A joint (articulation) is where two or more bones meet. Joints can be classified in two ways: by what they're made of (structural classification) and by how much they move (functional classification).

Structural Classification:

• Fibrous joints: Bones connected by fibrous connective tissue. Usually immovable or slightly movable.
  • Sutures: Skull bones — essentially immovable
  • Syndesmoses: Tibia-fibula connection — slightly movable
  • Gomphoses: Teeth in sockets — peg-in-socket
• Cartilaginous joints: Bones connected by cartilage.
  • Synchondroses: Epiphyseal plates — hyaline cartilage
  • Symphyses: Intervertebral discs, pubic symphysis — fibrocartilage, slightly movable
• Synovial joints: Most common type. Bones separated by a fluid-filled cavity. Freely movable.

Functional Classification:

• Synarthroses: Immovable joints (e.g., skull sutures)
• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs)
• Diarthroses: Freely movable joints (all synovial joints)

Synovial joints are the most complex and most important for movement. They share common features:

Articular Cartilage:

The ends of bones are covered with smooth, glassy hyaline cartilage. This provides a low-friction surface for movement and absorbs shock. Importantly, cartilage is avascular (no blood supply), so it heals poorly when damaged.

Joint Cavity:

A small space between the articulating bones. This "potential space" allows movement without friction.

Synovial Membrane:

The inner layer of the joint capsule. This specialized tissue secretes synovial fluid — a viscous, egg-white-like fluid that:

• Lubricates the joint surfaces
• Nourishes the articular cartilage
• Absorbs shock

Fibrous Capsule:

The outer layer of the joint capsule. It's continuous with the periosteum of the bones and provides stability. It may be thickened in places to form ligaments.

Reinforcing Ligaments:

Ligaments connect bone to bone and reinforce the joint capsule. They limit excessive movement and provide stability. Some are thickened parts of the capsule (intrinsic); others are separate (extrinsic).

Additional Features:

Some synovial joints have menisci (fibrocartilage pads that improve fit between bones), bursae (fluid-filled sacs that reduce friction), or fat pads for cushioning.

Synovial joints are classified by the shape of their articulating surfaces, which determines what movements they allow:

Plane (Gliding) Joints:

Flat or nearly flat surfaces that glide across each other. Movement is slight in any direction.

Examples: Intercarpal joints (wrist bones), intertarsal joints (ankle bones), facet joints of vertebrae

Hinge Joints:

Like a door hinge — allows movement in one plane (flexion and extension only).

Examples: Elbow joint, ankle joint, interphalangeal joints (fingers and toes)

Pivot Joints:

A rounded bone fits into a ring, allowing rotation around a single axis.

Examples: Atlantoaxial joint (C1-C2, shaking head "no"), radioulnar joint (turning palm up/down)

Condyloid (Ellipsoid) Joints:

An oval-shaped surface fits into an oval cavity. Allows movement in two planes (flexion/extension, abduction/adduction) but no rotation.

Examples: Wrist joint, metacarpophalangeal joints (knuckles)

Saddle Joints:

Both surfaces are saddle-shaped. Allows movement in two planes plus some rotation — the most versatile of biaxial joints.

Example: Carpometacarpal joint of the thumb (allows opposition)

Ball-and-Socket Joints:

A ball-shaped head fits into a cup-like socket. Allows movement in ALL planes — flexion/extension, abduction/adduction, rotation. The most mobile joint type.

Examples: Shoulder joint, hip joint

Joints allow specific types of movement. Here are the key terms:

Angular Movements:

• Flexion: Decreasing the angle between bones (bending elbow)
• Extension: Increasing the angle (straightening elbow)
• Hyperextension: Extending beyond anatomical position
• Abduction: Moving a limb away from the midline
• Adduction: Moving a limb toward the midline
• Circumduction: Moving in a cone shape (arm circles)

Rotation:

• Rotation: Turning around the longitudinal axis
• Pronation: Turning palm downward (forearm)
• Supination: Turning palm upward (forearm)

Special Movements:

• Dorsiflexion: Toes up (standing on heels)
• Plantar flexion: Toes down (standing on tiptoes)
• Inversion: Sole inward
• Eversion: Sole outward
• Protraction/Retraction: Jaw forward/backward
• Elevation/Depression: Shoulder shrug/lower
• Opposition: Thumb touching other fingers

As we age, the musculoskeletal system undergoes significant changes that affect mobility and increase fall risk:

Bone Changes:

• Osteoporosis: Bone density decreases, especially in postmenopausal women. Bones become brittle and fracture easily.
• Common fracture sites: vertebral bodies, femoral neck (hip), distal radius (wrist)

Joint Changes:

• Osteoarthritis: Wear and tear on articular cartilage. Joints become stiff and painful.
• Cartilage thins, bone spurs form, synovial fluid decreases
• Weight-bearing joints most affected: knees, hips, spine

Muscle Changes:

• Sarcopenia: Progressive loss of muscle mass and strength
• Begins around age 30-50, accelerates after 65
• Fast-twitch fibers lost preferentially
• Muscle replaced by fat and connective tissue

Balance and Fall Risk:

Balance depends on multiple systems working together:

• Vision: Provides spatial orientation
• Vestibular system: Inner ear balance organs
• Proprioception: Sensory receptors in joints, muscles, tendons
• Muscle strength: For correcting balance
• Reaction time: Nervous system processing speed

Age-related deterioration in ANY of these systems increases fall risk. Falls are the leading cause of injury in older adults, with hip fractures being particularly devastating.

Prevention: Weight-bearing exercise, strength training, balance exercises, and adequate calcium/vitamin D can slow these changes.