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of the hip and thigh | Main Anatomy
Index | Muscles of the leg
Last updated 30 March 2006
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
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
- 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
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
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
- 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.
The Articular Capsule of the Knee
- The fibrous capsule is
strong, especially where local thickenings of it form
- Superiorly, the fibrous capsule is attached to the femur,
just proximal to the articular margins of the condyles
and to the intercondylar line
- 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
- 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).
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
- 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
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.
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
- The fibular collateral ligament is
fused with the fibrous capsule of the knee joint
superiorly; hence, this part of it is an
- Inferiorly the fibular collateral ligament is separated
from the fibrous capsule by fatty tissue; hence this part
of it is an extrinsic ligament.
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.
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
- 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).
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
- 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
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
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
- 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.
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.
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
- 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.
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
- 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
- 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
- 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.
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
- The medial meniscus is firmly
attached to the deep surface of the tibial collateral
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.
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.
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
Injuries involving the suprapatellar
- 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).
Popliteus Bursa (p. 482)
- This extension of the synovial cavity lies between the
tendon of the popliteus muscle and the lateral condyle of
- 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.
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
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.
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
- 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
The Subcutaneous Infrapatellar Bursa (p. 483)
- This bursa is located between the skin and the tibial
- 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
Subcutaneous infrapatellar bursitis (p.
- 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
Deep Infrapatellar Bursa (p. 483)
- This small bursa is lies between the patellar ligament
and the anterior surface of the tibia, superior to the
- 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.