Lock knee is a generic term used to describe the inability to either flex or extend the knee. The normal flexion of the knee is about 135 degrees. The normal straightening (extension) of the knee is a fully locked position or zero degrees.
There are two primary causes of the inability to fully flex or extend the knee.
First, severe pain will cause a person to have a locked knee no matter what the cause. The pain is so severe that one can no longer use the knee medical help should be sought out immediately.
The second cause of locked knee is mechanical block of the normal knee motion. If there is something caught in the mechanism of the knee such as a large piece of torn cartilage the knee will not be able to flex or extend which will cause the knee to “lock up” until the blockage is removed.