Joints (Articulations)

Definition and Functions

Joints, also known as articulations, are sites where two or more bones meet. They play a crucial role in the human skeletal system by providing both mobility and stability.

• Give skeleton mobility: Joints allow for a wide range of movements necessary for daily activities.

• Hold skeleton together: Joints maintain the integrity and structure of the skeleton by connecting bones.

Classification of Joints

Functional Classification

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

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

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

• Diarthroses: Freely movable joints, also known as synovial joints. These are the most abundant type of joints in the body.

Structural Classification

Structural classification is based on the material binding the bones together and the presence or absence of a joint cavity:

• Fibrous Joints: Bones joined by fibrous tissues; no joint cavity.

• Cartilaginous Joints: Bones united by cartilage; no joint cavity.

• Synovial Joints: Bones separated by a fluid-filled joint cavity; only type with a joint cavity.

Fibrous Joints

General Features

Fibrous joints are connected by dense connective tissue and lack a joint cavity. Most are immovable.

• Sutures: Found only in the skull; bones are tightly bound by minimal fibrous tissue, allowing for growth during youth.

• Syndesmoses: Bones connected by a ligament; movement varies from immovable to slightly movable. Example: Distal connection between tibia and fibula, and between radius and ulna.

• Gomphoses: Peg-in-socket fibrous joint; only example is the articulation of a tooth with its socket (alveolus) in the jawbone.

Table: Types of Fibrous Joints

Cartilaginous Joints

General Features

In cartilaginous joints, bones are united by cartilage and lack a joint cavity. They allow for little to no movement.

• Synchondroses: A bar or plate of hyaline cartilage unites the bones. All are synarthrotic (immovable). Examples: Epiphyseal plates in children, costal cartilage of the first rib and sternum.

• Symphyses: The articulating surfaces of the bones are covered with hyaline cartilage, which is fused to a pad of fibrocartilage. All are amphiarthrotic (slightly movable). Examples: Intervertebral joints, pubic symphysis.

Table: Types of Cartilaginous Joints

Key Terms

• Articulation: The site where two or more bones meet.

• Synarthrosis: An immovable joint.

• Amphiarthrosis: A slightly movable joint.

• Diarthrosis: A freely movable joint.

• Hyaline cartilage: A type of cartilage found on many joint surfaces; it is pearly bluish in color with a firm consistency.

• Fibrocartilage: A tough, very strong tissue found in intervertebral discs and joint capsules.

Additional info: The notes above cover the first part of the chapter, focusing on the definition, classification, and basic structure of joints. Later sections (not shown in the images) would likely cover synovial joints, joint injuries, and clinical conditions.