Knee Joint

The knee joint is made up of the articulations between the femur, tibia, and patella bones, and is one of the largest and most complex joints of the human body. The knee is classified as a synovial hinge joint, which primarily allows for flexion and extension with a more limited degree of translation and rotation. The supporting structures of the knee joint include a joint capsule, the lateral and medial menisci, and multiple ligaments that help ensure mobility and stability of the knee.

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Bony Structure of the Knee

Femur

  • The longest and strongest bone in the human body
  • The femur articulates with the hip bone proximally and the patella and the tibia distally.
  • Articulations of the distal femur:
    • With the tibia: forms medial and lateral tibiofemoral articulations
    • With the patella: forms the patellofemoral articulation
  • Notable bony landmarks of the distal femur:
    • Medial and lateral condyles: 
      • Covered with articular cartilage
      • Articulate with the tibia
    • Intercondylar notch separates the distal femur condyles posteriorly.
    • Adductor tubercle: 
      • Arises from the superior portion of the medial epicondyle
      • Serves as the attachment for adductor muscles
    • Femoral trochlea: 
      • Distal, anterior depression of the femur 
      • Surface on which the patella glides during flexion and extension

Tibia

  • Also known as the shinbone 
  • Connects the femur to the ankle joint and foot
  • Knee articulations:
    • With the femur: forms medial and lateral tibiofemoral articulations
    • With the fibula: proximal tibiofibular syndesmosis
  • Notable bony landmarks of proximal tibia:
    • Medial and lateral tibial plateaus: correspond anatomically with the femoral condyles
    • Gerdy’s tubercle: distal insertion of the iliotibial band on the proximal anterolateral tibia
    • Tibial tuberosity: site of distal insertion of the patellar tendon

Patella

  • Largest sesamoid bone in the body
  • Located within the quadriceps femoris tendon
  • Serves as a pulley and increases mechanical advantage during knee extension
  • Articulations: with femur to form the patellofemoral articulation

Joints of the Knee

Tibiofemoral joint

The knee is a modified hinge joint; a double condyloid articulation. Although the motions of the knee are primarily flexion and extension, it has a complex movement pattern consisting of 6 degrees of motion during dynamic activities:

  • 3 rotations:
    • Flexion/extension: 
      • Primary movement
      • Range of motion of the knee is approximately 0–130 degrees.
    • Internal/external rotation: important when “locking” and “unlocking” the knee, moving from extension to flexion
    • Varus/valgus angulation
  • 3 translations:
    • Compression/distraction
    • Anterior/posterior glide: necessary because of the larger surface of the distal femoral condyles gliding on the smaller surfaces of the tibial plateau
    • Medial/lateral translation
Motions of the knee

Motions of the knee

Image by BioDigital, edited by Lecturio

Patellofemoral joint

  • The trochlear groove of the distal femur articulates with the patella.
  • Patella increases the mechanical advantage of the knee.
  • Stability is provided by the joint anatomy plus the presence of multiple patellofemoral ligaments.

Proximal tibiofibular syndesmosis

  • A relatively immobile joint in which the proximal tibia and fibula are joined by ligaments
  • Arthrodial plane joint between the lateral tibial condyle and the head of the fibula
  • Stabilized by a sturdy capsule and multiple ligaments. Joint capsule receives additional support from:
    • Anterior and posterior superior tibiofibular ligaments: span the region between the fibular head and lateral tibial condyle
    • Lateral collateral ligament of the knee joint
    • Biceps femoris: provides reinforcement as it inserts onto the fibular head
Leg Anterior and Posterior view

Anterior and posterior views of the tibia, fibula, and tibiofibular joints

Image by BioDigital, edited by Lecturio

Menisci of the Knee

The menisci are semilunar-shaped fibrocartilage wedges between the femur and tibia and made of type I collagen fibers. Menisci are shock absorbers, improve the congruence of the tibiofemoral joint, and are vital for the normal functioning and health of the knee.

Overview of meniscal anatomy

  • The inner ⅓ is thin and avascular.
  • The outer ⅓ is thicker and vascularized.
  • Increases the stability of the joint, improves lubrication, limits extremes of flexion and extension
  • Ligamentous fixation of the meniscus increases stability:
    • Coronary ligaments (meniscotibial ligaments) attach the peripheral meniscus to the tibial plateaus.
    • Transverse ligaments connect the anterior horns of the medial and lateral meniscus.
    • Patellomeniscal ligaments connect the anterior horns of the meniscus to the patella.
    • 2 meniscofemoral ligaments (Wrisberg and Humphrey) attach the posterior lateral meniscus to the medial femoral condyle.

Medial meniscus

  • Overlies the medial tibial plateau, C shaped
  • Attached to the tibial collateral ligament, also known as the medial collateral ligament (MCL)

Lateral meniscus

  • Overlies the lateral tibial plateau, more circular
  • More mobile than the medial meniscus

Movement of menisci

  • During flexion: Menisci move posteriorly, and the lateral meniscus moves more than medial.
  • During extension: Menisci get pushed anteriorly by the femoral condyles.
Image displaying the menisci and their relation with other articular surfaces that compose the knee joint

Image displaying the menisci and their relation with other articular surfaces that compose the knee joint

Image by BioDigital, edited by Lecturio

Supporting Structures of the Knee

Membranes

A 2-layered joint capsule provides support to the knee. These layers are the external fibrous membrane and internal synovial membrane.

  • External fibrous membrane:
    • Most superficial layer: Thickened areas make up the intrinsic ligaments of the knee.
    • Provides stability
  • Internal synovial membrane: 
    • Lines all surfaces of the articular cavity within the fibrous layer, except those covered by articular cartilage
    • Provides lubrication; synovial fluid lubricates joint movements
    • Inserts on the periphery of the articular cartilage of the femur and tibia 
Articular capsule and components

Articular capsule and its components

Image by Lecturio. License: CC BY-NC-SA 4.0

Ligaments and tendons

Ligaments and tendons of the knee provide stability to the knee during motion and increase the efficiency of the knee.

Table: Ligaments and tendons of the knee
Ligament/tendonOriginInsertionFunction
Anterior cruciate ligament (ACL)
is composed of 2 bundles:
  • Anteromedial bundle
  • Posterolateral bundle
Inner lateral femoral condyleRoof of the intercondylar fossa
  • Prevents anterior displacement of the tibia
  • Prevents hyperextension of the knee
  • Prevents posterior sliding of the femur on the tibia
  • Prevents anterior sliding of the tibia on the femur
Posterior cruciate ligament (PCL)
is composed of 2 bundles:
  • Anterolateral bundle
  • Posteromedial bundle
Inner surface of the medial femoral condylePosterior intercondylar area of the tibia
  • Prevents posterior displacement of the tibia
  • Prevents hyperflexion of the knee
  • Prevents anterior sliding of the femur on the tibia
  • Prevents posterior sliding of the tibia on the femur
Medial collateral ligament (MCL)Medial femoral epicondyleMedial condyle of the tibiaStabilizes the knee joint against valgus stress
Lateral/fibular collateral ligamentLateral femoral epicondyleHead of the fibulaStabilizes the knee joint against varus stress
Patellar ligamentDistal patellaTuberosity of the tibiaComponent of the extensor mechanism of the knee
Quadriceps tendonQuadriceps musclesProximal patellaComponent of the extensor mechanism of the knee

Bursae

Bursae are serous membranes/synovial fluid-filled sacs with a small amount of fluid facilitating movement around a joint. There are multiple bursae described around the knee joint.

Table: Bursae of the knee joint
BursaeLocation
SuprapatellarBetween the femur and tendon of the quadriceps muscle
PrepatellarBetween the patella and skin
Infrapatellar (superficial and deep)
  • Superficial infrapatellar: between the tibial tubercle and skin
  • Deep infrapatellar: between the posterior patellar tendon and tibia
Pes anserinusMedial knee/proximal medial tibia

Vascular Supply of the Knee

  • Vascular supply is accomplished via branches of the popliteal artery:
    • Located in the popliteal fossa posterior to the posterior capsule
    • Multiple branches to the knee
    • Becomes the tibioperoneal trunk after take-off of the anterior tibial artery
    • Tibioperoneal trunk continues as the posterior tibial artery and fibular artery to the lower leg.
  • Arterial anastomosis about the knee include:
    • Multiple genicular branches from the popliteal artery
    • Anterior and posterior tibial artery branches
    • Branch of the lateral femoral circumflex artery 
  • The venous system:
    • Primarily the popliteal and femoral veins
    • The veins typically course with their corresponding arteries in the knee.

Innervation of the Knee

  • Nerve supply to the knee is primarily accomplished via:
    • Femoral nerve to the vastus lateralis, intermedius, and medialis
    • Sciatic nerve:
      • Genicular branches of the common fibular nerve to the rectus femoris and biceps femoris short head 
      • Tibial nerve to the biceps femoris long head, semimembranosus, and semitendinosus
    • Posterior division of the obturator nerve via an unnamed articular branch to the knee joint
  • Hilton’s law: Innervation of a joint is typically via a branch from a motor nerve that innervates a muscle, which extends across and acts on that joint.

Clinical Relevance

The following common conditions are associated with the knee:

  • Anterior cruciate ligament (ACL) injury: a common knee injury during sports activities. The most frequent etiology is a noncontact pivoting injury. More common in women. The majority of ACL injuries are treated with surgical reconstruction.
  • Posterior cruciate ligament (PCL) injury: an injury resulting primarily from a direct blow to the anterior proximal tibia in a flexed knee (dashboard injury). A PCL injury may also occur from hyperextension. Treatment is with bracing and rehabilitation. Some athletes may require surgical reconstruction.
  • Knee dislocations: generally result from secondary to high-energy trauma such as a fall from a height or motor vehicle accidents. Morbid obesity is a risk factor for low-energy knee dislocations. Dislocations are most commonly either anterior or posterior, depending on the mechanism of injury. Hyperextension injury leads to anterior dislocations. Treatment involves immediate reduction and evaluation of the vascular system. Vascular evaluation includes serial exams and may involve surgical exploration of the arterial system. Interruption of the blood supply can lead to acute ischemia, gangrene, and even amputation.
  • Dislocation of the patella: can occur acutely from trauma or chronically due to ligament laxity. The patella usually dislocates laterally. Recurrent dislocation can result in patellofemoral arthritis. Common risk factors include generalized ligamentous laxity and increased Q angle. Common surgical treatments include medial patellofemoral ligament reconstruction and/or tibial tubercle osteotomy.
  • Meniscal injuries: meniscus tears are seen acutely in younger patients and are generally attributed to degeneration in older patients. In general, medial meniscus tears are more common. Description of meniscus tears is based on the location and pattern of the tear.
  • Patellofemoral pain syndrome: a common disorder of the knee that manifests as pain arising from the patellofemoral joint or the soft tissues around the joint. There are many etiologies of patellofemoral pain syndrome.
  • Pes anserinus bursitis/tendinopathy (goose’s foot): the anatomic name given to the conjoined tendons of the sartorius, gracilis, and semitendinosus muscles that attach to the anteromedial surface of the proximal tibia. The tendons may become painful because of bursitis or tendinopathy.

References

  1. Meyler, Z. (2018). Knee Anatomy. Arthritis-health. Retrieved May 24, 2021, from https://www.arthritis-health.com/types/joint-anatomy/knee-anatomy
  2. Drake, R.L., Vogl, A.W., Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
  3. Moore, K.L., Dalley, A.F., Agur, A.M.R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

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