Articular or hyaline cartilage is the tissue that covers the adjacent bone surfaces of the knee and creates a smooth surface for motion. Articular cartilage often gets damaged because of direct injury to the knee, and can result swelling, stiffness, locking and limited movement. Articular cartilage has minimal capacity to repair itself. Generalised wearing out of cartilage is termed “osteoarthritis (OA)”, however any damage to cartilage represents part of the osteoarthritis process.
Because cartilage does not heal itself well, surgical techniques have been developed to stimulate the growth of new cartilage. Restoring articular cartilage can relieve pain and allow better function. Most importantly, it can delay or prevent the onset of arthritis. These surgical techniques to repair damaged cartilage are still evolving and the surgeons at Sydney Knee Specialists are experienced in these operations. It is hoped that as more is learned about cartilage and the healing response, surgeons will be better able to restore an injured joint. At this stage, there are no treatments that restore the cartilage to a completely normal structure. At best, the current techniques seem to stimulate a mix of normal cartilage (hyaline cartilage) and a less resilient but still useful tissue called fibrocartilage.
In many cases, patients who have joint injuries, such as meniscal or ligament tears, will also have cartilage damage. This damage may be hard to diagnose because hyaline cartilage does not contain calcium and cannot be seen on an X-ray. The diagnosis is usually established by performing an MRI scan of the knee.
Articular cartilage in the knee damaged in a single, or focal, location.
Who Are Candidates for Cartilage Surgery?
Most candidates for cartilage repair are young adults with a single injury, or lesion. The size of the lesion, site of the lesion and the status of other structures in the knee will determine whether surgery is possible for you. Often, additional operations such as knee re-alignment (osteotomy) and ligament reconstructions may also be required to improve the chance of success. Older patients, or those with many lesions in one joint, are less likely to benefit from the surgery, as this process is more representative of OA.
What Surgical Procedures Are Available?
Many procedures to restore articular cartilage are done arthroscopically. During arthroscopy, the procedure is done through 2 small incisions (key-holes). Some procedures require direct access to the affected area via an open incision (arthrotomy). In general, recovery from an arthroscopic procedure is quicker and less painful than traditional, open surgery. Your doctor will discuss the options with you to determine what kind of procedure is right for you.
The most common procedures for damaged cartilage are:
- Matrix-Induced Autologous Chondrocyte
- Implantation (MACI)
- Osteochondral Autograft Transplantation
This term simply means smoothing out any unstable areas of cartilage and is performed arthroscopically. The chondroplasty is performed using fine mechanical shavers and thermal devices to stabilise loose areas of cartilage. The benefit of chondroplasty is that it is not invasive and as such recovery is quicker than other techniques. However, unlike the other procedures listed below, it does not allow the cartilage to regenerate.
Steps of the microfracture technique. Left: Damaged cartilage is removed. Center: Awl is used to make holes in the subchondral bone. Right: Healing response brings new, healthy cartilage cells.
(Reproduced with permission from Mithoefer K, Williams RJ III, Warren RF, et al: Chondral resurfacing of articular cartilage defects in the knee with the microfracture technique. J Bone Joint Surg Am 2006;88(suppl 1):294-304.
The goal of microfracture is to stimulate the growth of fibrocartilage by creating a new blood supply. A sharp tool called an awl is used to make multiple holes in the joint surface. The holes are made in the bone beneath the cartilage, called subchondral bone. This creates a healing response. Your own stem cells (mesenchymal cells) from the underlying bone marrow move in to the area and then cells differentiate into specific cells that can create a fibrocartilage cover. The best candidates are young patients with single lesions and healthy subchondral bone. Smaller lesions under 2cm respond best to this treatment. The recovery is usually slower than a chondroplasty as specific rehabilitation protocols are required to allow the new fibrocartilage to regenerate.
Normal healthy articular cartilage in the knee (left). A large cartilage defect in the knee joint surface (center). During microfracture, an awl is used to penetrate the defect (right).
Matrix-Induced Autologous Chondrocyte Implantation (MACI)
MACI is a two-step procedure. New cartilage cells are grown and then implanted in the cartilage defect. First, healthy cartilage tissue is removed from a non-weightbearing area of the bone. This step is done as an arthroscopic procedure. The tissue which contains healthy cartilage cells, or chondrocytes, is then sent to the laboratory. The cells are cultured, and rapidly expanded in number from approximately 50,000 cells to up to 20 million cartilage cells over a period of 6 weeks. These cells are cultured on to a biologic porcine membrane (Matrix).
An arthrotomy is then done to implant the matrix implant. The cartilage defect is prepared and the matrix implant is glued in to position. Over time, the cells on this matrix attempt to restore the normal structure of cartilage. The theoretical advantage of this procedure is that it may regenerate more of the normal hyaline cartilage, as opposed to a microfracture, which tends to produce fibrocartilage.
MACI is most useful for younger patients who have single defects larger than 2 cm in diameter. MACI has the advantage of using the patient’s own cells, so there is no danger of a patient rejecting the tissue. It does have the disadvantage of being a two-stage procedure that requires an open incision. It also takes several weeks to complete.
Currently, the costs of treatment for a MACI procedure are very expensive, as the private health industry no longer covers the costs of manufacturing the implant.
Post-operative physiotherapy will be required following cartilage repair surgery. The amount and length of time undertaking this is dependent on the type of surgery as well as other factors. It is important for the knee joint to be protected whilst the new cartilage has time to regenerate. This often involves restricted weight bearing, using crutches for up to 6 weeks so that the cartilage can heal. Often, your surgeon will also recommend use of a knee-bending device called a Controlled Passive Motion (CPM) machine. This needs to be used for between 4 and 6 hours per day to achieve the best results in terms of smooth cartilage regeneration. Recent research also suggests that injection of your own blood (PRP) in to the knee joint following this procedure may benefit the quality of cartilage repair. As healing progresses, your therapy will focus on strengthening the joint and the muscles that support it. It may be several months before you can safely return to sports activity.
Results and Complications
Most patients who have cartilage repair notice a significant improvement in symptoms and are able to return to most activities. However, as the knee joint cartilage has been damaged, activities such as long distance running are not recommended as this can result in rapid deterioration of the knee joint cartilage. The best way to keep your knee joint healthy is to keep a low body weight and perform regular low impact exercises such as bike riding, swimming, gym exercises and core exercises (eg pilates).
Some patients find no improvement in their symptoms following cartilage repair surgery. The quality of the cartilage tissue that regenerates can vary between patients and affect the result of surgery. Some times, the cartilage repair becomes too thick (hypertrophy) and requires further surgery to perform a chondroplasty and reduce symptoms. Less commonly, the cartilage repair can fail completely. Other symptoms that may arise include swelling and clicking (crepitus).