Joints (Articulations)

Definition and Function of Articulations

Articulations, or joints, are sites where two or more bones meet. They serve two primary functions: providing the skeleton with mobility and holding the skeleton together. In some cases, joints also play a protective role by absorbing shock or limiting movement to prevent injury.

• Mobility: Joints allow for a range of movements necessary for daily activities and complex motions.

• Stability: Joints maintain the integrity of the skeletal system by holding bones together.

Classification of Joints by Function (Movement)

Types of Joints Based on Movement

Joints are classified functionally by the amount of movement they permit:

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

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

• Diarthroses: Freely movable joints (predominate in the appendicular skeleton, e.g., shoulder, knee).

Classification of Joints by Structure

Structural Types of Joints

Joints are also classified by their structural characteristics, which relate to the material binding the bones and the presence or absence of a joint cavity:

• Suture: Held together by very short, interconnecting fibers; interlocking bone edges. Found only in the skull. Immovable (synarthrosis).

• Syndesmosis: Bones connected by a ligament; fibrous tissue is longer than in sutures. Allows more movement than a suture (can be amphiarthrotic or synarthrotic).

• Gomphosis: "Peg-in-socket" fibrous joint. Example: tooth held in socket by the periodontal ligament. Immovable.

• Synchondrosis: Bones united by hyaline cartilage (e.g., epiphyseal plates in children, joint between first rib and sternum). Immovable or slightly movable.

• Symphysis: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis). Slightly movable (amphiarthrosis).

• Synovial: Articulating bones are separated by a fluid-containing joint cavity. Freely movable (diarthrosis).

Synostosis

Definition and Examples

Synostosis is a completely ossified (fused) joint where the original joint has been replaced by bone. Examples include the lambdoid and sagittal sutures of the skull in adults.

Structure of Synovial Joints

Key Components and Their Functions

• Articular Cartilage: Hyaline cartilage covering bone ends; reduces friction and absorbs shock.

• Joint (Synovial) Cavity: Space containing synovial fluid; allows for free movement.

• Articular Capsule: Encloses the joint cavity; has two layers:

  • Fibrous Layer: Dense irregular connective tissue; strengthens joint to prevent separation.

  • Synovial Membrane: Loose connective tissue; lines the internal surface (except where cartilage is present) and produces synovial fluid.

• Synovial Fluid: Viscous, egg-white-like fluid; lubricates and nourishes articular cartilage, reduces friction, and prevents overheating and tissue destruction.

• Reinforcing Ligaments: Strengthen and stabilize the joint; can be:

  • Capsular: Thickened parts of the fibrous layer.

  • Extracapsular: Located outside the capsule.

  • Intracapsular: Located deep within the capsule.

• Nerves and Blood Vessels: Provide sensory input and nourishment, especially to the synovial membrane.

• Articular Discs (Menisci): Fibrocartilage pads that improve the fit between articulating bone ends, increase joint stability, and minimize wear and tear.

Bursae and Tendon Sheaths

Definitions and Functions

• Bursa (plural: bursae): Flattened fibrous sacs lined with synovial membrane and containing synovial fluid. Found where ligaments, muscles, skin, tendons, or bones rub together; reduce friction.

• Tendon Sheath: An elongated bursa that wraps around a tendon subjected to friction (e.g., in the wrist and ankle).

Factors Affecting Joint Stability and Mobility

Key Factors

• Articular Surface Shape: Determines possible movements and stability.

• Ligaments: The number and position of ligaments reinforce the joint.

• Muscle Tone: The most important stabilizing factor; keeps tendons taut as they cross joints.

Note: Increased stability often means decreased mobility, and vice versa.

Movements at Synovial Joints

Types of Movements

• Gliding: Flat bone surfaces slide over each other (e.g., intercarpal, intertarsal joints).

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

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

• Hyperextension: Extension beyond the anatomical position.

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

• Adduction: Movement toward the midline.

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

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

• Supination: Rotating the forearm laterally so the palm faces anteriorly.

• Pronation: Rotating the forearm medially so the palm faces posteriorly.

• Dorsiflexion: Lifting the foot upward toward the shin.

• Plantar Flexion: Pointing the toes downward.

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

• Protraction: Moving a body part forward (e.g., jutting the jaw).

• Retraction: Moving a body part backward.

• Elevation: Lifting a body part superiorly (e.g., shrugging shoulders).

• Depression: Moving an elevated part inferiorly (e.g., lowering the jaw).

• Opposition: Movement of the thumb to touch the fingertips (saddle joint of thumb).

Types of Synovial Joints and Their Movements

Anatomy of Major Joints

Shoulder (Glenohumeral) Joint

• Type: Ball and socket

• Features: Thin, loose articular capsule; allows greatest range of motion but less stability.

• Stabilization: Muscle tendons (especially biceps brachii) and rotator cuff muscles.

Elbow Joint

• Type: Hinge

• Features: Formed by humerus, radius, and ulna; trochlear notch of ulna grips trochlea of humerus tightly.

• Movement: Flexion and extension only; very stable.

Hip (Coxal) Joint

• Type: Ball and socket

• Features: Deep socket (acetabulum) and strong ligaments; allows all movements but less range than shoulder.

• Stability: High, due to deep socket and strong ligaments.

Knee Joint

• Type: Largest, most complex; primarily a hinge (tibiofemoral), with a plane joint (femoropatellar).

• Features: Three joints in one; menisci improve fit and stability; allows flexion, extension, and some rotation when flexed.

• Stability: Provided by ligaments and menisci; side-to-side movement and rotation are limited when extended.

Temporomandibular Joint (TMJ)

• Type: Modified hinge

• Features: Condylar process of mandible articulates with mandibular fossa of temporal bone; allows rotation (opening/closing) and gliding (protrusion/retraction).

Joint Injuries and Disorders

Common Joint Injuries

• Sprain: Ligaments are stretched or torn; slow healing due to poor blood supply; causes pain and immobilization.

• Dislocation: Bones are forced out of alignment; often accompanied by sprains, inflammation, and difficulty moving the joint; requires reduction (realignment).

• Bursitis: Inflammation of a bursa, usually due to trauma or friction; causes pain and swelling.

• Tendonitis: Inflammation of tendon sheaths, typically from overuse; causes pain and restricted movement.

Types of Arthritis

Additional info: In osteoarthritis, enzymes such as metalloproteinases degrade cartilage; in rheumatoid arthritis, the immune system attacks joint tissues; in gout, uric acid crystals trigger an inflammatory response.