Describe Bone Grafts — Types & Properties in detail.

WHY a Graft Must Do Three Things

A concept that clarifies the whole subject is understanding why bone grafts are judged by three separate properties — and why the ideal graft has all three. Osteogenesis: the graft itself contains living osteoblasts and osteoprogenitor cells which survive transplantation and directly form new bone. Only a fresh autograft (particularly cancellous bone, which is rich in cells and has a large surface area) has this property. Osteoinduction: the graft contains growth factors — chiefly the bone morphogenetic proteins (BMPs) — which recruit the host's own mesenchymal stem cells and induce them to differentiate into osteoblasts. Autograft and demineralised bone matrix have this; and recombinant BMP is available commercially. Osteoconduction: the graft acts as an inert scaffold — a passive lattice over which the host's new bone can creep and grow ('creeping substitution'). Allograft, and synthetic materials such as hydroxyapatite and tricalcium phosphate, do this, but nothing more. Hence the gold standard remains the autologous cancellous graft from the iliac crest — the only graft that is osteogenic, osteoinductive and osteoconductive — at the cost of donor-site morbidity (pain, bleeding, infection, and injury to the lateral cutaneous nerve of the thigh).

WHY a Graft Needs a Good Bed

A principle that determines success is understanding why even the finest bone graft will fail if it is placed in the wrong environment — and why grafting is never a substitute for correcting the underlying problem. A graft is not a piece of bone that is simply 'stuck in': it must be incorporated — revascularised by ingrowth of host capillaries, and then progressively resorbed and replaced by host bone ('creeping substitution'). This process makes three demands. First, the graft needs a well-vascularised bed — so it will not take in irradiated, scarred, ischaemic or infected tissue; and this is precisely why an atrophic non-union with a dead, sclerotic bone end must have the ends freshened back to bleeding bone ('paprika sign') before grafting. Second, it needs stable fixation — a graft in a mobile fracture will simply be resorbed; grafting without stabilising is a wasted operation. Third, it must be free of infection — grafting an infected non-union is futile and harmful. And for large segmental defects, where a conventional graft cannot bridge the gap, the options become a vascularised graft (a free fibula, which brings its own blood supply and survives as living bone), or distraction osteogenesis (the Ilizarov technique — in which new bone is grown by slowly distracting an osteotomy at about 1 mm per day).

Autograft alone provides all three properties.

• The three properties: osteogenesis (the graft's own living osteoblasts form bone — only fresh autograft); osteoinduction (BMPs recruit and differentiate the host's stem cells — autograft, demineralised bone matrix, recombinant BMP); osteoconduction (a passive scaffold for creeping substitution — allograft, hydroxyapatite, tricalcium phosphate).
• Autograft (from the patient): the gold standard — the only graft with all three properties; no immune rejection and no disease transmission. Cancellous (iliac crest — rich in cells, rapidly incorporated, but no structural strength) vs cortical (fibula — provides structural support, but is slowly incorporated). Drawback: donor-site morbidity (pain, bleeding, infection, nerve injury) and limited quantity.
• Allograft (from a cadaver donor/bone bank): available in large quantity and any shape — but it is only osteoconductive (processing kills the cells), incorporates slowly, and carries a small risk of disease transmission and immune reaction.
• Synthetic substitutes (hydroxyapatite, tricalcium phosphate, calcium sulphate, bioactive glass): osteoconductive only; unlimited supply; no donor-site morbidity or disease risk.
• Indications: non-union and delayed union (especially atrophic), bone defects and loss, arthrodesis, revision arthroplasty, and to fill a cavity after tumour or cyst curettage. A graft needs a well-vascularised, infection-free bed and stable fixation to incorporate.