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How the Knee Works and The Overall Impact of OA

24th September 2021

Knee osteoarthritis can manifest differently in every individual, depending on various factors, including their knee structure. The first step to combat knee osteoarthritis is to learn how the knee works. That’s why we prepared a comprehensive guide about the human knee.

The Anatomy of the Knee

The figure below illustrates the components of a healthy knee. In its simplest form, the knee is composed of bones and the connective tissue in between.

Bones

The bones that make up the knee are the femur (the thigh bone), tibia (shin bone), and patella (kneecap).

The femur and tibia are two of the longest and heaviest bones in our body, so it comes as no surprise that they serve a weight-bearing function.

Although not directly connected to the knee, the fibula (the smaller lower leg bone behind the tibia) brings additional stability while supporting the force transfer from the knee to the ankle joint.

Patella - the kneecap - is a small triangular bone sitting in front of the lower part of the femur called the patellofemoral groove, surrounded by thigh muscles.

The function of the kneecap is to act as a lever during knee extension. To extend your knee, your quadriceps (thigh muscles) contract, pulling on the kneecap, which pulls on the tibia. Without the kneecap, your quadriceps would have to exert a tremendous force to raise your leg. In the long run, this would have even caused tears in the tendons that pull the tibia.

Connections

Besides the bones, you have components that connect these bonds, such as ligaments, tendons, and cartilage.

The knee owes its moving abilities to tendons, fibrous connective structures between bones and leg muscles. The main tendon in the knee is the patellar tendon that connects the kneecap to the quadriceps.

Ligaments connect two bones to provide stability. There are four ligaments in the knee. While medial collateral and lateral collateral prevent the femur from sliding sideways, two cruciate ligaments (anterior and posterior) prevent the lower or the upper leg from sliding too far in either direction.

Knee cartilage covers the bone surfaces at the joint and has numerous tasks for knee protection. Not only does it absorb shock and reduce the strain on your knee, but it also prevents friction between the femur and the tibia during the bending of the knee.

The two types of knee cartilage are meniscus and articular cartilage.

The meniscus is the collection of the disks in the knee that absorb shock and provide an even weight distribution over the leg bones. This is crucial for balance while walking. You have two types of menisci: the medial on the inside and lateral on the outside.

Articular cartilage is found on femur and tibia surfaces to protect against friction. It owes its slippery texture to the synovial membrane within the joint, which secretes the lubricant, synovial fluid for the cartilage.

The Functions of the Knee

With such a complex yet well-coordinated mechanism, the knee joint brings significant improvements to your range of motion, as well as stability to the leg as a whole.

The knee provides 5-10 degrees of extension and 120-150 degrees of flexion. When flexed, it also allows 20 degrees of rotational motion that reassures stability. Both directional rotations work with seamless coordination through internal and external rotational mechanisms collectively called the screw-home mechanism.

Especially with the help of the kneecap-patella, the knee joint acts like a pulley and transmits forces from your quadriceps with ease.

For the impact from the ground (e.g., while jumping and running), the knee helps to absorb, distribute, and dissipate shock caused by the forces from the ground.

Knee OA and Anatomy Change

Knee osteoarthritis stems from the wear and tear of articular cartilage and disrupts the rest of the knee functionality in a domino effect. Without the lubricant effect, the femur and the tibia start to rub against each other, resulting in stiffness and pain.

When the articular cartilage wears away, the shock-absorbing capabilities of the knee are lost. This manifests in the knee as symptoms, such as joint swelling, difficulty in flexion-extension, pain (exacerbated in the morning or even when resting), and decreased stability.

The common causes of knee OA include:

  • Age - since torn cartilage will take longer to heal as you get older
  • Weight - since the increased weight will put extra pressure on your knee and increase the chances of cartilage damage
  • Heredity
  • Stress injuries - workplace or sports-related

How Knee OA Impacts the Other Joints

As your knee OA symptoms worsen, you might notice differences in the way you move. More specifically, your legs will try to adjust their coordination to minimize the pressure and pain in the knee. However, this puts extra strain on other joints like the hip, ankle, and even the spine in the long run.

A clinical study clearly demonstrated that patients with knee OA walked more slowly and with a different stance. The different movement patterns in knee OA patients increased not only contraction in quadriceps & hamstrings but also joint loading in the hips.

Another study showed that knee osteoarthritis could cause the narrowing of lumbar spine disc space, increasing spine OA risk.

The collection of evidence emphasises the importance of early osteoarthritis treatment for knee health, as well as the whole musculoskeletal health.



[1] Metcalfe, A. J., et al. "The effect of osteoarthritis of the knee on the biomechanics of other joints in the lower limbs." The bone & joint journal 95.3 (2013): 348-353.

[2] Hassett, G., et al. "The relation between progressive osteoarthritis of the knee and long term progression of osteoarthritis of the hand, hip, and lumbar spine." Annals of the rheumatic diseases 65.5 (2006): 623-628.

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