On the other hand, true "popping out of place" or catching in the knee after an injury can indicate an entirely separate problem. It's important to know what symptoms warrant a trip to the doctor. Two common injuries that can lead to these sensations include kneecap (patella) dislocations and meniscus tears.
Knee cap dislocations indicate that the patella moves abnormally out of its groove, usually to the lateral or outside of the knee. This is often associated with an injury such as twisting or planting the knee awkwardly. Most patients who experience this injury describe severe pain and an abnormality in the way the knee looks - the knee cap points to the left or right instead of straight ahead. On occasion, this dislocation will go back "into place" on its own. Many times though, this requires a push - sometimes even by a doctor in the emergency room. Associated symptoms include bruising, swelling and pain around the knee cap.
This type of injury should be addressed by your doctor or surgeon to make sure that the bones or cartilage did not break during the injury. Some of the these injuries become chronic and feelings of instability can persist for years - wearing a brace and working with a physical therapist can be helpful in many situations.
The meniscus is the soft cartilage "cushion" located in the knee joint. It is a true shock absorber that is important in preventing arthritis later in life. Unfortunately, it also is one of the most commonly injured structures in the knee - often after a twisting injury or some other trauma. Later in life, these injuries can occur with prolonged simple activities like kneeling or running. The classic symptoms of a large meniscus tear include catching, locking, and giving way or buckling sensations in the knee. Swelling, pain, and stiffness are also common. Treatment is personalized to each patient, but true mechanical feelings in the knee attributed to a meniscus tear are often treated surgically.
In general, any injury associated with pain, swelling, and "popping out of place" should be addressed by your physician.