Introduction
Fusion is an important part in spinal surgery, be it in the cervical spine or in the lumbosacral spine. Also, in large deformity cases fusing all the spinal segments involved is necessary. This needs large amount of bone graft. Procurement of bone graft from the patient (autograft) is standard practice. However, substituting with other products have taken importance in recent years. This article gives an overview of different types of bone graft and its substitutes available for spinal surgeons during spinal surgery.
Bone graft properties
Before going into the intricacies of bone graft and its substitutes it is important for the reader to understand how each product is helping to achieve spinal fusion. Each product may have one or more of the following properties all of which is needed in achieving a strong solid fusion.
- Osteoconduction - Provides scaffolding and matrix needed for new bone to grow on
- Osteoinduction - Proteins and growth factors induce or stimulate bone to grow
- Osteogenicity - Transmittal of osteoblasts or live bone cells
- Structural support - Giving strength to hold until new bone gets strength
Types of Bone graft & its substitutes
- Autograft
- Allograft
- Bone graft substitutes, and
- Osteoinductive agents
Autograft
Autograft is the gold standard for fusion in spinal surgery (like in other specialties) against which the others are measured. It is osteoconductive, osteoinductive and also osteogenic. It can be cancellous or cortical or corticocancellous. Commonly, the cancellous bone graft is procured from the posterior iliac crest. Also, the iliac crest gives corticocancellous graft as well when it can be used in cervical spine. Further, during posterior spinal decompression the posterior elements of spine removed as part of the process can be nibbled and packed as bone graft. Cancellous autograft has the disadvantage of poor structural support. Cortical bone graft has all the qualities like a cancellous bone graft, however, as they are biologically less active, they are inferior compared to the cancellous bone graft except structural support. Cancellous autograft gets rapidly incorporated than a cortical autograft. Cortical autograft can be either nonvascular or vascular. In spinal surgery frequently it is nonvascular fibular graft which is used to give structural support. Fibular struts or rib struts may be used to fill in the space created by voids resulting from vertebrectomies and also in transacral fixation of high grade spondylolisthesis. Also to note, autogenous bone grafting is the only source of osteoprogenitor cells apart from bone marrow aspiration. In normal marrow, 1 per 50000 nucleated bone marrow cells is an osteoprogenitor cell. This can be increased by concentrating them in a centrifuge. The limitations of autogenous bone graft are its limited supply, and the donor site morbidity associated with its procurement.
Allograft
Allograft can be cancellous, cortical or corticocancellous like autograft. They are available as fresh, fresh frozen or freeze-dried. The disadvantage in general for any allograft is infection (esp. Slow virus infection). Unlike autograft, allograft does not have osteogenic cells, but has the advantage of avoiding donor site morbidity.
Fresh allograft:
This is highly antigenic and has got little time to test for infection or immunogenicity. This is seldom used in spinal surgery. The rare indication may be to fill in the void created by en bloc vertebrectomy for tumour in a young child where the fibular graft can be procured from the mother.
Fresh frozen allograft:
This is relatively less antigenic. There is plenty of time to test for infection and immunogenicity. The important advantage is that it preserves the biomechanical properties of the bone graft.
Freeze-dried (lyophilised) allograft:
This is processed by removing water through a vacuum process, and thus reduces the immunogenicity significantly. However, this also degrades the mechanical properties of the bone graft.
Demineralised bone matrix (DBX)
The cortical allograft is subjected to acid extraction to produce decalcification thereby yielding a composite of collagen, noncollagenous proteins and also a low concentration of growth factors. This is the least immunogenic form of the allograft because of extensive preparation. DBX is available in different forms like powder, putty, crushed granules, chips or gel-packed syringes.
In general, the success of allograft fusion depends on its placement, i.e., anterior or posterior. Allograft fusion is quite successful when placed anteriorly (in compression) compared to posterior spinal fusion (neutral load or in tension).
Bone graft substitutes
Bone graft substitutes can act as an extender for autogenous bone graft