Knee injuries can involve any of the several different tissues that make up the knee joint: cartilage, ligaments or bone. They may also involve the supporting tissues: muscles and tendons. When injured, these components commonly become inflamed or torn, causing pain, stiffness, limited range of motion and/or swelling.
Merck Manuals, an online medical library, reports that runner's knee is a condition often caused by the pressure exerted on the knee joint during running, especially when the runner has weak thigh muscles. The downward pressure on the knee joint causes the kneecap to rub the top of the thighbone, possibly damaging the bone. This type of injury usually triggers pain first when running downhill. As the injury becomes more severe, walking at a normal pace may become painful.
The Mayo Clinic describes how the knee is made up of four ligaments, which connect the thighbone to the lower leg bone. Injuries, especially falls or high-impact trauma, can cause any of these four ligaments to tear. The signs and symptoms of a ligament injury include acute pain, especially when trying to bend the knee, a popping sound, inability to put weight on the knee and a feeling of weakness in the knee.
While ligaments connect bone to bone, tendons connect muscle to bone. Knee tendinitis is characterized by inflammation of the tendons of the knee. The Mayo Clinic notes that the patellar tendon in the knee is especially susceptible to tendinitis, which connects the thigh muscles to the top of the lower leg bone. This type of tendinitis is especially common among people who frequently run, ski and bicycle. The symptoms of patellar tendinitis are swelling in the area below the kneecap; pain, especially while running; jumping, climbing stairs, or squatting; and being unable to straighten the knee.
Bursitis is inflammation of the bursa in the knee, which are small sacs that provide additional cushioning to the knee joint, according to the Mayo Clinic. Intact bursas are necessary for smooth gliding of the ligaments and tendons of the knee joint. Symptoms of bursitis include swelling, warmth, redness and stiffness. Commonly, there is pain in the knee even at rest, and the pain may get much worse when descending stairs or kneeling.
The meniscus is made up of two pieces of cartilage, which act as cushioning between the thighbone and the lower leg bone. Sudden tearing of the meniscus is a common sports-related injury, according to the American College of Orthopedic Surgeons. Squatting and twisting abruptly can result in a meniscus tear, as can being tackled, such as during football. The symptoms of meniscal tears are pain, stiffness, swelling, weakness; locking of the knee; and inability to flex the knee normally.