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Anatomy of the knee

The anatomy of the knee joint is the most complex of the organism from the anatomical and functional point of view, because it must be stable and at the same time allow a good mobility.

Bones and Joints

Composed of three bones: femur, tibia and patella, it would be more convenient to divide it into two joints:

  • Femoro-patellar (between the anterior patella area and the anterior-distal part of the femur)
  • Femoral-tibial (between the distal epiphysis of the femur and the proximal epiphysis of the tibia)

The distal epiphysis of the femur is formed by two asymmetrical condyles of oval form, posteriorly separated by a clearly visible intercondylar fossa. 
In the front, the condyles are united between them by the same area that is a unique articular surface. 
On the sides, there are two bony protrusions, the  epicondyles,  so called because they are above the femoral condyles. 
The condyles are articular surfaces of the femur, while the epicondyles are not. 
The tibia is characterized by a wider head, and there is also the turbulence of the tibia which can easily be found touching the front of the leg. 
In the upper part there is the tibial tuberosity which is the insertion of the patellar ligament.
The patella is located in the lower tendon of the femoral quadriceps. 
This tendon has an upper and an inferior part called the patellar ligament that inserts into the tibial tuberosity. 
The patella is shaped like a chestnut, in the outer area it is wrinkled with vertical striae, while the inner surface is smooth and covered by articular cartilage . 
The patella is included in the joint capsule.
The femur and the tibia form a gingivium, that is, a joint in which the only possible movement is flexion-extension in the sagittal plane; with the folded knee a rotational movement can also be made because the tibial joint surface is not exactly complementary to that of the femur and does not have an important concavity that blocks some movements.
The articular surfaces of the femur are the condyles, which have a convex shape. 
The tibia articulates with the upper part of the bone with two condyles; the inner ring is concave, the outer ring is convex. 
Read also: Cartilage injury


The menisci are like two “C” shaped fibro-cartilage bearings 
inserted between the two bones. 
The thickness of these two structures is greater on the outside than on the medial side. 
The external meniscus has a nearly closed circular shape and adheres to the joint capsule for almost its entire length except for a small area in which the tendon of the popliteal muscle runs. 
The meniscus horns have about the same volume. 
The medial meniscus adheres to the capsule in all its extension and has different horns between them; the anterior one is narrower and smaller than the posterior one. 
The functions of the meniscus are:

  • Absorb shocks.
  • Transmit body weight over tibial plateau area; if it did not exist, the weight would be supported by an area of ​​smaller amplitude, consequently causing a greater wear of the knee.
  • Spread the synovial fluid in order to make the movement more fluid because it decreases friction.
  • Improve the congruence of the joints between the tibia and the femur.
  • Limit the rotation of the tibia.

The external meniscus gives mobility to the knee, while the inner one makes the joint more stable. 
In flexion, the external meniscus is pulled back by the popliteal muscle and the internal muscle moves less, by the action of the semimembranosus muscle. 
In the extension movement, the meniscus enters its anatomical position moved by the ligaments that attach the menisci to the kneecap. 
The patella is a sesamoid bone between the femur and the patellar tendon, serves to protect the knee joint, to centralize the forces exerted and to facilitate the extension of the leg. 
All the joint surfaces of these bones are covered by smooth cartilage which serves to reduce friction during movement and make joint movement smooth and painless.
During movement, the cartilage is collected and the synovial fluid is pushed towards the capsule, instead when the joint is no longer under load or to, the cartilaginous tissue reabsorbs the synovium, acting like a sponge. 
See also: meniscus injury

The knee ligaments

Anterior view of the knee with transparent patellar tendon 
© axel kock – Fotolia.com

View to nterior

The knee has strong ligaments that are fundamental for biomechanics and physiology. 
The cruciate ligaments have a role of “Central Pivot”, that is, the pivot in which it moves the joint. 
The anterior cruciate ligament (ACL) at the level of the tibia inserts before the tibial spine and ends at the medial aspect of the external condyle. 
ACL is not very vascularized and is defined as fragile because it is subject to injury more often than LCP. 
It plays a key role in keeping the joint stable by limiting internal rotation and hyperextension. During flexion, it reduces forward displacement of the tibia from the femoral condyles.
The posterior cruciate ligament originates in the intercondylar tibial cavity and is inserted into the medial condyle of the femur on the inner side, has a good perfusion and is much more robust than the ACL. 
The ACL is fundamental in the control of rotation and during extension reduces the posterior displacement of the tibia in relation to the femoral condyles. 
The cruciate ligaments are “X” shaped, cross-linked in all three dimensions of space. 
Externally in the knee there are two ligaments: lateral collateral (LCL) and medial collateral (LCM). 
The first has insertion in the lateral epicondyle of the femur and inserts in the external part of the fibular head; has a rope-like shape. 
The LCL is divided into a deep and a superficial layer and does not join the capsule or the meniscus.
The medial collateral ligament has insertion on the outer surface of the medial condyle and is inserted into the inner surface of the tibia.

The LCM is thinner than the LCL and has the shape of an elastic. It is located inside the capsule and is connected to the medial meniscus, being longer than the lateral collateral.

The collaterals are important because they block the lateral tilt movements of the tibia in the knee. The ends of the two ligaments reach the maximum tension with the knee extended, while during flexion there is no tension. 
In the anterior part of the knee is located the transverse ligament of the knee that connects the meniscus between them and the medial meniscus with the intercondylar artery.

At the side of the patella are the retinaculars, the lateral part of the external retinaculum and the inner part of the medulla, its task is to avoid excessive lateral displacement of the patella. 
At the back of the knee, the posterior meniscus-femoral ligament connects the medial face of the posterior horn of the lateral meniscus to the outer face of the lateral femoral condyle and has an oblique orientation. 
At the level of the patella there are fibrous extensions coming from the vastus medialis and lateral muscles that are inserted in the outer edge of the patella. 
This tissue continues to the lateral collateral and the tibial condyle on the same side of the knee.

The capsule and the synovial membrane

The fibrous capsule is a connective membrane that surrounds the joint as a cuff; is born in the posterior part of the femur further up the articular cartilage. In the anterior part it connects to the sides of the patella and in the inferior part it is inserted in the articular surface of the tibia. 
The capsule is thicker in some stretches where it connects to the knee ligament. In the anterior part of the knee, the capsule increases in thickness and forms the patellar ligament that joins the patella to the tibia. 
The synovial membrane is in contact with the inner surface of the capsule and is composed of fibrous connective tissue. It is very useful because it produces the synovial fluid that nourishes the cartilage and lubricates the joint to reduce friction during movement.

In the knee joint, behind the tendons, there are numerous synovial bursae (bags), ie sacs of synovial fluid between the skin and bones or between the ligaments and muscles. 
The major one is located in the anterior part of the knee between the skin and the patella, it is called pre-patellar synovial bursa; between the quadriceps tendon and the femur is located the patellar bursa, and between the patellar ligament and the tibia there is the infra-patellar bursa. 
These structures are small cushions located within a joint, between the muscles and tendons. The bursa reduces the friction between the tissues.

See also: synovitis



Goose paw

Among the muscles of the medial posterior part, three have a common tendon called the “goose leg” that is inserted into the inner surface of the proximal epiphysis of the tibia. 
These muscles are the sartorius, the semitendinosus muscle, and the gracilis. 
This large tendon acts close to the medial collateral in stabilizing the knee and prevents excessive external rotation.

Hoffa’s fat

Hoffa’s body is an adipose tissue below the patellar ligament and has a dual function to cushion shocks and facilitate movement, reducing friction.

Muscles of the knee

Side view of the leg © Andreas Meyer – Fotolia.com


Tensile muscle of the fascia lata

It is a long, narrow muscle that is located on the outside of the thigh.
It originates in the SIAS (superior anterior iliac spine) and inserts on the external condyle of the tibia, it is biarticular because it crosses and controls two joints: hip and knee.
The tendon that enters the tibia is very long, begins just below the pectine line of the femur and the gluteal tuberosity and joins the fascia lata or femoral forming the iliotibial tract.
The action of the fascia lata tensor is the abduction of the thigh, it also helps in the movement of extension of the leg over the thigh.

Sartorius muscle

View of the medial lower limb. 
steve estvanik – Fotolia.com

It is the longest muscle we have, has an “S” shape and runs superficially to the quadriceps.

It originates in the SIAS (anterior superior iliac spine) and is inserted in the medial aspect of the tuberosity of the tibia, joining to the tendons of the gracile and semitendinosus that form a firm structure called “goose leg”.
The sartorius  is a muscle that allows to assume the typical position of the tailor with the crossed legs, hence its name; in fact, it folds, rotates externally and does the abduction.

Femur quadriceps muscle

It is the most important knee extensor and is key to walking.
It is located on the front of the thigh, is a muscle formed by four heads:
  1. Wide lateral
  2. Vast intermediary
  3. Vast medial
  4. Straight muscle of the thigh.
The femoral rectus is biarticular and originates from the SIAI (inferior anterior iliac spine) and the upper part of the circumference of the acetabulum. The vastus lateralis originates from the lateral side of the great trochanter and the rough line, the vastus medialis has its insertion in the anterolateral aspect of the femoral diaphysis and the vastus medialis its insertion in the medial part of the rough line.
These four muscles progress toward the patellar tendon, forming a common tendon called the quadriceps tendon that engages the patella. Some fibers continue anteriorly to the patella, and terminate in the tuberosity of the tibia.
The quadriceps covers the entire anterior part of the femur, serves to extend the leg. With the rectus femoris it contributes to the flexion of the thigh.

On both sides of the patella, the quadriceps tendon is reinforced by two retinacles, that is, fibrous connective tissue fibers that attach the patella to the tibial condyles. 
The right quadriceps muscle should be the same as the left quadriceps muscle, or almost, while the arms have a more pronounced difference.

The hamstrings

There are three muscles of the posterior region of the thigh: the femoral biceps, the semimembranosus and the semitendinosus, are all biarticular, have in common the insertion in the ischial tuberosity, the action of flexion of the leg over the thigh and extension of the hip.
They play a very important role in knee kinematics because they protect the anterior cruciate ligament from injury .

Femoral biceps

As the name implies, it is clear that this muscle has two heads, the long one inserts into the ischial tuberosity along with the semimembranous muscle and the short one has its insertion in the distal half of the rough line and the lateral intermuscular septum.
After covering the back and side of the thigh, it inserts into the head of the fibula and the lateral condyle of the tibia.
The action of the femoral biceps is the flexion of the leg over the thigh and the external rotation, it is the only muscle that acts as an external rotator of the knee; also extends the thigh.

 The semitendium

Leg rearview mirror 
© Patrick Hermans – Fotolia.com

The semitendinosus has its insertion in the ischial tuberosity, but in comparison with other hamstrings it is in superior and internal position. 
It is part of the posterior and medial part of the thigh and is inserted into the medial fascia of the medial condyle of the tibia. 
The action of the semitendinosus is the flexion and internal rotation of the leg over the thigh.

The semimembranous

The semimembranosus has its insertion in the ischial tuberosity along with the femoral biceps, it is located in the posterior and medial part of the thigh. Distally, the tendon divides into three fascias: the “descending” branch terminates over the posterior fascia of the medial condyle of the tibia, the “recurring” branch continues through the lateral femoral condyle constituting the oblique popliteal ligament, and the “anterior” branch is inserted in the anterior aspect of the internal condyle of the tibia. 
The action of the semimembranosus is the internal flexion and rotation of the leg over the thigh and the extension of the leg.

The gracilis
As the name suggests, this muscle is small and tight, it is located in the medial region of the thigh. 
It originates in the anterior area of ​​the ischium-pubic branch and is inserted into the medial and anterior fascia of the medial condyle of the tibia with the sartorius muscle and the semitendinosus, forming the goose’s paw. 
The gracile attaches to the thigh, folds and swivels the leg over the thigh.



The Popliteus Muscle The popliteus
muscle is a thin, triangular muscle that is located at the back of the leg. 
It originates in the external area of ​​the lateral femoral condyle and is inserted in the superior part of the oblique line and on the posterior area of ​​the tibia. 
The popliteal muscle flexes and rotates medially in the leg. 
The gastrocnemius
consists of two heads of symmetrical muscle, one medial arising from the internal femoral condyle and the internal position of the capsule, and a lateral that originates from the lateral lateral femoral condyle and the external part of the capsule; is a biarticular muscle. 
It is inserted with the stout Achilles tendon in the posterior-superior area of ​​the calcaneus.
The action of the gastrocnemius is the plantar flexion of the foot and the internal rotation; also contributes to the flexion of the leg over the thigh.

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