Introduction:
The menisci are C-shaped fibrocartilaginous structures in the knee that play a crucial role in shock absorption, load transmission, joint stability, and lubrication. MRI is the modality of choice for evaluating meniscal pathology, offering high soft-tissue contrast and detailed anatomical resolution without ionizing radiation.
Meniscal Anatomy on MRI:
- Medial meniscus: C-shaped, less mobile, firmly attached to the capsule → more prone to injury
- Lateral meniscus: Almost circular, more mobile, loosely attached
- Composed of:
- Anterior horn
- Body
- Posterior horn
📌 Normal MRI Appearance:
- Low signal intensity on all sequences
- Triangular-shaped on sagittal images
- No extension of signal to articular surface
MRI Sequences Used:
- Sagittal PD/T2-weighted with fat saturation – most useful
- Coronal PD/T2 fat sat
- Axial T2-weighted (for ramp lesions, radial tears)
- Optional: 3D isotropic sequences for multiplanar reconstructions
MRI Criteria for Meniscal Tear:
A tear is diagnosed when:
- Increased intrameniscal signal extends to an articular surface
- Abnormal meniscal morphology or fragmentation
- Displaced meniscal fragment within the joint space
Types of Meniscal Tears on MRI:
| Tear Type | MRI Appearance |
|---|---|
| Longitudinal (Vertical) | Vertical linear signal, may progress to bucket-handle |
| Horizontal | Horizontal cleavage plane, often degenerative |
| Radial | Perpendicular to the tibial plateau, truncates meniscus |
| Bucket-handle | Displaced longitudinal tear, may show “double PCL sign” |
| Flap tear | Displaced meniscal fragment (parrot-beak appearance) |
| Complex | Combination of multiple patterns |
| Root tear | Seen near meniscal root insertion, best in coronal plane |
Special Signs on MRI:
- Double PCL sign: Bucket-handle tear of medial meniscus
- Ghost meniscus sign: Absent meniscal body in sagittal plane (displaced fragment)
- Truncated triangle sign: Seen in radial tears
- Meniscal extrusion: Displacement of meniscus >3 mm beyond tibial margin (esp. medial meniscus)
Grading of Intrameniscal Signal (Not Diagnostic Alone):
- Grade I: Focal or globular intrameniscal signal – no surface contact
- Grade II: Linear signal within meniscus – no surface contact
- Grade III: Signal extending to articular surface – diagnostic of tear
Associated Findings:
- Joint effusion
- Parameniscal cysts (esp. horizontal tears)
- Chondral lesions
- Subchondral bone marrow edema
- Ligamentous injuries (especially ACL/MCL)
Importance of MRI:
- Differentiates degenerative vs traumatic tears
- Evaluates surgical repairability
- Detects occult and complex tears
- Identifies associated pathology (ligaments, cartilage)
- Helps in pre-operative planning
Conclusion:
MRI is a sensitive, non-invasive imaging modality for detecting and characterizing meniscal tears. Understanding tear types, typical MRI signs, and associated injuries is crucial for appropriate management and surgical planning.
