To clarify, articular cartilage is the kind which wraps around the end of the bones and facilitates ease of movement, and the menisci are pads between the bones that improve stability and act as shock absorbers. A tear in the meniscus is a well-known injury, as it often happens to athletes. This is frequently the result of a bad fall that twists the knee at an odd angle as they land, or sometimes from a traumatic blow to the area; it can also be the result of small tears building up over time. Damage to the meniscus causes severe swelling and pain, and it limits mobility; treatment depends on the severity of the damage – from a sprain to a sizable tear – but icing and elevation are the first steps, with surgery as a possibility for a more extreme injury.
Damage to the articular cartilage, like menisci, can be caused by an active injury such as a fall or twist, but they are more often the result of wear and tear, especially from repetitive use over time. The cartilage wears away, growing thinner as we age and put pressure on it. However, other medical causes can contribute, such as hormonal conditions like osteochondritis dessicans.
Regardless of the cause, deteriorating cartilage is painful and uncomfortable; as it wears away, it increases friction in the knee as the bones move past each other, making the motion far less smooth. With less padding, the bones rub together more, increasing discomfort, and reducing stability.
If you have swelling or limited motion in the knee and suspect a cartilage injury, apply ice and elevate; you may also take an over the counter anti-inflammatory medication. If the pain, stiffness, or swelling continues, see a doctor, because with an ongoing injury, the greater the likelihood that more serious treatment may be necessary.