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Research Papers

A New PMHS Model for Lumbar Spine Injuries During Vertical Acceleration

[+] Author and Article Information
Brian D. Stemper1

Steven G. Storvik, Narayan Yoganandan

Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI 53226 Department of Biomedical Engineering, Marquette University, Milwaukee, WI 53201  Veterans Affairs Medical Center, Milwaukee, WI 53295

Jamie L. Baisden

Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI 53226;  Veterans Affairs Medical Center, Milwaukee, WI 53295

Ronald J. Fijalkowski

Department of Neurosurgery,  Medical College of Wisconsin, Milwaukee, WI 53226

Frank A. Pintar

Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI 53226; Department of Biomedical Engineering, Marquette University, Milwaukee, WI 53201;  Veterans Affairs Medical Center, Milwaukee, WI 53295

Barry S. Shender, Glenn R. Paskoff

 Naval Air Warfare Center Aircraft Division, Patuxent River, MD 20670

1

Corresponding author.

J Biomech Eng 133(8), 081002 (Aug 30, 2011) (9 pages) doi:10.1115/1.4004655 History: Received November 18, 2010; Revised July 08, 2011; Posted July 21, 2011; Published August 30, 2011; Online August 30, 2011

Ejection from military aircraft exerts substantial loads on the lumbar spine. Fractures remain common, although the overall survivability of the event has considerably increased over recent decades. The present study was performed to develop and validate a biomechanically accurate experimental model for the high vertical acceleration loading to the lumbar spine that occurs during the catapult phase of aircraft ejection. The model consisted of a vertical drop tower with two horizontal platforms attached to a monorail using low friction linear bearings. A total of four human cadaveric spine specimens (T12-L5) were tested. Each lumbar column was attached to the lower platform through a load cell. Weights were added to the upper platform to match the thorax, head-neck, and upper extremity mass of a 50th percentile male. Both platforms were raised to the drop height and released in unison. Deceleration characteristics of the lower platform were modulated by foam at the bottom of the drop tower. The upper platform applied compressive inertial loads to the top of the specimen during deceleration. All specimens demonstrated complex bending during ejection simulations, with the pattern dependent upon the anterior-posterior location of load application. The model demonstrated adequate inter-specimen kinematic repeatability on a spinal level-by-level basis under different subfailure loading scenarios. One specimen was then exposed to additional tests of increasing acceleration to induce identifiable injury and validate the model as an injury-producing system. Multiple noncontiguous vertebral fractures were obtained at an acceleration of 21 g with 488 g/s rate of onset. This clinically relevant trauma consisted of burst fracture at L1 and wedge fracture at L4. Compression of the vertebral body approached 60% during the failure test, with -6,106 N axial force and 168 Nm flexion moment. Future applications of this model include developing a better understanding of the vertebral injury mechanism during pilot ejection and developing tolerance limits for injuries sustained under a variety of different vertical acceleration scenarios.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Lumbar spine vertical acceleration model. Axes for coordinate systems follow the right hand rule.

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Figure 2

Compressive mechanical properties of the pulse-shaping foam

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Figure 3

Vertical acceleration versus time of the lower platform during a simulated ejection test versus acceleration measured during ejection of a Naval ejection seat

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Figure 4

Representative time-based loading history for test condition S3.5

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Figure 5

Segmental kinematics obtained from subfailure testing (flexion: positive; extension: negative). Each column of plots represents a different testing condition with S1.0, S3.5, and S6.0 corresponding to tests with the torso load applied 1.0, 3.5, and 6.0 cm anterior to the posterior aspect of the L3 vertebral body. Kinematic corridors were defined as the mean experimental response ±1 standard deviation.

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Figure 6

Sagittal CT of the specimen subjected to higher severity acceleration pulses demonstrating burst fracture at L1 and anterior wedge fracture at L4. Top row images were obtained prior to dynamic testing and bottom row images were obtained immediately following the last test. The two images in each column were obtained from approximately the same medial-lateral position.

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Figure 7

Axial CT of the specimen subjected to higher severity acceleration pulses demonstrating laminar fracture at L1 (left) and intact posterior column at L4 (right)

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Figure 8

Time-histories of (a) the relative vertical displacement of three-vertebrae lumbar segments during the injury test (negative: compression), and (b) compression of vertebral bodies during the injury test presented as a percentage of the original displacement between the two targets within the vertebra’s local axis system

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