The symptom of a knee giving out is most often due to a ligament injury. The knee is held together by ligaments, and the sensation of instability, or the knee giving out, is often dues to a tear in one of the knee ligaments.
Injuries to one or multiple ligaments can lead to the two sides of the joint not being held tightly enough in position. Usually symptoms of instability occur with twisting or side-to-side movements. This may occur in sports activities, or may occur with simple tasks such as twisting your knee to get in and out of a car.
Can other injuries cause instability?
Yes. It is common for people to experience instability symptoms with injuries that cause knee pain. Often the body protects itself from pain with involuntary movements. This may 'feel' as though the knee wants to give out, causing a sensation of instability, but it is not due to a ligament injury as described above.
The key difference between true ligamentous instability and a sensation of instability can be detected on examination where your doctor can test the stability of each ligament and determine if it is functioning properly.