Differences Between Types of Bone Graft
Bone grafts are categorized into four main types based on their origin: autografts (patient's own bone), allografts (human donor bone), xenografts (animal-derived bone), and alloplasts/synthetic materials (manufactured substitutes), each with distinct biological properties, clinical performance, and specific indications.
Classification by Origin and Biological Properties
Autograft
- Contains living osteogenic cells with all three critical properties: osteogenesis (bone-forming cells), osteoinduction (ability to induce new bone formation), and osteoconduction (scaffold for bone growth) 1
- Gold standard for bone grafting due to superior incorporation and no risk of disease transmission or immunologic rejection 2, 3
- Available as block grafts (from mandibular symphysis, ramus, or retromolar area) or particulate form 1
- Major limitation: donor site morbidity including pain, limited availability, and need for secondary surgical site 2, 4
- Healing time: 4-6 months for block autografts with expected lateral bone gain of 4-6 mm 1
Allograft
- Human donor bone that is carefully processed and screened to minimize disease transmission risk 2
- Available as freeze-dried bone allograft (FDBA) or demineralized freeze-dried bone allograft (DFDBA) 1
- Lacks osteogenic capacity (no living cells) but retains osteoconductive properties; DFDBA has some osteoinductive potential 2, 3
- Eliminates donor site morbidity but has slower incorporation compared to autograft 2, 5
- Block allograft healing time: 5-6 months with lateral bone gain of 3-6 mm (average 4 mm) 1
- Resorption rates of 20-50% for block allografts; particulate allograft shows 13.9% resorption 1
- Complications include membrane exposure (30.7%), incision line opening (30%), and infection (13%) 1
Xenograft
- Animal-derived bone (typically bovine or porcine) with chemical composition similar to human bone 6, 7
- Primarily osteoconductive with minimal to no osteoinductive properties 3, 6
- Known for extremely slow turnover, making it ideal for limiting rapid resorption when mixed with autogenous grafts 1
- Block xenograft shows variable outcomes: bovine block with rhBMP-2 achieved 3 mm width gain versus 5 mm with autograft block at 4 months 1
- Rarely used alone due to highly variable results; typically combined with autograft or allograft 1
- Particulate xenograft combined with collagen membrane yields bone gains of 3.0-5.6 mm 1
Alloplasts (Synthetic Materials)
- Manufactured materials including calcium phosphate salts (β-tricalcium phosphate, hydroxyapatite), calcium sulfate, and bioactive glass 2, 8
- Purely osteoconductive - provide scaffold for bone growth but lack osteogenic and osteoinductive properties 2, 3
- Eliminate concerns about disease transmission, immunologic response, and donor site morbidity 4, 6
- Cannot regenerate live bone alone; rarely used as sole material for lateral ridge augmentation 1
- β-tricalcium phosphate supplemented with local autograft achieved 100% fusion rate at 36 months in spinal fusion studies 8
- Calcium phosphate ceramics recommended as bone graft extenders, especially when combined with autologous bone 2
Key Clinical Distinctions
Biological Activity Hierarchy
- Autograft: Osteogenic + Osteoinductive + Osteoconductive 3, 4
- DFDBA (demineralized allograft): Osteoinductive + Osteoconductive 2, 3
- FDBA (non-demineralized allograft): Osteoconductive only 1, 3
- Xenograft: Osteoconductive only 3, 6
- Synthetic materials: Osteoconductive only 2, 3
Incorporation and Remodeling
- Small amounts of cancellous allograft are usually remodeled completely in humans 3
- Large allografts become incorporated by limited surface bone formation, functioning primarily as osteoconductive scaffolds 3
- Synthetic materials show remodeling characteristics similar to allograft 3
- Xenografts have the slowest remodeling rate, which can be advantageous for maintaining volume 1
Common Clinical Pitfalls
Autograft Considerations
- Resorption risk of 20-50% even with autograft blocks; combining with particulate grafts and barrier membranes reduces this risk 1
- Computer-aided design for block harvest can prevent neurologic complications 1
Allograft Limitations
- Lack of osteoinductive capacity limits utility in some applications 2
- Cost, availability, and disease transmission risk (though minimal with modern processing) 2
- Fusion rates comparable to autograft in instrumented procedures but may show delayed fusion (63.1% at 6 months versus 89.2% for autograft) 9
Material Selection Strategy
- For optimal outcomes: Use autograft when available and donor site morbidity is acceptable 2, 3
- For avoiding donor site complications: Allograft or synthetic materials with rhBMP-2 in appropriate settings 2
- For graft extenders: Calcium phosphate ceramics combined with autologous bone 2, 8
- For slow resorption: Xenograft mixed with autograft or allograft 1
Augmentation with Biologics
- rhBMP-2 is the best-studied biological agent with Class I evidence supporting use as autograft substitute in anterior lumbar interbody fusion with titanium cage 2
- rhBMP-2 achieved 94.5% fusion rate versus 88.7% with autograft, with advantages in operating time (24 minutes shorter) and decreased blood loss (44 ml less) 2
- Biologics can enhance incorporation of allogenic and xenogenic block grafts 1