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Allograft
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Allograft

Allografting or, to give its full name, ‘fresh osteochondral allograft transplantation (OCA)’ is an operation in which a damaged or diseased area of a joint is reconstructed using a bone and cartilage transplant. The cartilage cells can survive the transplantation only if the tissue is ‘fresh’, which means it has not been exposed to radiation or prolonged freezing.

OCA was pioneered at the beginning of the 20th century, and has had a long and successful history. It is becoming increasingly popular as a treatment for large traumatic injuries, osteochondritis dissecans, and bone death (osteonecrosis) resulting from lack of blood flow to the bone supporting the joint cartilage.

The most common reasons for performing OCA are:

  • A focal cartilage lesion greater than 2 cm2
  • Re-treatment (revision or salvage) of previous cartilage surgery, such as microfracture, autologous osteochondral transfer (OAT) or autologous chondrocyte transplantation (ACI)
  • Severe (type III or IV) osteochondritis dissecans
  • Osteonecrosis (bone death)
  • Joint reconstruction after a fracture, known as post-traumatic reconstruction

The surgery itself is fairly straightforward but the patient should understand that they will receive living human tissue that has been donated. Fortunately, patients do not need to take anti-rejection drugs after surgery, as the immune response from these grafts is, in the majority of cases, mild or non-existent.

However, the tissue being donated means it is often difficult to predict availability and patients must therefore be prepared for surgery with a few days’ notice.

Intended audience

This article is intended for anyone suffering from damage to their articular cartilage and their families who would like to find out about allograft, as well as anyone interested in cartilage problems.

What is Allograft?

The scientific basis of OCA is the transplantation of fully developed, or mature hyaline (joint) cartilage containing living cartilage cells (‘chondrocytes’) that survive the transplant and support the production of the cartilage matrix indefinitely.

Theoretically, this maintains the tissue balance (‘homeostasis’) of the joint cartilage. Studies have shown chondrocytes living as long as 29 years after transplant. The graft often includes a portion of bone to help restore missing bone.

Candidates for OCA first have a careful history taken and clinical examination, and the size and type of the lesion, or defect, within the joint is documented. This involves either a photo or video taken during diagnostic arthroscopy, or an MRI (magnetic resonance imaging), along with plain X-rays. At this point, if the patient is a candidate for OCA, the size of the joint is measured. The patient is then placed on a waiting list, which can vary depending on the region or country in which the patient lives.

Donor tissue is obtained from an accredited tissue bank that specialises in the complex process of recovering and preparing fresh, living osteochondral (bone and cartilage) tissue. No tissue-type matching is performed because the allograft causes a minimal immune reaction. This is because the graft is considered to be relatively ‘immuno-privileged’. This means that cartilage is not fed by blood vessels, and the cartilage cells (‘chondrocytes’) are protected from surveillance by the immune system.

Donor–recipient matching is primarily by size to ensure the best fit of the graft. When a donor becomes available and is matched to the appropriate recipient, the patient is contacted and scheduled for surgery, ideally within 48–72 hours. The tissue is transplanted fresh (within 28 days of being taken from the donor), and is not processed like other tissue grafts. This allows the cartilage cells to survive. However, the tissue may be frozen before transplanting, as the cells within the tissue are still viable after thawing.

Surgery

The procedure takes approximately 1–3 hours and consists, essentially, of replacing the damaged surface of the joint with a carefully fitted graft from a donor joint. In summary, the surgical technique is as follows. A mini or standard ‘arthrotomy’ technique is performed and then:

  • The defect is exposed and measured, and a guide pin placed through the centre of the lesion, perpendicular (90º, or at a right angle) to the surface of the joint
  • The lesion is widened (reamed) to a modest depth to remove the diseased cartilage and a small (3–6 mm) amount of bone.
  • Depth measurements are taken from the prepared site where the transplant will be placed
  • A ‘graft plug’ is removed from the donor tissue using a special tool called a coring reamer
  • Depth measurements are marked on the plug and any excess bone removed, creating an graft matching the size and depth of the prepared site
  • The graft is washed to remove blood and debris, and the bony edges are trimmed to help insertion
  • The graft is gently inserted either with a special device or simply by moving the joint, which compresses it
  • Loose grafts are then fixed with absorbable pins or screws, if necessary.

Follow-up

Patients either go home the same day as the operation or stay in hospital for up to 3 days, depending on circumstances. Crutches are used for protected weight bearing for 4–12 weeks and physical therapy is started immediately. Patients are encouraged to complete a rehabilitation program, including range of motion and muscle exercises, which begins immediately after surgery. Follow-ups are scheduled for 4–6 weeks, 3 months, 6 months and yearly.

X-rays are taken to check graft healing. If the graft appears to be functioning and incorporating into the joint, then a progressive weight-bearing programme is started. At 4–6 months, if the individual has healed appropriately, he/she can begin more strenuous activity and return to sports or other athletic pursuits.

Most patients feel they have not completed their full recovery until up to a year after surgery. Transplant patients are followed on a routine basis every year indefinitely. This is important, as the long-term outcome of OCA procedures is not completely known.

Figures

Osteochondral allograft transplantation (OCA)

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