Joints (Articulations): Classification, Structure, and Function

Introduction to Joints

Joints, also known as articulations, are the locations where two or more bones meet. They are essential for body movement and play a critical role in determining the balance between mobility and strength in the skeletal system.

• Joints allow for movement and flexibility in the skeleton.

• There is a tradeoff between mobility (range of movement) and strength (stability) at joints.

Classification of Joints

Joints are classified using two main schemes: structural and functional. The structure of a joint determines its function.

• Structural classification (based on anatomy):

  • Fibrous – bones joined by fibrous connective tissue

  • Cartilaginous – bones joined by cartilage

  • Bony – bones fused by ossification

  • Synovial – bones separated by a fluid-filled cavity

• Functional classification (based on range of motion):

  • Synarthrosis – immovable joint

  • Amphiarthrosis – slightly movable joint

  • Diarthrosis – freely movable joint

Synarthrosis (Immovable Joints)

Synarthroses are very strong joints that allow no movement. They are critical for protecting internal organs and providing stability.

• Edges of bones may touch or interlock.

• Can be fibrous or cartilaginous in structure.

• Four main types:

  • Suture

  • Gomphosis

  • Synchondrosis

  • Synostosis

Suture

• Found only between bones of the skull.

• Edges of bones interlock and are bound by dense fibrous connective tissue.

• Example: Coronal suture between frontal and parietal bones.

Gomphosis

• Binds teeth to bony sockets in the maxillae and mandible.

• Fibrous connection is the periodontal ligament.

Synchondrosis

• Bones are bridged by rigid hyaline cartilage.

• Examples:

  • Joint between vertebrosternal ribs and sternum.

  • Epiphyseal cartilage of growing long bones.

Synostosis

• Created when two bones fuse and the boundary disappears.

• Examples:

  • Left and right mandibular bones in infants.

  • Epiphyseal lines of mature long bones.

Additional info: Synarthroses provide protection and support, especially in areas like the skull, where movement would be detrimental to function.