Definition:
A joint is a junction between two or more bones or cartilages. It serves as a device to permit movement, though some joints may be immovable and mainly assist in growth and stability.
In infants, there are more joints than in adults because several bones fuse as growth proceeds — for example:
- Ilium, ischium, and pubis fuse to form the pelvic bone
- Two halves of the infant frontal bone and mandible unite
- Five sacral and four coccygeal vertebrae fuse during development
Classification of Joints
Joints can be classified in several ways:
A. Structural Classification
- Fibrous Joints – Bones are united by fibrous tissue and allow little or no movement.
- Sutures: Found in the skull (e.g., interparietal suture).
- Syndesmosis: Bones joined by interosseous ligament (e.g., inferior tibiofibular joint).
- Gomphosis: Peg-and-socket joint (e.g., tooth root in its socket).
- Cartilaginous Joints – Bones are joined by cartilage and allow limited movement.
- Primary cartilaginous joints (Synchondrosis): Temporary, immovable (e.g., epiphyseal plate, costochondral joints).
- Secondary cartilaginous joints (Symphysis): Permanent, slightly movable (e.g., pubic symphysis, intervertebral joints).
- Synovial Joints – Most advanced and freely movable joints.
- Ball and Socket (Spheroidal): e.g., shoulder, hip joint
- Saddle (Sellar): e.g., first carpometacarpal joint
- Condylar (Bicondylar): e.g., knee joint
- Ellipsoid: e.g., wrist joint
- Hinge: e.g., elbow, ankle joint
- Pivot (Trochoid): e.g., atlanto-axial joint
- Plane: e.g., intercarpal joints
B. Functional Classification (According to Degree of Mobility)
- Synarthrosis (Immovable): Found in fibrous joints like skull sutures.
- Amphiarthrosis (Slightly Movable): Found in cartilaginous joints such as intervertebral discs.
- Diarthrosis (Freely Movable): Found in synovial joints like shoulder and hip joints.
C. Regional Classification
- Skull type – immovable
- Vertebral type – slightly movable
- Limb type – freely movable
D. According to Number of Articulating Bones
- Simple Joint: Two bones articulate (e.g., interphalangeal joint)
- Compound Joint: More than two bones articulate (e.g., elbow, wrist joint)
- Complex Joint: Joint cavity divided by a disc (e.g., temporomandibular joint)
Structural Details of Major Joint Types
1. Fibrous Joints
- Sutures: Found in skull; types include plane, serrate, squamous, denticulate, and schindylesis.
- Fontanelles: Soft gaps in newborn skull; close by 18 months.
- Syndesmosis: Slight movement possible (e.g., inferior tibiofibular joint).
- Gomphosis: Peg-in-socket type (e.g., tooth root).
2. Cartilaginous Joints
(a) Primary Cartilaginous Joints (Synchondrosis)
- Joined by hyaline cartilage
- Immovable and temporary
Examples:
Epiphyseal plate, spheno-occipital joint, first chondrosternal joint
(b) Secondary Cartilaginous Joints (Symphysis)
- Articular surfaces covered by hyaline cartilage with fibrocartilage between
- Slightly movable and permanent
Examples:
Pubic symphysis, manubriosternal joint, intervertebral discs
3. Synovial Joints
Main Characteristics:
- Articular surfaces covered with hyaline cartilage
- Presence of a joint cavity filled with synovial fluid
- Surrounded by fibrous capsule and synovial membrane
- Reinforced by ligaments (capsular and accessory)
- Permit free movement
Synovial Fluid: Viscous, nutrient-rich fluid that lubricates and nourishes the cartilage.
Classification and Movements of Synovial Joints
| Type of Joint | Shape | Example | Movement |
|---|---|---|---|
| Plane | Flat | Intercarpal | Gliding |
| Hinge | Pulley-shaped | Elbow, ankle | Flexion & extension |
| Pivot | Peg & ring | Atlanto-axial | Rotation |
| Condylar | Two condyles | Knee | Flexion, extension, slight rotation |
| Ellipsoid | Oval | Wrist | Flexion, extension, abduction, adduction |
| Saddle | Concavo-convex | 1st carpometacarpal | Flexion, extension, rotation |
| Ball & Socket | Spherical | Shoulder, hip | All movements |
Movements at Synovial Joints
Angular Movements:
- Flexion: Decreasing angle between two bones
- Extension: Increasing angle between two bones
- Abduction: Movement away from midline
- Adduction: Movement toward midline
Rotary Movements:
- Rotation: Turning around an axis
- Circumduction: Circular movement forming a cone shape
Gliding Movements:
- One surface slides over another (e.g., in plane joints)
Mechanism of Joint Movements
- Spin: Bone rotates around its own axis
- Slide: Surfaces glide in the same direction
- Roll: One bone rolls over another
- Swing: Curved motion combining spin and glide
Lubrication Mechanism of Synovial Joints
- Synovial Fluid: Viscous and lubricating (contains hyaluronic acid)
- Articular Cartilage: Smooth surface reducing friction
- Menisci/Discs: Spread synovial fluid
- Fat Pads (Haversian pads): Help distribute fluid
- Bursa: Fluid-filled sacs reducing friction; inflammation causes bursitis
Disorders of Joints
1. Inflammatory Diseases (Arthritis)
Rheumatoid Arthritis (RA)
- Chronic autoimmune inflammation of synovial joints
- More common in females (ages 35–55)
- Involves hands, feet, and other joints
- May lead to deformity and stiffness
Features:
- Synovial hypertrophy
- Pannus formation (granulation tissue)
- Cartilage erosion
- Fibrosis and ankylosis
2. Other Types of Polyarthritis
- Ankylosing Spondylitis: Ossification of vertebral joints
- Psoriatic Arthritis: Occurs with psoriasis
- Reiter’s Syndrome: Arthritis with urethritis and conjunctivitis
- Rheumatic Fever: Polyarthritis with fever; usually self-limiting
3. Infective Arthritis
- Caused by bacteria reaching joints via bloodstream
- May lead to pus formation or chronic infection (e.g., tuberculosis, gonorrhea)
4. Traumatic Joint Injuries
- Sprains & Strains: Ligament or tendon damage
- Dislocations: Bone displacement
- Penetrating Injuries: May lead to chronic degeneration or infection
5. Osteoarthritis (Degenerative Joint Disease)
- Non-inflammatory, age-related wear and tear
- Cartilage thins, bones rub together, forming osteophytes
- Common in weight-bearing joints (hip, knee, spine)
Types:
- Primary: Due to aging and overuse
- Secondary: Due to injury or underlying disease
6. Gout
- Caused by uric acid crystal deposition in joints
- Leads to redness, pain, and swelling (often in the big toe)
- May be due to excessive uric acid production or poor kidney excretion
7. Connective Tissue Disorders
- Systemic Lupus Erythematosus (SLE): Autoimmune disorder with joint pain and rash
- Scleroderma: Excess collagen causing joint stiffness and skin tightening
- Polyarteritis Nodosa: Inflammation of arteries affecting joints and organs
8. Carpal Tunnel Syndrome
- Compression of the median nerve at the wrist
- Causes pain, tingling, and numbness in the thumb, index, and middle fingers
- Common in repetitive wrist use or due to conditions like RA or diabetes
Detailed Notes:
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