Bones

Dr. B P Kumar

Introduction

  1. Bones are specialized connective tissues that form the internal framework of the body.
  2. They are vital for multiple roles, including support, movement, protection, storage of minerals, and housing bone marrow for hematopoiesis (blood cell production).
  3. The human skeletal system is dynamic, undergoing continuous remodeling, influenced by growth, mechanical stress, nutrition, and hormonal control.
  4. By adulthood, the skeletal system stabilizes at 206 bones, divided into axial and appendicular skeletons.

 

Functions of Bones:

  1. Support: Provide structural support for the body and attachment sites for muscles.
  2. Protection: Shield vital organs (e.g., skull for the brain, ribcage for the heart and lungs).
  3. Movement: Act as levers for muscles to generate movement.
  4. Mineral Storage: Reservoir for calcium, phosphorus, and other minerals.
  5. Hematopoiesis: Produce red and white blood cells and platelets in red bone marrow.
  6. Energy Storage: Yellow marrow stores lipids as an energy reserve.

Bone Structure

Bones are complex organs with distinct regions and components, each contributing to their strength, flexibility, and functionality.

Gross Anatomy

  • Diaphysis:
    • The central shaft of long bones, composed of compact bone surrounding a medullary cavity filled with yellow marrow.
  • Epiphysis:
    • The rounded ends of long bones, composed of spongy bone with red marrow, covered by articular cartilage to reduce friction in joints.
  • Metaphysis:
    • The transitional zone between the diaphysis and epiphysis; in growing bones, it contains the epiphyseal plate (growth plate).
  • Medullary Cavity:
    • A hollow space within the diaphysis, containing yellow marrow in adults.
  • Periosteum:
    • A dense, fibrous membrane covering the bone’s surface, rich in nerves and blood vessels, providing nourishment and anchoring for tendons/ligaments.
  • Endosteum:
    • A thin membrane lining the medullary cavity and internal bone surfaces.

Microscopic Anatomy

  • Bone Matrix:
    • Organic components (30%): Collagen fibers for flexibility and tensile strength.
    • Inorganic components (70%): Hydroxyapatite crystals for hardness and compressive strength.
  • Bone Cells:
    • Osteoblasts: Secrete bone matrix and promote bone formation.
    • Osteocytes: Mature bone cells residing in lacunae, maintaining bone health.
    • Osteoclasts: Multinucleated cells that resorb bone tissue during remodeling.
  • Haversian System (Osteon):
    • Structural unit of compact bone with concentric lamellae surrounding a central canal for blood vessels and nerves.
    • Connected by Volkmann’s canals for vascular communication.
  • Trabeculae:
    • The lattice-like network of spongy bone, oriented to withstand stress.

Bone Marrow

  • Red Marrow:
    • Found in flat bones (sternum, pelvis) and epiphyses of long bones in adults.
    • Produces blood cells.
  • Yellow Marrow:
    • Dominates the medullary cavity in adults, primarily storing fats.

 

Classification of Bones

Bones can be classified based on their shape, location, and structure.

Based on Shape

  1. Long Bones:
    • Longer than they are wide; designed for leverage.
    • Examples: Femur, tibia, humerus.
  2. Short Bones:
    • Cube-shaped, providing stability and support with little movement.
    • Examples: Carpals (wrist), tarsals (ankle).
  3. Flat Bones:
    • Thin, often curved, providing protection and broad surfaces for muscle attachment.
    • Examples: Skull bones, ribs, scapulae.
  4. Irregular Bones:
    • Complex shapes for specialized functions.
    • Examples: Vertebrae, facial bones.
  5. Sesamoid Bones:
    • Embedded within tendons, reducing friction and modifying pressure.
    • Example: Patella.

Based on Location

  1. Axial Skeleton (80 Bones):
    • Includes skull, vertebral column, and thoracic cage.
    • Functions in protection and support.
  2. Appendicular Skeleton (126 Bones):
    • Includes limbs and girdles (pectoral and pelvic).
    • Facilitates movement.

Type of bones

Long Bones

  • Description:
    • These bones are longer than they are wide and primarily consist of a shaft (diaphysis) and two ends (epiphyses).
    • They are designed for strength, structure, and leverage, facilitating movement.
  • Composition:
    • Compact bone forms the outer layer, while spongy bone is present at the ends.
    • The medullary cavity houses yellow marrow.
  • Examples:
    • Upper Limb: Humerus, radius, ulna.
    • Lower Limb: Femur, tibia, fibula.
    • Others: Metacarpals, metatarsals, phalanges.

Short Bones

  • Description:
    • Cube-shaped bones that are as wide as they are long.
    • Provide stability and support with limited motion.
  • Composition:
    • Mostly spongy bone with a thin layer of compact bone.
  • Examples:
    • Carpals (wrist bones): Scaphoid, lunate.
    • Tarsals (ankle bones): Calcaneus, talus.

Flat Bones

  • Description:
    • Thin, flattened bones that often have a curved shape.
    • They provide protection for internal organs and broad surfaces for muscle attachment.
  • Composition:
    • Two layers of compact bone surrounding a layer of spongy bone (called diploë in the skull).
  • Examples:
    • Skull bones: Frontal, parietal, occipital.
    • Thoracic cage: Ribs, sternum.
    • Shoulder girdle: Scapula.

Irregular Bones

  • Description:
    • Bones with complex shapes that do not fit into other categories.
    • Provide support, protection, and attachment points for muscles.
  • Composition:
    • Primarily spongy bone enclosed by a thin layer of compact bone.
  • Examples:
    • Vertebrae (spinal bones).
    • Bones of the pelvis: Ilium, ischium, pubis.
    • Certain skull bones: Sphenoid, ethmoid.

Sesamoid Bones

  • Description:
    • Small, round bones embedded within tendons.
    • Reduce friction, modify pressure, and protect tendons from stress.
  • Composition:
    • Usually formed in areas of high mechanical stress.
  • Examples:
    • Patella (kneecap) – the largest sesamoid bone.
    • Other small sesamoid bones in the hands and feet.

Special Categories

Some bones are not classified strictly into these types but have unique characteristics:

  • Wormian (Sutural) Bones:
    • Extra bone pieces that occur within the sutures of the skull.
  • Accessory Bones:
    • Supernumerary bones, often found in the hands or feet, due to variations in development.

 

Clinical Aspects of Bones

Bones are prone to various disorders, trauma, and diseases, impacting their structure and function.

Disorders

  1. Osteoporosis:
    • Characterized by decreased bone density and increased fragility.
    • Common in postmenopausal women due to estrogen deficiency.
  2. Osteoarthritis:
    • Degenerative joint disease involving cartilage wear and bone remodeling.
  3. Rickets and Osteomalacia:
    • Caused by vitamin D deficiency, leading to defective bone mineralization.
  4. Paget’s Disease:
    • Abnormal bone remodeling results in enlarged but weak bones.
  5. Bone Infections (Osteomyelitis):
    • Often caused by bacteria, leading to inflammation and destruction.
  6. Fractures:
    • Breaks in bones are classified by pattern (e.g., simple, compound, comminuted).
  7. Bone Tumors:
    • Benign: Osteochondroma, osteoma.
    • Malignant: Osteosarcoma, Ewing’s sarcoma.

Diagnostic Tools

  1. X-Rays:
    • Identify fractures, deformities, and bone density issues.
  2. CT/MRI Scans:
    • Provide detailed imaging of bones and surrounding tissues.
  3. Bone Density Tests (DEXA):
    • Assess the risk of osteoporosis.

Treatments

  1. Fracture Management:
    • Includes immobilization, surgical fixation (plates, screws), or traction.
  2. Osteoporosis Management:
    • Medications like bisphosphonates, calcium, and vitamin D supplements.
  3. Surgical Interventions:
    • Bone grafting, joint replacements.
  4. Physical Therapy:
    • Promotes recovery and maintains function.

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