The knee joint is a hinge joint (at least two bones with articular surfaces covered by hyaline cartilage and lubricated by synovial fluid) between the femur, tibia, and patella. [1]

Knowledge of basic knee anatomy is crucial for accurate and complete radiological diagnosis. Figure 1 illustrates a few of the anatomical landmarks commonly seen affected on knee MRIs.

Fig 1: Basic anatomy of the knee. Illustrative representations of a few of the anatomical landmarks, including bones, menisci, ligaments, tendons, bursae, and fat pads.

MRI PROTOCOL AND SEQUENCES (Figure 2)

Knee MRI protocols typically include axial, coronal, and sagittal imaging with fluid-sensitive sequences. [2]

T1-weighted images evaluate soft tissues and tissues that contain more fat.

• tissues that predominantly contain fat (bone marrow) - high T1 signal
• tissues that contain more water (ligaments, cartilage, synovial fluid) - low T1 signal [3]

T2-weighted images evaluate tissues that contain more water.

• tissues such as ligaments, cartilage and fluids - high T2 signal 
• bone marrow - high T2 signal [3]

Proton density (PD) images minimize the contribution of both T1 and T2 and have better contrast by enhancing structures with a higher proton density.

• fatty bone marrows and hyaline cartilage - high PD signal
• muscles - intermediate PD signal
• ligaments - low PD signal [3]

Intravenous contrast is useful for evaluating synovitis, tumours, and infection, and can be used to obtain an indirect arthrogram. [2]

Fig 2: MRI protocol and sequences. PD weighted, T2 weighted, and T1 weighted - scanning planes, the purpose of each sequence, and example images. FS - fat-saturated. ACL - anterior cruciate ligament. Images - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania.

BONES (Figure 3)

Common pathologies affecting the knee joint bones (distal femur, proximal tibia, fibular head, and patella) include:

Degenerative changes [4]

• altered shape of the tibial plateau and femoral condyles
• subchondral sclerosis 
• subchondral bone marrow oedema and cysts (geodes)
• marginal osteophytes

Deformities and anatomical variants [5, 6]

• trochlear dysplasia 
• a shallow, flattened or convex trochlear groove and a hypoplastic or convex lateral femoral condyle (a sulcus angle of >145-150 degrees)
• associated with patellofemoral instability and recurrent patellar dislocation
• bipartite patella (kneecap divided into two separate bones)
• sesamoid bones: fabella (in the lateral head of the gastrocnemius tendon)

Bone marrow changes [6]

• red marrow: low T1 and high T2 signal, high FS signal
• yellow marrow: high T1 and T2 signal, low FS signal
• marrow reconversion (in increased hematopoietic demand): red marrow signal
• marrow oedema: intermediate T1 signal, high T2 FS signal

Focal lesions [7]

• chondrogenic tumours (eg. osteochondroma)
• osteogenic tumours (eg. osteoma, osteosarcoma)
• fibrogenic tumours (eg. fibroma, fibrosarcoma)
• vascular tumours (eg. hemangioma, angiosarcoma)
• osteoclastic giant cell-rich tumours (eg. aneurysmal bone cyst, non-ossifying fibroma)
• metastases

Post-traumatic lesions

• fractures
• dislocations
• bone marrow oedema

Post-operative changes

• metallic knee arthroplasty implants

Fig 3: Bones. Examples of different knee pathologies affecting the bone that can be visible on MRIs (degenerative changes, deformities, bone marrow changes, focal lesions, post-traumatic lesions, and post-operative changes). Images A, B - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania. Image C - Tan TJL, et al. Sequential Proximal Tibial Stress Fractures associated with Prolonged usage of Methotrexate and Corticosteroids: A Case Report. Malaysian Orthopaedic Journal. 2015;9. 65-67. Image D - Diederichs G, et al. MR imaging of patellar instability: injury patterns and assessment of risk factors. Radiographics. 2010 Jul-Aug;30(4):961-81. Image E - Department of Radiology, Clinical Emergency Hospital for Children Cluj-Napoca, Romania Image F - Musculoskeletal Key. Magnetic Resonance Imaging in Total Knee Replacement. [Internet] 2016 [cited 2025 Feb 11]. Available from: https://musculoskeletalkey.com/magnetic-resonance-imaging-in-total-knee-replacement/

ANTERIOR COMPARTMENT (Figure 4)

The structures most commonly affected in the anterior compartment of the knee are:

Tendons (distal quadriceps, patellar) [2, 8]

• traumatic injury (partial tear, rupture) - linear interfibrillar high T2 signal intensities
• tendinopathy (reactive, tendon disrepair, degenerative)
• false positive tendinopathy - magic angle artefact (collagen fibres are oriented 55 degrees relative to the magnetic field)
• patellar height 
• Insall-Salvati ratio (ISR) - the ratio of the patella tendon length to the length of the patella
• patella alta (high patella) - ISR > 1.5
• patella baja (low patella) - ISR < 0.8

Fat pads [6]

• impingement syndromes
• anterior suprapatellar fat pad (quadriceps)
• infrapatellar fat pad (Hoffa) - repetitive traumas, patella alta
• femoral fat pad impingement syndrome - osteophytosis, patellar tendon-lateral femoral condyle friction syndrome

Fig 4: Anterior compartment. Tendinopathies and fat pad impingement syndromes. Images - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania.

MEDIAL AND LATERAL COMPARTMENTS (Figure 5)

Collateral ligaments - stabilisation of side-to-side movements [2, 9]

• MCL (medial collateral ligament) - prevention of inward movement
• LCL (lateral collateral ligament) - prevention of outward movement

Iliotibial band syndrome [2, 9]

• chronic inflammation of the fat adjacent to the iliotibial band
• associated with limb length discrepancy, genu varum, overpronation

Fig 5: Medial and lateral compartments. Collateral ligaments injuries and iliotibial band syndrome. MCL - medial collateral ligament. Images A, B, D - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania. Image C - Alaia EF, et al. Stener-Like Lesions of the Superficial Medial Collateral Ligament of the Knee: MRI Features. AJR Am J Roentgenol. 2019 Dec;213(6):W272-W276.

JOINT SPACE (Figure 6)

Joint effusion [2, 10]

• physiological small amount of intra-articular fluid
• causes: trauma, inflammation, infection
• loose bodies (post-traumatic, osteoarthritis)
• lipo-hemarthrosis (fat-fluid level) - in cases of intraarticular fractures due to marrow fat leakage
• synovitis - synovial hyperplasia and enhancement on post-contrast imaging

Cysts [2]

• ganglion cysts - lined by fibrous connective tissue
• synovial cysts - lined by synovium
• posterior knee - popliteal cyst (Baker’s cysts) - between the medial head of the gastrocnemius muscle and the semimembranosus tendon
• complications: haemorrhage, leakage, rupture

Fig 6: Joint space. Joint effusion and popliteal (Baker) cysts. Images - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania.

CARTILAGES (Figure 7)

Chondral lesions [11]

• usually progress slowly and clinical manifestations appear with time
• high PD and T2 signal
• chondral tapering
• loss of definition of the cartilage margins
• surface irregularities
• most frequent locations: medial femoral condyle, lateral tibial plateau

Osteochondral injury [11]

• focal areas of cartilage injury and damage to the adjacent subchondral bone plate and subchondral cancellous bone
• rim sign (high signal line) outlining bone fragment - unstable lesion

Fig 7: Cartilages. Chondromalacia (Modified Outerbridge Classification) and osteochondral injury staging. Images - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania.

MENISCI (Figure 8)

Tears [12]

• meniscal distortion in the absence of prior surgery
• increased signal intensity in contact with the articular surface
• “two-slice-touch” rule - findings can be seen on at least two consecutive images in the same plane or at least two planes
• increased intradiscal high signal is often not associated with a tear intraoperatively 
• indirect signs of meniscus tear
• parameniscal cysts (in contact with the torn meniscus)
• meniscus extrusion (>3 mm extension of the meniscus beyond the tibial plateau)
• subchondral marrow oedema (superficial, adjacent to the meniscus attachment, parallel to the articular surface, <5 mm deep)

Anatomic variants [12]

• discoid meniscus - enlarged meniscus with central extension onto the tibial plateau
• meniscal flounce - “wavy” appearance of the non-anchored inner edge of the meniscus
• meniscal ossicle
• chondrocalcinosis - increased diffuse intradiscal signal intensity

Fig 8: Menisci. Meniscal injury grading.

CRUCIATE LIGAMENTS (Figure 9)

Cruciate ligament tears [6]

• ACL (anterior cruciate ligament)
  • ligament swelling, discontinuity, non-visualization, or mass replacement (oedema, haemorrhage)
  • ACL angle (between intercondylar Blumensaat line and ACL) >15 degrees - ACL rupture
  • empty notch sign (avulsion of the femoral attachment)
  • scarring of torn ACL to the PCL, roof of the intercondylar notch, or lateral femoral condyle
  • bone contusion (lateral femoral condyle, lateral tibial plateau)
  • anterior tibial translation
  • positive PCL line sign 
  • Segond fracture (avulsion fracture of the lateral tibial plateau)
• PCL (posterior cruciate ligament)
  • anterior meniscofemoral ligament (ligament of Humphrey)
  • posterior meniscofemoral ligament (ligament of Wrisberg)
  • usually remains contiguous
  • absent PCL - high T1 and T2 signal
  • ligament swelling (>7 mm)
  • posterior tibial translation
  • false positive PCL tear - variant anatomy

Mucoid degeneration

• thickening with a “celery stalk” appearance
• can mimic acute or chronic interstitial partial tears

Ligament grafts

• complications: impingement, rupture, rejection

Fig 9: Cruciate ligaments. ACL and PCL. Images - Department of Radiology, Clinical County Emergency Hospital Cluj-Napoca, Romania.

SOFT TISSUES

Bursitis

• prepatellar
• infrapatellar - superficial or deep to the distal insertion of the patellar tendon

Muscular pathologies [13]

• atrophy (fatty infiltration)
• muscle injury
• musculotendinous injury
• chronic disuse
• denervation
• myopathy (eg. muscular dystrophy)
• corticosteroid use
• oedema (low T1 signal and high T2 signal)
• traumatic injury
• muscular exertion
• rhabdomyolysis
• vascular insults (eg. compartment syndrome)
• myositis (eg. autoimmune, infectious, early myositis ossificans)
• focal lesions
• infection (eg. abscess)
• trauma (eg. haematoma)
• myonecrosis
• neoplasms (eg. lipoma, leiomyosarcoma)

Vascular pathology [14]

• anatomy variants 
• trauma 
• posterior knee dislocations - traumatic dissection of the popliteal artery or vein
• anterior knee dislocations - intimal tractions and tears
• popliteal artery entrapment syndrome - under the medial head of the gastrocnemius muscle
• popliteal vein thrombosis
• soft tissue oedema around the vessel
• vein distension
• venous collaterals
• contrast-enhanced images - filling defect, intraluminal web
• arterial atherosclerotic stenoses
• popliteal artery aneurysms 
• atherosclerotic disease, trauma, Behcet disease, Marfan syndrome
• complications: rupture, thrombosis (multiple concentric rings of signal abnormality), distal embolisation - risk of acute leg ischemia
• cystic adventitial disease -nonatherosclerotic benign lesion
• vascular malformations