Measurement of canal occlusion during the thoracolumbar burst fracture process

doi:10.1016/S0021-9290(01)00180-4

Journal of Biomechanics

Volume 35, Issue 3, March 2002, Pages 381-384

https://doi.org/10.1016/S0021-9290(01)00180-4 Get rights and content

Abstract

Post-injury CT scans are often used following burst fracture trauma as an indication for decompressive surgery. Literature suggests, however, that there is little correlation between the observed fragment position and the level of neurological injury or recovery. Several studies have aimed to establish the processes that occur during the fracture using indirect methods such as pressure measurements and pre/post impact CT scans. The purpose of this study was to develop a direct method of measuring spinal canal occlusion during a simulated burst fracture by using a high-speed video technique. The fractures were produced by dropping a mass from a measured height onto three-vertebra bovine specimens in a custom-built rig. The specimens were constrained to deform only in the impact direction such that pure compression fractures were generated. The spinal cord was removed prior to testing and the video system set up to film the inside of the spinal canal during the impact. A second camera was used to film the outside of the specimen to observe possible buckling during impact. The video images were analysed to determine how the cross-sectional area of the spinal canal changed during the event. The images clearly showed a fragment of bone being projected from the vertebral body into the spinal canal and recoiling to the final resting position. To validate the results, CT scans were taken pre- and post-impact and the percentage canal occlusion was calculated. There was good agreement between the final canal occlusion measured from the video images and the CT scans.

Section snippets

Introduction to high-speed video analysis of thoracolumbar burst fractures

Every year, more than 10,000 people in the United States alone incur a spinal cord injury (SCI) (Go et al., 1995) and a recent survey put the average cost of healthcare at over $180,000 per person for the first year after injury (DeViro, 1997). About 15% of spinal injuries are caused by burst fractures (Denis, 1983), and surgery is often recommended in these cases to decompress the spinal canal.

Post-injury computed tomography (CT) scans are often used as an indicator for decompressive surgery,

Preparation of specimen

Thoracolumbar spinal specimens from 21-day-old male Holstein calves were retrieved from an abattoir and frozen at −20°C. After defrosting for 24 h, the specimens were cut into three vertebra segments with T13 in the centre (the bovine spine has 13 thoracic segments). Excess para-vertebral muscle was removed and the spinal cord and meninges extracted from the spinal canal. The ends of the specimen were set in 80 mm diameter polymethyl-methacrylate (PMMA) endplates to produce flat, parallel

Validation

Nine specimens were tested once each at an impact energy in the range from 60 to 140 J. In every case, the video images clearly showed the fragment projecting into the spinal canal before recoiling to its final resting position (Fig. 2, Fig. 3). The entire event lasted under 20 ms. For each test, the relative cross-sectional canal area during fracture was plotted against time (Fig. 2).

The images from the second high-speed camera showed that there was no visible flexing of the specimen during the

Discussion

Although several studies have used indirect methods to determine spinal canal occlusion during burst fractures, this study provides the first technique for filming the occlusion directly. Since the video can be run throughout the event, image analysis of the frames can be used to calculate the change in canal area with time along with other quantifiable data such as fragment velocity.

Although the impact energies used in this study are within the range reported by Panjabi et al. (1995b), a

Acknowledgments

This work was supported by grants from the Yorkshire Children's Spine Foundation, The Wishbone Trust and The Engineering and Physical Sciences Research Council (EPSRC).

References (16)

J.M. Bland et al.
Measuring agreement in method comparison studies
Statistical Methods in Medical Research
(1999)
T.O. Boerger et al.
Does canal clearance affect neurological outcome after thoracolumbar burst fractures?
Journal of Bone and Joint Surgery, British Volume
(2000)
K.J. Bozin et al.
Three-dimensional finite element modelling of a cervical vertebraan investigation of the burst fracture mechanism
Journal of Spinal Disorders
(1994)
D.G. Chang et al.
Geometric changes in the cervical spinal canal during impact
Spine
(1994)
F. Denis
The three column spine and its significance in the classification of acute thoracolumbar spinal injuries
Spine
(1983)
M.J. DeViro
Causes and costs of spinal cord injury in the united states
Spinal Cord
(1997)
B.K. Go et al.
The epidemiology of spinal cord injury
Hedman, T.P., Liao, W.L., Yu, J., Watkins, R., Liker, M., 1999. Strength reduction of the posterior intervertebral disc...

There are more references available in the full text version of this article.

Cited by (52)

Smoothed particle hydrodynamics implementation to enhance vertebral fracture finite element model in a cervical spine segment under compression
2024, Journal of the Mechanical Behavior of Biomedical Materials
Spinal cord injuries (SCIs) can arise from compression loading when a vertebra fractures and bone fragments are pushed into the spinal canal. Experimental studies have demonstrated the importance of both fracture initiation and post-fracture response in the investigation of vertebral fractures and spinal canal occlusion resulting from compression. Finite element models, such as the Global Human Body Models Consortium (GHBMC) model, focused on predicting the initiation location of fractures using element erosion to model hard tissue fracture. However, the element erosion method resulted in a loss of material and structural support during compression, which limited the ability of the model to predict the post-fracture response. The current study aimed to improve the post-fracture response by combining strain-based element erosion with smoothed particle hydrodynamics (SPH) to preserve the volume of the trabecular bone during compression fracture. The proposed implementation was evaluated using a model comprising two functional spinal units (FSUs) (C5–C6–C7) extracted from the GHBMC 50th percentile male model, and loaded under central compression. The original and enhanced models were compared to experimental force-displacement data and measured occlusion of the spinal canal. The enhanced model with SPH improved the shape and magnitude of the force-displacement response to be in good agreement with the experimental data. In contrast to the original model, the enhanced SPH model demonstrated occlusion on the same order of magnitude as reported in the experiments. The SPH implementation improved the post-fracture response by representing the damaged material post-fracture, providing structural support throughout compression loading and material flow leading to occlusion.
Occlusion of the lumbar spine canal during high-rate axial compression
2020, Spine Journal
Citation Excerpt :
However, canal occlusion under dynamic compression in the absence of a severe vertebral fracture has not been measured in previous research. Experimental studies of canal occlusion induced by vertebral fracture have primarily focused on the cervicothoracic and thoracolumbar junctions [8–11] since these are common fracture sites clinically encountered in civilian trauma [2,3]. However, morphometric variation along the spine is associated with changes in canal geometry [20,21], and it is unclear if these anatomical variations influence canal occlusion susceptibility.
While burst fracture is a well-known cause of spinal canal occlusion with dynamic, axial spinal compression, it is unclear how such loading mechanisms might cause occlusion without fracture.
To determine how spinal canal occlusion during dynamic compression of the lumbar spine is differentially caused by fracture or mechanisms without fracture and to examine the influence of spinal level on occlusion.
A cadaveric biomechanical study.
Twenty sets of three-vertebrae specimens from all spinal levels between T12 and S1 were subjected to dynamic compression using a hydraulic loading apparatus up to a peak velocity between 0.1 and 0.9 m/s. The presence of canal occlusion was measured optically with a high-speed camera. This was repeated with incremental increases of 4% compressive strain until a vertebral fracture was detected using acoustic emission measurements and computed tomographic imaging.
For axial compression without fracture, the peak occlusion (O_max) was 29.9±10.0%, which was deduced to be the result of posterior bulging of the intervertebral disc into the spinal canal. O_max correlated significantly with lumbar spinal level (p<.001), the compressive displacement (p<.001) and the cross-sectional area of the vertebra (p=.031).
Spinal canal occlusion observed without vertebral fracture involves intervertebral disc bulging. The lower lumbar spine tended to be more severely occluded than more proximal levels.
Clinically, intermittent canal occlusion from disc bulging during dynamic compression may not show any radiographic features. The lower lumbar spine should be a focus of injury prevention intervention in cases of high-rate axial compression.
Development of a traumatic cervical dislocation spinal cord injury model with residual compression in the rat
2019, Journal of Neuroscience Methods
Preclinical spinal cord injury models do not represent the wide range of biomechanical factors seen in human injuries, such as spinal level, injury mechanism, velocity of spinal cord impact, and residual compression. These factors may be responsible for differences observed between experimental and clinical study results, especially related to the controversial issue of timing of surgical decompression.
Somatosensory Evoked Potentials were used to: a) characterize residual compression depths in a dislocation model, and b) evaluate the physiological effect of whether or not the spinal cord was decompressed following the initial injury, prior to the application of residual compression. Modifications to vertebral clamps and the development of a novel surgical frame allowed us to conduct surgical and injury procedures in a controlled manner without the risk of additional damage to the spinal cord. Behavioural outcomes were evaluated following varying dislocation displacements, in addition to the survivability of 4 h of residual compression following a traumatic injury.
Residual compression immediately following the initial dislocation demonstrated significantly different electrophysiological response compared to when the residual compression was delayed.
There are currently no other residual compression models that utilize a dislocation injury mechanism. Many residual compression studies have demonstrated the effectiveness of early decompression, however the compression of the spinal cord is often not representative of clinical traumatic injuries. Preclinical studies typically model residual compression using a sustained force through quasi-static application, when human injuries often occur at high velocities, followed by a sustained displacement occlusion of the spinal canal.
This study has validated several novel procedural approaches and injury parameters, and provided critical details to implement in the development of a traumatic cervical dislocation SCI model with residual compression.
Engineering approaches to understanding mechanisms of spinal column injury leading to spinal cord injury
2019, Clinical Biomechanics
Citation Excerpt :
However, it is limited to tests with a small number of spinal segments, and those in which vertical canal alignment is maintained to ensure the passage of light. Importantly, the study provided measures of bone fragment velocity, mass and cross-sectional area (which informed the design of in vivo animal (Jones et al., 2012b) and physical (Jones et al., 2012a) models of SCI) and provided evidence that in clinical burst fracture the post-injury occlusion is likely lower than the peak occlusion (Wilcox et al., 2002). The results from this approach were later combined with a model in which the canal was filled with gelatin and an embedded pressure transducer, to determine transient pressures, and the results used to develop and validate a finite element model (see Section 4) (Wilcox et al., 2003; Wilcox et al., 2004).
The mechanical interactions occurring between the spinal column and spinal cord during an injury event are complex and variable, and likely have implications for the clinical presentation and prognosis of the individual.
The engineering approaches that have been developed to better understand spinal column and cord interactions during an injury event are discussed. These include injury models utilising human and animal cadaveric specimens, in vivo anaesthetised animals, finite element models, inanimate physical systems and combinations thereof.
The paper describes the development of these modelling approaches, discusses the advantages and disadvantages of the various models, and the major outcomes that have had implications for spinal cord injury research and clinical practice.
The contribution of these four engineering approaches to understanding the interaction between the biomechanics and biology of spinal cord injury is substantial; they have improved our understanding of the factors contributing to the spinal column disruption, the degree of spinal cord deformation or motion, and the resultant neurological deficit and imaging features. Models of the injury event are challenging to produce, but technological advances are likely to improve these models and, consequently, our understanding of the mechanical context in which the biological injury occurs.
Basic biomechanics of spinal cord injury — How injuries happen in people and how animal models have informed our understanding
2019, Clinical Biomechanics
The wide variability, or heterogeneity, in human spinal cord injury is due partially to biomechanical factors. This review summarizes our current knowledge surrounding the patterns of human spinal column injury and the biomechanical factors affecting injury. The biomechanics of human spinal injury is studied most frequently with human cadaveric models and the features of the two most common injury patterns, burst fracture and fracture dislocation, are outlined. The biology of spinal cord injury is typically studied with animal models and the effects of the most relevant biomechanical factors - injury mechanism, injury velocity, and residual compression, are described. Tissue damage patterns and behavioural outcomes following dislocation or distraction injury mechanisms differ from the more commonly used contusion mechanism. The velocity of injury affects spinal cord damage, principally in the white matter. Ongoing, or residual compression after the initial impact does affect spinal cord damage, but few models exist that replicate the clinical scenario. Future research should focus on the effects of these biomechanical factors in different preclinical animal models as recent data suggests that treatment outcomes may vary between models.
Compressive mechanical characterization of non-human primate spinal cord white matter
2018, Acta Biomaterialia
Citation Excerpt :
More recently, whole cords have been characterized using non-linear viscoelastic models for low strains (<5%) and moderate strain rates (0.1/sec) [36]. However, during a typical traumatic SCI the spinal cord undergoes large deformations at strain rates of approximately 110/sec [37,38]. Knowing that the impact velocity substantially affects the pattern and severity of the SCI [39], characterizing the tissue at higher strain rates is crucial for more accurate SCI models.
The goal of developing computational models of spinal cord injury (SCI) is to better understand the human injury condition. However, finite element models of human SCI have used rodent spinal cord tissue properties due to a lack of experimental data. Central nervous system tissues in non human primates (NHP) closely resemble that of humans and therefore, it is expected that material constitutive models obtained from NHPs will increase the fidelity and the accuracy of human SCI models. Human SCI most often results from compressive loading and spinal cord white matter properties affect FE predicted patterns of injury; therefore, the objectives of this study were to characterize the unconfined compressive response of NHP spinal cord white matter and present an experimentally derived, finite element tractable constitutive model for the tissue. Cervical spinal cords were harvested from nine male adult NHPs (Macaca mulatta). White matter biopsy samples (3 mm in diameter) were taken from both lateral columns of the spinal cord and were divided into four strain rate groups for unconfined dynamic compression and stress relaxation (post-mortem <1-hour). The NHP spinal cord white matter compressive response was sensitive to strain rate and showed substantial stress relaxation confirming the viscoelastic behavior of the material. An Ogden 1st order model best captured the non-linear behavior of NHP white matter in a quasi-linear viscoelastic material model with 4-term Prony series. This study is the first to characterize NHP spinal cord white matter at high (>10/sec) strain rates typical of traumatic injury. The finite element derived material constitutive model of this study will increase the fidelity of SCI computational models and provide important insights for transferring pre-clinical findings to clinical treatments.
Spinal cord injury (SCI) finite element (FE) models provide an important tool to bridge the gap between animal studies and human injury, assess injury prevention technologies (e.g. helmets, seatbelts), and provide insight into the mechanisms of injury. Although, FE model outcomes depend on the assumed material constitutive model, there is limited experimental data for fresh spinal cords and all was obtained from rodent, porcine or bovine tissues. Central nervous system tissues in non human primates (NHP) more closely resemble humans. This study characterizes fresh NHP spinal cord material properties at high strains rates and large deformations typical of SCI for the first time. A constitutive model was defined that can be readily implemented in finite strain FE analysis of SCI.

View all citing articles on Scopus

View full text

Technical noteMeasurement of canal occlusion during the thoracolumbar burst fracture process

Abstract

Section snippets

Introduction to high-speed video analysis of thoracolumbar burst fractures

Preparation of specimen

Validation

Discussion

Acknowledgments

Measuring agreement in method comparison studies

Statistical Methods in Medical Research

Does canal clearance affect neurological outcome after thoracolumbar burst fractures?

Journal of Bone and Joint Surgery, British Volume

Three-dimensional finite element modelling of a cervical vertebraan investigation of the burst fracture mechanism

Journal of Spinal Disorders

Geometric changes in the cervical spinal canal during impact

Spine

The three column spine and its significance in the classification of acute thoracolumbar spinal injuries

Spine

Causes and costs of spinal cord injury in the united states

Spinal Cord

The epidemiology of spinal cord injury

Technical note
Measurement of canal occlusion during the thoracolumbar burst fracture process