Joints

Last updated: February 2, 2021

Summary

Joints are structures in the musculoskeletal system that regulate the type and range of motion between two or more adjacent bones, enabling body parts to move harmoniously. They can be grouped according to their structure (synovial, cartilaginous, and fibrous joints) or their degree of mobility (diarthroses and synarthroses). Diarthroses (synovial joints) are freely movable and consist of elements (e.g., intraarticular space, synovial fluid, joint capsule) that allow low-friction movements between opposing articular surfaces and supporting structures (e.g., ligaments, menisci, articular discs). The range and axes of motion of a synovial joint are determined by the shape of its constituent parts. Examples of synovial joints include hinge joints, saddle joints, plane joints, and ball and socket joints. Synarthroses (fibrous joints) are immovable and connected by thick connective tissue. Depending on the type of tissue connecting the articular surfaces, synarthroses are further classified into four types: syndesmoses, synchondroses, symphyses, and synostoses.

Types of joints

Joints are connections between the ends of neighboring bones that enable specific motions. They can be classified based on their type of connective tissue (structural) or amount of mobility between bones (functional).

Functional classification	Freely movable (diarthrosis)	Slightly movable joint (amphiarthrosis)	Fixed joint (synarthrosis)
Structural classification	Synovial joint	Cartilaginous joint	Fibrous joint
Connection	Intraarticular space with an articular cavity	Connected by cartilage	Held together by thick connective tissue (usually collagen)
Range of motion	Generally high	Slightly movable (linear motion)	No motion
Examples	See types of synovial joints below	Tibiofibular joint Sacroiliac joint Carpus Tarsus	See “Synarthrosis” below Sutures between Diarhtrosisbones of the skull Syndesmoses between: Ulna and radius Tibia and fibula Gomphosis: peg and socket joint between tooth and jawbone

Structure of ball and socket synovial joints

Synovial joints

Components of synovial joints

Components		Definition	Function
Body of the joint	Articular head	Convex end of bone that moves within a socket joint	Joint motion is determined by how the body of the joint is shaped.
Body of the joint	Socket joint	Concave end of bone in which the articular head moves
Articular surface		Surface of articulating bone that is covered with articular cartilage	Articulating bones are connected to one another via articular surfaces.
Articular cartilage		Layer of hyaline cartilage covering the articular surface of bones	Enables low friction sliding and rotational motion in the joint The resilience of cartilage protects the subchondral bones against mechanical stress.
Intraarticular space		Space between articulating bones that is filled with synovial fluid (synovia) and is several millimeters in width	Enables bones to articulate with each other
Joint capsule		Envelope surrounding the joint that consists of an outer fibrous membrane and inner synovial membrane	External closure of the intraarticular space, resulting in the formation of a closed articular cavity Formation of synovial fluid (synovia) through the joint capsule mucosa
Articular cavity		Space completely enclosed by the joint capsule in which the articulating bone ends are located	Enables sliding and rotational motion free of interference
Synovial fluid		Clear viscous fluid in the articular cavity that is formed by the synovial cells of the joint capsule	Reduces frictional forces caused by motion between articular surfaces Nourishes cartilage, which is a bradytrophic tissue

Most articular surfaces are covered with hyaline cartilage! There are two exceptions: the articular surfaces of the temporomandibular joint and sternoclavicular joint are covered with fibrocartilage!

Structure of ball and socket synovial joints

Structure of the joint capsule

An envelope surrounding the joint that consists of an outer fibrous membrane and inner synovial membrane.

Fibrous membrane
- Composed of dense connective tissue (partially strengthened by ligaments)
Synovial membrane
- Composed of the tunica intima and tunica subintima
- Contains two types of synovial cells, or synoviocytes
  - Type A cells = macrophage-like synoviocytes that maintain synovial fluid by removing debris
  - Type B cells = fibroblast-like synoviocytes that produce glycoproteins such as hyaluronan, a major component of synovial fluid

Supporting structures of synovial joints

Structure	Definition	Function	Occurrence
Ligament (joint ligament)	Poorly elastic strand of connective tissue from parallel arrays of dense collagen fibers, which insert into the joint body or joint capsule Type 1: intracapsular ligament (runs within the articular cavity) Type 2: extra-articular ligament (runs external of the articular cavity)	Direction: guides joint motion Restraint: limits joint motion Strengthens: strengthens the joint capsule	Ubiquitous in synovial joints
Meniscus	Two crescent-shaped, intraarticular discs of fibrocartilage attached to the medial and lateral joint capsule of the knee joint	Enlarges articulating surfaces: results in reduced mechanical pressure and a decreased risk of dislocation	Only in the knee joint
Articular disc	An intraarticular disc of fibrocartilage and dense connective tissue that spans the entire articular cavity of specific joints and is fused to the joint capsule margin		Temporomandibular joint Sternoclavicular joint Radiocarpal joint
Labrum	Intraarticular wedge-shaped ring of fibrocartilage that surrounds the edges of joint surfaces, which are connected medially to articular cartilage and laterally with the joint capsule		Glenohumeral joint: glenoid labrum Hip joint: acetabular labrum
Joint recess	Protrusions of the joint capsule	Reserve zone of the joint capsule (enables large deviations in motion)	E.g., at the glenohumeral joint (axillary recess)
Synovial bursa	Cushioned sac filled with synovial fluid. It is located between tendons and muscles and also bone structures. Type 1: communicating bursas (direct connection to the articular cavity) Type 2: non-communicating bursas (no connection to the articular cavity)	Helps tendons slide more easily over the bone	E.g., at the knee joint (infrapatellar bursa)

In the musculoskeletal system, ligaments connect bones to one another, whereas tendons connect muscle to bone!

Accessory structures in synovial joints

Types of synovial joints

Type of joint	Degrees of freedom	Range of motion	Examples of joints
Ball and socket joint	3	Flexion & extension Abduction & adduction Internal rotation & external rotation	Hip joint (acetabulofemoral joint) Glenohumeral joint (shoulder joint)
Condyloid joint	2	Flexion & extension Side-to-side motion Lateral flexion Abduction & adduction	Atlanto-occipital joint Radiocarpal joint
Saddle joint	2	Flexion & extension Abduction & adduction	Carpometacarpal joint
Hinge joint	1	Flexion & extension	Humeroulnar joint Proximal interphalangeal joint Distal interphalangeal joint
Pivot joint	1	Internal & external rotation (terms for the radioulnar joint: pronation & supination)	Atlantoaxial joint Proximal radioulnar joint Distal radioulnar joint
Modified hinge joint	2	Flexion & extension Internal & external rotation	Humeroradial joint with the humeroulnar joint Tibiofemoral joint
Plane joint	1–3	Varies, depending on the accessory muscles and ligaments: rotation and/or linear motion	Patellofemoral joint Facet joints

Types of synovial joints

Synarthroses (fixed joints)

There are four types of synarthroses, categorized based on the type of tissue connected to the articulating bone surface: syndesmosis, synchondrosis, symphysis, and synostosis.

Type of synarthrosis	Definition	Examples
Syndesmosis	Bones are joined together by connective tissue	Fontanelles of the skull Vertebral body connections: ligamenta flava, interspinous ligament, supraspinous ligament Interosseous membrane Inferior tibiofibular joint (connection between the distal tibia and distal fibula) Gomphosis: A special peg-and-socket joint between the root of a tooth and alveolar process that is connected via collagenous connective tissue fibers.
Synchondrosis	Bones are joined together by hyaline cartilage	Costal cartilage Epiphyseal plates in bone growth
Symphysis	A special type of synchondrosis in which bones are joined together by fibrocartilage	Pubic symphysis Intervertebral discs
Synostosis	Bones fused together via ossification	Sacrum Ossified (closed) epiphyseal plates

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Evidence-based content, created and peer-reviewed by physicians. Read the disclaimer