I saw that with the supposed x-rays of the area. I don't know the forehead is one of the strongest areas of bone in the body from what I understand. Kind of weird that much damage was done to the forehead and nothing at all happened to the knee of the other guy. You would think the knee cap would have broke as well.
Did some Google-Fu and found it takes 1900 lbs to break the bone in the forehead for an average person. But another source says 30 Lbs per sq in. I would think unless he had some deficiency in his bones due to age or disease that the knee of the striker should be messed up as well.
With all that being said that injury was something nasty and I hope he heals up quick. Reminds me of Koscheck's injuries when he broke his orbital bones.