In vitro torsion-induced stress distribution changes in porcine intervertebral discs.

STUDY DESIGN: A cadaveric porcine spine motion segment experiment was conducted. OBJECTIVE: To test the hypothesis that small vertebral rotations cause increased stress in the anulus while decreasing stress in the nucleus through stiffening of the anulus. SUMMARY OF BACKGROUND DATA: Stress profiles of the intervertebral disc reportedly depend on degeneration grade and external loading. Increased stress in the anulus was found during asymmetric loading. In addition, depressurization of the nucleus combined with an instantaneous disc height increase was found when small (<2 degrees ) axial vertebral rotations were applied. METHODS: Seven lumbar porcine cadaveric motion segments consisting of two vertebrae and the intervening disc with ligaments were loaded in the neutral position with 340 N of compression. Stress profiles were obtained in the neutral position, then after 0.5 degrees and 1 degrees axial rotation of the bottom vertebral body. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc along a straight path in the midfrontal plane. Stress profiles were measured in vertical (0 degrees ) and horizontal (90 degrees ) orientation. RESULTS: Deformation of the anulus by small axial rotations of the lower vertebra instantaneously decreased the horizontally and vertically measured stress in the nucleus while increasing stress in the anulus. A 1-hour period of creep loading decreased the stresses in the nucleus and the anulus 20% to 30%, depending on the orientation, but the effect of an increasing stress in the anular region after axial rotation persisted. CONCLUSIONS: The compressive Young's modulus of the composite anulus tissue increases instantaneously when small axial rotations are applied to porcine spine motion segments. This is accompanied by decreased stress in the nucleus pulposus, increased stress in the anulus fibrosus, changes in the stress profile superimposed on and independent of prolonged viscoelastic creep and dehydration, and changes in stress distribution independent of horizontal and vertical orientation.     

Another chance: a non-seatbelt related fracture of the lumbar spine

A previously undescribed upper lumbar spine fracture configuration is reviewed in three patients. These flexion/distraction injuries were not associated with seatbelt use. The anterior vertebral body underwent significant compression. The distraction component created a horizontal fracture through the pedicles and lamina, with avulsion of the spinous process of the adjacent cephalad vertebra.     

Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation.

Cervical spine disorders such as spondylotic radiculopathy and myelopathy are often related to osteophyte formation. Bone remodeling experimental-analytical studies have correlated biomechanical responses such as stress and strain energy density to the formation of bony outgrowth. Using these responses of the spinal components, the present study was conducted to investigate the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C5-C6 cervical spine was used to simulate progressive disc degeneration at the C5-C6 level. Slight degeneration included an alteration of material properties of the nucleus pulposus representing the dehydration process. Moderate degeneration included an alteration of fiber content and material properties of the anulus fibrosus representing the disintegrated nature of the anulus in addition to dehydrated nucleus. Severe degeneration included decrease in the intervertebral disc height with dehydrated nucleus and disintegrated anulus. The intact and three degenerated models were exercised under compression, and the overall force-displacement response, local segmental stiffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints, and strain energy density and stress in the vertebral cortex were determined. The overall stiffness (C4-C6) increased with the severity of degeneration. The segmental stiffness at the degenerated level (C5-C6) increased with the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density and stress in vertebral cortex increased adjacent to the degenerated disc. Specifically, the anterior region of the cortex responded with a higher increase in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes.     

Anheftungspunkte des Lig. longitudinale posterius und deren Bedeutung für Wirbelkörperfrakturen

Spine fractures with damage of the posterior wall of the vertebra often can be anatomically reconstructed by indirect reduction. Whether the posterior longitudinal ligament (PLL) is responsible for the reduction is still subject to debate. The aim of our investigation was to ascertain the role of the PLL in closed reduction of spine fractures by identifying the bony attachment points of this ligament. We performed a gross anatomical dissection, a light- and polarized microscopic investigation on 22 human cadaverous thoracic and lumbar spines to determine the points of attachment of the PLL. We found two layers of the PLL. The superficial layer runs from the first thoracic down to the third lumbar vertebra with a width of 0.4-1.0 cm and from there descends as a thin rudiment to the sacrum. The deep layer shows a segmental rhomboid structure. Lateral fibers are attached to the annulus fibrosus and at the rim of the adjacent vertebrae. Medial fibers are attached additionally to the posterior wall of the vertebral bodies by bridging the foramina basivertebralia. Since these foramina become enlarged in the caudal parts of the vertebral column, the number of attachment points at the posterior wall of the vertebral bodies decreases caudally. Good results for reconstruction of the posterior wall in vertebral fractures of the thoracic and upper lumbar spine can be explained by the anatomical situation of the PLL and stress the important role of the PLL in indirect reduction of spine fractures.     

Lumbar vertebral haemangioma causing pathological fracture, epidural haemorrhage, and cord compression: a case report and review of literature

CONTEXT: Vertebral haemangiomas are recognized to be one of the commonest benign tumours of the vertebral column, occurring mostly in the thoracic spine. The vast majority of these are asymptomatic. Infrequently, these can turn symptomatic and cause neurological deficit (cord compression) through any of four reported mechanisms: (1) epidural extension; (2) expansion of the involved vertebra(e) causing spinal canal stenosis; (3) spontaneous epidural haemorrhage; (4) pathological burst fracture. Thoracic haemangiomas have been reported to be more likely to produce cord compression than lumbar haemangiomas. FINDINGS: A forty-nine year old male with acute onset spinal cord compression from a pathological fracture in a first lumbar vertebral haemangioma. An MRI delineated the haemangioma and extent of bleeding that caused the cord compression. These were confirmed during surgery and the haematoma was evacuated. The spine was instrumented from T12 to L2, and a cement vertebroplasty was performed intra-operatively. Written consent for publication was obtained from the patient. Clinical Relevance: The junctional location of the first lumbar vertebra, and the structural weakness from normal bone being replaced by the haemangioma, probably caused it to fracture under axial loading. This pathological fracture caused bleeding from the vascularized bone, resulting in cord compression.     

Lumbar spine endplate fractures: Biomechanical evaluation and clinical considerations through experimental induction of injury

Lumbar endplate fractures were investigated in different experimental scenarios, however the biomechanical effect of segmental alignment was not outlined. The objectives of this study were to quantify effects of spinal orientation on lumbar spine injuries during single‐cycle compressive loads and understand lumbar spine endplate injury tolerance. Twenty lumbar motion segments were compressed to failure. Two methods were used in the preparation of the lumbar motion segments. Group 1 (n = 7) preparation maintained pre‐test sagittal lordosis, whereas Group 2 (n = 13) specimens had a free‐rotational end condition for the cranial vertebra, allowing sagittal rotation of the cranial vertebra to create parallel endplates. Five Group 1 specimens experienced posterior vertebral body fracture prior to endplate fracture, whereas two sustained endplate fracture only. Group 2 specimens sustained isolated endplate fractures. Group 2 fractures occurred at approximately 41% of the axial force required for Group 1 fracture (p < 0.05). Imaging and specimen dissection indicate endplate injury consistently took place within the confines of the endplate boundaries, away from the vertebral periphery. These findings indicate that spinal alignment during compressive loading influences the resulting injury pattern. This investigation identified the specific mechanical conditions under which an endplate breach will take place. Development of endplate injuries has significant clinical implication as previous research identified internal disc disruption (IDD) and degenerative disc disease (DDD) as long‐term consequences of the axial load‐shift that occurs following a breach of the endplate. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1084–1091, 2016.     

Vertebral burst fractures: an experimental, morphologic, and radiographic study.

Spinal burst fractures are produced by rapid compressive loading, and may result in spinal cord injury from bone fragments forced from the vertebral body into the spinal canal. This fracture is one of the most difficult injuries of the spine to successfully treat, in part because the biomechanics of reduction and the exact mechanism by which the distraction forces are transmitted to the intracanal fragments of the burst fracture have not been adequately investigated. The authors developed a reproducible technique for creating these fractures in vitro. The fractures produced were identical to those observed in clinical practice, and were used for investigating the mechanics of this fracture and its reduction. This work describes the pathologic anatomy of the burst fracture both on the gross structure and also on microtome sections of the vertebrae, and examines the biomechanics of fracture reduction. The margins of the vertebral bone fragment, which was forced posteriorly into the spinal canal during fracture, were noted to extend far laterally beyond the pedicles. The authors also found extensive damage not only to the disc above the injured level, but also to that below, explaining the clinical observation that disc degeneration frequently occurs at both levels. Examination of anatomic data provided by microtome section supported the hypothesis that the fibers that actually reduce the intracanal fragment originate in the anulus of the superior vertebra in the midportion of the endplate and insert into the lateral margins of the intracanal fragment. Investigations using magnetic resonance imaging confirmed that these obliquely directed fibers account for the indirect reduction of the fragment. The authors' studies demonstrate that the posterior longitudinal ligament provides only a minor contribution in the reduction of the fracture in comparison to the attachments of the posterior portion of the anulus fibrosus. The forces required to reduce this fragment were studied. Distraction was found to be the predominant force required for indirect posterior reduction. This was confirmed by a series of tests using devices that provided segmental fixation. The application of uniform distraction forces was most effective in the posterior reduction of the intracanal fragment.     

Fractures of the posterior apophyseal ring of the lumbar vertebral body in young patients. Three cases

Three cases of posterior apophyseal fracture of a lumbosacral vertebral body are described. This disco-apophyseal lesion is typical of adolescents and sports-related trauma. Like all acute and chronic apophysiolyses it is probably enhanced by a constitutional weakness of the apophyseal area, closely fused with the anulus fibrosus of the disc.     

Effect of changes in lordosis on mechanics of the lumbar spine-lumbar curvature in lifting.

Using a realistic nonlinear three-dimensional finite element model, biomechanics of the entire lumbar spine L1-S1, risk of tissue injury, and required local lumbar muscle exertion in extended and flexed postures are investigated under moderate to relatively large compression loads as great as 2800 N as the lumbar lordosis is altered from the undeformed value of -46 degrees by + 15 degrees in extension or by as much as 38 degrees in flexion. To prevent the instability of the passive structure in compression, the changes in segmental rotations are prescribed and the required sagittal/lateral moments at each level calculated. The effect of load distribution is considered by applying the whole compression on the L1 vertebra alone or among all vertebral levels with 90% or 80% of the compression on the L1 and the remaining evenly shared by the rest. The results are markedly affected by the postural changes and load distributions. The primary global displacement responses are stiffened in the presence of combined loads. The axial compression load substantially increases the intradiscal pressure, facet loads, and disc fiber strains. The large facet loads at the caudal L5-S1 level causes large differential sagittal rotations at vertebral posterior and anterior bony structures, resulting in large stresses in the pedicles and pars interarticularis. The contribution of the passive structures in carrying the load is influenced by the lumbar lordosis and compression load magnitude. Slight flattening of the lumbar spine under large compression reduces the maximum disc fiber strains and required equilibrating moments without adversely affecting the disc pressure and ligament forces. During lifting tasks, the passive spinal structures are protected by slight to moderate flattening in the lumbar curvature, whereas larger flexion angles impose significantly higher risk by increasing the disc pressure, disc anulus fiber strains, ligamentous forces, and facet forces. Changes in lordosis also markedly affect the stabilizing sagittal moments; the required moments diminish in small flexion angles, thus requiring smaller forces in local lumbar muscles. Thus, the lumbar posture during heavy lifting could be adjusted to minimize the required moments generated by lumbar muscle exertions and the risk of tissue injury.     

Brust- und Lendenwirbelsäulenverletzungen im Kindes- und Jugendalter

Spinal injuries are generally very rare in childhood. Fractures of the thoracic and lumbar spine occur mainly in older children and adolescents. Exact knowledge of the anatomy is essential for accurate diagnosis in still incomplete ossification. With increasing age the classification can be performed by using the AO classification over the age of 8 years. Neurological symptoms in the thoracic and lumbar spine occur mainly in adolescence. Conventional radiography is the standard diagnostic tool for thoracic and lumbar spinal injuries. With the appearance of abnormal neurological deficits magnetic resonance imaging (MRI) diagnostics should also be performed and for operative cases computed tomography (CT) scans are mandatory. The most common fractures of the thoracic and lumbar spine are compression fractures (type A) which can generally be treated conservatively due to the stable situation but unstable fractures of the thoracic and lumbar spine (types B and C) are stabilized dorsally (internal fixation). Ventral stabilization with vertebral body replacement is occasionally necessary in adolescents. Spinal injuries in children have a good overall prognosis.     

Modeling of the naked facet sign in the lumbar spine

The study design is a computer visualization model that simulated flexion deformities about the lumbar spine for evaluation of the naked facet sign (NFS). The objectives were to ascertain the angles of rotation required for NFS to occur in the lumbar spine with various centers of rotation about the vertebral body and to assess whether NFS correlates with unstable flexion-distraction injuries in the lumbar spine. The presence of the NFS on axial computed tomography (CT) images occurs when the inferior articulating facet of the cephalad vertebra is not paired with an adjacent superior articulating facet of the caudal vertebra. This sign, when evidenced in the lumbar spine, is suggestive of significant injury secondary to a flexion-distraction force. A previous study using a computer-generated spine model challenged the utility of the NFS in the thoracolumbar spine. The NFS may prove to be more diagnostic of an unstable injury in the lumbar spine because of its normal lordotic resting position. A commercial spine computer visualization model was used to simulate various degrees of flexion injury in the lumbar spine. Lumbar functional spinal units (FSU) L2-L5 were each examined separately. The model simulated two CT scan slices (each 2 mm thick), which were created parallel to the inferior endplate of the cephalad vertebra of each FSU. The cephalad vertebra was rotated in 0.5 degrees increments until NFS was produced. The appearance of NFS required >/=11 degrees kyphotic angulation in more than two thirds of simulated centers of rotation about the lumbar vertebral bodies. The NFS was produced between a range of 8-24.5 degrees. For rotations about a point located 3 cm anterior to the vertebral body (to simulate seat-belt-type flexion-distraction injuries), the minimum angle required for NFS was 7.5 degrees. Our data correlate well with previously published results from in vitro and cadaveric studies. As opposed to the thoracolumbar spine, which normally rests in a neutral position, the lumbar spine normally rests in a lordotic position. Therefore, NFS in the lumbar spine may be more suggestive of an unstable injury and would warrant closer examination of the patient and additional radiographic studies.     

Injuries of the lumbosacral segments of the spine

Having studied 470 case histories of patients with injuries of the spine the author found traumas of the 5th lumbar vertebra in 23 cases (4.9%). The characteristic injuries were compression wedge-shaped fractures, multiple fractures of the vertebrae, comminuted and traumatic spondylolistheses and dislocation fractures. A typical mechanism of the injury was a blow in the lumbar region or in the back with subsequent bending of the trunk. The tactics of surgical treatment of such lesions is presented, the main tasks of which are the reconstruction of the shape and the volume of the vertebral canal, the stabilization of the basis compartment of the spine and the prevention of subsequent degenerative changes.     

Prediction of peripheral tears in the anulus of the intervertebral disc

STUDY DESIGN: The mechanical behavior of age-related degeneration in the anulus of lumbar spine segments was investigated under extension with compressive preload by the finite-element method. OBJECTIVE: To investigate why peripheral tears are initiated in the anterior outer anulus in the early stage of life. SUMMARY OF BACKGROUND DATA: Age-related changes in the geometry, loading conditions, and material-mechanical properties of lumbar spine would change mechanical behavior of the anulus. METHODS: Two finite element models of the human lumbar segments (L3-L4), a young spine model for young adults and an old spine model for elder adults, were constructed. The anulus was modeled as laminate composite elements with 16 layers and six materials. The microfailure modes such as fiber breakage, fiber folding, layer (ply) failure, layer folding, and interlaminar delamination were predicted by principal strain of the anulus layer. RESULTS: Excessively high tensile principal strain in the transverse direction of the anulus layer, indicating layer failure, was predicted in the anterior outer anulus. The strain level was much higher in the young model than in the old model. Compressive principal strain in the transverse direction of the anulus layer, predicting layer folding, was found in the posterior and posterolateral anulus. CONCLUSIONS: Layer failure in the anterior outer anulus could occur in the early stage of life during extension with preload. This could then progress to formation of peripheral tears oriented at right angles to the fiber's direction.     

Spinal loads after osteoporotic vertebral fractures treated by vertebroplasty or kyphoplasty

Vertebroplasty and kyphoplasty are routine treatments for compression fractures of vertebral bodies. A wedge-shaped compression fracture shifts the centre of gravity of the upper body anteriorly and generally, this shift can be compensated in the spine and in the hips. However, it is still unclear how a wedge-shaped compression fracture of a vertebra increases forces in the trunk muscle and the intradiscal pressure in the adjacent discs. A nonlinear finite element model of the lumbar spine was used to estimate the force in the trunk muscle, the intradiscal pressure and the stresses in the endplates in the intact spine, and after vertebroplasty and kyphoplasty treatment. In this study, kyphoplasty represents a treatment with nearly full fracture reduction and vertebroplasty one without restoration of kyphotic angle although in reality kyphoplasty does not guarantee fracture reduction. If no compensation of upper body shift is assumed, the force in the erector spine increases by about 200% for the vertebroplasty but by only 55% for the kyphoplasty compared to the intact spine. Intradiscal pressure increases by about 60 and 20% for the vertebroplasty and kyphoplasty, respectively. In contrast, with shift compensation of the upper body, the increase in muscle force is much lower and increase in intradiscal pressure is only about 20 and 7.5% for the vertebroplasty and kyphoplasty, respectively. Augmentation of the vertebral body with bone cement has a much smaller effect on intradiscal pressure. The increase in that case is only about 2.4% for the intact as well as for the fractured vertebra. Moreover, the effect of upper body shift after a wedge-shaped vertebral body fracture on intradiscal pressure and thus on spinal load is much more pronounced than that of stiffness increase due to cement infiltration. Maximum von Mises stress in the endplates of all lumbar vertebrae is also higher after kyphoplasty and vertebroplasty. Cement augmentation has only a minor effect on endplate stresses in the unfractured vertebrae. The advantages of kyphoplasty found in this study will be apparent only if nearly full fracture reduction is achieved. Otherwise, differences between kyphoplasty and vertebroplasty become small or vanish. Our results suggest that vertebral body fractures in the adjacent vertebrae after vertebroplasty or kyphoplasty are not induced by the elevated stiffness of the treated vertebra, but instead the anterior shift of the upper body is the dominating factor.     

Computed tomography of the spine as an important diagnostic tool in the management of war missile spinal trauma

Twenty-two patients with spinal injury were evaluated by plain radiography immediately after hospital admission. In 14 patients whose condition was stable, we performed computed tomography (CT) scanning through the involved segments. To provide better planning before neurosurgical management, we divided the vertebral column in thirds. According to this division, we concluded that these injuries are mostly extensive, severely damaging all three thirds of the vertebral column and accompanying neural structures in the majority of cases. The information acquired by CT concerning bony fragments, bone destruction, dural tear, spinal cord and nerve root compression, and neural damage directly influenced the surgical management. All patients except one underwent surgery while associated injuries of other organs were given priority in management. Injuries of the thoracic and the lumbar spine were the most common ones, frequently found in association with lesions of nearby organs. Penetrating injuries with a dural lesion were present in the majority of cases, while spinal cord injury was obvious in some. They were all well visualized using spinal CT scanning. Our view is that the role of CT is essential in guiding surgical management of war missile injuries to the spine. Received: 18 February 1997     

Biomechanical changes after the augmentation of experimental osteoporotic vertebral compression fractures in the cadaveric thoracic spine.

BACKGROUND CONTEXT: Osteoporotic compression fractures are an important public health concern, leading to significant morbidity, mortality and economic burden. Cement augmentation procedures used to treat these fractures alter the biomechanics of the fractured segment, which could promote adjacent failure. However, if alignment is improved or restored, there will be less risk of adjacent failure. PURPOSE: To determine the effects of load (compression/flexion), adjacent vertebral location (superior/inferior) and augmentation on vertebral segment stiffness and adjacent vertebral strain in the upper and lower thoracic spine. STUDY DESIGN: Human cadaveric thoracic spine segments were tested under load control before and after the creation of experimentally augmented vertebral compression fractures. METHODS: Six T1-T5 and six T8-T12 segments were obtained from eight thoracic spines with known bone mineral density (BMD). Rosette strain gauges were applied to T2, T4, T9 and T11 to measure strain adjacent to the experimental fracture sites T3 and T10. Two compression fractures were created in succession, the first in flexion preceded by a weakening defect in T3 and T10 and the second created in an adjacent vertebra in compression without prior weakening. The first fracture was reduced with the inflatable bone tamp (IBT) and augmented with cement. Compression and flexion tests were performed before and after the first fracture while measuring vertebral cortical shear strain on T2, T4, T9 and T11 and stiffness of the entire segment. Strain and stiffness were compared by using a repeated measures analysis using adjacent vertebral location (superior/inferior), augmentation and load (compression/flexion) as factors. RESULTS: The mean BMD was 0.61+/-0.11 g/cm(2) (T1-T5) and 0.78+/-0.07 g/cm(2) (T8-T12). Stiffness in compression and flexion increased with load (p<.05, and p>.27, respectively). Augmentation reduced compressive and bending stiffness (p=.23, and p=.19, respectively), whereas the adjacent vertebral strain increased (p>.11). The adjacent strain in flexion was much greater than in compression (p<.03). Cement augmentation caused greater amounts of inferior than superior adjacent strain (p>.19). The applied moment at first fracture was 2.98+/-1.28 Nm (T1-T5) and 8.44+/-1.02 Nm (T8-T12). The compressive load at second fracture was 1122+/-993 N (T1-T5) and 2906+/-1008 N (T8-T12). Adjacent vertebral strain during the second compression and flexion tests exceeded that during the first compression and flexion tests (p=.11). Adjacent vertebral strain at second fracture exceeded that at first fracture (p=.007) and was greater on the superior adjacent vertebra than the inferior (p=.47). CONCLUSION: With axial compressive loads, the addition of flexion increases fracture risk. Cement augmentation of a fractured vertebral segment reduces stiffness while increasing both the superior and inferior adjacent cortical strain. This increment in strain that is greatest on the inferior adjacent vertebra effectively redistributes loads from the superior adjacent vertebra to the inferior adjacent vertebra, sparing the superior adjacent vertebra from failure.     

Chance-type flexion-distraction fracture of lumbar spine

A case of a twenty years old male who had been hit by a van at the lower back, presented two weeks later with complete paraplegia and double incontinence is presented. Radiological imaging revealed shearing of spine with fracture line slicing through the second lumbar (L2) vertebra going across all the three vertebral columns with complete retrolisthesis of upper fragment. He was managed conservatively with immobilization and rehabilitation.     

胸腰椎脊髓损伤侧前方减压的适应证与术式选择

胸腰椎骨折脱位并截瘫主要系椎管前方的椎体骨拆块、椎体后上角与椎间盘突出及大于２０°的脊柱后弓成角所致，是侧前方减压的适应证。经后正中入路椎弓根侧前方减压的优点是手术创伤较小，可同时探查脊髓与安置后方内固定；缺点是不能直视脊髓前方，对侧减压可能不彻底或需对侧辅助减压。适于Ｔ１０以上的胸椎与Ｌ２以下腰椎，亦适于胸腰段，特别是已行椎板切除者。     

Rationale for the management of flexion-distraction injuries of the thoracolumbar spine based on a new classification

A new classification of flexion-distraction injuries of the spine is described based on the bony and soft tissue injuries to the posterior complex and the anterior column. In addition, the classification includes the status of the vertebral body, that is, the association of a wedge-compression fracture or a burst injury. The soft tissue component provides a rationale for surgical intervention. Most injuries were treated by compression instrumentation, but it is recommended that those injuries associated with a burst component require distraction instrumentation.