Lateral and posterior aspects of right knee
Articular surfaces of femur.
Articular surfaces of tibia.
The knee is a modified
hinge joint, a type of
synovial joint, which is composed of three functional compartments: the patellofemoral articulation, consisting of the
patella, or "kneecap", and the
patellar groove on the front of the
femur through which it slides; and the medial and lateral tibiofemoral articulations linking the femur, or thigh bone, with the
tibia, the main bone of the lower leg.
 The joint is bathed in
synovial fluid which is contained inside the
synovial membrane called the
joint capsule. The
posterolateral corner of the knee is an area that has recently been the subject of renewed scrutiny and research.
The knee is the largest joint and one of the most important joints in the body. It plays an essential role in movement related to carrying the body weight in horizontal (running and walking) and vertical (jumping) directions.
At birth, the
kneecap is just formed from
cartilage, and this will
ossify (change to
bone) between the ages of three and five years. Because it is the largest
sesamoid bone in the human body, the
ossification process takes significantly longer.
The main articular bodies of the femur are its
condyles. These diverge slightly distally and posteriorly, with the lateral condyle being wider in front than at the back while the medial condyle is of more constant width.
 The radius of the condyles' curvature in the
sagittal plane becomes smaller toward the back. This diminishing radius produces a series of
involute midpoints (i.e. located on a spiral). The resulting series of transverse axes permit the sliding and rolling motion in the flexing knee while ensuring the collateral ligaments are sufficiently lax to permit the rotation associated with the curvature of the medial condyle about a vertical axis.
The pair of tibial condyles are separated by the intercondylar eminence
 composed of a lateral and a medial tubercle.
patella also serves an articular body, and its posterior surface is referred to as the trochlea of the knee.
 It is inserted into the thin anterior wall of the joint capsule.
 On its posterior surface is a lateral and a medial articular surface,
 both of which communicate with the
patellar surface which unites the two femoral condyles on the anterior side of the bone's distal end.
The articular capsule has a
synovial and a
fibrous membrane separated by fatty deposits. Anteriorly, the synovial membrane is attached on the margin of the cartilage both on the femur and the tibia, but on the femur, the suprapatellar
bursa or recess extends the joint space proximally.
 The suprapatellar bursa is prevented from being pinched during extension by the
articularis genus muscle.
 Behind, the synovial membrane is attached to the margins of the two femoral condyles which produces two extensions similar to the anterior recess. Between these two extensions, the synovial membrane passes in front of the two cruciate ligaments at the center of the joint, thus forming a pocket direct inward.
bursae surround the knee joint. The largest communicative bursa is the
suprapatellar bursa described above. Four considerably smaller bursae are located on the back of the knee. Two non-communicative bursae are located in front of the patella and below the patellar
tendon, and others are sometimes present.
Cartilage is a thin, elastic
tissue that protects the
bone and makes certain that the
joint surfaces can slide easily over each other. Cartilage ensures supple knee movement. There are two types of joint cartilage in the knees: fibrous cartilage (the
hyaline cartilage. Fibrous cartilage has tensile strength and can resist pressure. Hyaline cartilage covers the surface along which the joints move. Cartilage will wear over the years. Cartilage has a very limited capacity for self-restoration. The newly formed tissue will generally consist of a large part of fibrous cartilage of lesser quality than the original hyaline cartilage. As a result, new cracks and tears will form in the cartilage over time.
articular disks of the knee-joint are called
menisci because they only partly divide the joint space.
 These two disks, the
medial meniscus and the
lateral meniscus, consist of connective tissue with extensive collagen fibers containing cartilage-like cells. Strong fibers run along the menisci from one attachment to the other, while weaker radial fibers are interlaced with the former. The menisci are flattened at the center of the knee joint, fused with the synovial membrane laterally, and can move over the tibial surface.
The menisci serve to protect the ends of the bones from rubbing on each other and to effectively deepen the tibial sockets into which the femur attaches. They also play a role in shock absorption, and may be cracked, or torn, when the knee is forcefully rotated and/or bent.
Anterolateral aspect of right knee.
Anteromedial aspect of right knee
The ligaments surrounding the knee joint offer stability by limiting movements and, together with the menisci and several bursae, protect the articular capsule.
The knee is stabilized by a pair of
cruciate ligaments. The
anterior cruciate ligament (ACL) stretches from the
lateral condyle of femur to the
anterior intercondylar area. The ACL is critically important because it prevents the tibia from being pushed too far anterior relative to the femur. It is often torn during twisting or bending of the knee. The
posterior cruciate ligament (PCL) stretches from
medial condyle of femur to the
posterior intercondylar area. Injury to this ligament is uncommon but can occur as a direct result of forced trauma to the ligament. This ligament prevents posterior displacement of the tibia relative to the femur.
transverse ligament stretches from the
lateral meniscus to the
medial meniscus. It passes in front of the menisci. It is divided into several strips in 10% of cases.
 The two menisci are attached to each other anteriorly by the ligament.
anterior meniscofemoral ligaments stretch from the posterior horn of the lateral meniscus to the medial femoral condyle. They pass posteriorly behind the posterior cruciate ligament. The posterior meniscofemoral ligament is more commonly present (30%); both ligaments are present less often.
meniscotibial ligaments (or "coronary") stretches from inferior edges of the mensici to the periphery of the tibial plateaus.
patellar ligament connects the
patella to the
tuberosity of the tibia. It is also occasionally called the patellar tendon because there is no definite separation between the
quadriceps tendon (which surrounds the patella) and the area connecting the patella to the tibia.
 This very strong ligament helps give the patella its mechanical leverage
 and also functions as a cap for the condyles of the femur. Laterally and medially to the patellar ligament the
lateral and medial retinacula connect fibers from the
vasti lateralis and
medialis muscles to the tibia. Some fibers from the
iliotibial tract radiate into the lateral retinaculum and the medial retinaculum receives some transverse fibers arising on the medial femoral epicondyle.
medial collateral ligament (MCL a.k.a. "tibial") stretches from the
medial epicondyle of the femur to the
medial tibial condyle. It is composed of three groups of fibers, one stretching between the two bones, and two fused with the medial meniscus. The MCL is partly covered by the
pes anserinus and the tendon of the
semimembranosus passes under it.
 It protects the medial side of the knee from being bent open by a stress applied to the lateral side of the knee (a
valgus force). The
lateral collateral ligament (LCL a.k.a. "fibular") stretches from the
lateral epicondyle of the femur to the
head of fibula. It is separate from both the joint capsule and the lateral meniscus.
 It protects the lateral side from an inside bending force (a
varus force). The
anterolateral ligament (ALL) is situated in front of the LCL.
Lastly, there are two ligaments on the dorsal side of the knee. The
oblique popliteal ligament is a radiation of the tendon of the
semimembranosus on the medial side, from where it is direct laterally and proximally. The
arcuate popliteal ligament originates on the apex of the head of the fibula to stretch proximally, crosses the tendon of the
popliteus muscle, and passes into the capsule.