Articular cartilage is a special type of cartilage characterized by a particular arrangement of collagen fibers and water-binding substances. Intact articular cartilage transmits the acting forces evenly and allows the bones moving against each other to slide with extremely low friction. Unfortunately, articular cartilage essentially does not have the ability to regenerate. Injuries to the cartilage lead to defects, which at best fill with replacement cartilage, so-called fibrocartilage, or remain as defects in the otherwise smooth surface. This does not ensure uniform pressure transmission, which leads to secondary damage to the surrounding cartilage, especially in weight-bearing joints such as the knee. Such localized cartilage damage must be distinguished from wear diseases of the whole or larger parts of the knee joint, so-called arthroses. Causes of such localized cartilage damage can be one-time force impacts or repeated micro-injuries leading to additive damage. Circulatory disorders of the cartilage-bearing bone can also lead to localized defects in the cartilage. Mostly younger people are affected, more often athletes, in whom overloading of the cartilage is not uncommon. In older age, arthrosis is found more frequently, although transitions between both forms of cartilage damage can be found, since local damage can naturally lead to arthrosis over time.

The extent of discomfort varies widely; pain under load or even at rest are the leading symptoms. Sheared cartilage scales can become trapped in the joint and lead to blocking symptoms.

Examinaiton

There are no specific tests that point the way during the physical examination. In most cases, rather non-specific signs such as pain during certain movements or fluid accumulation in the joint can be detected. X-rays can show cartilage damage only indirectly, since cartilage is not imaged. Magnetic resonance imaging is most likely to detect local cartilage damage, but with limitations. Sometimes it is necessary to inject a contrast medium into the knee joint to make cartilage defects visible. Ultimately, the extent of the defects in particular can often only be determined by direct visualization through arthroscopy.

Treatment

Since articular cartilage hardly heals, as mentioned at the beginning, treatment is correspondingly problematic. There are currently no procedures that are capable of restoring the cartilage layer to its natural microstructure.

One approach is to harvest bone-cartilage cylinders of variable size from less stressed areas of the knee joint and graft them into defects localized in the main stress zone. This procedure is referred to as "osteochondral transfer" (OCT) or mosaicplasty or OATS.
In the case of small defects, a single cylinder is sufficient, but more often, it is necessary to arrange several next to each other in order to cover the defect adequately. It goes without saying that this procedure is primarily limited by the size of the defect, since only limited cartilage can be obtained from other areas.

While the actual articular cartilage does not regenerate, a fibrocartilage regenerate can develop, which may completely fill the defect. Fibrocartilage is biomechanically inferior to the actual articular cartilage because of the non-directional fiber arrangement, but it can form a functionally adequate substitute. Therefore, the bone lying in the defect area is sometimes deliberately perforated (so-called micro-fractures) to facilitate the ingrowth of stem cells from the bone marrow and thus promote the development of fibrocartilage.

For several years now, a procedure has also entered clinical practice in which attempts are made to obtain cartilage material to fill the defect by tissue cultivation. This is referred to as "autologous chondrocyte transplantation (ACT)". There are various procedures for the propagation of cartilage cells outside the body with a range of advantages and disadvantages.

What they all have in common is that cartilage cells have to be removed from the joint in order to subsequently multiply them in the laboratory and finally transplant them back into the joint. This means that two operations are required in each case, although the first is a minor arthroscopic procedure. Ideally, the cartilage will grow into the defect and restructure. The procedure is still imperfect and it is uncertain whether permanent healing of the cartilage will occur. In addition, the cultivation is still relatively expensive, so that funding by the responsible payer can be problematic.Therefore, ACT is most appropriate in younger patients in whom other alternatives are not an option because of the extent and location of the defect.

A reliable therapy for cartilage defects is not yet available. All the above-mentioned procedures should rather be considered in terms of damage limitation. ACT may have the potential to allow true restoration of cartilage cover in the future. Above mentioned interventions are only suitable for localized cartilage defects of limited extent, extensive cartilage defects in terms of arthrosis require other measures. The optimal therapy must ultimately be decided on a case-by-case basis.

Follow-up Treatment

As a rule, it is necessary to relieve the affected knee for a certain time after the operation. Nevertheless, physiotherapy is of course necessary to maintain mobility and muscle function.

Late Complications

All the procedures mentioned are relatively new and lack long-term experience. In principle, it cannot be ruled out that the damage to the cartilage will continue to progress despite surgery and that further interventions will be necessary.