Bone grafting

Indications

Category Description
Structural support Bone loss, support of depressed joint surface, deformity (e.g., scaphoid)
Aid bone formation Aid fusion of a joint or in the spine
Non-union Stimulate fracture healing in atrophic non-union

Properties of Bone Grafts

  • Osteogenic: Graft contains living cells capable of differentiating into bone. Independent of the state of the graft bed (e.g., mesenchymal stem cells, bone marrow, or cancellous autograft).
  • Osteoconductive: Graft provides a scaffold for the ingrowth of capillaries and cells.
  • Osteoinductive: Graft provides biologic stimulus for host cells to differentiate to form bone (e.g., BMPs, TGF-beta). The graft material itself does not contain these cells.

Classification

  1. Autograft
    • Graft harvested from and implanted in the same individual.
    • Contains viable host cells.
    • Best graft option.
    • Volume available is limited, leading to graft site morbidity.
  2. Allograft
    • Graft taken from another individual of the same species.
    • Allograft may be fresh, frozen, freeze-dried, or demineralized.
    • Cells are destroyed in the process.
  3. Xenograft
    • Graft obtained from a different species, often bovine or porcine.
  4. Synthetic graft

Sources

  • Cancellous bone: Best option but volume available is limited.
  • Cortical bone
  • Osteochondral grafts
  • Bone marrow: Contains osteogenic and osteoinductive factors.

Dangers

Graft Type Risks
Autograft Graft site morbidity (scar, pain, infection, fracture)
Allograft Infection transmission (2 in a million risk of Hepatitis C or HIV), increased infection risk, immune reaction, malignant cell implantation

Graft Incorporation

  • Host bone grows into and replaces the graft.
  • Allograft incorporates more slowly due to being less osteogenic and causing an immune response. Allograft may also be rejected.
  • Cancellous graft incorporates quicker than cortical graft due to a greater cell volume.

Cancellous Autograft Incorporation Process

  1. Inflammatory response
  2. Creeping substitution
    • Graft stimulates osteoblast activity (osteoinduction).
    • Osteoclastic resorption of the graft follows.
    • The graft is eventually completely replaced by creeping substitution.
    • X-rays at this stage appear denser, and bone is at its strongest.
  3. Remodeling
    • Bone density reduces, as does strength over time, following Wolff’s law.
  • Cancellous grafts achieve early structural strength as osteoblasts lay down bone onto the graft early on.

Cortical Graft Incorporation

  1. Inflammatory response
  2. Osteoclast cutting cones: All donor bone must be removed before appositional bone formation occurs. Mechanical strength is reduced by 50% during this process.
  3. Osteoblasts: With vessels, migrate into cutting cone channels and lay down lamellar bone.
  • There is no remodeling phase.
  • Mechanical strength takes 1-2 years to recover.
  • Incorporation is often incomplete.

Allograft Incorporation (Cortical or Cancellous)

  • Follows a similar process but is much slower.
  • Higher chance of rejection due to a pronounced immune response, which is cellular (not humoral) and mediated by host T cells.

Vascularized Graft Incorporation

  • Incorporates like a healing fracture since the biological and mechanical environment is similar.

Bone Graft Substitutes

Type Description
Calcium Phosphates e.g., tricalcium phosphate, Norion (injectable). Undergoes partial conversion to hydroxyapatite. Degrades slowly, providing structural support (e.g., tibial plateau). Good osteoconductivity.
Hydroxyapatite Does not incite any immune response.
Other Various types of calcium sulfate (e.g., Osteoset), ceramic, silicone, and synthetic polymers.

Osteoinductive Agents

  • TGF Beta: Induces the formation of type 2 collagen.
  • BMPs (Bone Morphogenic Proteins):
    • 20 types; only BMP-2 and BMP-7 approved for use in orthopedic surgery (in the USA).
    • BMP-2 is the most osteogenic and primarily osteoinductive.
    • OP-1 (BMP 7) proven effective in tibial non-union.
  • PDGF: Platelet-derived growth factor.
  • FDGF: Fibroblast-derived growth factor.

Bone Banking

Principles

  • Written consent is required if the donor is living.
  • For cadavers, no objection from the next of kin is required.

Screening

  • Blood tests and history are used to highlight contraindications.

Exclusions: - HIV, Hepatitis, malignancy, chronic diseases (e.g., rheumatoid arthritis, autoimmune diseases), steroid treatment, Alzheimer’s, CJD, MS.

Available Products from Bone Banks

Type Description
Fresh Frozen Femoral Heads Available only to hospitals that collect them.
Cancellous Cubes Freeze-dried in 1cm cubed size; purely osteoconductive with no viable cells.
Cortical Struts From femoral shafts, ranging from 2-22 cm, freeze-dried or gamma-irradiated.
Massive Allografts Proximal and distal femora and tibiae available; frozen and gamma-irradiated.

Allograft Processing Techniques

  • Physical debridement.
  • Ultrasonic and ethanol treatments.
  • Antibiotic soaks.
  • Irradiation (reduces mechanical strength).
  • Demineralization.

Allograft Storage

Type Details
Fresh Immunogenic, therefore not typically done.
Fresh Frozen (-70°C) Preserves BMPs, little impact on mechanical strength, reduces immunogenicity.
Freeze-Dried Safest regarding disease transmission and immune response, eliminates BMPs, reduces mechanical strength.

Other Methods of Augmenting Bone Healing

  • Distant Corticotomy: Within the same bone, it stimulates healing elsewhere in the bone.
  • Electromagnetic effect: Piezoelectric currents.
  • Ultrasound: Low-intensity ultrasound shown to affect gene expression, increase osteoblast activity, and stimulate blood flow.
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