Anterior longitudinal ligament injuries in whiplash may lead to cervical instability
Introduction
Whiplash disorders represent a significant societal problem in the United States and other industrialized nations. A complicating factor in the treatment of whiplash injuries is that the exact injury mechanism is unknown and affected tissues are not easily identified using contemporary imaging techniques. Whiplash-associated disorder is most commonly attributed to soft-tissue injury of the cervical spine, resulting from increased distraction without catastrophic failure. However, the specific tissues involved and factors affecting injury severity remain unclear.
Whiplash biomechanics were previously studied using computational modeling [1], [2], [3], [4], [5], [6], [7], [8], [9]. A typical whiplash event involves automotive rear impact, wherein the patient's thorax is accelerated anteriorly due to interaction with the seatback. Because no forces are placed directly on the head during the initial stage, it remains stationary due to its inertia. Differential translation between the head and thorax, commonly referred to as retraction, forces the cervical spine to compensate with non-physiologic S-curvature characterized by flexion in upper and extension in lower segments. This bi-phasic curvature is an abnormal orientation of the spine, which normally displays a single lordotic curvature in the cervical region. It was hypothesized that whiplash injury occurs during the retraction phase due to abnormal loading of soft-tissue spinal components [10], [11]. After the retraction phase, the entire head–neck complex transitions into overall extension, eventually limited by contact with the head restraint or reflex contraction of the neck musculature. These factors result in rebound of the head and neck, characterized by overall head–neck flexion. The kinematic phases of whiplash are represented in Fig. 1.
The degree to which anterior spinal soft-tissue distortion during retraction and extension phases may be injurious remains unknown. Head restraints were added to all new passenger vehicles in the United States in 1969 to prevent hyperextension of the head–neck complex. However, localized distraction of the anterior longitudinal ligament (ALL) at specific spinal levels may exceed injury thresholds, resulting in sub-catastrophic injury. Clinical studies have documented injuries to the ALL resulting from automotive loading [12], [13], [14], [15]. In addition, Yoganandan et al. [16] experimentally produced ALL injuries in human cadavers exposed to single rear impacts as low as 4.4 m/s. The present study was conducted to test the hypothesis that the ALL sustains localized distraction magnitudes during the retraction phase that may approach or exceed injury thresholds.
Section snippets
Methods
The hypothesis was tested using a head–neck computational model, exercised using MADYMO software [2], [7], [17]. The model consisted of the head, seven cervical vertebrae, first thoracic vertebra, and all relevant soft-tissue components of the spinal column (Fig. 1). The head and vertebrae were modeled as rigid bodies, incorporating level-specific mass and inertial properties (Table 1). The coordinate system was right-handed with positive x-axis in the posterior to anterior direction, positive y
Sensitivity analysis
The material property sensitivity analysis indicated that the model is relatively insensitive to changes in ALL stiffness (Fig. 2). In particular, altering this value by ±25% had a minimal effect on segmental angulations during the whiplash simulation. The largest change in segmental angulation between the lowest (−25%) and highest (+25%) ALL stiffness was 7.0% at the C2–C3 level, with a mean change in segmental angulation of 2.9% for C2–C3 through C6–C7 levels.
Spinal kinematics
Increasing impact severity
Discussion
Ligament distraction magnitudes approached failure values reported in literature (Fig. 4). Although magnitudes of maximum ALL distraction were approximately uniform across all investigated spinal levels, distraction in the middle to lower cervical spine (C3–C4 to C5–C6) approached failure values due to decreased ligament stiffness thresholds. In particular, maximum C3–C4 ALL distraction at higher impact severities was within one standard deviation for ligament failure, indicating susceptibility
Conclusions
The present study examined level-by-level temporal distraction of the cervical anterior longitudinal ligament in whiplash. Results demonstrated susceptibility of middle to lower cervical regions to failure, particularly at increasing impact severities. In particular, C3–C4 through C5–C6 levels sustained maximum distraction magnitudes approaching experimental failure values. Lower cervical regions (C5–C6 and C6–C7) sustained maximum ligament distractions earlier in the event, correlating with
Acknowledgements
This study was supported in part by PHS CDC Grant R49CCR-515433 and the Department of Veterans Affairs Medical Research.
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