Bonegraft
Discussion
Biology of Incorporation of Bone Grafts
Remodelling Cycle
Resorption
Formation of new bone
Regulation of bone resorption & formation
Regulation of bone resorption & formation
Histology of Incorporation of Grafts
Osteoconduction
Osteoinduction
Cortical bone
Cancellous bone grafts
Repair of Allografts
Revascularized Autografts
Disadvantages
Pitfalls in Clinical Application
Recipient-Related Considerations
Systemic factors
Graft-Related Considerations
Additional Factors
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