Your knees have lots of moving parts, and you use them a lot, so lots of things can go wrong. Too much of one kind of motion, especially if you don't work up to it, can lead to "overuse" injuries. Simple wear and tear is a problem, especially as you age. Accidents can crack bones and tear tissue. With some conditions, your body attacks its own joints. Your doctor can help you sort out what's going on with your knee when it doesn't feel right.
Dislocations and Fractures
If your knee hurts intensely after a bump, bang, or fall, you may have broken one of the bones that meet up there -- the thigh, shin, and kneecap -- or shifted one out of place. Go to the emergency room or see a doctor as soon as possible. Sometimes fractures happen more slowly, causing tiny cracks at the ends of the leg bones. This can happen when you've started using your knee more.
Torn Anterior Cruciate Ligament
You hear a pop and can't move after you suddenly change direction -- often while playing soccer, football, or basketball. You may have torn your ACL, which connects the femur and the tibia and prevents the tibia from moving too far forward. Your knee will hurt and swell and feel unstable.
You can tear or strain any of the tissues that hold your knee together: Ligaments connect bones to each other; tendons connect muscle to bone. Irritated tendons from using them too much? That's tendinitis.
Iliotibial Band Syndrome
The "IT band," a ligament that runs along the outside of your thigh, can rub against the bone and get irritated and swollen. You're more likely to get this when you run or ride your bike for exercise. It might hurt more if you go downhill or sit for a while. You may feel better after you warm up, but if you don't rest the injury and give it a chance to heal, it could get worse.
It's tough to bend or stretch your leg. Your knee feels stiff and sore and may swell after you've been moving around. You've probably worn down the cartilage, the stuff that helps cushion the ends of your bones. Because that takes time, this kind of arthritis is more common in people 65 and older. If you're younger, you may get it because you've used your knee a lot after an injury, like a ligament tear.
Besides pain and swelling, you may also feel tired, sick, or feverish. When you have an autoimmune disease, your immune system, which is meant to fight off germs, may attack your knees. For example, rheumatoid arthritis tends to affect hands and other joints in pairs on both sides of the body. Lupus goes after joints, muscles, and organs all over. Psoriatic arthritis often causes thick, discolored patches of skin, along with joint pain.
This makes your knee swollen, stiff, and warm or tender to the touch, usually because you've overworked it. The condition is also known as "housemaid's knee" or "clergyman's knee" because people with these jobs are kneeling so much. It happens when small, fluid-filled sacs called bursa that help cushion your knee joint get irritated and swollen. You may ache even when you're resting.
You'll feel this in the front of your knee, around the kneecap. Your knee may hurt after you sit with it bent for a while or when you try to kneel. It may pop or crack when you climb stairs. Typically, patellofemoral pain syndrome comes from overuse, misalignment between your hip and ankle, a weak thigh muscle, or the breakdown of cartilage behind your kneecap -- or a combination of these. It's common in women and young athletes.
A sudden twist or pivot -- especially with your full weight on your knee -- can tear a meniscus, the rubbery cartilage that acts as a cushion between the bones of your thigh and shin. You have one on each side of your ACL. They may be more susceptible to tears because of arthritis or age. The pain can be hard to pinpoint and describe. Your knee may get stiff, swollen, and hard to move and extend.
Baker's Cyst (Popliteal Cyst)
This fluid-filled sac at the back of your knee may bulge out or get so tight that it's hard to fully bend or stretch your leg. It may be caused by another problem, like arthritis or a tear in your meniscus. The cyst itself usually doesn't hurt unless it bursts, which can make the back of your knee and calf swell and bruise.
Gout and Pseudogout
Both have similar symptoms: The pain and swelling are often intense and hit you fast. Your knee may be stiff, red, and hot. It happens when crystals gather in the joint. Gout is from a buildup of uric acid and often affects the big toe. Pseudogout is caused by calcium pyrophosphate.
Another illness or a direct injury to the joint can cause septic arthritis. The pain will come on quickly, and you'll also be sick, cranky, and running a fever. Your doctor may use a needle to take some fluid from your knee to figure out which bacteria is causing it so they know how to treat it. Though it doesn't happen as much, viruses and parasites can infect your joints, too.
Problems somewhere else -- in your back, hip, or foot, for example -- can make your knee sore. Nerves can move pain from one area to another, or your brain may get confused about the source of pain signals. The feeling is real, but there may be nothing wrong with your knee itself.
What You Can Do
Take over-the-counter NSAID drugs like ibuprofen and naproxen to ease pain and swelling. RICE -- rest, ice, compression, and elevation -- can often help, too: Get off your feet. Raise your leg so it's higher than your heart. Put a cold pack in a thin cloth or towel on your knee for 10-20 minutes at a time, several times a day. Wrap an elastic bandage around your knee when you're up and about, snug but not tight.
When to See Your Doctor
Don't wait if your knee pain is sudden and intense. Pick up the phone if it won't go away or gets worse.
To make a diagnosis, your doctor might take X-rays or other images of your knee. Blood or knee fluid samples can help confirm or rule out certain conditions. Treatment may include medication, special exercises, braces, or in some cases surgery. Losing weight could help lessen pressure and strain on your knees.