It is widely accepted that a bicycle helmet will in many cases prevent or reduce skull fractures and superficial head injuries. Less clear is how effective helmets are at preventing injuries to the brain.
Brain injuries can be either focal or diffuse.
It is the diffuse brain injuries which are the most serious. It is erroneous to associate skull fracture directly with brain injury. Obviously in an impact severe enough to cause a skull fracture, other forces are likely to be present. Holbourn used two-dimensional jelly moulds to compare the effects of these two forces. It was possible to see the damage caused to the jelly, and compare that with the autopsies of actual head injury victims.