Meniscal MRI

Normal MRI anatomy of the menisci

The menisci are two C-shaped fibrocartilaginous structures located between the articular cartilages of the femoral and tibial condyles. Anatomically, they consist of the anterior horn, body, and posterior horn. The ends of the horns attach to the tibial plateau in the intercondylar eminence area—these attachment sites are called the meniscal roots.

Morphologically, the menisci differ:

• The medial meniscus is larger than the lateral, with its posterior horn being noticeably larger than the anterior.
• The lateral meniscus has horns of approximately equal size. About 5% of people have a congenital variant — the discoid lateral meniscus, which is predisposed to early degeneration.

On normal tomograms across all MRI sequences, the menisci show a uniform low signal. In children, a slight increase in signal intensity in the posterior horn is acceptable. The geometry of the meniscus on sagittal slices depends on the scanning level: at the periphery, it appears as a solid «bow-tie» (bow-tie configuration), while closer to the intercondylar notch, it divides into two dark triangles corresponding to the anterior and posterior horns.

Assessment of pathologies: degeneration vs tear

For targeted detection of meniscal injuries, sequences with a short echo time (TE) are the most sensitive — T1-weighted or proton density-weighted (PD), including fat-suppressed sequences (PD FS).

The key rule for differential diagnosis on MRI:

1. Degeneration manifests as intrameniscal signal hyperintensity («myxoid changes» — myxoid change). Most commonly, this process affects the posterior horn of the medial meniscus. The signal remains confined within the substance of the cartilage.
2. A true tear is diagnosed only when the high signal from the lesion clearly reaches the articular surface of the meniscus (one or both).

MRI classification of tears

Tears are divided into horizontal and vertical (depending on whether they extend to the articular surfaces). If two or more configurations are present simultaneously, the tear is called complex (complex tear).

Horizontal tears (Horizontal tears) Often degenerative in nature and asymptomatic. They appear as a linear high signal running parallel to the tibial plateau. Synovial fluid may leak through the defect, accumulating at the periphery and forming a parameniscal cyst.

Vertical tears (Vertical tears) Divided into two fundamentally different types:

• Longitudinal (Longitudinal tears): run within the meniscal tissue along its circumference, parallel to the outer margin. Since they often involve the peripheral zone with good blood supply, they have the potential for spontaneous healing.
• Radial (Radial tears): directed radially from the free edge into the tissue. A small oblique cleft gives the pattern of a «parrot beak» tear (parrot beak tear). A complete radial tear can split the meniscus in two. A specific MRI pattern of a complete radial tear is the «ghost meniscus» sign (ghost meniscus). It occurs when the sagittal slice passes precisely through the gaping cleft (often in the posterior root zone of the medial meniscus), causing the cartilage on the tomogram to locally «disappear».

Displaced tears

Displaced meniscal fragments can cause mechanical locking (locking) of the knee joint. On MRI (especially on coronal and sagittal slices in T2 FS or PD FS mode), it is important to evaluate the location of the torn fragments.

• Bucket-handle type (bucket handle tear): occurs with an extensive longitudinal tear. The torn fragment remains attached at the roots, but its central part is displaced into the intercondylar notch.
• «Double PCL» sign (double PCL sign): on a sagittal slice, the displaced bucket-handle fragment lies directly beneath the posterior cruciate ligament, mimicking a second ligament.
• «Flipped meniscus» sign (flipped meniscus sign): the torn posterior horn (often of the lateral meniscus) folds anteriorly and lies directly on top of the anterior horn.

Torn fragments can also migrate into the superior or inferior meniscal recesses of the joint capsule.

Class and exam questions

What else to study on this topic

• Anterior cruciate ligament (ACL) injuries and O'Donoghue's unhappy triad
• MRI findings of intrasubstance tears of the posterior cruciate ligament (PCL)
• «Pivot Shift» bone marrow contusion pattern in ligamentous injuries
• Osteochondral fractures and osteochondrosis dissecans (König's disease)
• Lipohemarthrosis and visualization of avulsion fractures in children
• Patellar pathologies: sleeve avulsion fractures and recurrent dislocations

Other topics in the "Musculoskeletal System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.