Types of joints

Types of joints are categories of connections between bones, classified by their structure or the movement they allow. Structurally, joints are fibrous, cartilaginous or synovial; functionally, they are immovable, slightly movable or freely movable. Synovial joints are further divided into six types according to their shape and permitted movements.

What are the three structural classes of joints?

Structural classification asks what joins the bones and whether there is a joint cavity: a space between the contacting bone surfaces. Fibrous and cartilaginous joints have no joint cavity. Synovial joints have a cavity containing fluid.

Fibrous joints

Fibrous joints connect bones through dense connective tissue, a tissue rich in strong protein fibres called collagen. The amount of movement depends partly on the length and arrangement of these fibres.

• Sutures are joints between skull bones, with a narrow layer of fibrous tissue between their edges.
• Syndesmoses connect bones through a ligament or interosseous membrane. A ligament is a band of connective tissue joining bones; an interosseous membrane is a sheet spanning the space between bones.
• Gomphoses attach teeth to their bony sockets through the periodontal ligament, the fibrous tissue surrounding each tooth root.

Cartilaginous joints

Cartilaginous joints connect bones through cartilage, a firm connective tissue with cells embedded in a supporting matrix. Their two main forms differ in the kind of cartilage joining the bones.

A synchondrosis joins bones through hyaline cartilage, which has a smooth, glass-like matrix. An example is the growth plate between the shaft and end of a developing long bone; this connection is later replaced by bone.

A symphysis joins bones through fibrocartilage, cartilage reinforced by prominent collagen fibres. The pubic symphysis connects the right and left pubic bones at the front of the pelvis. Discs between adjacent vertebral bodies are another example; vertebral bodies are the main weight-bearing portions of the bones of the spine.

Synovial joints

Synovial joints have a fluid-containing cavity enclosed by a capsule. Their contacting surfaces are covered by articular cartilage, meaning cartilage at a joint surface. This arrangement allows movement while maintaining a connection between the bones.

How are joints classified by movement?

Functional classification describes the amount of movement rather than the tissue connecting the bones. Its terms are:

• Synarthrosis: an immovable joint, such as a skull suture.
• Amphiarthrosis: a slightly movable joint, such as the pubic symphysis.
• Diarthrosis: a freely movable joint; all synovial joints belong to this class.

The structural and functional classifications are related, but they are not interchangeable. Fibrous joints may be immovable or slightly movable, while cartilaginous joints may also fall into either category.

“Freely movable” does not mean unrestricted movement in every direction. The shape of the bones, tension in ligaments and surrounding muscles determine the directions and extent of movement at each synovial joint.

What are the main features of a synovial joint?

A synovial joint has a capsule surrounding its cavity, cartilage covering its contacting surfaces and a membrane that produces synovial fluid. Ligaments support the joint, while some joints also have fibrocartilage structures within them or fluid-filled sacs nearby.

• Joint capsule: the sleeve enclosing the joint cavity. Its outer fibrous layer attaches to the bones and helps hold them together.
• Synovial membrane: the inner lining of the capsule. It lines the internal surface of the fibrous capsule and covers some other intracapsular surfaces, but does not cover articular cartilage, articular discs or menisci. It contributes to the production of synovial fluid.
• Synovial fluid: the fluid within the cavity. It lubricates the contacting surfaces and helps nourish articular cartilage, which has no blood vessels.
• Articular cartilage: the smooth covering over the contacting bone surfaces, usually hyaline cartilage. It reduces friction and helps distribute loads.
• Ligaments: bands that reinforce the connection between bones and guide or limit movement. They may lie outside the capsule, form part of it or lie within it.
• Bursae: small sacs lined by synovial membrane that reduce friction between neighbouring tissues, such as a tendon and bone. A tendon is the connective tissue attaching a muscle to its attachment site; bursae are not present around every joint.
• Menisci: fibrocartilage structures that partly divide a joint cavity and improve the fit between contacting surfaces. The knee joint has menisci that also help distribute loads.

Some synovial joints contain an articular disc, a plate of fibrocartilage that divides the cavity more completely than a meniscus. Menisci, discs and bursae are accessory structures, not features shared by every synovial joint.

What are the six types of synovial joints?

Synovial joint types describe the shape of the contacting surfaces and the movements that shape permits. An axis is an imaginary line around which movement occurs; some joints move mainly around one axis, while others allow movement around several.

These categories simplify joint geometry. The knee joint is commonly described as a modified hinge because it permits some rotation as well as bending and straightening.

In the 3D model beside this article, you can rotate and zoom, select a structure, and isolate it or hide neighbouring structures to compare joint relationships.

What do flexion, extension, abduction and rotation mean?

Movement terms usually refer to anatomical position: standing upright, facing forwards, with the arms at the sides and palms facing forwards. They describe movement relative to the body or between neighbouring segments.

• Flexion generally decreases the angle between body segments, as when bending the elbow.
• Extension generally increases that angle, as when straightening the elbow.
• Abduction moves a body part away from the body's midline, as when raising an arm sideways.
• Adduction moves it towards the midline, as when lowering that arm towards the trunk.
• Rotation turns a bone around its long axis. Medial rotation turns the front of a limb towards the midline; lateral rotation turns it away.
• Circumduction combines flexion, abduction, extension and adduction so that the far end of a limb traces a circle.

Circumduction is not the same as rotation: the limb sweeps through a cone-shaped path rather than simply turning around its own length. For fingers and toes, abduction and adduction are defined relative to reference digits rather than the body's midline: the middle finger and second toe, respectively.

What is the difference between pronation and supination?

Pronation and supination are rotational movements of the forearm occurring at the proximal and distal radioulnar joints, the connections between the radius and ulna near the elbow and wrist. The ulna is the forearm bone on the little-finger side in anatomical position.

Pronation turns the palm backwards from anatomical position as the radius crosses over the ulna. Supination turns the palm forwards again, with the radius and ulna lying parallel.

With the elbow bent and the forearm held horizontally, pronation turns the palm down and supination turns it up. These movements belong to the forearm rather than to the wrist alone.