Etiology and Epidemiology

Sacral fractures present with a bimodal age distribution. It may present in young adults after high-energy trauma or in elderly and osteoporotic patients after lower-energy falls. Whether from increased survivability or improved radiographic imaging, the incidence of sacral fractures increased significantly from 0.67 per 100,000 persons in 2002 to 2.09 per 100,000 in 2011. Identification of a sacral fracture increases in the presence of a neurologic injury. In a retrospective review, Denis and associates found only 51% of neurologically intact patients with sacral fractures had the fractures detected during the initial hospitalization. Close to 75% of patients who present to the hospital with sacral fractures are neurologically intact; thus, the diagnosis is often missed on the initial visit, and patients do not receive optimal treatment.

The most common mechanism of injury leading to sacral fractures typically is one of high energy. Of all sacral fractures, 57% result from motor vehicle accidents, 18% from pedestrian versus automobile accidents, 9% from motorcycle accidents, and 9% from a fall from an elevation. Most of the patients in Denis’ series had vertically oriented sacral fractures. However, fractures of the sacrum may be unilateral or bilateral; in addition, they may have a transverse component.

Isolated sacral fractures are uncommon and represent 5% to 10% of all traumatic injuries from high-energy accidents. Most isolated fractures are transverse and result from direct trauma, such as a fall from a height.

On the other end of the spectrum, insufficiency fractures of the sacrum can occur either spontaneously or after ground level falls. Patients who suffer insufficiency fractures of the sacrum often have comorbidities such as osteoporosis, pelvic irradiation, or chronic steroid use. Case series have been reported, but the incidence of insufficiency fractures of the sacrum is unknown because the diagnosis is frequently under-recognized. There is still little epidemiologic data, but the incidence of pelvic fractures in osteoporotic patients seems to be increasing.

Anatomy

The sacrum is a large triangular bone at the base of the spine that is wedged between two innominate bones to make up the pelvis. The osseous sacral anatomy is composed of five kyphotically aligned, fused vertebral segments, with significant variability in upper sacral anatomy in the form of transitional vertebrae and sacral dysplasia. This variability can alter the relationships among the sacrum, pelvis, and spinal column relative to their adjacent neurovascular structures. These variations must be recognized, especially if surgical treatment of sacral fractures is being considered.

The greatest density of sacral cancellous bone exists in the first sacral body, particularly adjacent to the superior S1 end plate. The sacral promontory is the most anterior aspect of the upper S1 body that projects superiorly into the pelvis. The sacral ala is the lateral portion of the sacrum, which articulates with the ilium through the sacroiliac joints. The ala is largely cancellous and formed by the coalescence of the sacral transverse processes. The cancellous bone of the sacral ala has a particularly lower density in older individuals, and an alar void is a consistent finding in middle-aged and older adults. This area of the sacrum is prone to fracture due to the relative difference in bone density. The hypodense ala is predisposed, particularly in osteopenic patients, to fracture line propagation. This problem is accentuated by the relative strength of the sacroiliac joint ligaments.

The posterior surface of the sacrum is convex and narrower than that of the pelvic surface. In the midline is the middle sacral crest, variably surmounted by three or four tubercles that correspond to the rudimentary spinous processes of the upper three or four sacral vertebrae. On either side of the middle sacral crest is a shallow groove, which gives origin to the multifidus muscle. Laterally, the intermediate sacral crests correspond to the zygapophyseal joints. The lowest one or two sacral segments have incompletely formed bony posterior elements, resulting in a hiatus in the posterior wall of the sacral canal known as the sacral hiatus. Enlargement of the sacral hiatus may weaken the sacrum and predispose it to fracture, and must also be recognized in order to prevent intraoperative iatrogenic sacral root injury.

Anatomic differences in the structure of the sacrum influence fixation strategies compared to the lumbar vertebra. Normal kyphosis of the thoracic spine ranges from 15 to 49 degrees, whereas normal lordosis of the lumbar spine is 60 degrees. The slope of the sacral base, which averages 45 degrees from the horizontal, influences these values. Consequently, the sacral slope is crucial in defining the shear force on the lumbosacral junction.

The sacrum forms both the lowest functional portion of the spinal column and the central portion of the pelvis. If its ligamentous structures are intact, the pelvis constitutes a stable ring. The sacrum forms the posterior aspect of the pelvic ring and serves as its keystone, as it maintains stability while transmitting forces from the lumbosacral articulation across the sacroiliac joints to the pelvis. This keystone function is particularly true in the pelvic outlet plane, in which the orientation of the sacrum relative to the ilium is such that axial forces lock the sacrum into the pelvic ring and further stabilize the sacroiliac articulation. In the pelvic inlet plane the sacrum is shaped like a “reverse keystone,” which is more inherently unstable and therefore requires substantial intrinsic and extrinsic ligamentous stabilization of the sacroiliac joints, while permitting required pelvic ring motion.

Lumbosacral motion occurs through the lumbosacral intervertebral disc and the paired zygapophyseal (facet) joints. The iliolumbar ligaments originating on the L5 transverse processes and inserting on the iliac crest uniquely stabilize the sacrum. The sacrolumbar ligaments have an origin contiguous with the iliolumbar ligament and insert on the sacroiliac joint and anterosuperior sacrum. The pelvic ligaments that maintain stability of the pelvic ring include the anterior, posterior, and pelvic floor ligaments. The major anterior ligament is the symphysis pubis. The posterior ligaments include the anterior sacroiliac, interosseous sacroiliac, and posterior sacroiliac ligaments. The pelvic floor is composed of the sacrotuberous and sacrospinous ligaments. These structures make the L5-S1 articulation more stable than the cephalad lumbar levels and allow for axial skeleton loading through the first sacral segment then out through the iliac wings.

The sacral roots (S2-4) are responsible for sexual function, urinary and fecal continence, micturition, and defecation. The autonomic nervous system also contributes to these functions. 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 the conus medullaris, the sacral canal, or within the foramina. The L5 nerve root exits the spinal canal beneath the L5 pedicle posterior to the sacrum, then courses anteriorly over the sacral ala. Fractures of the sacral ala or iliosacral screw fixation that breeches anteriorly can result in L5 nerve root injury. The lumbosacral plexus crosses the sacrum and courses laterally at the S1 level. Nerve root injury at the ventral foramen is more likely at S1 and S2 than at S3 and S4, due to the decreased foramen to root ratio at these levels. In evaluating functional deficit, a unilateral nerve root injury of the sacral roots does not result in a loss of sphincter function, whereas bilateral injury results in loss in voluntary bowel and bladder control as well as in sexual dysfunction.

Sacral anatomy presents a distinctive challenge for internal and percutaneous fixation. The bony thickness rapidly decreases with progression from the proximal to distal sacral segments. At S3 and S4, the thickness is often less than 20 mm. Only at the S1 level can pedicle screws safely be angled medially. S1 screws angled medially can safely breach the anterior cortex to achieve bicortical fixation while avoiding neurovascular structures that lie on the lateral edge of the body. Alternatively, S1 “alar” screws can be angled laterally in line with the sacroiliac joint and should exit lateral to the neurovascular structures. Pedicle screws can also be placed at the S2 level in most patients but anterior breaching on the left side risks injury to the sigmoid colon. Finally, percutaneous placement of lateral to medial sacroiliac screws can be placed for fixation of sacral fractures. The sacroiliac joint has an L-shaped configuration; one limb runs about 1 cm above the inferior border of the iliac wing; the second limb runs almost vertically for about 3 cm in length and is positioned about 4 cm from the posterior inferior iliac spine (PIIS) and about 5.5 cm from the posterior superior iliac spine (PSIS). The concavity of the sacral ala can cause a false projection of the anterior border of the ala on a lateral fluoroscopic view. This can mislead the surgeon into believing the sacroiliac screw is intraosseous. Careful fluoroscopic imaging is essential to identify the superior and anterior borders of the sacrum in order to avoid exiting the ala anteriorly and then reentering it more medially.

Evaluation

As noted previously, sacral fractures occur in two distinctly different patient groups. Most commonly, sacral fractures are the result of high-energy trauma. However, they are increasingly observed in patients with osteopenic bone disorders that are predisposed to pathologic fractures. In both groups of patients, diagnosis is frequently delayed, which may result in further displacement or neurologic deterioration. The causes of diagnostic delay are multifactorial. The presence of distracting injuries in the trauma patient, the relative difficulty in identifying these fractures on screening anteroposterior pelvic radiographs, and low clinical suspicion in patients with insufficiency fractures all account for missed sacral fractures.

In the trauma patient, the energy necessary to cause a fracture of the sacrum often causes other injuries including life-threatening head, thorax, and abdominal trauma. The primary objective in these patients is emergent resuscitation. Advanced Trauma Life Support (ATLS) protocol necessitates a primary survey during which immediately life-threatening conditions are addressed. Resuscitation is focused on maintaining cardiopulmonary and hemodynamic stability. Following appropriate evaluation and stabilization of the cardiovascular and pulmonary systems, a secondary survey, composed of examination of the patient to identify additional injuries, should be completed. Attention to the details of the accident should be focused on high-energy deceleration mechanisms. Mechanisms such as jumps/falls from a height or motor vehicle accidents should alert the examining physician to evaluate the pelvic ring for injury.

Precautions to maintain spinal column stabilization are necessary, and patients should be initially maintained on a flat surface and log-rolled side to side to prevent spinal column displacement. Physical examination should include inspection and palpation of the patient’s back over the entire spinal column, including the sacrum. Commonly, areas associated with sacral fractures can have skin discoloration or laceration, palpable step off or crepitus, localized tenderness, and hematoma, which can indicate the presence of a sacral fracture. Significant soft tissue contusion or internal degloving, such as a Morel-Lavallée lesion, can have implications on subsequent treatment. Manual compression over the iliac crests, both anteroposteriorly and laterally, can also help identify a sacral fracture. Perforations through the rectum, bladder, or vagina must be considered. Rectal and vaginal manual examination as well as the use of a speculum and proctoscope allows for detection of occult open sacral fractures.

The patient with a sacral fracture associated with a pelvic ring disruption may require massive resuscitation. These injuries are associated with significant intrapelvic hemorrhage as result of disruption of sacral vasculature. Associated vascular injury, particularly to the hypogastric arterial system, may require embolization or pelvic packing to adequately control arterial hemorrhage. Provisional methods of pelvic ring stabilization may be necessary to reduce pelvic volume and provide provisional stability. These methods include the application of a pelvic clamp, circumferential pelvic antishock sheet, or anterior external fixator in open book type injury patterns. Alternatively, skeletal traction can be of benefit in displaced vertical shear fracture patterns.

Radiologic Evaluation

Radiologic imaging is mandatory if the physician suspects that a patient has sustained a pelvic ring fracture, and is normally dictated by standard trauma protocols in situations involving high-energy trauma. (ATLS) guidelines recommend an anteroposterior (AP) radiograph of the pelvis in all polytrauma patients.

A pelvic ring fracture must be excluded after sufficient trauma or if symptoms seem to indicate a fracture. After a general radiologic assessment of a polytrauma patient is completed, a more specialized evaluation of the sacrum and pelvis can be performed. Whereas plain radiographs historically were performed, trauma centers are increasingly turning to the routine use of abdominopelvic computed tomography (CT) scans with reconstruction of the bony anatomy. The scans thus establish the diagnosis of pelvic ring disruption while also being able to assess for visceral or vascular injuries. The different radiologic techniques are described next.

Radiographs

Although careful inspection of the anteroposterior pelvis radiograph can detect the majority of sacral fractures, they are frequently missed. The sagittal 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 the identification of fractures.

Aside from the standard anteroposterior pelvis radiograph, several additional projections can yield important information. The inlet view allows for evaluation of the pelvic brim, the pubic rami, the sacroiliac joints, the sacral ala and the body of the sacrum. Displacement of the hemipelvis in the transverse (axial) plane can be identified on this view. The patient is positioned supine. The craniocaudal beam is directed at the level of the anterior superior iliac spines and the middle of the radiographic plate at an angle of approximately 40 degrees relative to the horizontal ( Fig. 134-1 ). The outlet view is orthogonal to the inlet view. The vertebral bodies of S1 and S2 can usually be clearly visualized. This view allows for evaluation of the symmetry of the sacroiliac joints and the pubic symphysis. Vertical displacement of the hemipelvis can be identified. Fractures extending into the obturator foramen can be detected more easily than on the AP view. The inferior aspect of the sacroiliac joint may not be visualized clearly because it is superimposed on the superior pubic rami. The patient is positioned supine, while the caudocranial beam is focused two to three fingerbreadths below the pubic symphysis with a 60-degree cephalad tilt ( Fig. 134-2 ).

The inlet view allows for evaluation of the pelvic.

The outlet view allows for evaluation of the pelvic.

Computed Tomography

CT has become the accepted gold standard for the evaluation of pelvic fractures and is considered to be mandatory for the evaluation of patients who have sustained high-energy injuries or in whom a posterior pelvic injury is suspected. The implementation of CT of the abdomen and pelvis in the routine 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 of the sacrum with 2-mm or less axial cuts and sagittal and coronal reformations. The dedicated CT provides the detail required for determining the fracture configuration, resulting instability pattern, and extent of sacral canal and neuroforaminal compromise. Scheyerer and colleagues recommended the use of single-photon emission computed tomography (SPECT)/CT on all patients with pubic rami fractures. They determined that all patients with public rami fractures in their series suffered additional lesions, none detectable previously by x-ray or CT, within the pelvic ring. In this context, SPECT/CT has proved to be very helpful for visualizing occult fractures and instability within the sacroiliac joint.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is not usually helpful in high-energy trauma except in cases of unclear neurologic deficits or discrepancies between skeletal and neurologic levels of injury. However, MRI can confirm early diagnosis of lumbosacral nerve root avulsion. According to many authors, magnetic resonance imaging (MRI) is the most sensitive screening methodology and it is considered the gold standard for the diagnosis of sacral stress fractures. MRI can detect a stress fracture surrounded by bone marrow edema by way of linear areas of low signal intensity on T ₁ weighted images. T ₂ -weighted images demonstrate a high-intensity signal region. T ₂ -weighted short tau inversion recovery (STIR) images are even more sensitive in showing the fracture line. Although rarely necessary, intravenous gadolinium may enhance MRI sensitivity. Coronal images are helpful in detecting a horizontal component of the sacral stress fracture.

Sacral Injury Classification

Lumbosacral fracture-dislocations and fractures at the lumbosacral junction with functional instability are most often classified in terms of sacral and pelvic ring fracture classifications. Informal terminology will frequently refer to sacral fractures in colloquial terms such as sacral U, T-type, lambda, or vertical shear fractures.

The American Orthopaedic Trauma Association (AO/OTA) 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. The Denis classification of sacral fractures correlates anatomic factors with neurologic injury risk in a progressive severity scale ( Fig. 134-3 ). It differentiates between alar fractures (zone I; 5.9% incidence of predominately 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 dysfunction). The L5 root, however, may be entrapped between the L5 transverse process and the displaced sacral ala, leading to a “traumatic far-out syndrome. However, this classification system fails to take lumbosacral stability into account. Isler recognized that vertical sacral fractures extend rostrally either lateral to, through, or medial to the S1 facet (Isler, 1990). Fractures that involve or extend medial to the L5/S1 facet result in lumbosacral instability.

Denis classification of sacral fractures.

U- and H-type fracture-dislocations at the lumbosacral junction are summarized in AO/OTA type 61-C3.3 pelvic fractures. According to the Denis classification, these fracture-dislocations are zone III injuries, as they involve the sacral canal. Unfortunately, these classifications do not take into account the mechanism of injury, nor the type, magnitude, or direction of displacement. Roy-Camille and associates have added a helpful subclassification system of Denis zone III injuries and lumbosacral fracture-dislocations, describing three types of transverse sacral fractures that are classified according to injury severity and presumed likelihood of neurologic injury. Type 1 injuries consist of a simple flexion deformity of the sacrum 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, consisting of a segmental comminuted S1 vertebral body caused by axial implosion of the lumbar spine into the upper sacrum. All of these injuries are caused by indirect forces to the lumbosacral junction. Unfortunately, the height of the transverse component of the sacral body fracture is not specified by this classification system. The added information of the sacral segment involved (“high” equaling S1-S2, “low” equaling S3-4 and coccyx) may add to the understanding of the type of neurologic injury commonly involved in these complex injuries.

Initial case reports often characterize the injury pattern as solely a transverse or vertical fracture, possibly owing to imaging limitations. The advent of CT reveals that most transverse fractures of the upper sacrum have complex fracture patterns. The majority of these injuries are now believed to consist of a transverse fracture of the sacrum with associated vertical injury components. The so-called “U” fracture usually consists of a transverse fracture with bilateral transforaminal fractures that extend rostral to the lumbosacral junction. Variations in fracture line propagation include the “H,” “Y,” and “lambda” fracture patterns. There is also a high rate of L5 transverse process fractures, indicating disruption of the iliolumbar ligament. The combination of multiple longitudinal and transverse fractures in the sacrum results in separation of the axial and appendicular skeleton, termed spinopelvic dissociation . These injuries may result in severe instability and cauda equina syndrome.

The high likelihood and variable grade of neurologic impairment in lumbosacral fractures is not accounted for in any of the above classification systems. In light of an absence of any spinal cord injury classification system (such as the one proposed by the ASIA group) addressing the sacral plexus injuries, Gibbons suggested a differentiation based on motor, sensory, and bowel/bladder control. He subdivided patients into those having (1) no injury, (2) lower extremity paresthesias only, (3) lower extremity motor deficit with intact bowel and bladder function, or (4) impaired bowel or bladder control. This classification system, though simple, unfortunately does not address incomplete injuries, nor does it take sexual function into consideration.

A revised AO sacral fracture classification has focused on categorizing sacral fractures based primarily on the extent and pattern of instability. Type A fractures are either inconsequential injuries or occur below the SI joint, and therefore they result in neither posterior pelvic nor spinopelvic instability. Type B fractures are generally vertical fracture patterns that result in posterior pelvic instability only. Type C injuries are complex sacral U fracture variants or bilateral vertical fractures that result in posterior pelvic and spinopelvic instability. Within each type there are three to four subtypes, categorized based on worsening potential prognosis or greater likelihood of operative intervention due to greater risk of neurologic deficit or instability.