The Knee Joint

Muscles of the hip and thigh | Main Anatomy Index | Muscles of the leg

The Knee Joint

• This is a hinge type of synovial joint that permits some rotation.
• Its structure is complicated because it consists of three articulations: an intermediate one between the patella and femur and lateral and medial ones between the femoral and tibial condyles.

Articular Surfaces of the Knee Joint

• The bones involved are the femur, tibia, and patella.
• The articular surfaces are the large curved condyles of the femur, the flattened condyles of the tibia, and the facets of the patella.

• The knee joint is relatively weak mechanically because of the configurations of its articular surfaces. It relies on the ligaments that bind the femur to the tibia for strength.

• On the superior surface of each tibial condyle, there is an articular area for the corresponding femoral condyle.
• These areas, commonly referred to as the medial and lateral tibial plateaux, are separated from each other by a narrow, nonarticular area, which widens anteriorly and posteriorly into anterior and posterior intercondylar areas, respectively.

Surface Anatomy of the Knee Joint

• This joint may be felt as a slight gap on each side between the corresponding femoral and tibial condyles. When the leg is flexed or extended, a depression appears on each side of the patellar ligament.
• The articular capsule is very superficial in these depressions. The knee joint lies deep to the apex of the patella.

Movements of the Knee Joint

• The principal movements occurring at this joint are flexion and extension of the leg, but some rotation also occurs in the flexed position.
• Flexion and extension of the knee joint are very free movements.

• Flexion normally stops when the calf contacts the thigh. The ligaments of the knee stop extension of the leg.
• When the knee is fully extended, the skin anterior to the patella is loose and can easily be picked up. This laxity of the skin helps flexion to occur.

• The knee "locks" owing to medial rotation of the femur on the tibia. This makes the lower limb a solid column and more adapted for weight bearing. To "unlock" the knee the popliteus muscle contracts, thereby rotating the femur laterally so that flexion of the knee can occur.

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The Articular Capsule of the Knee

• The fibrous capsule is strong, especially where local thickenings of it form ligaments.

• Superiorly, the fibrous capsule is attached to the femur, just proximal to the articular margins of the condyles and to the intercondylar line posteriorly.
• It is deficient on the lateral condyle, which allows the tendon of the popliteus muscle to pass out of the joint and insert into the tibia.
• Inferiorly the fibrous capsule is attached to the articular margin of the tibia, except where the tendon of the popliteus muscle crosses the bone.
• Here the fibrous capsule is prolonged inferolaterally over the popliteus to the head of the fibula, forming the arcuate popliteal ligament.

• The fibrous capsule is supplemented and strengthened by five intrinsic ligaments; patellar ligament, fibular collateral ligament, tibial collateral ligament, oblique popliteal ligament, and arcuate popliteal ligament.
• These are often called the external ligaments to differentiate them from the internal ligaments (e.g., the cruciate ligaments, which are internal to the fibrous capsule).

The Patellar Ligament or Ligamentum Patellae (pp. 478-9)

• This very strong, thick band is the continuation of the tendon of the quadriceps femoris muscle.
• The patella is a sesamoid bone in this tendon.
• The patella is continuous with the fibrous capsule of the knee joint and is most easily felt when the leg is extended.

• The superior part of its deep surface is separated from the synovial membrane of the knee joint by a mass of loose fatty tissue called the infrapatellar fatpad. The inferior part of the patellar ligament is separated from the anterior surface of the tibia by the deep infrapatellar bursa.

Patellar Reflex

With the leg flexed, the patellar ligament is struck to elicit a knee jerk. This patellar reflex or knee reflex results in the extension of the leg. This reflex is blocked by damage to the femoral nerve, which supplies the quadriceps muscle. Similarly, damage to the reflex centres in the spinal cord (L2, L3, and L4) will affect the patellar reflex.

The Fibular Collateral Ligament (p. 479)

• The fibular collateral ligament (lateral ligament) is a round pencil-like cord about 5 cm long.
• It extends inferiorly from the lateral epicondyle of the femur to the lateral surface of the head of the fibula.

• The tendon of the popliteus muscle passes deep to the fibular collateral ligament, separating it from the lateral meniscus.
• The biceps femoris muscle is also split into two parts by this ligament.

• The fibular collateral ligament is fused with the fibrous capsule of the knee joint superiorly; hence, this part of it is an intrinsic ligament.
• Inferiorly the fibular collateral ligament is separated from the fibrous capsule by fatty tissue; hence this part of it is an extrinsic ligament.

The Tibial Collateral Ligament (p.479-50)

• This ligament (also known as the medial ligament) is a strong, flat band, 8 to 9 cm long, which extends from the medial epicondyle of the femur to the medial condyle and superior part of the medial surface of the tibia.
• It is a thickening of the fibrous capsule of the knee joint and is partly continuous with the tendon of the adductor magnus muscle.

• The medial inferior genicular vessels and nerve separate the inferior end of the ligament from the tibia.
• The deep fibres of the tibial collateral ligament are firmly attached to the medial meniscus and the fibrous capsule of the knee.

Injuries, the collateral ligaments and the knee joint

• The tibial and fibular collateral ligaments normally prevent disruption of the sides of the knee joint.
• They are tightly stretched when the leg is extended and prevent the rotation of the tibia laterally or the femur medially.
• As the collateral ligaments are slack during flexion of the leg, they permit some rotation of the tibia on the femur in this position.

• The fibular collateral ligament is not commonly torn because it is very strong. However, lesions (e.g., strains or tears) or the fibular collateral ligament can have serious consequences.
• Usually, it is the distal end of the ligament that tears, and sometimes the head of the fibular is pulled off because the ligament is stronger than the bone. Complete tears are associated with stretching of the common fibular (peroneal) nerve. This affects the muscles of the anterior and lateral compartments of the leg and may produce foot-drop owing to paralysis of the dorsiflexor and eversion muscles of the foot.

• The firm attachment of the tibial collateral ligament to the medial meniscus is of considerable clinical significance because injury to the tibial collateral ligament frequent results in concomitant injury to the medial meniscus.
• Rupture of the tibial collateral ligament, often associated with tearing of the medial meniscus and anterior cruciate ligament, is a common type of football injury. The damage is frequently caused by a blow to the lateral side of the knee.

• When considering soft tissue injuries of the knee, always think of the three Cs which indicate those structures that may be damaged: Collateral ligaments, Cruciate ligaments, and Cartilage (menisci).

The Oblique Popliteal Ligament (p. 482)

• The broad band is an expansion of the tendon of the semimembranosus muscle. The oblique popliteal ligament strengthens the fibrous capsule of the knee joint posteriorly.
• It arises posterior to the medial epicondyle of the tibia and passes superolaterally to attach to the central part of the posterior aspect of the fibrous capsule of the knee joint.

The Arcuate Popliteal Ligament (p. 482)

• This Y-shaped band of fibres also strengthens the fibrous capsule posteriorly. The stem of the ligament arises from the posterior aspect of the head of the fibula. As it passes superomedially over the tendon of the popliteus muscle, the arcuate popliteal ligament spreads out over the posterior surface of the knee joint.
• It inserts into the intercondylar area of the tibia and the posterior aspect of the lateral epicondyle of the femur.

Cruciate Ligaments of the Knee Joint (pp. 483-5)

• These are very strong ligaments within the capsule of the joint but are outside the synovial cavity.
• Joining the femur and tibia, they are located between they are located between the medial and lateral condyles and are separated from the joint cavity by the synovial membrane. The synovial capsule lines the fibrous capsule, except posteriorly where it is reflected anteriorly around the cruciate ligaments.

• The cruciate (L. resembling a cross) ligaments are strong, rounded bands that cross each other obliquely in a manner similar to an X. They are named anterior and posterior according to their site of attachment to the tibia, i.e., the anterior cruciate ligament attaches to the tibia anteriorly and the posterior cruciate ligament attaches to it posteriorly. These ligaments are essential to the anteroposterior stability of the knee joint, especially when it is flexed.

The Anterior Cruciate Ligament (p. 483)

• The weaker of the two ligaments, the anterior cruciate ligament arises from the anterior part of the intercondylar area of the tibia, just posterior to the attachment of the medial meniscus.
• It extends superiorly, posteriorly, and laterally to attach to the posterior part of the medial side of the lateral condyle of the femur.

• The anterior cruciate ligament, which is slack when the knee is flexed and taut when it is fully extended, prevents posterior displacement of the femur on the tibia hyperextension of the knee joint. When the joint is flexed at a right angle, the tibia cannot be pulled anteriorly because it is held by the anterior cruciate ligament.

The Posterior Cruciate Ligament (p. 483-5)

• This is the stronger of the two ligaments. It arises from the posterior part of the intercondylar area of the tibia and passes superiorly and anteriorly on the medial side of the anterior cruciate ligament to attach to the anterior part of the lateral surface of the medial condyle of the femur.
• The posterior cruciate ligament is the first structure observed when the knee joint is surgically opened posteriorly.

• The posterior cruciate ligament, which tightens during flexion of the knee joint, prevents anterior displacement of the femur on the tibia or posterior displacement of the tibia.
• It also helps to prevent hyperflexion of the knee joint. In the weight bearing flexed knee, it is the main stabilising factor for the femur, e.g., when walking downhill or downstairs.

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The Menisci of the Knee Joint

• The medial and lateral menisci (G. crescents) are crescentic plates of fibrocartilage on the articular surface of the tibia.
• These act like shock absorbers. Because they are basically C-shaped, they were formally called semilunar cartilages. They are wedge-shaped in the transverse section.

• The menisci are firmly attached at their ends to the intercondylar area of the tibia. The menisci deepen the articular surfaces of the tibia where they articulate with the femoral condyles.
• Their superior surfaces are slightly concave for reception of their condyles, whereas their inferior surfaces that rest on the tibial condyles are flatter.
• The menisci are thick at their peripheral attached margins and thin at their internal unattached edges.

• Being smooth and slightly movable, the menisci fill the gaps between the femur and tibia that would otherwise be present during movements of the knee joint. Their external margins are attached to the fibrous capsule of the knee joint and through it to the edges of the articular surfaces of the tibia.

• The capsular fibres that attach the thick, convex margins of the menisci to the tibial condyles are called coronary ligaments.
• A slender fibrous band, called the transverse ligament of the knee, joins the anterior edges of the two menisci. This connection allows them to move together during movements of the femur on the tibia. The thickness of this ligament varies in different people; sometimes it is absent.

• The thick peripheral margins of the menisci are vascularised by genicular branches of the popliteal artery, but the thin unattached edges of the interior of the joint are avascular.

The Medial Meniscus (p. 485)

• This C-shaped cartilage is broader posteriorly than anteriorly.
• Its anterior end or horn (L. cornu) is attached to the anterior intercondylar area of the tibia, anterior to the attachment of the anterior cruciate ligament.
• The posterior end or horn is attached to the posterior intercondylar area, anterior to the attachment of the posterior cruciate ligament and between the attachments of the lateral meniscus and the posterior cruciate ligament.

• The medial meniscus is firmly attached to the deep surface of the tibial collateral ligament.

The Lateral Meniscus (p. 485)

• This C-shaped cartilage is nearly circular and conforms to the rather circular lateral tibial condyle.
• The lateral meniscus is smaller and more freely movable than the medial meniscus, but it covers a larger area of articular surface than does the medial meniscus.

• The tendon of the popliteus muscle and bursa separate the lateral meniscus from the fibular collateral ligaments.

• The anterior and posterior horns of the lateral meniscus are attached close together in the anterior and posterior intercondylar areas. A strong tendinous slip, called the posterior meniscofemoral ligament, joins the lateral meniscus to the posterior cruciate ligament and the medial femoral condyle.

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Bursae around the Knee

• Several bursae are present around the knee because most tendons around the knee joint run parallel to the bones and pull lengthwise across the joint.
• Four bursae communicate with the synovial cavity of the knee joint; they lie deep to the tendons of the quadriceps femoris, the popliteus, and the medial head of the gastrocnemius muscle.

The Suprapatellar (Quadriceps) Bursa (p. 482)

• This large saccular extension of the synovial capsule passes superiorly between the femur and the tendon of the quadriceps femoris muscle.
• The clinically important suprapatellar bursa extends about 8 cm superior to the base of the patella.

• The suprapatellar bursa permits free movement of the quadriceps tendon over the distal end of the femur and facilitates full extension and flexion of the knee joint.
• The bursa is held in position by the part of the vastus intermedius muscle called the articular genus muscle.

Injuries involving the suprapatellar bursa

• Because the suprapatellar bursa communicates freely with the synovial cavity of the knee joint, it is regarded as a part of it.
• Stab or puncture wounds superior to the patella may infect the knee joint via the suprapatellar bursa. It may also be involved in fractures of the femur resulting in hemarthrosis of the knee joint (blood in the joint cavity).

The Popliteus Bursa (p. 482)

• This extension of the synovial cavity lies between the tendon of the popliteus muscle and the lateral condyle of the tibia.
• The bursa opens into the lateral part of the synovial cavity, inferior to the lateral meniscus.
• Sometimes the bursa is also continuous with the synovial cavity of the proximal tibiofibular joint as a result of perforation of the partition between the popliteus bursa and its joint cavity.

The Anserine Bursa (Bursa anserina) (p. 482)

• This complicated bursa has several diverticula and separates the tendons of the sartorius, gracilis and semitendinosus muscles from the proximal part of the medial surface of the tibia and from the tibial collateral ligament.

The Gastrocnemius Bursa (p. 482)

• This extension of the synovial cavity of the knee joint lies deep to the proximal attachment of this the tendon of the medial head of the gastrocnemius muscle. As it separates the tendon from the femur, it is often called the subtendinous bursa of the gastrocnemius.

The Semimembranosus Bursa (p. 482)

• This bursa is related to the distal attachment of the semimembranosus muscle related to the distal attachment of the semimembranosus muscle and is located between the medial head of the gastrocnemius and the semimembranosus tendon.
• Frequently it is a prolongation of the gastrocnemius bursa and communicates with the knee joint cavity.

The Subcutaneous Prepatellar Bursa (p. 483)

• This bursa lies between the skin and the anterior surface of the patella. It allows free movement of the skin over the patella during flexion and extension of the leg.
• Because of its superficial and exposed position, this bursa may become inflamed after prolonged periods of weight bearing on the hands and knees.

Prepatellar bursitis (p. 483)

• This is a friction bursitis caused by friction between the skin and the patella. If the inflammation is chronic, the bursa becomes distended with fluid and forms a soft, fluctuant swelling anterior to the knee.
• This condition is commonly called "housemaid's knee".

The Subcutaneous Infrapatellar Bursa (p. 483)

• This bursa is located between the skin and the tibial tuberosity.
• It allows the skin to glide over the tibial tuberosity and withstand pressure when kneeling with the trunk upright (e.g., when one kneels or genuflects during praying).

Subcutaneous infrapatellar bursitis (p. 483)

• This results from excessive friction between the skin and the tibial tuberosity. The swelling occurs over the proximal end of the tibia.
• This condition has been called "clergyman's knee".

The Deep Infrapatellar Bursa (p. 483)

• This small bursa is lies between the patellar ligament and the anterior surface of the tibia, superior to the tibial tuberosity.
• It is separated from the knee joint by the infrapatellar fatpad, a mass of fatty tissue between the superior part of the patellar ligament and the synovial capsule.

Deep infrapatellar bursitis (p. 483)

• It results in a swelling between the patellar ligament and the tibia, superior to the tibial tuberosity. The swelling is less pronounce than that associated with subcutaneous prepatellar bursitis.
• Enlargement of this bursa obliterates the dimples one each side of the patellar ligament when the leg is extended. Obliteration of these dimples may also result from synovial effusion.

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