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Biomechanics of Vertebral Fractures and Their Treatment
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BIOMECHANICS OF VERTEBRAL FRACTURES AND THEIR TREATMENT
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Richard A. Lindtner, Werner Schmoelz Medical University of Innsbruck, Innsbruck, Austria
INTRODUCTION Traumatic lesions of the spine are relatively common and encompass a wide spectrum of injuries ranging from minor fractures of spinous or transverse processes to complex, multidirectionally unstable injuries. These lesions may involve osseous or disco-ligamentous structures or both. Spinal injuries usually are the result of high-energy trauma, and associated injuries are common. Moreover, traumatic injury to the spine frequently leads to neurological deficits with potentially devastating consequences for patients, ranging from mild, transient symptoms to complete tetraplegia. Other potential sequelae of spinal injuries include progressive deformity, persistent pain, and disability. The most appropriate treatment strategy for spinal injuries varies depending on the degree to which spinal structures have been compromised. Treatment options thus range from conservative to combined posteroanterior surgical approaches depending on the individual injury pattern. General goals of treatment, however, include restoration of anatomic alignment, decompression of neural elements (if required), adequate stabilization to allow for early mobilization, and maximization of clinical outcome and neurologic recovery. The majority of spinal injuries affect the thoracic and lumbar spine. This chapter will therefore focus on thoracolumbar trauma, and special reference will be made to important aspects of injury mechanism, classification, and biomechanics of fracture stabilization.
INJURY MECHANISMS In young and middle-aged individuals, thoracolumbar fractures typically result from high-energy blunt trauma. Common injury mechanisms include falls from height, motor vehicle accidents, and sportsrelated injuries. In elderly people, low-energy mechanisms and even spontaneous fractures without relevant trauma are more common as a consequence of diminished bone quality caused by osteoporosis. Bone disorders other than osteoporosis may also increase the susceptibility to fracture even after a low-energy impact. Ankylosing spinal disorders (i.e., ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis), for example, lead to ankylosis/fusion of normally mobile spinal segments via progressive ossification of spinal ligaments and/or intervertebral discs. As a consequence of ankylosis, the normal biomechanics of the spine is massively altered, and forces can act on longer lever arms during trauma. The ankylosed thoracolumbar spine is thus much more susceptible to injury even after low-energy trauma and typically shows an otherwise uncommon hyperextension injury pattern. Biomechanics of the Spine. https://doi.org/10.1016/B978-0-12-812851-0.00022-7 © 2018 Elsevier Ltd. All rights reserved.
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In high-energy trauma, usually complex and multidirectional forces act on the spine at the time of injury. These forces may involve compressive and distractive forces; rotational moments in the sagittal (flexion and extension), coronal (left and right side bending), and axial (left and right axial rotation) planes; as well as shear forces. For the most part, the spine is exposed to a combination of several of these forces and moments during trauma. The forces and moments acting on the spine can result in a great variety of injury patterns, including bony injuries without ligamentous disruption, ligamentous injury without bony injury, or often a combination of both. Depending on the direction of the resultant force vector, characteristic types of injury patterns result. The extent of disco-ligamentous and/or bony disruption again varies as a function of the magnitude of the resulting force vector acting on the spine. From a biomechanical point of view, the spinal column can be divided into an anterior and a posterior column, as suggested by Holdsworth (1970). The anterior column consists of the vertebra and intervertebral discs (including the anterior and posterior longitudinal ligaments), resists compression and axial loading, and carries about 60%–90% of the load in the normal upright position (Adams and Dolan, 2005). The posterior column consists of the bone structures posterior to the posterior longitudinal ligament (posterior vertebral arch) as well as the posterior ligamentous complex (PLC), maintains tension, and carries about 10%–40% of the load in the normal upright position. The PLC is critical for spinal stability and consists of the supraspinous and interspinous ligaments, the ligamentum flavum, and the facet capsules. Depending on the resultant force vector and resultant moments, characteristic patterns of anterior and/ or posterior column involvement result. A traumatic axial load (i.e., a load applied along the spinal long axis) results in excessive compression and subsequent failure of the anterior column in the form of vertebral body fracture without significant injury to the posterior column. A traumatic forward bending moment leads to flexion of the spinal column in the sagittal plane and causes tension of the posterior column as well as compressive loading of the anterior part of the vertebral body. In contrast, a traumatic backward bending moment leads to hyperextension of the spinal column and causes tension of the anterior column (particularly of the anterior annulus fibrosus and anterior longitudinal ligament) as well as compression of the posterior column. Excessive and complex traumatic loading may cause a complete disruption of both columns that permits fracture dislocation with translational displacement in one or more planes. The thoracolumbar junction (T11-L2) is the most common site of spinal fractures and is particularly vulnerable because it constitutes an anatomical and biomechanical transition zone between the subjacent lumbar and superjacent thoracic spine. At the thoracolumbar junction, the relatively rigid thoracic kyphosis transitions to the more flexible lumbar lordosis. The rib cage constrains motion of the thoracic spine and increases resistance to bending moments in the sagittal and coronal planes as well as axial rotation. The “free floating” ribs of T11 and T12 provide markedly less stability at the thoracolumbar junction because they are not connected to the sternum. Furthermore, the coronal orientation of the thoracic facet joints limits flexion and extension and provides significant resistance to anterior translation, whereas the more sagittal-oriented lumbar facet joints allow for increased motion in flexion and extension.
DIAGNOSTIC IMAGING Initial evaluation of a patient with suspected thoracolumbar injury consists of clinical assessment including a thorough neurological examination as well as diagnostic imaging. Imaging modalities include conventional anteroposterior and lateral radiographs, computed tomography (CT), and magnetic resonance imaging (MRI). CT is considered the “gold standard” for imaging of spinal trauma. It offers higher spatial resolution than conventional plain radiographs and allows visualization of the spine in the axial, sagittal, and coronal planes.
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CT is superior for quantifying the extent of vertebral body comminution as well as of retropulsion of bone fragments into the spinal canal, for distinguishing wedge compression fractures from burst fractures, and for detecting subtle fractures of posterior column elements. In general, high-quality CT imaging is the imaging method of choice to characterize the bony injury pattern and forms the basis for injury classification. MRI is time-consuming and is not routinely used for initial evaluation of a trauma patient. Nevertheless, MRI allows for direct visualization of spinal soft tissues, including intervertebral discs, spinal ligaments, and the spinal cord, and may provide valuable information in selected cases, such as suspected but undetermined disc and/or ligament disruption after CT imaging, presence of neurological deficits without evidence of spinal injury in CT imaging, and neurological dysfunction not correlating with level of spinal injury. Moreover, MRI permits differentiation between recent and old fractures, particularly in osteoporotic patients, which can be difficult when relying on CT images alone, and allows detection of bone marrow edema caused by microtrabecular fractures without cortical breaks, which are not detectable by CT imaging.
SPINAL FRACTURE CLASSIFICATION Numerous classification systems have been developed in the past decades in an attempt to better define and characterize thoracolumbar injury patterns as well as to aid in making decisions about treatment. However, none of these classification systems has been uniformly adopted. The two classification systems that have gained the most widespread use in the past two decades are the AO Magerl classification and the Thoracolumbar Injury Classification and Severity Score (TLICS). Magerl et al. (1994) proposed a comprehensive, hierarchical classification system for thoracic and lumbar spinal injuries after analysis of 1445 consecutive cases over a 10-year period. This systematic and detailed system is based primarily on the pathomorphological characteristics of the injuries and consists of three main injury types: − Type A: Compression injuries (vertebral body compression injuries with intact PLC; result from axial compression; 66% of all fractures analyzed); − Type B injuries: Distraction injuries (either posterior or anterior element injury with distraction; result from a flexion-distraction or a hyperextension-distraction mechanism, respectively; 15% of all fractures analyzed); and − Type C: Rotational injuries (anterior and posterior element injury with rotation; result from axial torque; 19% of all fractures analyzed). Each of these three main types is divided into three groups (resulting in nine basic injury types); each of these groups is further divided into subgroups (resulting in 25 subgroup patterns), and each subgroup is divided into subdivisions. Injury severity and incidence of neurological deficit increases from type A to B to C as well as within groups. The Magerl system has been criticized for being overly complex and for being based only on a morphology and injury mechanism and not taking into account the degree of neurological injury and other clinical factors that impact decisions about treatment. In 2005, Vaccaro et al. (2005) introduced the Thoracolumbar Injury Classification and Severity Score (TLICS) system as a practical guide to facilitate making decisions about operative versus nonoperative care and in an effort to overcome the shortcomings of the AO Magerl classification. The TLICS is based on three injury characteristics: (1) injury morphology determined by radiographic appearance (compression fracture, burst fracture, translational/rotational injury and distraction injury), (2) integrity of the posterior ligamentous complex (PLC), and (3) neurologic status of the patient. Each characteristic is assigned points according to the severity of injury, and based on the final score (sum of points),
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the proposed treatment algorithm guides decisions regarding operative versus nonoperative care. The TLICS takes into account the neurological status and has been shown to provide improved intra- and interobserver reliability compared to the more detailed and complex Magerl system. The TLICS, however, has been criticized for oversimplifying fracture morphology and for not sufficiently reflecting its influence on instability and treatment decisions. Moreover, the feasibility and reproducibility of evaluating PLC integrity by MRI have been questioned (Vaccaro et al., 2009; Rihn et al., 2010). Therefore in 2013 the AOSpine Spinal Cord Injury and Trauma Knowledge Forum, an international group of academic spine surgeons, developed the AOSpine thoracolumbar spine injury classification system (Vaccaro et al., 2013) (Fig. 1). This new classification system is based on the TLICS and Magerl systems and is intended to combine the strengths of both as well as to overcome their shortcomings. This new classification system consists of a morphological injury classification, a grading system for the neurological status, and two clinical modifiers. It was developed in an attempt to provide a simple and reproducible combined morphologic and clinical classification system of thoracolumbar injuries for universal international adoption. This new classification system has been shown to provide better inter- and intraobserver reliability than the existing TLICS and Magerl system (Vaccaro et al., 2013; Kepler et al., 2016a; Kaul et al., 2017; Wood et al., 2005). Moreover, the thoracolumbar AOSpine Injury score was developed (Kepler et al., 2016b), and a surgical algorithm was established (Vaccaro et al., 2016). The morphological injury classification of this new system basically constitutes an adapted and simplified version of the AO Magerl classification and distinguishes three main types of injuries according to the predominant mode of failure of the spinal column: type A (compression injuries; subtypes A0– A4), type B (distraction injuries; subtype B1–B3), and type C (translation injuries; no subtypes). These three basic types and eight subtypes have been defined as follows (Fig. 1): Type A—Compression injuries of the vertebral body: Type A injuries are injuries of the anterior elements (vertebral body and/or disc) without tension band involvement as well as without any translation/displacement. Type A injuries result from failure under axial compression. This type also includes minor, insignificant fractures of the vertebra (subtype A0). The term “burst fracture” refers to a specific subtype of compression fractures. The pathognomonic feature of burst fractures is fracture involvement of the posterior wall of the vertebral body. The resulting posterior wall fragment(s) are commonly retropulsed into the spinal canal leading to spinal canal compromise and may cause spinal cord or cauda equina compression. A burst fracture may involve only one endplate (incomplete burst fracture) or both endplates (complete burst fracture), and the degree of vertebral body comminution varies. Type A injuries are divided into five subtypes: − Subtype A0: Minor, insignificant fractures of the vertebra that do not compromise the structural integrity of the spinal column (such as transverse process or spinous process fractures). − Subtype A1: Wedge-compression or impaction fractures: Vertebral fracture involving a single endplate but without involvement of the posterior wall of the vertebral body. − Subtype A2: Split fractures or pincer-type fractures: Vertebral fracture involving both endplates but without involvement of the posterior wall of the vertebral body. − Subtype A3: Incomplete burst fracture: Vertebral fracture involving the posterior wall as well as only a single endplate. − Subtype A4: Complete burst fracture: Vertebral fracture involving the posterior wall as well as both endplates.
FIG. 1 AOSpine thoracolumbar spine injury classification system. Image reproduced with permission from AO Foundation, Switzerland.
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In subtypes A3 and A4, a vertical fracture of the lamina is often present. In contrast to type B injuries (with horizontal osseo-ligamentous disruption of the PLC or facet joints and resulting posterior tension band failure), the vertical lamina fracture does not constitute a posterior tension band failure; it is rather a consequence of splaying of the posterior arch of the vertebra under axial loading. The splaying of the posterior arch and resulting interpedicular widening is caused by a wedging effect of the inferior articulate processes of the upper next vertebra. As a result of excessive axial loads, the vertebral body bursts, and the inferior articulate processes of the upper next vertebra displace downward and wedge into the superior articulate processes of the fractured vertebra because of the facet joint orientation at the lower thoracic and lumbar region. If a type B or a type C injury is combined with a type A fracture of the vertebral body, the latter should be mentioned separately (e.g., “T12/L1 (=level of injury) type B2 injury with L1 type A3 vertebral body fracture”). Type B—Tension band injuries (distraction injuries): Type B injuries are injuries affecting either the posterior tension band (i.e., PLC, and posterior arch) or the anterior tension band (i.e., anterior longitudinal ligament, or ALL, and anterior part of the disc) without evidence of (gross) translational displacement. The posterior tension band resists forward bending moments (flexion), whereas the anterior tension band resists backward bending moments (extension). Consequently, posterior tension band injuries typically result from a flexion-distraction mechanism, whereas anterior tension band injuries result from a hyperextension-distraction mechanism. Subtypes of type B injuries include − Subtype B1: Transosseous posterior tension band disruption (Chance fracture): Monosegmental pure transosseous failure of the posterior tension band extending into the vertebral body. − Subtype B2: Posterior tension band disruption: Disruption of the posterior tension band (i.e., PLC) with or without osseous involvement. Subtype B2 injuries are usually combined with a type A fracture of the vertebral body. − Subtype B3: Anterior tension band disruption: Hyperextension injury through the disc or vertebral body with disruption of the anterior tension band (particularly of the anterior longitudinal ligament, or ALL) resulting in a hyperextension deformity of the spinal column. Typically seen in ankylosing spinal disorders. Anterior structures, especially the ALL, are ruptured but an intact posterior hinge prevents translational displacement. Type C—Translation injuries: Type C injuries are injuries involving dislocation or translational displacement beyond the physiological range of the cranial and caudal parts of the spinal column in any plane. These injuries usually implicate disruption of the anterior as well as posterior tension bands. Type C injuries are not divided into subtypes because of various possible injury configurations due to dissociation/dislocation and may be combined with subtypes of type A or B. The neurological status is graded as follows: N0: neurologically intact; N1: transient neurological deficit, which is no longer present; N2: symptoms or signs of radiculopathy; N3: incomplete spinal cord injury or cauda equina injury; N4: complete spinal cord injury; and Nx: neurological status is unobtainable. Moreover, two patient-specific clinical modifiers relevant to treatment decision-making were included: M1: indeterminate injury to the tension band based on spinal imaging (with or without MRI), and M2: patient-specific comorbidity which might argue either for or against surgery for patients with relative surgical indications (e.g., ankylosing spondylitis). These two modifiers may not be relevant to every patient and therefore were stated to be optional.
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T REATMENT PRINCIPLES AND BIOMECHANICS OF SPINAL FRACTURE STABILIZATION GENERAL REMARKS The optimal treatment for thoracolumbar injuries is still controversial. Treatment options range from conservative to combined posteroanterior surgical stabilization depending on the individual fracture pattern. Despite substantial regional and institutional variations in treatment, the general aims of spinal fracture treatment include fracture reduction and restoration of sagittal alignment, decompression of neural elements (in patients with neurological deficits caused by neural impingement), adequate stabilization to allow for early mobilization and to prevent short- and long-term deformity, and maximization of clinical outcome and neurologic recovery. There is a general consensus that unstable injuries must be stabilized. However, spinal instability is difficult to assess based on imaging that captures only an isolated moment in time and typically with the patient in a supine position. Furthermore, “spinal instability” continues to be one of the most debated terms among spine surgeons, although numerous definitions have been proposed. White and Panjabi perhaps provided the most comprehensive yet practical definition: “Clinical instability is defined as a loss in the ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots. In addition there is no development of incapacitating deformity or pain due to structural changes” (White and Panjabi, 1978). As a general rule of thumb, conservative treatment is recommended for subtypes A0, A1, and A2, whereas operative treatment is indicated in type B and type C injuries (Vaccaro et al., 2016). However, considerable controversy still exists regarding the optimal treatment for A3 and A4 fractures; depending on the neurological status, presence or absence of clinical modifiers, and the individual fracture configuration, either conservative or operative treatment may be more appropriate. If surgical treatment is envisioned, several issues must be considered prior to surgery, including, among others: Is neural decompression required due to neurological deficits owing to spinal canal compromise? How will adequate reduction be achieved and maintained? Which column needs to be stabilized (posterior and/or anterior column)? Which surgical approach will be most appropriate? How many spinal segments will be included to ensure sufficient stability of the construct and to prevent loss of reduction (monosegmental versus bisegmental; short-segment versus long-segment instrumentation)? Is fusion via bone grafting necessary as a supplement to instrumentation? Therefore, in the following sections, the different options for surgical stabilization of an injured thoracolumbar spine will be outlined and discussed.
POSTERIOR-ONLY STABILIZATION Posterior pedicle screw-based instrumentation with and without fusion represents the most commonly used technique to stabilize thoracolumbar fractures. Modern pedicle screw/rod systems allow for reliable fixation and also facilitate fracture reduction and indirect decompression of the spinal canal via distraction and ligamentotaxis (i.e., reduce retropulsed bone fragments encroaching on the spinal canal by an intact posterior longitudinal ligament and posterior anulus fibrosus via distraction and/or traction). One of the most fundamental considerations in posterior spinal instrumentation is the number of levels to be included in the instrumentation. On the one hand, saving motion segments by limiting the number of fixed and/or fused segments to preserve the normal biomechanics of the spine constitutes a
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fundamental principle of spinal surgery. On the other hand, the posterior fixation construct has to provide sufficient stability to prevent construct failure and to maintain reduction. Motion segment preservation of shorter constructs and improved resistance to deforming forces of longer constructs therefore have to be weighed against each other. Preserving motion segments is critical in the more mobile lumbar and thoracolumbar junction region, but of less importance in the less mobile thoracic spine because little motion is lost. Thus posterior instrumentation of thoracic fractures typically involves two levels above and below the injured vertebra (Fig. 2D). In the lumbar and thoracolumbar junction region, however, short segment fixation (i.e., bisegmental stabilization with pedicle screws placed in the vertebra one level cephalad and one level caudal to the fractured vertebra) (Fig. 2B) has become the standard of care. Adding additional transpedicular screws in the fractured vertebra (six-screw construct) (Fig. 2C) has been reported to provide additional construct stiffness, to enable improved reduction and to reduce postoperative loss of correction (Norton et al., 2014; Li et al., 2016). In selected cases, such as monosegmental pure ligamentous type B2 injuries or type A3 injuries with minor vertebral body comminution, monosegmental stabilization may be feasible and sufficient (Wei et al., 2010) (Fig. 2A). Fixed-angle screw/rod systems using monoaxial screws are standard for spinal fracture stabilization in order to prevent loss of correction caused by polyaxial screw slippage (Marino, 2010; Kubosch et al., 2016). Cross links connecting both rods add torsional stability to the screw/rod construct (Wahba et al., 2010) and therefore are deemed advisable in rotationally unstable type C injuries, where there is severe anterior column instability, after vertebral body replacement and if facet joint resection is performed as part of spinal canal decompression or anterior column reconstruction from the posterior column. Posterior or posterolateral fusion of the injured spinal segment using bone grafting is indicated in type B injuries with PLC disruption due to the poor healing ability of the PLC, but may not provide an advantage with regard to postoperative loss of reduction and functional outcome in burst fractures (type A injuries) with an intact PLC (Chou et al., 2014; Dai et al., 2009b). However, if satisfactory neural decompression and spinal canal clearance necessitate facet joint resection (at the expense of spinal stability), fusion is generally considered necessary even in type A injuries.
FIG. 2 Posterior fixation constructs: monosegmental (A), bisegmental (B), bisegmental with additional transpedicular screws in the fractured vertebra (six-screw construct) (C), and long-segment (two levels above and two levels below) (D) posterior fixation constructs.
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FIG. 3 Anterior und posterior load-sharing characteristics under physiological conditions (A), after posterior only fixation (B), as well as after combined posteroanterior stabilization using a vertebral body replacement device (C).
Minimally invasive percutaneous posterior instrumentation techniques may be considered to reduce blood loss and approach-related morbidity (McAnany et al., 2016) and may be appropriate particularly if fusion and decompression are not required. From a biomechanical point of view, pedicle screw/rod constructs act as a tension band (or compression) fixation in type B1 and type B2 injuries without significant vertebral body comminution: The construct compresses the posterior column and thereby resists flexion moments and prevents displacement of the posterior column. In burst fractures with severe vertebral body comminution, however, pedicle screw/rod constructs act as a cantilever beam fixation. The construct alters the physiological anterior and posterior column load sharing (Fig. 3A) by unloading the anterior column (Fig. 3B) and shifting the loads posteriorly until healing of the vertebral body fracture has occurred. The axial load applied to the screws creates a bending moment in the sagittal plane that is resisted at the bonescrew interface. Excessive axial loading and/or too little anterior column load sharing caused by s evere vertebral body comminution thus results in bone-screw interface failure and subsequent kyphotic deformity. Using longer and larger diameter screws improves the resistance to the sagittally oriented bending moment by improving the bending resistance and by increasing the screw-bone contact area.
COMBINED POSTEROANTERIOR STABILIZATION In cases of severe vertebral body comminution (Fig. 4A), such as subtype A4 vertebral body injury, posterior-only stabilization is at risk of failure and is usually not able to maintain sagittal alignment postoperatively. Therefore, in these cases, anterior column reconstruction (ACR) has been advocated in order to prevent posterior fixation failure due to insufficient anterior column support and to ensure adequate stability. ACR enables restoration of the load-bearing capacity of the anterior column. Consequently, a more physiological load sharing between the posterior and anterior spinal column is reestablished, and stresses at the bone-pedicle screw interface are reduced (Fig. 3C).
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(A)
(B)
(C)
FIG. 4 Combined posteroanterior stabilization. Exemplary severe anterior column compromise (type A4 vertebral body injury) (A), combined posteroanterior stabilization using a vertebral body replacement device (B), and alternative anterior column reconstruction from posterior using monocortical strut grafts from the posterior iliac crest (C).
ACR may be achieved by different strategies, with combined posteroanterior stabilization being one of the most commonly used strategy. First, posterior fixation with or without neural decompression is performed to reduce and stabilize the unstable spinal injury. Second, the comminuted vertebral body as well as the adjacent intervertebral discs are partially resected and replaced by a vertebral body replacement device via an anterior thoracoscopic approach (Knop et al., 2005). The vertebral body replacement device is usually placed between the inferior endplate of the cephalad intact vertebra and the superior endplate of the caudal intact vertebra (Fig. 4B). This combined posteroanterior stabilization offers high primary biomechanical stability and is associated with only minimal postoperative loss of kyphosis correction (Knop et al., 2009; Schnake et al., 2014). Alternatively, the anterior column can be reconstructed from a posterior approach via a PLIF (posterior lumbar interbody fusion) or TLIF (transforaminal interbody fusion) procedure (Schmid et al., 2010a,b). This strategy allows obviating the need for an additional anterior approach and achieves mechanical anterior column support by placing monocortical strut grafts harvested from the posterior iliac crest in the disc space adjacent to the fractured vertebra (Fig. 4C). Similarly to combined posteroanterior stabilization, this procedure aims at intervertebral fusion of the fractured with the adjacent vertebra. The clinical and radiological outcome has been reported to be comparable to that of combined posteroanterior stabilization (Schmid et al., 2012). Another viable strategy may be to augment the compromised anterior column using calcium phosphate cement in addition to posterior fixation (Verlaan et al., 2015; Marco, 2009). Anterior-only stabilization using a vertebral body replacement device, titanium mesh cage, or bone autograft (without posterior instrumentation) (Dai et al., 2009a), however, has not gained widespread acceptance, and biomechanical studies have indicated that anterior-only stabilization is less favorable compared to a combined posteroanterior approach (Wilke et al., 2001; Pflugmacher et al., 2004).
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SPECIFIC ASPECTS OF OSTEOPOROTIC FRACTURE STABILIZATION For patients suffering from osteoporosis and/or having reduced bone quality, the problem of fracture stabilization is not satisfactorily solved and is still a controversial topic. Standard established implants for the treatment of fractures in younger patients (e.g., pedicle screws, vertebral body replacements, and plates) cannot always be anchored sufficiently in the compromised bone stock of the elderly. Therefore various approaches to improving the treatment of fractures in geriatric patients have been developed. New developments to improve pedicle screw anchorage range from expandable screw designs to pedicle screw augmentations with various types of bone cement to changes in the screw trajectory. Techniques have been developed to help with fracture stabilization, ranging from minimally invasive procedures such as vertebroplasty and kyphoplasty (Fig. 5A) with various types of bone cements to extensions of short to long instrumentations of multiple segments with augmentation of pedicle screws (Fig. 5B). Despite all these improvements in techniques and implants, an optimal solution for the stabilization of fractures in geriatric patients is still a strong focus in research studies.
FIG. 5 Treatment options for osteoporotic fractures: vertebroplasty (A), short posterior instrumentation with augmented pedicle screws (B).
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Acknowledgments We would like to acknowledge the AO Foundation, Switzerland, and AO Spine for permission to reproduce the AOSpine thoracolumbar spine injury classification system (Fig. 1). AOSpine is a clinical division of the AO Foundation, an independent medically guided nonprofit organization. The AOSpine Knowledge Forums are pathology-focused working groups acting on behalf of AOSpine in its domain of scientific expertise. Each forum consists of a steering committee of up to 10 international spine experts who meet on a regular basis to discuss research, assess the best evidence for current practices, and formulate clinical trials to advance spine care worldwide. Study support is provided directly through AOSpine’s Research department and AO’s Clinical Investigation and Documentation unit.
REFERENCES Adams, M.A., Dolan, P., 2005. Spine biomechanics. J. Biomech. 38 (10), 1972–1983. Chou, P.-H., et al., 2014. Fusion may not be a necessary procedure for surgically treated burst fractures of the thoracolumbar and lumbar spines. J. Bone Joint Surg. Am. 96 (20), 1724–1731. Dai, L.Y., Jiang, L.S., Jiang, S.D., 2009a. Anterior-only stabilization using plating with bone structural autograft versus titanium mesh cages for two-or three-column thoracolumbar burst fractures: a prospective randomized study. Spine 34 (14), 1429–1435. Dai, L.Y., Jiang, L.S., Jiang, S.D., 2009b. Posterior short-segment fixation with or without fusion for thoracolumbar burst fractures. A five to seven-year prospective randomized study. J. Bone Joint Surg. 91 (5), 1033–1041. Holdsworth, F., 1970. Fractures, dislocations, and fracture-dislocations of the spine. J. Bone Joint Surg. Am. 52 (8), 1534–1551. Kaul, R., et al., 2017. Reliability assessment of AOSpine thoracolumbar spine injury classification system and thoracolumbar injury classification and severity score (TLICS) for thoracolumbar spine injuries: results of a multicentre study. Eur. Spine J. 26 (5), 1470–1476. Kepler, C.K., Vaccaro, A.R., Koerner, J.D., et al., 2016a. Reliability analysis of the AOSpine thoracolumbar spine injury classification system by a worldwide group of naïve spinal surgeons. Eur. Spine J. 25 (4), 1082–1086. Kepler, C., Vaccaro, A., Schroeder, G., et al., 2016b. The thoracolumbar AOSpine injury score. Global Spine J. 6 (4), 329–334. Knop, C., et al., 2005. Vertebral body replacement with Synex in combined posteroanterior surgery for treatment of thoracolumbar injuries. Oper. Orthop. Traumatol. 17 (3), 249–280. Knop, C., et al., 2009. Combined posterior–anterior stabilisation of thoracolumbar injuries utilising a vertebral body replacing implant. Eur. Spine J. 18 (7), 949–963. Kubosch, D., et al., 2016. Biomechanical investigation of a minimally invasive posterior spine stabilization system in comparison to the universal spinal system (USS). BMC Musculoskelet. Disord. 17 (1), 134. Li, K., et al., 2016. Pedicle screw fixation combined with intermediate screw at the fracture level for treatment of thoracolumbar fractures. Medicine 95 (33), e4574–8. Magerl, F., et al., 1994. A comprehensive classification of thoracic and lumbar injuries. Eur. Spine J. 3 (4), 184–201. Marco, R.A.W., 2009. Thoracolumbar burst fractures treated with posterior decompression and pedicle screw instrumentation supplemented with balloon-assisted vertebroplasty and calcium phosphate reconstruction. J. Bone Joint Surg. Am. 91 (1), 20. Marino, J.F., 2010. Nonfusion short fixation of A3 burst fractures, loss of fixation attributable to polyaxial screw slippage? Spine J. 10 (5), 459–460. Author reply 460. McAnany, S., et al., 2016. Open versus minimally invasive fixation techniques for thoracolumbar trauma: a metaanalysis. Global Spine J. 6 (2), 186–194.
REFERENCES
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Norton, R.P., et al., 2014. Biomechanical analysis of four- versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures. Spine J. 14 (8), 1734–1739. Pflugmacher, R., et al., 2004. Biomechanical comparison of expandable cages for vertebral body replacement in the thoracolumbar spine. Spine 29 (13), 1413–1419. Rihn, J.A., et al., 2010. Using magnetic resonance imaging to accurately assess injury to the posterior ligamentous complex of the spine: a prospective comparison of the surgeon and radiologist. J. Neurosurg. Spine 12 (4), 391–396. Schmid, R., et al., 2010a. Mid-term results of PLIF/TLIF in trauma. Eur. Spine J. 20 (3), 395–402. Schmid, R., et al., 2010b. PLIF in thoracolumbar trauma: technique and radiological results. Eur. Spine J. 19 (7), 1079–1086. Schmid, R., et al., 2012. Combined posteroanterior fusion versus transforaminal lumbar interbody fusion (TLIF) in thoracolumbar burst fractures. Injury 43 (4), 475–479. Schnake, K.J., Stavridis, S.I., Kandziora, F., 2014. Five-year clinical and radiological results of combined anteroposterior stabilization of thoracolumbar fractures. J. Neurosurg. Spine 20 (5), 497–504. Vaccaro, A.R., et al., 2005. A new classification of thoracolumbar injuries: the importance of injury morphology, the integrity of the posterior ligamentous complex, and neurologic status. Spine 30 (20), 2325–2333. Vaccaro, A.R., et al., 2009. Injury of the posterior ligamentous complex of the thoracolumbar spine a prospective evaluation of the diagnostic accuracy of magnetic resonance imaging. Spine 34 (23), E841–E847. Vaccaro, A.R., et al., 2013. AOSpine thoracolumbar spine injury classification system. Spine 38 (23), 2028–2037. Vaccaro, A.R., et al., 2016. The surgical algorithm for the AOSpine thoracolumbar spine injury classification system. Eur. Spine J. 25 (4), 1087–1094. Verlaan, J.-J., et al., 2015. Clinical and radiological results 6 years after treatment of traumatic thoracolumbar burst fractures with pedicle screw instrumentation and balloon assisted endplate reduction. Spine J. 15 (6), 1172–1178. Wahba, G.M., et al., 2010. Biomechanical evaluation of short-segment posterior instrumentation with and without crosslinks in a human cadaveric unstable thoracolumbar burst fracture model. Spine 35 (3), 278–285. Wei, F.-X., et al., 2010. Transpedicular fixation in management of thoracolumbar burst fractures: monosegmental fixation versus short-segment instrumentation. Spine 35 (15), E714–20. White, A.A., Panjabi, M.M., 1978. Clinical Biomechanics of the Spine. Lippincott, Philadelphia. Wilke, H.J., et al., 2001. Combined anteroposterior spinal fixation provides superior stabilisation to a single anterior or posterior procedure. J. Bone Joint Surg. Br. 83 (4), 609–617. Wood, K.B., et al., 2005. Assessment of two thoracolumbar fracture classification systems as used by multiple surgeons. J. Bone Joint Surg. Am. 87 (7), 1423–1429.
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Richard A. Lindtner, Werner Schmoelz Medical University of Innsbruck, Innsbruck, Austria
INTRODUCTION Traumatic lesions of the spine are relatively common and encompass a wide spectrum of injuries ranging from minor fractures of spinous or transverse processes to complex, multidirectionally unstable injuries. These lesions may involve osseous or disco-ligamentous structures or both. Spinal injuries usually are the result of high-energy trauma, and associated injuries are common. Moreover, traumatic injury to the spine frequently leads to neurological deficits with potentially devastating consequences for patients, ranging from mild, transient symptoms to complete tetraplegia. Other potential sequelae of spinal injuries include progressive deformity, persistent pain, and disability. The most appropriate treatment strategy for spinal injuries varies depending on the degree to which spinal structures have been compromised. Treatment options thus range from conservative to combined posteroanterior surgical approaches depending on the individual injury pattern. General goals of treatment, however, include restoration of anatomic alignment, decompression of neural elements (if required), adequate stabilization to allow for early mobilization, and maximization of clinical outcome and neurologic recovery. The majority of spinal injuries affect the thoracic and lumbar spine. This chapter will therefore focus on thoracolumbar trauma, and special reference will be made to important aspects of injury mechanism, classification, and biomechanics of fracture stabilization.
INJURY MECHANISMS In young and middle-aged individuals, thoracolumbar fractures typically result from high-energy blunt trauma. Common injury mechanisms include falls from height, motor vehicle accidents, and sportsrelated injuries. In elderly people, low-energy mechanisms and even spontaneous fractures without relevant trauma are more common as a consequence of diminished bone quality caused by osteoporosis. Bone disorders other than osteoporosis may also increase the susceptibility to fracture even after a low-energy impact. Ankylosing spinal disorders (i.e., ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis), for example, lead to ankylosis/fusion of normally mobile spinal segments via progressive ossification of spinal ligaments and/or intervertebral discs. As a consequence of ankylosis, the normal biomechanics of the spine is massively altered, and forces can act on longer lever arms during trauma. The ankylosed thoracolumbar spine is thus much more susceptible to injury even after low-energy trauma and typically shows an otherwise uncommon hyperextension injury pattern. Biomechanics of the Spine. https://doi.org/10.1016/B978-0-12-812851-0.00022-7 © 2018 Elsevier Ltd. All rights reserved.
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In high-energy trauma, usually complex and multidirectional forces act on the spine at the time of injury. These forces may involve compressive and distractive forces; rotational moments in the sagittal (flexion and extension), coronal (left and right side bending), and axial (left and right axial rotation) planes; as well as shear forces. For the most part, the spine is exposed to a combination of several of these forces and moments during trauma. The forces and moments acting on the spine can result in a great variety of injury patterns, including bony injuries without ligamentous disruption, ligamentous injury without bony injury, or often a combination of both. Depending on the direction of the resultant force vector, characteristic types of injury patterns result. The extent of disco-ligamentous and/or bony disruption again varies as a function of the magnitude of the resulting force vector acting on the spine. From a biomechanical point of view, the spinal column can be divided into an anterior and a posterior column, as suggested by Holdsworth (1970). The anterior column consists of the vertebra and intervertebral discs (including the anterior and posterior longitudinal ligaments), resists compression and axial loading, and carries about 60%–90% of the load in the normal upright position (Adams and Dolan, 2005). The posterior column consists of the bone structures posterior to the posterior longitudinal ligament (posterior vertebral arch) as well as the posterior ligamentous complex (PLC), maintains tension, and carries about 10%–40% of the load in the normal upright position. The PLC is critical for spinal stability and consists of the supraspinous and interspinous ligaments, the ligamentum flavum, and the facet capsules. Depending on the resultant force vector and resultant moments, characteristic patterns of anterior and/ or posterior column involvement result. A traumatic axial load (i.e., a load applied along the spinal long axis) results in excessive compression and subsequent failure of the anterior column in the form of vertebral body fracture without significant injury to the posterior column. A traumatic forward bending moment leads to flexion of the spinal column in the sagittal plane and causes tension of the posterior column as well as compressive loading of the anterior part of the vertebral body. In contrast, a traumatic backward bending moment leads to hyperextension of the spinal column and causes tension of the anterior column (particularly of the anterior annulus fibrosus and anterior longitudinal ligament) as well as compression of the posterior column. Excessive and complex traumatic loading may cause a complete disruption of both columns that permits fracture dislocation with translational displacement in one or more planes. The thoracolumbar junction (T11-L2) is the most common site of spinal fractures and is particularly vulnerable because it constitutes an anatomical and biomechanical transition zone between the subjacent lumbar and superjacent thoracic spine. At the thoracolumbar junction, the relatively rigid thoracic kyphosis transitions to the more flexible lumbar lordosis. The rib cage constrains motion of the thoracic spine and increases resistance to bending moments in the sagittal and coronal planes as well as axial rotation. The “free floating” ribs of T11 and T12 provide markedly less stability at the thoracolumbar junction because they are not connected to the sternum. Furthermore, the coronal orientation of the thoracic facet joints limits flexion and extension and provides significant resistance to anterior translation, whereas the more sagittal-oriented lumbar facet joints allow for increased motion in flexion and extension.
DIAGNOSTIC IMAGING Initial evaluation of a patient with suspected thoracolumbar injury consists of clinical assessment including a thorough neurological examination as well as diagnostic imaging. Imaging modalities include conventional anteroposterior and lateral radiographs, computed tomography (CT), and magnetic resonance imaging (MRI). CT is considered the “gold standard” for imaging of spinal trauma. It offers higher spatial resolution than conventional plain radiographs and allows visualization of the spine in the axial, sagittal, and coronal planes.
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CT is superior for quantifying the extent of vertebral body comminution as well as of retropulsion of bone fragments into the spinal canal, for distinguishing wedge compression fractures from burst fractures, and for detecting subtle fractures of posterior column elements. In general, high-quality CT imaging is the imaging method of choice to characterize the bony injury pattern and forms the basis for injury classification. MRI is time-consuming and is not routinely used for initial evaluation of a trauma patient. Nevertheless, MRI allows for direct visualization of spinal soft tissues, including intervertebral discs, spinal ligaments, and the spinal cord, and may provide valuable information in selected cases, such as suspected but undetermined disc and/or ligament disruption after CT imaging, presence of neurological deficits without evidence of spinal injury in CT imaging, and neurological dysfunction not correlating with level of spinal injury. Moreover, MRI permits differentiation between recent and old fractures, particularly in osteoporotic patients, which can be difficult when relying on CT images alone, and allows detection of bone marrow edema caused by microtrabecular fractures without cortical breaks, which are not detectable by CT imaging.
SPINAL FRACTURE CLASSIFICATION Numerous classification systems have been developed in the past decades in an attempt to better define and characterize thoracolumbar injury patterns as well as to aid in making decisions about treatment. However, none of these classification systems has been uniformly adopted. The two classification systems that have gained the most widespread use in the past two decades are the AO Magerl classification and the Thoracolumbar Injury Classification and Severity Score (TLICS). Magerl et al. (1994) proposed a comprehensive, hierarchical classification system for thoracic and lumbar spinal injuries after analysis of 1445 consecutive cases over a 10-year period. This systematic and detailed system is based primarily on the pathomorphological characteristics of the injuries and consists of three main injury types: − Type A: Compression injuries (vertebral body compression injuries with intact PLC; result from axial compression; 66% of all fractures analyzed); − Type B injuries: Distraction injuries (either posterior or anterior element injury with distraction; result from a flexion-distraction or a hyperextension-distraction mechanism, respectively; 15% of all fractures analyzed); and − Type C: Rotational injuries (anterior and posterior element injury with rotation; result from axial torque; 19% of all fractures analyzed). Each of these three main types is divided into three groups (resulting in nine basic injury types); each of these groups is further divided into subgroups (resulting in 25 subgroup patterns), and each subgroup is divided into subdivisions. Injury severity and incidence of neurological deficit increases from type A to B to C as well as within groups. The Magerl system has been criticized for being overly complex and for being based only on a morphology and injury mechanism and not taking into account the degree of neurological injury and other clinical factors that impact decisions about treatment. In 2005, Vaccaro et al. (2005) introduced the Thoracolumbar Injury Classification and Severity Score (TLICS) system as a practical guide to facilitate making decisions about operative versus nonoperative care and in an effort to overcome the shortcomings of the AO Magerl classification. The TLICS is based on three injury characteristics: (1) injury morphology determined by radiographic appearance (compression fracture, burst fracture, translational/rotational injury and distraction injury), (2) integrity of the posterior ligamentous complex (PLC), and (3) neurologic status of the patient. Each characteristic is assigned points according to the severity of injury, and based on the final score (sum of points),
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the proposed treatment algorithm guides decisions regarding operative versus nonoperative care. The TLICS takes into account the neurological status and has been shown to provide improved intra- and interobserver reliability compared to the more detailed and complex Magerl system. The TLICS, however, has been criticized for oversimplifying fracture morphology and for not sufficiently reflecting its influence on instability and treatment decisions. Moreover, the feasibility and reproducibility of evaluating PLC integrity by MRI have been questioned (Vaccaro et al., 2009; Rihn et al., 2010). Therefore in 2013 the AOSpine Spinal Cord Injury and Trauma Knowledge Forum, an international group of academic spine surgeons, developed the AOSpine thoracolumbar spine injury classification system (Vaccaro et al., 2013) (Fig. 1). This new classification system is based on the TLICS and Magerl systems and is intended to combine the strengths of both as well as to overcome their shortcomings. This new classification system consists of a morphological injury classification, a grading system for the neurological status, and two clinical modifiers. It was developed in an attempt to provide a simple and reproducible combined morphologic and clinical classification system of thoracolumbar injuries for universal international adoption. This new classification system has been shown to provide better inter- and intraobserver reliability than the existing TLICS and Magerl system (Vaccaro et al., 2013; Kepler et al., 2016a; Kaul et al., 2017; Wood et al., 2005). Moreover, the thoracolumbar AOSpine Injury score was developed (Kepler et al., 2016b), and a surgical algorithm was established (Vaccaro et al., 2016). The morphological injury classification of this new system basically constitutes an adapted and simplified version of the AO Magerl classification and distinguishes three main types of injuries according to the predominant mode of failure of the spinal column: type A (compression injuries; subtypes A0– A4), type B (distraction injuries; subtype B1–B3), and type C (translation injuries; no subtypes). These three basic types and eight subtypes have been defined as follows (Fig. 1): Type A—Compression injuries of the vertebral body: Type A injuries are injuries of the anterior elements (vertebral body and/or disc) without tension band involvement as well as without any translation/displacement. Type A injuries result from failure under axial compression. This type also includes minor, insignificant fractures of the vertebra (subtype A0). The term “burst fracture” refers to a specific subtype of compression fractures. The pathognomonic feature of burst fractures is fracture involvement of the posterior wall of the vertebral body. The resulting posterior wall fragment(s) are commonly retropulsed into the spinal canal leading to spinal canal compromise and may cause spinal cord or cauda equina compression. A burst fracture may involve only one endplate (incomplete burst fracture) or both endplates (complete burst fracture), and the degree of vertebral body comminution varies. Type A injuries are divided into five subtypes: − Subtype A0: Minor, insignificant fractures of the vertebra that do not compromise the structural integrity of the spinal column (such as transverse process or spinous process fractures). − Subtype A1: Wedge-compression or impaction fractures: Vertebral fracture involving a single endplate but without involvement of the posterior wall of the vertebral body. − Subtype A2: Split fractures or pincer-type fractures: Vertebral fracture involving both endplates but without involvement of the posterior wall of the vertebral body. − Subtype A3: Incomplete burst fracture: Vertebral fracture involving the posterior wall as well as only a single endplate. − Subtype A4: Complete burst fracture: Vertebral fracture involving the posterior wall as well as both endplates.
FIG. 1 AOSpine thoracolumbar spine injury classification system. Image reproduced with permission from AO Foundation, Switzerland.
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In subtypes A3 and A4, a vertical fracture of the lamina is often present. In contrast to type B injuries (with horizontal osseo-ligamentous disruption of the PLC or facet joints and resulting posterior tension band failure), the vertical lamina fracture does not constitute a posterior tension band failure; it is rather a consequence of splaying of the posterior arch of the vertebra under axial loading. The splaying of the posterior arch and resulting interpedicular widening is caused by a wedging effect of the inferior articulate processes of the upper next vertebra. As a result of excessive axial loads, the vertebral body bursts, and the inferior articulate processes of the upper next vertebra displace downward and wedge into the superior articulate processes of the fractured vertebra because of the facet joint orientation at the lower thoracic and lumbar region. If a type B or a type C injury is combined with a type A fracture of the vertebral body, the latter should be mentioned separately (e.g., “T12/L1 (=level of injury) type B2 injury with L1 type A3 vertebral body fracture”). Type B—Tension band injuries (distraction injuries): Type B injuries are injuries affecting either the posterior tension band (i.e., PLC, and posterior arch) or the anterior tension band (i.e., anterior longitudinal ligament, or ALL, and anterior part of the disc) without evidence of (gross) translational displacement. The posterior tension band resists forward bending moments (flexion), whereas the anterior tension band resists backward bending moments (extension). Consequently, posterior tension band injuries typically result from a flexion-distraction mechanism, whereas anterior tension band injuries result from a hyperextension-distraction mechanism. Subtypes of type B injuries include − Subtype B1: Transosseous posterior tension band disruption (Chance fracture): Monosegmental pure transosseous failure of the posterior tension band extending into the vertebral body. − Subtype B2: Posterior tension band disruption: Disruption of the posterior tension band (i.e., PLC) with or without osseous involvement. Subtype B2 injuries are usually combined with a type A fracture of the vertebral body. − Subtype B3: Anterior tension band disruption: Hyperextension injury through the disc or vertebral body with disruption of the anterior tension band (particularly of the anterior longitudinal ligament, or ALL) resulting in a hyperextension deformity of the spinal column. Typically seen in ankylosing spinal disorders. Anterior structures, especially the ALL, are ruptured but an intact posterior hinge prevents translational displacement. Type C—Translation injuries: Type C injuries are injuries involving dislocation or translational displacement beyond the physiological range of the cranial and caudal parts of the spinal column in any plane. These injuries usually implicate disruption of the anterior as well as posterior tension bands. Type C injuries are not divided into subtypes because of various possible injury configurations due to dissociation/dislocation and may be combined with subtypes of type A or B. The neurological status is graded as follows: N0: neurologically intact; N1: transient neurological deficit, which is no longer present; N2: symptoms or signs of radiculopathy; N3: incomplete spinal cord injury or cauda equina injury; N4: complete spinal cord injury; and Nx: neurological status is unobtainable. Moreover, two patient-specific clinical modifiers relevant to treatment decision-making were included: M1: indeterminate injury to the tension band based on spinal imaging (with or without MRI), and M2: patient-specific comorbidity which might argue either for or against surgery for patients with relative surgical indications (e.g., ankylosing spondylitis). These two modifiers may not be relevant to every patient and therefore were stated to be optional.
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T REATMENT PRINCIPLES AND BIOMECHANICS OF SPINAL FRACTURE STABILIZATION GENERAL REMARKS The optimal treatment for thoracolumbar injuries is still controversial. Treatment options range from conservative to combined posteroanterior surgical stabilization depending on the individual fracture pattern. Despite substantial regional and institutional variations in treatment, the general aims of spinal fracture treatment include fracture reduction and restoration of sagittal alignment, decompression of neural elements (in patients with neurological deficits caused by neural impingement), adequate stabilization to allow for early mobilization and to prevent short- and long-term deformity, and maximization of clinical outcome and neurologic recovery. There is a general consensus that unstable injuries must be stabilized. However, spinal instability is difficult to assess based on imaging that captures only an isolated moment in time and typically with the patient in a supine position. Furthermore, “spinal instability” continues to be one of the most debated terms among spine surgeons, although numerous definitions have been proposed. White and Panjabi perhaps provided the most comprehensive yet practical definition: “Clinical instability is defined as a loss in the ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots. In addition there is no development of incapacitating deformity or pain due to structural changes” (White and Panjabi, 1978). As a general rule of thumb, conservative treatment is recommended for subtypes A0, A1, and A2, whereas operative treatment is indicated in type B and type C injuries (Vaccaro et al., 2016). However, considerable controversy still exists regarding the optimal treatment for A3 and A4 fractures; depending on the neurological status, presence or absence of clinical modifiers, and the individual fracture configuration, either conservative or operative treatment may be more appropriate. If surgical treatment is envisioned, several issues must be considered prior to surgery, including, among others: Is neural decompression required due to neurological deficits owing to spinal canal compromise? How will adequate reduction be achieved and maintained? Which column needs to be stabilized (posterior and/or anterior column)? Which surgical approach will be most appropriate? How many spinal segments will be included to ensure sufficient stability of the construct and to prevent loss of reduction (monosegmental versus bisegmental; short-segment versus long-segment instrumentation)? Is fusion via bone grafting necessary as a supplement to instrumentation? Therefore, in the following sections, the different options for surgical stabilization of an injured thoracolumbar spine will be outlined and discussed.
POSTERIOR-ONLY STABILIZATION Posterior pedicle screw-based instrumentation with and without fusion represents the most commonly used technique to stabilize thoracolumbar fractures. Modern pedicle screw/rod systems allow for reliable fixation and also facilitate fracture reduction and indirect decompression of the spinal canal via distraction and ligamentotaxis (i.e., reduce retropulsed bone fragments encroaching on the spinal canal by an intact posterior longitudinal ligament and posterior anulus fibrosus via distraction and/or traction). One of the most fundamental considerations in posterior spinal instrumentation is the number of levels to be included in the instrumentation. On the one hand, saving motion segments by limiting the number of fixed and/or fused segments to preserve the normal biomechanics of the spine constitutes a
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fundamental principle of spinal surgery. On the other hand, the posterior fixation construct has to provide sufficient stability to prevent construct failure and to maintain reduction. Motion segment preservation of shorter constructs and improved resistance to deforming forces of longer constructs therefore have to be weighed against each other. Preserving motion segments is critical in the more mobile lumbar and thoracolumbar junction region, but of less importance in the less mobile thoracic spine because little motion is lost. Thus posterior instrumentation of thoracic fractures typically involves two levels above and below the injured vertebra (Fig. 2D). In the lumbar and thoracolumbar junction region, however, short segment fixation (i.e., bisegmental stabilization with pedicle screws placed in the vertebra one level cephalad and one level caudal to the fractured vertebra) (Fig. 2B) has become the standard of care. Adding additional transpedicular screws in the fractured vertebra (six-screw construct) (Fig. 2C) has been reported to provide additional construct stiffness, to enable improved reduction and to reduce postoperative loss of correction (Norton et al., 2014; Li et al., 2016). In selected cases, such as monosegmental pure ligamentous type B2 injuries or type A3 injuries with minor vertebral body comminution, monosegmental stabilization may be feasible and sufficient (Wei et al., 2010) (Fig. 2A). Fixed-angle screw/rod systems using monoaxial screws are standard for spinal fracture stabilization in order to prevent loss of correction caused by polyaxial screw slippage (Marino, 2010; Kubosch et al., 2016). Cross links connecting both rods add torsional stability to the screw/rod construct (Wahba et al., 2010) and therefore are deemed advisable in rotationally unstable type C injuries, where there is severe anterior column instability, after vertebral body replacement and if facet joint resection is performed as part of spinal canal decompression or anterior column reconstruction from the posterior column. Posterior or posterolateral fusion of the injured spinal segment using bone grafting is indicated in type B injuries with PLC disruption due to the poor healing ability of the PLC, but may not provide an advantage with regard to postoperative loss of reduction and functional outcome in burst fractures (type A injuries) with an intact PLC (Chou et al., 2014; Dai et al., 2009b). However, if satisfactory neural decompression and spinal canal clearance necessitate facet joint resection (at the expense of spinal stability), fusion is generally considered necessary even in type A injuries.
FIG. 2 Posterior fixation constructs: monosegmental (A), bisegmental (B), bisegmental with additional transpedicular screws in the fractured vertebra (six-screw construct) (C), and long-segment (two levels above and two levels below) (D) posterior fixation constructs.
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FIG. 3 Anterior und posterior load-sharing characteristics under physiological conditions (A), after posterior only fixation (B), as well as after combined posteroanterior stabilization using a vertebral body replacement device (C).
Minimally invasive percutaneous posterior instrumentation techniques may be considered to reduce blood loss and approach-related morbidity (McAnany et al., 2016) and may be appropriate particularly if fusion and decompression are not required. From a biomechanical point of view, pedicle screw/rod constructs act as a tension band (or compression) fixation in type B1 and type B2 injuries without significant vertebral body comminution: The construct compresses the posterior column and thereby resists flexion moments and prevents displacement of the posterior column. In burst fractures with severe vertebral body comminution, however, pedicle screw/rod constructs act as a cantilever beam fixation. The construct alters the physiological anterior and posterior column load sharing (Fig. 3A) by unloading the anterior column (Fig. 3B) and shifting the loads posteriorly until healing of the vertebral body fracture has occurred. The axial load applied to the screws creates a bending moment in the sagittal plane that is resisted at the bonescrew interface. Excessive axial loading and/or too little anterior column load sharing caused by s evere vertebral body comminution thus results in bone-screw interface failure and subsequent kyphotic deformity. Using longer and larger diameter screws improves the resistance to the sagittally oriented bending moment by improving the bending resistance and by increasing the screw-bone contact area.
COMBINED POSTEROANTERIOR STABILIZATION In cases of severe vertebral body comminution (Fig. 4A), such as subtype A4 vertebral body injury, posterior-only stabilization is at risk of failure and is usually not able to maintain sagittal alignment postoperatively. Therefore, in these cases, anterior column reconstruction (ACR) has been advocated in order to prevent posterior fixation failure due to insufficient anterior column support and to ensure adequate stability. ACR enables restoration of the load-bearing capacity of the anterior column. Consequently, a more physiological load sharing between the posterior and anterior spinal column is reestablished, and stresses at the bone-pedicle screw interface are reduced (Fig. 3C).
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(A)
(B)
(C)
FIG. 4 Combined posteroanterior stabilization. Exemplary severe anterior column compromise (type A4 vertebral body injury) (A), combined posteroanterior stabilization using a vertebral body replacement device (B), and alternative anterior column reconstruction from posterior using monocortical strut grafts from the posterior iliac crest (C).
ACR may be achieved by different strategies, with combined posteroanterior stabilization being one of the most commonly used strategy. First, posterior fixation with or without neural decompression is performed to reduce and stabilize the unstable spinal injury. Second, the comminuted vertebral body as well as the adjacent intervertebral discs are partially resected and replaced by a vertebral body replacement device via an anterior thoracoscopic approach (Knop et al., 2005). The vertebral body replacement device is usually placed between the inferior endplate of the cephalad intact vertebra and the superior endplate of the caudal intact vertebra (Fig. 4B). This combined posteroanterior stabilization offers high primary biomechanical stability and is associated with only minimal postoperative loss of kyphosis correction (Knop et al., 2009; Schnake et al., 2014). Alternatively, the anterior column can be reconstructed from a posterior approach via a PLIF (posterior lumbar interbody fusion) or TLIF (transforaminal interbody fusion) procedure (Schmid et al., 2010a,b). This strategy allows obviating the need for an additional anterior approach and achieves mechanical anterior column support by placing monocortical strut grafts harvested from the posterior iliac crest in the disc space adjacent to the fractured vertebra (Fig. 4C). Similarly to combined posteroanterior stabilization, this procedure aims at intervertebral fusion of the fractured with the adjacent vertebra. The clinical and radiological outcome has been reported to be comparable to that of combined posteroanterior stabilization (Schmid et al., 2012). Another viable strategy may be to augment the compromised anterior column using calcium phosphate cement in addition to posterior fixation (Verlaan et al., 2015; Marco, 2009). Anterior-only stabilization using a vertebral body replacement device, titanium mesh cage, or bone autograft (without posterior instrumentation) (Dai et al., 2009a), however, has not gained widespread acceptance, and biomechanical studies have indicated that anterior-only stabilization is less favorable compared to a combined posteroanterior approach (Wilke et al., 2001; Pflugmacher et al., 2004).
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SPECIFIC ASPECTS OF OSTEOPOROTIC FRACTURE STABILIZATION For patients suffering from osteoporosis and/or having reduced bone quality, the problem of fracture stabilization is not satisfactorily solved and is still a controversial topic. Standard established implants for the treatment of fractures in younger patients (e.g., pedicle screws, vertebral body replacements, and plates) cannot always be anchored sufficiently in the compromised bone stock of the elderly. Therefore various approaches to improving the treatment of fractures in geriatric patients have been developed. New developments to improve pedicle screw anchorage range from expandable screw designs to pedicle screw augmentations with various types of bone cement to changes in the screw trajectory. Techniques have been developed to help with fracture stabilization, ranging from minimally invasive procedures such as vertebroplasty and kyphoplasty (Fig. 5A) with various types of bone cements to extensions of short to long instrumentations of multiple segments with augmentation of pedicle screws (Fig. 5B). Despite all these improvements in techniques and implants, an optimal solution for the stabilization of fractures in geriatric patients is still a strong focus in research studies.
FIG. 5 Treatment options for osteoporotic fractures: vertebroplasty (A), short posterior instrumentation with augmented pedicle screws (B).
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CHAPTER 22 VERTEBRAL FRACTURES AND THEIR TREATMENT
Acknowledgments We would like to acknowledge the AO Foundation, Switzerland, and AO Spine for permission to reproduce the AOSpine thoracolumbar spine injury classification system (Fig. 1). AOSpine is a clinical division of the AO Foundation, an independent medically guided nonprofit organization. The AOSpine Knowledge Forums are pathology-focused working groups acting on behalf of AOSpine in its domain of scientific expertise. Each forum consists of a steering committee of up to 10 international spine experts who meet on a regular basis to discuss research, assess the best evidence for current practices, and formulate clinical trials to advance spine care worldwide. Study support is provided directly through AOSpine’s Research department and AO’s Clinical Investigation and Documentation unit.
REFERENCES Adams, M.A., Dolan, P., 2005. Spine biomechanics. J. Biomech. 38 (10), 1972–1983. Chou, P.-H., et al., 2014. Fusion may not be a necessary procedure for surgically treated burst fractures of the thoracolumbar and lumbar spines. J. Bone Joint Surg. Am. 96 (20), 1724–1731. Dai, L.Y., Jiang, L.S., Jiang, S.D., 2009a. Anterior-only stabilization using plating with bone structural autograft versus titanium mesh cages for two-or three-column thoracolumbar burst fractures: a prospective randomized study. Spine 34 (14), 1429–1435. Dai, L.Y., Jiang, L.S., Jiang, S.D., 2009b. Posterior short-segment fixation with or without fusion for thoracolumbar burst fractures. A five to seven-year prospective randomized study. J. Bone Joint Surg. 91 (5), 1033–1041. Holdsworth, F., 1970. Fractures, dislocations, and fracture-dislocations of the spine. J. Bone Joint Surg. Am. 52 (8), 1534–1551. Kaul, R., et al., 2017. Reliability assessment of AOSpine thoracolumbar spine injury classification system and thoracolumbar injury classification and severity score (TLICS) for thoracolumbar spine injuries: results of a multicentre study. Eur. Spine J. 26 (5), 1470–1476. Kepler, C.K., Vaccaro, A.R., Koerner, J.D., et al., 2016a. Reliability analysis of the AOSpine thoracolumbar spine injury classification system by a worldwide group of naïve spinal surgeons. Eur. Spine J. 25 (4), 1082–1086. Kepler, C., Vaccaro, A., Schroeder, G., et al., 2016b. The thoracolumbar AOSpine injury score. Global Spine J. 6 (4), 329–334. Knop, C., et al., 2005. Vertebral body replacement with Synex in combined posteroanterior surgery for treatment of thoracolumbar injuries. Oper. Orthop. Traumatol. 17 (3), 249–280. Knop, C., et al., 2009. Combined posterior–anterior stabilisation of thoracolumbar injuries utilising a vertebral body replacing implant. Eur. Spine J. 18 (7), 949–963. Kubosch, D., et al., 2016. Biomechanical investigation of a minimally invasive posterior spine stabilization system in comparison to the universal spinal system (USS). BMC Musculoskelet. Disord. 17 (1), 134. Li, K., et al., 2016. Pedicle screw fixation combined with intermediate screw at the fracture level for treatment of thoracolumbar fractures. Medicine 95 (33), e4574–8. Magerl, F., et al., 1994. A comprehensive classification of thoracic and lumbar injuries. Eur. Spine J. 3 (4), 184–201. Marco, R.A.W., 2009. Thoracolumbar burst fractures treated with posterior decompression and pedicle screw instrumentation supplemented with balloon-assisted vertebroplasty and calcium phosphate reconstruction. J. Bone Joint Surg. Am. 91 (1), 20. Marino, J.F., 2010. Nonfusion short fixation of A3 burst fractures, loss of fixation attributable to polyaxial screw slippage? Spine J. 10 (5), 459–460. Author reply 460. McAnany, S., et al., 2016. Open versus minimally invasive fixation techniques for thoracolumbar trauma: a metaanalysis. Global Spine J. 6 (2), 186–194.
REFERENCES
407
Norton, R.P., et al., 2014. Biomechanical analysis of four- versus six-screw constructs for short-segment pedicle screw and rod instrumentation of unstable thoracolumbar fractures. Spine J. 14 (8), 1734–1739. Pflugmacher, R., et al., 2004. Biomechanical comparison of expandable cages for vertebral body replacement in the thoracolumbar spine. Spine 29 (13), 1413–1419. Rihn, J.A., et al., 2010. Using magnetic resonance imaging to accurately assess injury to the posterior ligamentous complex of the spine: a prospective comparison of the surgeon and radiologist. J. Neurosurg. Spine 12 (4), 391–396. Schmid, R., et al., 2010a. Mid-term results of PLIF/TLIF in trauma. Eur. Spine J. 20 (3), 395–402. Schmid, R., et al., 2010b. PLIF in thoracolumbar trauma: technique and radiological results. Eur. Spine J. 19 (7), 1079–1086. Schmid, R., et al., 2012. Combined posteroanterior fusion versus transforaminal lumbar interbody fusion (TLIF) in thoracolumbar burst fractures. Injury 43 (4), 475–479. Schnake, K.J., Stavridis, S.I., Kandziora, F., 2014. Five-year clinical and radiological results of combined anteroposterior stabilization of thoracolumbar fractures. J. Neurosurg. Spine 20 (5), 497–504. Vaccaro, A.R., et al., 2005. A new classification of thoracolumbar injuries: the importance of injury morphology, the integrity of the posterior ligamentous complex, and neurologic status. Spine 30 (20), 2325–2333. Vaccaro, A.R., et al., 2009. Injury of the posterior ligamentous complex of the thoracolumbar spine a prospective evaluation of the diagnostic accuracy of magnetic resonance imaging. Spine 34 (23), E841–E847. Vaccaro, A.R., et al., 2013. AOSpine thoracolumbar spine injury classification system. Spine 38 (23), 2028–2037. Vaccaro, A.R., et al., 2016. The surgical algorithm for the AOSpine thoracolumbar spine injury classification system. Eur. Spine J. 25 (4), 1087–1094. Verlaan, J.-J., et al., 2015. Clinical and radiological results 6 years after treatment of traumatic thoracolumbar burst fractures with pedicle screw instrumentation and balloon assisted endplate reduction. Spine J. 15 (6), 1172–1178. Wahba, G.M., et al., 2010. Biomechanical evaluation of short-segment posterior instrumentation with and without crosslinks in a human cadaveric unstable thoracolumbar burst fracture model. Spine 35 (3), 278–285. Wei, F.-X., et al., 2010. Transpedicular fixation in management of thoracolumbar burst fractures: monosegmental fixation versus short-segment instrumentation. Spine 35 (15), E714–20. White, A.A., Panjabi, M.M., 1978. Clinical Biomechanics of the Spine. Lippincott, Philadelphia. Wilke, H.J., et al., 2001. Combined anteroposterior spinal fixation provides superior stabilisation to a single anterior or posterior procedure. J. Bone Joint Surg. Br. 83 (4), 609–617. Wood, K.B., et al., 2005. Assessment of two thoracolumbar fracture classification systems as used by multiple surgeons. J. Bone Joint Surg. Am. 87 (7), 1423–1429.