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Bone grafting in spinal fusion

In spinal fusion a joint or motion segment at which movement is present or at least possible is made solid by laying materials across it which grow into solid bone.

In such an operation the healing properties of the body are put to the test in this challenging biological process.

“Bioactive materials” are placed in various positions across the spinal joint or motion segment to be stabilized. A substance is said to be bioactive if it participates in biological reactions in the body and affects them. Inert materials do not do this - an example is the components of a hip replacement

In a hip replacement we do not want the metal and plastic to react with the body tissues but rather to have no effect on bodily reactions. In spinal fusion we want the materials put into the body to react inside and form solid bone.

A successful spinal fusion encourages the formation of new bone across an area which normally does not support bone.

Five necessities for the regeneration of bony tissue

  1. The presence of bone-forming (osteogenic) cells, called osteoblasts. These are only found in freshly taken bone graft and bone marrow.
  2. A framework or scaffolding, which acts as a physical guide for bone growth. Bone itself, and many other materials, have this property.
  3. A favourable biomechanical environment, without excessive movement or physical stresses applied to the area.
  4. The re-formation of blood supply to the graft.
  5. The presence of specific chemicals, which have been called morphogenetic proteins. These encourage various types of cells to change into osteoblasts, which make bone.

Freshly taken bone graft from a patient is a good material, satisfying 1, 2 and 3 above. However, this technique is limited by problems. If the patient is have a fusion at several spinal levels or has had bone grafting before, there may be difficulty in finding enough bone graft.

Taking the bone adds to the surgical trauma, there is often pain at the site for up to two years and bone healing after operation is not certain in the fusion.

These problems have stimulated a search for materials which could act as good substitutes for bone graft and thereby avoid taking bone from the hip areas.

Passive scaffold materials

Various materials have been used as passive scaffolding for bone cells it the area to gradually grow into and replace with solid bone. They can reduce the amount of bone graft needed from the patient.

These have included ceramics, with or without collagen (bodily fibrous tissue) and freeze-dried bone from other people from a bone bank.

These materials do not encourage or improve the rate of bone growth, nor do they have any active cells or BMPs to encourage or stimulate growth. Overall they do not work as well as the patients’ own bone in lumbar fusions.

Techniques to harvest active cells

One way to improve the chances of a bone graft taking is to add cells which have the potential to turn into bone making cells. Aspirating these cells from bone is unpredictable and it is not possible to give any accurate view of how well it works.

Concentrating the correct marrow cells is also possible but again there is no good evidence yet it is useful. Many cells may die because of the low oxygen and blood supply in the bone grafted areas initially.

Chemical factors

Two main groups of chemicals exist here, those promoting general cell growth and multiplication and those which promote bone formation. It is possible to concentrate the first group from a patient’s plasma but the results of adding this to the bone grafts is again not clear.

Using sterilized bone

Ostoeinduction is the name given to the ability to transform uncommitted cells into bone producing osteoblasts, producing bone at an non-bony site. BMPs are the only factors known to be able to do this.

Removing bony material from and sterilizing donated bone leaves mostly collagen with some low, and rather unpredictable levels of BMPs. The necessary levels of BMPs for best bone formation are not known.

The sterilized bone material is not as good as live bone graft from the patient and is used best to bulk out live bone graft so the material can cover more area.

Bone morphogenetic proteins (BMPs)

These chemicals, first discovered by Urich, are an important part of normal bone formation and fracture healing response. 24 different proteins have been identified so far.

Summary

It’s challenging to promote a fusion in an area where bony union does not naturally occur. It must reduce or abolish motion in the area and relieve the pain of the abnormal motion segment. It is a race between the graft being absorbed and the new bone growing into the graft to stabilize the joint.

Sterilised bone can be useful to add to live bone graft, but using cells from the patient has not been shown to be effective. BMPs have great potential to encourage bone growth but their promise has not yet been realized.


Sources

Burkus, J Kenneth MD
Surgical Treatment of the Painful Motion Segment: Matching Technology With Indications.
Spine. Painful Motion Segment. 30(16S) (Supplement):S7-S15, August 15, 2005.

 


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