Compact/Cortical bone structure

• Compact/cortical/lamella bone reminder:

  • Dense, smooth, homogenous
  • forms external covering of all bones & predominant component of shafts
  • Variable thickness
  • Resists torsional stress

  A) compact bone

  
• Compact bone – Microscopic structure

  • made up of many basic units called Osteon (previously called an Haversian system)
  • Osteon - Concentric cylinders of mineralised collagen
  • Enclosing a nutrient vessel – the central canal

  
• Compact bone – Microscopic structure

  • Each layer = a lamella – formed into a tube
  • Tubes – arranged concentrically
  • All collagen fibres in a lamella are orientated unidirectionally
  • All collagen fibres in adjacent lamellae are orientated counter direction to neighbours = ability to resist / cope with torsional forces
  • Osteons are orientated parallel to the long axis of bone
• • Osteocytes sit within the lacuna which are located within the walls of lamellae
  • the dendrites of osteocyte cells join and make connections with neighbouring osteocytes within the same/adjacent lamella of an osteon
  • this allows the osteocytes to keep structural health/integrity of the osteon

  
• Circumferential lamellae

  • plates (lamellae) of mineralised collagen which are not organised into concentric tubes in the same way that is seen in osteon
  • but is organised into plates which continue around the whole circumference of bone, providing a structure of compact bone around the outer aspect of the bone shaft

  
• Microscopic Structure of compact bone - Interstitial Lamellae:

  • Remnants of earlier osteons - fillers between osteon
  • Enhances internal structural integrity
• Microscopic Structure of compact bone - Circumferential Lamellae:

  • Extends around whole bone circumference
  • Enhances external structural integrity
• Microscopic Structure of compact bone - Volkmann's Canals

  • Run perpendicular to the long axis of the bone – connecting to central canals
  • Act as conduits for blood vessels - periosteum to central canal

  A) Volkmanns canals

  
• Microscopic Structure of compact bone - Volkmann's Canals

  • Why are blood vessels necessary?
  • exchange of nutrients & waste products
  • exchange of calcium ions

  A) Volkmanns canals

  
• Compact bone - Blood Supply:

  • Diffusion impossible via dense calcified tissue
  • Cells within matrix need to be nourished etc
  • calcium homeostasis needs to occur
  • The canal system provides a geometrically efficient system for supplying the maximum amount of bone with the minimum number of vessels

  
• Summary re: structure compact bone

  • Main component of bone is collagen = inherent tensile strength
  • Collagen is organised into osteons
  • Osteons & stress management:
  • excellent – torsional stress ( see twisting of the diaphysis)
  • good for tensile strength – (see bending of diaphysis)
  • good for compression (see WB pillars along the long axis of the diaphysis)
• Canaliculi:

  • what osteocytes use to communicate with each other
• Volkmanns canals:

  • connect central canals together
• • Osteocytes are found within the lacunae
• • Osteons are the structural unit of compact bone
• Collagen arrangement within the osteon means that compact bone is best able to cope with forces of these natures:

  • bending
  • twisting
  • compressional