This evening my husband was just walking down the hallway and felt a snap in the back of his right knee. Immediately the vein in the back of his knee became swollen and the skin became bruised. this was several hours ago and the swelling has gone down but the bruising looks a bit worse. He is complaining of tenderness to the touch and soreness when sitting or bending at the knees. What could this be? He was not doing any abnormal activity - just simply walking from the restroom to the kitchen!