Joints (Articulations)

Definition and Overview

Joints, also known as articulations or arthroses, are the sites where two or more skeletal components meet. They may allow movement or be immobile, depending on their structure and function. Joints are essential for providing mobility, stability, and support to the skeletal system.

Classification of Joints

Joints are classified in two main ways: structurally and functionally.

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

  • Fibrous joints

  • Cartilaginous joints

  • Synovial joints

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

  • Synarthrosis: Immobile

  • Amphiarthrosis: Slightly movable

  • Diarthrosis: Freely movable

Fibrous Joints

General Structure

Fibrous joints are characterized by the absence of a synovial cavity and the presence of dense irregular connective tissue binding the bones. These joints are generally immobile or allow very limited movement.

• Suture: Found only in the skull, sutures are joints held together by very short, interconnecting fibers. The bone edges interlock, providing stability.

• Syndesmosis: Bones are united by a ligament or interosseous membrane, allowing more movement than sutures. Example: Inferior tibiofibular joint.

• Gomphosis: A peg-in-socket joint where the root of a tooth fits into the socket of the jaw, held by fibrous tissue (periodontal ligament).

Cartilaginous Joints

General Structure

Cartilaginous joints lack a synovial cavity and are held together by cartilage. They are classified as primary or secondary cartilaginous joints.

• Primary Cartilaginous Joint (Synchondrosis): Bones are united by hyaline cartilage, typically immobile and temporary. Example: Epiphyseal plates in growing bones.

• Secondary Cartilaginous Joint (Symphysis): Bones are united by fibrocartilage and hyaline cartilage, usually located in the midline, permanent, and slightly movable. Example: Pubic symphysis.

Synovial Joints

General Structure

Synovial joints are the most movable type of joint and are characterized by the presence of a synovial cavity. The bones are held together by dense irregular connective tissue and are surrounded by an articular capsule.

• Articular hyaline cartilage: Covers the ends of bones, reducing friction and absorbing shock.

• Synovial cavity: Space between the bones filled with synovial fluid.

• Synovial fluid: Lubricates the joint and nourishes the cartilage.

• Articular capsule: Encloses the joint cavity, consisting of a fibrous layer and a synovial membrane.

• Ligaments: Strengthen and stabilize the joint.

• Intra-articular structures: Include cartilages, ligaments, and muscles within the joint.

Classification of Synovial Joints by Shape and Movement

Synovial joints are classified according to the shape of their articular surfaces and the type of movement they allow:

• Plane Joint: Flat or slightly curved surfaces; allows gliding movements. Example: Intercarpal joints.

• Hinge Joint: Convex surface fits into concave surface; uniaxial movement (flexion and extension). Example: Elbow joint.

• Pivot Joint: Rounded or pointed surface articulates with a ring; uniaxial rotatory movement. Example: Atlantoaxial joint.

• Ellipsoid (Condylar) Joint: Oval-shaped projection fits into oval depression; biaxial movement (flexion-extension, abduction-adduction). Example: Metacarpophalangeal joints.

• Saddle Joint: One bone is saddle-shaped, the other fits like a rider; biaxial movement. Example: Thumb carpometacarpal joint.

• Ball & Socket Joint: Ball-like surface fits into a cuplike depression; multiaxial movement. Example: Shoulder and hip joints.

Joint Movements

Types of Movement

• Gliding: Occurs in plane joints; one bone slips over another.

• Angular: Change in angle between bones.

  • Flexion and extension: Occur around transverse axis.

  • Abduction and adduction: Occur around anteroposterior axis.

• Rotation: Movement around vertical/longitudinal axis. Example: Circumduction at shoulder and hip joints.

• Special Movements: Include elevation, depression, protraction, retraction, supination, pronation, inversion, eversion, dorsiflexion, and plantar flexion.

Factors Limiting Range of Movement

• Shape of articulating bones: Disproportionate surfaces affect movement freedom.

• Tension of ligaments: Ligament tension restricts movement.

• Tension of antagonistic muscles: Muscle tension can arrest movement.

• Approximation of soft parts: Soft tissue contact limits movement.

• Hormones: Hormones like relaxin increase flexibility, especially in the pelvis during childbirth.

Factors Maintaining Stability of Joints

• Bony configuration

• Ligaments

• Muscles

• Atmospheric pressure

Bursae and Tendon Sheaths

Bursae

Bursae are closed fibrous sacs lined with a smooth membrane and filled with viscous fluid. They reduce friction where tendons rub against bones, ligaments, or other tendons, commonly found near joints.

Synovial Sheath

A synovial sheath is a tubular bursa that surrounds a tendon, reducing friction as the tendon passes under ligaments or through osseofibrous tunnels. The mesotendon allows blood vessels to enter the tendon.

Aging and Joints

Effects of Aging

• Decreased production of synovial fluid

• Thinning of articular cartilage

• Shortening and loss of flexibility in ligaments

• Degenerative changes may begin as early as age 20, with most people affected by their 80s

• Knee, hip, elbow, shoulder, and vertebral column are commonly affected

Common Joint Injuries and Disorders

Arthritis

Arthritis is a group of disorders characterized by inflammation of joints, leading to pain, stiffness, and reduced mobility. Common types include osteoarthritis, rheumatoid arthritis, and gout.

Lyme Disease

Lyme disease is caused by the bacterium Borrelia burgdorferi, transmitted by ticks. It can lead to joint pain, swelling, and long-term complications if untreated.

References

• Hole’s Anatomy and Physiology, 12th edition

• Gray’s Anatomy for Students

• Tortora’s Principles of Anatomy & Physiology – Global edition