Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the locations where two bones connect. They are essential for movement and provide structural support. The structure of a joint determines its function, with increased mobility often resulting in decreased strength.

Classification of Joints

Structural Classification

Joints are classified based on their anatomical structure:

• Fibrous Joints: Held together by dense fibrous connective tissue or ligaments. Types include:

  • Suture: Joint between skull bones.

  • Gomphosis: Joint between a tooth and its bony socket.

  • Syndesmosis: Bones connected by a ligament (e.g., distal tibia and fibula).

• Cartilaginous Joints: Held together by cartilage. Types include:

  • Synchondrosis: Rigid cartilaginous connection (e.g., first rib and sternum).

  • Symphysis: Bones connected by a pad of fibrocartilage (e.g., pubic symphysis).

• Bony Joints: Ossified joints where two bones fuse and the boundary disappears (e.g., epiphyseal lines).

• Synovial Joints: Enclosed by a capsule filled with synovial fluid (e.g., knee joint).

Functional Classification

Joints are also classified by their range of motion:

• Synarthrosis: Immovable joints (e.g., suture, gomphosis, synostosis, synchondrosis).

• Amphiarthrosis: Slightly movable joints (e.g., syndesmosis, symphysis).

• Diarthrosis: Freely movable joints; all synovial joints fall into this category.

Synovial Joints

Basic Structure

Synovial joints (diarthroses) are freely movable and found at the ends of long bones. They are surrounded by a joint capsule, which consists of an inner synovial membrane and an outer fibrous capsule. Synovial fluid fills the joint cavity, and articular cartilage covers the surfaces to prevent direct bone contact.

Synovial Fluid

• Lubrication: Reduces friction between articulating surfaces.

• Nutrient Distribution: Supplies nutrients to chondrocytes in articular cartilage.

• Shock Absorption: Cushions joints under compression.

Accessory Structures

• Meniscus: Fibrocartilage pad between opposing bones.

• Fat Pads: Masses of adipose tissue covered by synovial membrane, protecting articular cartilages.

• Ligaments: Support and strengthen joints; can be extracapsular or intracapsular.

• Tendons: Attach muscles to bones and stabilize joints.

• Bursae: Small pockets filled with synovial fluid, reducing friction and acting as shock absorbers.

Stabilization Factors

• Collagen fibers of the joint capsule and ligaments

• Shapes of articulating surfaces and menisci

• Other bones, muscles, or fat pads around the joint

• Tension in tendons attached to articulating bones

Movements at Synovial Joints

Axes of Motion

Movement at joints is described by the number of axes:

• Monaxial: Movement around one axis (e.g., elbow).

• Biaxial: Movement around two axes (e.g., wrist).

• Triaxial: Movement around three axes (e.g., shoulder).

Types of Movement

• Gliding: Two flat surfaces slide past each other (e.g., carpal bones).

• Angular Movements: Change the angle between bones.

  • Flexion: Decreases the angle in the anterior-posterior plane.

  • Extension: Increases the angle in the anterior-posterior plane.

  • Hyperextension: Extension beyond the normal limit.

  • Abduction: Movement away from the body's longitudinal axis.

  • Adduction: Movement toward the body's longitudinal axis.

  • Circumduction: Circular movement without rotation.

• Rotational Movements:

  • Medial Rotation: Internal rotation toward the trunk's axis.

  • Lateral Rotation: External rotation away from the trunk's axis.

  • Pronation: Rotates forearm so palm faces posteriorly.

  • Supination: Rotates forearm so palm faces anteriorly.

• Special Movements:

  • Inversion/Eversion: Twisting the sole of the foot medially/laterally.

  • Dorsiflexion/Plantar Flexion: Flexion/extension at the ankle.

  • Opposition/Reposition: Movement of thumb toward/away from fingers.

  • Protraction/Retraction: Moving a body part forward/backward.

  • Depression/Elevation: Moving a structure down/up.

  • Lateral Flexion: Bending the vertebral column to the side.

Classification of Synovial Joints

Intervertebral Joints

Structure and Function

Intervertebral joints connect adjacent vertebral bodies and articular processes. The first two cervical vertebrae are joined by a pivot synovial joint (atlanto-axial). Plane synovial joints connect articular processes, while symphyses connect vertebral bodies.

Intervertebral Discs

• Anulus Fibrosus: Tough outer layer attaching disc to vertebrae.

• Nucleus Pulposus: Soft, elastic core acting as a shock absorber.

• Vertebral End Plates: Hyaline and fibrocartilage covering disc surfaces.

• Bulging Disc: Anulus fibrosus bulges into vertebral canal.

• Herniated Disc: Nucleus pulposus breaks through anulus fibrosus, compressing spinal nerves.

Intervertebral Ligaments

• Ligamenta Flava: Connect laminae of adjacent vertebrae.

• Posterior Longitudinal Ligament: Connects posterior surfaces of vertebral bodies.

• Anterior Longitudinal Ligament: Connects anterior surfaces of vertebral bodies.

• Interspinous Ligaments: Connect spinous processes.

• Supraspinous Ligament: Connects tips of spinous processes, continuous with ligamentum nuchae.

Vertebral Movements

• Flexion: Bending anteriorly.

• Extension: Bending posteriorly.

• Lateral Flexion: Bending laterally.

• Rotation: Rotational movement.

Elbow and Knee Joints

Elbow Joint

The elbow is a complex hinge joint involving the humerus, radius, and ulna. The humero-ulnar joint is the largest and strongest, while the humeroradial joint is between the capitulum and the head of the radius. The elbow is stabilized by interlocking bony surfaces, a thick joint capsule, and strong ligaments.

• Radial Collateral Ligament: Stabilizes lateral surface.

• Annular Ligament: Binds head of radius to ulna.

• Ulnar Collateral Ligament: Stabilizes medial surface.

Knee Joint

The knee is a complex hinge joint involving the femur, tibia, and patella. It transfers weight from femur to tibia and has three articulations. The joint capsule is thin and incomplete, with medial and lateral menisci providing cushioning and stabilization.

• Patellar Ligament: Supports anterior surface.

• Popliteal Ligaments: Support posterior surface.

• Anterior and Posterior Cruciate Ligaments (ACL, PCL): Limit anterior and posterior movement.

• Tibial and Fibular Collateral Ligaments: Reinforce medial and lateral surfaces.

Shoulder and Hip Joints

Shoulder Joint (Glenohumeral Joint)

The shoulder is a ball-and-socket diarthrosis between the humerus and the glenoid cavity of the scapula. It has the greatest range of motion and is frequently dislocated. The joint is supported by muscles, tendons, ligaments, and the glenoid labrum.

• Acromioclavicular, Coracoclavicular, Coraco-acromial, Coracohumeral, Glenohumeral Ligaments: Stabilize the joint.

• Rotator Cuff Muscles: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Bursae: Subdeltoid, subcoracoid, subacromial, subscapular.

Hip Joint

The hip is a strong ball-and-socket diarthrosis between the femur and the acetabulum. It allows flexion, extension, adduction, abduction, rotation, and circumduction. The acetabular labrum increases the depth of the cavity and seals in synovial fluid.

• Iliofemoral, Pubofemoral, Ischiofemoral Ligaments: Thicken the capsule and stabilize the joint.

• Transverse Acetabular Ligament: Fills the gap in the acetabulum.

• Ligamentum Teres: Attaches acetabulum to femoral head.

Effects of Aging on Joints

Degenerative Changes

• Rheumatism: Pain and stiffness in the musculoskeletal system.

• Arthritis: Joint inflammation.

• Osteoarthritis: Degenerative joint disease due to wear and tear or genetic factors.

• Rheumatoid Arthritis: Autoimmune disease causing joint inflammation.

• Gouty Arthritis: Uric acid crystals form in synovial fluid, often affecting the great toe.

• Joint immobilization reduces synovial fluid flow, increasing degeneration.

• Risk of fractures and dislocations increases with age due to decreased bone mass and weakened bones.

Integration with Other Systems

Functional Relationships

• Bone remodeling involves formation (osteoblasts) and resorption (osteoclasts).

• Balance depends on age, activity, hormones, calcium/phosphorus, genetics, and environment.

• Muscles attach to bones for movement.

• Endocrine system controls bone growth and maintenance.

• Skeletal system produces blood cells for the cardiovascular system.

• Digestive and urinary systems supply minerals for bone growth.

• Skeleton serves as a mineral reserve.