Joints: Structure, Classification, and Function in Human Anatomy

Joints (Articulations)

Definition and Functions

Joints, also known as articulations, are sites where two or more bones meet. They serve two main functions:

• Secure bones together to provide stability to the skeleton.

• Allow rigid skeleton to move by providing points of movement between bones.

With the exception of the hyoid bone, every bone forms a joint with at least one other bone.

Classification of Joints

Structural Classification

Joints are classified structurally based on the material binding the bones together and the presence or absence of a joint cavity. The main types are:

• Fibrous joints

• Cartilaginous joints

• Synovial joints

Functional Classification

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

• Synarthroses – immovable joints

• Amphiarthroses – slightly movable joints

• Diarthroses – freely movable joints

Freely movable joints predominate in the limbs. Immovable and slightly movable joints are found in the axial skeleton. Fibrous joints are immovable, synovial joints are freely movable, and cartilaginous joints have both rigid and slightly movable examples.

Fibrous Joints

Types and Characteristics

• Bones are joined by fibrous tissue.

• No joint cavity is present.

• Most are synarthrotic (immovable).

• Types are based on length of intervening fibers: sutures, syndesmoses, gomphoses.

Type	Description	Example
Sutures	Interlocking edges of bones, very short connective tissue fibers	Skull bones
Syndesmoses	Bones connected by a ligament or interosseous membrane	Distal tibiofibular joint, radius-ulna interosseous membrane
Gomphoses	Peg-in-socket fibrous joint	Teeth in alveolar sockets

Cartilaginous Joints

Types and Characteristics

• Articulating bones are united by cartilage.

• Lack a joint cavity.

• Two types: synchondroses and symphyses.

Type	Description	Example
Synchondroses	Bones united by hyaline cartilage; usually immovable	Epiphyseal plates, first rib and sternum
Symphyses	Bones united by fibrocartilage; amphiarthrotic (slightly movable)	Pubic symphysis, intervertebral discs

Synovial Joints

Features and Types

Synovial joints are separated by a fluid-filled joint cavity, allowing free movement (diarthroses). Examples include most limb joints.

Distinguishing Features

1. Articular cartilage – covers opposing bone surfaces.

2. Joint cavity – filled with synovial fluid.

3. Articular capsule – double-layered capsule enclosing the joint cavity.

4. Synovial fluid – slippery fluid that lubricates and nourishes articular cartilage.

5. Reinforcing ligaments – strengthen the joint.

6. Nerves and blood vessels – supply the joint, detect pain, and monitor joint position.

Types of Synovial Joints (by shape and movement)

• Plane joints – flat surfaces, allow gliding (intercarpal joints).

• Hinge joints – cylindrical end fits into a trough, allow flexion/extension (elbow).

• Pivot joints – rounded end fits into a ring, allow rotation (proximal radioulnar joint).

• Condyloid joints – oval articular surface fits into a depression (knuckle joints).

• Saddle joints – similar to condyloid but with greater movement (thumb).

• Ball-and-socket joints – spherical head fits into a cup-like socket (shoulder, hip).

Stability of Synovial Joints

Factors Contributing to Stability

• Nature of articular surfaces – shapes determine possible movements and stability.

• Number and positioning of ligaments – more ligaments usually mean greater stability.

• Muscle tone – most important stabilizing factor, keeps tendons taut.

Movements Allowed by Synovial Joints

Axes and Planes of Movement

• Uniaxial – movement in one plane (elbow).

• Biaxial – movement in two planes (wrist).

• Multiaxial – movement in or around all three planes (shoulder, hip).

Types of Movements

• Gliding – flat bone surfaces slide past each other.

• Angular – increase or decrease the angle between bones (flexion, extension, abduction, adduction, circumduction).

• Rotation – turning a bone around its own long axis.

Special Movements: pronation, supination, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Selected Articulations of the Body

Knee Joint

• Multiple joints in one – patella/femur; tibiofemoral joint.

• Menisci (cartilage discs) prevent side-to-side rocking of femur on tibia.

• Primarily a hinge joint (flexion/extension).

• Ligaments: collateral (prevent unwanted movement), cruciate (prevent displacement).

Shoulder Joint

• Head of humerus articulates with glenoid cavity of scapula.

• Most freely movable joint; less stability.

• Stabilized by rotator cuff muscles and ligaments.

Elbow Joint

• Distal humerus articulates with radius and ulna.

• Hinge joint, permits flexion and extension.

Joint Injuries and Disease Processes

Common Injuries

• Sprain – ligaments are stretched or torn; slow healing due to poor vascularization.

• Cartilage injuries – tearing of menisci or articular cartilage; often requires surgery.

• Dislocation – bones forced out of alignment; must be reduced (returned to proper position).

Inflammatory and Degenerative Conditions

• Bursitis and Tendonitis – inflammation of bursae or tendons due to overuse or friction.

• Arthritis – over 100 types; most common are osteoarthritis (wear and tear), rheumatoid arthritis (autoimmune), and gouty arthritis (uric acid crystals).

Example: Osteoarthritis is a degenerative joint disease characterized by the breakdown of articular cartilage, leading to pain and reduced mobility.

Additional info: Synovial fluid is produced by the synovial membrane and acts as a lubricant and nutrient source for articular cartilage.