Effect of Loading Rate on Compressive Failure Mechanics of the Pediatric Cervical Spine

This study investigated the effect of loading rate on the compressive failure mechanics of the pediatric cervical spine, using baboons of a controlled age group as a human surrogate. Cervical spines were obtained from 12 male baboons (9 ± 1 human equivalent years) and dissected into 35 2-FSU segments. All specimens underwent cyclic preconditioning to 100 N for 50 cycles at 1 Hz prior to failure testing at displacement rates of 5, 50, 500, or 5,000 mm/sec. Load-displacement curves were plotted and analyzed to determine stiffness, failure load, and displacement at failure for the various loading rates. Stiffness showed a significant increase as loading rate was increased, with mean stiffness increasing 30 % between rates of 5 and 5,000 mm/sec. Failure load showed an increasing, though not statistically significant, trend with increasing loading rate, while displacement at failure showed no rate dependence. The results of this research may be useful for developing better pediatric automotive safety standards, improved biofidelic test dummies, and more accurate computational models. INTRODUCTION o date, the effort towards prevention of pediatric cervical spine injuries has been hindered by a lack of data on pediatric spine mechanics. Although pediatric injuries to the cervical spine are not particularly common, they may be associated with significant rates of mortality and disability (Brown et al., 2001). To reduce such injuries, various crash-test dummies, computational models, and safety standards have been developed by scaling adult injury tolerance data to children. While animal models have been used to establish pediatric tensile (Ching et al., 2001, Pintar et al., 2000) and compressive (Nuckley et al, 2002a) scaling factors, the viscoelastic behavior of biologic tissues requires that loading rate be taken into account for a thorough understanding of cervical spine T Injury Biomechanics Research 186 mechanical properties. As such, this study aims to contribute data on the compressive failure mechanics of the cervical spine with an emphasis on the effect of loading rate. The adult human cervical spine has been widely studied. Several groups have used full cervical spine preparations to investigate mechanical properties in compression (Panjabi et al., 1998, Pintar et al., 1998, Nightingale et al., 1996, Pintar et al., 1995a). Compressive tolerances and functional properties have been established, and the effect of loading rate has been documented. Pintar et al. (1995) reported a force at failure of 3.8 kN for adult males, and later showed significantly increasing mechanical properties with loading rate (1998). Full cervical spine preparations, however, make it difficult to apply a purely compressive load and to measure properties for different segments of the cervical spine. Individual motion segments have also been used to study the adult cervical spine. Several studies have tested segments consisting of 2 function spinal units because they allow failure in the hard or soft tissues without buckling (Nuckley et al., 2002a, Carter et al., 2000, Zhu et al., 1999, Yingling et al., 1997, Pintar et al., 1995b). Of these studies, only Yingling et al. attempted to determine the effect of loading rate over a wide range of rates. Using a porcine model, Yingling et al. tested cervical spine segments at rates ranging from 100 to 16,000 N/s. However, the highest rate tested corresponded to a displacement rate of not more than 50 mm/sec, which is likely lower than the rates experienced in severe car crashes. Additionally, their data sampling rate of 50 Hz may have been inadequate for the higher rates. A baboon model has been chosen because of both the demonstrated similarities between the baboon and human cervical spine (Tominaga et al., 1995, Swindler and Wood, 1973), and the scarcity of available healthy human pediatric tissues. Although baboons exhibit smaller size, thinner pedicles, and non-bifid spinous processes relative to humans, the geometry, anatomy, and mechanical properties compare very well overall. These characteristics, along with its phylogenetic proximity to humans, have made the baboon model an excellent human surrogate. This study aims to test pediatric baboon cervical spine segments over a range of loading rates up to 5,000 mm/sec. With loading rates and sampling rates more than two orders of magnitude greater than Yingling et al, these tests will allow measurements in a scenario more similar to an actual car crash. This data on the compressive failure mechanics of the pediatric cervical spine may be useful in the development of better pediatric safety standards, computational models, and crash-test dummies.