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Knee Joint

  1. Knee joint is the largest joint of the body.
  2. It is condylar type of synovial joint.
  3. It is a compound joint as more than two bones (more than one articulation) are involved in its formation, i.e. patella, femur and tibia (patellofemoral articulation and femorotibial articulation). Condyles of tibia and femur are separated from each other partially by menisci; hence it is a complex joint.
  4. Knee joint is compound synovial joint, incorporating two condylar joints between the condyles of femur and one saddle joint between femur and patella.




The knee joint is supported by following ligaments:-

  1. Fibrous capsule (articular capsule) with synovial membrane
    1. It is very thin capsule and deficient anteriorly being replaced by quadriceps femoris tendon, patella and ligamentum patellae (patellar tendon). There are two opening in the capsule: (i) One leading to suprapatellar bursa, (ii) another for exit of the tendon of popliteus (Thus popliteus is intracapsular/intra-articular.
    2. Coronary Iigament is the part of capsule that connects the peripheral part of menisci with corresponding tibial condyles.
    3. The synovial membrane lines the inner surface of the capsule, except posteriorly where it is reflected forward by the cruciate ligament, forming a common covering for both the ligaments. In front, It is absent from patella. Above the patella, it is prolonged upward as the suprapatellar bursa. Below patella, it covers the deep surface of infrapatellar fat pad (Hoffa's fat pad), which separates it from the ligamentum patellae. A median fold, the infrapatellar synovial fold is there inferior to patella.
  2. Ligamentum patellae (patellar tendon)
    1. It is considered to be continuation of the quadriceps femoris tendon which extends from apex of patella to tibial tuberosity.
  3. Tibial collateral (medial collateral) ligament
    1. Superiorly it is attached to medial epicondyle of femur just below the adductor tubercle. Inferiorly it divides into superficial (anterior) and deep (posterior) parts.
      1. Superficial (anterior) part is attached below the medial condyle and upper part of the medial border of shaft of tibia. It covers the inferior medial genicular vessels and nerve and the anterior part of the tendon of the semimembranosus. The lower part of the ligament is crossed by tendons of sartorius, gracilis and semitendinosus muscles, which are separated from it by anserine bursa.
      2. Deep (posterior) part is short and blends with the capsule and peripheral margin of medial meniscus. It is attached to medial condyle of the tibia above the groove for semimembranosus.
    2. Phylogenetically this ligament represents the degenerated tendon of adductor magnus muscles.
  4. Fibular (lateral) collateral ligament
    1. It is thick round cord like ligament which extends from lateral epicondyle of femur to fibular head. In comparison to deep part of tibial collateral ligament it is not attached to capsule and lateral meniscus. Popliteus tendon separates it from lateral meniscus and inferior lateral genicular vessels and nerve separate it from the capsule.
    2. Phylogenetically, it represents the degenerated tendon of origin of peroneus longus, i.e. femoral attachment of peroneus longus.
  5. Oblique popliteal ligament
    1. It is an expansion from the tendon of semimembranosus attachment to intercondylar line of femur. It is closely related to popliteal artery and is pierced by middle genicular vessels and nerve and the terminal part of the posterior division of the obturator nerve. 
  6. Arcuate popliteal ligament
    1. It is a posterior expansion from the short lateral ligament. It extends from the head of the fibula to the posterior border of the tibial intercondylar area.
  7. Anterior cruciate ligament (ACL)
    1. ACL is one of the most important ligaments of the knee joint.
    2. ACL begins from anterior part of intercondylar area of tibia just behind the anterior horn of medial meniscus and runs upwards, backwards &laterally to attach the posterior part of medial surface of lateral condyle of femur:
    3. ACL is intrasynovial i.e., it lies inside the synovium.
    4. AGL provides antero-posterior stability and prevents anterior gliding of tibia on femur and prevents hyperextension.
    5. ACL also provides proprioceptive function of knee joint.
    6. ACL is taut during extension.
  8. Posterior cruciate ligament
    1. PCL begins from posterior part of intercondylar area of tibia and runs upwards, forwards and medially to attach the anterior part of the lateral surface of medial condyle of femur.
    2. PCL is extra synovial but intracapsular, i.e., lies between synovium and capsule of the knee joint.
    3. It provides antero-posterior stability and prevents posterior gliding of tibia on femur.
    4. It is taut in flexion.
    5. Blood supply of cruciate (anterior & posterior) ligaments is from: -
      1. Middle genicular artery (major supply)
      2. Inferior genicular (medial &lateral) artery (less important). 
      3. Nerve supply of cruciate ligaments (ACL & PCL) is from posterior articular branch of tibial nerve.


  1. These are two semilunar cartilages which are crescent-shaped plates of fibrocartilage that are placed on the condylar surface of tibia.
  2. There are two menisci: Medial meniscus and lateral meniscus.
  3. The medial meniscus is nearly semilunar and its peripheral margin is attached to the deep part of medial (tibial) collateral ligaments.
  4. The lateral meniscus is nearly circular. The tendon of the popliteus and the capsule separate this meniscus from the fibular collateral ligament.
  5. The peripheral part of menisci is vascular while inner part is avascular and is nourished by synovial fluid.
    Medial meniscus is more frequently injured than lateral meniscus because: -
    1. The medial meniscus is securely attached around the entire periphery of the joint capsule, which makes it less mobile.
    2. Whereas the lateral meniscus is more mobile and has no weak point between a movable and relatively fixed point.
    3. Popliteus muscle sends few fibers into the posterior margin of lateral meniscus. Thus muscle contraction withdraws and protects the lateral meniscus by drawing it posterolaterally during flexion of the knee and medial rotation of the tibia.           

Medial Meniscus   

Lateral Meniscus

Semilunar in shape (Less circular)                                     

Semi-circular in shape (C shaped; more circular)

Larger in diameter but narrower in body                        

Smaller in diameter but wider in body

Anterior horn is small while posterior horn is large

Anterior horn and posterior horn are uniform in size

Covers less tibial articular surface than lateral

Covers more tibial articular surface than medial

Entire periphery of the meniscus is attached to the joint capsule.

Entire periphery of meniscus is not attached to joint capsule (Area where the popliteus tendon crosses the joint through the popliteus hiatus is not attached)

Is attached to the medial collateral ligament

Is not attached to the lateral collateral ligament

Less mobile (due to firmer attachment with joint capsule and medial collateral ligament)

More mobile (due to gaps in attachment wit joint capsule and lateral collateral ligament)

More prone to injury (due to reduced mobility)

Less prone to injury (due to increase mobility)


Ligaments related to menisci

Following are the important ligaments in relation to menisci: -

  1. Coronary ligaments: - These ligaments attach the periphery of both the menisci to the periphery of tibial condyles.
  2. Menisco-femoral ligaments: - These ligaments attach the posterior part of lateral meniscus to the femur. These are of two types: -
    1. Anterior menisco-femoral ligament (ligament of Humphrey): - Runs anterior to PCL.
    2. Posterior menisco-femoral ligament (Ligament of Wrisberg): - Runs posterior to the PCL.
  3. Transverse ligaments: - Attach the anterior edges of the medial and lateral meniscus to each other.


  • Active movements performed at the knee joint are extension, flexion, medial and lateral rotation.


Principal muscles

Accessory muscles


Biceps femoris, semitendinosus, semimembranosus (Hamstring)

Gracilis, sartorius, popliteus


Quadriceps femoris (Rectus femoris), Vastus medialis, lateralis and intermedius)


Medial rotation

of flexed knee

semimembranosus, semitendinosus, popliteus


Lateral rotation

of flexed knee

Biceps femoris



Locking (screw home movement) and unlocking (unscrewing)

Physiological locking occurs in extension when the femur is internally (medially) rotated on a fixed tibia. Locking is a mechanism that allows the knee to remain in the position of full extension as in standing without much muscular efforts and is caused by quadriceps femoris. Unlocking of knee is required when flexion is initiated from a fully extended position.

  1. It involves External rotation/ Lateral rotation of femur on a fixed tibia.
  2. Unlocking is brought about by the action of popliteus muscle.
  3. When a person attempts to sit from a standing position the femur rotates externally a fixed tibia.

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