human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

thigh In human anatomy, the thigh is the area between the hip ( pelvis) and the knee. Anatomically, it is part of the lower limb. The single bone in the thigh is called the femur. This bone is very thick and strong (due to the high proportion of ...

leg A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ca ...

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having ...

flexion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

extension Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * E ...

osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the w ...

fibular collateral ligament The lateral collateral ligament (LCL, long external lateral ligament or fibular collateral ligament) is a ligament located on the lateral (outer) side of the knee, and thus belongs to the extrinsic knee ligaments and posterolateral corner of the k ...

hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having ...

synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulat ...

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

patellar groove The intercondylar fossa of femur (intercondyloid fossa of femur, intercondylar notch of femur) is a deep notch between the rear surfaces of the medial and lateral epicondyle of the femur, two protrusions on the distal end of the femur (thigh bone ...

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.posterolateral corner Posterolateral corner injuries (PLC injuries) of the knee are injuries to a complex area formed by the interaction of multiple structures. Injuries to the posterolateral corner can be debilitating to the person and require recognition and treatment ...

kneecap The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

ossify Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...

sesamoid bone In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for ' sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be pres ...

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

condyle A condyle (;Entry "condyle"
in
The radius of the condyles' curvature in the

sagittal plane The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...

becomes smaller toward the back. This diminishing radius produces a series of

involute In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the locus of a point on a piece of taut string as the string is either unwrapped from or ...

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. The

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

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.

Articular capsule

The articular capsule has a synovial and a

fibrous Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or Fiber#Artificial fibers, artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The stronge ...

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, it communicates with the suprapatellar

bursa ( grc-gre, Προῦσα, Proûsa, Latin: Prusa, ota, بورسه, Arabic:بورصة) is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the ...

or recess and extends the joint space proximally. The suprapatellar bursa is prevented from being pinched during extension by the

articularis genus muscle The articularis genus (also known as the subcrureus muscle) is a small skeletal muscle located anteriorly on the thigh just above the knee. Structure It arises from the anterior surface of the lower part of the body of the femur, deep to the v ...

. Behind, the synovial membrane is attached to the margins of the two femoral condyles which produces two extensions (semimembranosus bursa under medial head of the gastrocnemius and popliteal bursa under lateral head of the gastrocnemius ) similar to the suprapatellar bursa. 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. Synovium lining the capsule and its bursae. The synovium also lines infrapatellar fat pad, the fat pad that lies below the ligamentum patellae. Synovium projecting into the fat pad as two foldings.

Bursae

Numerous bursae surround the knee joint. The largest communicative bursa is the

suprapatellar bursa The knee bursae are the fluid-filled sacs and synovial pockets that surround and sometimes communicate with the knee joint cavity. The bursae are thin-walled, and filled with synovial fluid. They represent the weak point of the joint, but also pr ...

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 A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

, and others are sometimes present.

Cartilage

Cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

is a thin, elastic tissue that protects the

bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...

and makes certain that the

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

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

meniscus Meniscus may refer to: *Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity *Meniscus (liquid) The meniscus (plural: ''menisci'', from the Greek for "crescent") is the curve in the upper surface ...

) and

hyaline cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...

. Fibrous cartilage has tensile strength and can resist pressure. Hyaline cartilage covers the surface along which the joints move. Collagen fibres within the articular cartilage have been described by Benninghoff as arising from the subchondral bone in a radial manner, building so called Gothic arches. On the surface of the cartilage these fibres appear in a tangential orientation and increase the abrasion resistance. There are no blood vessels inside of the hyaline cartilage, the alimentation is performed per diffusion. Synovial fluid and the subchondral bone marrow serve both as nutrition sources for the hyaline cartilage. Lack of at least one source induces a degeneration. 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.

Menisci

The

articular disk The articular disk (or disc) is a thin, oval plate of fibrocartilage present in several joints which separates synovial cavities. This separation of the cavity space allows for separate movements to occur in each space. The presence of an articula ...

s of the knee-joint are called menisci because they only partly divide the joint space. These two disks, the

medial meniscus The medial meniscus is a fibrocartilage semicircular band that spans the knee joint medially, located between the medial condyle of the femur and the medial condyle of the tibia.Platzer (2004), p 208 It is also referred to as the internal semilun ...

and the

lateral meniscus The lateral meniscus (external semilunar fibrocartilage) is a fibrocartilaginous band that spans the lateral side of the interior of the knee joint. It is one of two menisci of the knee, the other being the medial meniscus. It is nearly circula ...

, 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 upper and lower surfaces of the menisci are free. Each meniscus have anterior and posterior horns that meet in the intercondylar area of the tibia. Medial meniscus is bigger, less curved, and thinner. Its posterior horn is thicker (14mm) than the anterior horn (6mm). The lateral meniscus is smaller, more curved (nearly circular), and has more uniform thickness than medial meniscus (10mm). The lateral meniscus is less attached to the joint capsule, because its posterolateral surface is grooved by the

popliteus The popliteus muscle in the leg is used for unlocking the knees when walking, by laterally rotating the femur on the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground). In open chain movements ...

tendon, separating the meniscus from the capsule. The popliteus tendon is not attached to the lateral meniscus.

Ligaments

The ligaments surrounding the knee joint offer stability by limiting movements and, together with the menisci and several bursae, protect the articular capsule.

Intracapsular

The knee is stabilized by a pair of

cruciate ligament Cruciate ligaments (also cruciform ligaments) are pairs of ligaments arranged like a letter X. They occur in several joints of the body, such as the knee joint and the atlanto-axial joint. In a fashion similar to the cords in a toy Jacob's la ...

s. These ligaments are both extrasynovial, intracapsular ligaments. The

anterior cruciate ligament The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. The two ligaments are also called "cruciform" ligaments, as they are arranged in a crossed formation ...

(ACL) stretches from the

lateral condyle of femur The lateral condyle is one of the two projections on the lower extremity of the femur. The other one is the medial condyle. The lateral condyle is the more prominent and is broader both in its front-to-back and transverse diameters. Clinical sign ...

to the anterior

intercondylar area The intercondylar area is the separation between the medial and lateral condyle on the upper extremity of the tibia. The anterior and posterior cruciate ligaments and the menisci attach to the intercondylar area. The intercondyloid eminence ...

. 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 The posterior cruciate ligament (PCL) is a ligament in each knee of humans and various other animals. It works as a counterpart to the anterior cruciate ligament (ACL). It connects the posterior intercondylar area of the tibia to the medial cond ...

(PCL) stretches from

medial condyle of femur The medial condyle is one of the two projections on the lower extremity of femur, the other being the lateral condyle. The medial condyle is larger than the lateral (outer) condyle due to more weight bearing caused by the centre of mass being med ...

to the posterior intercondylar area. This ligament prevents posterior displacement of the tibia relative to the femur. Injury to this ligament is uncommon but can occur as a direct result of forced trauma to the ligament. The

transverse ligament A transverse ligament is a ligament on a transverse plane, orthogonal to the anteroposterior or oral-aboral axiscan of the body. In human anatomy, examples are: * Flexor retinaculum of the hand or transverse carpal ligament (ligamentum carpi tra ...

stretches from the

lateral meniscus The lateral meniscus (external semilunar fibrocartilage) is a fibrocartilaginous band that spans the lateral side of the interior of the knee joint. It is one of two menisci of the knee, the other being the medial meniscus. It is nearly circula ...

to the

medial meniscus The medial meniscus is a fibrocartilage semicircular band that spans the knee joint medially, located between the medial condyle of the femur and the medial condyle of the tibia.Platzer (2004), p 208 It is also referred to as the internal semilun ...

. 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., p. 200 The posterior (of Wrisberg) and

anterior meniscofemoral ligament The anterior meniscofemoral ligament (ligament of Humphry) is a small fibrous band of the knee joint. It arises from the posterior horn of the lateral meniscus and passes superiorly and medially in front of the posterior cruciate ligament to atta ...

s (of Humphrey) stretch from the posterior horn of the lateral meniscus to the medial femoral condyle. They pass anterior and posterior to the posterior cruciate ligament respectively. The meniscotibial ligaments (or "coronary") stretches from inferior edges of the menisci to the periphery of the tibial plateaus.

Extracapsular

The

patellar ligament The patellar tendon is the distal portion of the common tendon of the quadriceps femoris, which is continued from the patella to the tibial tuberosity. It is also sometimes called the patellar ligament as it forms a bone to bone connection when ...

connects the

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

to the

tuberosity of the tibia The tuberosity of the tibia or tibial tuberosity or tibial tubercle is an elevation on the proximal, anterior aspect of the tibia, just below where the anterior surfaces of the lateral and medial tibial condyles end. Structure The tuberosity o ...

. It is also occasionally called the patellar tendon because there is no definite separation between the

quadriceps tendon In human anatomy, the quadriceps tendon works with the quadriceps muscle to extend the leg. All four parts of the quadriceps muscle attach to the shin via the patella (knee cap), where the quadriceps tendon becomes the patellar ligament. It att ...

(which surrounds the patella) and the area connecting the patella to the tibia. This very strong ligament helps give the patella its mechanical leverage , p. 194 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 The iliotibial tract or iliotibial band (ITB; also known as Maissiat's band or the IT band) is a longitudinal fibrous reinforcement of the fascia lata. The action of the muscles associated with the ITB ( tensor fasciae latae and some fibers of ...

radiate into the lateral retinaculum and the medial retinaculum receives some transverse fibers arising on the medial femoral epicondyle. The

medial collateral ligament The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist out ...

(MCL a.k.a. "tibial") stretches from the

medial epicondyle of the femur The medial epicondyle of the femur is an epicondyle, a bony protrusion, located on the medial side of the femur at its distal end. Located above the medial condyle, it bears an elevation, the adductor tubercle,Platzer (2004), p 192 which serves ...

to the

medial tibial condyle The medial condyle is the medial (or inner) portion of the upper extremity of tibia. It is the site of insertion for the semimembranosus muscle. See also * Lateral condyle of tibia * Medial collateral ligament The medial collateral ligament ...

. 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 The semimembranosus muscle () is the most medial of the three hamstring muscles in the thigh. It is so named because it has a flat tendon of origin. It lies posteromedially in the thigh, deep to the semitendinosus muscle. It extends the hip joint ...

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 The lateral epicondyle of the femur, smaller and less prominent than the medial epicondyle, gives attachment to the fibular collateral ligament of the knee-joint. Directly below it is a small depression from which a smooth well-marked groove curv ...

to the

head of fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity is ...

. 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 The anterolateral ligament (ALL) is a ligament on the lateral aspect of the human knee, anterior to the fibular collateral ligament. Perhaps the earliest account of the ALL was written by French surgeon Paul Segond in 1879, in which he describe ...

(ALL) is situated in front of the LCL. Lastly, there are two ligaments on the dorsal side of the knee. The

oblique popliteal ligament The oblique popliteal ligament (posterior ligament) is a broad, flat, fibrous band on the posterior knee representing an expansion of the tendon of the semimembranosus muscle. It attaches onto the intercondylar fossa and lateral condyle of the f ...

is a radiation of the tendon of the

semimembranosus The semimembranosus muscle () is the most medial of the three hamstring muscles in the thigh. It is so named because it has a flat tendon of origin. It lies posteromedially in the thigh, deep to the semitendinosus muscle. It extends the hip joint ...

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 The popliteus muscle in the leg is used for unlocking the knees when walking, by laterally rotating the femur on the tibia during the closed chain portion of the bipedal gait cycle, gait cycle (one with the foot in contact with the ground). In op ...

, and passes into the capsule.

Muscles

The muscles responsible for the movement of the knee joint belong to either the anterior, medial or posterior compartment of the thigh. The extensors generally belong to the anterior compartment and the flexors to the posterior. The two exceptions to this is gracilis, a flexor, which belongs to the medial compartment and sartorius, a flexor, in the anterior compartment.

Extensors

Flexors

Posterior compartment Medial compartment:

Blood supply

The

femoral artery The femoral artery is a large artery in the thigh and the main arterial supply to the thigh and leg. The femoral artery gives off the deep femoral artery or profunda femoris artery and descends along the anteromedial part of the thigh in the fem ...

and the

popliteal artery The popliteal artery is a deeply placed continuation of the femoral artery opening in the distal portion of the adductor magnus muscle. It courses through the popliteal fossa and ends at the lower border of the popliteus muscle, where it branches ...

help form the arterial network or

plexus In neuroanatomy, a plexus (from the Latin term for "braid") is a branching network of vessels or nerves. The vessels may be blood vessels (veins, capillaries) or lymphatic vessels. The nerves are typically axons outside the central nervous syste ...

, surrounding the knee joint. There are six main branches: two superior genicular arteries, two

inferior genicular arteries The inferior genicular arteries (inferior articular arteries), two in number, arise from the popliteal beneath the gastrocnemius. On the inside of the knee, is the medial inferior genicular artery, and on the outer side is the lateral inferior g ...

, the

descending genicular artery The descending genicular artery (highest genicular artery) arises from the femoral artery just before it passes through the adductor hiatus. It immediately divides into two branches: * a saphenous branch, which classically joins with the medial i ...

and the recurrent branch of anterior tibial artery. The medial genicular arteries penetrate the knee joint.

Function

The knee permits

flexion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

and

extension Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * E ...

about a virtual transverse axis, as well as a slight medial and lateral rotation about the axis of the lower leg in the flexed position. The knee joint is called "mobile" because the femur and

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

meniscus Meniscus may refer to: *Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity *Meniscus (liquid) The meniscus (plural: ''menisci'', from the Greek for "crescent") is the curve in the upper surface ...

move over the tibia during rotation, while the femur rolls and glides over both menisci during extension-flexion. The center of the transverse axis of the extension/flexion movements is located where both collateral ligaments and both cruciate ligaments intersect. This center moves upward and backward during flexion, while the distance between the center and the articular surfaces of the femur changes dynamically with the decreasing curvature of the femoral condyles. The total range of motion is dependent on several parameters such as soft-tissue restraints, active insufficiency, and hamstring tightness.

Extended position

With the knee extended, both the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

and

medial collateral ligament The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist out ...

s, as well as the anterior part of the

anterior cruciate ligament The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. The two ligaments are also called "cruciform" ligaments, as they are arranged in a crossed formation ...

, are taut. During extension, the femoral condyles glide and roll into a position which causes the complete unfolding of the

tibial collateral ligament The medial collateral ligament (MCL), or tibial collateral ligament (TCL), is one of the four major ligaments of the knee. It is on the medial (inner) side of the knee joint in humans and other primates. Its primary function is to resist outw ...

. During the last 10° of extension, an obligatory terminal rotation is triggered in which the knee is rotated medially 5°. The final rotation is produced by a lateral rotation of the tibia in the non-weight-bearing leg, and by a medial rotation of the femur in the weight-bearing leg. This terminal rotation is made possible by the shape of the medial femoral condyle, assisted by contraction of the popliteus muscle and the

iliotibial tract The iliotibial tract or iliotibial band (ITB; also known as Maissiat's band or the IT band) is a longitudinal fibrous reinforcement of the fascia lata. The action of the muscles associated with the ITB ( tensor fasciae latae and some fibers of ...

and is caused by the stretching of the anterior cruciate ligament. Both cruciate ligaments are slightly unwinded and both lateral ligaments become taut.

Flexed position

In the flexed position, the collateral ligaments are relaxed while the cruciate ligaments are taut. Rotation is controlled by the twisted cruciate ligaments; the two ligaments get twisted around each other during medial rotation of the tibia—which reduces the amount of rotation possible—while they become unwound during lateral rotation of the tibia. Because of the oblique position of the cruciate ligaments, at least a part of one of them is always tense and these ligaments control the joint as the collateral ligaments are relaxed. Furthermore, the dorsal fibers of the tibial collateral ligament become tensed during extreme medial rotation and the ligament also reduces the lateral rotation to 45–60°.

Clinical significance

Knee pain Knee pain is pain in or around the knee. The knee joint consists of an articulation between four bones: the femur, tibia, fibula and patella. There are four compartments to the knee. These are the medial and lateral tibiofemoral compartments, the ...

is caused by trauma, misalignment, and degeneration as well as by conditions like

arthritis Arthritis is a term often used to mean any disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, swelling, and decreased range of motion of the affected joints. In som ...

. The most common knee disorder is generally known as

patellofemoral syndrome Patellofemoral pain syndrome (PFPS; not to be confused with jumper's knee) is knee pain as a result of problems between the kneecap and the femur. The pain is generally in the front of the knee and comes on gradually. Pain may worsen with sitti ...

. The majority of minor cases of knee pain can be treated at home with rest and ice but more serious injuries do require

surgical Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...

care. One form of patellofemoral syndrome involves a tissue-related problem that creates pressure and irritation in the knee between the patella and the trochlea ( patellar compression syndrome), which causes pain. The second major class of knee disorder involves a tear, slippage, or dislocation that impairs the structural ability of the knee to balance the leg ( patellofemoral instability syndrome). Patellofemoral instability syndrome may cause either pain, a sense of poor balance, or both.

Prepatellar bursitis Prepatellar bursitis is an inflammation of the prepatellar bursa at the front of the knee. It is marked by swelling at the knee, which can be tender to the touch and which generally does not restrict the knee's range of motion. It can be extreme ...

also known as ''housemaid's knee'' is painful inflammation of the

prepatellar bursa The prepatellar bursa is a frontal bursa of the knee joint. It is a superficial bursa with a thin synovial lining located between the skin and the patella. Pathology Prepatellar bursitis, also known as housemaid's knee, is a common cause of s ...

(a frontal knee bursa) often brought about by occupational activity such as roofing. Age also contributes to disorders of the knee. Particularly in older people, knee pain frequently arises due to osteoarthritis. In addition, weakening of tissues around the knee may contribute to the problem. Patellofemoral instability may relate to hip abnormalities or to tightness of surrounding ligaments.

Cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

lesions can be caused by: *Accidents (fractures) *Injuries *The removal of a

meniscus Meniscus may refer to: *Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity *Meniscus (liquid) The meniscus (plural: ''menisci'', from the Greek for "crescent") is the curve in the upper surface ...

*

Anterior cruciate ligament The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. The two ligaments are also called "cruciform" ligaments, as they are arranged in a crossed formation ...

injury *

Posterior cruciate ligament The posterior cruciate ligament (PCL) is a ligament in each knee of humans and various other animals. It works as a counterpart to the anterior cruciate ligament (ACL). It connects the posterior intercondylar area of the tibia to the medial cond ...

injury *

Posterolateral corner Posterolateral corner injuries (PLC injuries) of the knee are injuries to a complex area formed by the interaction of multiple structures. Injuries to the posterolateral corner can be debilitating to the person and require recognition and treatment ...

injury *

Medial knee injuries Medial knee injuries (those to the inside of the knee) are the most common type of knee injury. The medial ligament complex of the knee is composed of the superficial medial collateral ligament (sMCL), deep medial collateral ligament (dMCL), and ...

*Considerable strain on the knee. Any kind of work during which the knees undergo heavy stress may also be detrimental to cartilage. This is especially the case in professions in which people frequently have to walk, lift, or squat. Other causes of pain may be excessive on, and wear off, the knees, in combination with such things as muscle weakness and

overweight Being overweight or fat is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary. , excess weight reached epidemic proportions globally, with mo ...

. Common complaints: *A painful, blocked, locked or swollen knee. *Sufferers sometimes feel as if their knees are about to give way, or may feel uncertain about their movement.

Overall fitness and knee injury

Physical fitness is related integrally to the development of knee problems. The same activity such as climbing stairs may cause pain from patellofemoral compression for someone who is physically unfit, but not for someone else (or even for that person at a different time). Obesity is another major contributor to knee pain. For instance, a 30-year-old woman who weighed 120lb at age 18 years, before her three pregnancies, and now weighs 285lb, had added 660lb of force across her patellofemoral joint with each step.

Common injuries due to physical activity

In sports that place great pressure on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages. Some of the most common knee injuries are those to the medial side:

medial knee injuries Medial knee injuries (those to the inside of the knee) are the most common type of knee injury. The medial ligament complex of the knee is composed of the superficial medial collateral ligament (sMCL), deep medial collateral ligament (dMCL), and ...

.

Anterior cruciate ligament injury

The

anterior cruciate ligament The anterior cruciate ligament (ACL) is one of a pair of cruciate ligaments (the other being the posterior cruciate ligament) in the human knee. The two ligaments are also called "cruciform" ligaments, as they are arranged in a crossed formation ...

is the most commonly injured ligament of the knee. The injury is common during sports. Twisting of the knee is a common cause of over-stretching or tearing the ACL. When the ACL is injured a popping sound may be heard, and the leg may suddenly give out. Besides swelling and pain, walking may be painful and the knee will feel unstable. Minor tears of the anterior cruciate ligament may heal over time, but a torn ACL requires surgery. After surgery, recovery is prolonged and low impact exercises are recommended to strengthen the joint.

Torn meniscus injury

The menisci act as shock absorbers and separate the two ends of bone in the knee

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

. There are two menisci in the knee, the medial (inner) and the lateral (outer). When there is torn cartilage, it means that the meniscus has been injured. Meniscus tears occur during sports often when the knee is twisted. Menisci injury may be innocuous and one may be able to walk after a tear, but soon swelling and pain set in. Sometimes the knee will lock while bending. Pain often occurs when one squats. Small meniscus tears are treated conservatively but most large tears require surgery.

Fractures

Knee fractures are rare but do occur, especially as a result of a road accident. Knee fractures include a

patella fracture A patella fracture is a break of the kneecap. Symptoms include pain, swelling, and bruising to the front of the knee. A person may also be unable to walk. Complications may include injury to the tibia, femur, or knee ligaments. It typically res ...

, and a type of

avulsion fracture An avulsion fracture is a bone fracture which occurs when a fragment of bone tears away from the main mass of bone as a result of physical trauma. This can occur at the ligament by the application of forces external to the body (such as a fall ...

called a

Segond fracture The Segond fracture is a type of avulsion fracture (soft tissue structures pulling off fragments of their bony attachment) from the lateral tibial plateau of the knee, immediately below the articular surface of the tibia (see photo). Clinical ...

. There is usually immediate pain and swelling, and a difficulty or inability to stand on the leg. The muscles go into

spasm A spasm is a sudden involuntary contraction of a muscle, a group of muscles, or a hollow organ such as the bladder. A spasmodic muscle contraction may be caused by many medical conditions, including dystonia. Most commonly, it is a muscle c ...

and even the slightest movements are painful.

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s can easily confirm the injury and surgery will depend on the degree of displacement and type of fracture.

Ruptured tendon

Tendons usually attach muscle to bone. In the knee the quadriceps and patellar tendon can sometimes tear. The injuries to these tendons occur when there is forceful

contraction Contraction may refer to: Linguistics * Contraction (grammar), a shortened word * Poetic contraction, omission of letters for poetic reasons * Elision, omission of sounds ** Syncope (phonology), omission of sounds in a word * Synalepha, merged ...

of the knee. If the tendon is completely torn, bending or extending the leg is impossible. A completely torn tendon requires surgery but a partially torn tendon can be treated with leg immobilization followed by physical therapy.

Overuse

Overuse injuries of the knee include

tendonitis Tendinopathy, a type of tendon disorder that results in pain, swelling, and impaired function. The pain is typically worse with movement. It most commonly occurs around the shoulder ( rotator cuff tendinitis, biceps tendinitis), elbow ( tennis e ...

, bursitis, muscle strains, and

iliotibial band syndrome Iliotibial band syndrome (ITBS) is the second most common knee injury, and is caused by inflammation located on the lateral aspect of the knee due to friction between the iliotibial band and the lateral epicondyle of the femur. Pain is felt most ...

. These injuries often develop slowly over weeks or months. Activities that induce pain usually delay healing. Rest, ice and compression do help in most cases. Once the swelling has diminished, heat packs can increase blood supply and promote healing. Most overuse injuries subside with time but can flare up if the activities are quickly resumed. Individuals may reduce the chances of overuse injuries by warming up prior to exercise, by limiting high impact activities and keep their weight under control.

Varus or valgus deformity

There are two disorders relating to an abnormal angle in the

coronal plane The coronal plane (also known as the frontal plane) is an anatomical plane that divides the body into dorsal and ventral sections. It is perpendicular to the sagittal and transverse planes. Details The coronal plane is an example of a longitud ...

at the level of the knee: *''

Genu valgum Genu, a Latin word for "knee," may refer to: * Genu of internal capsule * Genu of the corpus callosum * Genu recurvatum * Genu valgum * Genu varum Genu varum (also called bow-leggedness, bandiness, bandy-leg, and tibia vara) is a varus def ...

'' is a

valgus deformity A valgus deformity is a condition in which the bone segment distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, des ...

in which the tibia is turned outward in relation to the femur, resulting in a knock-kneed appearance. *''

Genu varum Genu varum (also called bow-leggedness, bandiness, bandy-leg, and tibia vara) is a varus deformity marked by (outward) bowing at the knee, which means that the lower leg is angled inward ( medially) in relation to the thigh's axis, giving th ...

'' is a

varus deformity Varus may refer to: * Var River or Varus, a river in France * Stura di Lanzo or Varus, a river in Italy * Varus deformity, a medical term for the inward angulation of the distal segment of a bone or joint ** Coxa vara, affecting the hip ** Genu varu ...

in which the tibia is turned inward in relation to the femur, resulting in a bowlegged deformity. The degree of varus or valgus deformity can be quantified by the ''hip-knee-ankle angle'', which is an angle between the femoral mechanical axis and the center of the

ankle joint The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular join ...

. It is normally between 1.0° and 1.5° of varus in adults. Normal ranges are different in children. File:Hip-knee-ankle angle by age.png, ''Hip-knee-ankle angle'' by age, with 95%

prediction interval In statistical inference, specifically predictive inference, a prediction interval is an estimate of an interval in which a future observation will fall, with a certain probability, given what has already been observed. Prediction intervals are ...

.

Surgical interventions

Before the advent of

arthroscopy Arthroscopy (also called arthroscopic or keyhole surgery) is a minimally invasive surgical procedure on a joint in which an examination and sometimes treatment of damage is performed using an arthroscope, an endoscope that is inserted into the ...

and

arthroscopic surgery Arthroscopy (also called arthroscopic or keyhole surgery) is a minimally invasive surgical procedure on a joint in which an examination and sometimes treatment of damage is performed using an arthroscope, an endoscope that is inserted into the ...

, patients having surgery for a torn ACL required at least nine months of rehabilitation, having initially spent several weeks in a full-length plaster cast. With current techniques, such patients may be walking without crutches in two weeks, and playing some sports in a few months. In addition to developing new surgical procedures, ongoing research is looking into underlying problems which may increase the likelihood of an athlete suffering a severe knee injury. These findings may lead to effective preventive measures, especially in female athletes, who have been shown to be especially vulnerable to ACL tears from relatively minor trauma.

Articular cartilage repair Articular cartilage repair treatment is focused on the restoration of the surface of an articular joint's hyaline cartilage. Over the last few decades, surgeons and researchers have made progress in elaborating surgical cartilage repair intervent ...

treatment: *

Arthroscopic Arthroscopy (also called arthroscopic or keyhole surgery) is a minimally invasive surgical procedure on a joint in which an examination and sometimes treatment of damage is performed using an arthroscope, an endoscope that is inserted into the jo ...

debridement Debridement is the medical removal of dead, damaged, or infected tissue to improve the healing potential of the remaining healthy tissue. Removal may be surgical, mechanical, chemical, autolytic (self-digestion), and by maggot therapy. In p ...

of the knee (

arthroscopic lavage Arthroscopic lavage is the washing out or cleaning out the contents (blood, fluid or loose debris) inside a joint space. Lavage is a general term referring to the therapeutic washing, cleaning or rinsing. Medical uses Excessive growth of irritated ...

) * Mosaïc-plasty *Microfracture (Ice-picking) *

Autologous chondrocyte implantation Autologous chondrocyte implantation (ACI, ATC code ) is a biomedical treatment that repairs damages in articular cartilage. ACI provides pain relief while at the same time slowing down the progression or considerably delaying partial or total join ...

*Osteochondral

Autograft Autotransplantation is the transplantation of organs, tissues, or even particular proteins from one part of the body to another in the same person ('' auto-'' meaning "self" in Greek). The autologous tissue (also called autogenous, autogene ...

and Allografts *

PLC Reconstruction Posterolateral corner injuries (PLC injuries) of the knee are injuries to a complex area formed by the interaction of multiple structures. Injuries to the posterolateral corner can be debilitating to the person and require recognition and treatment ...

Imaging

MRI

Both anterior cruciate ligament (ACL) and posterior cruciate ligaments (PCL) are hypointense on both T1 and T2 weighted images of MRI. However, some high signal striations are often seen at the distal part of the ACL, making ACL higher intensity than PCL on MRI scans. File:Knee MRI PD TSE FS Sagittal.jpg, Knee MRI (PD TSE FS sagittal) File:Knee MRI T1 TSE Sagittal.jpg, Knee MRI (T1 TSE sagittal) File:Knee MRI tse fs sag 320-R.ogg, Knee MRI (sagittal TSE FS) File:Knee MRI t1 tse sag-R.ogg, Knee MRI (sagittal T1 TSE) File:Knee MRI t2 tse fs cor 320.ogg, Knee MRI (coronal T2 TSE FS) File:Knee MRI pd tse fs tra 320.ogg, Knee MRI (traverse PD TSE FS) File:Knee MRI osteoarthritis pd tse fs sag 320.ogg, Knee MRI osteoarthritis (sagittal TSE FS) File:Knee MRI osteoarthritis t1 tse sag R.ogg, Knee MRI osteoarthritis (sagittal T1 TSE) File:Knee MRI osteoarthritis t2 tse fs cor 320.ogg, Knee MRI osteoarthritis (coronal T2 TSE FS) File:Knee MRI osteoarthritis pd tse fs tra 320.ogg, Knee MRI osteoarthritis (traverse PD TSE FS) File:Real-time MRI - Knee (central).ogv,

Real-time MRI Real-time magnetic resonance imaging (RT-MRI) refers to the continuous monitoring ("filming") of moving objects in real time. Because MRI is based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or ...

- Knee File:Knie ct.gif, Knee MR

X-ray

File:Knee plain X-ray.jpg, Knee X-ray File:Knee plain X-ray weight bearing.jpg, Knee X-ray (weight bearing) File:Knee plain X-ray weight bearing flexion.jpg, Knee X-ray (weight bearing, flexion)

Illustrations

File:Legamenti crociati.jpg, Cruciate ligaments File:Gray348.png, Left knee-joint from behind, showing interior ligaments. File:Gray351.png, Capsule of right knee-joint (distended). Lateral aspect. File:Knee skeleton lateral anterior views.svg, Anterior and lateral view of knee. File:Slide2CAC.JPG, Anterior view of knee.

Other animals

In humans, the term "knee" refers to the joints between the femur, tibia, and patella, in the leg. In

quadruped Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...

s such as dogs, horses, and mice, the homologous joints between the femur, tibia, and patella, in the

hind leg A hindlimb or back limb is one of the paired articulated appendages (limbs) attached on the caudal ( posterior) end of a terrestrial tetrapod vertebrate's torso.http://www.merriam-webster.com/medical/hind%20limb, Merriam Webster Dictionary-Hindl ...

, are known as the

stifle joint The stifle joint (often simply stifle) is a complex joint in the hind limbs of quadruped mammals such as the sheep, horse or dog. It is the equivalent of the human knee and is often the largest synovial joint in the animal's body. The stifle joint ...

. Also in quadrupeds, particularly horses,

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, cam ...

s, and elephants, the layman's term "knee" also commonly refers to the forward-facing joint in the

foreleg A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead ...

, the

carpus In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

, which is homologous to the human

wrist In human anatomy, the wrist is variously defined as (1) the Carpal bones, carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known ...

. In

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

, the "knee" refers to the joints between the femur and tibiotarsus, and also the patella (when present). The layman's term "knee" may also refer to the (lower and often more visible due to not being covered by feathers) joint between the tibiotarsus and tarsometatarsus, which is homologous to the human

ankle The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular join ...

. In insects and other animals, the term knee widely refers to any

hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having ...

.

References

External links

*
Animation of bones and ligament in the kneeMRI anatomy of a normal knee
{{Authority control Articles containing video clips *Knee

Structure

Articular bodies

Articular capsule

Bursae

Cartilage

Menisci

Ligaments

Intracapsular

Extracapsular

Muscles

Extensors

Flexors

Blood supply

Function

Extended position

Flexed position

Clinical significance

Overall fitness and knee injury

Common injuries due to physical activity

Anterior cruciate ligament injury

Torn meniscus injury

Fractures

Ruptured tendon

Overuse

Varus or valgus deformity

Surgical interventions

Imaging

MRI

X-ray

Illustrations

Other animals

See also

References

External links