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Topic

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Knee Joint

The knee joint is the largest and most complex synovial joint in the human body. It functions as a hinge-type joint with rotational capability, formed primarily between the femur, tibia, and patella. It is stabilized by an intricate system of ligaments, tendons, muscles, and menisci, allowing weight-bearing, locomotion, and dynamic stability.

The knee allows flexion, extension, slight internal and external rotation, and acts as a major shock absorber during movement. Its complexity and high mechanical demand make it one of the most frequently injured joints in the body.

Articulating Surfaces

  • Femoral condyles: Convex surfaces of distal femur

  • Tibial plateaus: Flat superior surface of tibia with medial and lateral menisci for congruency

  • Patella: Glides within the trochlear groove of the femur

Ligaments

  • Intracapsular ligaments

    • Anterior cruciate ligament (ACL): Prevents anterior tibial translation

    • Posterior cruciate ligament (PCL): Prevents posterior tibial translation

  • Extracapsular ligaments

    • Medial collateral ligament (MCL): Resists valgus forces

    • Lateral collateral ligament (LCL): Resists varus forces

    • Oblique popliteal ligament: Strengthens posterior capsule

    • Arcuate ligament complex: Reinforces posterolateral corner

Tendon Attachments

  • Quadriceps tendon: Inserts into superior patella

  • Patellar ligament (tendon): Extends from patella to tibial tuberosity

  • Iliotibial band: Inserts on Gerdy’s tubercle, stabilizes laterally

  • Pes anserinus tendons: Sartorius, gracilis, semitendinosus insert on anteromedial tibia

  • Semimembranosus tendon expansions: Strengthen posteromedial capsule

Muscles Acting on Knee

  • Extensors (anterior): Quadriceps femoris group

  • Flexors (posterior): Hamstrings (biceps femoris, semitendinosus, semimembranosus), gastrocnemius, plantaris

  • Rotators: Popliteus (internal rotation, unlocks knee), biceps femoris (external rotation when flexed)

Menisci

  • Medial meniscus: C-shaped, less mobile, more prone to injury

  • Lateral meniscus: O-shaped, more mobile, frequently associated with discoid variants

Nerve Supply

  • Femoral nerve (via saphenous branch): Anteromedial capsule

  • Sciatic nerve branches (tibial and common fibular): Posterior and lateral capsule

  • Obturator nerve: Medial capsule

Arterial Supply

  • Genicular branches of the popliteal artery (superior medial, superior lateral, inferior medial, inferior lateral, middle genicular)

  • Anastomotic network ensures perfusion during knee flexion

Venous Drainage

  • Popliteal vein receives venous drainage from the periarticular genicular venous plexus

  • Drains into femoral vein proximally

Function

  • Flexion and extension: Primary movements

  • Rotation: Internal and external rotation possible in flexion

  • Shock absorption: Menisci and cartilage reduce load on bone

  • Weight-bearing stability: Provided by ligaments, menisci, and muscles

  • Locomotion: Allows smooth walking, running, and jumping

Clinical Significance

  • Injuries: ACL/PCL tears, meniscal tears, collateral ligament injuries common in sports

  • Degeneration: Osteoarthritis frequent due to chronic overload

  • Dislocation: Rare but limb-threatening due to popliteal artery injury

  • Patellofemoral syndrome: Anterior knee pain from maltracking

  • Surgical relevance: Arthroscopy, ligament reconstruction, meniscal repair critical for joint preservation

MRI Appearance

  • T1-weighted images:

    • Joint structures: ligaments and menisci appear as low signal (dark)

    • Bone marrow: intermediate to high signal intensity

    • Fat planes: bright, delineating capsule and tendons

  • T2-weighted images:

    • Synovial fluid: bright hyperintense signal

    • Cartilage: intermediate-to-dark signal against dark bone

    • Ligament/tendon tears: bright hyperintense clefts or discontinuity

  • STIR:

    • Suppresses fat, highlights edema, effusion, and marrow contusion

    • Meniscal tears and capsular edema appear hyperintense

  • Proton Density Fat-Saturated (PD FS):

    • Excellent for menisci, cruciate ligaments, and cartilage defects

    • Normal menisci and ligaments: dark

    • Pathology: bright signal abnormalities at tears or degeneration sites

  • T1 Fat-Sat Post-Contrast:

    • Synovitis, infection, or postoperative scarring enhances

    • Ligament reconstruction sites enhance during healing

MRI Arthrogram Appearance

  • Contrast distends the joint capsule and outlines intra-articular structures

  • Tears in menisci or cartilage defects: contrast fills clefts or fissures

  • Enhances evaluation of subtle labral or root pathology

  • Useful in postoperative knees to differentiate scar vs recurrent tear

CT Appearance

Non-Contrast CT:

  • Bones, osteophytes, and fractures well visualized

  • Menisci and ligaments poorly defined

  • Indirect signs: joint space narrowing, sclerosis, osteophyte formation

Post-Contrast CT (standard):

  • Capsule and soft-tissue enhancement may show synovitis or tumors

  • Intra-articular detail inferior to MRI

CT Arthrogram Appearance

  • Contrast outlines menisci, cartilage, and cruciate ligament stumps

  • Meniscal tears: contrast-filled clefts

  • Chondral defects: contrast extends to subchondral bone

  • Superior to conventional CT in patients with MRI contraindications

MRI image

Knee Joint coronal cross sectional anatomy 3T MRI AI enhanced radiology anatomy image-img-00000-00000

MRI image

Knee Joint sagittal cross sectional anatomy 3T MRI AI enhanced radiology anatomy image-img-00000-00000

CT image

Knee Joint Anatomy ct coronal

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Knee Joint Anatomy ct sagttal image

CT 3D VRT image

knee joint 3d