Bone Grafting in Orthopaedics
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Direct answer
Every nonunion tray and every fusion case asks the same three-part question: does the graft bring living cells, growth signals, or only a scaffold? Osteogenesis means transplanted osteogenic cells (fresh cancellous autograft), osteoinduction means factors that recruit host cells down bone-forming lines (BMPs, demineralised bone matrix), and osteoconduction means a passive scaffold for ingrowth (cancellous matrix, ceramics). Non-vascularised cancellous iliac crest autograft remains the gold standard because it supplies all three properties; allografts and synthetics trade biology for availability and volume. The graft, however, is only the third leg of the stool — stability and vascularity do the other two-thirds of the work.
What you must remember
- The three properties (the exam's frame): osteogenesis — living osteoblasts and progenitors, unique to fresh autograft and marrow; osteoinduction — BMP-2, BMP-7 and demineralised bone matrix recruiting mesenchymal cells; osteoconduction — passive scaffolds such as cancellous chips, hydroxyapatite and tricalcium phosphate.
- Autograft menu: cancellous iliac crest — gold standard, all three properties, fastest incorporation; non-vascularised cortical graft (fibula strut) gives structure but incorporates slowly by creeping substitution; vascularised free fibula for large or hostile defects; reamer-irrigator-aspirator (RIA) harvests large volumes from the femur with less site morbidity.
- Iliac crest donor morbidity: pain, superior cluneal nerve or lateral femoral cutaneous nerve injury, haematoma, hernia — the reason local bone from decompression and marrow aspirate are popular alternatives.
- Allograft forms: fresh-frozen (structural strength, osteoconductive, mostly dead cells), freeze-dried (conduction only, weaker), massive structural allografts (complications: nonunion, fracture, infection); no viable cells means incorporation depends entirely on host ingrowth.
- Synthetics: calcium phosphate ceramics resemble bone mineral and resorb slowly; calcium sulphate resorbs fast and carries antibiotics; none gives structural support alone — they supplement fixation, never replace it.
- BMP reality: rhBMP-2 has trial support in selected tibial nonunions and spinal fusions, but cost keeps it rare in India; anterior cervical swelling reports limit enthusiasm.
- Masquelet technique (modern viva favourite): stage 1 — radical debridement and a cement spacer that induces a vascularised "membrane"; stage 2 after six to eight weeks — remove the cement and fill the induced membrane with cancellous graft; the membrane secretes growth factors and blocks fibrous ingrowth. Cement is cheap, which is why the technique flourishes in India.
- By problem: atrophic nonunion — decortication plus cancellous autograft; contained defects — chips; segmental defects — Masquelet, Ilizarov transport or vascularised graft; infected sites — graft only after infection control.
Matching graft to defect
Three cases fix the logic. First, an atrophic tibial nonunion after nailing: a reamed exchange nail plus decortication and iliac crest chips, because the biology was missing and the graft that carries all three properties is the one to graft with. Second, a 6 cm post-traumatic tibial defect after infected fixation: a Masquelet — debride, cement, wait for the membrane, then graft through it — because a membrane-wrapped autograft outperforms a bare graft in a hostile bed, and the alternative, bone transport, needs months in a frame the patient may not tolerate. Third, a two-level posterior lumbar fusion: local bone from the laminectomy, mixed with a calcium phosphate extender, spares the crest without measurable loss of union in most hands — donor-site morbidity is a price, not a principle. Notice that in all three, fixation and vascularity were settled before graft material was chosen; a graft poured into an unstable or avascular bed is an expensive dressing.
Conduction is not induction
The MCQ "which property does a hydroxyapatite block have?" wants osteoconduction only — no cells, no growth factors — and the follow-up "which graft alone carries osteogenesis?" wants fresh cancellous autograft or marrow. The deeper trap is assuming graft substitutes heal fractures: they scaffold nothing in an unstable bed, and examiners close the loop by asking what completes the triad — fixation and blood supply. Allograft incorporation deserves its own sentence in answers: creeping substitution, host osteoclasts tunnelling into dead matrix, means massive structural grafts stay weak for years — the reason a vascularised graft, despite its donor-site cost, wins for large defects where strength is needed early.
Frequently asked questions
What are the three biological properties of a bone graft?
Osteogenesis (living bone cells), osteoinduction (growth factors recruiting host cells) and osteoconduction (a scaffold for ingrowth).
Why is cancellous iliac crest autograft the gold standard?
It uniquely provides all three properties — cells, inducive factors and scaffold — and incorporates faster than any substitute.
What is creeping substitution?
The process by which host osteoclasts and osteoblasts slowly replace dead graft matrix with living bone — how cortical and structural allografts incorporate.
Describe the two stages of the Masquelet technique.
Stage 1: debridement plus cement spacer that induces a vascularised membrane; stage 2, six to eight weeks later: remove the cement and pack cancellous autograft inside the membrane.
Why are calcium sulphate beads used in infected bone defects?
They resorb rapidly, can be antibiotic-loaded, and leave space for new bone while delivering high local antibiotic concentrations.