Chapter 8: Joints (Articulations) – Structure, Function, and Clinical Aspects

Joints (Articulations)

Definition and Functions

Joints, or articulations, are sites where two or more bones meet. They are essential for providing the skeleton with mobility and for holding the skeleton together, ensuring structural integrity and movement.

• Articulation: The point of contact between bones.

• Functions: Mobility and stability of the skeleton.

Classification of Joints

• Functional Classification: Based on the amount of movement allowed.

  • Synarthroses: Immovable joints

  • Amphiarthroses: Slightly movable joints

  • Diarthroses: Freely movable joints

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

  • Fibrous joints

  • Cartilaginous joints

  • Synovial joints

Fibrous Joints

General Features

Fibrous joints are connected by dense fibrous connective tissue and lack a joint cavity. Most are immovable (synarthrotic), though movement depends on the length of connective tissue fibers.

• Sutures: Rigid, interlocking joints found only in the skull. Allow for growth during youth and ossify in middle age (synostoses).

• Syndesmoses: Bones connected by ligaments. Movement varies with fiber length (e.g., tibiofibular joint – little movement; interosseous membrane of radius and ulna – more movement).

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets), connected by the periodontal ligament.

Cartilaginous Joints

General Features

Cartilaginous joints unite bones with cartilage and lack a joint cavity. They are not highly movable.

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

• Symphyses: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis). Amphiarthrotic (slightly movable).

Synovial Joints

General Features

Synovial joints are characterized by bones separated by a fluid-filled joint cavity. All are diarthrotic (freely movable) and include nearly all limb joints.

• Six distinguishing features:

  1. Articular cartilage: Hyaline cartilage covering bone ends, preventing crushing.

  2. Joint (synovial) cavity: Small, fluid-filled space.

  3. Articular (joint) capsule: Two layers – external fibrous layer (dense irregular connective tissue) and inner synovial membrane (loose connective tissue, produces synovial fluid).

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

  5. Reinforcing ligaments: Capsular (thickened fibrous layer), extracapsular (outside capsule), intracapsular (deep to capsule).

  6. 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 that improves fit, stabilizes joint, and reduces wear.

• Bursae: Sacs lined with synovial membrane, reduce friction where structures rub together.

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

Stabilizing Factors at Synovial Joints

• Shape of articular surfaces (minor role)

• Ligament number and location (limited role)

• Muscle tone (most important): Keeps tendons taut, especially important in shoulder, knee, and foot arches.

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.

Illustration	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
Intervertebral	Between articular processes	Synovial; plane	Diarthrotic; gliding
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 (ribs 2–7)	Sternum and ribs II–VII	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
Elbow	Ulna and radius with humerus	Synovial; hinge	Diarthrotic; uniaxial
Proximal radioulnar	Radius and ulna	Synovial; pivot	Diarthrotic; uniaxial
Wrist	Radius and proximal carpals	Synovial; condyloid	Diarthrotic; biaxial
Carpometacarpal (thumb)	Carpal (trapezium) and metacarpal I	Synovial; saddle	Diarthrotic; biaxial
Metacarpophalangeal (knuckle)	Metacarpal and proximal phalanx	Synovial; condyloid	Diarthrotic; biaxial
Interphalangeal (finger)	Adjacent phalanges	Synovial; hinge	Diarthrotic; uniaxial

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
Knee (tibiofemoral)	Femur and tibia	Synovial; modified hinge	Diarthrotic; biaxial
Knee (femoropatellar)	Femur and patella	Synovial; plane	Diarthrotic; gliding
Inferior tibiofibular	Tibia and fibula	Fibrous; syndesmosis	Synarthrotic; slight "give" during dorsiflexion
Ankle	Tibia and fibula with talus	Synovial; hinge	Diarthrotic; uniaxial
Intertarsal	Adjacent tarsals	Synovial; plane	Diarthrotic; gliding
Tarsometatarsal	Tarsal and metatarsal	Synovial; plane	Diarthrotic; gliding
Metatarsophalangeal	Metatarsal and proximal phalanx	Synovial; condyloid	Diarthrotic; biaxial
Interphalangeal (toe)	Adjacent phalanges	Synovial; hinge	Diarthrotic; uniaxial

Movements at Synovial Joints

General Principles

• Muscles attach to bones at two points: origin (immovable) and insertion (movable).

• Muscle contraction moves the insertion toward the origin.

• Movements occur along transverse, frontal, or sagittal planes.

Range of Motion

• Nonaxial: Slipping movements only

• Uniaxial: Movement in one plane

• Biaxial: Movement in two planes

• Multiaxial: Movement in or around all three planes

Types of Movements

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

• Angular Movements: Change the angle between bones.

  • Flexion: Decreases the angle of the joint

  • Extension: Increases the angle of the joint

  • Hyperextension: Movement beyond anatomical position

• Movements along the frontal plane:

  • Abduction: Movement away from the midline

  • Adduction: Movement toward the midline

  • Circumduction: Limb describes a cone in space (combines flexion, abduction, extension, and adduction)

• Rotation: Turning of a bone around its own long axis (e.g., between C1 and C2, rotation of humerus and femur).

Special Movements at Synovial Joints

• Supination and Pronation: Rotation of the radius and ulna (forearm).

• Dorsiflexion and Plantar Flexion: Upward and downward movement of the foot at the ankle.

• Inversion and Eversion: Turning the sole of the foot medially or laterally.

• Protraction and Retraction: Anterior and posterior movement (e.g., mandible).

• Elevation and Depression: Lifting or lowering a body part (e.g., mandible).

• Opposition: Movement of the thumb to touch the tips of other fingers.

Types of Synovial Joints

Synovial joints are classified by the shape of their articular surfaces and the movements they allow:

• Plane: Nonaxial, gliding (e.g., intercarpal joints)

• Hinge: Uniaxial, flexion and extension (e.g., elbow)

• Pivot: Uniaxial, rotation (e.g., proximal radioulnar joint)

• Condylar: Biaxial, flexion/extension, abduction/adduction (e.g., knuckle joints)

• Saddle: Biaxial, similar to condylar but with greater movement (e.g., thumb)

• Ball-and-socket: Multiaxial, all movements (e.g., shoulder, hip)

Common Joint Injuries

• Sprains: Ligaments stretched or torn; common in ankle, knee, and lumbar region. Partial tears heal slowly; complete tears may require surgery or immobilization.

• Dislocations (luxations): Bones forced out of alignment, often with sprains and inflammation. Treated by reduction (realignment).

• Subluxation: Partial dislocation of a joint.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of a bursa, usually from trauma or friction. Treated with rest, ice, and anti-inflammatory drugs.

• Tendonitis: Inflammation of tendon sheaths, typically from overuse. Similar treatment to bursitis.

• Arthritis: Over 100 types; most common crippling disease in the U.S. Symptoms include pain, stiffness, and swelling.

  • Osteoarthritis (OA): Degenerative, "wear-and-tear" arthritis. Cartilage is destroyed faster than replaced, leading to bone spurs and restricted movement. Treated with moderate activity and pain relievers.

  • Rheumatoid Arthritis (RA): Chronic, autoimmune disease. Immune system attacks joint tissues, leading to inflammation, swelling, and joint deformity (ankylosis). Treated with anti-inflammatory drugs and immune suppressants.

  • Gouty Arthritis: Uric acid crystal deposition in joints, especially the big toe. Can lead to joint fusion if untreated. Treated with drugs, hydration, and dietary changes.

  • Lyme Disease: Bacterial infection from tick bites, can cause joint pain and arthritis. Treated with antibiotics.

Developmental Aspects of Joints

• By embryonic week 8, synovial joints resemble adult joints.

• Joint size, shape, and flexibility are modified by use.

• Aging leads to shortened and weakened ligaments/tendons, increased risk of herniated discs, and higher prevalence of osteoarthritis.

• Full-range-of-motion exercise helps postpone joint problems.