The compressive stiffness of human pediatric heads

Abstract

Head injury is a persistent and costly problem for both children and adults. Globally, approximately 10 million people are hospitalized each year for head injuries. Knowing the structural properties of the head is important for modeling the response of the head in impact, and for providing insights into mechanisms of head injury. Hence, the goal of this study was to measure the sub-injurious structural stiffness of whole pediatric heads. 12 cadaveric pediatric (20-week-gestation to 16 years old) heads were tested in a battery of viscoelastic compression tests. The heads were compressed in both the lateral and anterior–posterior directions to 5% of gauge length at normalized deformation rates of 0.0005/s, 0.01/s, 0.1/s, and 0.3/s. Because of the non-linear nature of the response, linear regression models were used to calculate toe region (<2.5%) and elastic region (>2.5%) stiffness separately so that meaningful comparisons could be made across rate, age, and direction. The results showed that age was the dominant factor in predicting the structural stiffness of the human head. A large and statistically significant increase in the stiffness of both the toe region and the elastic region was observed with increasing age (p<0.0001), but no significant difference was seen across direction or normalized deformation rate. The stiffness of the elastic region increased from as low as 5 N/mm in the neonate to >4500 N/mm in the 16 year old. The changes in stiffness with age may be attributed to the disappearance of soft sutures and the thickening of skull bones with age.

Introduction

Head injury is a persistent and costly problem. Globally, approximately 10 million people are hospitalized each year for head injuries—in the United States alone, 1.1 million people are hospitalized annually (Langlois et al., 2006b, Murray and Lopez, 1996). Children and young adults under the age of 20 suffer the majority of these injuries (Langlois et al., 2006a). It is estimated that 3% of all children will experience a brain injury by the time they turn 16 years old (Kraus, 1995). Despite this, there is very limited quantitative data on the structural properties of the pediatric head (Prange et al., 2004). Knowing the head׳s structural properties is essential for understanding its response to loads and, consequently, the mechanical exposures for which skull anatomical changes with age are based (Lapeer and Prager, 2001, Prange et al., 2004).

In previous work on adult heads, McElhaney conducted quasi-static (~0.0005/s) compression failure tests on 23 specimens. He found that the average peak force at fracture was 5115 N and 5954 N in the lateral and anterior–posterior directions, respectively, with compression strains of ~2% of head length and ~4% of head width (McElhaney, 1976, McElhaney et al., 1972). Additionally, head stiffness was observed to be nonlinear (McElhaney, 1976, McElhaney et al., 1972). Yoganandan et al. (1995) studied adult head compression using a hemispherical anvil while the lower portion of the head was rigidly fixed (Yoganandan et al., 1995). The heads were loaded locally with direct contact to the vertex, temporal, occiput, and frontal bones at rates of 0.0025 m/s and at 7.1–8.0 m/s. This experiment showed that head stiffness and fracture force increased with deformation rate, while the deflection to failure decreased with deformation rate (Yoganandan et al., 1995). To date, no study has evaluated how head stiffness changes with age or evaluated the stiffness of heads between the ages of 1 month and 18 years old.

The goal of this study was to quantify the structural stiffness of human pediatric heads from birth through age 16. The variables examined include age, compression direction, compression amount, and normalized deformation rate. These data are valuable for understanding the compressive response of the pediatric head, and are useful in developing computational models, anthropomorphic test devices (ATDs), and scaling relationships.