Summarize this article in one sentence.
A Baker's cyst occurs when fluid builds up in the back of the knee. This happens when something is wrong or injured inside the joint. However, this can also occur spontaneously without any trauma. Some people just get them and there is no known explanation. This cyst points to a larger underlying problem, like a torn meniscus, that needs to be found soon because the swelling can cause damage to the knee. Bursitis is inflammation or an injury to the prepatellar bursa. Bursa, a round, fluid-filled structure, help the kneecap move smoothly across other tissues without friction. If the bursa gets inflamed, it can cause surrounding tendons to become inflamed and pinched, resulting in pain. Bursitis can cause stiffness or pain in the knee, as well as tenderness of the knee when touched and pain with movement. Pain is usually worsened by bending the knee and improved with extending the knee. The knee may also be swollen and red. Bursitis can be caused by repetitive motions, like bending or stooping. You can also get it by putting pressure on the joint by kneeling for too long on a hard surface. Patellar tendinitis occurs when you do the same motion on your knee, like running or cycling. Patellar tendinitis is when the tendon between your kneecap and shin becomes inflamed. Pain is the main symptom of patellar tendinitis. The pain is located beneath your kneecap, near the place it attaches to your shin. The pain may surround a workout, either as you begin or after you finish one. Eventually, the pain will make it difficult to stand or take stairs. Arthritis of the knee occurs when the knee joint swells. Arthritis generally occurs with age or due to a knee injury. It can make everyday tasks difficult, like standing and sitting, walking, or taking stairs. Pain, swelling, and stiffness are common symptoms of arthritis of the knee. Osteoarthritis (OA) occurs due to age. It can start around age 50, but may affect younger people. OA occurs due to the wear and aging of the cartilage of the knee, which causes less protection for the bones as they rub against one another. Pain may also become worse as you go through your day. Rheumatoid arthritis (RA) is a chronic autoimmune disease that will not only affect the knee, but joints over the entire body. RA usually starts affecting people between 50 to 75 years old, it is more common in women, and pain is often worse in the morning and improves with moving throughout the day. Post traumatic arthritis occurs after a knee injury. Sometimes this happens years after the injury. Broken bones, ligament injuries, and meniscal damage can cause this type of arthritis.

Summary:
Determine if you have a Baker's cyst. Decide if you have bursitis. Monitor for tendinitis. Check to see if you have arthritis of the knee.