Autograft | Allograft | Bone Substitutes | Biologics
- Autograft: ONLY graft with all 3 properties (osteogenic, osteoinductive, osteoconductive)
- Allograft: Osteoconductive ONLY (scaffold). No living cells. Minimal osteoinduction.
- DBM: Demineralized bone matrix. Osteoinductive + osteoconductive. Exposes BMPs.
- Ceramics (HA/TCP): Osteoconductive ONLY. Variable resorption rates.
- BMPs: Bone morphogenetic proteins. Pure osteoinduction. No scaffold.
- “Autograft = all 3 properties (GIC)
- “Allograft = scaffold only
- “DBM exposes BMPs = osteoinductive
- “ICBG donor site morbidity 10-30%
Osteogenic + Osteoinductive + Osteoconductive. Only graft with living cells. Gold standard but limited quantity and donor site morbidity.
Osteoconductive only. No living cells (processing kills them). Minimal osteoinduction. No disease transmission with proper processing.
Purely osteoinductive. rhBMP-2 and rhBMP-7. No scaffold - needs carrier. Complications: swelling, heterotopic bone.
ICBG: 10-30% chronic pain. Nerve injury (LFCN), hematoma, fracture. Consider RIA for large volumes.
- Osteogenic
- YES
- Osteoinductive
- YES
- Osteoconductive
- YES
- Key Use
- Gold standard for non-union
- Osteogenic
- No
- Osteoinductive
- Minimal
- Osteoconductive
- YES
- Key Use
- Large structural defects
- Osteogenic
- No
- Osteoinductive
- Minimal
- Osteoconductive
- YES
- Key Use
- Impaction grafting
- Osteogenic
- No
- Osteoinductive
- YES
- Osteoconductive
- YES
- Key Use
- Graft extender with autograft
- Osteogenic
- No
- Osteoinductive
- No
- Osteoconductive
- YES
- Key Use
- Metaphyseal defects
- Osteogenic
- No
- Osteoinductive
- YES
- Osteoconductive
- No
- Key Use
- Non-union, spine fusion
ILDRAutograft Sources
Hook:ILDR = Iliac crest (most common), Local bone, Distal radius, RIA from femur!
Overview and Epidemiology
Bone graft science is essential basic science for the exam. You must know the three properties (osteogenic, osteoinductive, osteoconductive) and which grafts possess which properties.
Bone Grafts are materials used to fill defects, augment healing, and reconstruct bone.
- Non-union: Augment biology
- Bone defects: Fill segmental loss
- Arthrodesis: Promote fusion
- Augmentation: Enhance fixation
- Revision surgery: Restore bone stock
Graft choice depends on indication and defect size.
- Autograft: Patient's own bone
- Allograft: Human donor bone
- Xenograft: Animal bone (rare)
- Synthetic: Ceramics, polymers
- Biologics: DBM, BMPs, PRP
Multiple options with different properties.
Pathophysiology and Mechanisms
Bone healing requires: (1) Cells to form bone (osteogenic), (2) Signals to induce bone formation (osteoinductive), and (3) Scaffold for bone to grow into (osteoconductive). Autograft provides all three; other grafts provide only some.
- Inflammation: Hematoma forms, cytokines released
- Repair: Callus formation (soft then hard)
- Remodeling: Wolff's law - bone remodels to stress
- Creeping substitution: Host bone resorbs graft and replaces
- Cancellous: Faster incorporation (more surface area)
- Cortical: Slower incorporation (dense structure)
Understanding biology guides graft selection.
The Diamond Concept of Bone Healing
The global-practice section quotes the AO Foundation's "diamond concept" but never explains it - yet it is the single framework that ties this whole topic together and the reason every management plan pairs biology with fixation.
The diamond concept (Giannoudis) states that successful bone healing requires several elements to be present together, like the facets of a diamond. If any one is missing, healing fails - which is why even autograft fails in an unstable or avascular site, and why stable fixation alone fails in an atrophic, biology-poor non-union.
- What it provides
- Living cells that form bone
- Supplied by
- Autograft, bone-marrow aspirate (the 'G' of GIC)
- What it provides
- A framework for ingrowth
- Supplied by
- Any graft, ceramics, allograft (the 'C')
- What it provides
- Growth factors that recruit/differentiate osteoprogenitors
- Supplied by
- Autograft, DBM, BMP (the 'I')
- What it provides
- A strain environment that permits union
- Supplied by
- The SURGEON's fixation - plate, nail, external fixator
- What it provides
- Blood supply to deliver cells, oxygen and nutrients
- Supplied by
- A healthy, non-irradiated, non-infected soft-tissue envelope; vascularised graft if the bed is poor
The three graft properties (GIC) supply only three corners; the surgeon must add mechanical stability and ensure vascularity. This is why the management sections insist on stable fixation before or with any graft, and why a graft placed into an infected or avascular bed will fail no matter how good its biology.
Use the diamond concept as your framework: bone healing needs osteogenic cells + osteoconductive scaffold + osteoinductive signals + mechanical stability + a vascular host bed, all together. A graft supplies the first three (GIC); the surgeon supplies stability and protects vascularity. It is the one-line reason every non-union plan is "biology plus fixation," never one alone.
GICThree Properties of Bone Graft
Hook:GIC = osteogenic (living cells), osteoinductive (growth factors), osteoconductive (scaffold). Only AUTOGRAFT has all 3!
Classification Systems
The Three Properties
- Definition
- Contains living cells that can form bone
- Which Grafts
- Autograft, BMA
- Example
- Osteoblasts, MSCs
- Definition
- Contains signals that induce new bone
- Which Grafts
- Autograft, DBM, BMP
- Example
- BMPs, growth factors
- Definition
- Provides scaffold for bone ingrowth
- Which Grafts
- All grafts, ceramics
- Example
- HA, TCP, allograft
Only autograft has ALL THREE properties.
Allograft Processing and Sterilisation
The classification tables list allograft "by processing" and note that "processing affects properties," and the complications table cites disease transmission - but how each processing step changes the graft is never explained. It is a favourite basic-science question because every step is a trade-off between safety (sterility, low immunogenicity) and biology/strength.
Why process at all? Fresh allograft carries the highest disease-transmission and immunogenic load. Processing reduces both - but at a cost to the graft's cells, growth factors and mechanical strength.
- What it does
- Retains viable cells and best mechanics
- Trade-off
- Highest immunogenicity and disease-transmission risk - reserved for osteochondral allograft where cartilage cell viability matters
- What it does
- Kills cells, lowers immunogenicity
- Trade-off
- Retains most structural strength and some growth factors - the usual structural allograft
- What it does
- Removes water for long shelf-life and lowest immunogenicity
- Trade-off
- REDUCES strength (more brittle); must be rehydrated; best for morselised/non-structural use
- What it does
- Acid extraction exposes the embedded BMPs
- Trade-off
- Becomes osteoINDUCTIVE but loses all structural strength; potency varies by donor/lot
- What it does
- Inactivates viruses/bacteria
- Trade-off
- Dose-dependent: higher doses cross-link/damage collagen, weaken the graft and can degrade BMP activity
- What it does
- Chemical sterilisation
- Trade-off
- Residues can provoke inflammation and impair incorporation
Disease transmission in numbers. With modern donor screening (serology and nucleic-acid testing) plus processing, viral transmission from banked allograft is exceedingly rare - the often-quoted residual HIV risk from screened, processed tissue is on the order of one in roughly 1.6 million. The more realistic hazard is bacterial contamination of insufficiently processed grafts; accredited tissue banks (e.g. AATB) mitigate both.
Allograft is processed to cut disease-transmission and immunogenicity, but each step costs biology or strength: fresh-frozen keeps strength (structural use); freeze-drying lowers immunogenicity and shelf-life but weakens the graft (morselised use); demineralisation exposes BMPs (osteoinductive, DBM) but removes strength; gamma irradiation sterilises but, at higher doses, weakens collagen and degrades BMP. Screened, processed allograft has a residual HIV risk around one in 1.6 million - bacterial contamination is the bigger practical concern.
Clinical Assessment
- Comorbidities: Diabetes, smoking, vascular disease
- Prior surgery: Available bone stock
- Infection: Must be excluded
- Soft tissue: Adequate coverage
- Host factors: Malnutrition, immunosuppression
Optimize patient factors before grafting.
- Size: Determines volume needed
- Location: Metaphyseal vs diaphyseal
- Biology: Atrophic vs hypertrophic non-union
- Vascularity: Deficient may need vascularized graft
- Structural needs: Load-bearing vs non-structural
Match graft to defect requirements.
Consider: (1) Defect size and location, (2) Structural requirements, (3) Biological requirements, (4) Donor site morbidity, (5) Cost and availability. For non-union, autograft + stable fixation is gold standard.
Investigations
Pre-Graft Investigation
Assess defect accurately. Size, location, bone stock. Plan graft volume and type.
For free vascularized grafts. Assess recipient vessels. Fibular flap planning.
ESR, CRP, aspiration. Grafting into infected bone will fail. Two-stage if needed.
Albumin, prealbumin, vitamin D, calcium. Malnutrition impairs graft incorporation.
Thorough pre-operative assessment guides graft selection.
Graft Option Selection (Differential)
- First-line Option
- Autograft (ICBG or RIA) plus stable fixation
- Why
- Needs cells, signal and scaffold; only autograft supplies all three
- Alternative / Avoid
- rhBMP if autograft contraindicated; avoid allograft alone
- First-line Option
- Stable fixation alone
- Why
- Biology is adequate; mechanical stability is the deficit
- Alternative / Avoid
- Graft usually unnecessary
- First-line Option
- Injectable calcium phosphate / TCP
- Why
- Osteoconductive void filler with structural support, no donor site
- Alternative / Avoid
- Autograft if biology compromised; avoid HA block as autograft replacement
- First-line Option
- Induced membrane (Masquelet) plus autograft
- Why
- 89% union; staged biology into a vascular membrane
- Alternative / Avoid
- Bone transport; vascularised fibula if poor vascularity
- First-line Option
- Impaction allograft (morselised)
- Why
- Restores bone stock as osteoconductive scaffold
- Alternative / Avoid
- Combine with autograft/DBM to add biology
- First-line Option
- Standard fixation; consider rhBMP-2 (on-label)
- Why
- Reduces secondary intervention, esp. type-III
- Alternative / Avoid
- Avoid off-label anterior cervical rhBMP
Management Algorithm
Graft Selection for Non-Union
Non-Union Approach
Atrophic vs hypertrophic. Atrophic needs biology. Hypertrophic needs stability only.
Stable fixation is prerequisite. Exchange nailing, plate revision, external fixation.
Autograft is gold standard. ICBG or RIA depending on volume. DBM as extender.
rhBMP-2 for difficult cases. Alternative to autograft. Consider cost and complications.
Biological non-union requires both stable fixation AND biological augmentation.
Surgical Technique
Iliac Crest Bone Graft Harvest
Anterior ICBG Technique
Supine. Bump under ipsilateral hip. Prep iliac crest.
2-3cm posterior to ASIS. Parallel to crest. Protects LFCN (runs 1-2cm medial to ASIS).
Split iliac apophysis. Subperiosteal elevation exposes outer table.
Curettes for cancellous. Osteotome for corticocancellous. Stay 2cm from ASIS and AIIS.
Haemostasis. Consider drain. Close in layers. Leave cancellous bed exposed.
Careful technique minimizes donor site morbidity.
Complications
- Incidence
- 10-30%
- Prevention/Management
- Meticulous technique, minimize disruption
- Incidence
- 5-15%
- Prevention/Management
- Incision 2cm posterior to ASIS
- Incidence
- 2-5%
- Prevention/Management
- Haemostasis, consider drain
- Incidence
- Less than 1%
- Prevention/Management
- Leave 2cm anterior margin
- Incidence
- 5-15%
- Prevention/Management
- Adequate fixation, optimize biology
- Incidence
- 1-5%
- Prevention/Management
- Sterile technique, prophylactic antibiotics
- Incidence
- Very rare
- Prevention/Management
- Proper screening and processing
Donor site morbidity is the main disadvantage of autograft. Consider RIA or synthetic alternatives if significant.
SCHOBMP Complications
Hook:SCHO = Swelling, Cancer concerns, Heterotopic ossification, Osteolysis - know BMP complications!
Postoperative Care
Post-Graft Management
Standard wound care. Watch for hematoma at donor site. Donor site often more painful than recipient.
Protected weight-bearing as per fixation. Donor site pain usually settles by 2-6 weeks.
Serial X-rays. Cancellous grafts incorporate faster than cortical. CT if union unclear.
Once incorporated. Graft remodels over months to years.
Graft incorporation takes 3-6 months for cancellous, longer for cortical.
Outcomes and Prognosis
Union Rates with Grafting:
- Autograft for non-union: 85-95%
- Allograft alone: 60-80%
- DBM + autograft: 85-90%
- BMP + fixation: 80-90%
Prognostic Factors:
- Better Outcome
- Non-smoker, healthy
- Worse Outcome
- Smoker, diabetic
- Better Outcome
- Small, contained
- Worse Outcome
- Large, segmental
- Better Outcome
- Good soft tissue
- Worse Outcome
- Scarring, irradiation
- Better Outcome
- Rigid stability
- Worse Outcome
- Motion at site
Optimize modifiable factors for best outcomes.
Guidelines, Registries & Global Practice
Global epidemiology:
- Bone grafting is among the most common transplantation procedures worldwide; over two million grafting procedures are performed annually, second only to blood transfusion among transplanted tissues.
- Autograft (chiefly iliac crest) still accounts for the majority of grafts; substitutes and biologics have grown steadily but supplement rather than replace autograft.
- Donor-site complication burden (ICBG ~19%, RIA ~6%) is a key global driver of substitute and RIA uptake.
Side-by-side guidance:
- Position on bone graft / biologics
- Autograft is the gold standard; substitutes/biologics are extenders. rhBMP-2 approved for open tibia (with IM nail) and selected anterior lumbar interbody fusion only
- Position on bone graft / biologics
- Emphasise stable fixation plus biology for nonunion; graft choice individualised, autograft preferred where biology is the limiting factor
- Position on bone graft / biologics
- Teaches the "diamond concept": cells, scaffold, growth factors and mechanical stability must all be addressed
- Position on bone graft / biologics
- Restricts rhBMP to approved indications; flags cost and adverse-event profile
- Position on bone graft / biologics
- rhBMP-2 and rhBMP-7 carry narrow approved (on-label) indications; off-label spinal use is widespread but not endorsed
- Tissue-bank registries (e.g. AATB-accredited banks, European tissue establishments) report disease transmission from screened, processed allograft as exceedingly rare.
- Post-marketing and FDA data (Carragee 2011) substantially revised upward the adverse-event estimates for rhBMP-2 in spine surgery relative to the original industry trials.
- Well-resourced settings: ready access to RIA systems, processed allograft, ceramics and rhBMP products; choice driven by biology, defect size and morbidity rather than availability.
- Limited-resource settings: autograft (iliac crest, local bone) predominates because it is effective and low-cost; allograft banking and rhBMP are often unavailable. Distraction osteogenesis (Ilizarov) is widely used for large defects where vascularised transfer or banked allograft is not feasible.
- Counsel on donor-site morbidity for autograft harvest, the very low allograft disease-transmission risk, and the off-label status / adverse-event profile when rhBMP is proposed.
- Record the biological and mechanical rationale for the graft chosen.
Controversies & Areas of Uncertainty
Early industry trials reported 0% device-related adverse events; re-analysis against FDA data estimated true rates of 10-50% depending on approach (Carragee 2011). Anterior cervical use is now widely avoided. The benefit-risk balance outside approved indications remains contested.
With RIA reducing harvest morbidity to ~6%, some argue large-volume autograft has been unfairly displaced by costlier substitutes whose comparative data are weak (Hagen 2012). HA blocks and allograft chips are not supported as routine autograft replacements.
Demineralized bone matrix osteoinductivity varies markedly between donors, processing methods and even lots; product potency is not standardised, so clinical performance is unpredictable and DBM is best used as a graft extender, not a stand-alone inductive agent.
Bone marrow aspirate concentrate (BMAC) and synthetic/ceramic composites are promoted to add osteogenic cells, but high-quality comparative evidence is limited and concentration techniques are not standardised.
MCQ Practice Points
Q: Which bone graft has all three properties (osteogenic, osteoinductive, osteoconductive)? A: Autograft. Only autograft has living cells. Allograft lacks cells. DBM has induction+conduction. BMPs have induction only.
Q: What is the main property of allograft? A: Osteoconductive (scaffold only). Processing removes cells. Provides structure for bone ingrowth but no biological activity.
Q: Why is demineralized bone matrix (DBM) osteoinductive? A: Removing mineral exposes BMPs. The demineralization process exposes bone morphogenetic proteins that were embedded in the mineral matrix.
Q: What is the incidence of chronic donor site pain after ICBG harvest? A: 10-30%. This is the main disadvantage of autograft. Consider RIA or alternatives if significant concern.
Q: What is the Masquelet technique? A: Two-stage induced membrane technique. Stage 1: cement spacer placement. Stage 2 (6-8 weeks): membrane preserved, cement removed, cavity filled with autograft.
Q: What are the main complications of BMP use? A: Significant swelling, heterotopic ossification, osteolysis. Contraindicated near neural structures (spinal canal). Very expensive.
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“You are treating an atrophic tibial non-union. The patient is a 45-year-old non-smoker. What graft would you use and why?”
“A 30-year-old motorcyclist has a 10cm tibial bone defect after debridement of an open fracture. How would you manage the bone defect?”
“What are the differences between autograft and allograft? When would you use each?”
Three Properties (GIC)
- osteoGenic: Living cells
- osteoInductive: Growth factors, BMPs
- osteoConductive: Scaffold
- Only AUTOGRAFT has all 3
Autograft
- Gold standard - all 3 properties
- ICBG most common source
- RIA for large volume
- 10-30% donor site pain
Allograft
- Osteoconductive ONLY
- No living cells
- Fresh-frozen or freeze-dried
- Structural or morselized
Synthetics and Biologics
- Ceramics (HA, TCP): Scaffold only
- DBM: Inductive + conductive
- BMPs: Pure induction, no scaffold
- Combine with autograft to enhance
Complications
- ICBG: Pain, LFCN injury, fracture
- BMP: Swelling, heterotopic ossification
- Allograft: Very low disease transmission
- All: Infection, non-incorporation
Evidence Base
- 122 patients with 124 tibial nonunions, IM nail plus rhOP-1 (BMP-7) in collagen carrier vs fresh iliac autograft
- Clinical success at 9 months: 81% (OP-1) vs 85% (autograft), no significant difference (p=0.52)
- No significant difference maintained at 2 years (p=0.94)
- Over 20% of the autograft group had chronic donor-site pain; OP-1 avoided harvest morbidity
- 450 patients with open tibial shaft fractures; IM nailing alone vs nailing plus rhBMP-2 on collagen sponge at wound closure
- 1.50 mg/mL rhBMP-2: 44% relative risk reduction in secondary intervention for delayed/nonunion (RR 0.56, 95% CI 0.40-0.78, p=0.0005)
- Significantly faster fracture and wound healing, fewer hardware failures
- Fewer infections specifically in Gustilo-Anderson type-III injuries (p=0.022)
- 92 articles, 6682 patients pooled
- Overall complication rate: 6% with RIA vs 19.4% with iliac crest harvest
- Anterior iliac crest had higher infection, haematoma and fracture rates; posterior crest had higher chronic pain and sensory disturbance
- RIA is a low-morbidity alternative for large-volume autograft
- 120 acute traumatic lower-limb bone-loss fractures across 4 Level-1 trauma centres
- Overall union 89.2% after staged spacer then bone grafting
- No significant difference between diaphyseal (95.3%) and metaphyseal (85.7%) defects
- Post-debridement defect size, not limb segment, was the main predictor of reoperation
- 14 RCTs synthesised; calcium phosphate (CaP) cements and bone-marrow composites showed advantages over autograft for some outcomes
- Hydroxyapatite blocks and allograft chips could not be recommended over autograft
- BMP-2 a viable option for open tibial fractures, cost-saving in Gustilo-Anderson IIIB injuries
- Overall evidence quality was modest, so conclusions are suggestive rather than definitive
- Re-analysis of 13 industry trials (780 patients) against FDA data and follow-up studies
- Original trials reported 0% rhBMP-2 adverse events; true rate estimated at 10-50% depending on approach
- Anterior cervical use linked to life-threatening airway swelling; lumbar use to retrograde ejaculation, osteolysis, radiculitis
- Higher doses associated with greater apparent malignancy risk
- Autograft remains the biological gold standard (osteogenic + osteoinductive + osteoconductive)
- Substitutes and biologics are graft extenders, not routine wholesale replacements for autograft
- rhBMP products should be used within approved on-label indications only
- Stable mechanical fixation is a prerequisite for any biological augmentation