The knee joint is a modified hinge joint between the femur, tibia, and patella. It is the largest synovial joint in the body and allows flexion and extension of the leg as well as some rotation in the flexed position.

Summary

• location: two condylar joints between femur and tibia; saddle joint between patella and femur

• blood supply: main supply are the genicular branches of the popliteal artery

• nerve supply: branches from the femoral, tibial, common peroneal, and obturator nerves

• movement: flexion to 150°, extension to 5-10° hyperextension; rotation whilst in the flexed position to 10° actively and 60° passively 

Gross anatomy 

Articulations

There are two condylar joints between the femur and tibia (tibiofemoral). There are medial and lateral articular facets on the tibial plateau and medial and lateral femoral condyles on the distal femur which are convex and circular shaped.

• medially: between a narrow and curved femoral condyle, and an oval tibial articular surface with a long anteroposterior length

• laterally: between a wide and flat  femoral condyle; and a circular tibial articular surface which overhangs the shaft posterolaterally 

• the knee menisci are shaped accordingly 

Saddle joint between the patella and femoral condyles: 

• medial, lateral and odd facet on the posterior surface of the patella articulate with the medial and lateral condyles of the femur

• on flexion, more parts of the bony surface are exposed to articulation (four below, odd facet) and are more proximal on the patella

• with extension, the contact area lessens and moves distally

Menisci

• fibrocartilaginous, C-shaped in appearance and triangular in cross-section

• the medial meniscus is attached to the medial collateral ligament and the lateral meniscus is attached to the popliteus tendon

• attached to the femur and tibia via the coronary ligaments

See menisci of the knee.

Joint capsule

• knee capsule

  • on the femur 

    • adheres below the epiphyseal line down to the articular margin except in two places 

      • posteriorly attached to the intercondylar ridge at the lower limit of the popliteal surface

      • on the lateral condyle it encloses a pit and groove for the popliteus tendon

  • on the tibia

    • attached around the margins of the tibial plateau except in two places 

      • posteriorly to the ridge between the two condyles at the lower end of the groove for the PCL

      • laterally the capsule is not attached to the tibia but is prolonged down over the popliteus tendon 

  • two main gaps 

    • lateroposteriorly allowing passage of the popliteus tendon ¹

    • anteriorly a circular gap whose margins attach to the patella allowing communication with suprapatellar bursa ¹

Synovial membrane

• joint capsule is lined by synovial membrane, however, the attachment of the synovial membrane does not coincide with the capsular attachments because of the intra-articular structures 

• the cruciate ligament and popliteus tendon are extrasynovial but intracapsular

• communicates with the suprapatellar bursa 

Fat pads

There are three anterior fat pads:

• infrapatellar fat pad (of Hoffa)

• posterior suprapatellar (prefemoral or supratrochlear) fat pad

• anterior suprapatellar (quadriceps) fat pad

Attachments

• intracapsular ligaments

  • anterior intermeniscal ligament

    • connects the anterior limbs of the two menisci

  • anterior (Humphrey) and posterior (Wrisberg) meniscofemoral ligaments: 

    • the lateral meniscus is attached to the medial femoral condyle via the anterior and posterior meniscofemoral ligament of Humphrey and Wrisberg

  • cruciate ligaments: cross each other to form an "x" shape. 

    • anterior cruciate ligament: from the anterior tibial plateau to the lateral femoral condyle

    • posterior cruciate ligament: from the posterior intercondylar area to the medial femoral condyle

• extracapsular ligaments 

  • patellar retinacular ligaments: medial and lateral portions of the quadriceps tendon pass down on either side of the patella and are inserted into the upper extremity of the tibia on either side of the tuberosity, merging into the capsule

  • medial collateral ligament

    • from the medial epicondyle to the medial surface of the tibia, which it is separated from by the passage of the inferior medial genicular arteries 

    • attached to the medial meniscus 

    • flat band like approximately 12 cm long 

    • has superficial and deep parts (thickening of the capsule)

  • lateral collateral ligament

    • from the lateral epicondyle to the fibular head 

    • not attached to the lateral meniscus 

    • thin cord like, approximately 5 cm long 

    • separated from the tibia within the joint by the popliteus tendon and outside the joint by the inferior lateral genicular artery 

  • oblique popliteal ligament

    • tendinous expansion of the semimembranosus muscle terminating on the popliteal surface of the femur 

    • perforated by the middle genicular artery 

  • arcuate popliteal ligament 

    • thickened part of the joint capsule that arches over the popliteus tendon as it emerges from the joint capsule and attached to the styloid process of the fibular head 

  • popliteofibular ligament

    • extends from the popliteus tendon near the myotendinous junction to the posterior aspect of the fibular styloid process, posteromedial to the biceps insertion

  • patellar ligament 

    • from the apex of the patella to the tibial tuberosity 

• other

  • anterolateral ligament

  • posterolateral ligamentous complex

• tendons

  • popliteal tendon

• knee menisci

• knee capsule

• knee synovial membrane

Bursa 

• suprapatellar - superior extension of the knee joint cavity

• prepatellar - communicates with the joint cavity, between the lower half of the patella and skin

• subcutaneous infrapatellar - between the patella ligament and skin

• deep infrapatellar - between the tibia and patella tendon

• posterior (between muscle and bone)

  • popliteal - communicates with the joint cavity, beneath the tendon of popliteus lying in the gutter between tibia and head of fibula

  • gastrocnemius

    • bursa beneath the medial head (and usually the lateral head) communicates with the joint cavity 

  • semimembranosus - may communicate with the bursa beneath the medial head of the gastrocnemius 

Relations 

• knee bursae

• anterior knee fat pads

Blood supply

The knee is supplied by anastomoses of:

• five genicular branches of the popliteal artery (main supply)

  • medial and lateral superior genicular arteries encircle the femoral condyle

  • medial and lateral inferior genicular arteries encircle the tibial condyle

  • middle genicular artery supplies the anterior and posterior cruciate ligaments 

• descending branch of the lateral circumflex femoral artery

• descending genicular branch of the femoral artery

• circumflex fibular branches of the posterior tibial artery

• anterior and posterior recurrent branches of the anterior tibial artery 

Innervation

Multiple articular branches are derived from several nerves (Hilton's law):

• branches of the femoral nerve to vastus medialis, and also intermedius and lateralis

• from the sciatic nerve by genicular branches of the tibial and common peroneal nerves

• from the obturator nerve by a branch from the posterior division 

Movements

• flexion

  • semimembranosus, semitendinosus, biceps femoris, gracilis, sartorius

  • also gastrocnemius, plantaris and popliteus

• extension

  • quadriceps femoris, iliotibial tract

  • also gluteus maximus, tensor fascia latae

• internal rotation (when flexed)

  • semimembranosus, semitendinosus, gracilis, sartorius

• external rotation (when flexed)

  • biceps femoris 

• unlocking 

  • popliteus externally rotates femur on tibia, locked ligaments loosen, hamstrings can then flex free

• locking

  • as the knee moves into full extension, the anterior cruciate ligament becomes taut, with no further extension of the lateral condyle possible

  • passive rotation forwards of the lateral condyle around the radius of the taut anterior cruciate ligament

  • medial femoral condyle is then able to glide backwards into full extension  

  • tightening of the oblique popliteal, lateral collateral and medial collateral ligaments

  • purely passive due to the skew pull of the obliquely set ligaments 

Radiographic features

Plain radiograph

See knee radiograph (an approach)

Related pathology

• absent patella

• ACL ganglion cyst

• ACL mucoid degeneration

• anterior tibial translocation sign

• arcuate sign

• avulsion fractures of the knee

• bipartite patella

• chondromalacia patellae

• contrecoup injury of the knee

• jumper's knee

• meniscal cyst

• Osgood-Schlatter disease

• patella alta

• patella baja

• patellar instability

• patella fracture

• patellar sleeve fractures

• patellar tendon lateral femoral condyle friction syndrome

• patella tendon rupture

• patellar tooth sign

• pes anserinus bursitis

• reverse Segond fracture

• Segond fracture

• semimembranosus tendon avulsion fracture

• Sinding-Larsen-Johansson syndrome

• Stieda fracture (MCL avulsion fracture)

• tibial plateau fracture

• tibial pseudoarthrosis

• tibial tuberosity avulsion fracture