Experimental determination of adult and pediatric neck scale factors

The purpose of this study was to determine scale factors for small, mid-size and large adults using a caprine model. In a previous study conducted in our lab, scaling relationships were developed to define cervical spine tolerance values of children using caprine specimens. In that study, tolerances were normalized with respect to an average adult. Because airbag-related injuries are associated with out-of-position children and small adult females, additional experimental data are needed to better estimate human tolerance. In the present study, cervical spine radiographs from the 5th, 50th and 95th percentile human adults were used to determine vertebral body heights for small, mid-size and large anthropometries. Mean human vertebral body heights were computed for each anthropometry and were normalized with respect to mid-size anthropometry. Similar measurements were calculated from caprine cervical spine radiographs and each caprine specimen was grouped into one of the three categories based upon vertebral body size. Seventy-two motion segments (OC-C2, C3-C4, C5-C6 and C7-T1) from 18 adult caprine cadavers were subjected to pure moment and distraction loads. Pure moment testing resulted in bending stiffness, and distraction testing resulted in failure force and linear stiffness. Data were normalized with respect to the mid-size anthropometric category. For the small, mid-size and large adult categories, tensile failure force yielded scaling ratios of 0.74, 1.00 and 1.13, linear stiffness yielded ratios of 0.78, 1.00 and 1.10 and bending stiffness resulted in ratios of 0.89, 1.00 and 1.03. For the one-year-old, three-year-old, six-year-old and 12-year-old, scaling ratios were 0.10, 0.16, 0.30 and 0.62 for the tension force, 0.13, 0.18, 0.38 and 0.66 for the linear stiffness and 0.13, 0.19, 0.42 and 0.76 for the bending stiffness. These scale factors are compared with FMVSS 208.