CSRS – 2014 Please click on the “Disclosure” link for author/participant financial disclosure information. E-Poster #41 Contiguous Spinal Instability Due to Tri-Phasic Kinematics from Underbody Blast Loading Narayan Yoganandan, PhD, Milwaukee, Wisconsin John R. Humm, MS, Milwaukee, Wisconsin Frank A. Pintar, PhD, Milwaukee, Wisconsin Michael Mumert, MD, Milwaukee, Wisconsin Dennis J. Maiman, MD, PhD, Milwaukee, Wisconsin Introduction: Inferior-to-superior loading of cervical spine coupled with/without helmet contact with roof/interior structure of vehicle is a postulated injury mechanism from underbody blasts from IEDs. Using a custom vertical accelerator device, this study applied simulated underbody blast loads to human cadaver head-neck complexes to determine forces, kinematics, injuries, injury mechanisms and instability at contiguous levels. Methods: Pretest BMD, X rays and CT of six head-T2 complexes were obtained. Disc and facet joints were graded (two surgeons). Specimens were prepared with retro-reflective targets at various levels. T2-T3 was fixed. C7-T1 joint was unconstrained. A six- axis load cell was attached at inferior end. An appropriate-size military-combat-helmet was used for each specimen, determined based on individual head size. Prepositioning: T1 was inferiorly oriented at 30-degrees from horizontal. Occipital condyles were anterior to cervico-thoracic disc (seven-degrees forward of vertical axis). ese were based on mean position data from a study of military volunteers. Tests were conducted using a vertical accelerator capable of imparting high-acceleration, short-duration pulses to specimen’s inferior end (Figure 1). Accelerometers, angular velocity and acoustic sensors, and strain gages were used on vertebrae. Tests were done at different velocities, applied sequentially such that low velocity initial baseline test was repeated in between two higher velocity tests. Roof structure of military vehicles was simulated at a distance of 0.13 m from top of helmet. Load cells recorded roof impact forces. High-speed videos were taken (5000 frames/second). X-rays and palpations were done in between tests. X-rays and CT were obtained following final test. Detailed dissection was performed. Results: Age, stature, BMI: 55±9 years, 183±5 cm, 21±3 kg/m2. Impact pulses were within 10-millisecond rise-time. Kinematics, force and acceleration analysis demonstrated tri-phasic response: initial acceleration/compression wave transmitted from T2 to occipital condyles during launching phase, followed by extension kinematics of the column, and in final phase, additional (re) compression occurred from roof contact. Entire tri-phasic event occurred within 120-milliseconds. Forces during initial compressive wave transmission phase were similar in all specimens, while during final compression phase, roof-contacted specimens sustained significantly greater forces than non-contacted specimens (Figure 2). Injuries were consistent with vertical inferior to superior loading mechanisms, forces, moments and accelerations: vertebra fractures, anterior intrevertebral annulus disruptions and ligament tears, and facet injuries. Conclusion: Tri-phasic response and inferior-to-superior loading is unique to military environments. Anterior, middle and posterior column injuries occurred in the third phase with helmet-head to roof contact associated with ‘buckling’ of cervical column, only possible through the local realign upward wave phenomenon to the entire fractured vertebra(e) cross section. Involvement of anterior column, disc and ligament injuries suggests: soſt tissue-related injuries occur due to local segmental extensions during second phase of wave transmission. Roof contact during third phase of the response injured spine, leading to clinical instability at contiguous levels. ese mechanistic features distinctly differ from neck injuries under civilian environments. Military neck injuries from underbody blast loads are unique, have complex tri-phasic phenomenon and involve additional contiguous segmental instability.