A dislocated knee is an extremely serious knee injury in which the thigh bone and shin bone completely lose contact with each other. It is not the same as a patellar dislocation, also called a “kneecap dislocation,” in which the kneecap slips out of its groove at the end of the thigh bone.  Symptoms. Symptoms of kneecap dislocation include: 1  Tenderness and swelling on and around your knee. 2  Discomfort when you are active. 3  Pain over the edges of the kneecap. 4  Collapsing or giving-way of the knee that sometimes causes you to fall. Causes and Risk Factors. It is common for a kneecap dislocation to occur after a high-impact injury. After the first time, the kneecap is more likely to dislocate again. Young people, such as adolescents who are more active, are at a greater risk of having a dislocated kneecap. Treatment of Knee Dislocation : If you have dislocated your knee for the first time, then doctors will splint the leg. Sometimes, the kneecap settles down in grove by straightening the leg and if this not happens, then doctors will use the method of reduction to manipulate the kneecap back to its original place.  Patella dislocation explained. The patella, or kneecap lies in front of the knee joint. It glides up and down a groove called the patellofemoral groove at the front of the thigh bone or femur as the knee bends. The patella normally lies within the patellofemoral groove and is only designed to slide vertically within it. Dislocation of the patella occurs when the patella moves or is moved to the outside of this groove and onto the bony head of the femur (lateral femoral condyle). A knee dislocation occurs when the bones that form your knee are out of place. The bones of your calf (the tibia and fibula) get moved in relation to the bone of your thigh (femur). The bones of your knee are held together by strong bands of tissue, called ligaments. Knee dislocations are rare. They usually happen only after major trauma, which could result from falls, car crashes, and other high-speed injuries. 1  If the knee is dislocated, it will look deformed. With a knee dislocation, You'll feel a lot of pain in the knee. Sometimes, there will be no feeling below the knee. If the knee relocates, it will become swollen. You may not be able to feel a pulse (your heartbeat) in your foot. Knee dislocations are rare. They usually happen only after major trauma, which could result from falls, car crashes, and other high-speed injuries. 1  If the knee is dislocated, it will look deformed. Patellar dislocation is an injury of the knee, typically caused by a direct blow or a sudden twist of the leg. It occurs when the patella (kneecap) slips out of its normal position in the patellofemoral groove, and generally causes intense pain with swelling of the knee.  Your child's kneecap (patella) is usually right where it should be—resting in a groove at the end of the thighbone (femur). When the knee bends and straightens, the patella moves straight up and down within the groove. Sometimes, the patella slides too far to one side or the other. When this occurs — such as after a hard blow or fall — the patella can completely or partially dislocate. When the patella slips out of place — whether a partial or complete dislocation — it typically causes pain and loss of function. Even if the patella slips back into place by itself, it will still require treatment to relieve painful symptoms.  A partially dislocated kneecap can be the result of a previous dislocation or an inherited tendency for the kneecap to slide to the outside. There could be tenderness under the kneecap and pain when the person twists the knee or climbs stairs. A feeling of the knee’s instability is a common complaint.  Your health care provider will examine the knee, which could confirm that the kneecap is dislocated. A knee x-ray and, sometimes, MRIs should be done to make sure that the dislocation did not cause a bone to break or cartilage to be damaged. 
http://www.theknee.com/knee-pain-causes/dislocated-knee/ https://www.cedars-sinai.edu/Patients/Health-Conditions/Kneecap-Patellar-Dislocation.aspx http://kneeortho.org/kneecap-dislocation-patella-exercises-treatment/ http://www.sportsinjuryclinic.net/sport-injuries/knee-pain/patellar-dislocation http://www.webmd.com/pain-management/knee-pain/knee-dislocation?print=true http://www.webmd.com/pain-management/knee-pain/knee-dislocation?print=true https://en.wikipedia.org/wiki/Patellar_dislocation http://orthoinfo.aaos.org/topic.cfm?topic=A00707 http://www.coreperformance.com/knowledge/injury-pain/dislocated-kneecap.html http://www.nytimes.com/health/guides/injury/knee-cap-dislocation/overview.html
what causes a dislocated kneecap