The possible list of causes for feelings of instability is long and runs the gamut from ligament injuries to loose pieces of cartilage in the knee to kneecap problems. A thorough history and physical exam can limit this list to one or two possible diagnoses though.
When a sudden event such as a skiing accident or sports twisting maneuver occurs, the possibility of a ligament injury to the anterior cruciate ligament (ACL) needs to be considered. A "pop" sound or sensation, coupled with immediate swelling and the inability to walk, are also associated with this type of problem. Alternatively, injuries to the shock absorber or pad in the knee called the meniscus can also cause these loose feelings, particularly if the tear causes the meniscus to be mobile and move in the knee joint.
If there is no recent event that caused the problem, then chronic potential diagnoses need to be considered. Loose fragments of cartilage can intermittently cause a "catch" in the knee and lead to a different type of "giving way" sensation. Patients describe a normal knee most of the time and then a sudden sharp catch or "locking" sensation if the piece moves and gets stuck. This is often a surgical problem, as the piece needs to be taken out for the symptoms to resolve.
More commonly, irritation of the front of the knee joint, called the patellofemoral region, can cause a sudden relaxation of the quadriceps (thigh) muscle, leading to a similar "giving way" sensation. This is very common. It is sometimes associated with an abnormal alignment of the knee and leg and often is seen with a poorly toned muscle. The one good thing about kneecap issues is that they almost always are treated without surgery.
An evaluation by an orthopedic specialist is often necessary in the setting of an unstable knee, particularly if the symptoms occur with an acute injury or if they don't get better with time.
If the history and exam findings are inconclusive, an MRI can be helpful in almost all of these situations.