In a complete rupture of a knee ligament a loud pop or snap may be heard, pain is often sudden and severe, with swelling, a reduced range of movement, a feeling or instability of the joint and an inability to put weight on the point without pain.

In the case of a mild sprain the symptoms are much milder, there may be some discomfort along the joint line on the side of the knee that was over stretched, mild swelling or no swelling, movement and stability of the knee may not be impeded.

If there is a partial tear the swelling and pain may be worse, and the knee may feel slightly loose and unstable. 

There are four ligaments that help to stabilize the knee joint, injuries to the ligaments most often occur when playing football, skiing, gymnastics and other athletes.  

The mechanism of injury is usually a sudden twisting movement while the foot is planted on the ground, a direct blow to the knee, over extension of the knee, suddenly stopping whilst running.  The severity of the injury may be mild (partial tear or sprain) or severe (complete rupture).