Joints of the Human Body: Structure, Classification, and Movement

Joints (Arthro-): Introduction

Definition and Overview

Joints, also known as articulations, are locations where two or more bones meet. They are essential for providing both mobility and stability to the human skeleton.

• Mobility: Joints allow for movement of the skeleton, enabling activities such as walking, running, and grasping.

• Stability: Joints also provide structural support, maintaining the integrity of the skeletal system.

• Inverse Relationship: There is an inverse relationship between mobility and stability in joints; as mobility increases, stability tends to decrease, and vice versa.

Classification of Joints

Overview

Joints can be classified in two main ways: by their function (degree of movement) and by their structure (type of tissue and presence of a cavity).

Functional Classification of Joints

This classification is based on the amount of movement a joint allows.

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

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

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

Structural Classification of Joints

This classification focuses on the material binding the bones together and whether a joint cavity is present.

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

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

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

Fibrous Joints

General Structure

• Composed of dense connective tissue (mainly collagen fibers).

• No joint cavity present.

• Types include sutures (skull), syndesmoses (between long bones), and gomphoses (teeth in sockets).

Cartilaginous Joints

General Structure

• Bones are united by cartilage (hyaline or fibrocartilage).

• No joint cavity present.

• Types include synchondroses (epiphyseal plates) and symphyses (pubic symphysis, intervertebral discs).

Synovial Joints

Structural Characteristics

• Articulating bones are separated by a fluid-filled joint cavity.

• Most common and most movable type of joint in the body.

• Key features include:

  • Articular cartilage: Hyaline cartilage covering bone surfaces.

  • Joint (articular) capsule: Two-layered capsule enclosing the joint cavity.

  • Synovial fluid: Lubricates and nourishes the articular cartilage.

  • Ligaments: Reinforce and strengthen the joint.

  • Nerves and blood vessels: Supply the joint tissues.

Bursae and Tendon Sheaths

• Bursae: Flattened sacs filled with synovial fluid; reduce friction between moving structures.

• Tendon sheaths: Elongated bursae that wrap around tendons; found where tendons are subjected to friction.

Types of Synovial Joints

Classification by Shape and Movement

Type	Shape	Movement	Example
Plane (Gliding)	Flat articular surfaces	Nonaxial (gliding)	Intercarpal joints, facet joints of vertebrae
Hinge	Cylindrical projection fits into a trough	Uniaxial (flexion/extension)	Elbow, knee
Pivot	Rounded end fits into a ring	Uniaxial (rotation)	Proximal radioulnar joint, atlantoaxial joint
Condyloid (Ellipsoid)	Oval articular surface fits into a depression	Biaxial (flexion/extension, abduction/adduction)	Metacarpophalangeal joints
Saddle	Articular surfaces both concave and convex	Biaxial (flexion/extension, abduction/adduction)	Carpometacarpal joint of thumb
Ball-and-Socket	Spherical head fits into a cup-like socket	Multiaxial (all movements)	Shoulder, hip

Common Body Movements at Synovial Joints

Types of Movements

• Gliding: Sliding of flat bone surfaces over each other (e.g., between carpals).

• Angular Movements:

  • Flexion: Decreases the angle between bones (e.g., bending the elbow).

  • Extension: Increases the angle between bones (e.g., straightening the knee).

  • Abduction: Movement away from the midline (e.g., raising the arm sideways).

  • Adduction: Movement toward the midline (e.g., lowering the arm to the side).

  • Circumduction: Circular movement combining flexion, extension, abduction, and adduction (e.g., arm circles).

• Rotation: Turning a bone around its own long axis (e.g., shaking the head 'no').

• Special Movements:

  • Supination/Pronation: Rotational movement of the forearm (palm up/palm down).

  • Dorsiflexion/Plantar Flexion: Movements at the ankle (toes up/toes down).

  • Inversion/Eversion: Movements of the sole of the foot (inward/outward).

  • Opposition: Movement of the thumb to touch the fingers.

Summary Table: Functional vs. Structural Classification

Functional Classification	Structural Classification	Example
Synarthrosis (Immovable)	Fibrous (Suture)	Sutures of the skull
Amphiarthrosis (Slightly movable)	Cartilaginous (Symphysis)	Intervertebral discs
Diarthrosis (Freely movable)	Synovial	Shoulder, knee

Key Terms and Definitions

• Articulation: Another term for a joint.

• Ligament: A band of dense connective tissue connecting bones.

• Bursa: A fluid-filled sac reducing friction between tissues.

• Tendon Sheath: An elongated bursa wrapping around a tendon.

Additional info:

• Understanding joint structure and function is essential for comprehending movement, injury, and disease in the musculoskeletal system.

• Clinical relevance includes conditions such as arthritis, dislocations, and ligament injuries.