Joints: Structural and Functional Classification, Synovial Joint Structure, and Movements

Joints: Classification and Function

Definition and General Function

Joints, also known as articulations, are the sites where two or more bones meet. They are essential for providing mobility to the skeleton and holding the skeleton together.

• Articulation: The location at which bones connect.

• Functions of joints:

  • Allow movement of the skeleton

  • Provide stability and support

Structural Classification of Joints

Basis of Classification

Structural classification is based on the material binding the bones together and whether or not a joint cavity is present. There are three main structural types:

• Fibrous joints (Fig. 8.1)

• Cartilaginous joints (Fig. 8.2)

• Synovial joints (Fig. 8.3)

Fibrous Joints & Examples

• Bones joined by fibrous tissues

• No joint cavity

• Mostly immovable

Types of fibrous joints:

• Sutures: Dense regular connective tissue, found between bones of the skull

• Syndesmoses: Bones connected by ligaments or interosseous membranes (e.g., between tibia and fibula)

• Gomphosis: Peg-in-socket joint, such as a tooth in its alveolar socket

Cartilaginous Joints & Examples

• Bones united by cartilage

• No joint cavity

• Not highly movable

Types of cartilaginous joints:

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates in children)

• Symphyses: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis)

Synovial Joints

• Bones separated by a fluid-filled joint cavity

• All are diarthroses (freely movable)

• Most joints of the body are synovial

Functional Classification of Joints

Basis of Classification

Functional classification is based on the amount of movement allowed by the joint. The three functional classes are:

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Synovial Joints: Structure and Characteristics

General Structure

Synovial joints are characterized by the presence of a joint cavity filled with synovial fluid. They are the most common and most movable type of joint in the body.

• Articular cartilage: Covers the ends of bones

• Joint (synovial) cavity: Space containing synovial fluid

• Articular capsule: Two-layered capsule enclosing the joint cavity

• Synovial fluid: Lubricates and nourishes articular cartilage

• Reinforcing ligaments: Strengthen the joint

Friction-Reducing Structures

• Bursae: Flattened fibrous sacs lined with synovial membrane and containing synovial fluid; reduce friction between moving structures

• Tendon sheath: Elongated bursa that wraps around a tendon

Stability of Synovial Joints

• Articular surfaces: Shape determines possible movements

• Ligaments: Unite bones and prevent excessive or undesirable motion

• Muscle tone: Muscle tendons crossing joints are important stabilizers

Movements at Synovial Joints

Types of Movements

Movements at synovial joints occur when muscles contract across joints and their insertion moves toward their origin. All muscles are attached at two points:

• Origin: Attachment to the immovable bone

• Insertion: Attachment to the movable bone

Common Movements

• Flexion: Decreases the angle of the joint

• Extension: Increases the angle of the joint

• Abduction: Movement away from the midline

• Adduction: Movement toward the midline

• Rotation: Turning of a bone around its own long axis

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction

• Supination: Rotation of the forearm so the palm faces anteriorly (thumb lateral)

• Pronation: Rotation of the forearm so the palm faces posteriorly (thumb medial)

• Dorsiflexion: Superior surface of foot approaches the shin

• Plantar flexion: Pointing the toes

• Inversion: Sole of foot faces medially

• Eversion: Sole of foot faces laterally

Summary Table: Structural and Functional Classification of Joints

Structural Type	Binding Material	Joint Cavity?	Functional Type	Examples
Fibrous	Dense fibrous connective tissue	No	Synarthroses (immovable), some amphiarthroses	Sutures (skull), syndesmoses (tibia-fibula), gomphoses (teeth)
Cartilaginous	Cartilage (hyaline or fibrocartilage)	No	Synarthroses or amphiarthroses	Epiphyseal plates, intervertebral discs, pubic symphysis
Synovial	Articular cartilage, synovial fluid	Yes	Diarthroses (freely movable)	Shoulder, knee, elbow, hip

Key Terms and Definitions

• Articulation: The site where two or more bones meet

• Synarthrosis: Immovable joint

• Amphiarthrosis: Slightly movable joint

• Diarthrosis: Freely movable joint

• Bursa: Fluid-filled sac that reduces friction between tissues

• Tendon sheath: Elongated bursa that wraps around a tendon

Example: Synovial Joint (Knee)

• The knee joint is a synovial joint, containing articular cartilage, a synovial cavity, and reinforcing ligaments.

• It allows flexion, extension, and limited rotation.

• Bursae and tendon sheaths reduce friction during movement.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.