The knee is a very complex joint held together by many ligaments and connective tissue surrounding the joint capsule and tendons. The specific injury to the joint can be very difficult to discern at first. There are several reasons for this.
First of all, if the patient has any amount of swelling or pain, the joint stiffens up. Muscle tension and guarding occurring around the joint prevent movement. This can make it seem as though the joint is still very stable, when in fact, it's not.
Second, sometimes ligaments are damaged but not ruptured. Further episodes of unexpected forces can result in further injury. So what appears as a strain or sprain can progress to a full rupture with continued use.
And for some injuries, more than one soft tissue structure is disrupted. If the correct tests aren't applied accurately, the examiner may see the knee as normal when it's not. This is a false-negative test.
Most of the tests used to assess the knee joint are hands-on, applied by the examiner. Correct technique and interpretation are needed for the diagnosis. The examiner can be misled if proper positioning isn't used. If the muscles around the joint are contracting during the test, instability may be masked or hidden.