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Structure
Intermediate
9 min read

The knee

Two poorly matched surfaces, an almost flat plateau under rounded condyles. Menisci, cruciate and collateral ligaments make up for that lack of fit.
3D scene
The knee, bones and joint tissues
The scene shows the femur, tibia, fibula and kneecap, with the capsule, menisci, cruciates and collaterals highlighted. Without the 3D, the idea fits in one sentence: two surfaces that do not match, held together by a set of cords and two cushions.
Open in the explorer

A joint that does not fit

The knee brings together three bones: the femur from above, the tibia from below, the kneecap in front. The fibula, on the side, takes no direct part in the joint but serves as an anchor.
The mechanical problem is obvious as soon as you look at the surfaces. The femur ends in two rounded condyles; the tibia receives them on an almost flat plateau. Two shapes that do not fit together. Where the hip holds by the depth of its socket, the knee holds only by what is added to it.
Condyle
A rounded swelling at the end of a bone, which rolls and glides on the surface facing it. The femur carries two, separated by a groove.
rounded articular surface

The menisci, to fill the gap

Meniscus
A crescent of fibrocartilage resting on the tibial plateau. Its triangular section is thick at the rim and thin at the centre, which hollows out a cup where the bone is flat.
semilunar fibrocartilage
Two menisci, one medial and one lateral, sit on the plateau. They increase the contact area, spread load over a wider surface, and absorb part of the stress. Without them, weight would concentrate on a few points of cartilage.
They are not simply laid there: their horns are moored to the tibia by short ligaments, and the medial meniscus is partly attached to the tibial collateral ligament. That attachment explains why it is less mobile than its lateral neighbour.

The cruciate ligaments, holding front to back

Two ligaments cross at the centre of the joint, in the space left between the condyles.
  • The anterior cruciate ligament runs from the front of the tibial plateau to the back of the lateral condyle. It stops the tibia sliding forwards under the femur.
  • The posterior cruciate ligament, thicker, does the reverse: it stops the tibia sliding backwards.
Together they also control rotation of the tibia under the femur, which can only occur through a narrow range.
Certainty level · Established
The cruciate ligaments limit front-to-back sliding of the tibia under the femur.
Descriptive anatomy and kinesiology describe the same mechanics, from the path of the fibres and their tension across the range of flexion. What varies between studies is the share taken by each bundle of a given ligament at different angles.
Standring S (2020) · Neumann DA (2016)

The collaterals, holding side to side

On either side, two ligaments oppose sideways movement. The tibial collateral, broad and attached to the capsule, limits opening inwards; the fibular collateral, narrow and free, limits opening outwards. Both tighten as the knee extends and slacken in flexion — which is why the knee rotates a little when bent and hardly at all when straight.

The kneecap, a pulley

The patella is embedded in the tendon of the quadriceps. It does not articulate the femur with the tibia: it holds the tendon away from the joint axis, which increases the leverage of extension.

What each part prevents

Structure
What it limits
When it is most taut
Anterior cruciate
The tibia sliding forwards
Knee near extension
Posterior cruciate
The tibia sliding backwards
Knee flexed
Tibial collateral
Opening inwards
Extension
Fibular collateral
Opening outwards
Extension
Menisci
Load concentrating on points
Under load, at any angle
Certainty level · Established
How stress is distributed in the knee depends on the angle of flexion and on trunk position.
Work on squat biomechanics measures the shift of stress between the patellofemoral joint and the tibiofemoral structures according to the depth reached. Published values depend on the protocol, the load and the population studied, and do not transfer directly to any given sporting movement.
Neumann DA (2016) · Escamilla RF (2001)

Seeing the knee

The knee, bones and joint tissues

The scene shows the femur, tibia, fibula and kneecap, with the capsule, menisci, cruciates and collaterals highlighted. Without the 3D, the idea fits in one sentence: two surfaces that do not match, held together by a set of cords and two cushions.
Current step
The knee
Two poorly matched surfaces — the rounded condyles of the femur on an almost flat tibial plateau. The menisci fill the gap, the cruciates hold the tibia front to back, the collaterals hold it side to side. Remove any one of these parts and the load shifts onto the others.
Scene description
A three-dimensional model of four joints from the left side of the body, presented one after another on a paper background. The shoulder shows the shoulder blade, collarbone and upper humerus in bone tones, with the joint capsule, the glenoid labrum and the glenohumeral ligaments in olive grey. The hip shows the hip bone and femur, the capsule wrapping the neck, the acetabular labrum and the three ligaments wound around it. The knee shows the femur, tibia, fibula and kneecap, with the two menisci resting on the tibial plateau, the cruciate ligaments at the centre and the collaterals on either side. The lumbar spine, seen from the side, shows the five vertebrae and the sacrum separated by five discs, with the longitudinal ligaments running in front of and behind the vertebral bodies. The joint tissues are highlighted at each step, the bones staying in the background.
Visible structures
  • Glenoid labrum
    A fibrocartilaginous rim fixed around the edge of the glenoid cavity. It deepens the socket and increases the contact area with the head of the humerus, whose diameter far exceeds that of the cavity.
  • Ligaments of the hip
    Three bands wound around the capsule — iliofemoral, pubofemoral, ischiofemoral. They tighten as the hip extends, which allows standing without continuous muscular effort.
  • Cruciate ligaments
    Two ligaments stretched across the centre of the knee, crossing one in front of the other. They limit the tibia sliding under the femur — forwards for the anterior, backwards for the posterior.
  • Intervertebral discs
    Five discs separate the lumbar vertebrae and the sacrum. Each pairs a tough fibrous ring with a more deformable core, which spreads load and allows slight movement between two neighbouring vertebrae.
  • Ligaments of the elbow
    Two collateral ligaments hold the elbow from the sides, and a ring encircles the head of the radius so it can pivot without leaving its place. The elbow thus combines a hinge and a pivot.
  • Lateral ligaments of the ankle
    Three slender bands link the fibula to the talus and the heel bone. They are markedly thinner than their medial counterparts, which is why the ankle is more easily forced inwards.
Guided steps
  • 1/6
    The head of the humerus is far wider than the socket that receives it. The glenoid labrum deepens the rim, the capsule wraps the whole, and three glenohumeral ligaments reinforce it in front. That is the price of mobility: little bone to hold, a great deal of soft tissue.
  • 2/6
    Unlike the shoulder, the femoral head sits deep in the socket, which the acetabular labrum extends further. The three ligaments wind around the neck and tighten in extension: standing, they carry part of the work the muscles no longer have to supply.
  • 3/6
    Two poorly matched surfaces — the rounded condyles of the femur on an almost flat tibial plateau. The menisci fill the gap, the cruciates hold the tibia front to back, the collaterals hold it side to side. Remove any one of these parts and the load shifts onto the others.
  • 4/6
    Five vertebrae, five discs, and a set of ligaments running the whole height. Each level moves only a few degrees; it is their sum that gives the trunk its range. The longitudinal ligaments limit flexion and extension, the ligamenta flava close the back of the canal.
  • 5/6
    The elbow does two things inside one capsule. The humerus and ulna form a hinge that flexes and extends in a single plane; the head of the radius pivots in place inside a ligamentous ring, which turns the palm over. The two collateral ligaments hold it all from the sides.
  • 6/6
    The talus is wedged between the two malleoli like a tenon in a mortise. On the inner side, a thick fan-shaped ligament; on the outer side, three markedly thinner bands. That asymmetry is no detail: it explains which way the ankle most often gives.
Model licence · Z-Anatomy et BodyParts3D — CC-BY-SA 4.0

What the model simplifies

Limit
What the image is missing
The articular cartilage is not represented. Yet it is what covers the condyles and the plateau, and it is on cartilage that the gliding happens: without it, the fit looks more abrupt than it really is.
The bursae, the fat pad under the kneecap and the synovial fluid are also absent. The model shows the left side only, in a reference anatomy that matches no particular person's build.

When to seek help

Caution
What this page cannot do
Describing the anatomy of a ligament does not tell you whether yours is injured. The terms used here — anterior cruciate, meniscus — circulate widely, and the temptation to attribute a pain to one of them is strong. Only an examination can say.
A knee that locks, swells or gives way, or pain that appeared suddenly during weight-bearing, belongs to a health professional.
The knee is moved by the quadriceps, which extends it, and by the hamstrings, which flex it. The calves cross its back surface. It works in the squat and in the lunge.

Sources

Main sources

  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Escamilla RF (2001). Knee biomechanics of the dynamic squat exercise. Medicine and Science in Sports and Exercise.
Put it into practice in Shapier

See the squat, which works the knee and quadriceps

Performing the squat is done with Shapier's guidance; this page only explains how the knee works in it.
See the squat, which works the knee and quadriceps
Body Lab explains; Shapier lets you act and track.

Check my understanding

Unlike the hip, the bone surfaces of the knee do not fit together. What holds the joint?
Why is the medial meniscus less mobile than the lateral one?
When can the knee rotate a little?
Choose an answer

Read next

  • The hip
    A sphere set deep in a socket, locked by three ligaments that tighten in extension. That is what makes standing possible without continuous muscular effort.
    With a 3D scene
  • Quadriceps
    The quadriceps brings together four muscles on the front of the thigh that share a common tendon. It extends the knee, and one of its heads also crosses the hip.
    With a 3D scene
  • Squat
    The squat is a simultaneous flexion, then extension, of the hips, knees and ankles with the feet planted on the ground. This page describes its phases, the joints involved, the muscles that contribute and the most widespread misconceptions about it.
    With a 3D scene

Where to go next

Available offline

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • The model shows the left side only, and without the cartilage covering the surfaces: it is what glides, and its absence makes the fit look more abrupt than it is.
  • The bursae and the infrapatellar fat pad are not represented, although they occupy real space in the joint.
  • This page describes an anatomy. It cannot identify an injury or judge a pain: discomfort that persists, locking, or a feeling of instability belong to a health professional.
Sources
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Escamilla RF (2001). Knee biomechanics of the dynamic squat exercise. Medicine and Science in Sports and Exercise.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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