Paul C. Ivancic

The Role of Tissue Damage in Whiplash Associated Disorders: Discussion Paper 1

Study Design. Nonsystematic review of cervical spine lesions in whiplash-associated disorders (WAD). Objective. To describe whiplash injury models in terms of basic and clinical science, to summarize what can and cannot be explained by injury models, and to highlight future research areas to better understand the role of tissue damage in WAD. Summary of Background Data. The frequent lack of detectable tissue damage has raised questions about whether tissue damage is necessary for WAD and what role it plays in the clinical context of WAD. Methods. Nonsystematic review. Results. Lesions of various tissues have been documented by numerous investigations conducted in animals, cadavers, healthy volunteers, and patients. Most lesions are undetected by imaging techniques. For zygapophysial (facet) joints, lesions have been predicted by bioengineering studies and validated through animal studies; for zygapophysial joint pain, a valid diagnostic test and a proven treatment are available. Lesions of dorsal root ganglia, discs, ligaments, muscles, and vertebral artery have been documented in biomechanical and autopsy studies, but no valid diagnostic test is available to assess their clinical relevance. The proportion of WAD patients in whom a persistent lesion is the major determinant of ongoing symptoms is unknown. Psychosocial factors, stress reactions, and generalized hyperalgesia have also been shown to predict WAD outcomes. Conclusion. There is evidence supporting a lesion-based model in WAD. Lack of macroscopically identifiable tissue damage does not rule out the presence of painful lesions. The best available evidence concerns zygapophysial joint pain. The clinical relevance of other lesions needs to be addressed by future research.

The Anatomy and Biomechanics of Acute and Chronic Whiplash Injury

Whiplash injury is the most common motor vehicle injury, yet it is also one of the most poorly understood. Here we examine the evidence supporting an organic basis for acute and chronic whiplash injuries and review the anatomical sites within the neck that are potentially injured during these collisions. For each proposed anatomical site—facet joints, spinal ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles—we present the clinical evidence supporting that injury site, its relevant anatomy, the mechanism of and tolerance to injury, and the future research needed to determine whether that site is responsible for some whiplash injuries. This article serves as a snapshot of the current state of whiplash biomechanics research and provides a roadmap for future research to better understand and ultimately prevent whiplash injuries.

Injury Mechanisms of the Cervical Intervertebral Disc During Simulated Whiplash

Study Design. A kinematic analysis of cervical intervertebral disc deformation during simulated whiplash using the whole cervical spine with muscle force replication model was performed. Objectives. To quantify anulus fibrosus fiber strain, disc shear strain, and axial disc deformation in the cervical spine during simulated whiplash. Summary of Background Data. Clinical studies have documented acute intervertebral disc injury and accelerated disc degeneration in whiplash patients, although there has been no biomechanical investigation of the disc injury mechanisms. Methods. A bench-top sled was used to simulate whiplash at 3.5, 5, 6.5, and 8 g using six specimens. The 30° and 150° fiber strains, disc shear strains, and axial disc deformations during whiplash were compared with the sagittal physiologic levels. Results. Increases over sagittal physiologic levels (P < 0.05) were first observed during the 3.5 g simulation. Peak fiber strain was greatest in the posterior 150° fibers (running posterosuperiorly), reaching a maximum of 51.4% at C5–C6 during the 8 g simulation. Peak disc shear strain was also greatest at the posterior region of C5–C6, reaching a maximum of 1.0 radian due to posterior translation during the 8 g simulation. Axial deformation at the anterior disc region exceeded physiologic levels at 3.5 g and above, while axial deformation at the posterior region exceeded physiologic limits only at C5–C6 at 6.5 g and 8 g. Conclusions. The cervical intervertebral discs may be at risk for injury during whiplash because of excessive 150° fiber strain, disc shear strain, and anterior axial deformation.

Soft Tissue Injury Threshold During Simulated Whiplash : A Biomechanical Investigation

Study Design. A newly developed biofidelic whole cervical spine (WCS) model with muscle force replication (MFR) was subjected to whiplash simulations of varying intensity, and the resulting injuries were evaluated through changes in the intervertebral flexibility. Objectives. To identify the soft tissue injury threshold based on the peak T1 horizontal acceleration and the association between acceleration magnitude and injury severity resulting from simulated whiplash using the WCS + MFR model. Summary of Background Data. Whiplash has been simulated using mathematical models, whole cadavers, volunteers, and WCSs. The measurement of injury (difference between prewhiplash and postwhiplash flexibilities) is possible only using the WCS model. Methods. Six WCS + MFR specimens (C0–T1) were incrementally rear-impacted at nominal T1 horizontal maximum accelerations of 3.5, 5, 6.5, and 8 g, and the changes in the intervertebral flexibility parameters of neutral zone and range of motion were determined. The injury threshold acceleration was the lowest T1 horizontal peak acceleration that caused a significant increase in the intervertebral flexibility. Results. The first significant increase (P <0.01) of 39.8% occurred in the C5–C6 extension neutral zone following the 5 g acceleration. At higher accelerations, the injuries spread among the surrounding levels (C4–C5 to C7–T1). Conclusions. A rear-end collision is most likely to injure the lower cervical spine by intervertebral hyperextension at a peak T1 horizontal acceleration of 5 g and above. These results may aid in the design of injury prevention systems and more precise diagnoses of whiplash injuries.

Dynamic Intervertebral Foramen Narrowing During Simulated Rear Impact

Study Design. A biomechanical study of intervertebral foraminal narrowing during simulated automotive rear impacts. Objectives. To quantify foraminal width, height, and area narrowing during simulated rear impact, and evaluate the potential for nerve root and ganglion impingement in individuals with and without foraminal spondylosis. Summary of Background Data. Muscle weakness and paresthesias, documented in whiplash patients, have been associated with neural compression within the cervical intervertebral foramen. To our knowledge, no studies have comprehensively examined dynamic changes in foramen dimensions. Methods. There were 6 whole cervical spine specimens (average age 70.8 years) with muscle force replication and surrogate head that underwent simulated rear impact at 3.5, 5, 6.5, and 8 g, following noninjurious baseline 2 g acceleration. Peak dynamic narrowing of foraminal width, height, and area were determined during each impact and statistically compared to baseline narrowing. Results. Significant increases (P < 0.05) in average peak foraminal width narrowing above baseline were observed at C5–C6 beginning with 3.5 g impact. No significant increases in average peak foraminal height narrowing were observed, while average peak foraminal areas were significantly narrower than baseline at C4–C5 at 3.5, 5, and 6.5 g. Conclusions. Extrapolation of the present results indicated that the highest potential for ganglia compression injury was at the lower cervical spine, C5–C6 and C6–C7. Acute ganglia compression may produce a sensitized neural response to repeat compression, leading to chronic radiculopathy following rear impact.

Cervical spine ligament injury during simulated frontal impact

Study Design. The supraspinous and interspinous ligaments, ligamentum flavum, and capsular and posterior longitudinal ligament strains were monitored during simulated frontal impact of whole cervical spine specimens with muscle force replication and compared with corresponding physiologic strain limits. Objectives. To quantify the strains in the cervical spine ligaments during simulated frontal impact and investigate injury mechanisms. Summary of Background Data. Clinical and biomechanical studies have documented injuries to cervical spine ligaments during frontal impact. There are no biomechanical studies investigating subfailure injury mechanisms to these ligaments during simulated frontal impacts of increasing severity. Methods. The whole cervical spine with muscle force replication model and a bench-top sled were used to simulate frontal impacts at 4, 6, 8, and 10g horizontal accelerations of the T1 vertebra. The peak ligament strains during frontal impacts were compared with physiologic strain limits determined during intact flexibility testing. Results. Significant increases (P < 0.05) in the supraspinous and interspinous ligaments and the ligamentum flavum strains beyond physiologic limits were observed throughout the cervical spine, with the highest strains occurring at C3–C4. Significant increases were observed in the capsular ligament strains only during the 10g impact, whereas the posterior longitudinal ligament strains did not exceed physiologic limits. Conclusions. The supraspinous and interspinous ligaments and the ligamentum flavum may be at risk for injury due to excessive strains during frontal impacts.

Mechanism of cervical spinal cord injury during bilateral facet dislocation.

Study Design. An in vitro biomechanical study. Objectives. The objectives were to: quantify dynamic canal pinch diameter (CPD) narrowing during simulated bilateral facet dislocation of a cervical functional spinal unit model with muscle force replication, determine if peak dynamic CPD narrowing exceeded that observed post-trauma, and evaluate dynamic cord compression. Summary of Background Data. Previous biomechanical models are limited to quasi-static loading or manual ligament transection. No studies have comprehensively analyzed dynamic CPD narrowing during simulated dislocation. Methods. Bilateral facet dislocation was simulated using 10 cervical functional spinal units (C3–C4: n = 4; C5–C6: n = 3; C7–T1: n = 3) with muscle force replication by frontal impact of the lower vertebra. Rigid body transformation of kinematic data recorded optically was used to compute the CPD in neutral posture (before dislocation), during dynamic impact (peak during dislocation), and post-impact (flexion rotation = 0°). Peak dynamic impact and post-impact CPD narrowing were statistically compared. Results. Average peak dynamic impact CPD narrowing significantly exceeded (P < 0.05) post-impact narrowing and occurred as early as 71.0 ms following impact. The greatest dynamic impact narrowing of 7.2 mm was observed at C3–C4, followed by 6.4 mm at C5–C6, and 5.1 mm at C7–T1, with average occurrence times ranging between 71.0 ms at C7–T1 and 97.0 ms at C5–C6. Conclusion. Extrapolation of the present results indicated dynamic spinal cord compression of up to 88% in those with stenotic canals and 35% in those with normal canal diameters. These results are consistent with the wide range of neurologic injury severity observed clinically due to bilateral facet dislocation.

Cervical Spine Loads and Intervertebral Motions During Whiplash

Objective. To quantify the dynamic loads and intervertebral motions throughout the cervical spine during simulated rear impacts. Methods. Using a biofidelic whole cervical spine model with muscle force replication and surrogate head and bench-top mini-sled, impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Inverse dynamics was used to calculate the dynamic cervical spine loads at the centers of mass of the head and vertebrae (C1-T1). The average peak loads and intervertebral motions were statistically compared (P < 0.05) throughout the cervical spine. Results. Load and motion peaks generally increased with increasing impact acceleration. The average extension moment peaks at the lower cervical spine, reaching 40.7 Nm at C7-T1, significantly exceeded the moment peaks at the upper and middle cervical spine. The highest average axial tension peak of 276.9 N was observed at the head, significantly greater than at C4 through T1. The average axial compression peaks, reaching 223.2 N at C5, were significantly greater at C4 through T1, as compared to head-C1. The highest average posterior shear force peak of 269.5 N was observed at T1. Conclusion. During whiplash, the cervical spine is subjected to not only bending moments, but also axial and shear forces. These combined loads caused both intervertebral rotations and translations.

Neck ligament strength is decreased following whiplash trauma

BackgroundPrevious clinical studies have documented successful neck pain relief in whiplash patients using nerve block and radiofrequency ablation of facet joint afferents, including capsular ligament nerves. No previous study has documented injuries to the neck ligaments as determined by altered dynamic mechanical properties due to whiplash. The goal of the present study was to determine the dynamic mechanical properties of whiplash-exposed human cervical spine ligaments. Additionally, the present data were compared to previously reported control data. The ligaments included the anterior and posterior longitudinal, capsular, and interspinous and supraspinous ligaments, middle-third disc, and ligamentum flavum.MethodsA total of 98 bone-ligament-bone specimens (C2–C3 to C7-T1) were prepared from six cervical spines following 3.5, 5, 6.5, and 8 g rear impacts and pre- and post-impact flexibility testing. The specimens were elongated to failure at a peak rate of 725 (SD 95) mm/s. Failure force, elongation, and energy absorbed, as well as stiffness were determined. The mechanical properties were statistically compared among ligaments, and to the control data (significance level: P < 0.05; trend: P < 0.1). The average physiological ligament elongation was determined using a mathematical model.ResultsFor all whiplash-exposed ligaments, the average failure elongation exceeded the average physiological elongation. The highest average failure force of 204.6 N was observed in the ligamentum flavum, significantly greater than in middle-third disc and interspinous and supraspinous ligaments. The highest average failure elongation of 4.9 mm was observed in the interspinous and supraspinous ligaments, significantly greater than in the anterior longitudinal ligament, middle-third disc, and ligamentum flavum. The average energy absorbed ranged from 0.04 J by the middle-third disc to 0.44 J by the capsular ligament. The ligamentum flavum was the stiffest ligament, while the interspinous and supraspinous ligaments were most flexible. The whiplash-exposed ligaments had significantly lower (P = 0.036) failure force, 149.4 vs. 186.0 N, and a trend (P = 0.078) towards less energy absorption capacity, 308.6 vs. 397.0 J, as compared to the control data.ConclusionThe present decreases in neck ligament strength due to whiplash provide support for the ligament-injury hypothesis of whiplash syndrome.

EFFECTS OF THE ABDOMINAL BELT ON MUSCLE-GENERATED SPINAL STABILITY AND L4/L5 JOINT COMPRESSION FORCE

The goals of this study were (1) to determine the effects of abdominal belts on muscle-generated active lumbar spine stability, (2) to determine their effect on the subsequent joint compression force at L4/L5 and (3) to determine whether the effective stability of the spine could be predicted by the active spine stability and belt condition. Electromyographic (EMG) and trunk stiffness data from a previously reported experiment in which 10 subjects performed quick-release tasks (pertubation) with and without an abdominal belt were used as inputs to biomechanical models to estimate the active spine stability and effective stability of the spine, respectively. The subjects exerted isometric trunk flexion, extension and lateral bending trials at 0 and 80% of maximum intra-abdominal pressure when the resisted force was suddenly released. Wearing an abdominal belt had no significant effect on either the muscle-generated lumbar spine stability or the L4/L5 joint compression force in any direction. The effective stability of the spine was adequately predicted by the active spine stability and the effect of the belt, which accounted for approximately 34% of the effective spine stability. The study demonstrated that the abdominal belt contributed to the passive stability of the lumbar spine and did not change the active stability for tests performed within the same experimental session.