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Articular cartilage is the substance on the end of the bones which allow the surfaces within a joint to move with very little friction. It is capable of deforming with stresses applied across the joint and returning to it original form afterwards. Integrity of the articular surface of a joint is essential to normal function. The tissue itself is a highly structured matrix of cells and collagen organised in such a way as to resist the forces and stresses it has to continually undergo. It has very limited capacity to repair itself if injured.
Where is articular cartilage in the knee?
There is a layer of articular cartilage on the lower end of the thigh (femur), upper end of the shin (tibia) and on the under surface of the kneecap (patella). There are two distinct contact areas between the femur and the tibia which are called the medial and lateral compartments. The patella runs in a shallow groove on the front of the lower end of the femur which is called the trochlea
What can happen to this articular cartilage which causes problems?
In the broadest terms, injury to articular cartilage is either an acute or chronic event. The words acute and chronic refer to the length of time that the condition takes to develop and not to the severity of any damage.
Chronic damage is that seen in osteoarthritis (for whatever cause) and symptoms are often experienced over many years with gradual progression of severity and may ultimately require treatment with an artificial knee. The area of damage is often widespread within the knee and may affect both the tibio-femoral and patellofemoral parts of the joint.
Acute damage is often the result of a specific injury to the knee during which shearing forces occur at the joint surface causing the articular cartilage to break away from the underlying bony surface. As a result the area of damage is often well circumscribed and the surrounding articular surface is in good order. This has huge implications for the treatment of these injuries.
If a ligament injury (eg cruciate rupture) goes untreated and a patient continues to experience episodes of instability then a situation can arise where the instability causes chronic articular cartilage problems superimposed upon those which may have been caused by the acute injury. Osteoarthritis secondary to ligament injury in the young patient is a particularly challenging condition to treat.
Worthy of mention here is the very specific condition of juvenile osteochondritis dissecans. This is a condition in which part of the bone and overlying articular cartilage which forms the joint becomes loose and can ultimately break away giving rise to a loose body in the knee. The exact cause of the condition remains uncertain despite many authors suggesting various mechanisms since it was first described in 1887. The condition should be considered in children with unexplained pain/swelling in the knee and the diagnosis can be easily confirmed on MRI scan. About 50% of patients will settle without operation. The adult form of the condition is far more aggressive and intervention is often required.
What treatment is available for chronic articular cartilage damage?
This is the treatment of osteoarthritis. For patients over 60-65 years of age the mainstay of treatment is an arthroplasty (replacement) of the knee. There are three parts to the knee which can become worn. The contact between the tibia (shin) and the femur (thigh) bones is in two parts called the medial (inner) and lateral (outer) compartments. The third part is the contact area between the kneecap and the underlying groove in the thigh which is called the patellofemoral joint.
If two, or all three, compartments are diseased then a Total Knee Replacement is performed where all the surfaces are replaced. In recent years partial knee replacements have been developed to treat patients where only one of the three compartments is affected. As a rule these operations are less traumatic than a total knee replacement and require less time in hospital and have a quicker recovery time.
Under the age of 50, the treatment is a little less clear cut. Knee replacements have only a finite lifespan and there are many difficulties in re-doing a replacement when it wears out. One solution is to try to address the underlying mechanical alignment problems that these patients have by surgically altering the shape of the leg to take pressure off the worst affected part of the knee and thus reduce pain. This type of surgery is called an osteotomy and although the principle is straightforward, meticulous attention to the technique of surgery is required and the postoperative rehabilitation will take three months or more.
Where osteoarthritic changes in the knee are causing ‘mechanical’ symptoms, then a simple arthroscopy and debridement can greatly help with symptoms but will usually not reverse the disease process. Mechanical symptoms are those caused by loose articular or meniscal cartilage catching during knee movement with consequent pain and swelling.
Non surgical treatments include anti-inflammatory medication, physical therapy, activity modification, bracing and injection into the joint of enzyme (hyaluronidase) or corticosteroid. There is little in the world orthopaedic literature to suggest that injection of expensive enzyme preparations is any better than a simple steroid injection.
There is increasing interest in injecting interleukin-1 receptor antagonist (IL-1Ra), which is naturally occurring substance that can be extracted and concentrated from a patients own blood and then injected into the affected joint. This is the Orthokine procedure and has been carried out in over 20,000 patients in Germany with reportedly good results. It has the attraction of not being a synthetic drug.
Treatment for acute articular cartilage injury.
In the acute setting the objective is to try to restore the integrity of the joint surface and function of the knee. To simply remove any damaged tissue (mechanical symptoms) or loose bodies may well improve symptoms but will not address the underlying problem. Where the joint surface has been lost the underlying bone marrow can be stimulated to produce a ‘repair tissue’ which is similar to (but not as good as) articular cartilage. This is called fibrocartilage. The marrow is stimulated by punching holes in the bone to produce bleeding. This method can be used for relatively small defects in the joint. For damage up to 3-4 cm2 grafts of bone/cartilage can be transplanted from elsewhere in the knee to fill the defect. This technique is called osteochondral grafting. Where there is substantial loss (possibly combined with loss of the underlying bone) then a patients own cells can be grown to fill the defect. A sample of articular cartilage (a few grammes) is taken by keyhole surgery and the cells are then grown in the laboratory and embedded in a matrix which can then be reimplanted into the knee at a second operation to fill the defect. The technique has shown promise for acute injury but the cell take many months to fill the defect properly after implantation and thus the rehabilitation is slow. Work continues in this field and future treatments will probably develop from stem cell technology.