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The goal of this study was to compute the dynamic neck loads during simulated high-speed bilateral facet dislocation and investigate the injury mechanism.Ten osteoligamentous functional spinal units (C3/4, n = 4; C5/6, n = 3; C7/T1, n = 3) were prepared with muscle force replication, motion tracking flags, and a 3.3-kg mass rigidly attached to the upper vertebra. Frontal impacts of increasing severity were applied to the lower vertebra until dislocation was achieved. Inverse dynamics was used to calculate the dynamic neck loads during dislocation. Average peak impact acceleration required to cause dislocation ranged between 7.6 and 11.6 g. This resulted in dynamic neck loads applied at average peak rates of 906 Nm/s for flexion moment, 8017 N/ for anterior shear, and 8100 N/s for axial compression. To determine the temporal event patterns, the average occurrence times of the load and motion peaks were statistically compared (P0.05).Among average peak loads, axial compression of 233.6 N was first to occur followed by anterior shear force of 73.1 N and flexion moment of 30.7 Nm. Among average peak motions, axial separation of 5.3 mm was first to occur followed by flexion rotation of 63.1 degrees and anterior shear of 21.5 mm. Subsequently, average peak posterior shear force of 110.3 N was observed as the upper facet became locked in the intervertebral foramina. Average peak axial compression of 6.6 mm occurred significantly later than all preceding events.During bilateral facet dislocation, the main loads included flexion moment and forces of axial compression and anterior shear. These loads caused flexion rotation, facet separation, and anterior translation of the upper facet relative to the lower. The present data help elucidate the injury mechanism of cervical facet dislocation.

Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that dynamic three-dimensional intervertebral motion beyond physiological limits may cause multiplanar injury of cervical spine soft tissues. Goals of this study, using a biofidelic whole human cervical spine model with muscle force replication and surrogate head in simulated side impacts, were to correlate IV-NIC with multiplanar injury and determine IV-NIC injury threshold for each intervertebral level.Using a bench-top apparatus, side impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Pre- and post-impact flexibility testing in three-motion planes measured the soft tissue injury, i.e., significant increase (p0.05) in neutral zone (NZ) or range of motion (RoM) at any intervertebral level, above corresponding physiological limit.IV-NIC in left lateral bending correlated well with total lateral bending RoM (R = 0.61, P0.001) and NZ (R = 0.55, P0.001). Additionally, the same IV-NIC correlated well with left axial rotation RoM (R = 0.50, P0.001). IV-NIC injury thresholds (95% confidence limits) varied among intervertebral levels and ranged between 1.5 (0.6-2.4) at C3-C4 and 4.0 (2.4-5.7) at C7-T1. IV-NIC injury threshold times were attained beginning at 84.5 ms following impact.Present results suggest that IV-NIC is an effective tool for determining multiplanar soft tissue neck injuries by identifying the intervertebral level, mode, time, and severity of injury.

Intervertebral Neck Injury Criterion (IV-NIC) hypothesizes that dynamic three-dimensional intervertebral motion beyond physiological limit may cause multiplanar soft-tissue injury. Present goals, using biofidelic whole human cervical spine model with muscle force replication and surrogate head in head-turned rear impacts, were to: (1) correlate IV-NIC with multiplanar injury, (2) determine IV-NIC injury threshold at each intervertebral level, and (3) determine time and mode of dynamic intervertebral motion that caused injury.Impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of T1 vertebra (n = 6; average age: 80.2 years; four male, two female donors). IV-NIC was defined at each intervertebral level and in each motion plane as dynamic intervertebral rotation divided by physiological limit. Three-plane pre- and post-impact flexibility testing measured soft-tissue injury; that is significant increase in neutral zone (NZ) or range of motion (RoM) at any intervertebral level, above baseline. IV-NIC injury threshold was average IV-NIC peak at injury onset.IV-NIC extension peaks correlated best with multiplanar injuries (P0.001): extension RoM (R = 0.55) and NZ (R = 0.42), total axial rotation RoM (R = 0.42) and NZ (R = 0.41), and total lateral bending NZ (R = 0.39). IV-NIC injury thresholds ranged between 1.1 at C0-C1 and C3-C4 to 2.9 at C7-T1. IV-NIC injury threshold times were attained between 83.4 and 150.1 ms following impact.Correlation between IV-NIC and multiplanar injuries demonstrated that three-plane intervertebral instability was primarily caused by dynamic extension beyond the physiological limit during head-turned rear impacts.

The Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that dynamic intervertebral motion beyond physiological limits may injure soft tissues. In contrast, the Neck Injury Criterion (NIC) hypothesizes that sudden change in spinal fluid pressure may cause neural injuries. The goals of this study, using the biofidelic whole human cervical spine model with muscle force replication, were to determine the IV-NIC injury threshold due to frontal impact at each intervertebral level, and to compare the IV-NIC and NIC in determining injury.Using a bench-top apparatus, frontal impacts were simulated at 4, 6, 8, and 10 g horizontal accelerations of the T1 vertebra. Pre- and post-trauma flexibility testing measured the soft tissue injury; that is, a significant increase (p0.05) in neutral zone or range of motion at any intervertebral level, above the corresponding physiological limit.Results indicated that the soft tissue injury occurred due to flexion mode of injury and its threshold was 8 g. The average IV-NIC injury threshold (95% confidence interval) was 2.0 (1.2-2.8) at C4-C5 and 2.3 (1.6-3.0) at C6-C7, while the average NIC injury threshold was 18.4 (17.9-19.0) m(2)/s(2). The NIC injury threshold was reached significantly earlier than all the IV-NIC injury thresholds, demonstrating that the NIC may be unable to predict facet and soft tissue injury caused by non-physiologic inververtebral rotation.Present results suggest that IV-NIC is an effective tool for determining soft tissue neck injuries by identifying the intervertebral level, mode, time, and severity of injury.