Knee Joint Anatomy
The knee joint is a synovial joint
between the distal end of the femur, proximal end of the tibia and posterior
aspect of the patella (Fig. 6.73). It is a hinge joint with a wide range of
flexion and extension and limited lateral and medial rotation. The joint is
relatively superficial anteriorly, medially and laterally where some of its
bony features are palpable. Posteriorly, the joint is
inaccessible, being deeply placed in the floor of the popliteal fossa (Fig.
6.74).
Anteriorly, the patellar ligament (Fig.
6.75) passes from the
apex of the patella to the tibial tubercle. It is easily palpable and is used
clinically for the stretch reflex (knee jerk, L3 & L4).
Articular surfaces
The medial and lateral condyles of
the femur have articular surfaces, covered by hyaline cartilage, which extend
over their inferior and posterior aspects and articulate with the respective
condyles on the tibia (Fig. 6.73). On the front of the femur lies the patellar
articular surface, while posteriorly a deep intercondylar fossa separates the two
condyles. The articular areas on the tibial condyles are separated by the
intercondylar eminence, the lateral articular area being flatter and smaller than
the medial area (Fig. 6.73).
Capsule
The capsule is thin anteriorly and
posteriorly but is reinforced on each side by strong collateral ligaments. On
the sides of the femur, the capsule attachment extends up to the epicondyles.
Posteriorly, it attaches along the superior margins of the condyles (Figs
6.76 &
6.78) and the
intercondylar line, while anteriorly it extends proximally (Figs 6.75, 6.79
& 6.81) to accommodate the synovial membrane that forms the suprapatellar
bursa (pouch).
On the sides of the tibia, the
capsule attaches close to the articular margins. Those parts of the capsule on
each side of the joint that loosely anchor the outer borders of the menisci to
the tibia are called the coronary ligaments. Posteriorly, the capsular
attachment inclines downwards to include the posterior part of the
intercondylar area, while anteriorly the attachment deviates inferiorly as far
as the tibial tubercle. On the patella, the capsule is attached close to the
articular margins.
Posteriorly, part of the insertion
of semimembranosus forms the oblique popliteal ligament, which passes upwards
and laterally (Figs 6.76 & 6.77), reinforcing the capsule. Where the posterior
part of the capsule is pierced by popliteus, it thickens to form the arcuate
ligament (Fig. 6.76).
On the medial side of the joint,
the thick tibial collateral ligament (Fig. 6.79) broadens as it descends from the medial
femoral epicondyle to the upper part of
the subcutaneous surface of the tibia. Its deep aspect is attached to the outer
margin of the medial meniscus (Fig. 6.78), which diminishes mobility of the meniscus,
making it more susceptible to tears. On the lateral side the cord like fibular
collateral ligament (Fig. 6.80) descends from the lateral epicondyle of the
femur to the styloid process and head of the fibula, separated from the lateral
meniscus by the popliteus tendon (Fig. 6.78).
Intracapsular ligaments
The intracapsular ligaments
comprise the anterior and posterior cruciate ligaments and the meniscofemoral
ligament.
The cruciate ligaments (Fig.
6.78) are named
according to their attachment to the intercondylar eminence of the tibia (Fig.
6.82). The anterior ligament passes upwards, backwards and laterally to attach
to the medial surface of the lateral condyle of the femur.
The posterior
ligament passes upwards, forwards and medially (Fig. 6.78) to attach to the lateral surface of the medial femoral
condyle. The meniscofemoral ligament is adjacent to the posterior cruciate
ligament (Fig. 6.77) and attaches the posterior border of the lateral meniscus
close to the femoral attachment of the posterior cruciate ligament. It
stabilizes the meniscus during rotation of the femur on the tibia. Although
within the capsule, the cruciate and meniscofemoral ligaments are covered by
reflections of synovial membrane and are therefore not within the synovial
cavity.
The medial and lateral menisci are
Cshaped (Fig. 6.82) with their anterior and posterior horns attached to the
intercondylar eminence of the tibia and their outer borders to the joint
capsule (coronary ligaments). The menisci differ in size and shape, the medial
being narrower though slightly larger, so that its horns embrace those of the
lateral meniscus. Also, the medial is attached to the medial collateral
ligament and in cross section is deeper than the lateral meniscus. A
transverse ligament (Fig. 6.82) connects the anterior horn of the medial
meniscus with the anterior aspect of the lateral meniscus.
Infrapatellar fat pad
Deep to the patellar ligament is a
quantity of fat (Fig. 6.74), which bulges the synovial membrane into the
interior of the joint. Folds of synovial membrane, the alar folds (Fig.
6.81), extend on either
side from the main pad. Another fold, the ligamentum mucosum, lies in the
midline between the anterior part of the intercondylar notch and the lower
margin of the patella.
Synovial membrane and bursae
Synovial membrane lines the
interior of the capsule but does not cover the menisci. On the femur, it is
attached to the margins of the intercondylar notch and covers the front and
sides of the cruciate ligaments. Synovial membrane also covers the
infrapatellar fat pad and the tendon of popliteus.
The suprapatellar bursa (Fig.
6.81; Fig. 6.74) is a
large pouch of synovial membrane, passing a hand’s breadth proximal to the
upper border of the patella, deep to quadriceps. Part of vastus intermedius
attaches to it. Since the bursa is continuous with the synovial cavity of the
joint, it provides a route for injecting fluid into or withdrawing fluid from
the joint. After injuries to the joint, fluid accumulates (effusion) in the
suprapatellar bursa, causing typical fullness around the knee and the basis for
the patellartap test. Several other bursae lie near the knee joint and may
enlarge, causing swelling. Bursae between the capsule and the two heads of
gastrocnemius often communicate with the knee joint, while that beneath the
medial head may also communicate with the overlying semimembranosus bursa.
Other bursae, which do not communicate with the joint, are the pre and
infrapatellar bursae beneath the skin covering the patella and patellar
ligament, respectively. These may also enlarge.
Popliteus
From its tibial attachment, the
popliteus passes upwards and laterally (Fig. 6.76), penetrating the posterior
aspect of the capsule of the knee joint deep to the arcuate ligament. Within
the joint, its tendon, covered by synovial membrane, attaches to the posterior
border of the lateral meniscus and to the femur (Fig. 6.77) immediatelybelow
the lateral epicondyle. Popliteus is supplied by the tibial nerve and its
actions are considered below.
Movements
The principal movements of the knee
joint are flexion and extension. Flexion is produced mainly by the hamstrings
(semimembranosus, semitendinosus and biceps) assisted by the two heads of
gastrocnemius. Extension is produced by quadriceps femoris acting through the
patellar ligament. Gluteus maximus, acting through the iliotibial tract,
maintains stability of the knee in the extended position.
Because of the shape of the
articular surfaces, the femur rotates medially during
the later stages of extension. The lateral condyle and meniscus (moving in
unison because of the meniscofemoral ligament) glide forwards on the lateral
tibial condyle, while the medial condyle completes its movement of extension on
the medial meniscus. Full extension is achieved with completion of medial
rotation and further movement is prevented by tension in the collateral and
oblique posterior ligaments.
During the early stages of flexion,
lateral rotation of the femur on the tibia is produced by popliteus, which also
pulls the lateral meniscus posteriorly. During flexion and extension the
patella glides over the patellar surface of the femur.
Slight active rotation of the tibia
on the femur can occur when the knee is in a flexed but nonweightbearing
position. Sartorius, gracilis and semitendinosus rotate medially, while biceps
femoris rotates laterally.
Stability
The knee joint is very stable. The
most important factors are muscle tone, especially in quadriceps and the
ligaments. The cruciate ligaments stabilize the femur on the tibia, preventing
excessive anteroposterior movement. The collateral ligaments assist medial and
lateral stability, while the iliotibial tract stabilizes the knee during
extension. All of these ligaments, together with the oblique posterior
ligament, prevent hyperextension. Cruciate and collateral ligament injuries
together with meniscal tears commonly occur in sports, particularly following twisting
movements, during which the foot is anchored to the ground.
Owing to angulation of the femur
relative to the tibia, contraction of quadriceps femoris tends to displace the
patella laterally. This displacement is prevented, first by the lowest fibres
of vastus medialis, which insert into the medial patellar border and whose
active contraction resists lateral movement of the patella, and second by the
large size and prominence of the lateral femoral condyle, making lateral patellar
movement mechanically difficult.
Occasionally, the lateral femoral
condyle fails to develop normally, resulting in patellar instability.
Innervation
Branches from the femoral,
obturator and sciatic nerves supply the joint, sensory fibres from the femoral
nerve travelling with the branches to the vasti and sartorius. Genicular
branches from the tibial and common fibular divisions of the sciatic nerve,
together with fibres from the posterior division of the obturator nerve, also
supply the joint.
Blood supply
The knee joint receives its blood
supply from the extensive genicular anastomosis derived mainly from branches of
the popliteal, anterior and posterior tibial
arteries (Fig. 1.27). Dislocation of the knee may damage the popliteal artery
and seriously compromise the blood supply to the leg and foot. It may also
damage the tibial and common peroneal (fibular) nerves causing loss of
sensation and movement below the level of the knee.
