Knee Menisci: Number, Types, and Essential Functions

How many menisci are there in each knee?

Each human knee joint contains two menisci: the medial meniscus and the lateral meniscus. These crescent-shaped cartilages are crucial for knee joint function, stability, and load distribution.

Understanding the Knee Joint and its Components

The knee is one of the largest and most complex joints in the human body, primarily functioning as a modified hinge joint. It connects the thigh bone (femur) to the shin bone (tibia) and the kneecap (patella). While often simplified as a simple hinge, its intricate structure allows for a combination of flexion, extension, and slight rotational movements. Key to its robust function and durability are specialized structures, including ligaments, tendons, and articular cartilage. Among these, the menisci play a particularly vital role in ensuring smooth, stable, and pain-free movement.

The Two Menisci of Each Knee

Within each knee joint, there are precisely two menisci, both composed of tough, rubbery fibrocartilage. These C-shaped or O-shaped structures sit between the condyles of the femur and the tibial plateau, effectively deepening the shallow articular surface of the tibia to better accommodate the rounded femoral condyles.

• Medial Meniscus: Located on the inner (medial) side of the knee, this meniscus is typically C-shaped. It is more firmly attached to the tibia and the joint capsule, particularly via the medial collateral ligament (MCL). This stronger attachment makes it less mobile and, consequently, more susceptible to injury, especially during twisting motions.

• Lateral Meniscus: Situated on the outer (lateral) side of the knee, the lateral meniscus is generally more O-shaped or circular. It is less firmly attached to the joint capsule and has a looser connection to the lateral collateral ligament (LCL), allowing for greater mobility within the joint. This increased mobility can sometimes make it less prone to tearing from direct impact but still vulnerable to rotational forces.

Essential Functions of the Menisci

The presence of two menisci in each knee is not arbitrary; their collective and individual functions are indispensable for optimal knee health and mechanics:

• Shock Absorption: The menisci act as crucial shock absorbers, distributing compressive forces across a wider surface area of the joint. This prevents concentrated stress on the articular cartilage of the femur and tibia, protecting them from premature wear and tear.
• Load Transmission: By increasing the contact area between the femur and tibia, the menisci efficiently transmit weight and forces across the joint during activities like walking, running, and jumping. They can bear up to 50% of the load in extension and up to 85% in 90 degrees of flexion.
• Joint Stability: The menisci help to stabilize the knee joint by deepening the tibial plateau, providing a better congruence for the femoral condyles. They also contribute to guiding the movement of the bones during flexion and extension, preventing excessive anterior-posterior translation and rotation.
• Joint Lubrication and Nutrition: Their presence aids in the distribution of synovial fluid, which lubricates the joint surfaces and provides nutrients to the articular cartilage, contributing to overall joint health.

Clinical Relevance: Why Their Number Matters

Understanding that there are two distinct menisci in each knee is critical for both anatomical comprehension and clinical practice. Injuries to the menisci, commonly known as meniscal tears, are frequent in athletes and active individuals, often resulting from sudden twists or direct impacts. The specific location and type of tear (medial vs. lateral, C-shaped vs. O-shaped) influence diagnosis, treatment, and rehabilitation strategies. The loss or significant damage to either meniscus can lead to increased stress on the articular cartilage, potentially accelerating the development of osteoarthritis in the knee joint.

Conclusion

In summary, each knee joint is equipped with two vital fibrocartilaginous structures: the medial meniscus and the lateral meniscus. These crescent-shaped pads are fundamental to the knee's ability to absorb shock, distribute load, enhance stability, and facilitate smooth movement. Their distinct anatomical features and critical physiological roles underscore their importance in maintaining long-term knee health and function.