About the Medial and the Lateral Meniscus

Information

The medial meniscus and the lateral meniscus are special structures in the knee joint. These crescent-shaped, shock-absorbing structures between the tibia and the femur play an important role in the functioning of a healthy knee. The process of mechanical arthrosis is set in motion when the menisci are routinely removed after a tear. We now know that they support the healthy knee, because they play an important role in joint stability, load transmission, and lubrication. When possible, in the case of injury, they can be repaired. There are even experimental attempts in which it is possible to restore damaged menisci, which is an important advance in orthopedics.

There are two categories of meniscal injuries - acute tears and degenerative tears.

A tear is usually considered acute when the knee is forcibly bent or twisted due to the action of an external force, while the limb is in a position bearing the weight of the body or a load. Statistics show that about 61 out of 100,000 people experience an acute tear of one of their menisci.

Degenerative meniscal tears occur at an advanced age. Sixty percent of the population over the age of 65 probably has some type of meniscal tear. As the meniscus ages, its elasticity weakens and it becomes more brittle. Degenerative tears can result from seemingly minor movements; they may or may not present with symptoms.

What are the menisci?

The two menisci of the knee are crescent-shaped, wedge-shaped structures that fill the gap between the tibia and the femur. The menisci provide joint stability by creating a bed in which the femur sits. Their outer edges are quite thick, while the inner surfaces are very thin. If the menisci are absent, the surfaces of the femur rub directly against the tibia.

The medial meniscus, located on the inner side of the knee, has a more elongated "C" shape, and the tibial surface is also larger on this side. The medial meniscus is more often injured because it is firmly attached to the medial collateral ligament and the joint capsule. The lateral meniscus, on the outer side of the knee joint, has a more circular shape. The lateral meniscus is more mobile than the medial meniscus, as there is no attachment there to the lateral collateral ligament or the joint capsule.

The outer edges of each meniscus attach to the tibia via short coronary ligaments. Other short ligaments attach the ends of the menisci to the tibial surface. The inner edges move freely because they are not attached to bone. This allows the menisci to change shape as the joint moves. The front part of the meniscus is referred to as the anterior horn, the back part as the posterior horn, and the middle part as the body.

Under a microscope, the meniscus is fibrocartilage, which derives its strength from collagen fibers as well. Its elasticity is due to the water located in the spaces between the cells. The menisci have a poor blood supply. Only the outer periphery is supplied by small arteries around the joint. The poor blood circulation on the inner side of the meniscus makes it difficult to treat.

What do the menisci do?

The meniscus acts as a load absorber for the knee, dispersing the compressive forces from the femur over the wide area of the tibia. The medial meniscus bears up to 50% of the load applied to the medial (inner) compartment of the knee. The lateral meniscus absorbs up to 80% of the load on the lateral (outer) compartment of the knee.

Throughout the various phases of the gait cycle, forces shift from one meniscus to the other, and the forces acting on the knee can increase to 2-4 times the person's body weight. While a person is running, these forces in the knee can increase to 6-8 times their body weight. Even greater forces occur, for example, when landing from a jump.

The important role of the meniscus in force transmission can be seen when the menisci are removed. If the menisci are removed, the forces are not distributed over the wide area of the tibia. Without the medial meniscus, the tibial contact area is reduced by 50-70%. This means that the same forces from the femur are transmitted over a smaller area of the tibia.

When the lateral meniscus is removed, there is a 45-50% reduction in the contact area. The result is a 200-300% increase in contact pressure, which can eventually damage the cartilage on the bone surfaces. This can lead to degenerative arthritis.

During the 1960s and 1970s, it was common practice to remove damaged menisci completely. This often led to early degenerative arthritis in many patients. Complete removal of the medial meniscus can cause varus deformity ("bow legs") and medial joint arthritis, while removal of the lateral meniscus can cause valgus deformity ("knock knees") and lateral joint arthritis.

What is a meniscal injury?

Patients describe meniscal tears in a variety of ways. Knowing where and how the meniscus was twisted helps the doctor choose the best treatment.

Location - The tear can be located in the anterior horn, the body, or the posterior horn. Tears in the posterior horn are more common. The meniscus can be torn from the inside out - in the middle, and the inner third. The third in which the tear is located will determine the likelihood of the tear healing, since the blood supply in that area is critical to the healing process. Tears in the outer 1/3 have the best chance of healing.

Types - Meniscal tears can have various shapes. The type of tear influences the doctor's treatment decision. Examples of different types of tears include:

• Longitudinal
• Bucket-handle
• Displaced bucket-handle
• Parrot-beak
• Radial
• Displaced flap
• Horizontal
• Degenerative

Complex tears involve more than one type.

Completeness - Tears can be complete or partial. A tear is considered complete when the tear line runs through the meniscus and the piece of tissue is separated from the rest of it. If it remains attached to the body of the meniscus, it is considered incomplete.

Stability - Tears can be stable or unstable. Stable tears do not move and can heal on their own. Unstable tears allow the meniscus to move abnormally and may cause complications if surgical correction is not performed.