Cartilage and the Knee Joint
The knee is the largest weight-bearing joint in the body. It is the junction between the femur (thigh bone) and the tibia (shin bone). The knee joint is technically a hinge joint, but there is also a great deal of twisting, turning, and rotating that occurs at the knee. All this movement and weight bearing can cause a great deal of wear and tear on the knee as you go through your normal daily activities. Increasing your activities, as you do with sports and exercise, can add to these forces.
To help absorb the excessive forces that occur at the joint, your body has two different kinds of cartilage. The first is called articular cartilage. This is cartilage that lines the ends of all your bones. Any place that two bones come together to form a joint, you will find articular cartilage. The purpose of this cartilage is to allow smooth, pain free gliding of one bone on the other.
The ends of your bones are very rough and uneven. Bones are very rich in both blood and nerve supply. Therefore, if you were to rub one bone on the other, the motion would not be smooth, and you would get a lot of pain and bleeding. The articular cartilage helps prevent this. It covers the end of your bones to give you a nice smooth surface to move one bone on the other. It has no direct nerve or blood supply, so when it comes in contact with another cartilage coated bone, there is no pain or bleeding. However, this comes with a price. Because there is no direct nerve or blood supply, articular cartilage cannot regenerate. In other words, once it’s gone, it’s gone. If the articular cartilage begins to wear away, or if it completely wear down to bone, there will be a significant increase in pain and swelling at the knee. This is osteoarthritis.
Because your knee is such a huge weight bearing joint, there is a second type of cartilage present that is called the meniscus. There are two of them, and they sit directly on top of the tibial plateau.. The inside one is called the medial meniscus, and the outside one is the lateral meniscus. The menisci help to absorb shock at the knee joint so all the compressive forces do not go directly on the articular cartilage.
If there are no menisci present, the potential for serious joint degeneration is extremely high. Studies show that people who have had their entire meniscus removed to have a very high rate of significant osteoarthritis 5-10 years following surgery. Luckily, a total meniscectomy (complete removal of the meniscus) is very rarely performed anymore. Like articular cartilage, menisci will wear down over time. However, they can also tear. Menisci are attached to the tibia along the outside edge, but the inside border is free floating. With increased joint compression and twisting, a tear can occur. If the knee is already loose or unstable from a previous ligament injury, this is even more likely to occur. This would be particularly true if a person with an unstable knee were participating in a sport that involves twisting or pivoting, such as soccer or any court sports. This potential for cartilage damage is why ligament injuries are often surgically repaired to prevent instability. After all, a ligament reconstruction is a much better alternative to a to a total joint replacement in the future!
A tear in the meniscus may require surgical intervention, especially if it is causing irritation to the joint. The meniscus only has a good blood supply in the outer 1/3 rd , otherwise know as the red zone. Little or no healing will occur in the mid 2/3 rd , known as the white zone. Surgery is performed arthroscopically, and will either involve removing the torn piece of cartilage if it is in the white zone, or repairing it if there is decent blood supply, like there is in the red zone. Meniscal repairs take longer to recover from, but the long-term benefits to preserving the meniscus far outweigh the short-term loss of function with recovery from the repair.
Some meniscal tears occur due to repetitive stresses on the knee joint, and are more degenerative in nature. These tears cannot be repaired, and many time do very well without surgical intervention. Small, degenerative type tears will not usually be a major irritant, and are often best left alone as long as there is no restriction to knee motion. These tears usually do well with physical therapy for strengthening and modification of activities.
There have been many advances in the treatment of cartilage deficient knees. It is now possible to do meniscal transplants, though it is uncommon and long-term research needs to be done to prove the efficacy. There are techniques in which articular cartilage cells are removed from a non weight-bearing surface of the knee, grown in a lab, and transplanted to an area of cartilage defect. This treatment option is for small set of people who have specific areas that are affected. Osteotomies can be performed where the joint is re-aligned to distribute forces to a different area. These are also knee replacement surgeries in which only part of the joint is replaced. However, all of these surgeries are very invasive and require long healing times. The bottom line is that the cartilage needs to be preserved as much as possible to avoid significant surgical intervention. There are many things you can do to help decrease the amount of compressive forces to put on the knee joint, and therefore slow the progression of degeneration. Footwear that increases shock absorption, especially for sports or for people who are on their feet a lot is very helpful. Running shoes are best, though any footwear that is designed to increase shock absorption is helpful. Controlling body weight will also significantly help reduce compressive force on the knee joint. Studies have shown that for every pound lost, there is a four-pound decrease in compression at the knee joint. Losing five pounds would translate into a 20 pound decrease in compressive forces at your knee.
Very strong muscle forces will also increase compression at the knee joint, especially when the knee is bent. Therefore, care should be taken with lifting very heavy weights, especially with squatting activities. This increased compression can also occur with riding a bike with very heavy resistance. Anyone with a history of meniscus problems or osteoarthritis should avoid these activities.
The following chart outlines high to low risk activities:
- High-Risk Activities:
- – weight lifting squats
- – catching position in baseball
- – going up stairs two at a time
- – biking uphill
- – walking on uneven surfaces
- – running, especially hills
- – court sports (tennis, basketball,
- – soccer
- – aerobics
- Medium Risk Activities:
- – biking with moderate resistance
- – Frog kick in
- – walking
- – elliptical
- Minimal Risk Activities:
- – straight leg raises
- – swimming with legs straight
- – biking with minimal resistance
- – Stairmaster with very small steps
- Beneficial Activities:
- – pool walking
- – exercises given to you by a health care professional