stability [2]. Extrusion of the medial meniscus subluxated from the tibial plateau margin can lead to early osteoarthritis due to a decreased tibiofemoral contact area and increased contact pressure [3].
MR is an excellent tool to diagnose meniscal degenerations, meniscal tears and meniscal extrusion: degenerative changes and tears result in local decreases of trapped water molecules, causing a signal increase in T2-WI and allowing detection of increased signal intensity on short-TE sequences. Meniscal extrusion is defined as significant displacement (>3mm) of the meniscus with respect to the central margin of the tibial plateau, and it can be easily evaluated on coronal plane. Treatment is surgical, usually with arthroscopic meniscal repair.

Differential Diagnosis List

Final Diagnosis

Medial meniscus posterior root tear with meniscal extrusion

URL:	https://www.eurorad.org/case/9688
DOI:	10.1594/EURORAD/CASE.9688
ISSN:	1563-4086