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British Medical Books Health Issues Computerized Tomography

Discussion

  • among the most commonly used orthopaedic procedures
  • used with repair of #s, arthrodeses, filling of cystic defects & for replacement of skeletal deficits after traumatic loss or ablation of tumor

Biology of Incorporation of Bone Grafts

  • bone heals by regeneration and replacement of bone, rather than scar tissue
  • the central physiological feature of bone maintenance and repair is the remodelling cycle

Remodelling Cycle

Activation phase

  • both local & systemic factors can initiate bone remodelling
  • physical modalities, including mechanical stress & electrical signals, may induce this same activity

Resorption

  • appearance on trabecular surfaces of multinucleated giant cells - osteoclasts
  • a portion of pre-existing mineral and matrix is removed

Formation of new bone

  • osteoblasts (also originating from bone marrow) deposit osteoid on bone surfaces
  • this matrix is subsequently mineralized, often engulfing bone-forming cells that persist as osteocytes

Regulation of bone resorption & formation

  • complex process
  • local humoral factors from bone matrix & cells, as well as systemic influences
  • growth and differentiation factors that affect bone
    • prostaglandins
    • osteocalcin
    • bone-derived growth factor
    • bone morphogenetic protein
    • platelet-derived growth factor
  • when resorption and formation are synchronous, bone mass remains constant

Histology of Incorporation of Grafts

  • differences between the incorporation of cortical and cancellous tissues
  • process varies if the blood supply is rapidly re-established by vascular re-anastomosis

Incorporation of autografts

  • fresh autogenous cortical grafts and cancellous grafts are identical
  • hematoma formed around the implanted bone
  • necrosis of the graft ensues & stimulates a local inflammatory response - fibrovascular stroma
  • recipient-derived blood vessels & osteogenic precursor cells to the graft
  • only few graft-derived cells near the surface remain viable by diffusion
    • major contributions - osteoconduction & osteoinduction

Osteoconduction

  • graft providing a scaffold on which new bone is deposited

Osteoinduction

  • graft-derived factors actively stimulate the recipient to invade the structure with osteogenic activity
  • bone morphogenetic protein or proteins

Cortical bone

  • bone repair requires invasion of vascular buds - occurs thru’ pre-existing Haversian or Volkmann canals
  • these channels widened by osteoclastic activity
  • graft can’t withstand substantial loads - adequate internal fixation
  • new-bone formation restores the net bone mass, along with the mechanical strength

Cancellous bone grafts

  • differ from cortical bone grafts in the rate and the completeness of repair
  • more porous nature - more rapid revascularization & more complete incorporation

Repair of Allografts

  • considerable experience with allografts
  • two major issues that appear to influence incorporation

Major histocompatibility locus

  • allografts different from the recipient
  • genetic disparity between donor & recipient - << incorporation of the graft
  • process may be delayed, less complete, or both

Allografts treated in a variety of ways

  • purpose of long-term preservation
  • may also alter biological or biomechanical properties
  • most allografts are used fresh, deep-frozen, or freeze-dried

Fresh allografts

  • vigorous inflammatory response as well as a specific immune response
  • resorptive activity may be overwhelming
  • clinical reports - efficacy of massive fresh allografts

Deep-frozen allografts

  • decreased immunogenicity
  • no change in mechanical properties
  • if bone is frozen to -70 C°
    • loss of activity of proteolytic enzymes - storage for prolonged time
  • incorporation slower and less complete

Freeze-dried allografts

  • reduced immunogenicity
  • undergo changes in mechanical properties - protected by internal fixation
  • incorporation similar to that for autografts, but it is delayed and less extensive

Irradiation

  • also changes the mechanical properties
  • ethylene oxide compatible with success of a graft
  • autoclaving has detrimental to incorporation of a graft in animals
  • significant osteogenic (osteoinductive) activity associated with these grafts
  • advantages of availability without morbidity at the donor site

Revascularized Autografts

  • BG immediately re-anastomosed to its blood supply
  • does not become necrotic or require incorporation
  • repairs at either end by a process that is analogous to fracture-healing
  • another advantage - graft does not depend on recipient bed (e.g. scar tissue)

Disadvantages

    • operative process lengthy and demands special expertise
    • limited to segments of the fibula, ribs, or the iliac crest

Pitfalls in Clinical Application

  • incorporation of BG requires
    • graft-derived factors (osteoinduction, osteoconduction, and cells)
    • contributions from the recipient (blood vessels and cells)

Recipient-Related Considerations

Local factors

  • anything that interferes with the ingrowth of blood vessels or availability of osteoprogenitor cells
    • purulent material, intense inflammatory response, or foreign objects (PMMA) interposed between the graft and the recipient bed
    • necrotic or densely scarred tissue unable to generate a significant vascular response
    • local irradiation will injure regional osteoprogenitor cells and harms the local blood supply
  • bone bed -- crucial that the bone itself is healthy
    • sclerotic bone - poor vascular response
    • osteopenic bone mechanically may be weak
    • neoplastic processes can destroy a bone graft
  • surgical technique (tissues handling) is an important factor

Systemic factors

  • chemotherapeutic, myelosuppressive or antimetabolic drugs - suppress bone remodelling
  • NSAID agents, diphosphonates, steroids, hormones also
  • malnutrition and other metabolic bone dx

Graft-Related Considerations

  • various techniques of preservation
  • auto or allograft, or vascular

Additional Factors

  • physiological loading applied to a bone graft will be of benefit
    • without stress tend to resorb
  • potential value of electrical stimulation