A helmet would not have stopped the entirety of the injury, but would have significantly reduced the penetration depth and brain insult.
Did you repeat the crash as a control?
Surgeons are more cautious about what difference a helmet might have made when they give evidence under oath.
http://www.cyclehelmets.org/1054.html
A friend of mine died from a head inury after falling on a stone staircase. My thoughts after that were not that a helmet might have saved him but that it was a pity he wore flip flops while carrying a box that obscured his vision.
There were 4 helmeted cyclist killed in this one accident. The fact is if a car hits you at speed there is a risk of death helmet or not.
http://news.bbc.co.uk/1/hi/wales/north_west/4592412.stm
Avoiding crashes is the important thing but I see helmeted cyclists riding in the doorzone, filtering at speed through traffic queues etc.