Anatomy & Physiology: Structure and Function of Joints

Joints: Structure and Function

Overview of Joints

Joints, also known as articulations, are the sites where two or more bones meet. They play a crucial role in providing mobility to the skeleton and holding the skeleton together. Joints are classified by both their structure and their function.

• Structural classification is based on the material binding the bones together and whether or not a joint cavity is present. The three main types are: fibrous, cartilaginous, and synovial joints.

• Functional classification is based on the amount of movement allowed at the joint. The main categories are: synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

Fibrous Joints

General Structure and Types

In fibrous joints, bones are joined by fibrous tissue and lack a joint cavity. These joints are generally immovable or only slightly movable.

• Sutures: Found only in the skull, where bones are tightly bound by minimal fibrous tissue.

• Syndesmoses: Bones are connected by ligaments; movement depends on the length of the connecting fibers (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints, such as the articulation of a tooth with its socket.

Cartilaginous Joints

General Structure and Types

In cartilaginous joints, bones are united by cartilage and lack a joint cavity. These joints allow more movement than fibrous joints but less than synovial joints.

• Synchondroses: Bones are united by hyaline cartilage (e.g., epiphyseal plates in children, joint between first rib and sternum).

• Symphyses: Bones are united by fibrocartilage, which acts as a shock absorber and allows limited movement (e.g., intervertebral discs, pubic symphysis).

Synovial Joints

General Structure and Features

Synovial joints are the most movable type of joint in the body and are characterized by the presence of a fluid-filled joint cavity. Most joints of the limbs are synovial.

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

• Joint (articular) cavity is a space filled with synovial fluid.

• Articular capsule encloses the joint cavity; it has two layers: an outer fibrous capsule and an inner synovial membrane.

• Synovial fluid lubricates the joint, nourishes the cartilage, and contains phagocytic cells.

• Reinforcing ligaments strengthen the joint.

• Nerves and blood vessels supply the joint, providing nutrients and monitoring joint position.

• Bursae and tendon sheaths reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

Movements Allowed by Synovial Joints

• Gliding: One flat bone surface glides or slips over another (e.g., intercarpal joints).

• Angular movements: Increase or decrease the angle between two bones.

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

  • Extension: Increases the angle (e.g., straightening the knee).

  • Abduction: Movement away from the midline.

  • Adduction: Movement toward the midline.

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

• Rotation: Turning a bone around its own long axis (e.g., turning the head side to side).

• Special movements: Include supination, pronation, inversion, eversion, protraction, retraction, elevation, depression, and opposition.

Types of Synovial Joints

Classification by Shape and Movement

Synovial joints are classified into six types based on the shapes of their articulating surfaces and the movements they allow.

Type	Example	Movement Allowed
Plane	Intercarpal joints	Gliding
Hinge	Elbow, knee	Flexion and extension
Pivot	Proximal radioulnar joint	Rotation
Metacarpophalangeal (knuckle) joints	Flexion, extension, abduction, adduction, circumduction
Saddle	Carpometacarpal joint of the thumb	Flexion, extension, abduction, adduction, circumduction, opposition
Ball-and-socket	Shoulder, hip	All movements, including rotation

Selected Synovial Joints

Temporomandibular Joint (TMJ)

• Combines hinge and gliding movements.

• Articular disc divides the synovial cavity into two compartments.

Shoulder (Glenohumeral) Joint

• Ball-and-socket joint; most freely moving joint in the body.

• Stabilized by the rotator cuff muscles and ligaments.

Elbow Joint

• Hinge joint formed by the humerus, ulna, and radius.

• Allows flexion and extension.

• Stabilized by strong ligaments and muscles.

Hip (Coxal) Joint

• Ball-and-socket joint; provides a wide range of motion.

• Stabilized by a deep socket (acetabulum) and strong ligaments.

Knee Joint

• Largest and most complex joint in the body.

• Consists of three joints in one: femoropatellar, lateral tibiofemoral, and medial tibiofemoral joints.

• Stabilized by ligaments (anterior and posterior cruciate, medial and lateral collateral) and muscles (quadriceps, semimembranosus).

Joint Injuries and Disorders

Common Injuries

• Cartilage tears: Occur when a meniscus is subjected to compression and shear stress simultaneously.

• Sprains: Result from stretching or tearing of ligaments; may require surgical repair.

• Dislocations: Bones are forced out of alignment, often accompanied by sprains and inflammation.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of a bursa, usually caused by a blow or friction.

• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse.

• Arthritis: Group of inflammatory or degenerative diseases that damage joints.

  • Osteoarthritis: Most common; due to breakdown of articular cartilage and bone thickening.

  • Rheumatoid arthritis: Chronic inflammatory disorder; autoimmune in nature.

  • Gouty arthritis: Uric acid crystals deposited in joints, causing inflammation.

• Lyme disease: Inflammatory condition caused by a spirochete bacterium transmitted by ticks.

Summary Table: Types of Joints

Structural Type	Functional Type	Example	Movement
Fibrous (suture)	Synarthrosis	Sutures of the skull	Immovable
Cartilaginous (symphysis)	Amphiarthrosis	Pubic symphysis	Slightly movable
Synovial (hinge)	Diarthrosis	Elbow	Freely movable

Key Equations and Concepts

• Range of Motion (ROM) is determined by the structure of the articular surfaces, strength and tension of ligaments, and action of the muscles.

• Synovial fluid is produced by the synovial membrane and acts as a lubricant, reducing friction between articular cartilages.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.