Bone Tissue and Formation

Bone Tissue

• Bone is a highly vascularized connective tissue that provides support and protection for the body.
• Bone tissue is comprised of bone matrix and bone cells.
• The bone matrix is made up of both organic and inorganic components:
  • Organic component: Collagen type I, proteoglycans and glycoproteins.
  • Inorganic component: Hydroxyapatite crystals (calcium and phosphate).
• Bone cells include:
  • Osteoblasts: Bone-forming cells
  • Osteocytes: Mature bone cells
  • Osteoclasts: Bone-resorbing cells
• The bone covering is comprised of two layers:
  • Periosteum: Outer layer, composed of fibrous connective tissue
  • Endosteum: Inner layer, lining the medullary cavity.

Bone Formation

• Bone formation, also known as ossification, is a process that involves the deposition of bone matrix by osteoblasts.
• There are two types of ossification:
  • Intramembranous ossification: Occurs directly within mesenchyme, forming flat bones like the skull.
  • Endochondral ossification: Occurs within a cartilage model, forming long bones like the femur.

Bone Structure

• Bone is organized into structural units known as osteons or Haversian systems.
• Osteons are cylindrical structures that contain a central Haversian canal, which houses blood vessels and nerves.
• Lamellae, concentric layers of bone matrix, surround the Haversian canal.
• Lacunae, small cavities within the lamellae, house osteocytes.
• Canaliculi, tiny canals that connect lacunae, facilitate nutrient and waste exchange between osteocytes.

Bone Remodeling

• Bone remodeling is a continuous process of bone resorption and bone formation that helps maintain bone health by adapting to mechanical stresses.
• It involves a balance between osteoclast and osteoblast activity.
• Bone resorption occurs when osteoclasts break down bone.
• Bone formation occurs when osteoblasts deposit new bone.
• Mechanical stress stimulates bone formation, making bones stronger.
• This is why exercise is important for maintaining bone health.

Types of Bone

• Bone tissue can be classified into two types based on its structure:
  • Compact bone: Dense and strong, found in the shafts of long bones.
  • Cancellous bone: Spongy and porous, found in the ends of long bones and in flat bones.

Behavior of Bone Under Loading

• Bone exhibits anisotropic behavior, meaning its mechanical properties vary depending on the direction of the applied load.
• Under various loading modes:
  • Tension: Bone withstands tensile loads (pulling forces) to a certain extent.
  • Compression: Bone is strong in resisting compressive loads (pushing forces).
  • Bending: Bone is susceptible to bending forces, especially at areas of stress concentration.
  • Shear: Bone can withstand shear forces (forces acting parallel to the surface).
  • Torsion: Bone is relatively weak under torsional loads (twisting forces) but can exhibit some resistance.

Stress-Strain Curve

• The stress-strain curve is a graphical representation of how a material deforms under increasing load.
• The curve's area under the curve represents the fracture toughness, a measure of the energy required to fracture the material.
• Young's modulus (E) is a material's stiffness, reflecting its ability to resist deformation under load.
• Factors influencing the stress-strain curve of bone include:
  • Loading characteristics: direction of applied load, type of load, loading rate, and amount of applied load.
  • Mechanical properties: compact versus cancellous bone, and age.
  • Structural properties: bone architecture, cross-sectional area, length of bone, muscle action, joint structure, and weak points in bone.

Pathomechanics of Bone

• Pathomechanics is the study of how mechanical factors contribute to bone injury.
• Common bone pathologies:
  • Osteonecrosis: Death of bone cells due to lack of blood supply.
  • Osteoporosis: Reduction in bone density, increasing fracture risk.
  • Fractures: Breaks in bone structure, caused by excessive loading or stress.

Fractures

• Fractures can be caused by various loading forces, including tension, compression, bending, shear, torsion, and combined loading.
• Fatigue fractures occur due to repetitive loading, causing microdamage over time.
• Healing a fracture requires specialized cells (osteoblasts and osteoclasts) to form a callus.

Trabecular Architecture

• Trabeculae (thin, bony struts) are a key component of cancellous bone.
• Their arrangement and orientation are critical for bone strength.
• In long bones, trabecular architecture is aligned to resist compressive and tensile forces.
• In short bones, like vertebrae, trabeculae are oriented for primarily vertical loads.
• Trabecular architecture plays a significant role in load transmission and bone strength.