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Bone structure, growth and repair

Bone structure, growth and repair. Peer Support 17/02/14 Prateek Choudhary , Mel Draper. Macroscopic structure of bone. 4. 1. ( trabecular ). 2. 5. 6. 3. (cortical). 2. 1. Components of bone. “Cells embedded in a mineralised ECM” “Has rigidity and resilience”

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Bone structure, growth and repair

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  1. Bone structure, growth and repair Peer Support 17/02/14 PrateekChoudhary, Mel Draper

  2. Macroscopic structure of bone 4 1 (trabecular) 2 5 6 3 (cortical) 2 1

  3. Components of bone • “Cells embedded in a mineralised ECM” • “Has rigidity and resilience” • Mineral component is hydroxyapatite (calcium and phosphate)  rigidity • Fibrous/Osteoid component is type I collagen  resilience • Cells… Bone matrix

  4. Cells in bone

  5. Types of bone • Woven bone: • Immature/mature? Immature • Arrangement of collagen? Haphazard • Strength? Weaker than lamellar bone • Examples? Fracture healing, foetus, Paget’s disease (pathological) • Lamellar bone: • Immature/mature? Mature (replaces woven) • Arrangement of collagen? Ordered layers (lamellae) • Strength? Strong • Woven/lamellar bone can adopt different densities: • Cortical/compact bone • Trabecular/spongy/cancellous bone trabecular cortical

  6. Structure of cortical bone 3 • Haversian systems/osteons - concentric lamellae around central canals (long axis of bone) • Interstitial lamellae – bony lamellae between Haversian systems • Circumferential lamellae – lamellae running along the external and internal sides • Volkmann’s canals– transverse canals carrying periosteal vessels • Density? Denser than trabecular • Strength? Stronger than trabecular • Weight? Heavier than trabecular 1 2 4 3

  7. Structure of trabecular bone • Made up of struts of collagen fibres • Direction of fibres? Along lines of stress • Density? Less dense than cortical • Strength? Weaker than cortical (compressive) • Weight? Lighter than cortical

  8. Bone growth…

  9. Bone repair and fracture healing • Fracture definition: • A breach in the integrity of part or the whole of a bone • Steps in fracture healing: • Haematoma • Granulation tissue formation • Callus formation • Woven bone deposition • Lamellar bone deposition • Remodelling

  10. Bone repair and fracture healing • Haematoma: • Bleeding from nutrient arteries and periosteal arteries which raises periosteum from cortical bone • Avascular necrosis occurs around # • Granulation tissue formation: • Macrophages invade and remove necrotic bone near # • Vascular connective tissue grows by angiogenesis in response to cytokines (VEGF) and growth factors • Periosteal callus forms: • A fibrous material rich in fibroblasts – bridges broken ends of bone

  11. Bone repair and fracture healing • Woven bone and cartilage forms: • Osteogenic cells near # differentiate into chondrocytes. These deposit new cartilage at # site – this is radiolucent on X-ray • Cartilage is replaced over time by endochondral ossification • Osteogenic cells further from # differentiate into osteoblasts. These deposit woven bone – this is opaque on X-ray • Woven bone imparts rigidity to # site but has haphazard arrangement of collagen fibres  irregular swelling • Lamellar bone replaces woven bone: • Callus of woven bone is replaced by lamellar bone which is stronger and more ordered in its structure • Remodelling: • Osteoclasts and osteoblasts act in concert to remodel lamellar bone according to the lines of stress (Wolff’s Law) • Excess callus us broken down and the medullary cavity is re-established by osteoclast action

  12. Fracture union • Terms: • Malunion – fracture heals in an unsatisfactory position e.g. at an angle or in a rotated position  impaired function of bone • Delayed union – fracture healing takes longer than expected • Non union – fracture fails to unite  fibrous union forms instead (pseudoarthrosis)

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