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Research Papers

Compressive Follower Load Influences Cervical Spine Kinematics and Kinetics During Simulated Head-First Impact in an in Vitro Model

[+] Author and Article Information
Peter A. Cripton

e-mail: cripton@mech.ubc.ca
Orthopaedic and Injury Biomechanics Group,
Department of Orthopaedics,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada
Department of Mechanical Engineering,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada
International Collaboration on
Repair Discoveries,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada

Eyal Itshayek

Orthopaedic and Injury Biomechanics Group,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada
Department of Orthopaedics,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada
International Collaboration on
Repair Discoveries,
University of British Columbia,
Vancouver, BC V5S 2X9, Canada
Department of Neurosurgery,
Hadassah—Hebrew University Hospital,
Jerusalem 91120, Israel

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received September 4, 2012; final manuscript received May 27, 2013; accepted manuscript posted June 17, 2013; published online September 24, 2013. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 135(11), 111003 (Sep 24, 2013) (11 pages) Paper No: BIO-12-1390; doi: 10.1115/1.4024822 History: Received September 04, 2012; Revised May 27, 2013; Accepted June 17, 2013

Current understanding of the biomechanics of cervical spine injuries in head-first impact is based on decades of epidemiology, mathematical models, and in vitro experimental studies. Recent mathematical modeling suggests that muscle activation and muscle forces influence injury risk and mechanics in head-first impact. It is also known that muscle forces are central to the overall physiologic stability of the cervical spine. Despite this knowledge, the vast majority of in vitro head-first impact models do not incorporate musculature. We hypothesize that the simulation of the stabilizing mechanisms of musculature during head-first osteoligamentous cervical spine experiments will influence the resulting kinematics and injury mechanisms. Therefore, the objective of this study was to document differences in the kinematics, kinetics, and injuries of ex vivo osteoligamentous human cervical spine and surrogate head complexes that were instrumented with simulated musculature relative to specimens that were not instrumented with musculature. We simulated a head-first impact (3 m/s impact speed) using cervical spines and surrogate head specimens (n = 12). Six spines were instrumented with a follower load to simulate in vivo compressive muscle forces, while six were not. The principal finding was that the axial coupling of the cervical column between the head and the base of the cervical spine (T1) was increased in specimens with follower load. Increased axial coupling was indicated by a significantly reduced time between head impact and peak neck reaction force (p = 0.004) (and time to injury (p = 0.009)) in complexes with follower load relative to complexes without follower load. Kinematic reconstruction of vertebral motions indicated that all specimens experienced hyperextension and the spectrum of injuries in all specimens were consistent with a primary hyperextension injury mechanism. These preliminary results suggest that simulating follower load that may be similar to in vivo muscle forces results in significantly different impact kinetics than in similar biomechanical tests where musculature is not simulated.

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Figures

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Fig. 1

(a) Schematic of the vertical drop-tower used to simulate axial head-first impact. The carriage translates along the Z-axis and is constrained in the X- and Y-axes by linear stanchions/bearings. The direction of translation is the Z-direction and impact occurs between the surrogate head and impact platen. The approximate location of the head center of mass is shown as a solid filled circle. (b) Schematic of the cervical spine and surrogate head complex showing mount-cup center to head-mount center distance (Dc-c) and head inclination.

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Fig. 2

Photograph of osteo-ligamentous cervical spine and surrogate head complex showing follower load and compression springs. The lordotic posture shown was typical of all specimens.

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Fig. 3

Typical kinetic results: compressive Z-head force (dotted line); compressive Z-neck force at mount-cup/six axis load cell interface (solid line); and the neck moment about the Z-axis of the six axis load cell (dashed line). Note that the Z-axis of the load cell is parallel to the Z-axis in Fig. 1 but originates at approximately the volumetric center of the load cell. Results shown are for specimen H1091 (FL group).

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Fig. 4

Positions of photo-reflective markers fixed to the vertebral bodies and the superior-most vertices of the specimen mount-cup for increasing chronological time for (a)–(c) specimen H1177 (FL group), and (d)–(f) H1062 (NFL group). The time of 0 ms, nominally, just prior to head impact. Solid lines span the inter-marker space of the anterior-most markers at each vertebral level and the space between the superior-most surface of the mount-cup.

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Fig. 5

X-positions (i.e., anteroposterior axis positions) of the anterior-most markers at each vertebral level plotted with respect to time: (a) specimens exhibiting hyperextension and hyperextension injuries have marker positions that trace parabolic loci through time, as shown for specimen H1177 (FL group), (b) specimen H1062 (NFL group) exhibited C1-C2 subluxation and the loci of markers on C1 and C2 convey the posterior to anterior transition of C2 relative to C1 (subluxation), and (c) specimen H1116 exhibited hyperextension and a fracture dislocation at C3/4, which is indicated by a relatively large difference in the translated distance between C3 and C4 relative to other levels

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