Bone structure, growth and repair

Peer Support 17/02/14Prateek Choudhary, Mel DraperBone structure, growth and repairMacroscopic structure of bone

(trabecular)(cortical)12341256Components of boneCells embedded in a mineralised ECMHas rigidity and resilienceMineral component is hydroxyapatite (calcium and phosphate) rigidityFibrous/Osteoid component is type I collagen resilienceCellsBone matrixCells in bone

Part of mesodermDerivativesParaxial mesodermAxial skeleton bones, axial cartilage, fibroblasts, skeletal muscle, adipose, skinIntermediate mesodermUrogenital structuresLateral plate mesodermSomatic layer: Limb bones, limb cartilageSplanchnic layer: Smooth muscle of circulatory system and gut wall, cardiac muscle, blood, pericardium, peritoneumTypes of boneWoven bone:Immature/mature? ImmatureArrangement of collagen? HaphazardStrength? Weaker than lamellar boneExamples? Fracture healing, foetus, Pagets disease (pathological)Lamellar bone:Immature/mature? Mature (replaces woven) Arrangement of collagen? Ordered layers (lamellae)Strength? StrongWoven/lamellar bone can adopt different densities:Cortical/compact boneTrabecular/spongy/cancellous bone

trabecularcorticalStructure of cortical boneHaversian systems/osteons - concentric lamellae around central canals (long axis of bone)Interstitial lamellae bony lamellae between Haversian systemsCircumferential lamellae lamellae running along the external and internal sidesVolkmanns canals transverse canals carrying periosteal vesselsDensity? Denser than trabecularStrength? Stronger than trabecularWeight? Heavier than trabecular

12334Structure of trabecular boneMade up of struts of collagen fibresDirection of fibres? Along lines of stressDensity? Less dense than corticalStrength?Weaker than cortical (compressive)Weight?Lighter than cortical

Bone growthBone repair and fracture healingFracture definition: A breach in the integrity of part or the whole of a boneSteps in fracture healing:HaematomaGranulation tissue formationCallus formationWoven bone depositionLamellar bone depositionRemodellingBone repair and fracture healingHaematoma: Bleeding from nutrient arteries and periosteal arteries which raises periosteum from cortical boneAvascular 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 factorsPeriosteal callus forms:A fibrous material rich in fibroblasts bridges broken ends of bone

Bone repair and fracture healingWoven bone and cartilage forms:Osteogenic cells near # differentiate into chondrocytes. These deposit new cartilage at # site this is radiolucent on X-rayCartilage is replaced over time by endochondral ossificationOsteogenic cells further from # differentiate into osteoblasts. These deposit woven bone this is opaque on X-rayWoven bone imparts rigidity to # site but has haphazard arrangement of collagen fibres irregular swellingLamellar bone replaces woven bone:Callus of woven bone is replaced by lamellar bone which is stronger and more ordered in its structureRemodelling:Osteoclasts and osteoblasts act in concert to remodel lamellar bone according to the lines of stress (Wolffs Law)Excess callus us broken down and the medullary cavity is re-established by osteoclast action

Fracture unionTerms:Malunion fracture heals in an unsatisfactory position e.g. at an angle or in a rotated position impaired function of boneDelayed union fracture healing takes longer than expectedNon union fracture fails to unite fibrous union forms instead (pseudoarthrosis)

Fast rate of # unionSlow rate of # unionStability of #Stable (reduction and splint)Extensive movement of bone endsBlood supplyRichPoorInitial positionApposition of bone endsSeparation of bone endsOthersInfection or foreign body