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

Joints (Articulations)

Introduction to Joints

Joints, or articulations, are the locations where two bones meet. They are essential for body movement and are sites where the skeleton can bend or rotate. There is a fundamental tradeoff between the strength and mobility of a joint: the more mobile a joint, the less stable it tends to be.

• Joints allow for movement and flexibility in the skeleton.

• They are classified based on their structure and function.

Classification of Joints

Structural and Functional Classification

Joints are classified in two main ways: by their anatomical structure and by their functional range of motion. The structure of a joint determines its function.

• Structural classifications:

  • Fibrous – Bones joined by fibrous tissue.

  • Cartilaginous – Bones joined by cartilage.

  • Bony – Bones fused together.

  • Synovial – Bones separated by a fluid-filled cavity.

• Functional classifications:

  • Synarthrosis – Immovable joint.

  • Amphiarthrosis – Slightly movable joint.

  • Diarthrosis – Freely movable joint.

Synarthrosis (Immovable Joints)

These joints are very strong and may be fibrous or cartilaginous. The edges of bones may touch or interlock.

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

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

• Synchondrosis: Rigid cartilaginous bridge between two bones (e.g., between vertebrosternal ribs and sternum, epiphyseal cartilage of growing long bones).

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

Amphiarthrosis (Slightly Movable Joints)

These joints allow more movement than synarthroses but are stronger than diarthroses. They may be fibrous or cartilaginous.

• Syndesmosis: Bones connected by a ligament.

• Symphysis: Bones connected by fibrocartilage (e.g., pubic symphysis).

Diarthrosis (Freely Movable Joints / Synovial Joints)

These are the most mobile joints, found at the ends of long bones. They are surrounded by a joint capsule and contain synovial fluid.

• Synovial joints are characterized by a joint cavity filled with synovial fluid and articular cartilage covering the bone surfaces.

Structure of Synovial Joints

Basic Structure

• Joint (articular) capsule: Encloses the joint; lined by synovial membrane.

• Synovial membrane: Produces synovial fluid.

• Synovial fluid: Egg-yolk consistency, contains proteoglycans; functions include lubrication, nutrient distribution, and shock absorption.

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

Accessory Structures

• Cartilages (Meniscus): Pads of fibrocartilage between bones, improving fit and cushioning.

• Fat pads: Adipose tissue covered by synovial membrane; protect articular cartilages.

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

• Tendons: Attach muscles to bones around the joint, providing support.

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

Stabilization of Synovial Joints

• Stabilized by joint capsule, ligaments, shapes of articulating surfaces, menisci, other bones, muscles, fat pads, and tendons.

• Stabilization prevents injury by limiting range of motion.

Movements at Synovial Joints

Planes and Types of Movement

Movements are described by the plane or direction and the relationship between structures.

• 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: Flat surfaces slide past each other (e.g., between carpal bones).

• Angular movements:

  • Flexion: Decreases angle between bones.

  • Extension: Increases angle between bones.

  • Hyperextension: Extension past anatomical position.

  • Abduction: Movement away from the longitudinal axis.

  • Adduction: Movement toward the longitudinal axis.

  • Circumduction: Circular movement without rotation.

• Rotational movements:

  • Medial rotation: Toward the body's long axis.

  • Lateral rotation: Away from the body's long axis.

  • Pronation: Rotates forearm so palm faces posteriorly.

  • Supination: Rotates forearm so palm faces anteriorly.

• Special movements:

  • Inversion/Eversion: Twisting sole of foot medially/laterally.

  • Dorsiflexion/Plantar flexion: Flexion/extension at ankle (lifting/pointing toes).

  • Opposition/Reposition: Movement of thumb toward/away from palm or fingers.

  • Protraction/Retraction: Moving a structure forward/backward in the horizontal plane.

  • Depression/Elevation: Moving a structure down/up.

  • Lateral flexion: Bending vertebral column to the side.

Classification of Synovial Joints by Shape

Type	Structure	Movement	Example
Plane (gliding)	Flattened/slightly curved surfaces	Nonaxial, limited	Intercarpal joints
Hinge	Cylindrical projection fits into trough-shaped surface	Monaxial (angular)	Elbow, knee
Condylar (ellipsoid)	Oval articular face within a depression	Biaxial	Radiocarpal joint
Saddle	Articular faces fit like a rider in a saddle	Biaxial	First carpometacarpal joint (thumb)
Pivot	Rounded or pointed surface fits into ring	Monaxial (rotation)	Atlantoaxial joint
Ball-and-socket	Round head in cup-shaped depression	Triaxial	Shoulder, hip

Effects of Aging on Joints

Degenerative Changes and Clinical Problems

• Rheumatism: General pain and stiffness in the musculoskeletal system.

• 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 where the immune system attacks joint tissues.

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

• Joint immobilization reduces synovial fluid flow, causing arthritis-like symptoms; treated by continuous passive motion (CPM).

• Aging leads to decreased bone mass, weakened bones, and increased fracture risk.

Integration with Other Systems

Functional Relationships

• Living bones undergo continuous remodeling via osteoblasts (formation) and osteoclasts (recycling).

• Muscles attach to bones, enabling movement.

• Bones are regulated by the endocrine system.

• Digestive and urinary systems supply calcium and phosphate for bone growth.

• The skeleton serves as a mineral reserve.

Factors Affecting Bone Remodeling

• Age

• Physical stresses

• Hormone levels

• Calcium and phosphorus uptake/excretion

• Genetic and environmental factors

Example: The knee joint is a synovial, hinge-type joint stabilized by ligaments, menisci, and surrounding muscles. It allows flexion and extension, and is susceptible to injuries such as sprains and arthritis, especially with aging.