IN KEEPING WITH the previous two articles in this series, the focus remains on articular cartilage. Yes my obsession with cartilage rivals the recent fervour with which popular culture has dedicated to the gluteus maximum, “all about that bass!”
What is so special about cartilage then that allows it to command this much attention? In truth articular cartilage is a marvel of biological engineering. The cartilage that covers our joints is “hyaline” cartilage.
This tissue is highly specialised and structured to perform at an optimum level. Hyaline cartilage strongly attracts water, which allows it to bear loads efficiently and dissipate stress on the underlying bone. Cartilage also provides a low-friction surface that allows smooth gliding of the joint surfaces.
When hyaline cartilage is injured it is not readily repaired or regenerated, because this tissue has no blood supply and very few cells. Lesions that pass all the way through cartilage into the underlying bone actually have better healing potential as they allow access to blood and repair cells in the bone marrow.
Unfortunately, the repair tissue formed is fibrocartilage. This tissue lacks the excellent mechanical qualities of hyaline cartilage and is less durable.
This knowledge has guided one popular method of treatment for defects in articular cartilage. The first strategy consists of attempts at repair stimulation. In these methods small lesions in cartilage are treated, by creating pathways into the underlying bone marrow.
The most popular technique for doing this is called microfracture. Defects in cartilage can be likened to potholes in the surface of the road. In this technique, any remaining cartilage in the base (bottom) of the defect is first scraped away to the underlying bone. A pointed awl (icepick) is the used to make holes into the underlying bone marrow approximately 3mm deep and spaced 2-3mm apart.
Blood, stem cells and growth factors are then released into the defect and a clot is formed and initiates repair. The tissue made is, unfortunately, fibrocartilage. Defects of 1 cm or less are best suited to this treatment, and it can be performed using the arthroscope.
Modern techniques of cartilage repair have been developed with the hope of producing hyaline cartilage. Autologous chondrocyte implantation (ACI) is a therapy in evolution aimed that doing just this. It requires two procedures, the first of which is performed to harvest a portion of the patient’s own cartilage. The cartilage is then treated to isolated cartilage cells (chondrocytes).
Chondrocytes are then grown in the laboratory with an attempt to isolate the type responsible for producing hyaline cartilage. These cells are then reimplanted into the joint to hopefully regrow mature hyaline cartilage. This technique is very expensive, requires two procedures, but does result in more hyaline-like tissue.
Unfortunately the cartilage generated to date is not of the standard of the cartilage we were born with. Recent advances in these techniques involve the addition of growth factors and implanting the cells on various matrices in an attempt to produce better quality cartilage.
Another strategy is to replace hyaline cartilage with hyaline cartilage. I know a genius concept to be sure! There are two arms of this strategy. Smaller lesions can be treated by autologous osteoarticular transplantation (OATS). In this technique small plugs of bone and cartilage are harvested from the patient’s knee. These plugs are taken from areas of the joint that “do not bear weight”, and they are then implanted into the cartilage defect.
This strategy is good for medium sized defects, but is limited by the amount of the patient’s healthy joint surface that can be sacrificed. We are robbing Peter to pay Paul!
For large lesions we can take osteochondral transplants from donor tissue (allograft). These transplants work well for larger lesions, but can be prohibitively expensive and carry a very small risk of disease transmission and rejection. A more recent and novel development is focused on the use of stem cells to regenerate joint surfaces. Bone marrow aspirate concentrate (BMAC) is another promising technique where bone marrow is aspirated from the hip and centrifuged.
This is rich in stem cells, which are then implanted into the defect in the form of a gel. This technique can be performed with one procedure unlike ACI and voids the time and expense of trying to grow chondrocytes in the laboratory.
Paul Adams is a local orthopaedic surgeon with a fellowship in orthopaedic sports medicine.
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