MRI Knee

MRI of the knee is the best test for the menisci, cruciate and collateral ligaments, articular cartilage, bone marrow and synovium. It is a non-invasive alternative to diagnostic arthroscopy and avoids unnecessary operations.

The study uses a 1.5 or 3 tesla scanner with a dedicated knee coil. The knee is slightly flexed (about 5 to 10 degrees) and held still. Slices are 3 to 4 mm thick. The examination takes 15 to 30 minutes.

The core sequences are proton density or T2-weighted fast spin echo with fat suppression in sagittal, coronal and axial planes, and T1-weighted images in the sagittal or coronal plane. Three dimensional isotropic sequences allow reformatting. Cartilage specific sequences and compositional mapping are used in some centres.

Contrast is given only for synovitis, infection, tumour or after surgery (as a direct arthrogram when a graft or meniscal repair needs assessment).

Knee pain accounts for a large share of musculoskeletal consultations. Meniscal tears are found in over 30% of adults over 50, and most are degenerative and symptom free.

MRI has a sensitivity and specificity of over 90% for medial meniscal tears and cruciate ligament rupture, and a negative study has a high negative predictive value.

Indications.

• Acute injury with a locked knee, haemarthrosis, instability or suspected ligament rupture.
• Suspected meniscal tear with mechanical symptoms (locking, catching) after failed conservative treatment.
• Suspected cruciate or collateral ligament injury, and multiligament or dislocation injuries.
• Patellar dislocation and instability, and extensor mechanism injury.
• Occult fracture, stress fracture and bone bruise when radiographs are normal.
• Osteochondral lesions, osteonecrosis and cartilage defects.
• Septic arthritis, osteomyelitis and soft tissue infection.
• Inflammatory arthritis, synovitis and crystal disease.
• Bone and soft tissue tumours, popliteal cysts and masses.
• Painful arthroplasty or ligament reconstruction, and assessment before surgery.
• Children with knee pain, growth plate injury and osteochondritis.

Not indicated for classic osteoarthritis with typical radiographs, or for uncomplicated knee pain under 6 weeks without red flags.

Contraindications and precautions.

• The usual MRI safety screen. Most orthopaedic implants are safe, but metal causes artefact.
• Gadolinium: renal precautions apply.
• Claustrophobia: knee studies are done feet first in most scanners, which helps.
• Pregnancy: non-contrast knee MRI is acceptable.
• Large body size may limit coil fit.

Preparation and technique.

No preparation is needed. The patient lies supine with the knee in the coil, slightly flexed, and the foot is fixed to reduce motion.

Sagittal, coronal and axial fat suppressed proton density images are taken, with a T1 sequence for marrow. Thin 3D images may be added for cartilage and menisci.

Metal artefact reduction sequences are used after arthroplasty or fixation. Slices are angled along the cruciate ligaments when needed.

Normal appearances.

The menisci are dark, wedge shaped (triangular on sagittal images) fibrocartilage structures. The anterior and posterior horns of the lateral meniscus are of similar size, while the posterior horn of the medial meniscus is larger than its anterior horn.

The anterior cruciate ligament is a taut, dark band of several fascicles running from the posterior lateral femoral condyle to the tibial spine. The posterior cruciate ligament is a thick, dark, curved band. Collateral ligaments and the patellar and quadriceps tendons are dark and continuous.

Hyaline cartilage is intermediate and bright with fat suppression, and is smooth over the femur, tibia and patella. A small amount of joint fluid is normal. The marrow is bright on T1.

Interpretation.

• Meniscal tear: high signal reaching the articular surface on two or more consecutive images, or a distorted shape. Types include horizontal, vertical (longitudinal), radial, oblique (flap), complex, bucket handle (displaced fragment within the notch), and root tears (loss of meniscal attachment, meniscal extrusion over 3 mm).
• Meniscal degeneration: intrasubstance signal that does not reach the surface. This is common and is not a tear.
• Meniscal cyst: a fluid collection next to a horizontal tear, mainly at the lateral side and in the medial posterior horn.
• Anterior cruciate ligament injury: discontinuity, wavy or absent fibres, abnormal angle, and secondary signs such as a bone bruise pattern at the lateral femoral condyle and posterolateral tibia, anterior tibial translation, and a deep lateral femoral notch. Chronic tears show a thin, scarred or absent ligament.
• Posterior cruciate ligament injury: thickened high signal ligament (partial) or discontinuity (complete), commonly with posterior translation.
• Collateral ligament injury: grade of sprain by thickening and periligamentous oedema (partial) or fibre discontinuity (complete). Posterolateral corner injury involves the lateral collateral ligament, popliteus tendon and popliteofibular ligament.
• Bone bruise: marrow oedema (low T1, high fat suppressed signal) from impaction, indicating mechanism. A lateral patellar and lateral femoral condyle pattern follows patellar dislocation, with tear of the medial patellofemoral ligament.
• Fractures: subtle tibial plateau, femoral condyle and avulsion fractures (lateral capsular avulsion associated with anterior cruciate ligament rupture), and stress fractures (linear low signal line with surrounding oedema).
• Cartilage: partial and full thickness defects, delamination, fissures and chondromalacia, graded by depth. Osteochondral lesions have a fragment with a rim of fluid or cyst underneath, indicating instability.
• Osteoarthritis: cartilage loss, marginal osteophytes, subchondral cysts and sclerosis, meniscal extrusion and degeneration, and synovitis.
• Osteonecrosis: serpiginous low signal margin with a double line, subchondral fracture and collapse in the spontaneous type, usually at the medial femoral condyle.
• Extensor mechanism: patellar and quadriceps tendinosis (thickening with intermediate signal), partial and complete tears, patellar tendon avulsion, and jumper's knee at the lower pole of the patella.
• Synovium and fluid: effusion, synovitis (thickened enhancing synovium), loose bodies, pigmented villonodular synovitis (low signal blooming on gradient echo), synovial chondromatosis, and popliteal cysts (between the gastrocnemius and semimembranosus).
• Infection: marrow oedema, abscess, effusion with a thickened enhancing synovium, and soft tissue collections.
• Tumours: chondral and osseous masses, and soft tissue sarcoma. Features of concern are size over 5 cm, deep location, heterogeneity, necrosis and rapid growth.
• Post-operative: graft integrity and tunnel position after cruciate reconstruction, meniscal repair (persistent signal can be normal), arthroplasty complications (loosening, osteolysis, effusion).
• Variants: discoid lateral meniscus, plica, bipartite patella, and accessory ossicles.

Critical values and findings needing immediate action.

• Septic arthritis or osteomyelitis: urgent aspiration or drainage and antibiotics.
• Knee dislocation with vascular injury risk: vascular assessment (pulses, ankle brachial index, CT angiography) and surgical review.
• Displaced bucket handle tear with a locked knee: arthroscopic surgery.
• Extensor mechanism rupture or patellar tendon avulsion: early surgical repair.
• Unstable osteochondral fragment or loose body: orthopaedic referral.
• Aggressive bone or soft tissue tumour: specialist sarcoma centre referral before biopsy.
• Fracture with displacement: orthopaedic fixation.

Limitations and artefacts.

• Findings correlate poorly with symptoms. Degenerative meniscal tears, cartilage change and bone marrow lesions are common in asymptomatic people, especially after 40 years.
• Magic angle effect causes false high signal in the posterior horn of the lateral meniscus, the popliteus tendon and the patellar tendon at about 55 degrees to the field.
• Motion, pulsation from the popliteal artery, and metal artefact limit assessment. Post-surgical meniscal and graft signal can mimic tears.
• Posterior horn lateral meniscal tears and root tears are easy to miss. Meniscal extrusion should be looked for.
• Partial anterior cruciate ligament tears and acute low grade injuries are hard to separate from sprain.
• Cartilage assessment is less reliable for early degeneration, and arthrography or specific sequences improve accuracy.
• Patellar tracking and dynamic instability cannot be assessed with a static supine study.
• Meniscal ossicles, vascular channels and the transverse ligament mimic tears.
• Over reading mild findings can lead to unnecessary arthroscopy, which has little benefit in degenerative knees.

Special groups.

• Children and adolescents: growth plate injuries, osteochondritis, discoid meniscus, tumours and infection. Sedation may be needed in young children.
• Older adults: degenerative tears and insufficiency fractures (subchondral) are common. MRI changes management mainly when there is a mechanical block or a suspected fracture.
• Athletes: acute multiligament injury and chondral injury, with early surgical planning.
• Patients with arthroplasty: metal artefact reduction sequences.
• Patients with inflammatory arthritis: contrast to show synovitis.
• Regions: septic arthritis, tuberculosis of the knee, trauma, and neglected injuries are common. MRI is scarce, and radiographs with clinical tests and ultrasound are used. Aspiration and culture decide infection.

Follow up tests.

• Weight bearing radiographs, including long leg alignment views.
• Ultrasound for effusion, popliteal cyst, tendon, and needle aspiration under ultrasound.
• CT for fractures and tunnel assessment.
• Joint aspiration for cell count, crystals, Gram stain and culture.
• Blood tests: full blood count, erythrocyte sedimentation rate, C-reactive protein, uric acid, rheumatoid factor, anti-citrullinated peptide antibody.
• Arthroscopy for therapeutic treatment.
• Referral to orthopaedics, sports medicine, rheumatology or physiotherapy.