Knee clicking can have more causes than you can imagine! Clicking in itself is rarely a problem, so if you do not feel pain when your knee is clicking it's probably not serious.
If you feel pain during the clicks, then you either had a knee injury which you probably remember, or the painful clicking emerged without an injury.
Overall there are three possibilities: clicking without pain (this is not an issue), painful clicking after an injury or painful clicking without an injury.
If your knee is clicking and you do not feel pain, this can be caused by your tendons getting caught up and then jumping over a small lump of the bone as you are moving your knee. This is harmless, and no cause for concern.
Some say that the clicking and cracking noise can come from bubbles forming in the joint fluid when the pressure decreases. However according to researchers this is common in the joints of the hand and not in the knee. There's more information here and here.
The two most common injuries which can lead to painful clicking are the tearing of the ACL, the ligament in the center of the knee, or the MCL, the ligament on the inner side of the knee. There are braces which can help with this, both for ACL and MCL tears.
The most common causes for the ACL injury are suddens stops or changes of directions during sporting such as soccer or basketball. When the ACL snaps, you hear a popping noise, your knee swells, becomes painful, and may not be able to bear weight. Training properly reduces the chances of this injury. Would it still happen it can be fixed by physiotherapy or surgery.
MCL injury is usually caused by contacts sports, where a hit or a blow to the outer side of the knee overstretches or tears the MCL on the inner side of the knee. You'll hear a popping sound when the MCL tears, the inner side of the knee becomes painful, and you'll feel that your knee is not stable anymore and wants to give in. Depending on how serious it is, the injury can be fixed by physical therapy and stopping the activity that caused the injury. More serious cases require surgery.
Another effective way to reduce the loading on your knee is to get a brace, such as the FitXpert or the SB SOX knee brace. For both type of injuries icing the knee can help reduce the initial swelling and pain. Healthline has great articles on the MCL and ACL injuries.
Without a clear injury painful clicking can have four typical causes: cartilage or meniscus tear, arthritis, runner's knee (also known as chondromalacia patella or strength imbalance) and synovial plicae.
Runner's knee bears many names, however what's behind it is the same. As you move your knee, the patella moves out to the outer side of your knee, and you hear clicking or grinding sound as this happens. The patella can move towards the inner side too, however this is rare. The causes for this disorder are a combination of imbalanced thigh muscles or tendons, or bone malformations which let the patella slide out of it's groove. This problem can usually be fixed by strengthening the muscle group on the opposite side of the knee - usually the teardrop shaped vastus medialis muscle is not strong enough and lets the patella slide outwards. Using a knee brace, or a simple knee strap can help too.
Meniscus tear happens when one of the menisci between the thigh and shin bone tears. The menisci help to distribute the body weight and the forces evenly within the knee. Without these organs the knee would get damaged early on and you would not be able to use them due the the pain and cracking.
As the menisci tears parts of it can get caught up in the knee joint, and they can produce pain and clicking sounds as you move your knee. You may also feel that your knee is locked, as it becomes harder to move.
It's fairly easy to tear the meniscus, and sports which require quick stops or turns such as soccer or basketball, or even simple squatting can damage them.
Meniscus tears often require a small operation, the surgeon can trim away the damaged parts of the meniscus using just a small incision on your knee.
Warming up before exercising, and using proper shoes and maybe a knee brace can help prevent meniscus tears.
Arthritis and osteoarthritis in particular is a common cause of painfully clicking knees. Joint malformation and injury are major factors for this disorder. Osteoarthritis can lead to pain and stiffness in the knee as the cartilage surrounding the knee gets damaged. The cartilage is a though tissue, however it tends to wear down with age, and this causes a painful bone-on-bone contact in the knee or other joints.
Osteoarthritis is usually diagnosed using X-ray or MRI.
While osteoarthritis can't be cured, light exercise, wearing a brace and using anti-pain medication leads to improvements.
Mayo Clinic has a great overview on osteoarthritis.
Plica syndrome is another cause for knee clicking when straightening your leg, or when bending it. It happens when the synovial plicae, a leftover from fetal development, gets inflamed from overuse or an injury.
Plica syndrome leads to cracking and clicking sounds in the pain, feeling of instability and you may feel that your knee does not want to extend after sitting for long time.
Plica syndrome is rarely serious and it's easy to correct with physiotherapy. The best exercises for this disorder are front thigh muscle strengthening (walking, biking, half-squats) and hamstring stretches, which help relax the muscles on the back of your thighs.
While knee clicking can have many causes, the patellar tracking disorder deserves more attention as it's the most common one, and you can get rid of this problem with simple, at-home exercises. Most of these exercises require only very little equipment, and help you relax your and balance your muscles.
The outer thigh muscles are stronger in some people than the inner thigh muscles. This could cause the muscles pulling your patellas towards the inner sides of the knees. Strengthening the outer muscles fixes this issue.
Use a medium-strength resistance band and pull it up to just bellow your knees. Lower yourself into a half-squat if you can. Move two steps left then two steps back right, work against the resistance band as hard as you can. Do three one-minute sets every other day. Youtube has quite a few helpful videos on this.
For some people the inner thigh muscles are weaker than the outer thigh muscles. This could pull the patellas towards the outer side of the knees. To compensate strengthen your inner thigh muscles.
Stand with your feet pointing outwards at 45 degrees. Step a bit outwards so the distance between your feet is about the same as the width of your shoulder. Slowly squat down as if you'd sit on a chair, and keep your knees pointing outwards. Go low so your thighs become level, but not lower. Repeat this exercise every other day, do three sets of 15 repetitions. If unsure how to do this exercise properly, check the videos on Youtube.
The vastus medialis oblique is the small teardrop-shaped muscle at the inner side of the knee. This little muscle is often not strong enough, so the outer thigh muscles can pull the patella towards the outer side of the knee. Strengthening these small muscles will make this issue disappear.
Stand with one of your legs a bit behind the other as if you wanted to make a large step. Slowly go down to a half-squat, keep your weight on the front leg. Keep your front knee right above your front ankle. While squatting twist your front leg towards the outer side, and twist it back while standing back up. Three sets of 15 reps for both legs every other day will help you strengthen your VMO. The get more ideas on how to strengthen your VMO, check youtube.
The iliotibial band connects the hip bone to the shin bone, as it runs along the outer side your thigh. The IT band stabilizes the knee during walking and running. If it's too tight from overuse it can pull your patellas towards the outer sides.
Lie down on the side you'd like to relax with a foam roller at the side of your thigh. The foam roller should be placed at the middle of your thigh. Slide up and down over the roller so that it moves between the top of your knee and your hip. Carefully work over the more tender areas using smaller movements. Slowly bending and straigtening your knee while the roller is on a tender area works well too. Do two sets of 30 seconds every other day. You can find great IT band release videos on Youtube.
The hip flexor muscles pull indirectly on the patella too. If these muscles tighten up the stress on your patella increases. Use simple release to fix this issue.
Use a large-sized spikey massage ball (such as this). Bend your knee a bit and lie down forward, holding yourself on your elbows. Place the ball at one of your hip flexors, where your thigh muscles connect to your hip. Let some weight on the ball and move around in circular fashion. Repeat twice, use one minute sessions. You can find hip flexor releas videos on youtube.
If your calf muscles are too tight, your body will compensate by pulling on the thigh muscles. This in turn can overload your patella. Relaxing your thigh muscles will make your knee feel lighter too.
Sit down with your legs crossed over a foam roller . Place the roller at the middle of your calf. Roll yourself forward and backward over the foam. If you found a tender spot, work it using small movements. Repeat this relaxing exercise twice every other day. See helpful videos on youtube.