Cartilage restoration techniques are recent orthopaedic innovations that attempt to repair damage to articular cartilage. The articular cartilage within the knee does not have a sufficient blood supply to heal itself. Thus, specialized methods have been employed to repair cartilage damage and restore normal joint mechanics.
Dr. Cunningham recommends biological repair of the knee whenever possible. In this way, we are repairing or augmenting what was once a normal tissue in the knee and restoring it to normal function. Repairing one's meniscus or reconstructing a torn ACL are both examples of successful biological tissue repairs.
Articular cartilage (coating cartilage) defects of the knee have been the most challenging areas for orthopaedic surgeons to achieve a biologic repair. This is primarily due to the fact that articular cartilage has no blood supply. Rather the synovial fluid circulating through the knee joint nourishes it. In contrast, if a patient breaks a bone, an orthopaedist can realign the bone with hardware and the bone will predictably heal as most bones have good blood supply.
When a patient loses a portion of their articular cartilage to injury, there are newer techniques that aim to restore cartilage cells to the injured area. For very small defects, a simple microfracture can be effective. However, for larger defects, there are some newer technologies have been shown to lead to better outcomes.
Autologous Chondrocyte Implantation (ACI)
Autologous Chondrocyte Implantation (ACI) is a technique that can successfully resurface large articular cartilage defects. It is typically done in younger patients, as older patients usually present with more widespread arthritis in their joint, not one focal area of articular cartilage loss.
An MRI is done to assess the area of articular cartilage loss. Dr. Cunningham also assesses the patient’s leg alignment on a special x-ray taken from the hip to ankle. If the patient's lower extremity is significantly in varus (bow legged) or knock kneed (valgus), then ACI may need to be combined with an osteotomy (bone cutting procedure) to unweight the defect to be treated with ACI.
ACI consists of 2 surgeries. In the first, a simple arthroscopy is performed and a biopsy of the patient’s articular cartilage is taken from the knee. Two 2x4mm pieces of cartilage (eraser head sized) are taken from the edge of the knee joint where there is “extra” cartilage. This is then placed in a sterile medium and sent to Genzyme, the company who has patented this technology.
At Genzyme the chondrocytes (articular cartilage cells) are isolated and then grown up and multiplied in culture. 4–6 weeks later the cells are ready for implantation. Currently in the U.S., these cells have to be implanted through an arthrotomy (open incision into the knee joint) and cannot be implanted arthroscopically. However, in the next few years these chondrocytes may come embedded in a carrier, which would allow the cells to be implanted in the cartilage defect arthroscopically.
At the time of the second surgery, the decision would have been made as to whether there will also be an osteotomy required in order to not place the implanted cells at a mechanical disadvantage. In other words, if a young patient was in severe varus (bow legged), Dr. Cunnigham would not wish to implant the cells into a defect on the inside of the knee without making the lower extremity straight with an osteotomy or else the cartilage graft would wear away and fail prematurely.
In addition to a possible osteotomy of the femur or tibia, a 2–3 inch incision is made over the area of cartilage loss in the knee. The cartilage defect is meticulously prepared with special instruments removing any surrounding loose and unstable articular cartilage. A biological tissue patch (usually obtained from pig intestine) is sewed over the defect and then human fibrin glue to injected along the edges to make the patch watertight.
The cells from Genzyme are then injected under this patch. The cells very quickly bond to the underlying bone and begin to form a new cartilage layer. The cartilage layer that forms is the closest orthopaedists have come to normal, human articular cartilage. However, it has been found to be a mixture of human articular cartilage and fibrocartilage (scar cartilage). The rehabilitation after ACI takes one year. Typically, the area implanted with cells is on one of the femoral condyles or weight bearing portions of the knee. Given this, the patient is on crutches and cannot put weight on the knee for 6–8 weeks. Furthermore, a continuous passive motion machine (CPM) is used 6–8 hours a day initially.
Although this is a relatively large surgical undertaking, patients who present with articular cartilage lesions have historically had very limited treatment options and went on to have worsening pain. Patients who are candidates for this treatment have ultimately done quite well in Dr. Cunningham’s experience.
DeNovo NT Graft
DeNovo NT graft is very similar to ACI as outlined above, but the cartilage cells are not grown and cultured from one’s own cells. Rather, this cartilage is particulated articular cartilage containing living cells harvested from juvenile donors (aged below 13 years), who have unfortunately died from other causes. The surgery is otherwise peformed in much the same way as ACI. The advantage of DeNovo is that only one surgery is necessary as there is no need to take a biopsy of one’s cartilage cells and then wait to have it grown up in culture. The disadvantage of DeNovo over ACI is that these cells come from a donor and there is therefore a very small risk of transmitting an infectious disease from the donor to the recipient. There is also some concern that as an allograft or donor tissue the cells will not incorporate as well as one’s own cartilage cells, but studies to date have shown results similar to ACI.
Ostearticular Autograft (or allograft) Transfer
Another method used to resurface an articular cartilage defect of the knee is to use plugs of cartilage and a segment of underlying attached bone. Much like a greens keeper changing the location of a hole on a putting green, a plug of cartilage and bone is taken from the edges of the patients own knee (autograft) or from a donor knee (allograft). In turn, the area of worn cartilage and some underlying bone of the same size as the donor plug is removed and the normal cartilage plug implanted.
The advantage of using autograft is that the procedure can be performed at the time the lesion is discovered. Furthermore, the body incorporates ones own tissue better than donor tissue. Depending on the location of the defect, it is done arthroscopically or through an arthrotomy (open incision into the knee).
The disadvantages of autograft OATS, is that there is only a small area of cartilage along the edge of the knee that is not needed and the harvest itself can do damage to that area of the knee.
The advantages of using allograft is that there is no risk of damaging the patient's own knee with the harvesting of plugs. Furthermore, when using a plug from a donor knee, you can take a bigger plug that more accurately matches the normal contour of the cartilage in the patient’s damaged knee.
The disadvantage of allograft cartilage plugs is that there is risk of disease transmission with any allograft. Moreover, it has been shown that unless the allograft is very fresh, many of the cartilage cells may no longer be viable. This technique requires an arthrotomy (open surgery with incision into the knee)
The rehabilitation requires the patient to be non-weight bearing on crutches for 6–8 weeks if autograft and up to 12 weeks if allograft. Range of motion exercises are started immediately after surgery initially with the help of a continuous passive motion machine (CPM).