Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the locations where two bones meet and where body movement occurs. The structure of a joint determines its function, and there is a tradeoff between strength and mobility: the more mobile a joint, the less stable it tends to be.

• Joints are essential for movement and flexibility in the skeletal system.

• There are two main methods of classifying joints:

  • Structural classification (based on anatomy)

  • Functional classification (based on range of motion)

• Joint structure determines joint function.

Classification of Joints

Structural Classifications

Joints are classified structurally according to the material binding the bones and the presence or absence of a joint cavity.

• Fibrous joints: Bones joined by dense connective tissue; no joint cavity.

• Cartilaginous joints: Bones joined by cartilage; no joint cavity.

• Bony joints: Bones fused together; no joint cavity.

• Synovial joints (diarthrosis): Bones separated by a fluid-filled joint cavity; freely movable.

Functional Classifications

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

• Synarthrosis: Immovable joint

• Amphiarthrosis: Slightly movable joint

• Diarthrosis (synovial): Freely movable joint

Synarthrosis (Immovable Joints)

Synarthroses are very strong joints where the edges of bones may touch or interlock. They may be fibrous or cartilaginous.

• Four types of synarthrotic joints:

  • Suture: Found only between bones of the skull; edges interlock and are bound by dense fibrous connective tissue.

  • Gomphosis: Binds teeth to bony sockets; fibrous connection via the periodontal ligament.

  • Synchondrosis: Rigid cartilaginous bridge between two bones; found between vertebrosternal ribs and sternum, and in the epiphyseal cartilage of growing long bones.

  • Synostosis: Created when two bones fuse; example is the epiphyseal lines of mature long bones.

Amphiarthrosis (Slightly Movable Joints)

Amphiarthroses allow limited movement and may be fibrous or cartilaginous.

• Two types of amphiarthroses:

  • Syndesmosis: Bones connected by a ligament (e.g., tibia/fibula, ulna/radius).

  • Symphysis: Bones connected by fibrocartilage (e.g., pubic symphysis, intervertebral discs).

Diarthrosis (Synovial Joints)

General Structure and Function

Synovial joints are freely movable and are the most common type of joint in the body, especially between long bones. They are surrounded by a joint capsule (articular capsule) and contain synovial fluid.

• Joint capsule has two layers:

  • Outer: fibrous membrane

  • Inner: synovial membrane

• Synovial fluid functions:

  • Made of proteoglycans

  • Lubrication

  • Nutrient distribution

  • Shock absorption

• Articular cartilage (hyaline cartilage) covers articulating surfaces to reduce friction and absorb shock.

Components of Synovial Joints

The following table summarizes the main components of a synovial joint, as shown in the sagittal section image:

Example: The knee joint is a classic synovial joint, containing all these components and allowing significant movement while maintaining stability.