Articulations (Joints): Structure, Function, and Clinical Relevance

Articulations (Joints)

Introduction to Articulations

Articulations, or joints, are the sites where two or more bones meet. They are essential for movement and provide varying degrees of mobility and stability. The structure of a joint determines its range of motion and strength, with joints always compromising between mobility and stability.

• Articulation: The point of contact between bones.

• Range of motion: The distance and direction a joint can move.

Classification of Joints

Functional Classification

Joints are classified functionally based on the amount of movement they allow:

• Synarthroses (immovable joints): Bound together by fibrous or cartilaginous connections. Example: Sutures in the skull.

• Amphiarthroses (slightly movable joints): May have fibrous or cartilaginous connections. Example: Symphysis between vertebrae.

• Diarthroses (freely movable joints): Subdivided by type of motion. Example: Shoulder, hip joints.

Structural Classification

Joints can also be classified by their structure:

• Bony

• Fibrous

• Cartilaginous

• Synovial

Types of Synarthroses

• Suture: Bones interlocked and bound by dense fibrous connective tissue. Example: Skull.

• Gomphosis: Connection between teeth and sockets (periodontal ligament).

• Synchondrosis: Rigid cartilaginous bridge between bones. Example: Epiphyseal cartilage of long bones.

• Synostosis: Fused bones, immovable. Example: Metopic suture of the skull.

Types of Amphiarthroses

• Syndesmosis: Bones connected by ligaments.

• Symphysis: Bones separated by fibrocartilage.

Diarthroses (Synovial Joints)

Structure and Function

Diarthroses are freely movable joints found at the ends of long bones. They are enclosed in articular capsules lined with synovial membrane.

• Articular cartilage: Pads the articulating surfaces, reducing friction and preventing bone-to-bone contact.

• Synovial fluid: Secreted by synovial membrane, contains slippery proteoglycans. Functions include:

  • Lubrication

  • Nutrient distribution

  • Shock absorption

Accessory Structures of Synovial Joints

• Cartilages and fat pads: Cushion the joint.

• Ligaments: Support and strengthen joints.

• Tendons: Attach muscles to bones, add support.

• Bursae: Small fluid-filled sacs that reduce friction.

Injury and Dislocation

• Dislocation (luxation): Articulating surfaces are forced out of position, damaging cartilage, ligaments, or joint capsule.

Axes and Planes of Movement

Describing Joint Movement

Joints are described by the number of axes or planes in which they move:

• Monaxial: Movement in one axis.

• Biaxial: Movement in two axes.

• Triaxial: Movement in three axes.

Types of Movement at Synovial Joints

• Linear motion (gliding): Surfaces slide past each other. Example: Carpal bones.

• Angular motion:

  • Flexion: Decreases the angle between elements.

  • Extension: Increases the angle between elements.

  • Hyperextension: Extension past anatomical position.

  • Abduction: Moves away from the longitudinal axis.

  • Adduction: Moves toward the longitudinal axis.

  • Circumduction: Circular motion without rotation.

• Rotational motion:

  • Rotation: Direction of rotation from anatomical position.

  • Medial rotation: Toward the axis.

  • Lateral rotation: Away from the axis.

  • Pronation: Rotates the forearm, radius over ulna.

  • Supination: Returns forearm to anatomical position.

• Special movements:

  • Inversion: Twists sole of foot medially.

  • Eversion: Twists sole of foot laterally.

  • Dorsiflexion: Flexion at ankle (lifting toes).

  • Plantar flexion: Extension at ankle (pointing toes).

  • Opposition: Thumb movement toward fingers or palm.

  • Reposition: Opposite of opposition.

  • Elevation: Moves in superior direction (up).

  • Depression: Moves in inferior direction (down).

  • Lateral flexion: Bends vertebral column to the side.

Structural Classification of Synovial Joints

Shapes of Articulating Surfaces

Type	Description	Axes
Gliding joints	Flattened or slightly curved faces	Limited motion (nonaxial)
Hinge joints	Angular motion in a single plane	Monaxial
Pivot joints	Rotation only	Monaxial
Ellipsoidal joints	Oval articular face within a depression	Biaxial
Saddle joints	Two concave faces, straddled	Biaxial
Ball-and-socket joints	Round articular face in a depression	Triaxial

Aging and Articulations

Effects of Aging

• Rheumatism: General term for pain and stiffness of skeletal and muscular systems.

• Arthritis: Includes all rheumatic diseases affecting synovial joints.

• Osteoarthritis: Caused by wear and tear or genetic factors affecting collagen formation; common in older adults.

• Rheumatoid arthritis: Inflammatory condition due to infection, allergy, or autoimmune disease.

• Gouty arthritis: Occurs when crystals (uric acid or calcium salts) form within synovial fluid.

• Joint immobilization: Reduces flow of synovial fluid and can cause arthritis-like changes.

• Osteoporosis: Decreases bone mass, increasing risk of fracture and dislocation.

Integration with Other Systems

Bone Remodeling and Systemic Effects

• Bone building (by osteoblasts) and bone recycling (by osteoclasts) are ongoing processes.

• Balance depends on:

  • Age

  • Physical stress

  • Hormone levels

  • Rates of calcium and phosphorus uptake/excretion

  • Genetic and environmental factors

• The skeletal system supports and protects other body systems, stores minerals, and manufactures immune cells.

• Other body systems can affect the skeletal system, leading to disorders such as osteoporosis and rickets.

Examples of Common Joints

Vertebral Column Joints

• Vertebrae articulate via symphysis joints (intervertebral discs) and gliding joints (articular processes).

• Intervertebral discs consist of:

  • Annulus fibrosus: Tough outer layer.

  • Nucleus pulposus: Elastic, gelatinous core.

• Ligaments stabilize the vertebral column:

  • Anterior longitudinal ligament

  • Posterior longitudinal ligament

  • Ligamentum flavum

  • Interspinous ligament

  • Supraspinous ligament

  • Ligamentum nuchae

• Damage to discs can cause herniation, compressing spinal nerves.

• Vertebral column moves in four ways: flexion, extension, lateral flexion, rotation.

The Shoulder Joint (Glenohumeral Joint)

• Most mobile joint; ball-and-socket diarthrosis.

• Stabilized by rotator cuff muscles and several ligaments.

• Common injury: shoulder dislocation (separation).

The Elbow Joint

• Stable hinge joint between humerus, radius, and ulna.

• Movement controlled by biceps brachii muscle and ligaments.

The Hip Joint (Coxal Joint)

• Ball-and-socket diarthrosis with wide range of motion.

• Head of femur fits into acetabulum, stabilized by ligaments and muscles.

The Knee Joint

• Complicated hinge joint transferring weight from femur to tibia.

• Stabilized by menisci, fat pads, and several ligaments:

  • Patellar ligament

  • Popliteal ligaments

  • Anterior and posterior cruciate ligaments

  • Tibial collateral ligament

  • Fibular collateral ligament

• Standing with legs straight "locks" the knees by jamming the lateral meniscus between tibia and femur.

Key Terms and Definitions

• Articulation: Joint; point of contact between bones.

• Synovial fluid: Lubricating fluid in synovial joints.

• Ligament: Connective tissue binding bone to bone.

• Bursa: Fluid-filled sac reducing friction.

• Meniscus: Fibrocartilage pad in knee joint.

Formulas and Equations

• Range of Motion: $\text{Range of Motion} = \text{Maximum angle of movement allowed by the joint}$

• Torque at a Joint: $\tau = F \times r$ where $\tau$ is torque, $F$ is force, and $r$ is the distance from the axis of rotation.

Additional info: Academic context and definitions have been expanded for clarity and completeness.