Jason F. Luck

Comparative strengths and structural properties of the upper and lower cervical spine in flexion and extension

The purpose of this study is to test the hypothesis that the upper cervical spine is weaker than the lower cervical spine in pure flexion and extension bending, which may explain the propensity for upper cervical spine injuries in airbag deployments. An additional objective is to evaluate the relative strength and flexibility of the upper and lower cervical spine in an effort to better understand injury mechanisms, and to provide quantitative data on bending responses and failure modes. Pure moment flexibility and failure testing was conducted on 52 female spinal segments in a pure-moment test frame. The average moment at failure for the O-C2 segments was 23.7+/-3.4Nm for flexion and 43.3+/-9.3Nm for extension. The ligamentous upper cervical spine was significantly stronger in extension than in flexion (p=0.001). The upper cervical spine was significantly stronger than the lower cervical spine in extension. The relatively high strength of the upper cervical spine in tension and in extension is paradoxical given the large number of upper cervical spine injuries in out-of-position airbag deployments. This discrepancy is most likely due to load sharing by the active musculature.

Tension and Combined Tension-Extension Structural Response and Tolerance Properties of the Human Male Ligamentous Cervical Spine

Tensile loading of the human cervical spine results from noncontact inertial loading of the head as well as mandibular and craniofacial impacts. Current vehicle safety standards include a neck injury criterion based on beam theory that uses a linear combination of the normalized upper cervical axial force and sagittal plane moment. This study examines this criterion by imposing combined axial tension and bending to postmortem human subject (PMHS) ligamentous cervical spines. Tests were conducted on 20 unembalmed PMHSs. Nondestructive whole cervical spine tensile tests with varying cranial end condition and anteroposterior loading location were used to generate response corridors for computational model development and validation. The cervical spines were sectioned into three functional spinal segments (Occiput-C2, C4-C5, and C6-C7) for measurement of tensile structural response and failure testing. The upper cervical spine (Occiput-C2) was found to be significantly less stiff, absorb less strain energy, and fail at higher loads than the lower cervical spine (C4-C5 and C6-C7). Increasing the moment arm of the applied tensile load resulted in larger head rotations, larger moments, and significantly higher tensile ultimate strengths in the upper cervical spine. The strength of the upper cervical spine when loaded through the head center of gravity (2417+/-215 N) was greater than when loaded over the occipital condyles (2032+/-250 N), which is not predicted by beam theory. Beam theory predicts that increased tensile loading eccentricity results in decreased axial failure loads. Analyses of the force-deflection histories suggest that ligament loading in the upper cervical spine depends on the amount of head rotation orientation, which may explain why the neck is stronger in combined tension and extension.

Tensile failure properties of the perinatal, neonatal, and pediatric cadaveric cervical spine.

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.

Effect of the mandible on mouthguard measurements of head kinematics.

Wearable sensors are becoming increasingly popular for measuring head motions and detecting head impacts. Many sensors are worn on the skin or in headgear and can suffer from motion artifacts introduced by the compliance of soft tissue or decoupling of headgear from the skull. The instrumented mouthguard is designed to couple directly to the upper dentition, which is made of hard enamel and anchored in a bony socket by stiff ligaments. This gives the mouthguard superior coupling to the skull compared with other systems. However, multiple validation studies have yielded conflicting results with respect to the mouthguard׳s head kinematics measurement accuracy. Here, we demonstrate that imposing different constraints on the mandible (lower jaw) can alter mouthguard kinematic accuracy in dummy headform testing. In addition, post mortem human surrogate tests utilizing the worst-case unconstrained mandible condition yield 40% and 80% normalized root mean square error in angular velocity and angular acceleration respectively. These errors can be modeled using a simple spring-mass system in which the soft mouthguard material near the sensors acts as a spring and the mandible as a mass. However, the mouthguard can be designed to mitigate these disturbances by isolating sensors from mandible loads, improving accuracy to below 15% normalized root mean square error in all kinematic measures. Thus, while current mouthguards would suffer from measurement errors in the worst-case unconstrained mandible condition, future mouthguards should be designed to account for these disturbances and future validation testing should include unconstrained mandibles to ensure proper accuracy.

Bandwidth and sample rate requirements for wearable head impact sensors

Wearable inertial sensors measure human head impact kinematics important to the on-going development and validation of head injury criteria. However, sensor specifications have not been scientifically justified in the context of the anticipated field impact dynamics. The objective of our study is to determine the minimum bandwidth and sample rate required to capture the impact frequency response relevant to injury. We used high-bandwidth head impact data as ground-truth measurements, and investigated the attenuation of various injury criteria at lower bandwidths. Given a 10% attenuation threshold, we determined the minimum bandwidths required to study injury criteria based on skull kinematics and brain deformation in three different model systems: helmeted cadaver (no neck), unhelmeted cadaver (no neck), and helmeted dummy impacts (with neck). We found that higher bandwidths are required for unhelmeted impacts in general and for studying strain rate injury criteria. Minimum gyroscope bandwidths of 300Hz in helmeted sports and 500Hz in unhelmeted sports are necessary to study strain rate based injury criteria. A minimum accelerometer bandwidth of 500Hz in unhelmeted sports is necessary to study most injury criteria. Current devices typically sample at 1000Hz, with gyroscope bandwidths below 200Hz, which are not always sufficient according to these requirements. With hard contact test conditions, the identified requirements may be higher than most soft contacts on the field, but should be satisfied to capture the worst contact, and often higher risk, scenarios relative to the specific sport or activity. Our findings will help establish standard guidelines for sensor choice and design in traumatic brain injury research.

THE HUMAN CERVICAL SPINE IN TENSION: EFFECTS OF FRAME AND FIXATION COMPLIANCE ON STRUCTURAL RESPONSES

There is little data available on the responses of the human cervical spine to tensile loading. Such tests are mechanistically and technically challenging due to the variety of end conditions that need to be imposed and the difficulty of strong specimen fixation. As a result, spine specimens need to be tested using fairly complex, and potentially compliant, apparati in order to fully characterize the mechanical responses of each specimen. This, combined with the relatively high stiffness of human spine specimens, can result in errors in stiffness calculations. In this study, 18 specimen preparations were tested in tension. Tests were performed on whole cervical spines and on spine segments. On average, the linear stiffness of the segment preparations was 257 N/mm, and the stiffness of the whole cervical spine was 48 N/mm. The test frame was found to have a stiffness of 933 N/mm. Assembling a whole spine from a series combination of eight segments with a stiffness of 257 N/mm results in an estimated whole spine stiffness of 32.1 N/mm (32% error). The segment stiffnesses were corrected by assuming that the segment preparation stiffness is a series combination of the stiffnesses of the segment and the frame. This resulted in an average corrected segment stiffness of 356 N/mm. Taking the frame compliance into account, the whole spine stiffness is 51 N/mm. A series combination of eight segments using the corrected stiffnesses results in an estimated whole spine stiffness of 45.0 N/mm (12% error). We report both linear and nonlinear stiffness models for male spines and conclude that the compliance of the frame and the fixation must be quantified in all tension studies of spinal segments. Further, reported stiffness should be adjusted to account for frame and fixation compliance.

Pediatric Head and Neck Dynamics in Frontal Impact: Analysis of Important Mechanical Factors and Proposed Neck Performance Corridors for 6- and 10-Year-Old ATDs

Objective: Traumatic injuries are the leading cause of death of children aged 1–19 in the United States and are principally caused by motor vehicle collisions, with the head being the primary region injured. The neck, though not commonly injured, governs head kinematics and thus influences head injury. Vehicle improvements necessary to reduce these injuries are evaluated using anthropomorphic testing devices (ATDs). Current pediatric ATD head and neck properties were established by scaling adult properties using the size differences between adults and children. Due to the limitations of pediatric biomechanical research, computational models are the only available methods that combine all existing data to produce injury-relevant biofidelity specifications for ATDs. The purpose of this study is to provide the first frontal impact biofidelity corridors for neck flexion response of 6- and 10-year-olds using validated computational models, which are compared to the Hybrid III (HIII) ATD neck responses and the Mertz flexion corridors. Methods: Our virtual 6- and 10-year-old head and neck multibody models incorporate pediatric biomechanical properties obtained from pediatric cadaveric and radiological studies, include the effect of passive and active musculature, and are validated with data including pediatric volunteer 3 g dynamic frontal impact responses. We simulate ATD pendulum tests—used to calibrate HIII neck bending stiffness—to compare the pediatric model and HIII ATD neck bending stiffness and to compare the model flexion bending responses with the Mertz scaled neck flexion corridors. Additionally, pediatric response corridors for pendulum calibration tests and high-speed (15 g) frontal impacts are estimated through uncertainty analyses on primary model variables, with response corridors calculated from the average ± SD response over 650 simulations. Results and Conclusions: The models are less stiff in dynamic anterioposterior bending than the ATDs; the secant stiffness of the 6- and 10-year-old models is 53 and 67 percent less than that of the HIII ATDs. The ATDs exhibit nonlinear stiffening and the models demonstrate nonlinear softening. Consequently, the models do not remain within the Mertz scaled flexion bending corridors. The more compliant model necks suggest an increased potential for head impact via larger head excursions. The pediatric anterioposterior bending corridors developed in this study are extensible to any frontal loading condition through calculation and sensitivity analysis. The corridors presented in this study are the first based on pediatric cadaveric data and provide the basis for future, more biofidelic, designs of 6- and 10-year-old ATD necks.

An apparatus for tensile and bending tests of perinatal, neonatal, pediatric and adult cadaver osteoligamentous cervical spines

Investigations of biomechanical properties of pediatric cadaver cervical spines subjected to tensile or bending modes of loading are generally limited by a lack of available tissue and limiting sample sizes, both per age and across age ranges. It is therefore important to develop fixation techniques capable of testing individual cadavers in multiple modes of loading to obtain more biomechanical data per subject. In this study, an experimental apparatus and fixation methodology was developed to accommodate cadaver osteoligamentous head-neck complexes from around birth (perinatal) to full maturation (adult) [cervical length: 2.5-12.5 cm; head breadth: 6-15 cm; head length: 6-19 cm] and sequentially test the whole cervical spine in tension, the upper cervical spine in bending and the upper cervical spine in tension. The experimental apparatus and the fixation methodology provided a rigid casting of the head during testing and did not compromise the skull. Further testing of the intact skull and sub-cranial material was made available due to the design of the apparatus and fixation techniques utilized during spinal testing. The stiffness of the experimental apparatus and fixation technique are reported to better characterize the cervical spine stiffness data obtained from the apparatus. The apparatus and fixation technique stiffness was 1986 N/mm. This experimental system provides a stiff and consistent platform for biomechanical testing across a broad age range and under multiple modes of loading.

The compressive stiffness of human pediatric heads

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.

The response of the adult and ATD heads to impacts onto a rigid surface

Given the high incidence of TBI, head injury has been studied extensively using both cadavers and anthropomorphic test devices (ATDs). However, few studies have benchmarked the response of ATD heads against human data. Hence, the objective of this study is to investigate the response of adult and ATD heads in impact, and to compare adult Hybrid III head responses to the adult head responses. In this study, six adult human heads and seven ATD heads were used to obtain impact properties. The heads were dropped from both 15cm and 30cm onto five impact locations: right and left parietal, forehead, occiput and vertex. One set of drops were performed on the human heads and up to four sets were carried out on the ATD heads. For each drop, the head was placed into a fine net and positioned to achieve the desired drop height and impact location. The head was then released to allow free fall without rotation onto a flat aluminum 34 -inch thick platen. The platen was attached to a three-axis piezoelectric load cell to measure the impact force. The peak resultant acceleration, head impact criterion (HIC) and impact stiffness were calculated using the force/time curve and drop mass. No statistical differences were found between the adult human heads and the adult Hybrid III head for 15cm and 30cm impacts (p>0.05). For the human heads, the mid-sagittal impact locations produced the highest HIC and peak acceleration values. The parietal impacts produced HICs and peak accelerations that were 26-48% lower than those from the mid-sagittal impacts. For the ATD heads, the acceleration and HIC values generally increased with represented age, except for the Q3, which produced HIC values up to higher than the other ATD heads. The impact responses of the adult Hybrid III onto different impact locations were found to adequately represent the impact stiffness of human adult head impacts from 30cm and below onto a rigid surface. The Q3 dummy consistently produced the highest HIC values of the ATD heads, and produced higher acceleration and HIC values than the adult human heads as well, which is contrary to neonatal data demonstrating that the head acceleration increases with age.