The styrofoam that is in these helmets should not stay compressed. It should bounce back, as its not actually getting really compressed. The main thing is that the looseness of the particles in the styrofoam (the reason its so light also)dissipate the energy from the blow. Kinda absorb alot of the energy from the impact. If you bite it hard enough to permanently damage the helmet you would be messed up pretty bad. I wouldn't worry about replacing it unless it had visible damage (cracks, etc).