Visualization of the menisci and their pathology
The menisci are two C-shaped fibrocartilaginous cushions attached to the tibial plateau in the region of the so-called roots. On MRI, they normally demonstrate a homogeneous low signal across all sequences. On sagittal tomograms, the peripheral part of the meniscus resembles a "bow tie", whereas closer to the center of the joint it divides into two triangles—the anterior and posterior horns. Optimal visualization of defects is provided by short echo time (T1 and proton density) sequences.
Meniscal lesions are divided into degenerative (intrameniscal signal) and true tears (signal reaching the articular surface). The main types of tears are:
- Horizontal. These are usually degenerative in nature. Synovial fluid can penetrate into the defect cleft, forming a parameniscal cyst.
- Longitudinal (vertical). These run parallel to the margin of the meniscus. Injuries in the well-vascularized peripheral zone are capable of spontaneous healing.
- Radial. These run from the free edge to the periphery. When an MRI slice passes exactly along the tear line, the "ghost meniscus" phenomenon occurs—lack of tissue visualization in the defect area.
Displacement tears causing joint blockage deserve special attention. In a longitudinal lesion, a meniscal fragment may dislocate into the intercondylar notch—this is a "bucket-handle" tear. On sagittal MRI, such a fragment often lies parallel to the posterior cruciate ligament, forming the "double PCL" sign.
Cruciate ligament injuries
The anterior cruciate ligament (ACL / ACL) restrains the tibia from excessive anterior displacement. On MRI, it appears as a fan-shaped bundle running parallel to Blumensaat's line (the roof of the intercondylar notch). Unlike the posterior ligament, the ACL has a more heterogeneous signal because loose connective tissue runs between its anteromedial and posterolateral bundles.
On tomograms, an ACL tear is revealed by direct signs: disruption of fiber integrity, increased MR signal, and ligament sagging. Secondary symptoms also play a key role:
- Bone marrow edema on the lateral femoral and tibial condyles ("kissing contusions" / kissing contusions), occurring via the Pivot Shift injury mechanism.
- Anterior tibial translation (displacement greater than 7 mm) and angular kinking of the intact posterior cruciate ligament.
- Deepening of the sulcus terminalis on the lateral femoral condyle.
The posterior cruciate ligament (PCL / PCL) appears on MRI as a homogeneous, thick bundle with a low signal. Its injuries (e.g., from hitting the dashboard in a car crash) more often manifest not as a complete tear, but as an intrasubstance injury: the ligament thickens, the signal within it increases, but contour continuity is outwardly preserved.
Collateral apparatus and patella
Lateral joint stability is provided by collateral ligaments. The medial collateral ligament (MCL / MCL) is visualized on coronal MRI slices as a thin low-signal band. Its injuries are graded from periligamentous edema (grade 1) to complete rupture (grade 3). Chronic trauma can lead to soft tissue ossification at the attachment site—Pellegrini-Stieda syndrome.
Lateral stability depends on a complex of structures including the lateral collateral ligament (LCL / LCL) and posterolateral corner elements (tendons of the biceps femoris and popliteus muscles). Tears in this zone frequently accompany ACL injury and lead to marked joint instability.
The extensor mechanism includes the patella, quadriceps tendon, and patellar ligament. Developmental anomalies (flattening of the trochlear groove or high-riding patella — Patella alta) predispose to dislocations. Patellar height is assessed using the Insall-Salvati index (normal up to 1.5). An acute dislocation with spontaneous reduction leaves a characteristic trace on MRI: zones of bone marrow edema on the medial facet of the patella and the lateral femoral condyle, frequently combined with a tear of the medial patellofemoral ligament.
Briefly: answers to topical questions
Which anatomical structures form the posterolateral corner of the knee joint?
The posterolateral corner of the knee joint is formed by a complex of ligaments, tendons, and capsular structures. This complex includes:
- Lateral collateral ligament (lateral collateral ligament) — a component of the supporting structures.
- Popliteus tendon (popliteus tendon) — an element of the complex.
- Biceps femoris tendon (biceps femoris muscle tendon) — shares an attachment site with the lateral ligament.
- Arcuate ligament (arcuate ligament) — attaches to the fibular head.
- Popliteofibular ligament (popliteofibular ligament) — a structure of the posterolateral corner.
- Posterior joint structures — the posterior capsule.
What are the grades of medial collateral ligament injury identified on MRI?
Three grades of severity of medial collateral ligament injury are distinguished on MRI. The classification includes the following stages:
- Grade 1 (Grade 1) — periligamentous edema (periligamentous oedema) without fiber disruption.
- Grade 2 (Grade 2) — partial tear with focal intrasubstance thickening and edema.
- Grade 3 (Grade 3) — complete rupture (complete rupture).
Class and exam questions
What does the "Unhappy triad of O'Donoghue" include?
This is a combined injury occurring with severe valgus stress. It includes a tear of the anterior cruciate ligament, medial collateral ligament, and medial meniscus.
What are the MRI signs of a bucket-handle meniscus tear?
On coronal slices, a displaced fragment is visible in the intercondylar notch. On sagittal slices, it lies beneath the posterior cruciate ligament, mimicking a second ligament (the "double PCL" sign), or shifts anteriorly, resting on top of the anterior horn (the "flipped meniscus" sign).
What is a Segond fracture and what does it indicate?
This is an avulsion fracture of the margin of the lateral tibial plateau. The presence of this fracture on radiographs or MRI indicates an accompanying anterior cruciate ligament tear with a very high probability.
How to differentiate tendinopathy ("jumper's knee") on ultrasound?
In "jumper's knee", ultrasound reveals a hypoechoic focal thickening in the central deep portion of the proximal patellar tendon, while dopplerography registers pathological neovascularization in this zone.
What else to study on this topic
- Pathogenesis and imaging findings of Osgood-Schlatter disease in adolescents.
- MRI staging of cartilage degeneration and patellar chondromalacia.
- Evaluation of TT-TG distance on CT and MRI to predict lateral dislocations.
- Difficulties in the differential diagnosis of posterolateral corner structures of the knee.
- Osteochondral lesions of the subchondral bone: cyst formation.
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