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Fusion is a surgically created solid bone bridge between two or more
adjacent, usually freely mobile bones. In the spine this is used to create
stability between levels of the spine (vertebrae) that usually have some
motion across the inter-vertebral disc and the facet joints that is abnormal
and causing pain. In order to achieve a fusion, bone must grow across
the desired area in a gradual and solid fashion. A number of techniques
can increase the chance for this to occur. The basic principle is to place
bone tissue (bone graft) into the prepared area of desired fusion, to
ensure sufficient immobility across that area (brace, cast, spinal instrumentation…),
and then to await fusion (6-9months or more). The bone graft may be transplanted
from one area of the body to another (autologous bone graft/autograft),
or may be from a human donor (allograft), or from an animal (bovine graft/xenograft)
or may be man made – as in a bone substitute
The ideal properties of bone graft/bone substitutes used in spinal fusions
include:
Osteoconduction - scaffolding that is needed for new bone to grow on.
Osteoinduction - where proteins and growth factors promote bone growth.
Osteogenesis - the implantation of live bone cells or osteoblasts.
Traditionally, to achieve a spinal fusion, bone harvested from the patient’s
iliac crest (pelvic bone) is transplanted into the fusion site. The autologous
bone (patient's bone) consists of mineralized tissue, which provides osteoconductive,
osteoinductive (variable) and a bone marrow component rich in osteogenic
(bone formation) precursor cells. Autologous bone graft for lumbar spine
fusion may be obtained from the iliac crest. Other sources of autologous
bone include the rib/clavicle (used for surgery in the lower neck or thoracic
spine), the fibular (in the leg) used mainly in the lumbo-sacral junction
and bone that is removed from the spine during a spinal decompression
(spinous process and laminae).
The iliac crest is the commonest source of bone used in bone graft procedures.
Harvesting of bone graft from patients iliac crest may result in significant
donor site morbidity.
Post operative pain is common, delaying patient mobilization and discharge
and increasing health costs.
Bleeding is a potential but unusual risk and infection of haematoma (blood
clot) can be avoided by careful haemostasis and use of a suction drain.
Retained swabs at the donor site are well documented resulting in infection.
Up to 30% of patients report chronic hip pain following surgery.
Obtaining adequate quantity of graft is important and sometimes due to
variations in anatomy, supplementation of bone by other methods (synthetic
bone, allograft and bone proteins used to stimulate bone formation, may
be necessary).
Because of the potential morbidity from autologous bonegraft and the
variable fusion rate, studies have been undertaken to:
1) Evaluate methods by which to concentrate specific cells, such as centrifugation
and culture-expansion, but the results have been inconsistent and highly
variable.
2) Use alternative sources of bone – cadaver bone/animal bone
3) Demineralized bone matrix
4) Morphogenic protein products
Autologous bone graft remains the gold standard but even so, creating
a structurally stable and solid fusion remains a persistent surgical challenge
with fusion rates varying from 60% to over 90%. Autograft remains one
of the safest sources of bone(low risk of disease transmission) it offers
a better chance of acceptance and effectiveness at the transplant site
and provides the greatest amount of the patients own bone growing cells
and proteins with a strong framework for new bone to grow onto
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