Many knee injuries are fairly straightforward. A few tests and some follow-up imaging studies to confirm the diagnosis are all that's needed. Other injuries are not so clear. For example, if both ligaments inside the knee are torn, the doctor will need to look for different problems than if just one ligament is damaged.

Instability in the back and outside of the knee joint, called posterolateral instability, can be very difficult to recognize. This is especially true during the early phase of the injury. The kind of surgery needed to repair the knee depends on an accurate diagnosis. Thus, a large number of tests as well as X-rays or MRIs are often necessary.