Lecture 4.1: Joints (Articulations)

Joints and Articulations

Introduction to Joints

Joints, also known as articulations, are the locations where two or more bones meet. They are essential for movement and provide mechanical support to the skeleton. The structure of a joint determines its range of motion and stability; generally, the greater the range of motion, the less stable the joint.

• Joint strength and stability decrease as the range of motion increases.

• Joints are classified by both their structure and function.

Classification of Joints

Functional Classification

• Synarthrosis: Immovable joints (e.g., sutures of the skull).

• Amphiarthrosis: Slightly movable joints (e.g., intervertebral discs).

• Diarthrosis: Freely movable joints (e.g., shoulder, knee).

Structural Classification

• Bony: Bones fused together (e.g., synostosis).

• Fibrous: Bones joined by fibrous tissue (e.g., sutures, syndesmoses).

• Cartilaginous: Bones joined by cartilage (e.g., symphysis, synchondrosis).

• Synovial: Bones separated by a fluid-filled cavity (e.g., most limb joints).

Structure and Types of Synovial Joints

Basic Structure of Synovial Joints

Synovial joints are the most mobile type of joint but are relatively weak. They are stabilized by accessory structures such as cartilages, fat pads, ligaments, tendons, and bursae.

• Articular cartilage covers the ends of bones, reducing friction.

• Synovial membrane produces synovial fluid for lubrication.

• Joint capsule encloses the joint cavity.

• Accessory structures include menisci, ligaments, and bursae.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the types of movement they allow.

Joint Type	Movement	Examples
Gliding (plane)	Gliding, slight nonaxial/multiaxial	Acromioclavicular, intercarpal joints
Hinge	Angular, monaxial	Elbow, knee, ankle joints
Condylar (ellipsoid)	Angular, biaxial	Radiocarpal, metacarpophalangeal joints
Saddle	Angular, biaxial	First carpometacarpal joint (thumb)
Pivot	Rotation, monaxial	Atlantoaxial, proximal radioulnar joints
Ball-and-socket	Angular, circumduction, rotation, triaxial	Shoulder, hip joints

Joint Motion and Axes

Simple Model of Articular Motion

Joint movement can be modeled by imagining a pencil standing upright on a desk. The pencil can move in three main ways: linear motion, angular motion, and rotation.

Axes of Motion

Joints can be described by the number of axes they rotate around:

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

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

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

Movements at Synovial Joints

Types of Movement

• Gliding: Sliding movements between flat surfaces.

• Angular: Change in angle between bones (flexion, extension, abduction, adduction).

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotation: Movement around a bone's long axis (medial/lateral rotation, pronation/supination).

• Special movements: Unique to certain joints (e.g., opposition, inversion, eversion).

Angular Movements

• Flexion: Decreases the angle between bones (e.g., bending the elbow).

• Extension: Increases the angle between bones (e.g., straightening the elbow).

• Hyperextension: Extension beyond the anatomical position.

Abduction, Adduction, and Circumduction

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Circumduction: Circular movement at a joint.

Rotational Movements

• Medial (internal) rotation: Anterior surface moves toward the midline.

• Lateral (external) rotation: Anterior surface moves away from the midline.

• Supination: Palm faces anteriorly (upward).

• Pronation: Palm faces posteriorly (downward).

Special Movements

• Eversion/Inversion: Movements of the sole of the foot outward/inward.

• Dorsiflexion/Plantar flexion: Upward/downward movement of the foot at the ankle.

• Opposition: Movement of the thumb to touch the fingertips.

• Retraction/Protraction: Moving a part of the body backward/forward.

• Depression/Elevation: Lowering/raising a body part.

• Lateral flexion: Bending the vertebral column to the side.

Major Synovial Joints

Elbow Joint

The elbow is a hinge joint formed by the humerus, radius, and ulna. It allows flexion and extension and is stabilized by strong ligaments and the bony structure of the articulating surfaces.

Knee Joint

The knee is a complex hinge joint that transfers weight from the femur to the tibia. It contains three articulations: two between the femur and tibia (medial and lateral condyles) and one between the patella and femur. The knee is stabilized by menisci, ligaments, and other accessory structures.

• Menisci: Fibrocartilage pads that cushion and stabilize the joint.

• Ligaments: Patellar, popliteal, cruciate, tibial collateral, and fibular collateral ligaments.

Shoulder and Hip Joints

• Shoulder joint: Ball-and-socket joint allowing flexion, extension, abduction, adduction, circumduction, and rotation.

• Hip joint: Ball-and-socket joint with similar movements but greater stability due to deeper socket and stronger ligaments.

Effects of Aging on Joints

Degenerative Changes

• Rheumatism: Pain and stiffness in the musculoskeletal system.

• Arthritis: Inflammation of joints, including osteoarthritis (wear and tear), rheumatoid arthritis (autoimmune), and gouty arthritis (uric acid crystals).

• Joint immobilization reduces synovial fluid flow, increasing arthritis risk.

• Aging decreases bone mass and increases fracture risk.

Integration with Other Systems

Skeletal System Interactions

• Bones undergo continuous remodeling by osteoblasts (formation) and osteoclasts (recycling).

• Factors affecting bone health: age, physical stress, hormones, calcium/phosphorus balance, genetics, and environment.

• Muscles attach to bones for movement; endocrine, digestive, and urinary systems regulate mineral supply.

• The skeleton serves as a mineral reserve and supports other organ systems.