Anatomy of the knee joint

The knee joint is the most complex joint in the body, both from an anatomical and functional point of view, because it must be stable but also very flexible.


The bones and joints

The knee is made up of three bones: thigh bone (femur), shin bone (tibia) and knee joint (patella); correctly it must be divided into two single joints:

  • patellar joint or femoropatellar joint (between the posterior patella and the anterior, distal side of the femur);
  • Popliteal joint or femorotibial joint (between the end of the thigh bone near the knee and the tibial plateau).

The femur ends at the knee (distal) with two articular condyles (femoral condyles) of ellipsoidal cross-section, which are separated from each other on the back by a clearly visible fossa (intercondylar fossa).
On the front, the condyles are united with each other and form a single articular surface.
On the sides of the condyles are two bony prominences called the epicondyles.

  • The condyles form the articular surface of the femur, the epicondyles are not part of it.
  • The tibia has a broad head and a crest that can be easily felt at the front of the tibia.
  • In the upper area is the shinbone bump ( Tuberositas tibiae ), which represents the attachment of the kneecap ligament.
  • The kneecap, or patella, is embedded in the lower quadriceps tendon of the thigh. This tendon is divided into an upper and a lower area; the latter is called the patellar ligament and attaches to the shinbone bump.
  • The patella is chestnut-shaped, the exterior rough, with vertical furrows; the inner surface, on the other hand, is smooth and covered with articular cartilage.
  • The kneecap is contained within the joint capsule.
  • The femur and tibia form a hinge joint, that is, the only possible movements are flexion and extension in the sagittal plane; Rotational movement is also possible when the knees are bent because the articular surface of the tibia does not exactly match that of the femur and does not have a pronounced cavity that would restrict certain movements.
  • The articular surfaces of the femur are formed by the convex-shaped femoral condyles.
  • The tibia also articulates in the form of two condyles, the inner condyle is concave (hollow), the outer condyle is convex (curved).

More articles: Cartilage damage in the knee .

The Menisci

The crescent-shaped menisci lie between two bones like two cushions of fibrocartilage.
The thickness of these two structures is greater on the outside than on the inside.
The external meniscus ( Meniscus lateralis ) is circular, almost closed, and is firmly attached to the joint capsule for almost its entire length, except for a small area where the popliteal muscle (Musculus popliteus ) runs; the horns have about the same volume.
The medial meniscus ( meniscus medialis ) is firmly attached to the joint capsule along its entire length and has differently shaped horns, the anterior horn is narrower and lower than the posterior.
The menisci have the following functions:

  • absorbing the impact.
  • Distribution of body weight over the entire surface of the tibial plateau; without menisci, weight would be concentrated in a smaller area, resulting in faster knee wear.
  • Distribution of synovial fluid to keep movement smooth and reduce friction.
  • Compensating for the articular surface incongruence of the tibia and femur.
  • Restriction on rotation.

The outer meniscus makes the knee more flexible, the inner meniscus increases the stability of the knee joint.
During flexion, the popliteal muscle pulls the lateral meniscus backwards, while the inner meniscus moves much less due to the work of the semimembranous muscle.

When stretched, they are pushed back into their anatomical seat by the ligaments that connect the menisci to the kneecap.
The patella is a sesamoid bone (ie, a bone embedded in a tendon) and is embedded within the quadriceps tendon and patellar ligament; its role is to centralize the forces exerted on the joint, protecting the knee and facilitating extension of the lower leg.
All articular surfaces of these bones are surrounded by smooth cartilage, which reduces friction during movement and allows bones to slide smoothly and painlessly.
When moving, part of the cartilage is compressed and the synovial fluid surrounding it is pushed towards the capsule; when the knee joint is no longer under stress, the cartilage absorbs the synovial fluid like a sponge.

More articles: Meniscus tear in the knee

The bands

The knee has strong ligaments that are essential for biomechanics and motor function.
The cruciate ligaments form the central pivot through which knee movement occurs.
The anterior cruciate ligament (ACL) is attached to the bony crest of the tibia in front and attaches to the median-facing surface of the external femoral articular condyle ( condylus lateralis femoris ); it is not particularly well supplied with blood and is called fragile because it is more prone to injury than the posterior cruciate ligament.
It is by far the largest contributor to joint strength and stability, restricting internal rotation and hyperextension of the knee, and preventing the tibia from moving too far forward of the femurs during flexion.
The posterior cruciate ligament (PCL) has its origin in the indentation of the tibial plateau ( area intercondylaris ) and attaches to the inner side of the inner thigh knot; it is well supplied with blood and is much more resistant than the ACL.
The PCL plays an important role in controlling rotation and during extension, it prevents the tibia from slipping backwards relative to the femoral condyles.

The cruciate ligaments have an x-shape in all three spatial dimensions.
The two collateral ligaments are located on the outside of the knee: the outer ligament or lateral collateral ligament (LCL) and the inner ligament or medial collateral ligament (MCB).
The outer ligament has its origin at the top of the outer thigh condyle ( Epicondylus lateralis femoris ) and is attached to the joint head of the fibula ( Caput fibulae ).
The LKB is divided into a deep and a superficial layer and is not connected to either the joint capsule or the meniscus.
The medial ligament originates on the outside of the inner thigh bone and attaches to the inside of the tibia.
The medial ligament is thinner than the medial ligament, triangular in shape, built into the joint capsule, fused with the medial meniscus, and longer than the medial ligament.

The collateral ligaments are important because they block lateral bending of the tibia to the knee; the ends of the band are stretched to their maximum when the knee is extended and loosen when the knee is bent. In the front knee area, a slanting retaining ligament ( ligamentum transversum genus )
connects the menisci to each other and the inner meniscus to the hollow of the shinbone plateau ( area intercondylaris ).

Laterally on the patella are the alar ligaments, the outer and inner alar ligaments; they prevent excessive lateral displacement of the kneecap.
The posterior meniscus-thigh bone ligament ( ligamentum meniscofemorale posterius ) is located in the rear knee area; this ligament structure runs obliquely and forms a connection between the meniscus and the femur.
At the level of the kneecap, there are fibrous extensions that are associated with the medial broad thigh muscle ( vastus medialis muscle ) and the lateral broad thigh muscle ( vastus lateralis muscle).) arise and attach to the edge of the kneecap. This tissue continues to the lateral ligament and tibial condyle on the same side of the knee.

Other articles:
Anterior cruciate ligament tear
Inner ligament tear of the knee joint

Joint capsule and synovium

The fibrous joint capsule is a horseshoe-shaped container that connects the bony structures of the knee; it originates posteriorly in the femur above the articular cartilage, anteriorly it connects to the edges of the kneecap and inferiorly it is attached to the articular surface of the tibia.
The capsule is thicker where it connects to the knee ligaments; it increases in thickness at the front of the knee and forms the patellar ligament that connects the kneecap to the tibia.

The inner skin of the joint, the Membrana synovialis , called Synovialis for short , forms the inner layer of the joint capsule and consists of fibrous connective tissue; it forms the joint fluid ( synovia ), which is important for nourishing the cartilage and, as synovial fluid, reduces the friction that occurs during movement.
There are numerous bursae in the knee joint, i.e. small sacs filled with synovial fluid that lie between bone and skin or between ligaments and muscles.

The largest bursa, the prepatellar bursa , lies in front of the knee, between the skin and the knee bone; The suprapatellar bursa is located between the quadriceps tendon and the femur and the infrapatellar bursa is located between the patellar ligament and the tibia .
In this way, the sliding movement of muscles and tendons is kept smooth because the movement mechanism is “lubricated” and friction is reduced.

Other articles: Synovitis


The internal posterior vertices (PAPE) and external (PAPI) are structures formed by ligaments, tendons and joint capsule and have a common action with the cruciate ligaments to stabilize the knee in the AP direction during rotation, also preventing subluxation of the tibia during flexion or extension.
The PAPE lies behind the lateral collateral. The posteroanterior external vertex is dynamically controlled by the popliteal muscles which include a tendon divided into 3 bundles that fits the first lateral epicondyle of the femur, the second end at the head of the fibula (arcuate ligament popliteal) and the third is connected to the external meniscus, this property also the tendon of the biceps femoris.

This triple insertion of the tendon can create a force on the meniscus during flexion and internal rotation, also acting as a stabilizer. between thigh and shin.
In the event of an injury to the rear cruciate ligament, in most cases even the PAPE acts.
The PAPI is located on the medial aspect of the knee joint, within the medial collateral ligament, is formed by the posteromedial aspect of the capsule, the semimembranosus tendon, the posterior oblique ligament and also includes the medial meniscus.
The Posteroanterior Inner apex angle serves to stabilize the medial portion of the joint and acts in conjunction with the ACL.

The goose foot

Three of the muscles of the posterior and inner thighs share a common tendon called the “goose foot” ( pes anserinus ) that inserts on the inside of the tibia near the knee; these are tailor’s muscle (sartorius), semitendon muscle (semitendinosus) and slender muscle (gracilis).
This large tendon structure works synergistically with the medial ligament to stabilize the knee and prevent excessive external rotation.

The Hoffa fat body

The corpus adiposum infrapatellare (also Hoff’sches fat pad) is a fat pad that lies under the patellar ligament and both cushions shocks and promotes sliding movement by reducing friction.

The muscles of the knee joint

 Thigh Band Tensioner

The thigh band tensioner ( musculus tensor fasciae latae ) is a long, narrow muscle on the outside of the thigh.
It originates from the anterior superior iliac spine ( Spina iliaca anterior superior) and attaches to the outer tibial condyle ( Condylus lateralis tibiae) ; it supplies and controls two joints: the hip and the knee joint.
The tendon that attaches to the tibia is very long and begins just below the pectine line and gluteal tuberosity ; it unites with the muscular fascia of the thigh ( fascia lata ) and forms the iliotibial band ( tractus iliotibialis ).
The function of the thigh band tensioner is to spread (abduct) the thigh; it also contributes to the extension of the knee joint.

Tailor muscle

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Four-headed thigh muscle (quadriceps)

The quadriceps (Musculus quadriceps femoris) is the most important extensor of the knee joint and fundamental for the walking movement.
It lies in the anterior thigh compartment and has four muscle heads:

  • Vastus lateralis muscle
  • Vastu intermedius muscle
  • Vastus medialis muscle
  • Rectus femoris muscle.

The rectus femoris , or straight muscle of the thigh, is involved in two joints and originates at the anterior superior iliac spine ( spina iliaca anterior superior) and at the top of the acetabulum ( acetabulum) ; the vastus lateralis , or external broad muscle, has its origin on the side of the greater trochanter and linea aspera ; the vastus intermedius or middle broad muscle arises on the lateral anterior surface of the femur and the vastus medialisor medial broad muscle originates on the medial surface of the femur in the medial aspect of the linea aspera .

These four muscles run towards the kneecap and form a common tendon, the quadriceps tendon, which attaches to the kneecap; some bundles continue in the anterior region and end at the shinbone bump.
The quadriceps covers the entire front surface of the thigh bone, provides extension of the knee joint and, together with the rectus femoralis , flexes the hip joint. On the sides of the kneecap, the quadriceps tendon is reinforced
by two retaining bands ( retinacula patellae ) of fibrous connective tissue that connect the kneecap to the tibial condyles.
The right and left quadriceps should be roughly the same size, with the arms having a more pronounced lateral difference.

Related articles:
Patellar tendon rupture
Patellar tendon inflammation
Quadriceps tendon inflammation


The ischiocrural muscles

The hamstrings or hamstrings consist of three muscles: hamstrings or hamstrings ( biceps femoris muscle ), semimembranous muscle ( semimembranosus ), and semitendon muscle ( semitendinosus ); they all move two joints, have a common origin, flex the knee joint and extend the hip joint.
They play a fundamental role in the kinematics of the knee because their activity protects the anterior cruciate ligament from injury.

The thigh biceps

As the name suggests, this muscle has two muscle heads; the long head arises together with the semimembranous muscle on the ischial tuberosity ( tuber ischiadicum ), the short head on the midbody-distant half of the linea aspera and on the lateral intermuscular septum.
It runs posteriorly and laterally along the thigh until it attaches to the head of the fibula ( Caput fibulae ) and the external tibial condyle.
The femur flexes the knee joint and externally rotates the lower leg; it is the only outward rotator of the knee joint and also extends the hip joint.

The semitendon muscle

The semitendinosus originates at the ischial tuberosity ( ischial tuberosity ) of the hipbone, but lies further up and outward with respect to the other hamstring muscles.
It is part of the posterior and medial thigh muscles and inserts on the medial side of the medial tibial condyle.
The semitendon muscle flexes the knee joint and rotates the lower leg inward, and also extends the hip joint.

The semimembranous muscle

The semimembranosus arises together with the femoral biceps at the ischial tuberosity ( tuber ischiadicum ) and is located in the posterior inner region of the thigh; the distal tendon is divided into three parts: the first branch ends at the back of the inner tibial condyle, the second branch extends to the outer femoral condyle and forms the oblique popliteal ligament (ligamentum popliteum obliquum), and the third branch attaches to the front surface of the inner tibial condyle.

The semi-membranous muscle flexes the knee joint and rotates the lower leg inward, and also extends the hip joint.

The Lean Muscle

As the name suggests, the gracilis is small and ribbon-shaped; it runs along the inside of the thigh.
It originates in the anterior region of the pubic bone ( Ramus inferior ossis pubis ) and, together with the Schneiderian and semitendon muscles, inserts on the inner anterior surface of the tibial condyle, forming the goose foot ( Pes anserinus) .
The slender muscle leads the thigh to the center of the body (adduction), flexes the knee joint and rotates the lower leg inwards.

popliteal muscle

The popliteus muscle is a broad and thin muscle that lies in the deep posterior flexor box of the lower leg.
It originates externally on the lateral femoral condyle and inserts on the superior side of the oblique line ( linea obliqua ) and on the posterior surface of the tibia.
The popliteal muscle flexes the knee joint and rotates the lower leg inward.

The two-headed calf muscle

The gastrocnemius has two symmetrical muscle heads: an inner head ( caput mediale) originating from the medial thighbone and the inner part of the capsule, and an outer head ( caput laterale ) originating from the outer thighbone and the outer part of the capsule; the muscle moves two joints.

It has its insertion along with the strong Achilles tendon in the upper back area of ​​the heel bone ( calcaneus ).
Its function is plantar flexion and inward rotation of the foot, as well as flexing the knee joint.

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