Allografts & Bone Banking

Overview & Rationale

Bone grafting is one of the oldest and most commonly performed procedures in orthopaedic surgery. Allografts (grafts from another individual of the same species) and bone banking provide an essential alternative to autograft when graft volume requirements exceed what can be safely harvested, or when donor site morbidity is unacceptable. Understanding graft biology, the principles of bone banking processing, and the specific clinical indications for different allograft types is essential for the orthopaedic surgeon.

Autograft remains the gold standard — its osteogenic, osteoinductive, and osteoconductive properties are unmatched; however, limited volume, donor site morbidity, and prolonged operative time are significant limitations
Allograft provides a scaffold (osteoconductive) and may retain some osteoinductive proteins (BMPs) depending on processing — lacks osteogenic potential (no viable cells survive processing)
The three properties of bone grafts: osteogenesis (viable cells producing new bone — autograft only); osteoinduction (stimulating host cells to differentiate into bone-forming cells — BMPs in DBM and allograft); osteoconduction (structural scaffold for bone ingrowth — all graft types provide this)
Global demand: bone is the second most transplanted tissue after blood; approximately 2.2 million bone allograft procedures performed annually worldwide

Graft Types & Biology

Graft Type	Source	Properties	Limitations
Autograft	Same patient (iliac crest, local bone, RIA)	Osteogenic + osteoinductive + osteoconductive; no immune rejection; best incorporation	Limited volume; donor site morbidity (pain, haematoma, nerve injury, fracture); additional operative time
Allograft (frozen)	Cadaveric or living donor; processed and stored	Osteoconductive; some osteoinductive proteins preserved; no osteogenic cells; large volume available	Disease transmission risk (minimised by screening); immune response; slower incorporation; non-union risk; higher cost
Demineralised bone matrix (DBM)	Allograft with mineral removed; exposes collagen and BMPs	Primarily osteoinductive (BMP exposure); osteoconductive scaffold; available in paste, putty, gel forms; easily combined with autograft	Variable BMP content between products and donors; no structural strength; not standalone for load-bearing defects
Synthetic bone substitutes (calcium phosphate, tricalcium phosphate, hydroxyapatite)	Manufactured; various forms	Osteoconductive only; no disease risk; unlimited availability; degradable (TCP) or non-degradable (HA)	No osteogenic or inductive properties; brittle; slower incorporation than biological grafts
Recombinant BMP (rhBMP-2, rhBMP-7)	Recombinant protein on collagen sponge carrier	Potently osteoinductive; stimulates de novo bone formation; approved for spinal fusion and tibial shaft fractures (rhBMP-2)	Expensive; heterotopic ossification risk; oedema; potential carcinogenesis concerns (long-term data emerging); not first-line in most sites

Bone Banking — Processing & Storage

Donor screening: cadaveric and living donors; extensive medical history, serological screening (HIV, HBV, HCV, HTLV, syphilis, CMV); tissue culture; exclusion criteria include active infection, malignancy (except primary CNS tumours), autoimmune disease, prion disease risk
Residual risk of HIV transmission from screened allograft: approximately 1 in 1.6 million — comparable to blood transfusion risk; disease transmission from properly screened and processed allograft is extremely rare but not zero; the window period between infection and detectable serology means screening cannot eliminate all risk
Processing methods:

Method	Effect on Bone	Effect on Pathogens
Deep freezing (−70°C to −80°C)	Kills all cells; reduces immunogenicity; preserves BMP activity partially; maintains structural properties	Reduces but does not eliminate bacteria and viruses; does not sterilise
Freeze-drying (lyophilisation)	Removes water; reduces immunogenicity further; further reduces BMP content; loses some structural strength; room temperature storage; long shelf life (5 years)	Reduces pathogen load; does not sterilise alone — used with terminal sterilisation
Gamma irradiation (terminal sterilisation)	Doses >25 kGy sterilise but significantly reduce mechanical strength and BMP activity; lower doses (15–20 kGy) used for structural grafts to preserve properties	Sterilises; kills bacteria, fungi, and most viruses; reduces but may not eliminate prions
Ethylene oxide (EtO)	Chemical sterilisation; preserves structural properties	Broad-spectrum sterilisation; residual EtO inflammatory response if not properly aerated

Frozen allograft: best preserves structural strength and BMP content; stored at −70°C to −80°C; shelf life approximately 5 years; must be transported on dry ice; thawed at 37°C immediately before use

Clinical Applications

Application	Graft Type	Notes
Intercalary allograft (diaphyseal replacement)	Massive structural frozen allograft	For diaphyseal tumour resection; fixed with plates and screws; biological incorporation over years; non-union and fracture are major complications
Osteoarticular allograft	Whole joint allograft (including cartilage)	Replaces both joint surface and bone; preserves cartilage surface; cartilage viability decreases with time; articular collapse and joint degeneration are long-term problems; used in young patients to avoid endoprosthesis; 10-year survival approximately 60–70%
Allograft-prosthesis composite (APC)	Structural allograft + endoprosthesis cemented within	Combines biological incorporation of allograft with immediate stability of prosthesis; allows soft tissue reattachment to allograft; used in proximal femur, proximal humerus, proximal tibia
Revision arthroplasty bone defects	Morselised cancellous allograft; impaction grafting; structural cortical struts	Impaction bone grafting (Ling technique) for acetabular and femoral defects in revision THA; restores bone stock; fully revascularised graft over time
Spinal fusion	Morselised or structural allograft; DBM; BMP	Used for interbody support (structural) and posterolateral fusion (morselised); DBM mixed with local bone for enhanced osteoinduction

Complications of Allografts

Non-union at graft-host junction: most common complication of structural allografts — occurs in approximately 15–20% of intercalary reconstructions; risk factors include inadequate fixation, large graft, poor host bone biology (chemotherapy), infection; management with bone grafting, fixation revision, or exchange to vascularised fibula
Allograft fracture: late complication of structural allografts — allograft undergoes creeping substitution but is never fully revascularised; remains biomechanically inferior to native bone long-term; fatigue fractures occur, particularly at stress risers (screw holes, host-graft junction); incidence approximately 15–25% at 10 years; management with plate fixation or exchange to endoprosthesis
Infection: deep infection of allograft is catastrophic — infected allograft usually requires complete removal; infection rates approximately 5–10% for massive structural allografts; higher risk with immunosuppression (chemotherapy); meticulous sterile technique and antibiotic prophylaxis essential
Immune response: frozen allograft retains some antigenic cells; immune response may impair incorporation; rarely clinically significant with modern processing; no systemic immunosuppression required
Disease transmission: see bone banking section — extremely rare with properly screened and processed allograft

Consultant-Level Considerations

RIA (Reamer-Irrigator-Aspirator) autograft: harvests large volumes of cancellous autograft from the femoral canal during reaming; yields 40–90 mL of graft with excellent biological properties (BMP content, MSCs, growth factors); superior to iliac crest autograft in volume; complications include cortical perforation and femoral fracture if used without care; increasingly popular for large defects requiring autograft
Vascularised fibula free flap: for massive structural defects where allograft biology is inadequate — particularly intercalary reconstruction in children after diaphyseal tumour resection; fibula provides living, vascularised cortical bone that incorporates and hypertrophies over time; can be combined with allograft (fibula in allograft technique); requires microsurgical anastomosis; technically demanding but provides superior biological outcome
Impaction bone grafting (IBG) in revision THA: morselised allograft packed under pressure into the acetabular or femoral defect; cement applied over the packed graft; provides immediate stability while allowing biological incorporation; Ling technique for femoral IBG; Combes/Gerber technique for acetabular IBG; long-term follow-up shows restoration of bone stock; risk of early graft displacement if technique inadequate
BMPs in clinical use: rhBMP-2 (InFUSE) and rhBMP-7 (OP-1) are the only FDA-approved recombinant BMPs; approved indications limited; significant concern about heterotopic ossification and potential cancer risk (FDA warning); off-label use common in spinal fusion; use only with clear indication and awareness of risks

Exam Pearls

Three graft properties: osteogenesis (autograft only — viable cells); osteoinduction (BMP — autograft, DBM, some allograft); osteoconduction (scaffold — all graft types)
Autograft = gold standard; limited volume; donor site morbidity; iliac crest or RIA from femoral canal
Allograft processing: deep freeze (−70–80°C) preserves most structural strength and BMP; freeze-drying reduces immunogenicity + room temperature storage; gamma irradiation sterilises but reduces strength
Disease transmission: HIV risk ≈ 1 in 1.6 million with screened allograft; window period means screening cannot eliminate all risk
Structural allograft complications: non-union (15–20%), fracture (15–25% at 10 years), infection (5–10%); allograft is never fully revascularised — vulnerable to fatigue fracture long-term
Osteoarticular allograft: preserves native cartilage; articular collapse and late degeneration main limitations; 10-year survival 60–70%
APC (allograft-prosthesis composite): biological soft tissue attachment to allograft + immediate prosthetic stability; used proximal femur/humerus/tibia
RIA: large volume autograft from femoral canal; 40–90 mL; superior to iliac crest for volume; risk of cortical perforation
DBM: demineralised bone matrix; osteoinductive (exposes BMPs); no structural strength; not standalone for load-bearing defects
Impaction bone grafting (IBG): revision THA; morselised allograft packed under pressure; restores bone stock; Ling technique femoral, Combes acetabular

Allografts & Bone Banking

References

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