Inside the knee, two meniscuses, one medial and one lateral, function as shock-absorbing pads, facilitating movement and protecting the entire joint.
What are the meniscuses
The most common injuries to the knee are those in the meniscus, two small fibrocartilaginous C-shaped structures placed between the femoral condyles and the tibia. During the movements the menisci allow to discharge 30-70% of the weight bearing on the articular cartilage stabilizing the knee. Their shape, slightly raised at the edges and concave inside also increases the congruence of the articular surfaces that form this important articulation.
Laterally, both menisci make contact with the joint capsule through a fibrous connective tissue called paramenisco. While the upper face, slightly hollow, makes contact with the femoral condyles, the lower one, flat, rests on the respective glenoid cavity of the tibia.
The menisci are made of whitish-colored fibrous cartilage which is particularly resistant to mechanical stress. The main component of the fibrous cartilage, called type I collagen, in turn is arranged along circular fibers in order to resist the loads exerted by the femur. A small part of fibers instead has a radial orientation and gives the meniscus a certain resistance to longitudinal tears.
The medial or internal meniscus resembles a half-moon while the lateral or external meniscus has a more circular appearance, more like an O. The lateral meniscus covers a greater portion of the articular surface of the tibia than the medial meniscus. It also has greater mobility.
Inside the knee the meniscuses are not free between the two articular surfaces but are stabilized by important connections. The transverse ligament of the knee connects the front horns of the two meniscuses to connect them to the patella. The two menisci also make contact with the fibers of the anterior and posterior cruciate ligaments thus accentuating their stabilizing function.
Laterally the two meniscuses are connected to a fibrous bundle coming from the respective lateral end of the patella. Finally, expansions of the tendons of the semimembranosus and popliteal muscles are connected respectively to the posterior edge of the internal meniscus and to the posterior edge of the external meniscus. These last connections described are very important because they give the menisci an active mobility and protect them from possible injury during movements.
Functions of the meniscuses
Once the meniscuses were considered important but not indispensable and were therefore removed in the event of injury. Although in the short term these interventions quickly restored the lost joint function, some subsequent studies showed a profound incidence of arthrosis and degenerative pathologies in patients who had undergone this operation (meniscectomy).
Today the old techniques have been almost completely replaced by arthroscopic surgery which in most cases does not remove but sutures the damaged part of the meniscus. A succession of numerous studies has clearly shown that the preservation of the meniscus protects the articular cartilage from degenerative processes and that these are directly proportional to the portion of the meniscus removed. So let's take a brief look at the many functions of the menisci:
- they absorb and distribute the loads applied to them uniformly
- help cartilage absorb shocks
- they cooperate with the tendons protecting the joint from hyperextension and hyperflexion damage
- increase the congruence of the joint
- if subjected to load they push the nutrient-rich synovial fluid into the articular cartilage
- stabilize the entire joint
The meniscus has no blood vessels except for its two ends. In young adults this vascular system penetrates inside the medial meniscus for about 10-30% of its length, while in the lateral one the penetration is slightly lower (10-25%). Over the years, there has been a progressive reduction of the meniscal capillaries. Nourishment is however guaranteed by the presence of synovial fluid.
Also the meniscal nerve endings have a distribution similar to the vascular one and are absent in the central portion. Their task is to transmit information on the position taken by the joint.
Beyond these subtleties it is important to remember that the meniscus is a structure largely free of blood capillaries. It follows that, with the exception of small peripheral lesions, in case of a strong trauma its repair capabilities if they exist are extremely low.
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