Bone Grafts, BMP, and Bone Substitutes

Bone Grafts, BMP, and Bone Substitutes

Prepared by Dr. Alper DUNKI

• Purpose: Enhance fracture healing, treat nonunion, arthrodesis, fill bone defects
• Mechanisms:
  Osteogenesis → viable bone-forming cells
  Osteoinduction → growth factors (e.g., BMP)
  Osteoconduction → scaffold for bone growth
• Gold Standard: Autograft (provides all three mechanisms)
• Alternatives: Allograft, DBM, ceramics (HA, β-TCP), collagen, bone marrow aspirate
• BMPs: rhBMP-2 & rhBMP-7 → used in spinal fusion, long bone nonunion
• Limitations: Risk of ectopic bone, swelling, variable efficacy, cost issues
• Adjuncts: Electromagnetic stimulation, low-intensity ultrasound (experimental)

  Bone Healing

Primary healing

• Occurs with fixation that provides      absolute stability.
• In open reduction and internal      fixation, disruption of the fracture hematoma may delay early healing.
• Direct healing occurs through      cortical remodeling.
• Areas lacking direct contact are      initially filled with woven bone, which later remodels into lamellar bone.

Secondary healing

• Takes place in the periosteum and      surrounding soft tissues.
• Both endochondral and      intramembranous ossification may occur simultaneously.

Phases of bone healing

1. Inflammation: Hematoma formation, migration of      inflammatory cells, angiogenesis, BMP-mediated mesenchymal stem cell      differentiation.
2. Reparative phase: Formation of woven bone (soft      callus), osteoid seam, cartilaginous bridge, and calcification.
3. Remodeling phase: Resorption and formation occur      concurrently; Wolff’s law is the guiding principle.

Factors impairing healing:Excessive instability, poor vascularity, periosteal damage, use of NSAIDs/corticosteroids, smoking, systemic metabolic bone disease.

2. Role of Bone Grafts

Indications: Fracture healing, delayed union/nonunion, arthrodesis, repair of bone defects.

Physiologic mechanisms:

• Osteogenesis: Provides viable bone-forming cells      (e.g., autograft, bone marrow aspirate).
• Osteoinduction: Provides factors stimulating cell      differentiation (e.g., BMP).
• Osteoconduction: Provides a scaffold for new bone      formation (e.g., cancellous bone).

3. Bone Graft Materials

Autograft

• Harvested from the same individual;      considered the gold standard with osteogenic, osteoinductive, and      osteoconductive properties.
• Types: Cortical (high structural      support, low cellular/factor content), cancellous (low structural support,      high cellular/factor content), corticocancellous.
• Iliac crest graft is most common;      complications include pain, hematoma, nerve injury, infection, fracture.
• Local autograft (e.g., lamina after      laminectomy) is limited in quantity.

Allograft

• Harvested from cadavers; weak      osteogenic potential; sterilization reduces osteoinductivity.
• Types: Fresh (rarely used), frozen,      freeze-dried, demineralized bone matrix (DBM).
• DBM: Mineral removed, collagen      structure preserved; osteoinductive potential varies with processing,      provides no structural support.

Autologous bone marrow aspirate

• Source of mesenchymal stem cells;      used alone or combined with grafts.

Collagen

• Supports mineralization and      vascularization; does not provide structural support; often used as a      carrier.

Inorganic compounds and bioceramics

• HA, β-TCP, bioactive glass;      compressive strength, osteoconductive; resorption rates differ.

4. Bone Morphogenetic Proteins (BMP)

• Members of the TGF-β superfamily;      only certain types are osteoinductive (e.g., BMP-2, BMP-7).
• Recombinant forms: rhBMP-2 (used in      spinal fusion, tibial fractures), rhBMP-7 (used in long bone nonunions).
• Efficacy: Comparable clinical outcomes to      ICBG; some studies report higher radiological fusion rates.
• Complications: Ectopic bone formation, regional      swelling, increased complication rates with some fusion techniques.

5. Combined Graft Use

• Different grafts may be combined to      complement properties or compensate for insufficient volume (e.g.,      cortical allograft + DBM + local bone).

6. Other Methods to Enhance Bone Healing

• Electromagnetic stimulation: Based on alteration of bioelectric      potentials at the healing site; clinical efficacy is variable.
  • Types: Pulsed electromagnetic       field, capacitive coupling, direct current stimulation.
• Low-intensity ultrasound: Potentially beneficial; not widely      used in clinical practice.

References

1. Zhang, J., Wang, Y., & Jin, D. (2025). Research progress of bone grafting: A comprehensive review. International Journal of Nanomedicine,

2. Von Benecke, J. P., & ark. (2024). A narrative review on recombinant human bone morphogenetic protein-2: Where are we now? Cureus, Article e.g. 2024.