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Biological and Mechanical Properties of External Fixation
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Biological and Mechanical Properties of External Fixation

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Biological and Mechanical Properties of External Fixation

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  • 1. Biological and Mechanical Properties of External Fixation Mr C.W. Oliver BSc MD FRCS(Orth) Royal Infirmary of Edinburgh
  • 2. Why do we have bones
    • ?
  • 3. Why do fractures heal
    • ?
  • 4. Evolution
    • endoskeleton
    • genetic pool
    • reproduction
    • intramembranous v endochrondral ossification
  • 5. Intramembranous Bone Formation
    • occurs at periosteal surface
    • pelvis, scapula, clavicle, skull
    • osteoblasts form a calcified matrix within collagen framework
    everyday stress fractures ~ primitive process
  • 6. Endochondral Bone Formation
    • occurs at growth plates and within # callus
    • osteoblasts form osteoid on cartilagenous framework
    fractures ~ mature process
  • 7. Fracture Treatment reduce Hold Rehabilitate
  • 8. Fracture Stability
    • impaction of bone ends
    • adjacent ligaments
    • soft tissue damage
  • 9. Fracture Immobilisation
    • nothing
    • plaster
    • cast brace
    • internal
      • plate
      • intramedullary nail
    • external fixator
    • mixture
  • 10. Fracture Behaviour
    • UNCERTAINTY ?
    TRUST ?
  • 11. Fracture Behaviour
    • STABILITY versus INSTABILITY
    STIFFNESS versus STRENGTH
  • 12. Why do bones fracture ?
    • related to crystal structure and collagen orientation
    • cortical bone is weakest in tension and shear
    • low energy # follow stress lines
  • 13. What types of bone healing ?
    • primary = union without callus
    • secondary = union with callus
  • 14. Compression Plating
    • PRIMARY BONE HEALING
    • rigid plating inhibits # callus
    • haversian remodelling
    • contact healing - contact areas
    • non-contact healing - non contact areas
  • 15. Haversian Remodelling
    • PRIMARY BONE HEALING
    • revascularise necrotic # ends
    • reconstitution of intercortical union
  • 16. Haversian Remodelling
    • REQUIRES
    • exact reduction
    • stable fixation
    • existence of a blood supply
  • 17. DCP Plates
    • compression decays with time
    • 50% within 2 months
    • same decay occurs with intact bone
    • due to haversian remodelling
  • 18. Gap Healing
    • incongruencies
    • fill with woven bone
    • secondary osteons scaffold the gaps
  • 19. Primary Bone Healing
    • oversimplification
    • often see small amount of callus opposite a plate
    • #’s healing without callus but have 2ndry healing
      • scaphoid and talus = little periosteum or soft tissues
  • 20. Healing under Plates
    • stress shielding
    • vascular disturbance
    • number of cortices
    • “ biological fixation”
  • 21. IM Nails
    • reamed versus non-reamed
    • extramedullary callus
    • slowed collapse and endochondral repair
  • 22. External Fixation
    • primary bone healing can be seen with ex-fix
    • less rigid fixation - “controlled” motion
  • 23. Mechanical Strength of Ex-Fix
    • NOT BASED SOLEY ON FIXATOR
    • composite structure
    • load sharing device
    • strength of the bone
    • HOW MUCH WEIGHT BEARING ?
  • 24. How Rigid Should an Ex-Fix be ?
    • Controlled micro-movement
    • Non-union - prevent periosteal new callus
  • 25. Ex-Fix Disassembly ?
    • Controlled removal
    • initial stiff frame then more elastic ?
  • 26. Ex-Fix Pins
    • pinholes < 30% of bone diameter
    • 5mm pin 144% stronger than 4mm pin
    • separate pins greater than 45º
  • 27. Ex-Fix Frames
    • 11mm tubular rod x2 stiffer than solid rods of 8mm diameter
    • increasing pin spread increases bending stiffness
    • decreasing bone to rod distance increases bending stiffness
  • 28. Rigidity of Ex-Fix
  • 29. How to improve pin bone interface ?
    • apply half pins rather than wires
    • wash pins at time of surgery
    • avoid thermal necrosis - hand drill
    • pre-loading
    • dynamization
  • 30. Axial Stimulation at the fracture
    • PASSIVE DYNAMISATION
    • due to pin bending
    • remove side bars and pins
  • 31. Axial Stimulation at the fracture
    • ACTIVE AXIAL DYNAMISATION
    • weightbearing - without pin bending
    • relaxation of axial constraint
  • 32. Axial Stimulation at the fracture
    • CONTROLLED AXIAL DYNAMISATION
    • controlled force/displacement actuator
    • cam device
  • 33. Circular Ex-Fix
    • recent innovation
    • 1.8 to 2mm wires
    • circular rings
    • anatomic considerations
    • bending stiffness independent of loading direction
  • 34. Half frame versus Ilizarov
    • Ilizarov half as rigid in compression
    • A-P and torsion comparable
    • M-L half frame x10 stiffer than Ilizarov
  • 35. Ilizarov Frame Stability
    • larger wire diameter
    • decrease ring size
    • use of olive wires
    • more wires/half pins
    • cross wires at 90º
    • increase wire tension
    • centre rings closer to fracture
    • closer adjacent rings
  • 36. Frame Stiffness ?
    • mono-planar
    • biplanar
    • circular
    • THE ULTIMATE QUESTION !
  • 37. Ex-Fix Summary
    • ideal construct for external fixation is not known
    • can allow for a change in compression/alignment
    • modulate bone healing between minimal and maximal callus formation
  • 38.