Joints and ligaments of the body

Joints and ligaments of the body are connections between skeletal structures and the strong connective-tissue bands that help support them. Joint shape helps determine the movements available, while ligaments guide and limit those movements. The major joints differ in how they balance mobility, stability and the transmission of loads.

What are the main types of joint?

A joint is a connection between skeletal structures. Fibrous joints join bones through dense connective tissue, as in the sutures between skull bones. Cartilaginous joints join bones through cartilage, a firm supporting tissue; an intervertebral disk forms part of the cartilaginous connection between neighbouring vertebral bodies, the weight-bearing portions of spinal bones.

A synovial joint has a cavity containing synovial fluid, which lubricates its moving surfaces. Articular cartilage covers the contacting bone surfaces, and a joint capsule encloses the cavity. The capsule has an outer fibrous layer and an inner synovial membrane that produces the fluid.

A ligament is a band of connective tissue; skeletal ligaments connect bones and help restrain movement. Muscles provide active support through tendons, the connective-tissue structures that attach muscles to other structures, usually bone.

Movement terms describe direction:

• Flexion and extension usually mean bending and straightening.
• Abduction and adduction generally mean movement away from and towards the body's midline; for the fingers and toes, these movements are described relative to the middle finger and second toe, respectively.
• Medial and lateral rotation turn a limb about its longitudinal axis, bringing its anterior surface towards and away from the midline, respectively.
• Circumduction combines movements so that the limb traces a cone.

Which joints move the shoulder, elbow and wrist?

The shoulder, specifically the glenohumeral joint, joins the head of the humerus, the upper-arm bone, to the glenoid cavity, a shallow socket on the scapula or shoulder blade. It is a ball-and-socket joint, permitting flexion, extension, abduction, adduction, rotation and circumduction. The glenohumeral and coracohumeral ligaments reinforce its capsule.

The elbow joint joins the humerus with the radius and ulna, the forearm bones. Its humeroulnar and humeroradial articulations chiefly permit flexion and extension. The proximal radioulnar articulation, between the upper ends of the radius and ulna, permits rotation and shares the elbow capsule.

The ulnar and radial collateral ligaments support the sides of the elbow. The annular ligament forms a ring around the radial head, holding it against the ulna while allowing rotation. Pronation and supination turn the forearm so that the palm faces backwards or forwards in anatomical position, the upright reference posture with palms facing forwards.

The wrist, or radiocarpal joint, is an ellipsoid joint: an oval convex surface meets a corresponding concave surface. The distal radius and an articular disc, a plate of fibrocartilage, meet the scaphoid, lunate and triquetrum, bones of the proximal wrist row. The ulna does not directly contact these bones; the disc intervenes. Radiocarpal and collateral ligaments support the joint and restrain excessive flexion, extension and movement towards either side.

Which joints move the hip, knee and ankle?

The hip joint is a ball-and-socket joint between the femoral head, the rounded upper end of the thigh bone, and the acetabulum, the socket of the hip bone. It permits the same broad movement categories as the shoulder. The iliofemoral, pubofemoral and ischiofemoral ligaments strengthen its capsule and become taut during extension.

The knee joint joins the femur with the tibia, the main weight-bearing bone of the leg, and with the patella or kneecap. It is a modified hinge joint, permitting flexion and extension with limited rotation when flexed. The patella moves against the anterior surface of the lower femur.

The anterior cruciate ligament chiefly resists forward displacement of the tibia relative to the femur. The posterior cruciate ligament chiefly resists backward displacement. These crossing ligaments lie inside the fibrous capsule but outside the synovial cavity; the tibial and fibular collateral ligaments support the sides, while the patellar ligament connects the patella to the tibia.

The ankle joint is a hinge joint between the lower tibia and fibula, the lateral leg bone, and the talus, a bone above the heel. The tibia and fibula form a socket around the talus. The medial ligament, also called the deltoid ligament, and the lateral ligament complex support it.

Dorsiflexion brings the top of the foot towards the leg; plantarflexion points the foot downwards. Inversion and eversion, turning the sole inwards and outwards, occur mainly at joints below and in front of the ankle rather than at the ankle itself.

How do the jaw and intervertebral joints work?

The temporomandibular joint connects the mandibular condyle, the rounded upper end of the lower jaw, with the mandibular fossa and articular tubercle of the temporal bone. These form the receiving surface and its anterior prominence. An articular disc divides the joint cavity into upper and lower compartments.

Rotation occurs chiefly in the lower compartment, while gliding occurs chiefly in the upper compartment. Together these permit elevation and depression, closing and opening the mouth, protrusion and retrusion, forward and backward movement, and side-to-side movement. The lateral ligament reinforces the capsule; the sphenomandibular and stylomandibular ligaments provide additional restraint.

Most neighbouring vertebrae connect through a cartilaginous joint between their bodies and synovial facet joints between their articular processes, the projecting parts that carry joint surfaces. Each disk has an outer fibrous ring, the annulus fibrosus, surrounding a gelatinous centre, the nucleus pulposus. Small movements at successive levels combine into bending and rotation of the spine.

The anterior and posterior longitudinal ligaments run along the front and back of the vertebral bodies. The ligamenta flava are elastic ligaments connecting neighbouring vertebral arches, the bony structures behind the bodies. Facet orientation helps determine the movement available in each spinal region.

How do the major joints compare?

The table summarises the principal connections and movements. A hinge chiefly moves in one plane, a pivot permits rotation, and a plane synovial joint permits gliding.

Why is the shoulder more mobile than the hip?

Stability means resistance to unwanted displacement; mobility means the range of available movement. The shoulder's shallow socket and relatively loose capsule permit extensive movement. Its stability depends strongly on surrounding muscles, particularly the rotator cuff, a group whose tendons blend with the capsule and help centre the humeral head.

The hip's deeper socket surrounds more of the femoral head. Its strong capsule and ligaments provide substantial restraint, especially in extension. Both joints also receive muscular support: their contrast reflects different proportions of bony, ligamentous and muscular restraint, not a complete separation between stability and mobility.

In the adjacent 3D model, you can rotate and zoom, select a structure, and isolate or fade it while comparing these relationships.

What are menisci, labra and bursae?

Menisci are crescent-shaped pads of fibrocartilage, cartilage reinforced with dense collagen fibres. The medial meniscus and lateral meniscus lie between the femur and tibia. They improve the fit of the joint surfaces and help distribute loads; the medial meniscus is attached to the capsule and the deep part of the tibial collateral ligament.

A labrum is a fibrocartilaginous rim around a socket. The glenoid labrum deepens the shoulder socket, while the acetabular labrum deepens the hip socket. Unlike menisci, labra primarily extend the socket margin rather than lying between broad opposing surfaces.

A bursa is a small sac lined by synovial membrane that reduces friction between adjacent tissues, such as a tendon and bone. Bursae lie around many joints but are not the joint cavity itself. Some communicate with a joint cavity, while others remain separate.