The knee joint
There are four bones around the area of the knee joint: the thigh bone (femur), the main shin bone (tibia), the outer shin bone (fibula) and the kneecap (patella). The main movements of the knee joint are between the femur, the tibia and the patella. Tough connective tissue (articular cartilage) covers the ends of the tibia and femur and the back of the patella around the knee joint. The articular cartilage reduces friction between the bones of the knee joint and helps smooth movement between them.
Each knee joint also contains an inner and outer meniscus (a medial and lateral meniscus). The menisci (plural of meniscus) are thick rubbery pads of cartilage tissue. They are C-shaped and become thinner towards the middle of the joint. The meniscal cartilages sit on top of, and are in addition to, the usual thin layer of articular cartilage which covers the top of the tibia. The menisci act like shock absorbers to absorb the impact of the upper leg on the lower leg. They also help to make the knee movements smooth and help to make the knee stable.
When people talk about a cartilage injury to a knee, they usually mean an injury to one of the menisci. However, the knee also has tough connective tissue covering the ends of the bones in the joint - this is called articular cartilage - and damage can occur here as well. The areas of articular cartilage can be seen in the side view of the knee joint in the diagram above.
How do you tear your meniscal cartilage?
The knee is commonly injured in sports, especially rugby, football and skiing. You may tear a meniscus by a forceful knee movement whilst you are weight bearing on the same leg. The typical injury is for a footballer to twist (rotate) the knee whilst the foot is still on the ground - for example, whilst dribbling around a defender. Another example is a tennis player who twists to hit a ball hard but with the foot remaining in the same position. The meniscus may tear fully or partially. How serious the injury is depends on how much is torn and the exact site of the tear.
Meniscal tears may also occur without a sudden severe injury. In some cases a tear develops due to repeated small injuries to the cartilage or to wear and tear (degeneration) of the meniscal cartilage in older people. In severe injuries, other parts of the knee may also be damaged in addition to a meniscal tear. For example, you may also sprain or tear a ligament. See separate leaflet called Knee Ligament Injuries for more details.
Meniscal cartilage does not heal very well once it is torn. This is mainly because it does not have a good blood supply. The outer edge of each meniscus has some blood vessels but the area in the centre has no direct blood supply. This means that although some small outer tears may heal in time, larger tears, or a tear in the middle, tend not to heal.
How do you injure the articular cartilage?
Damage to the cartilage covering the end of the bones at the knee joint is called a chondral injury. If the underlying bone itself is also damaged, this is called an osteochondral injury.
Injuries to the articular cartilage usually happen in combination with other injuries to the knee, either to the meniscus (as above) or the knee ligaments or bones. See separate leaflet called Knee Ligament Injuries for more details.
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