Bone grafting

Indications

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 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

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

Allograft Processing Techniques

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

Allograft Storage

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.