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As is the case with fractures involving other parts of the spine, the long-term goals of treating sacral fractures include restoring the best possible neurologic and musculoskeletal function while minimizing problems associated with chronic pain and deformity. Because of constraints related to the unusual shape of the sacrum and its unique positioning at the caudal-most end of the spine, fixation of the sacrum has posed unique challenges among spinal injuries and a higher tolerance for non-operative care than is typical of other spinal injuries.1 Recent advances in diagnostic imaging and instrumentation techniques have enabled sacral injuries to be treated according to principles similar to those for more rostral injuries of the spine. As with many other spine and pelvis injury types, strong and objectively validated indications for surgical intervention remain elusive. However, current paradigms regarding the treatment of sacral fractures are described here, including the rationale for the use of spinopelvic versus iliosacral or other forms of sacral fracture stabilization.

Anatomy

The sacrum is a large triangular bone positioned at the junction of the spine and pelvis. In addition to its important role as the foundation of the spine, it also articulates with the two innominate bones to form the central posterior aspect of the pelvic ring. If its ligamentous structures are intact, the pelvis constitutes a stable ring and the sacrum serves as the “keystone” of the pelvis, because it maintains stability while it transmits forces from the pelvis, across the sacroiliac joints to the lumbosacral spine. This keystone function is purest in the pelvic outlet plane, in which the bony orientation of the sacrum relative to the ilium enables axial forces to lock the sacrum into the pelvis to further stabilize the sacroiliac articulation. In the pelvic inlet plane, however, because the sacrum is shaped like a “reverse keystone,” the sacroiliac articulation is inherently unstable and depends heavily on ligamentous support of the sacroiliac joints.

The sacrum is composed of five kyphotically aligned and unsegmented vertebral segments. The considerable variability in upper sacral anatomy (i.e., transitional vertebrae or sacral dysplasia) is important to appreciate, as it can significantly impact the selection of instrumentation techniques and their safe application.2,3

The body of S1 contains the densest cancellous bone in the sacrum, particularly adjacent to its superior end plate. The sacral promontory, which is the most anterior aspect of the upper S1 body, projects superiorly into the posterior aspect of the pelvic inlet and is an important radiographic landmark as the anteriormost margin for placing percutaneous iliosacral or transiliac-transsacral screws. The sacral alae are positioned lateral to the sacral foramina and articulate with the ilium through the sacroiliac joints at the level of S1 and S2. The largely cancellous bone of the sacral alae has an even lower bone density in older individuals,⁴ making this area prone to insufficiency fracture.

The convex posterior surface of the sacrum is narrower than the anterior surface and contains the middle sacral crest, a coalescence of three or four midline tubercles that correspond to the rudimentary spinous processes of the upper sacral vertebrae. More laterally, the intermediate sacral crests correspond to the fused sacral zygapophyseal joints. Importantly, the lowest one or two sacral segments have incompletely formed bony posterior elements, creating a defect in the posterior wall of the sacral canal known as the sacral hiatus. The importance of the sacral hiatus is twofold: (1) enlargement of the sacral hiatus may weaken the sacrum and predispose it to fracture; and (2) failure to recognize the dimensions of the sacral hiatus can result in intraoperative iatrogenic sacral root injury.

Lumbosacral motion occurs through the L5-S1 intervertebral disk and the paired zygapophyseal (facet) joints. The iliolumbar ligaments, which originate on the L5 transverse processes and insert onto the iliac crest, also serve as important stabilizers of the spino-pelvic junction, along with the sacrolumbar ligaments, which originate contiguous with the iliolumbar ligaments and insert onto the anterior sacroiliac joint and anterosuperior sacrum. These structures combine to make the L5-S1 articulation more stable than more cephalad intervertebral lumbar levels. Forces applied to the axial skeleton through the lumbosacral junction are projected across the sacroiliac joints and the iliac wings by the anterior pelvic, posterior pelvic, and pelvic floor ligaments.⁵ The main anterior ligament is the symphysis pubis. The primary posterior ligaments are the anterior sacroiliac, interosseous sacroiliac, and posterior sacroiliac ligaments. The pelvic floor is composed of the sacrotuberous and sacrospinous ligaments.

The sacral roots (S2–S4), along with the autonomic nervous system, are responsible for urinary, bowel, and sexual function. The paired sacral nerve roots originate from the conus medullaris and exit the ventral and dorsal foramina. Injury to the sacral roots can occur at any point throughout their course, starting from the conus medullaris, through the sacral canal and the foramina, or even within the pelvis. The L5 nerve root exits the spinal canal beneath the L5 pedicle on the superior surface of the sacral alae, where it is vulnerable to injury from displaced sacral alae fractures or from anteriorly malpositioned iliosacral screws. Nerve root injury is more likely at the S1 and S2 foramina because of their higher proportion of sacral root occupancy than at the S3 and S4 ventral foramina.⁶ Bilateral injury to the lower sacral roots is thought to be required for loss of voluntary bowel and bladder function, and for sexual dysfunction.⁷

Differences in the orientation of the spinopelvic junction may influence the type of strategy selected for sacral fracture stabilization. The pelvic incidence, in particular, can significantly influence the shear forces exerted across the spinopelvic junction, and may therefore dictate the type of instrumentation required to best stabilize a given sacral fracture,8 with more biomechanically robust constructs potentially being favored if pelvic incidence is high.

Etiology and Epidemiology

High-energy sacral fractures generally occur in young adults, whereas the increasingly prevalent lower energy insufficiency fractures occur primarily in elderly and osteoporotic patients. The incidence of sacral fractures has tripled in the past decade, from 0.67 per 100,000 persons in 2002 to 2.09 per 100,00 in 2011.⁹ High-energy mechanisms are most common, including motor vehicle accidents (57%), pedestrians struck by motor vehicles (18%), motorcycle accidents (9%), and high-energy falls (9%).⁶

Prior to the routine use of total-body computed tomography (CT) scans in polytrauma patients, the diagnosis of sacral fractures was generally made more promptly and reliably in the presence of a neurologic injury, with delayed diagnosis documented in approximately half of neurologically intact patients with sacral fractures.⁶ Causes include the presence of distracting injuries, difficulty identifying these fractures on screening anteroposterior (AP) pelvic radiographs, the absence of (obvious) lower extremity neurologic deficits, and lack of standard rectal or genitourinary evaluation in polytrauma patients.

Most sacral fractures are components of pelvic fractures. This influences both their initial management and their definitive treatment. Isolated sacral fractures with no pelvic ring involvement are uncommon, constituting only 5 to 10% of all high-energy sacral fractures. Most are transverse fractures of the lower sacrum below the sacroiliac joint, which result from direct trauma.¹⁰

Insufficiency fractures of the sacrum can occur either spontaneously or after low-energy mechanisms such as ground-level falls,¹¹ particularly in patients with comorbidities such as osteoporosis, previous pelvic irradiation, or chronic steroid treatment.^11,12 Although various case series have been reported,¹³ the incidence of insufficiency fractures of the sacrum is unknown because the condition is likely to be underdiagnosed, but the general consensus is that the incidence of pelvic fractures in osteoporotic patients seems to be increasing.

Management Principles

Clinical Evaluation

Patients with high-energy sacral fractures often have injuries to several organ systems, including life-threatening intracranial, thoracic, and abdominal injuries. The immediate objective in these patients is emergent resuscitation. The Advanced Trauma Life Support (ATLS) protocol mandates a primary survey, during which immediately life-threatening cardiopulmonary problems are addressed, focusing on hemodynamic resuscitation. A secondary survey then enables identification of injuries that are not immediately life threatening.¹⁴

Standard spinal column injury precautions should be undertaken by initially keeping the patient on a flat surface and log-rolling from side to side, when necessary, to prevent spinal column displacement. Physical examination requires inspection and palpation of the patient’s posterior spine over the entire length of the spinal column, including the sacrum. Sacral fractures commonly have overlying skin discoloration or lacerations, palpable step-offs, crepitus, localized tenderness, and hematomas. Significant soft tissue contusion or internal degloving, analogous to Morel-Lavallée lesions seen with acetabular fractures, can have implications for subsequent treatment. Manual anteroposterior and mediolateral compression over the iliac crests, with or without fluoroscopic visualization, may also help identify a sacral fracture. Perforations of the rectum or vagina can represent open sacral fractures, and can be detected with rectal and vaginal digital examination and with the use of a speculum and proctoscope.

If the sacral fracture is associated with a pelvic ring disruption, massive fluid resuscitation may be necessary if hemodynamic instability is profound secondary to disruption of the intrapelvic vasculature.15 Injury to the hypogastric arterial system may require embolization or pelvic packing to adequately control arterial hemorrhage.¹⁵ Provisional methods of pelvic ring stabilization, such as the application of a pelvic resuscitation clamp, circumferential pelvic antishock sheet,² or anterior external fixator for open book pelvic injuries, including skeletal traction for displaced vertical shear fractures, may be necessary to reduce pelvic volume and provide temporary stability.

Because early determination of the extent of soft tissue injuries and the patient’s neurologic status are of the utmost importance in patients with sacral fractures, a rectal examination must be performed, including evaluation of perianal sensation, anal sphincter tone, and voluntary perianal contraction. The bulbocavernosus reflex is particularly useful in evaluating sacral root function, as the absence of the reflex without spinal cord trauma indicates sacral root injury.

Extremity motor function is graded on a scale of 0 to 5 according to the American Spinal Injury Association (ASIA), and a sensory level is obtained. The assessment of the level of neurologic injury by motor examination in sacral fractures is limited to the L5 and S1 levels unless a rectal exam is performed, which more specifically identifies sacral root injury. The classification described by Gibbons and coauthors¹⁶ is useful as a means of grading and monitoring sacral root function.

Radiological Evaluation

A pelvic ring fracture needs to be excluded after all high-energy injuries. Over the past decade, trauma centers have gravitated toward the routine use of reconstructed abdominopelvic CT as the primary screening method for pelvic fractures, because of their ability to identify pelvic ring injuries while simultaneously searching for visceral or vascular injuries. If injuries to the pelvis and sacrum are identified, additional detail can be obtained with multiplanar reconstructions of the pelvis and sacrum, although plain AP and inlet and outlet radiographs may still play a role. The different radiographic techniques are described below.

Plain Radiographs

Sacral fractures are frequently missed on the AP pelvic radiograph, which is actually an oblique projection of the pelvis, in which the sagittal pelvic inclination and the juxtaposition of the iliac wings make it difficult to visualize sacral fractures. Sacral dysmorphism and osteopenic bone can obscure landmarks, increasing the difficulty in identifying fractures. Additional radiographic plain film projections can therefore provide important information.

The inlet view enables evaluation of the pelvic brim, the pubic rami, the sacroiliac joints, the sacral alae, and the body of the sacrum. Displacement of the hemipelvis in the transverse (axial or AP) plane can be identified on this view (Fig. 13.1a,b). The outlet view is orthogonal to the inlet view, and represents a true AP view of the sacrum, parallel to the L5-S1 disk space. The vertebral bodies of S1 and S2 and the sacral foramina can be clearly visualized. This view enables evaluation of the symmetry of the sacroiliac joints and the pubic symphysis, and of any vertical (coronal or craniocaudal displacement of the hemipelvis (Fig. 13.1c,d).

Computed Tomography

Computed tomography (CT) has become the accepted gold standard for the evaluation of pelvic and sacral fractures, and is mandatory for the evaluation of patients who have sustained high-energy injuries or in whom a posterior pelvic injury is suspected.¹⁷ The routine use of abdominopelvic CT in the initial assessment of the trauma patient’s visceral injuries has increased detection of previously unrecognized sacral fractures. Identification of a sacral fracture on initial screening studies necessitates a dedicated CT scan of the sacrum with fine (2 mm or less) axial slices and sagittal and coronal reconstructions, to provide the detail required for determining fracture configuration, resulting instability patterns, and the extent of sacral canal and neuroforaminal compromise.18

Fig. 13.1a–d (a) The pelvic inlet view, which is used to identify pelvic ring displacement in the transverse (axial) plane, is obtained by angling the X-ray beam caudally, and (b) 40 degrees to the horizontal axis. (c) The pelvic outlet view, which is used to identify pelvic ring displacement in the coronal plane, is obtained by directing the X-ray beam cranially, and (d) 60 degrees to the horizontal axis.

Magnetic resonance imaging (MRI) is not usually helpful in the screening of high-energy injuries except in patients with unclear neurologic deficits or discrepancies between skeletal and neurologic levels of injury. However, MRI can confirm early diagnosis of lumbosacral nerve root avulsion, which may impact the timing of surgical intervention.19 MRI is also considered the most sensitive screening test, for the diagnosis of sacral stress fractures, particularly the T2-weighted short tau inversion recovery (STIR) sequences.²⁰

Sacral Injury Classification

Sacral fracture-dislocations and fractures at the lumbosacral junction with spinopelvic instability are most often classified in terms of existing sacral and pelvic ring fracture classifications, although classification systems based on both pelvic and spinopelvic instability are currently being refined.

The AO/Orthopaedic Trauma Association (OTA) pelvis fracture classification, which is the most commonly used fracture classification in orthopaedic and trauma surgery, describes vertical sacral fractures as 61-C1.3, -C2.3, -C3.2, and -C3.3 fractures, depending on overall pelvic ring stability in the horizontal and vertical planes.²¹ The Denis classification of sacral fractures (Fig. 13.2) correlates anatomic factors with neurologic injury risk, which in turn correlates with both injury severity and prognosis. It differentiates between alar fractures (zone I; 5.9% incidence of predominantly L5 root injuries), transforaminal fractures (zone II; 28.4% incidence of mainly L5/S1 root injuries), and central fractures, which include any fracture extending into the spinal canal (zone III; 56.7% incidence of neurologic injury mostly consisting of sacral plexus/cauda equina dysfunction).⁶ The L5 root, however, may be incarcerated between the L5 transverse process and the displaced sacral ala, leading to the “traumatic far-out syndrome.” However, the classification proposed by Denis fails to take spinopelvic stability into account. Isler²² recognized that vertical sacral fractures can extend rostrally, either lateral to (type 1), through (type 2), or medial to (type 3) the S1 superior facet (Fig. 13.3), which can have implications for spinopelvic stability. Fractures that involve or extend medial to the L5/S1 facet joint (types 2 and 3) result in a form of spinopelvic instability.

Fig. 13.2 Denis and coauthors6 categorized sacral fractures according to the location of fractures relative to the sacral foramina. More medially located fractures have a higher risk of neurologic deficits and a worse prognosis.

U- and H-type fracture-dislocations at the lumbosacral junction and their variants are considered Denis zone III injuries and are included in AO/OTA type 61-C3.3 pelvic fractures, although neither of these classifications provides insight into the mechanism of injury or displacement and instability patterns. Roy-Camille and coauthors²³ developed a helpful subclassification of Denis zone III injuries lumbosacral fracture-dislocations, classifying three types of transverse sacral fracture displacement and angulation patterns according to injury severity and presumed likelihood of neurologic injury. Type 1 injuries consist of a flexion deformity of the sacrum without translation, and are thought to be the result of axial loading injury with the spine in flexion; type 2 injuries are characterized by flexion and posterior translation of the upper sacrum, also presumably caused by axial loading injury with a flexed spine; type 3 injuries demonstrate complete anterior translation of the upper sacrum, typically caused by an axial loading force in extension. A type 4 injury was later added by Strange-Vognsen and Lebech,²⁴ consisting of a comminuted S1 vertebral body caused by axial loading of the upper sacrum (Fig. 13.4). All these injuries are caused by indirect forces to the lumbosacral junction. This classification system, however, does not distinguish the location of the transverse component of the sacral fractures. Defining the transverse sacral fracture as high (involving S2 or above) or low (involving S3 or below) can be helpful from both a biomechanical and a neurologic standpoint, and consequently as a useful guide to treatment and prognosis.^25,26

Rather than being exclusively vertical²⁷ or transverse, most Denis zone III sacral fractures have complex, multiplanar fracture patterns consisting of a transverse fracture of the sacrum with associated vertical injury components.²⁵ These so-called U fractures usually consist of a transverse fracture at the level of S2 or above, with bilateral transforaminal fractures that extend rostrally to the lumbosacral junction. Slight variations in fracture patterns result in the sacral H, Y, and lambda fracture variants, which have similar instability profiles to the sacral U fracture, in which the combination of longitudinal and transverse fractures of the sacrum results in separation of the axial and appendicular skeleton, termed lumbopelvic or spinopelvic dissociation.25 These injuries may result in severe instability and cauda equina syndrome.²⁶

Fig. 13.3 Isler classified vertical sacral fractures according to the location of the fracture relative to the L5-S1 facet joint. Fracture patterns that extend medial to the L5-S1 facet joint (types 2 and 3) result in spinopelvic instability.

The high likelihood and variable grade of neurologic impairment in lumbosacral fractures is inferred in the classification of Denis et al,6 but not directly accounted for in any of the above classification systems. Because the ASIA spinal cord injury grading system is geared toward sensorimotor function of the extremities, and therefore not suitable for addressing the severity of sacral root injuries, Gibbons et al16 suggested a sacral root injury grading system based on motor, sensory, and bowel/bladder function. They categorized patients as those having (1) no injury, (2) lower extremity paresthesias only, (3) lower extremity motor deficit with intact bowel and bladder function, and (4) impaired bowel and/or bladder function. This classification system, although simple to use, unfortunately does not address severity or completeness of bowel or bladder dysfunction and makes no reference to sexual function.

Fig. 13.4 The Roy-Camille classification,23 as modified by Strange-Vognsen and Lebech,²⁴ categorizes Denis⁶ zone III fractures according to sagittal plane angulation and displacement.

A recent AO-led effort has focused on categorizing sacral fractures based primarily on the extent and type of instability, specifically taking into account the sacrum’s role in posterior pelvic and spinopelvic stability (Fig. 13.5). Type A fractures are characterized by the absence of posterior pelvic and spinopelvic instability, and range from inconsequential injuries to severely displaced transverse fractures that occur below the SI joint, which may even entail severe sacral root injuries. Type B fractures are vertical fracture patterns that result in posterior pelvic instability, generally without spino-pelvic instability. Type C injuries are defined by the presence of both posterior pelvic and spino pelvic instability, and include vertical fractures, with Isler types 2 and 3 L5-S1 facet compromise, complex sacral U fracture types and bilateral vertical fractures. Type A and B are divided into three subtypes and type C is divided into four subtypes, categorized according to injury severity based on risk of neurologic deficit or of instability. Ideally, this fracture classification would be combined with neurologic grading to provide a more definitive guide to treatment and prognosis.

Fig. 13.5a–c Recent efforts have been undertaken by AOSpine to categorize sacral fractures according to their effect on posterior pelvic and spinopelvic instability. SI, sacroiliac.

Despite the tendency to consider the presence of a neurologic deficit as an indication for operative intervention, the effectiveness of surgery in improving neurologic outcomes after fracture of the sacrum remains unproven, because the literature on this topic consists primarily of small, heterogeneous case series without consistent grading and definitions of neurologic dysfunction.28 Small series have demonstrated that nerve root decompression combined with reduction of high transverse sacral fractures with kyphotic deformity can improve neurologic function,^1,23,25 but strictly controlled studies have not been conducted. Moreover, many of the factors on which recovery is contingent are difficult to identify preoperatively. For example, similar neurologic deficits can arise from neurapraxias, which frequently respond to conservative treatment, as from root avulsions, which obviously have an unfavorable prognosis. Nonetheless, when faced with high-grade sacral fractures and corresponding neurologic deficits, we tend to adhere to the same general principles as with injuries to other areas of the spine and recommend operative decompression (both direct and indirect via fracture realignment) and stabilization.

Most Denis zone III fractures of the sacrum result in kyphotic deformities, and may be accompanied by translation or rotational malalignment. The presence of a major angular or translational deformity often heralds an unstable injury. Severe angulation of a transverse sacral fracture results in increased pelvic incidence and potential problems with sagittal plane malalignment, neural compression at the level of the foramina and the spinal canal, where nerve roots can be draped over a severe kyphotic deformity, and may also result in tenting of the overlying soft tissues, with resulting skin breakdown. Although restoration of sagittal balance is an important factor in outcomes of lumbosacral spine surgery for degenerative conditions, the acceptable tolerance for sacral deformity after fracture is likely to be multifactorial and patient-specific, and has not been extensively studied. However, because the caudal location of the sacrum results in greater translational sagittal plane malalignment for any given angular deformity, it is likely that improved clinical outcomes can be achieved by minimizing sacral kyphosis. Hart and coauthors29 reported that restoration of appropriate sagittal alignment of the sacral fracture decreases pain by preventing compensatory lumbar hyperlordosis, enabling a more physiological alignment of the lumbar spine, and suggested that pelvic incidence could be used as an intraoperative guide to adequate restoration of lumbopelvic alignment.

Instability

The third indication for fixation of sacral fractures is the presence of instability, which is distinguished from the above discussion regarding deformity in that the concern pertains not to the existing malalignment but to whether the existing (presumably acceptable) alignment can be maintained without surgical stabilization. Careful examination of the fracture pattern is essential to determining whether the sacral fracture is associated with instability of the spinal column, the posterior pelvic ring, the spinopelvic weight-bearing axis, or a combination of these factors. Unilateral vertical fractures through the foraminal zone or ala maintain continuity of the contralateral weight-bearing axis, allowing weight bearing on that side. Bilateral displaced vertical fractures, as well as U- and H-shaped fractures, dissociate the spinal column from the pelvic ring, resulting in complete disruption of the weight-bearing axis.23 In these instances of spinopelvic dissociation, weight bearing on either lower extremity or even sitting is likely to cause displacement. Conversely, transverse fractures below the sacroiliac joint have no implication on the weight-bearing axis.

Nonoperative Treatment

Specific guidelines for operative versus non-operative treatment of sacral fractures have not yet been validated. The stability of unilateral, vertically oriented sacral fractures that constitute posterior pelvic fractures is largely subjective, and is based on the extent of displacement, the type and degree of the associated anterior pelvic ring injury, the amount of displacement during diagnostic fluoroscopically visualized manipulation, the presence of injury to the iliolumbar ligaments or their anchor points (e.g., L5 transverse process), and the presence of neurologic deficits, among others. Minimally displaced fractures with preserved neurologic function and favorable injury patterns that are deemed stable can be treated with toe-touch weight bearing for a minimum of 6 weeks, with close clinical and radiographic follow-up to monitor the presence of displacement. More widely displaced (e.g., > 1 cm) high-energy sacral fractures that either demonstrate motion with pelvic manipulation or are considered complex sacral fractures with spinopelvic instability are not typically treated nonoperatively. In the rare case of a truly nondisplaced high-energy fracture with a spinopelvic instability pattern, nonoperative treatment can be considered, consisting of a period of recumbency and possibly femoral traction (unilateral vs bilateral), 6 to 8 weeks of non–weight bearing, followed by progressive mobilization in a hip-thoracolumbosacral orthosis (HTLSO) brace to minimize load transfer to the sacrum, before allowing progressive weight bearing as tolerated.³⁰

The treatment of displaced, high-energy sacral fractures by closed treatment methods entails many potential complications. Pulmonary and thromboembolic events are associated with prolonged immobilization in the polytraumatized patient. Additional potential complications include the development of decubitus ulcers, inadequate neurologic decompression, and the potential for late instability causing deformity and late neurologic deficits,³¹ potentially resulting in the need for complex pelvic osteotomy and reconstructive procedures with a more unfavorable complication profile than for more acute fracture reconstruction.

Insufficiency fractures are usually amenable to nonoperative treatment with bed rest and gradual weight bearing in spite of frequent bilateral involvement, but may require surgical stabilization in the presence of intractable pain or if nonunion develops.

Operative Treatment

The goals of surgical treatment depend on the patient’s symptomatology and fracture instability pattern. In general terms, the objectives are (1) to achieve neurologic decompression in patients with neurologic deficits; (2) to restore pelvic and spinopelvic alignment; and (3) to obtain reliable fracture stabilization to provide the best possibility for fracture healing and to prevent late deformity. The timing of surgery is most often a function of the polytraumatized patient’s physiological status. Emergent operative intervention should be considered primarily in patients with (1) open fractures, either externally or into the alimentary or genital tracts; (2) dorsal soft tissue compromise caused by displaced fracture fragments; or (3) a deteriorating neurologic exam. The literature pertaining to the timing of surgery in patients with cauda equina syndrome caused by disk herniation and stenosis cannot be extrapolated to the circumstances surrounding sacral fracture. However, certain circumstances support earlier operative intervention if the patient’s physiological status permits doing so, including the presence of a neurologic deficit or of an open fracture, either externally or into the alimentary or genital tracts, or if the sacral kyphotic deformity places the overlying skin at risk of necrosis due to tenting.

Injuries with spinopelvic instability are considered less amenable to these limited fixation options, because they are more unstable injuries that are characterized by more complex multidirectional instability, in which the primary instability pattern involves flexion, anterior translation, and shortening.23 The absence of a stable hemipelvis that can bear weight postoperatively and to which the unstable side can be secured adds to the challenges of treating these injury types with more traditional, percutaneous iliosacral techniques. Spinopelvic instability exists when vertical sacral fractures causing posterior pelvic instability are (1) complete and bilateral, or (2) incomplete and associated with transverse fractures. In each of these circumstances the spine is functionally disconnected from both hemipelves. With these injury patterns, the previously described primary deforming flexion force has a center of rotation located at the anterior aspect of the S1 and S2 vertebral bodies. Instability patterns more typical of isolated vertical shear-type fractures are also present. Operative fixation of these injuries, therefore, needs to counteract these large and multiply directed deforming forces, requiring a bony anchor sufficiently stable to preferably allow immediate patient mobilization and full early weight bearing.

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