Joints (Articulations): Structure, Classification, and Function

Joints (Articulations)

Introduction to Joints

Joints, or articulations, are sites where two or more bones meet. They are essential for providing the skeleton with mobility and stability. Joints are classified based on their structure and the amount of movement they allow.

• Articulation: The location where bones connect.

• Functions of joints: Enable movement and hold the skeleton together.

• Classification: Joints are classified functionally (by movement) and structurally (by binding material and cavity presence).

Classification of Joints

Functional Classification

Functional classification is based on the degree of movement permitted by the joint.

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

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

• Diarthroses: Freely movable joints (e.g., most limb joints).

Structural Classification

Structural classification is based on the material binding the bones and the presence or absence of a joint cavity.

• Fibrous joints: Bones joined by dense fibrous connective tissue; no joint cavity; mostly immovable.

• Cartilaginous joints: Bones united by cartilage; no joint cavity; not highly movable.

• Synovial joints: Bones separated by a fluid-filled joint cavity; all are freely movable.

Fibrous Joints

Types of Fibrous Joints

• Sutures: Rigid, interlocking joints found only in the skull; allow for growth during youth and ossify in adulthood (synostoses).

• Syndesmoses: Bones connected by ligaments; movement varies with fiber length (e.g., distal tibiofibular joint, interosseous membrane between radius and ulna).

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets); periodontal ligament is the fibrous connection.

Cartilaginous Joints

Types of Cartilaginous Joints

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates, first rib and manubrium); mostly immovable.

• Symphyses: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis); strong, flexible, and slightly movable.

Synovial Joints

General Structure and Features

Synovial joints are the most common and movable type of joint in the body. They are characterized by the presence of a synovial cavity filled with fluid.

• Articular cartilage: Hyaline cartilage covering bone ends to prevent crushing.

• Joint (synovial) cavity: Small, fluid-filled space between bones.

• Articular capsule: Two layers—external fibrous layer (dense irregular connective tissue) and inner synovial membrane (produces synovial fluid).

• Synovial fluid: Viscous, slippery fluid that lubricates and nourishes cartilage; contains phagocytes.

• Reinforcing ligaments: Capsular (thickened part of capsule), extracapsular (outside capsule), and intracapsular (deep to capsule).

• Nerves and blood vessels: Nerve fibers detect pain and monitor position; capillaries supply filtrate for synovial fluid.

Other Features of Synovial Joints

• Fatty pads: Cushioning between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage pads that improve fit, stabilize joint, and reduce wear.

• Bursae: Sacs filled with synovial fluid that reduce friction where structures rub together.

• Tendon sheaths: Elongated bursae that wrap around tendons subjected to friction.

Stabilization of Synovial Joints

Factors Influencing Stability

• Shape of articular surfaces: Minor role in stability.

• Ligament number and location: Limited role; more ligaments usually mean more stability.

• Muscle tone: Most important factor; keeps tendons taut and stabilizes joints, especially in the shoulder and knee.

Summary Table: Structural and Functional Characteristics of Body Joints

The following tables summarize the main joints, their articulating bones, structural and functional types, and movements allowed.

Joint	Articulating Bones	Structural Type	Functional Type; Movements Allowed
Skull	Cranial and facial bones	Fibrous; suture	Synarthrotic; no movement
Temporomandibular	Temporal bone of skull and mandible	Synovial; modified hinge	Diarthrotic; gliding and uniaxial rotation
Atlanto-occipital	Occipital bone of skull and atlas	Synovial; condyloid	Diarthrotic; biaxial, flexion, extension, lateral flexion, circumduction of head on neck
Atlantoaxial	Atlas (C1) and axis (C2)	Synovial; pivot	Diarthrotic; uniaxial, rotation of the head
Intervertebral	Between adjacent vertebral bodies	Cartilaginous; symphysis	Amphiarthrotic; slight movement
Costovertebral	Vertebrae and ribs	Synovial; plane	Diarthrotic; gliding of ribs

Joint	Articulating Bones	Structural Type	Functional Type; Movements Allowed
Sternoclavicular	Sternum and clavicle	Synovial; shallow saddle	Diarthrotic; multiaxial
Sternocostal (first)	Sternum and rib 1	Cartilaginous; synchondrosis	Synarthrotic; no movement
Sternocostal (2-7)	Sternum and ribs 2-7	Synovial; double plane	Diarthrotic; gliding

Joint	Articulating Bones	Structural Type	Functional Type; Movements Allowed
Shoulder (glenohumeral)	Scapula and humerus	Synovial; ball-and-socket	Diarthrotic; multiaxial, flexion, extension, abduction, adduction, rotation
Elbow	Ulna and humerus	Synovial; hinge	Diarthrotic; uniaxial, flexion, extension
Proximal radioulnar	Radius and ulna	Synovial; pivot	Diarthrotic; uniaxial, rotation of radius around ulna
Wrist	Radius and carpals	Synovial; condyloid	Diarthrotic; biaxial, flexion, extension, abduction, adduction, circumduction
Carpometacarpal (thumb)	Carpal (trapezium) and metacarpal I	Synovial; saddle	Diarthrotic; biaxial, flexion, extension, abduction, adduction, opposition

Joint	Articulating Bones	Structural Type	Functional Type; Movements Allowed
Pubic symphysis	Pubic bones	Cartilaginous; symphysis	Amphiarthrotic; slight movement
Hip (coxal)	Hip bone and femur	Synovial; ball-and-socket	Diarthrotic; multiaxial, flexion, extension, abduction, adduction, rotation
Knee (tibiofemoral)	Femur and tibia	Synovial; modified hinge	Diarthrotic; flexion, extension, some rotation
Ankle	Tibia and fibula with talus	Synovial; hinge	Diarthrotic; uniaxial, dorsiflexion, plantar flexion

Movements at Synovial Joints

Types of Movement

• Gliding: Flat bone surfaces slide over each other (e.g., intercarpal joints).

• Angular movements: Change the angle between bones (flexion, extension, hyperextension, abduction, adduction, circumduction).

• Rotation: Bone turns around its own long axis (e.g., rotation of the head, humerus, femur).

• Special movements: Supination/pronation, dorsiflexion/plantar flexion, inversion/eversion, protraction/retraction, elevation/depression, opposition.

Knee Joint: Structure and Injuries

Anatomy of the Knee Joint

The knee is the largest and most complex joint in the body, consisting of three joints within a single cavity: the femoropatellar joint and the lateral and medial tibiofemoral joints. It allows flexion, extension, and some rotation.

• Menisci: C-shaped fibrocartilage pads that improve fit and absorb shock.

• Ligaments: Collateral, cruciate, and patellar ligaments stabilize the joint.

• Bursae: At least 12 bursae reduce friction.

Knee Joint Injuries

• Common injuries: Collateral ligament tears, cruciate ligament tears, and meniscal injuries (the "three C's").

• Mechanism: Often caused by horizontal blows to the knee, especially when extended.

• Cartilage tears: Due to compression and shear stress; cartilage heals poorly and may require arthroscopic surgery.

• Sprains: Ligaments stretched or torn; partial tears heal slowly due to poor blood supply.

• Dislocations (luxations): Bones forced out of alignment, often with sprains and inflammation; require reduction.

Summary

• Joints are classified by structure and function, with synovial joints being the most movable and complex.

• Stability is provided by articular surfaces, ligaments, and muscle tone.

• Common joint injuries include sprains, dislocations, and cartilage tears, especially in the knee.