Dynamic Bending Mechanics of the Pediatric Cervical Spine

While pediatric cervical spine injuries are not extremely common, when they occur, the life-long debilitating consequences for the child, their family, and society are catastrophic. In order to mitigate these injuries to children, we must understand the mechanics of the child neck and the inputs which create these deleterious injuries. Automotive crashes represent a significant percentage of these injuries and often involve high rates of speed and inertial loading of the head and neck complex. Thus, in an effort to prevent child neck injuries, we set out to investigate the bending mechanics of the maturing cervical spine as a result of dynamic loading. Sixteen baboon cadaver specimens were utilized spanning the pediatric populace from 2 to 23-human equivalent years. The C5-C6 functional spinal units of these specimen were dissected free and rigidly fixed to a custom dynamic bending apparatus. This device applied dynamic angular displacements(mean of 27.6-rad/sec) to the superior vertebrae while minimizing the shear forces traveling through the specimen. The specimens were divided into a flexion and extension group for testing to failure and the loads, displacements, and accelerations of the event were recorded. The flexion and extension stiffness increased with maturation as did the failure moments for both flexion and extension. Further, these data were utilized to generate scaling from the child to adult for spinal mechanics. The raw data and these scaling values provide data for computational models and anthropomorphic test devices which may lead to a meaningful neck injury prevention scheme for children. INTRODUCTION lthough cervical spine injuries in children account for less than 10% of all cervical injuries, the fatalities resulting from cervical trauma to children are four times the rate of adult fatalities (Myers and Winkelstein, 1995). A large percentage of these injuries are a result of motor vehicle accidents which induce dynamic bending of the cervical spine. Unfortunately, the mechanical response of the pediatric cervical spine to dynamic bending inputs is not well understood, making prevention of these injuries unfeasible. A Injury Biomechanics Research 12 Previous research has been performed to understand the response of adult cervical spine tissues to pure bending moments both quasi-statically and dynamically. A number of studies have investigated the quasi-static bending range of motion of the adult cervical spine (Dvorak et al., 1992; Lind et al., 1989; White and Panjabi, 1990; Panjabi et al., 2001); however, only a few studies have investigated the stiffness and failure characteristics (Nightingale et al., 2002; Voo et al, 1998). Nightingale et al. (2002) applied pure bending moments quasi-statically to both the upper and lower cervical spine segments. They found the failure moment to be significantly greater for extension compared with flexion of the upper cervical spine. Further, when comparing the upper and lower cervical spine, they measured significantly larger extension failure moments in the upper cervical spine. This result which is not consistent with epidemiological data, they explained, was likely due to active musculature load sharing. In a study by Voo et al. (1998), dynamic bending moments (18 to 35-rad/sec) were applied to cervical spine tissues and their stiffness values were compared with quasi-static tests performed on the same tissues. They discovered that the dynamic stiffness was statistically greater than the quasi-static stiffness. Therefore, data exists for the adult populace in quasi-static and dynamic loading rates for range of motion and failure experiments. An examination of the pediatric literature reveals but one study examining the bending mechanics of the cervical spine (Pintar et al., 2000). Pintar et al. (2000) utilized a goat model to examine the effects of maturation on the stiffness of isolated functional spinal units in flexion and extension. These quasi-static experiments provided the first data to estimate the mechanics of the immature spine in bending. In an effort to supplement this data set for child injury prevention, this research project was initiated to investigate the dynamic stiffness and failure mechanics of maturing tissues. Therefore, the objective of this study is to examine the pediatric cervical spine dynamic bending mechanics. These data will enable accurate child neck injury prediction in computational models and anthropomorphic test devices (ATD). To our knowledge, this is the first study examining the dynamic bending mechanics in the pediatric population.