Are headforms a poor surrogate for helmet fit?

International standards organizations require ice hockey helmets to be impact tested while mounted to a surrogate headform, with anthropometrics of a 50th percentile male. However, human head shapes are not identical, nor are there consistent guidelines for fitting a helmet to the ordinary user. The interaction between head shape and helmet fit impacts helmet safety: the contact area between a headform and helmet interior has been identified as a critical determinant of protection afforded by a helmet. The objective of this study was to compare quantitative measures of helmet fit between an adult male sample and three 50th percentile male headforms. This study recruited 42 adult male participants who wore a medium-sized ice hockey helmet (560-600 mm interior circumference) in an attempt to compare their quantitative helmet fit to those of three 50th percentile adult male headforms. Through three-dimensional modeling, fit was quantified by assessing dimensional differences in two transverse cross-sectional planes of the head and using principal component analysis to determine the largest components of fit. Significant differences were found between the headforms and the participants' heads in anthropometrics and dimensional differences. The headforms were smaller than the participants' heads, demonstrating average gapping with the interior of the helmet. The principal components of fit extracted included mediolateral deformation, gapping/compression at the rear aspect of the head-helmet interface, and general congruence of the head shape to the helmet liner. These findings demonstrated a vast discrepancy between helmet fit on the 50th percentile headforms and the ordinary helmet user.