There are 5 types of plicae in the knee. The two most well-known plicae are the mediopatellar (medial) plica placed on the inner site of the kneecap and the other one is the infrapatellar plica (ligamentum mucosum) placed in the front of the knee. All the five plicae have no known functions, but they are residuals from the fetal life.
An impact on your knee or a wrong use, can cause your plica to get inflamed and then it gets swollen. The medial plica are known to catch between the kneecap and the femoral condyle and this typically causes symptoms of inner site pain and catching. The Infrapatellar plica is attached to the Hoffa Fad and moves significantly when your knee straightens and bends. Sometimes scar tissue in the infrapatellar plica can cause anterior knee pain based upon scar tissue in the plica makes it shorten up.
First of all the treatment focus rest and anti-inflammatory medicine (NSAID or steroids). When your plica has restored it self, a physiotherapist can help by restoring balance and correct movement patterns to prevent recurrence. Sometimes those treatments are not sufficient and your plicae may have to be taken care of by arthroscopic surgery. Here it is very easy to remove. Some call the Infrapatellar plica problems arthrofibrosis and call the procedure arthroscopic removal of the plica - anterior interval release. Personally I do not think this is arthrofibrosis, which is a much more serious condition giving you a stiff knee.
The three other plicae are the Suprapatellare plica that occasionally can give rise to pain above the patella in the Quadriceps tendon. The Laterale plica, that can cause outer site pain. And finally there is a plica that is localized in the rear of the knee and act as valve in front of a Baker cyst.