Knee Meniscus

Written By: Chloe Wilson, BSc(Hons) Physiotherapy
Reviewed by: KPE Medical Review Board

The knee meniscus is a special layer of cartilage that lines the knee joint.

The function of the meniscus is to cushion the knee joint and transfer forces between the tibia and femur, the thigh and shin bones.

Most of the joints in our body are lined with a thin layer of articular cartilage, made of collagen and chondroitin. This provides a smooth surface over the bones allowing smooth movement. 

The knee joint also has an additional layer of special cartilage, called the meniscus. The reason that the knee needs this extra thick layer is to protect it from the huge forces that go through the joint as we move.

The meniscus has two distinct areas:

• White Zone: the inner portion of the knee meniscus. It is avascular, meaning it does not have a blood supply
• Red Zone: the outer portion of the knee meniscus. This part does have a reasonable blood supply being vascularised by the joint capsule.

As the inner portion of the knee meniscus doesn't have a blood supply, tears to the White Zone do not usually heal naturally, often requiring surgery.

Tears to the outer zone can potentially heal as the Red Zone receives nutrients, oxygen and chemicals via the blood from the joint capsule, however healing tends to be a slow process.

Some people are born with anatomical variations, the most common being a discoid meniscus. This is a thicker, more disc-shaped meniscus, most often seen in the lateral meniscus. It’s more prone to tearing due to its shape and limited flexibility.

The knee meniscus has several essential functions:

1. Joint Congruency: helps the tibia and femur to fit better to each other, increasing surface area contact by 40-60%, making the joint more stable and allowing smooth movement
   
   
2. Load Distribution: Helps ensure even weight distribution across the knee between the tibia and femur to reduce the stress through the joint
   
   
3. Shock Absorption: cushions the forces transmitted through the knee during walking, running and jumping, reducing the force through the knee bones by about 30%
   
   
4. Joint Stability: Contains nerves which help improve balance and stability and supports the anterior cruciate ligament (ACL)

Knee meniscus injuries are a common problem and there are two main ways that damage can occur:

1. Injury: The menisci are often injured when the knee twists suddenly e.g. when playing sports or during a fall. This tends to tear part of the cartilage and can cause bleeding in the joint resulting in swelling
   
   
2. Wear and Tear: As we age, our cartilage becomes more brittle and can start to wear away. This also makes them more prone to injury. This is a common feature of arthritis

Meniscus tears can occur in any part of the cartilage (as shown in the diagram) and can take a long time to recover from. Small blood vessels feed the outer edges of the knee meniscus but the middle parts have no direct supply which means it is very slow to heal following injury.