Allograft
Allografting is, perhaps, the most widely used orthopaedic transplantation technique. Its main use is in the field of revision joint replacement, particularly the failed total hip replacement. The problem is created due to the phenomenon of osteolysis, when small particles of debris from a joint replacement cause bone to weaken and disappear. At the time of the revision procedure, a surgeon must naturally reconstitute this lost bone stock so that the necessary support exists for the new prostheses.
Allograft tissue is derived from two sources:
- Primary joint replacements (living donors)
- Cadaveric donors
It is essential that all donors are screened for transmissible diseases, particularly HIV and hepatitis. Tissue Banks involved with allografting programmes take this aspect of the procedure very seriously. Donors are not only screened, but it is usual to irradiate allograft tissue as well, to be certain that all transmissible disease, and bacterial contamination, is eliminated. Not all Tissue Banks irradiate their grafts, but the majority do so.
Allograft tissue is normally stored in bone banks. These are essentially glorified freezers, able to care for allograft tissue for as long as five years. Temperatures are usually at most -23°Celsius but can be as low as -150°Celsius if articular cartilage is being preserved.
Orthopaedic allograft tissue is of two types:
- Structural
- Morcellised
Structural grafts are blocks of bone or articular cartilage that can be strapped (or wired) onto the sides of defects. They act as bulk supports to prostheses or other types of graft tissue. Morcellised allograft comprises tiny pieces of bone tissue that can be either sprinkled, or impacted into a defect. Impaction allografting is now a widely used technique in revision joint replacement circles, where a large volume of allograft morcels is impacted tightly into a cavity - the tighter the better. In time the morcels can incorporate into the recipient, though a prolonged period of limited weight-bearing may be needed.
Gross osteolysis - loss of femoral bone stock due to a hip replacement.
A perfect candidate for bone allografting.
Articular cartilage defects can also be treated with allograft tissue. The articular cartilage is harvested from a cadaveric donor and specially treated in order to keep the cartilage cells (chondrocytes) alive for as long as possible. Blocks of allograft tissue may then be plugged, screwed, pinned or wired onto or into a defect. Results are usually good, though limited weight-bearing for several months is often required.
Allograft tendon can also be used, perhaps to reconstruct a deficient anterior cruciate ligament (ACL) or stabilise an ankle. Achilles tendon, hamstring tendon, and patellar tendon are common allograft tissues taken from cadaveric donors for subsequent use.
Incorporation of allograft tissue is not always full, probably because of slight incompatibility between donor and recipient. However, true rejection is not seen, it is thought because of the presence of blocking factors within a recipient's system. An immune response can be seen, but anti-rejection drugs are not required. Nevertheless, at present allografting is probably the best available technique for filling large bone defects that is available to orthopaedic surgeons. There are obvious limitations as no-one really wants to have someone else's tissue inside them unless there is no alternative. There is also the donor availability problem in those countries where an 'opt out' system does not exist. Laboratory research worldwide is thus currently being aimed at various artificial substitutes that can be used instead of allograft. Despite the attractions of bone and cartilage substitutes, such research is still in its infancy. By 2010, however, life may be very different.