Articulations (Joints)

Introduction

Articulations, or joints, are the connections between bones in the skeletal system. They play a crucial role in providing mobility and stability to the body. Joints are classified based on their structure and function, which determines their range of motion and strength.

Classification of Joints

Functional Classification

Functional classification is based on the range of motion permitted by the joint. There are three main types:

• Synarthrosis (immovable joint): Joints that allow no movement. They are very strong and often found where bones are tightly bound together.

• Amphiarthrosis (slightly movable joint): Joints that allow limited movement. They provide both strength and flexibility.

• Diarthrosis (freely movable joint): Joints that allow a wide range of motion. These are the most mobile but also the weakest joints.

Key Principle: A joint cannot be both highly mobile and very strong. The greater the mobility, the weaker the joint. For example, the knees and shoulders are highly mobile but prone to injury and rely on muscles and ligaments for support.

Structural Classification

Structural classification is based on the anatomical organization of the joint. The main types are:

• Fibrous joints: Bones are joined by dense connective tissue (e.g., sutures in the skull).

• Cartilaginous joints: Bones are joined by cartilage (e.g., intervertebral discs).

• Synovial joints: Bones are separated by a fluid-filled cavity and enclosed in a capsule (e.g., knee, shoulder).

Types of Joints by Functional and Structural Classification

Synarthroses (Immovable Joints)

These joints are very strong, with edges of bones that may touch or interlock. Types include:

• Suture: Interlocked bones bound by dense fibrous connective tissue; found only in the skull.

• Gomphosis: A fibrous connection (periodontal ligament) that binds teeth to their sockets.

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

• Synostosis: Fused bones, immovable (e.g., metopic suture of the frontal bone, epiphyseal lines of long bones).

Amphiarthroses (Slightly Movable Joints)

These joints are somewhat strong and allow limited movement. Types include:

• Syndesmosis: Bones connected by ligaments (e.g., distal joint between tibia and fibula).

• Symphysis: Bones separated by fibrocartilage (e.g., pubic symphysis, intervertebral discs).

Diarthroses (Freely Movable Joints / Synovial Joints)

These are the most mobile joints, typically located at the ends of long bones. They are enclosed within a joint (articular) capsule lined with a synovial membrane.

• Joint (articular) capsule: Encloses the joint cavity and provides stability.

• Synovial membrane: Lines the capsule and produces synovial fluid for lubrication.

Synovial joints permit a wider range of motion than other types of joints and are commonly found in the limbs.

Types of Synovial Joints and Their Movements

Classification by Movement (Planes of Motion)

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

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

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

Types of Synovial Joints

• Gliding (Plane) Joints: Flattened or slightly curved faces; limited motion (nonaxial). Example: intercarpal joints.

• Hinge Joints: Permit angular motion in a single plane (monoaxial), like opening or closing a door. Example: elbow joint.

• Pivot Joints: Permit rotation only (monoaxial). Examples: atlantoaxial joint (between C1 and C2 vertebrae), proximal radioulnar joint.

• Condylar (Ellipsoid) Joints: Oval articular face within a depression; motion in two planes (biaxial). Examples: radiocarpal joint, metacarpophalangeal joints 2-5, metatarsophalangeal joint.

• Saddle Joints: Each articular face is concave along one axis and convex along the other (biaxial). Example: first carpometacarpal joint (thumb).

• Ball-and-Socket Joints: Round articular face in a depression (triaxial). Examples: shoulder joint, hip joint.

Summary Table: Functional and Structural Classification of Joints

Key Points and Examples

• Joints with greater mobility are more prone to injury (e.g., knees, shoulders).

• Mobile joints are supported primarily by muscles and ligaments, not by direct bone-to-bone connections.

• Examples of synovial joints include the shoulder (ball-and-socket), elbow (hinge), and wrist (condylar).

Additional info:

• Synovial fluid within the joint capsule reduces friction and nourishes the articular cartilage.

• Ligaments connect bone to bone, while tendons connect muscle to bone, both providing stability to joints.