Biomechanics and patterns of spine injuries associated with spinal cord injury





List of abbreviations


CCS


Central cord syndrome


PLC


Posterior ligamentous complex


TSCI


Traumatic spinal cord injury


Introduction


Worldwide, approximately 30 million individuals live with a traumatic spinal cord injury (TSCI), and about 1 million new cases of TSCI occur annually, and these numbers are expected to increase in view of population growth. Annually, 150,000 spine fractures are treated in the United States. Among those, close to 20% are associated with spinal cord injury (SCI) ( ). A spinal fracture and/or dislocation occurs in about 65% of all TSCI.


TSCI contributes to a considerable burden on patient’s lives because of lifelong functional disabilities and major psychosocial challenges. Accordingly, the severity of the spine injuries is usually associated with the severity of the SCI, in line with increased burden of healthcare resource utilization. Although management of the underlying spine injuries further contributes to the economic burden of TSCI, it is strongly influencing the clinical outcomes of patients.


TSCIs are most often due to a traumatic event, mainly falls and motor vehicle accidents. While motor vehicle accidents remain the most common cause of TSCI, there is an increasing incidence of TSCI due to low-energy trauma such as falls ( ; ).


Because of physiological changes due to aging, elderly individuals are particularly at increased risk of sustaining a TSCI from low-energy trauma such as a fall from their height.


In keeping with the aging of the general population, a demographic shift is observed toward older TSCI patient’s population ( ) with pre-existing spondylotic spinal stenosis and without significant traumatic spinal instability.


Understanding the biomechanics and patterns of the common types of spine injuries associated with SCI is central to the management of individuals with TSCI, and will be featured in this chapter.


Pathogenesis of TSCI: The critical role of spine biomechanics


Several studies ( ; ; ) demonstrated the importance of the mechanism and level of energy of the traumatic event as predictors of the severity of TSCI. When compared with low-velocity trauma (e.g., fall from standing height), high-velocity mechanisms of trauma (e.g., motor-vehicle accidents) more often result in increased spinal cord damage, particularly in the white matter ( ). The mechanism and level of energy of the trauma are only moderately associated with the outcomes, because they only partially reflect the level of energy that was actually transferred to the spinal cord during the trauma. During the trauma, the primary injury to the spinal cord occurs due to the physical insult and local energy transferred to the spinal cord, thereby causing direct damage to neural tissues and its blood vessels. It is believed that the morphological pattern of the spine injury is a better reflection of the underlying dynamics and local energy sustained by the spinal cord during the trauma than the mechanism of trauma and global level of energy ( ). During the trauma, energy is transferred to the spinal cord by direct or indirect mechanisms originating from the spine injury. Direct injury occurs from contusion of the spinal cord by vertebral bone and soft tissues (intervertebral disc and/or ligaments) in direct contact with the spinal cord.


Indirect injury to the spinal cord results from the motion (e.g., flexion, hyperextension, axial compression, distraction, dislocation, etc.) between spinal elements during the trauma, causing excessive tension, shear, torsion, compression, and/or bending loads on the spinal cord.


The type of loads sustained by the spinal cord will influence the extent of tissue damage ( ), contributing to the heterogeneity of the clinical outcomes observed following a TSCI ( ). Contusion causes a relatively localized damage, as opposed to dislocation (causing mainly shear stress) and distraction which result in more asymmetrical damage that can extend further rostrally and caudally ( ).


The acute surgical management is critical in TSCI patients and serves two purposes: neurological decompression and spinal stabilization. Neurological decompression entails removal of bone, blood, and disco-ligamentous structures that impinges the spinal cord, whereas spinal stabilization is achieved by spinal instrumentation rigidly connecting the unstable segments of the spine. Acute surgery ultimately aims at limiting the secondary injury to the spinal cord. The pattern of spine injury contributes to the secondary injury of the spinal cord through direct and indirect mechanisms. The residual compression of the spinal cord for example from the typical vertebral bone fragment retropulsed in the spinal canal with thoracolumbar burst fractures directly exerts loads on the spinal cord. In parallel, residual misalignment or instability between spinal elements can also exert undue loads on the spinal cord through an indirect mechanism, further contributing to the secondary injury of the spinal cord if it has not been addressed properly. Careful and prompt assessment of the clinical stability is therefore key after a TSCI.


In line with the definition by , we define traumatic spinal instability to be the loss of the ability of the spine to maintain normal alignment and motion between spinal segments in such a way that a secondary injury to the spinal cord will not occur.


Patterns of spine injuries associated with spinal cord injuries


TSCI can be associated with a stable or unstable spine, and we suggest that TSCI should be categorized accordingly to guide its management.


Traumatic spinal cord injury in a stable spine


Different terminologies have been used previously to describe TSCI not associated with overt spinal instability. The term SCIWORA (Spinal Cord Injury Without Radiographic Abnormality) was proposed by in 1982 to describe TSCI occurring in children without evidence of spine injuries such as fracture or dislocation on plain radiographs or tomography. Given the inherent elasticity of the vertebral column in children, they suggest that the vertebral column can deform in flexion, hyperextension or distraction in such a way that the spinal cord can be injured without concomitant damage of the vertebral column. SCIWORA more frequently involves the cervical spinal cord because spinal mobility is greater in the cervical than the thoracic and lumbar spine. In the adult population, SCIWORA has also been used to describe TSCI associated with underlying spondylotic changes, spinal stenosis, ossification of posterior longitudinal ligament, ligamentous abnormalities, and/or traumatic disc herniation ( ; ; ; ), in the absence of fracture or dislocation. Many authors ( ; ) warn about the misapplication of the term SCIWORA in adults considering that these patients will commonly have underlying canal stenosis and significant degenerative changes—which are radiographic abnormalities per se—and instead propose the use of the term SCIWOCTET (Spinal Cord Injury Without Computed Tomography Evidence of Trauma) or SCIWORET (Spinal Cord Injury Without Radiographic Evidence of Trauma) in these patients.


For the cervical spine, proper identification of TSCI with a stable spine is important because they are typically associated with improved recovery ( ). While the presence or absence of radiographic/tomographic findings can help to understand the pathomechanism of the injury and guide treatment, the central aspect to SCIWORA, SCIWORET, and SCIWOCTET is that they usually are not associated with overt spinal instability. For these conditions, the spinal cord is at higher risk of injury either due to the altered biomechanics of the spine or to the decreased space available for the spinal cord. The current authors believe that the overarching concept for these aforementioned terms is the absence of spinal instability, and that these terms should be used with caution because they underestimate the importance of spinal trauma and biomechanics in the context of TSCI. Radiological evidence of trauma can include a wide range of findings, and should not be interpreted without consideration of the spinal stability. The authors instead propose to use the term TSCI with a stable spine in order to account for the biomechanical and surgical aspects involved in the management of TSCI. Indeed, TSCI with a stable spine does not preclude the presence of minor spinal injuries such as spinous process fractures that do not impair the clinical stability of the spine and do not require surgical stabilization.


In adults, underlying degenerative changes are usually observed, but the literature does not provide specific outcome predictors based on the evaluation of the underlying spondylosis.


Animal models repeatedly showed a strong correlation between the amount of spinal cord compression and severity of the neurological impairment. However, in adult trauma patients the correlation between cord compression observed on MRI and the neurological impairment remains unclear. Interestingly, normal aging of the spine commonly implies degenerative changes that cause reduction of the diameter of the spinal canal called spinal stenosis, especially at the cervical level. Literature does not provide specific outcome predictors based on the evaluation of the underlying spondylosis. In a population of adult low-velocity cervical trauma—CCS— found a somewhat counterintuitive association between maximal cord compression on MRI and neurological recovery.


The CCS is a common clinical syndrome observed after a cervical TSCI and is typically observed in the presence of a stable spine. It has been described initially by and is characterized by a disproportionate greater motor impairment in the upper extremities and varying degree of bladder dysfunction and sensory loss. The central cord syndrome is mainly prevalent in the elderly population with pre-existing spondylotic cervical stenosis ( ; ). The mechanism of trauma typically involves a fall from someone’s height with cervical hyperextension. During hyperextension of the cervical spine, spinal cord damage associated with the central cord syndrome results from posterior compression from inward bulging of the ligamentum flavum and from anterior compression from marginal osteophytes and/or bulging discs ( ). The greater impairment in the upper extremities results from the injury to the spinal gray matter and lateral corticospinal tracts ( ). More specifically, it is believed that the predominant involvement of the upper extremities is due to the somatotopic arrangement of the lateral corticospinal tracts, such that the medial (more central) portion of the corticospinal tract controlling upper extremity motor function sustains a greater injury than the lateral portion controlling lower extremity motor function ( ). However, this hypothesis has been challenged by different authors who observed a uniform damage to the lateral corticospinal tract, suggesting that motor fibers descending to the upper and lower extremities are interlaced ( ; ).


In a finite-element analysis, Bailly et al. suggest that the presence of hypertrophic ligamentum flavum induced specific stress and strain distributions consistent with previous histologic findings related to central cord syndrome. The presence of a disc bulging alone was not associated with such a pattern, but when combined with hypertrophic ligamentum flavum, increased the overall level of stress and strain in the lateral corticospinal tract ( Fig. 1 )




Fig. 1


Hyperextension cervical trauma in a stable spine with degenerative changes. CCS. (*) red asterisks, Bulging disc C3–C4, C4–C5, C5–C6. (**) White asterisks, thickened ligamentum flavum multilevel mainly C4–C5. Yellow arrow showing spinal cord edema and hypersignal in C3 level.


Traumatic spinal cord injury in an unstable spine


While TSCI in a stable spine occurs predominantly at the cervical level due to the increased range of motion and frequent preexisting canal stenosis, unstable injuries occur commonly at all spinal levels. The neurological level of injury strongly relates to the neurological recovery, and it is common to distinguish between high cervical (C1–C4), low cervical (C5–C8), thoracic (T1–T10), and thoracolumbar (T11–L2) TSCI ( ). These categories account for the functional activity of the spinal cord, including breathing, upper versus lower extremity function, trunk control, and bowel/bladder function. However, other anatomical and morphological criteria should also be considered to further delineate the level of injury and characterize the trauma biomechanics when classifying and managing the unstable spine associated with TSCI.


There are three basic categories or types used in a similar manner to the AO thoracolumbar fracture classification system to describe primary injury morphology. The “Type A” injuries are fractures that result in compression of the vertebra with intact tension band. “Type B” injuries include failure of the posterior or anterior tension band through distraction with physical separation of the subaxial spinal elements while maintaining continuity of the alignment of the spinal axis without translation or dislocation. “Type C” includes those injuries with displacement or translation of one vertebral body relative to another in any direction; anterior, posterior, lateral translation, or vertical distraction. Injuries are first classified by their level and either C, B, or A in this order ( ).


Cervical spine injuries (C0–C7)


TSCI with an unstable spine are highly prevalent in the cervical spine for different reasons. The cervical spine is vulnerable to several types of trauma mechanisms, including motor vehicle accidents, diving, sports injuries, and falls. When compared to the thoracic and lumbar spine, the bony elements in the cervical spine are smaller and the disco-ligamentous structures are weaker so that they are more exposed to injuries.


In line with the demographic shift to an older population, the increasing prevalence of cervical degenerative changes and pre-existing spondylotic stenosis increases the likelihood to sustain a TSCI in association with a cervical trauma.


Upper cervical spine injuries (C0–C2)


TSCI following traumatic spine injuries occurring at the occipito-cervical or atlanto-axial level are highly distinct from subaxial injuries due to the increased risk of mortality and poor recovery. It is thought that between 8% and 19% of fatal cervical spine injuries result from occipito-cervical dislocation ( ). In addition, occipito-cervical and atlanto-axial levels have a highly distinct bony/soft tissue anatomy and biomechanical behavior, which require particular attention for evaluation and management. The spinal canal is also particularly large at these levels; it is widest between the C1 and C3 levels (AP diameter 16–30 mm) and progressively narrows caudally (14–23 mm).


Occipito-cervical dislocation is associated with high mortality rates because a high-energy trauma—and high level of energy transferred to the spinal cord—is required to damage the strong ligamentous structures at the occipito-cervical junction, which comprise the tectorial membrane and alar ligament ( ).


TSCI is seen infrequently with isolated C1 fracture ( ). A TSCI occurs when the transverse ligament is torn and atlanto-axial instability occurs. The mechanism of SCI is similar to that occurring with isolated rupture of the transverse ligament or odontoid fracture: the spinal cord will be compressed between the posteriorly displaced odontoid and the posterior arch of C1.


Subaxial cervical spine injuries (C3–C7)


The Spinal Trauma Study Group ( ) provided new insight on the morphological features of subaxial cervical trauma that need to be considered in TSCI, through the proposal of a novel classification system. In line with previous studies ( ), they consider that the severity and outcome of TSCI depend on the morphology of the lesion, which results in different loading conditions in the spinal cord. Based on their concept, we propose that TSCI with unstable subaxial cervical spine be classified following this increasing order of severity:



  • (1)

    Compression injury without involvement of the posterior vertebral wall.


  • (2)

    Burst injury with involvement of the posterior vertebral wall.


  • (3)

    Distraction injury such as a hyperextension injury involving rupture of anterior longitudinal ligament and intervertebral disc or a flexion-distraction injury involving disruption of posterior ligaments and facet capsules.


  • (4)

    Translation/rotation injury which is typical of bilateral facet dislocation.



TSCI is unlikely with a compression injury sparing the posterior vertebral wall, unless significant kyphosis occurs ( ). A TSCI is more likely with burst fractures when significant energy is transferred to the spinal cord from retropulsion of the posterior wall into the spinal canal ( Fig. 2 )


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Nov 9, 2024 | Posted by in NEUROLOGY | Comments Off on Biomechanics and patterns of spine injuries associated with spinal cord injury

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