Biomedical Engineering Reference
In-Depth Information
Bone regeneration is considered the future for bone repair, but in some
ways it is already the present.
Half a million bone graft operations are performed in the USA every
year, and just over half that number are carried out in Europe. Bone
defects are caused by a range of clinical indications: congenital defects
(e.g. cleft palates), trauma, tumour removal or non-union of fractures.
Another common procedure that needs bone graft is spinal fusion. In
this case, there is not really a bone defect. The clinical indication is
a slipped (herniated) intervertebral disc (IVD). The IVD is made of
cartilage, which regenerates at a much, much lower rate than bone.
When the IVD is severely herniated and has to be removed, treatment is
to replace the IVD with a metal or polymer cage filled with bone graft.
The bone grows into the cage and fuses with the bone graft. The idea
is that the implant fuses the neighbouring vertebrae, immobilising them.
This reduces pain but also restricts movement.
Surgeons currently routinely perform bone regeneration operations,
but the scaffold is the patient's own bone, termed autograft. The bone
is usually harvested from the pelvis, but in the case of spinal fusion
operations, bone spurs are often harvested from the vertebrae. Autograft
bone is an ideal material for regenerating bone.
Unfortunately, there are many drawbacks to using the patient's bone.
The most important is that there is limited supply of bone that can
be utilised without causing problems at the host site. The body is not
wasteful in bone production, so there is not much excess bone available.
One of the main functions of bone is to support load. If a situation occurs
where excess bone is made, the excess bone is likely not to be under
load. If this happens, the body will take the bone away by osteoclast
action. This is why astronauts need to exercise excessively in space to
prevent loss of bone density, as the lack of gravity means bones are not
loaded. This is an important consideration for bone graft design.
Harvesting healthy bone for grafting also creates another defect that
needs to be healed (without the help of more bone). The healing of this
defect is extremely painful and long. Patients generally feel a lot of pain
at the donor site and one in four patients will experience complications
at the defect site long after the operation. Some will require revision
surgery of the harvest site, which is not ideal for the patient, the
healthcare service or the economy.
These problems are the driving force for the need for synthetic bone
grafts that can perform as well as (or better than) autograft. The devices
must regenerate bone defects without the need for graft operations so
