Classification and Structure of Joints in the Human Body

Joint Classification

Structural Classification

Joints are classified structurally based on the presence or absence of a joint cavity and the type of connective tissue that binds the bones together.

• Fibrous Joints:

  • No joint cavity

  • Bones are joined by fibrous connective tissue

• Cartilaginous Joints:

  • No joint cavity

  • Bones are joined by cartilage

• Synovial Joints:

  • Most common type of joint

  • Has a synovial cavity

  • Articular capsule and ligaments, along with other specific structures, hold bones together

Functional Classification

Joints are also classified by the amount of movement they allow.

• Synarthroses: Immovable joints (e.g., sutures in the skull)

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs)

• Diarthroses: Freely movable joints (e.g., most synovial joints)

Note: 'Synarthroses' is plural, 'synarthrosis' is singular, and 'synarthrotic' is the adjective form.

Fibrous Joints

Sutures

Sutures are immovable joints found only in the skull, where bones are connected by dense fibrous connective tissue (usually dense irregular or dense regular connective tissue).

• Functionally synarthrotic: No movement between bones once mature

• Bones in babies may not completely join until around 2 years old

• Some sutures may fuse completely later in life, becoming synostoses (bone fused directly to bone)

Example: Skull sutures in adults

Syndesmoses

Syndesmoses are fibrous joints where bones are connected by longer and more connective tissue fibers than in sutures. The tissue is often a ligament or an interosseous membrane.

• Functionally amphiarthrotic: Slight movement, depending on fiber length

• Example: Distal tibiofibular joint, interosseous membrane between radius and ulna

Gomphoses

Gomphoses are peg-in-socket fibrous joints. The only examples in the human body are the articulations of the teeth with the sockets of the maxilla and mandible.

• Functionally synarthrotic: No movement

• Periodontal ligament holds the tooth in the socket

Cartilaginous Joints

Synchondroses

Synchondroses are cartilaginous joints where bones are united by hyaline cartilage.

• Functionally synarthrotic: Immovable

• All long bones in children have synchondroses at the epiphyseal plates (growth plates)

• Once growth is complete, the epiphyseal plate becomes a synostosis (bone fused to bone)

• Example: Sternum and first rib (only the first rib forms a synchondrosis with the sternum; other ribs form synovial joints)

Symphyses

Symphyses are cartilaginous joints where bones are united by fibrocartilage, often with a thin layer of hyaline cartilage on either side.

• Functionally amphiarthrotic: Slightly movable

• Fibrocartilage provides strength and flexibility

• Examples: Pubic symphysis, intervertebral discs between vertebral bodies

• Important: When discussing joints between vertebrae, the vertebral discs are symphyses, but there are also other points of articulation (synovial joints) between vertebrae

Summary Table: Structural and Functional Classification of Joints

Structural Type	Connective Tissue	Joint Cavity	Functional Type	Examples
Fibrous	Dense fibrous connective tissue	No	Synarthrosis or Amphiarthrosis	Sutures, syndesmoses, gomphoses
Cartilaginous	Hyaline or fibrocartilage	No	Synarthrosis or Amphiarthrosis	Synchondroses, symphyses
Synovial	Articular cartilage, synovial membrane	Yes	Diarthrosis	Shoulder, knee, hip, etc.

Key Terms and Definitions

• Synarthrosis: An immovable joint

• Amphiarthrosis: A slightly movable joint

• Diarthrosis: A freely movable joint

• Synostosis: A completely fused joint where bone is fused directly to bone

• Interosseous membrane: A broad and thin plane of fibrous tissue that separates many of the bones of the body

Examples and Applications

• Sutures: Skull bones in adults

• Syndesmoses: Distal tibiofibular joint, interosseous membrane between radius and ulna (allows pronation and supination of the forearm)

• Gomphoses: Teeth in their sockets

• Synchondroses: Epiphyseal plates in children, first sternocostal joint

• Symphyses: Pubic symphysis, intervertebral discs

Additional info: Synovial joints, while not detailed in these notes, are the most common and most movable type of joint in the body, characterized by a synovial cavity and articular cartilage.