Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are sites where two or more bones meet. They play a crucial role in providing the body with mobility and stability, and are essential for skeletal movement and integrity.

• Functions of joints:

  • Skeletal mobility: Allow movement of body parts.

  • Hold skeleton together: Maintain structural integrity.

Classification of Joints

Functional Classification of Joints

Joints are classified functionally based on the amount of movement they permit.

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

Structural Classification of Joints

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

• Fibrous joints

• Cartilaginous joints

• Synovial joints

Fibrous Joints

General Features

Fibrous joints are joined by dense fibrous connective tissue and lack a joint cavity. Most are synarthrotic (immovable).

• Types:

  • Sutures: Interlocking joints with short connective tissue fibers, found in the skull. Allow growth during youth; in middle age, sutures ossify and become synostoses.

  • Syndesmoses: Bones connected by ligaments; movement varies from immovable to slightly movable. Examples: Distal tibiofibular joint (synarthrotic), interosseous membrane between radius and ulna (diarthrotic).

  • Gomphoses: Peg-in-socket joints of teeth in alveolar sockets; the fibrous connection is the periodontal ligament.

Cartilaginous Joints

General Features

Cartilaginous joints are united by cartilage and lack a joint cavity.

• Types:

  • Synchondroses: Plate of hyaline cartilage unites bones; all are synarthrotic. Example: Epiphyseal plate in children.

  • Symphyses: Articulating surfaces covered with hyaline cartilage fused to a pad of fibrocartilage; strong, flexible amphiarthroses. Example: Intervertebral discs, pubic symphysis.

Synovial Joints

General Features

Synovial joints are diarthrotic and include all limb joints; they are the most common type in the body.

• Distinguishing features:

  1. Articular cartilage: Hyaline cartilage covers bone ends.

  2. Joint (synovial) cavity: Small potential space between bones.

  3. Articular capsule: Two layers—fibrous capsule (outer) and synovial membrane (inner).

  4. Synovial fluid: Viscous filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage.

  5. Reinforcing ligaments:

     • Capsular (intrinsic): Part of the fibrous capsule.

     • Extracapsular: Outside the capsule.

     • Intracapsular: Deep to capsule, covered by synovial membrane.

  6. Nerves and blood vessels: Nerve fibers detect pain, monitor joint position and stretch; capillary beds produce filtrate for synovial fluid.

Friction-Reducing Structures

• Bursae: Flattened, fibrous sacs lined with synovial membranes containing synovial fluid; reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae that wrap around tendons.

Stabilizing Factors at Synovial Joints

• Articular surfaces: Minor role in stability.

• Ligament number and location: Limited role.

• Muscle tone: Major role in reinforcing joints, especially shoulder and knee joints and arches of the foot.

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.

Summary Table: Characteristics of Body Joints

Movements at Synovial Joints

Angular Movements

Angular movements change the angle between bones.

• Flexion: Decreases the angle of the joint.

• Extension: Increases the angle of the joint.

• Hyperextension: Extension beyond normal range of motion.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Circumduction: Flexion + abduction + extension + adduction of a limb to describe a cone in space.

Rotation

Rotation is the turning of a bone around its own long axis.

• Examples: Rotation between C1 and C2 vertebrae; rotation of humerus and femur.

Special Movements

• Supination: Turning hand backward (palm up).

• Pronation: Turning hand forward (palm down).

• Dorsiflexion: Upward movement of the foot.

• Plantar flexion: Downward movement of the foot.

• Inversion: Turning sole of foot medially.

• Eversion: Turning sole of foot laterally.

• Protraction: Moving a body part anteriorly.

• Retraction: Moving a body part posteriorly.

• Elevation: Moving a body part superiorly.

• Depression: Moving a body part inferiorly.

• Opposition: Movement in the saddle joint so that the thumb touches the tips of the other fingers.

Examples and Applications

• Shoulder joint: Ball-and-socket synovial joint; allows multiaxial movement.

• Knee joint: Hinge synovial joint; allows flexion and extension.

• Intervertebral joints: Symphyses; allow slight movement and absorb shock.

Additional info: The above notes expand on the original content by providing definitions, examples, and a summary table for clarity and completeness.