Peer Support 17/02/14 Prateek Choudhary, Mel Draper Bone structure, growth and repair

Macroscopic structure of bone (trabecular) (cortical)

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

Cells in bone

Part of mesodermDerivatives Paraxial mesodermAxial skeleton bones, axial cartilage, fibroblasts, skeletal muscle, adipose, skin Intermediate mesodermUrogenital structures Lateral plate mesodermSomatic layer: Limb bones, limb cartilage Splanchnic layer: Smooth muscle of circulatory system and gut wall, cardiac muscle, blood, pericardium, peritoneum

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

Structure of cortical bone 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

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

Bone growth…

Bone repair and fracture healing Fracture definition: A breach in the integrity of part or the whole of a bone Steps in fracture healing: 1. Haematoma 2. Granulation tissue formation 3. Callus formation 4. Woven bone deposition 5. Lamellar bone deposition 6. Remodelling

Bone repair and fracture healing 1. Haematoma: Bleeding from nutrient arteries and periosteal arteries which raises periosteum from cortical bone Avascular necrosis occurs around # 2. Granulation tissue formation: Macrophages invade and remove necrotic bone near # Vascular connective tissue grows by angiogenesis in response to cytokines (VEGF) and growth factors 3. Periosteal callus forms: A fibrous material rich in fibroblasts – bridges broken ends of bone

Bone repair and fracture healing 4. 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 5. Lamellar bone replaces woven bone: Callus of woven bone is replaced by lamellar bone which is stronger and more ordered in its structure 6. 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

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) Fast rate of # unionSlow rate of # union Stability of #Stable (reduction and splint) Extensive movement of bone ends Blood supplyRichPoor Initial position Apposition of bone endsSeparation of bone ends OthersInfection or foreign body