Abstract
Four cases in which properly fastened, approved helmets became detached during an accident are described. In three cases fatal injuries resulted. The mechanism of the helmet coming off was as follows: the fixing bolts of the chin strap were near the base and the front of the helmet. This allowed the helmet to pivot forward on the strap, the pivot point lying anterior to the angle of the mandible. This is more likely to happen if the rider has a flattened occiput. When this was tested on a random group of 14 motorcyclists wearing apparently well fitting, comfortable and properly fastened helmets, four of the helmets were removed by pushing from behind.