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8 min read

What holds a joint together — the video

An animation comparing the shoulder and the hip, then showing what the menisci and ligaments make up for at the knee. The page repeats the content in text, with sources.
3D scene
Four joints, layer by layer
The scene shows the same structures in three dimensions, joint by joint. Without the 3D, the video is enough: it shows the same relationships, on diagrams rather than on extracted geometry.
Open in the explorer
A joint has to do two contradictory things: let movement through, and stop the bones coming apart. None manages both equally, and it is that impossibility which explains their differences.

What holds a joint together

Duration: 1:20

Chapters

  • 0:00
    Two ways of holding
  • 0:14
    What shape decides
  • 0:36
    What the tissues make up for
  • 0:58
    What this does not say
Transcript
A joint has to do two contradictory things: let movement through, and stop the bones coming apart. Each joint sets that balance differently, and its shape is often enough to say where.
The shoulder rests a wide head on a shallow socket: its range is the greatest in the body. The hip instead sets a sphere into a hollow socket: it moves less and holds better. Neither is better designed; they do not have the same job.
The knee has neither the depth of the hip nor its locking: rounded condyles rest on an almost flat plateau. The menisci fill the gap, the cruciate ligaments hold the front-to-back axis and the collaterals hold the sides. The less a joint holds by its shape, the more it depends on what is added to it.
Three levels of restraint stack up: the shape of the bones, the tissues joining them, and the muscles working continuously. Knowing the anatomy of a ligament does not tell you whether yours is involved: pain that persists, locking or instability call for an examination, not for educational content.

What the video shows

It works by comparison rather than description. Two spherical joints that sit side by side in textbooks — the shoulder and the hip — behave in opposite ways, and the reason lies entirely in the depth of their socket.
Joint fit
How closely two joint surfaces match each other's shape. The higher it is, the more the joint holds by its geometry and the less it depends on the tissues around it.
articular congruence
The shoulder rests a wide head on a shallow socket: its range is the greatest in the body, and its restraint depends almost entirely on the capsule, the ligaments and the rotator cuff. The hip does the reverse: the femoral head sits deep, and its three ligaments tighten in extension.
Certainty level · Established
The stability of a joint depends jointly on the shape of the bony surfaces, the tissues joining them and the muscles crossing it.
Descriptive anatomy and kinesiology describe these three levels the same way. What varies between studies is the share attributed to each for a given joint and angle, which depends on the measurement method.
Standring S (2020) · Neumann DA (2016)

The knee, an in-between case

The knee has neither the depth of the hip nor its locking. Rounded condyles rest on an almost flat plateau, and everything holding the joint has been added to it: the menisci fill the gap in shape, the cruciate ligaments hold the front-to-back axis, the collaterals hold the sides.
That is what makes it the most discussed joint in the body. The less a joint holds by its shape, the more it depends on parts that can be named separately — and the more tempting it becomes to attribute a pain to one of them.

What the video does not allow

Limit
Three levels, and what no video replaces
The bars comparing shoulder and hip convey an order of magnitude, not a measurement. They serve to contrast two trade-offs, not to quantify them.
Cartilage, bursae and synovial fluid are not represented, although they occupy real space in every joint. Above all, knowing the anatomy of a ligament does not tell you whether yours is involved: locking, swelling, instability or pain that persists call for an examination.

Four joints, layer by layer

The scene shows the same structures in three dimensions, joint by joint. Without the 3D, the video is enough: it shows the same relationships, on diagrams rather than on extracted geometry.
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

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.
Put it into practice in Shapier

See the squat exercise

Open the squat sheet in Shapier to train the quadriceps; the page above only explains what holds the knee during the movement.
See the squat exercise
Body Lab explains; Shapier lets you act and track.

Read next

  • 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.
    With a 3D scene
  • The shoulder
    The most mobile joint in the body, and the one that holds least by its shape. A wide head resting on a shallow socket, retained by soft tissue.
    With a 3D scene
  • The shoulder in depth
    The most mobile joint in the body is also the one that holds least by its shape. Six steps to understand what follows from that, from the glenoid cavity to reaching overhead.
    Described without a scene

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 bars comparing shoulder and hip convey an order of magnitude, not a measurement: they serve to contrast two trade-offs, not to quantify them.
  • The video shows neither cartilage, nor bursae, nor synovial fluid, which nonetheless occupy real space in every joint.
  • This video describes an anatomy. It cannot attribute a pain to a structure: locking, swelling or 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.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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