The knee is the joint between the femur (thigh-bone) and the tibia (shin-bone). A
third bone, the patella (kneecap) lies across
the front of the joint.
|At the knee the femur (1) and tibia (2) are joined by ligament (3). Further ligaments (4), attached to the patella (5), form the tendon for the quadriceps muscle (6). The synovial membrane forms the infrapatellar (7) and suprapatellar (8) bursae. Cartilage (9) covers the articular surfaces and there are two crescents of cartilage (10) between the femur and the tibia. Synovial membrane and fluid (11) lubricate; fatty pads (12) pack the joint. Biceps (13) and gastrocnemius (14) muscles are shown.
The knee is a modified hinge joint,
which is capable of bending, straightening, and slight rotation in the bent
|Cross-sectional view of the
knee joint from the side.
Credit: Lucile Packard Children's Hospital
Cartilages of the knee
The articular surfaces of the knee bones are covered with hyaline
cartilage which reduces friction between the bones. In addition the
knee contains two cartilaginous tissues, known as the lateral meniscus and medial meniscus, that are wedged horizontally between
the femur and the tibia. These menisci act as shock absorbers and also give
structural integrity to the knee when it bends or twists. The menisci used to be called semilunar cartilages because of their
half-moon shape. This old terminology often leads to confusion today. It
is important to be clear, especially when discussing knee injuries with
a doctor, that there is a big difference between the menisci and the hyaline
cartilage that covers the ends of the femur and tibia.
The knee joint is partly surrounded by a fibrous capsule lined with synovial
membrane – the largest in the body – which secretes a lubricating
Ligaments of the knee
|Ligaments of the left knee
There are four main ligaments in the knee.
Two of these, the medial and lateral collateral ligaments, lie external
to the knee joint, on the inner and outer side of the knee, respectively.
They act to stabilize the knee's sideways motion. The other two main ligaments
of the knee lie internal to the joint. These are the anterior and posterior cruciate ligaments, so-called
because the cross over each other (the Latin crux means "cross")
as they run diagonally between the femur and the tibia. The cruciate ligaments
control the backward and forward motion of the knee. The anterior cruciate
in particular restrains excessive forward motion of the knee as well as
the inward twisting or rotation of the knee.
associated with the knee
Fluid-filled sacs called bursas are situated
above and below the patella and behind the knee. The quadriceps muscles (which run along the front of the thigh) straighten the knee; the hamstring muscles at the back
of the thigh bend the knee.