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Joints

Last updated: February 2, 2021

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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.

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
  • Cartilaginous joint
  • Fibrous joint
Connection
Range of motion
  • Generally high
  • Slightly movable (linear motion)
  • No motion
Examples

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.

Socket joint

Articular surface
  • Articulating bones are connected to one another via articular surfaces.
Articular cartilage
  • 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

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
  • 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 the joint capsule

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

Supporting structures of synovial joints

Structure Definition Function Occurrence
Ligament (joint ligament)
  • Direction: guides joint motion
  • Restraint: limits joint motion
  • Strengthens: strengthens the joint capsule
Ubiquitous in synovial joints
Meniscus
  • Enlarges articulating surfaces: results in reduced mechanical pressure and a decreased risk of dislocation
Articular disc
Labrum
Joint recess
  • Reserve zone of the joint capsule (enables large deviations in motion)
Synovial bursa
  • Helps tendons slide more easily over the bone

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

Types of synovial joints

Type of joint Degrees of freedom Range of motion Examples of joints
Ball and socket joint

3

Condyloid joint

2

Saddle joint

2

Hinge joint

1

Pivot joint

1

Modified hinge joint

2

Plane joint 1–3
  • Varies, depending on the accessory muscles and ligaments: rotation and/or linear motion

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

Synchondrosis

Bones are joined together by hyaline cartilage

Symphysis

A special type of synchondrosis in which bones are joined together by fibrocartilage
Synostosis Bones fused together via ossification