Joints (Articulations)

Introduction to Joints

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 it together. Understanding joint structure and function is crucial for diagnosing and treating musculoskeletal injuries and diseases.

Classification of Joints

Joints are classified in two main ways: structural and functional.

• Structural Classification: Based on the material binding the bones and the presence or absence of a joint cavity.

• Functional Classification: Based on the degree of movement allowed.

Functional Classification

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Structural Classification

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

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; not highly movable.

• Synovial Joints: Bones separated by a fluid-filled joint cavity; all are diarthrotic (freely movable).

Fibrous Joints

• Sutures: Rigid, interlocking joints of the skull; allow for growth during youth; ossify in middle age.

• Syndesmoses: Bones connected by ligaments; e.g., inferior tibiofibular joint.

• Gomphoses: Peg-in-socket joints; e.g., tooth in socket.

Cartilaginous Joints

• Synchondroses: Bar or plate of hyaline cartilage unites bones; e.g., epiphyseal plate, joint between first rib and sternum.

• Symphyses: Fibrocartilage unites bone; e.g., intervertebral joints, pubic symphysis.

Synovial Joints

Synovial joints are the most common and most movable type of joint in the body. They have several unique structural features:

• Articular cartilage: Hyaline cartilage covering bone ends, preventing crushing.

• Joint (synovial) cavity: Small, fluid-filled space unique to synovial joints.

• Articular capsule: Two layers—external fibrous layer (dense irregular connective tissue) and inner synovial membrane (loose connective tissue).

• Reinforcing ligaments: Capsular, extracapsular, and intracapsular ligaments.

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage.

• Nerves and blood vessels: Nerves detect pain and monitor joint position; capillary beds supply filtrate for synovial fluid.

Other Features of Synovial Joints

• Fatty pads: Cushioning between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage improves fit, stabilizes joint, reduces wear.

• Bursae: Bags of synovial fluid that reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae wrapped around tendons subjected to friction.

Joint Stability

Three factors determine the stability of joints:

• Shape of articular surfaces: Shallow surfaces are less stable than ball-and-socket.

• Ligament number and location: More ligaments provide stronger joints.

• Muscle tone: Keeps tendons taut; most important for reinforcing shoulder, knee, and foot arches.

Types of Synovial Joint Movements

Synovial joints allow several types of movements, classified as nonaxial, uniaxial, biaxial, and multiaxial.

• Gliding: Flat bone surfaces glide over each other; e.g., intercarpal joints.

• Angular Movements: Change the angle between bones.

  • Flexion: Decreases the angle.

  • Extension: Increases the angle.

  • Hyperextension: Extension beyond anatomical position.

• Abduction: Movement away from midline.

• Adduction: Movement toward midline.

• Circumduction: Limb describes a cone in space.

• Rotation: Turning bone around its own long axis.

  • Medial rotation: Toward midline.

  • Lateral rotation: Away from midline.

Special Movements

• Supination and Pronation: Rotation of radius and ulna; supination (palms anterior), pronation (palms posterior).

• Opposition: Movement of thumb to touch other fingers.

• Dorsiflexion and Plantar Flexion: Dorsiflexion (foot toward shin), plantar flexion (pointing toes).

• Inversion and Eversion: Inversion (sole medially), eversion (sole laterally).

• Elevation and Depression: Elevation (lifting body part), depression (lowering body part).

• Protraction and Retraction: Protraction (moving forward), retraction (moving backward).

Types of Synovial Joints

Synovial joints are categorized based on the shape of their articular surfaces and the movement they allow:

• Plane Joints: Nonaxial, gliding movements; e.g., intercarpal joints.

• Hinge Joints: Uniaxial, flexion and extension; e.g., elbow.

• Pivot Joints: Uniaxial, rotation; e.g., proximal radioulnar joint.

• Condylar (Ellipsoid) Joints: Biaxial, flexion/extension and abduction/adduction; e.g., metacarpophalangeal joints.

• Saddle Joints: Biaxial, articular surfaces both concave and convex; e.g., carpometacarpal joint of thumb.

• Ball-and-Socket Joints: Multiaxial, flexion/extension, abduction/adduction, rotation; e.g., shoulder and hip.

Major Synovial Joints of the Body

• Temporomandibular Joint (TMJ): Shallow socket, prone to dislocation; symptoms include pain, stiffness, and popping sounds.

• Shoulder Joint (Glenohumeral): Most freely moving joint; ball-and-socket; stability sacrificed for mobility.

• Elbow Joint: Hinge joint; allows flexion and extension; ligaments restrict side-to-side movement.

• Hip Joint: Ball-and-socket; deep socket provides stability; dislocations are rare.

• Knee Joint: Largest, most complex; three joints surrounded by single cavity; vulnerable to injury.

Clinical Implications

• Cartilage Tears: Due to compression and shear stress; rarely repair themselves; may require surgery.

• Sprains: Ligaments stretched or torn; common in ankle, knee, lumbar region; slow repair due to poor vascularization.

• Dislocations: Bones forced out of alignment; must be reduced; subluxation is partial dislocation.

• Bursitis: Inflammation of bursa; treated with rest, ice, and anti-inflammatory drugs.

• Tendonitis: Inflammation of tendon sheaths; similar treatment to bursitis.

• Arthritis: Over 100 types; symptoms include pain, stiffness, swelling; acute forms treated with antibiotics; chronic forms include osteoarthritis, rheumatoid arthritis, and gouty arthritis.

Summary Table: Types of Joint Movements

Summary Table: Types of Synovial Joints

Example: Ankle sprains are common injuries involving the stretching or tearing of ligaments, often requiring immobilization and physical therapy for recovery.

Additional info: The notes have been expanded to provide academic context and definitions for all key terms and concepts, ensuring completeness and clarity for exam preparation.