Spondylopelvic dissociation is a complex injury pattern resulting in multiplanar instability of the lumbopelvis. These injuries have traditionally been known as “suicide jumper’s fractures” and have recently increased in prevalence as a result of under-vehicle explosions seen in the past decade of military conflicts in the Middle East. The hallmarks of spondylopelvic dissociation are bilateral vertical sacral fractures with a horizontal component, resulting in lumbosacral instability in the sagittal and axial planes. Surgical treatment has evolved greatly and both percutaneous and open options are available, with triangular osteosynthesis being the most relied on method of fixation.
Key points
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Spondylopelvic dissociation is a rare and highly complex injury pattern resulting in multiplanar instability of the lumbopelvis. Hallmarks include bilateral vertical sacral fractures with a horizontal component. Iliolumbosacral instability occurs in both the coronal and the axial planes.
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Extreme axial load is required to produce this injury pattern resulting in bony deformity as well as soft tissue trauma. Common mechanisms of injury include suicide jumps, motor vehicle and motorcycle collisions, and under-vehicle explosions.
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Diagnosis relies on thin-section computed tomographic scan with coronal and sagittal reconstructions.
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Surgical treatment has evolved greatly over the past 15 to 20 years and both percutaneous and open options are available. Triangular osteosynthesis is the most relied on method of fixation as it controls deformity in both sagittal and axial planes.
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The role of fracture reduction and nerve root decompression is debated in literature.
Spectrum of disease: historical perspective
Spondylopelvic dissociation is a complex injury pattern that has recently increased in prevalence as the United States Armed Forces have engaged in conflict in Iraq and Afghanistan. These high-energy injuries are the least understood of the sacral fractures and inconsistencies in nomenclature have contributed to the confusion within the literature. They are most frequently seen in suicide jumpers, blast injuries, and high-speed motor vehicle collisions. Spondylopelvic dissociation is at the extreme end of the spectrum of atypical sacral fractures (eg, transverse and U-type sacral fractures) and is a relatively new term. It was preceded by the term transverse sacral fracture and the first case report of a transverse sacral fracture was published in 1969. Since that time, their representation in the literature has been somewhat limited. Roy-Camille and colleagues published their landmark paper describing in detail the anatomy, pathoanatomy, and biomechanics of transverse sacral fractures in 1985. In addition, they proposed the first classification scheme for this fracture pattern.
As advanced imaging modalities and surgeons’ understanding of transverse sacral fractures improved, the term U-shaped sacral fracture emerged. This fracture pattern was introduced in 1996 by Ebraheim and colleagues in which they described the injury as a “transverse fracture at the S2-S3 segment and the longitudinal fractures running on either side through the sacral foramina.” Since that time, there have been few published reports in the English language describing the U-shaped sacral fracture pattern, with all publications being either case reports, case series, or expert opinion. In addition to U-shaped sacral fractures, H-shaped, T-shaped, and Y-shaped sacral fractures have been described, all of which include both transverse and vertical components.
Around the time U-shaped sacral fractures surfaced in the literature, spondylopelvic dissociation made its appearance as a distinct and highly complex type of U-shaped sacral fracture in which considerable displacement exists between the cephalad superior-central sacrum segment/lumbar spine and the caudal bilateral sacral alae and inferior sacral segments. In similar fashion to U-shaped sacral fractures there is scant literature beyond case series and literature reviews describing this injury pattern.
A close examination of the literature suggests the above-mentioned entities are in fact a spectrum of injury patterns with considerable overlap. Furthermore, there is spectrum of variability with respect to the nomenclature of these injuries. Reports describing similar injury patterns use multiple terms including transverse sacral fractures, U-shaped sacral fractures, spondylopelvic dissociation, spinopelvic dissociation, lumbopelvic dissociation, and lumbosacral dissociation.
Spectrum of disease: historical perspective
Spondylopelvic dissociation is a complex injury pattern that has recently increased in prevalence as the United States Armed Forces have engaged in conflict in Iraq and Afghanistan. These high-energy injuries are the least understood of the sacral fractures and inconsistencies in nomenclature have contributed to the confusion within the literature. They are most frequently seen in suicide jumpers, blast injuries, and high-speed motor vehicle collisions. Spondylopelvic dissociation is at the extreme end of the spectrum of atypical sacral fractures (eg, transverse and U-type sacral fractures) and is a relatively new term. It was preceded by the term transverse sacral fracture and the first case report of a transverse sacral fracture was published in 1969. Since that time, their representation in the literature has been somewhat limited. Roy-Camille and colleagues published their landmark paper describing in detail the anatomy, pathoanatomy, and biomechanics of transverse sacral fractures in 1985. In addition, they proposed the first classification scheme for this fracture pattern.
As advanced imaging modalities and surgeons’ understanding of transverse sacral fractures improved, the term U-shaped sacral fracture emerged. This fracture pattern was introduced in 1996 by Ebraheim and colleagues in which they described the injury as a “transverse fracture at the S2-S3 segment and the longitudinal fractures running on either side through the sacral foramina.” Since that time, there have been few published reports in the English language describing the U-shaped sacral fracture pattern, with all publications being either case reports, case series, or expert opinion. In addition to U-shaped sacral fractures, H-shaped, T-shaped, and Y-shaped sacral fractures have been described, all of which include both transverse and vertical components.
Around the time U-shaped sacral fractures surfaced in the literature, spondylopelvic dissociation made its appearance as a distinct and highly complex type of U-shaped sacral fracture in which considerable displacement exists between the cephalad superior-central sacrum segment/lumbar spine and the caudal bilateral sacral alae and inferior sacral segments. In similar fashion to U-shaped sacral fractures there is scant literature beyond case series and literature reviews describing this injury pattern.
A close examination of the literature suggests the above-mentioned entities are in fact a spectrum of injury patterns with considerable overlap. Furthermore, there is spectrum of variability with respect to the nomenclature of these injuries. Reports describing similar injury patterns use multiple terms including transverse sacral fractures, U-shaped sacral fractures, spondylopelvic dissociation, spinopelvic dissociation, lumbopelvic dissociation, and lumbosacral dissociation.
Classification
Classically, sacral fractures have been classified by either the Roy-Camille system or the Denis system, although modifications exist. Roy-Camille and colleagues presented the first classification system for transverse sacral fractures in 1985. In this scheme there are 3 subtypes of injuries based on the mechanism and orientation of displacement ( Fig. 1 ). Type 1 injury is a flexion deformity of the upper sacral segments onto the lower sacral segments without displacement. Type 2 injury is a flexion injury with posterior displacement of the superior segments relative to the inferior segments. Type 3 injury is an extension type injury with anterior displacement of the superior segments relative to the inferior segments. The Roy-Camille system fails to consider the level at which the transverse fracture takes place and this has important implications with respect to lumbopelvic stability, neurologic injury, and treatment algorithms.
The well-known Denis Classification of sacral fractures does not specifically address transverse sacral fractures; however, it incorporates these injuries into the zone 3 pattern, involving the central sacral canal. Similarly, this system fails to describe the complex pathoanatomy seen in the spectrum of spondylopelvic dissociation effectively.
Only recently was the complex spectrum of spondylopelvic dissociation addressed in a comprehensive classification system that incorporates multiple facets of this disease. Lehman and colleagues proposed the Lumbosacral Injury Classification System (LSICS) to address the deficiencies of the classification systems described above. Their work followed the large increase they observed in soldiers sustaining spondylopelvic dissociation injuries related to conflicts in the Middle East. The LSICS is based on a composite injury severity score that takes into account injury morphology, integrity of the posterior ligament complex, neurologic status, and the following 3 clinical modifiers: systemic injury load, soft tissue status, and expected mobility status. LSICS should be used to make management decisions for transverse sacral fracture with associated bilateral longitudinal components (U-/H-/T-/Y-shaped sacral fractures), traumatic spondylolisthesis at L5/S1, and vertically unstable sacral fracture. This system does not consider bilateral sacroiliac joint dislocations, because these injuries classically are considered pelvic ring injuries. A composite injury severity score is determined based on the injury morphology, integrity of the posterior ligament complex, and neurologic status. Operative treatment is then recommended based on this score in conjunction with the presence or absence of clinical modifiers ( Fig. 2 ).
Anatomy and biomechanics
The sacrum forms from the fusion of 5 vertebral bodies and intervertebral disks. Completion of fusion takes place between ages 25 and 33 years of age. In the nontraumatized patient the lumbosacropelvic articulations are inherently stable due to bony engineering and ligamentous restraints. The V-shaped sacrum is wedged between the iliac bones bilaterally in both the axial and the coronal planes. The broad sacroiliac articulation superiorly gives way to narrowing distally. This articulation includes sacral segments 1 and 2. Anteroinferiorly there is a shelf of dense cortical ilium blocking translation of the sacrum onto the pelvis.
Furthermore, the dense posterior ligamentous structures provide support in addition to the bony restraints described. These structures are critical to the prevention of anterior translation and flexion of the sacrum as well as radially directed instability throughout the pelvis. The ligamentous structures are many in number. Pelvic ring stability is provided by the posterior, interosseous (and to a lesser extent anterior) sacroiliac ligaments, sacrospinous ligaments, and sacrotuberous ligaments. Lumbosacral stability is imparted by the supraspinous ligament, ligamentum flavum, interspinous ligament, iliolumbar ligament, lateral lumbosacral ligament, and the L5/S1 facet joint capsule. In concert, these dense soft tissue restraints behave like a posterior tension band, stabilizing the iliolumbosacral junctions from progressive deformity. Assessment and understanding of the integrity of these structures are critical to managing spondylopelvic dissociation properly.
Sacral neural foraminal anatomy has important clinical implications as well. S1 and S2 nerve roots occupy between one-quarter and one-third of the entire cross-sectional area of the respective foramina. Meanwhile, the lower nerve roots occupy roughly one-sixth of cross-sectional area of their foramina, which has important implications when considering nerve root decompression and removal of bony fracture debris.
König and colleagues describe the 2 overriding biomechanical components of spondylopelvic dissociation. The first is the high-energy axial load resulting in the bilateral vertical components. This vertical instability causes the central component to pivot in flexion or extension, resulting in the transverse fracture line, usually occurring between S1 and S2 and causing the upper sacral segments to remain attached to the spine and the lower sacral segment to remain attached to the pelvis.
Critically important to fracture anatomy is the mechanism of injury and the forces imparted through the sacrum at the time of impact. Transverse sacral fractures can be thought of as flexion type (types 1 and 2) or extension type (type 3). As previously mentioned, Roy-Camille and colleagues initially proposed this classification in 1985. They correlated their cadaveric biomechanical findings with clinical findings. From this they proposed that an axial load on the body with the lumbar spine in kyphosis results in flexion deformity. The superior sacral segment may translate posteriorly if enough energy is imparted through the system. Conversely, with the lumbar spine in lordosis, the sacrum extends and the superior segment translates anteriorly. Pure axial compression of the sacrum occurs when the lumbar spine maintains its neutral alignment during impact. The blast or shear pattern has only recently been described. Blast morphology is characterized by extensive comminution of the proximal sacral segments. The typical mechanism for this injury is an under-vehicle explosion resulting in a superiorly directed force to the dorsal aspect of the sacrum. There is often massive soft tissue injury and they are frequently open injuries.
Clinical evaluation and related considerations
Because of the rarity of spondylopelvic dissociation, the initial diagnosis can be easily missed. For instance, transverse sacral fractures account for less than 5% of all sacral fractures. Furthermore, greater than 95% of patients with this injury are polytraumatized with various other visceral and musculoskeletal injuries. Additional pelvic ring injuries and spine injuries are common and seen in 44.4% and 31.7% of patients, respectively. The communicative patient may describe sacrococcygeal pain and discomfort. As always, a detailed trauma evaluation following the Advance Trauma Life Support guidelines is essential. A detailed examination of the pelvic soft tissue envelope and musculoskeletal system should be performed, paying particular attention to the posterior soft tissue. Morel-Lavallee (internal degloving) injury is common and may be suggested by a subcutaneous fluid collection or large ecchymotic patch over the sacrum. Not only is this injury important to consider because it is known to greatly increase infection risk with and without surgery, but operative approaches and orthopedic instrumentation should avoid crossing these lesions. Palpation of the sacrum for a step-off and crepitation should be carefully performed. As with all high-energy lumbopelvic injuries, an anogenital examination must be performed to assess for occult open fractures. Open fracture through rectal or vaginal tissue mandates urgent debridement.
As expected, there is an exceedingly high rate of neurologic injury seen in spondylopelvic dissociation. Motor and sensory radicular deficits should be expected. In their seminal work, Roy-Camille and colleagues reported some element of neurologic deficit in 100% of patients in their series. Similarly, high numbers have been reported in more recent reports of U-shaped sacral fractures. In their systematic review, König and colleagues describe greater than 94% of patients having some form of abnormal neurologic examination—44% of which developed bowel and/or bladder dysfunction—and included sexual dysfunction and dysesthesias. The significance of this finding cannot be overstated, mandating the vigilant practice of thorough rectal and perirectal examination on presentation to the trauma bay. In addition, neural involvement may present as a mononeuropathy, oligoneuropathy, or cauda equina syndrome.
The ubiquitous neurologic injuries described may only be applicable to transverse component fractures that involve the upper sacral segments. Sabiston and Wing reported on 11 patients who sustained low sacral transverse fractures. Only 1 patient developed a neurologic deficit, whereas 5 of 5 patients with upper segment fractures developed deficits. To further complicate the clinical picture, significant nerve root injury may be present with normal lower extremity motor findings. The lower sacral nerve roots cannot be simply evaluated by testing lower extremity strength. A cursory skeletal trauma examination may miss neurologic injury below S1. As such, once stabilized, the patient should receive a thorough tertiary neurologic examination.
Complete neurologic recovery occurs in only 46.5% of patients with abnormal immediate postinjury neurologic examination, whereas failure to recover any lost function may be seen in upward of 21.9%. The need and utility of surgical nerve root decompression in the acute setting continue to be debated in the literature.
Radiographic assessment
The standard radiographic evaluation of spondylopelvic dissociation is notoriously difficult, which has accounted for the high rate of missed injuries before current advanced imaging techniques that are now becoming commonplace in all modern trauma centers. Nearly all of the early publications reporting on transverse sacral fractures describe these diagnostic difficulties as they relate to standard film interpretation. That being said, there are subtle clues that the attuned observer can pick up to aid in the diagnosis. As part of the trauma series of radiographs, the anteroposterior view of the pelvis should be examined closely for 4 characteristic yet subtle findings, including L5 transverse process fractures, bilateral transforaminal or vertical alar sacral fractures, irregularities in the sacral foraminal lines, and paradoxic inlet view of the upper sacral bodies ( Fig. 3 ). A lateral view of the sacrum can confirm the diagnosis and will show angulation or translation at the fracture site.
