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Study Design: Biomechanical tensile testing of perinatal, neonatal, and pediatric cadaveric cervical spines to failure., Objective: To assess the tensile failure properties of the cervical spine from birth to adulthood., Summary of Background Data: Pediatric cervical spine biomechanical studies have been few due to the limited availability of pediatric cadavers. Therefore, scaled data based on human adult and juvenile animal studies have been used to augment the limited pediatric cadaver data. Despite these efforts, substantial uncertainty remains in our understanding of pediatric cervical spine biomechanics., Methods: A total of 24 cadaveric osteoligamentous head-neck complexes, 20 weeks gestation to 18 years, were sectioned into segments (occiput-C2 [O-C2], C4-C5, and C6-C7) and tested in tension to determine axial stiffness, displacement at failure, and load-to-failure., Results: Tensile stiffness-to-failure (N/mm) increased by age (O-C2: 23-fold, neonate: 22 ± 7, 18 yr: 504; C4-C5: 7-fold, neonate: 71 ± 14, 18 yr: 509; C6-C7: 7-fold, neonate: 64 ± 17, 18 yr: 456). Load-to-failure (N) increased by age (O-C2: 13-fold, neonate: 228 ± 40, 18 yr: 2888; C4-C5: 9-fold, neonate: 207 ± 63, 18 yr: 1831; C6-C7: 10-fold, neonate: 174 ± 41, 18 yr: 1720). Normalized displacement at failure (mm/mm) decreased by age (O-C2: 6-fold, neonate: 0.34 ± 0.076, 18 yr: 0.059; C4-C5: 3-fold, neonate: 0.092 ± 0.015, 18 yr: 0.035; C6-C7: 2-fold, neonate: 0.088 ± 0.019, 18 yr: 0.037)., Conclusion: Cervical spine tensile stiffness-to-failure and load-to-failure increased nonlinearly, whereas normalized displacement at failure decreased nonlinearly, from birth to adulthood. Pronounced ligamentous laxity observed at younger ages in the O-C2 segment quantitatively supports the prevalence of spinal cord injury without radiographic abnormality in the pediatric population. This study provides important and previously unavailable data for validating pediatric cervical spine models, for evaluating current scaling techniques and animal surrogate models, and for the development of more biofidelic pediatric crash test dummies.

Study Design: Survival analyses of a large cohort of published lumbar spine compression fatigue tests., Objective: To produce the first large-scale evaluation of human lumbar spine tolerance to repetitive compressive loading and to evaluate and improve guidelines for human exposure to whole-body vibration and repeated mechanical shock environments., Summary of Background Data: Several studies have examined the effects of compressive cyclic loading on the lumbar spine. However, no previous effort has coalesced these studies and produced an injury risk analysis with an expanded sample size. Guidelines have been developed for exposure limits to repetitive loading (e.g., ISO 2631-5), but there has been no large-scale verification of the standard against experimental data., Methods: Survival analyses were performed using the results of 77 male and 28 female cadaveric spinal segment fatigue tests from 6 previously published studies. Segments were fixed at each end and exposed to axial cyclic compression. The effects of the number of cycles, load amplitude, sex, and age were examined through the use of survival analyses., Results: Number of cycles, load amplitude, sex, and age all are significant factors in the likelihood of bony failure in the spinal column. Using a modification of the risk prediction parameter from ISO 2631-5, an injury risk model was developed, which relates risk of vertebral failure to repeated compressive loading. The model predicts lifetime risks less than 7% for industrial machinery exposure from axial compression alone. There was a 38% risk for a high-speed planing craft operator, consistent with epidemiological evidence., Conclusion: A spinal fatigue model which predicts the risk of in vitro lumbar spinal failure within a narrow confidence interval has been developed. Age and sex were found to have significant effects on fatigue strength, with sex differences extending beyond those accounted for by endplate area disparities.

Study Design: A uniaxial tensile loading study of 13 lumbar porcine ligaments under varying environmental temperature conditions., Objectives: To investigate a possible temperature dependence of the material behavior of porcine lumbar anterior longitudinal ligaments., Summary of Background Data: Temperature dependence of the mechanical material properties of ligament has not been conclusively established., Methods: The anterior longitudinal ligaments (ALLs) from domestic pigs (n = 5) were loaded in tension to 20% strain using a protocol that included fast ramp/hold and sinusoidal tests. These ligaments were tested at temperatures of 37.8 degrees C, 29.4 degrees C, 21.1 degrees C, 12.8 degrees C, and 4.4 degrees C. The temperatures were controlled to within 0.6 degrees C, and ligament hydration was maintained with a humidifier inside the test chamber and by spraying 0.9% saline onto the ligament. A viscoelastic model was used to characterize the force response of the ligaments., Results: The testing indicated that the ALL has strong temperature dependence. As temperature decreased, the peak forces increased for similar input peak strains and strain rates. The relaxation of the ligaments was similar at each temperature and showed only weak temperature dependence. Predicted behavior using the viscoelastic model compared well with the actual data (R2 values ranging from 0.89 to 0.99). A regression analysis performed on the viscoelastic model coefficients confirmed that relaxation coefficients were only weakly temperature dependent while the instantaneous elastic function coefficients were strongly temperature dependent., Conclusions: The experiment demonstrated that the viscoelastic mechanical response of the porcine ligament is dependent on the temperature at which it is tested; the force response of the ligament increased as the temperature decreased. This conclusion also applies to human ligaments owing to material and structural similarity. This result settles a controversy on the temperature dependence of ligament in the available literature. The ligament viscoelastic model shows a significant temperature dependence on the material properties; instantaneous elastic force was clearly temperature dependent while the relaxation response was only weakly temperature dependent. This result suggests that temperature dependence should be considered when testing ligaments and developing material models for in vivo force response, and further suggests that previously published material property values derived from room temperature testing may not adequately represent in vivo response. These findings have clinical relevance in the increased susceptibility of ligamentous injury in the cold and in assessing the mechanical behavior of cold extremities and extremities with limited vascular perfusion such as those of the elderly.

Study Design: The failure responses of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum were examined in vitro under large strain-rate mechanical loading., Objective: To quantify the failure properties for 3 cervical spinal ligaments at strain rates associated with traumatic events., Summary of Background Data: There exists little experimentation literature for fast-rate loading of the cervical spine ligaments. The small amount of available information is framed only in extensive experimental coordinates, and not in the context of strains., Methods: Bone-ligament-bone complexes were strained at fast rates, in an incrementally increasing loading protocol using a servohydraulic mechanical test frame. Failure loads and displacements were converted to engineering and true stress and strain values, and compared for the different ligaments (anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum), spinal levels (C3-C4, C5-C6, and C7-T1), and for male versus female specimens., Results: There were no significant differences in force or true stress for gender or spinal level. There was a significant difference in force and true stress for ligament type. A difference was found between the posterior longitudinal ligament and ligamentum flavum for failure force, and between the ligamentum flavum and both the anterior and posterior longitudinal ligaments for failure true stress. No significant differences were found in true strain for ligament, gender, or spinal level. The mean ligament failure true strain was 0.81. Failure true strains were approximately 57% of the failure engineering strains., Conclusions: Once the injury mechanisms of the cervical spine are fully understood, computational models can be employed to understand the potentially traumatic effects of clinical procedures, and mitigate injury in impact, falls, and other high-rate scenarios. The soft tissue failure properties in this study can be used to develop failure tolerances in fast-rate loading scenarios. Failure properties of the anterior and posterior longitudinal ligaments were similar, and the same properties can be used to model both ligaments.