Chapter 9: Joints (Articulations) – Structure, Function, and Clinical Significance

Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the locations where two bones meet. They are essential for body movement and represent a balance between strength and mobility. The structure of a joint determines its function and range of motion.

• Definition: A joint is a point of contact between two bones, allowing for movement or providing stability.

• Tradeoff: Joints with greater mobility tend to be less stable, while more stable joints are less mobile.

Classification of Joints

Structural and Functional Classification

Joints are classified by their anatomical structure and their functional range of motion. Understanding these classifications helps explain how joints operate and their role in the skeletal system.

• Structural Classification:

  • Fibrous Joints: Bones connected by fibrous tissue.

  • Cartilaginous Joints: Bones connected by cartilage.

  • Bony Joints: Bones fused together.

  • Synovial Joints: Bones separated by a fluid-filled cavity.

• Functional Classification:

  • Synarthrosis: Immovable joints; very strong.

  • Amphiarthrosis: Slightly movable joints; stronger than diarthrosis.

  • Diarthrosis: Freely movable joints; most mobile.

Types of Synarthrotic Joints (Immovable)

Synarthroses are joints that do not allow movement. They are very strong and may be fibrous or cartilaginous.

• Suture: Found only between bones of the skull; edges interlock and are bound by dense fibrous connective tissue.

• Gomphosis: Binds teeth to bony sockets via the periodontal ligament.

• Synchondrosis: Rigid cartilaginous bridge between bones; e.g., between vertebrosternal ribs and sternum, epiphyseal cartilage in growing bones.

• Synostosis: Created when two bones fuse; e.g., metopic suture of frontal bone, epiphyseal lines in mature bones.

Types of Amphiarthrotic Joints (Slightly Movable)

Amphiarthroses allow limited movement and are stronger than diarthroses. They may be fibrous or cartilaginous.

• Syndesmosis: Bones connected by a ligament.

• Symphysis: Bones connected by fibrocartilage.

Synovial Joints (Diarthroses)

Structure and Accessory Components

Synovial joints are freely movable and found at the ends of long bones. They are surrounded by a joint capsule and contain synovial fluid, which lubricates and nourishes the joint.

• Joint Capsule (Articular Capsule): Encloses the joint cavity and contains the synovial membrane.

• Synovial Fluid: Egg yolk-like consistency; contains proteoglycans. Functions include lubrication, nutrient distribution, and shock absorption.

• Articular Cartilage: Covers articulating surfaces, preventing direct bone contact.

Accessory Structures

• Cartilages: Meniscus is a fibrocartilage pad between opposing bones.

• Fat Pads: Adipose tissue covered by synovial membrane; protects articular cartilages. -helps cushion and protect

• Ligaments: Support and strengthen joints; a sprain is a ligament with torn collagen fibers.

  - supports and strengthen joint

  -sprain- ligament in

• Tendons: Attach muscles to bones around the joint. - connected tissues that go on end of organ

• Bursae: Small pockets of synovial fluid that cushion areas where tendons or ligaments rub against other tissues.

Factors Stabilizing Synovial Joints

• Collagen fibers of joint capsule and ligaments

• Shapes of articulating surfaces and menisci

• Other bones, muscles, or fat pads

• Tendons attached to articulating bones

Movements at Synovial Joints

Planes and Types of Movement

Movements at synovial joints are described by their plane or direction and the relationship between structures. Joints may be monaxial (one plane), biaxial (two planes), or triaxial (three planes).

• Monaxial: Movement in one plane (e.g., elbow).

• Biaxial: Movement in two planes (e.g., wrist).

• Triaxial: Movement in three planes (e.g., shoulder).

Types of Movement

• Gliding: Two flat surfaces slide past each other (e.g., carpal bones).

• Angular: Changes angle between bones.

  • Flexion: Decreases angle between bones.

  • Extension: Increases angle between bones.

  • Hyperextension: Extension past anatomical position.

  • Abduction: Movement away from longitudinal axis.

  • Adduction: Movement toward longitudinal axis.

• Circumduction: Complete circular movement without rotation.

• Rotational: Rotation relative to anatomical position.

  • Medial Rotation: Internal rotation toward long axis. (towards midline)

  • Lateral Rotation: External rotation away from body. (away from midline)

  • Pronation: Rotates forearm so radius rolls across ulna; palm faces posteriorly.

  • Supination: Turns palm anteriorly; forearm is supinated in anatomical position. (ex: like you are holding out a bowl of soup )

• Special Movements:

  • Inversion: Twists sole of foot medially. (foot bent towards midline)

  • Eversion: Twists sole of foot laterally. (foot bent away from midline)

  • Dorsiflexion: Flexion at ankle (lifting toes).

  • Plantar Flexion: Extension at ankle (pointing toes).

  • Opposition: Movement of thumb toward palm or other fingers. (holding up 4 fingers- use thumb to touch other fingers in this joint)

  • Reposition: Opposite of opposition. ()

  • Protraction: Anterior movement in horizontal plane. (ex: bringer your neck forward)

  • Retraction: Opposite of protraction. (bring ur neck back opposite of protraction)

  • Depression: Moving a structure inferiorly.

  • Elevation: Moving a structure superiorly.

  • Lateral Flexion: Bending vertebral column to the side. (decreasing angles between side of frontal plane)

Classification of Synovial Joints by Shape

Synovial joints are further classified by the shape of their articulating surfaces, which determines their movement capabilities.

Joint Type	Articular Surface	Movement	Example
Plane (Gliding)	Flattened/slightly curved	Limited (nonaxial)	Intercarpal joints
Hinge	Cylindrical	Angular (monaxial)	Elbow, knee
Condylar (Ellipsoid)	Oval face in depression	Biaxial	Radiocarpal joint
Saddle	Concave/convex faces	Biaxial	Thumb (carpometacarpal joint)
Pivot	Round/pointed surface	Rotation (monaxial)	Atlantoaxial joint
Ball-and-Socket	Round head in cup	Triaxial	Shoulder, hip

Intervertebral joints 9-4 ?missing

Elbow & Knee 9-5 ?missing

A First 2 cervical vertebrae

Effects of Aging on Joints

Degenerative Changes and Clinical Problems

Aging affects joints through degenerative changes, leading to various clinical conditions. Rheumatism and arthritis are common age-related joint problems.

• Rheumatism: Pain and stiffness in the musculoskeletal system. -[ncc recording #2 (4/8) slide.91]

• Arthritis: Inflammation of joints; includes all rheumatic diseases affecting synovial joints.

• Osteoarthritis: Caused by wear and tear or genetic factors; common in people over 60.

• Rheumatoid Arthritis: Inflammatory condition; immune system attacks joint tissues.

• Gouty Arthritis: Uric acid crystals form within synovial fluid.

• Joint Immobilization: Reduces synovial fluid flow, causing arthritis symptoms; treated by continuous passive motion (CPM).

• Bone Mass Decreases: Bones weaken and risk of fractures increases with age.

Integration with Other Systems

Functional Relationships

The skeletal system interacts with other body systems to maintain bone health and function. Living bones undergo continuous remodeling, influenced by various factors.

• Bone Formation: Osteoblasts build new bone tissue.

• Bone Recycling: Osteoclasts break down bone tissue.

• Factors Affecting Balance: Age, physical stress, hormone levels, calcium and phosphorus uptake/excretion, genetic/environmental factors.

• Muscle Attachment: Muscles attach to bones for movement.

• Endocrine Control: Hormones regulate bone growth and maintenance.

• Mineral Supply: Digestive and urinary systems provide calcium and phosphate for bone growth.

• Mineral Reserve: Skeleton stores calcium, phosphate, and other minerals.

Example:

Continuous passive motion (CPM) is used in rehabilitation to prevent joint immobilization and maintain synovial fluid flow, reducing arthritis symptoms.

Additional info: The notes have been expanded to include definitions, examples, and a summary table for synovial joint types, as well as context for clinical conditions and integration with other systems.