Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the locations where two or more rigid elements of the skeleton meet. The Greek root arthro means joint. The structure of joints enables the skeleton to resist crushing, tearing, and other mechanical forces, while also allowing for movement.

• Articulation: The point of contact between bones.

• Function: Provides both stability and mobility to the skeleton.

Classification of Joints

Functional Classification

Joints are classified functionally based on the amount of movement they allow:

• Synarthroses: Immovable joints; commonly found in the axial skeleton.

• Amphiarthroses: Slightly movable joints; also common in the axial skeleton.

• Diarthroses: Freely movable joints; most common in the appendicular skeleton (all synovial joints).

Structural Classification

Structural classification is based on the material binding the bones and the presence or absence of a joint cavity. The main structural types are:

• Fibrous Joints

• Cartilaginous Joints

• Synovial Joints

Summary Table: Joint Classes

The following table summarizes the main joint classes, their structural characteristics, types, and mobility:

Fibrous Joints

General Features

Fibrous joints are connected by fibrous connective tissue and do not have a joint cavity. Most are immovable or only slightly movable. The three main types are:

• Sutures

• Syndesmoses

• Gomphoses

Sutures

Sutures are joints where bones are tightly bound by a minimal amount of fibrous tissue. They occur only between the bones of the skull and allow bone growth during childhood. In middle age, the fibrous tissue ossifies, forming synostoses (closed sutures).

• Location: Skull

• Function: Immovable, protection of the brain

• Synostosis: The process of suture closure by ossification

Syndesmoses

In syndesmoses, bones are connected exclusively by ligaments. The amount of movement depends on the length of the connecting fibers.

• Tibiofibular joint: Immovable synarthrosis (short fibers)

• Interosseous membrane between radius and ulna: Freely movable diarthrosis (longer fibers)

Gomphoses

Gomphoses are joints where a tooth is anchored in its socket by the periodontal ligament.

• Location: Teeth in alveolar sockets

• Function: Immovable joint

Cartilaginous Joints

General Features

Cartilaginous joints unite bones by cartilage and lack a joint cavity. There are two main types:

• Synchondroses

• Symphyses

Synchondroses

In synchondroses, bones are united by hyaline cartilage. These joints are typically immovable.

• Examples: Epiphyseal plates in growing bones; joint between first rib and manubrium of sternum

Symphyses

Symphyses are joints where bones are united by fibrocartilage, which resists tension and compression. These joints are slightly movable and provide strength with flexibility.

• Examples: Intervertebral discs; pubic symphysis

• Articular cartilage: Hyaline cartilage present as articular cartilage on bone surfaces

Synovial Joints

General Features

Synovial joints are the most movable type of joint and are all diarthroses. Each contains a fluid-filled joint cavity.

• Articular cartilage: Hyaline cartilage covers the ends of opposing bones, absorbing compression.

• Joint (articular) cavity: Unique to synovial joints; a potential space holding a small amount of synovial fluid.

• Articular capsule: Encloses the joint cavity; consists of two layers:

  • Fibrous layer: Dense irregular connective tissue, strengthens the joint.

  • Synovial membrane: Loose connective tissue, lines the capsule and covers internal joint surfaces; produces synovial fluid.

• Synovial fluid: Viscous, similar to raw egg white; filtrate of blood containing glycoproteins secreted by fibroblasts. Provides lubrication (weeping lubrication).

• Reinforcing ligaments: May be thickened parts of the fibrous layer, extracapsular (outside capsule), or intracapsular (inside capsule).

• Nerve and blood supply: Highly supplied with sensory nerves (detect pain, monitor stretch) and rich blood supply (capillary beds produce synovial fluid).

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow:

• Plane

• Hinge

• Pivot

• Condylar

• Saddle

• Ball-and-socket

Example: The shoulder joint is a ball-and-socket synovial joint, allowing a wide range of movement.

Additional info: Synovial joints are essential for most voluntary movements and are the most common joint type in the limbs.