(Radiographics. 2001;21:83-104.)
© RSNA, 2001
The Sacrum: Pathologic Spectrum, Multimodality Imaging, and Subspecialty Approach1
Jack Diel, MD,
Orlando Ortiz, MD,
Richard A. Losada, MD,
Donald B. Price, MD,
Michael W. Hayt, MD and
Douglas S. Katz, MD
1 From the Department of Radiology, Winthrop University Hospital, 259 First St, Mineola, NY 11501 (J.D., O.O., R.A.L., D.B.P., M.W.H., D.S.K.); the Department of Radiology, State University of New York School of Medicine, Stony Brook (J.D., O.O., R.A.L., D.B.P., M.W.H., D.S.K.); the Department of Radiology, University of Maryland School of Medicine, Baltimore (O.O.); and the Department of Radiology, Yale University School of Medicine, New Haven, Conn (M.W.H.). Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received March 7, 2000; revision requested April 5 and received May 31; accepted June 1. Address correspondence to D.S.K. (e-mail: dsk2928@pol.net).
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Abstract
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The sacrum is a structure that is imaged by both general and subspecialty radiologists. A wide variety of disease processes can involve the sacrum either focally or as part of a systemic process. Plain radiographs, although limited in evaluation of the sacrum, should be carefully examined when abnormalities of the sacrum are suspected. Cross-sectional imaging, particularly computed tomography and magnetic resonance (MR) imaging, plays a crucial role in identification, localization, and characterization of sacral lesions. Congenital lesions of the sacrum, including sacral agenesis and meningocele, are optimally imaged with MR. The most common sacral neoplasm is metastatic disease. Primary neoplasms of the sacrum, which include giant cell tumor, chordoma, and teratoma, are infrequent. Infection of the sacrum or sacroiliac joint is most often due to contiguous spread from a suppurative focus. A wide variety of arthritic disorders such as ankylosing spondylitis and osteoarthritis can involve the sacroiliac joints as part of a localized or systemic process. Sacral fractures related to acute trauma or repetitive stress are difficult to diagnose and treat. Knowledge of these abnormalities and familiarity with the imaging of these processes will allow radiologists of all subspecialties to contribute to the diagnosis and management of sacral disorders.
Index Terms: Sacrum, 33.92 • Sacrum, fractures, 33.41 • Sacrum, neoplasms, 33.30 • Spine, arthritis, 337.70 • Spine, developmental defect, 33.14 • Spine, infection, 33.20
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
- Discuss the development of the sacrum and its relationship to developmental lesions.
- Describe the anatomy of the sacrum.
- Recognize various pathologic conditions that may be encountered at evaluation of the sacrum with different imaging modalities.
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Introduction
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The sacrum, by virtue of its anatomic location, is a structure that presents itself to the attention of multiple medical specialists as well as imaging generalists and subspecialists. Neuroradiologists study the sacrum as a continuation of the spinal axis; musculoskeletal, trauma, and pediatric radiologists evaluate the sacrum as part of the skeletal system; and cross-sectional abdominal radiologists often assess the sacrum as a posterior border of the pelvis. The objective of this article is to discuss the embryology and anatomy of the sacrum and the common and unusual pathologic conditions of this structure, which include congenital lesions, neoplastic lesions, infectious lesions, noninfectious arthropathies, and traumatic lesions. Plain radiographs, although notoriously limited in the evaluation of this complex region, should always be carefully scrutinized by all radiologists to minimize diagnostic delays, even when sacral abnormalities are not specifically suspected clinically. Cross-sectional imaging with computed tomography (CT) and magnetic resonance (MR) as well as skeletal scintigraphy play a crucial role in identifying, localizing, and characterizing lesions of the sacrum, thereby contributing to the correct diagnosis and facilitating treatment planning.
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Embryology
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Development of the human spine commences during embryogenesis and lasts until the 3rd decade of life (1). Vertebral column development occurs in four overlapping stages: the mesenchymal stage, chondrification, primary ossification, and secondary ossification. The notochord forms on the 17th gestational day, during the mesenchymal stage. The major role of the notochord is to induce ectodermal and mesodermal differentiation. The former process results in formation of the neural plate, which folds to form the neural tube. The notochord not only determines the spinal axis but subsequently makes a major contribution to the formation of the nucleus pulposus of the intervertebral disks (1). Notochord cell remnants have been shown to persist within the cranial and caudal portions of the spinal axis, thus accounting for the high prevalence of chordomas in these locations (1). After development of the notochord and neural tube, the mesoderm lateral to these structures thickens to form the paraxial mesoderm. By the end of the 5th gestational week, due to the process of segmentation, 42–44 somites will have arisen from the paraxial mesoderm (2). Each somite consists of three cell elements: the sclerotome or future vertebra, the myotome, and the dermatome. The adjacent halves of two contiguous somites contribute to one vertebra during formation of the membranous vertebral column (2).
Chondrification is initiated in the 5th gestational week and results in a cartilaginous vertebral column. Primary or enchondral ossification occurs in three primary ossification centers (central, neural, and costal) and forms the axial skeleton. In the sacrum, the costal ossification centers form a portion of the lateral mass. A total of six centers produce the sacral alae (Fig 1) (1). Bilateral neural ossification centers contribute to the neural arch and the posterolateral vertebral body. The central ossification center forms the midportion of the vertebral body. With secondary ossification, two epiphyseal plates provide accessory ossification to the superior and inferior portions of each sacral vertebral body. Disks separate the sacral vertebrae during childhood (Fig 1b). The S3-4 and S4-5 disks fuse in late adolescence, and the remaining levels fuse during the 3rd decade of life.
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Figure 1a. Sacrum in a newborn. (a) Frontal radiograph shows two of the three ossification centers (black arrows) that form the sacral ala. The central ossification centers contribute to the vertebral bodies of the sacrum (S1-S5). The costal ossification centers (white arrows) form a portion of the lateral mass. (b) Lateral radiograph shows the intervertebral disk spaces (arrows) of the sacrum. The neural ossification centers are seen posteriorly (arrowheads).
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Figure 1b. Sacrum in a newborn. (a) Frontal radiograph shows two of the three ossification centers (black arrows) that form the sacral ala. The central ossification centers contribute to the vertebral bodies of the sacrum (S1-S5). The costal ossification centers (white arrows) form a portion of the lateral mass. (b) Lateral radiograph shows the intervertebral disk spaces (arrows) of the sacrum. The neural ossification centers are seen posteriorly (arrowheads).
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Anatomy
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Bones
The sacrum is a large, triangular bone located between the lumbar and coccygeal portions of the caudal spinal axis. It resembles an inverted triangle and has a concave inner or pelvic surface and a convex outer surface. The sacrum articulates superiorly with L5, inferiorly with the coccyx, and bilaterally with the iliac bones at the sacroiliac joints. At lateral inspection of the spine, the sacrum is noted to project posteriorly, forming the lumbosacral angle. As a result of this angle, this articulation is subject to shearing forces.
The sacrum consists of five vertebrae, which are fused both anteriorly and posteriorly. Posteriorly, the fused spinous processes form the median sacral crest, which extends caudally to the sacral hiatus. The sacral hiatus is a defect in the posterior wall of the sacrum at the S5 level (Fig 2). S1 is the largest of the sacral vertebral bodies; it is composed of a wide body that contains the greatest density of trabeculae (3). These trabeculae are arranged in a cruciate pattern. S1 possesses a prominent anterosuperior lip of bone referred to as the sacral promontory. Therefore, S1 is designed to provide support during axial loading.
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Figure 2. Sacrum in an adult. Frontal radiograph shows the lateral mass (LM), sacral ala (SA), sacral hiatus (SH), sacroiliac joint spaces (SI), and anterior neural foramina (1-4) with well-defined neural arches.
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The sacral canal is the caudal continuation of the lumbar spinal canal. The contents of the sacral canal include the meninges, the lower portion of the cauda equina (sacral and coccygeal nerve roots), the filum terminale, and fibrous and fatty tissue (Fig 3) (4). The epidural space terminates at the sacral hiatus. Four pairs of foramina with openings on the anterior and posterior surfaces of the sacrum transmit the ventral and dorsal rami of the sacral nerve roots (S1–S4), respectively (5).
The lateral masses are paired blocks of bone located lateral to the sacral foramina. The sacral ala is the wing-shaped bone found lateral to the S1 vertebral body. Histologic study of the sacral alae demonstrates decreased trabecular density ("alar voids") within each of these structures (3). This observation may explain the propensity for sacral stress fractures to occur in this location. In general, the sacrum demonstrates uniform cortical thickness.
Joints
The sacroiliac joints are located between the lateral aspects of the sacral alae and the iliac bones. The joints consist of ligamentous and synovial components (6). The ligamentous portion comprises the superior two-thirds to one-half of the joint and is formed by the interosseous sacroiliac ligament. The inferior one-half of the joint is a true synovial joint. The sacral surface of the synovial sacroiliac joint is lined by 3–5-mm-thick hyaline cartilage. The iliac side of the joint is lined by 1-mm-thick fibrocartilage. This differential lining allows sacroiliac joint disorders to involve the iliac bone first (6).
Ligaments
Three major ligamentous structures serve to stabilize the sacrum and are present on each side of the structure. These are the sacroiliac (ventral, dorsal, and interosseous), sacrotuberous, and sacrospinous ligaments. The interosseous ligament forms the ligamentous portion of the sacroiliac joint, and the ventral and dorsal sacroiliac ligaments surround the entire sacroiliac joint. The sacrotuberous ligament extends from the sacrum, posterior iliac spine, and coccyx to insert on the ischial tuberosity (4). The sacrospinous ligament extends from the lower sacrum and coccyx to insert on the ischial spine.
Nerves
The sacral plexus is formed by the ventral rami of L4 to S4 (Fig 4). It is composed of a larger superior band and a smaller inferior band. The superior band contains the lumbosacral trunk (L4 and L5) and S1. The inferior band contains S2, S3, and S4. The superior and inferior bands join to form the sciatic nerve. The lumbosacral trunk lies medial to the psoas muscle and sacroiliac joint and anterior to the sacral ala (7). The sacral plexus lies posterior to the iliac vessels and anterior to the piriformis muscle. These latter anatomic landmarks are readily identifiable on MR images, and the sacral plexus is well visualized on axial and coronal images (true coronal images or coronal images angled relative to the long sagittal axis of the sacrum) (7). The nerve roots of the sacral plexus join at the greater sciatic foramen to form the sciatic nerve.
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Figure 4. Sacral plexus. Anterior drawing shows the lumbosacral trunk (LST), pudendal nerve (P), and sciatic nerve (SN). The superior band is formed by the ventral rami of L4 and L5 (the lumbosacral trunk) and S1. The ventral rami of S2, S3, and S4 form a smaller inferior band, which joins with the superior band to form the sciatic nerve.
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Congenital Lesions
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Transitional Vertebra
Transitional vertebrae are frequently encountered developmental variants of the spine. They are found in approximately 20% of human skeletons and often involve the sacrococcygeal and lumbosacral junctions (8). The L5 vertebra can be incorporated into the sacrum (ie, "sacralized"), or the S1 vertebra can be incorporated into the lumbar spine (ie, "lumbarized"). The transitional vertebra retains partial features of the segments above and below it so that the total number of vertebrae in the spinal column remains relatively constant. Transitional vertebrae are usually incidental findings during radiologic evaluation but rarely, when an enlarged transverse process irritates adjacent tissues, can be symptomatic (8). However, the most clinically important aspect of transitional vertebrae is the potential for confusion over the labeling or assignment of vertebral levels during medical or surgical treatment planning. Thus, to avoid inappropriate treatment of the wrong vertebral or disk space level, complete imaging evaluation of the spinal axis with fluoroscopy, radiography, or MR is required.
Sacral Agenesis
Sacral agenesis (caudal regression syndrome) is a severe form of abnormal sacral development. This condition is rare, occurring in 0.005%–0.01% of the population. A higher frequency, in the range of 0.1%–0.2%, has been observed in children of diabetic mothers (Fig 5). Approximately 16%–20% of children with sacral agenesis have mothers with diabetes mellitus (8). There is also an increased frequency of spinal cord abnormalities in patients with sacral agenesis, including syrinx, tethered cord, lipoma, and lipomyelomeningocele (9). The spectrum of sacral agenesis can be categorized into four types (8). In type 1, partial unilateral agenesis is localized to the sacrum or coccyx (Fig 6). In type 2, there are partial but bilaterally symmetric defects in the sacrum. The iliac bones articulate with S1, and distal segments of the sacrum and coccyx fail to develop. In type 3, there is total sacral agenesis and the iliac bones articulate with the lowest available segment of the lumbar spine. In type 4, there is total sacral agenesis and the iliac bones are fused posteriorly along the midline. The MR imaging findings in total sacral agenesis include absence of the sacrum and coccyx and variable absence of a portion of the lumbar spine, with a distinct club-shaped configuration of the conus medullaris (Fig 5) (8).
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Figure 5. Sacral agenesis in a female infant with a diabetic mother. Midline sagittal T1-weighted MR image shows complete absence of the sacrum and a club-shaped conus medullaris (arrow). Extensive adipose tissue is seen in the gluteal region.
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Figure 6a. Type 1 sacral agenesis associated with a lipomyelomeningocele in a young adult. Axial CT scans of the pelvis show absence of the left side of the sacrum, as well as a fatty mass extending from the spinal canal (arrow) into the paraspinal soft tissues (arrowheads in a) via an incompletely formed neural arch.
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Figure 6b. Type 1 sacral agenesis associated with a lipomyelomeningocele in a young adult. Axial CT scans of the pelvis show absence of the left side of the sacrum, as well as a fatty mass extending from the spinal canal (arrow) into the paraspinal soft tissues (arrowheads in a) via an incompletely formed neural arch.
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Meningocele
Meningoceles are protrusions of the membrane-lined spinal canal contents through a ventral or dorsal osseous defect in the vertebral column (9). An anterior meningocele consists of herniation of a cerebrospinal fluid–containing sac through a sacral defect or foramen. Anterior meningoceles manifest as asymptomatic or symptomatic pelvic masses (Fig 7). They may be associated with partial sacral agenesis. Posterior meningoceles are more common than their anterior counterparts and may be classified as myeloceles, myelomeningoceles, or lipomyelomeningoceles according to the contents of the herniated sac (Fig 8). Posterior meningoceles are frequently associated with a tethered spinal cord. These developmental lesions of the sacrum are optimally imaged with MR, although plain radiography or CT is used to assess the associated osseous defects. Spinal dysraphism may necessitate examination of the entire spinal axis (9).
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Figure 7. Anterior sacral meningocele in a newborn. Sagittal T2-weighted MR image shows a large, hyperintense cystic mass (straight white arrow) anterior to a deficient sacrum, which is continuous with a somewhat expanded spinal canal (curved white arrows). Note the cerebrospinal fluid flow artifact (black arrow) within the connection between the distal spinal canal and the meningocele. (Courtesy of Mauricio Castillo, MD, University of North Carolina, Chapel Hill.)
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Figure 8. Lipomyelomeningocele in a newborn. Sagittal T1-weighted MR image shows a large, dorsal soft-tissue mass that consists of a peripheral fatty component and a cerebrospinal fluid-containing cyst (curved arrow), which is continuous with the lumbar subarachnoid space. Neural elements (arrowhead) are seen entering the cyst. Only a vestigial sacrum is present (open arrow).
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Meningeal Cyst
Sacral meningeal cysts are common and are often observed during cross-sectional imaging or myelographic evaluation of the lumbosacral spine. These developmental lesions are referred to by various terms in the medical literature, including perineural cyst, Tarlov cyst, sacral arachnoid cyst, and occult intrasacral meningocele. They were reported in 5% of 500 consecutive patients who underwent MR imaging of the lumbar spine (10). Sacral meningeal cysts are abnormal dilatations of the meninges within the sacral canal or foramina. These cysts can be categorized according to whether they communicate with the subarachnoid space. Those that communicate freely with the subarachnoid space are referred to as perineural or Tarlov cysts. The less common variant lacks free communication with the subarachnoid space and can be referred to as a sacral meningeal cyst. In either type of cyst, the pulsations of the cerebrospinal fluid or raised intraspinal pressure can erode and remodel the sacral canal or involved sacral foramen.
The enlarged sacral canal or foramen and the relationship to the sacral nerve roots is well demonstrated on MR images (Fig 9). However, CT does show the surrounding thinned cortical margins. Moreover, it is not uncommon to encounter these cysts during CT evaluation of the pelvis (Fig 10). Although the cysts are usually asymptomatic, large cysts can manifest as neurologic symptoms. Furthermore, symptomatic cysts tend not to communicate with the subarachnoid space in MR imaging studies of cerebrospinal fluid flow (11). Percutaneous treatment strategies for symptomatic sacral cysts include cyst aspiration and fibrin glue therapy (10,12).
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Figure 9a. Sacral meningeal cyst in a 51-year-old man with lower back pain. (a) Midline sagittal T1-weighted MR image shows an expansile cystic mass within the sacral canal. The lesion is slightly hyperintense relative to the cerebrospinal fluid in the lumbar spinal canal. It erodes the sacrum and encroaches on the presacral fat (arrow). (b) Coronal T2-weighted MR image shows a hyperintense sacral cyst devoid of neural contents. The sacral nerve roots are displaced laterally by the cyst (arrows).
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Figure 9b. Sacral meningeal cyst in a 51-year-old man with lower back pain. (a) Midline sagittal T1-weighted MR image shows an expansile cystic mass within the sacral canal. The lesion is slightly hyperintense relative to the cerebrospinal fluid in the lumbar spinal canal. It erodes the sacrum and encroaches on the presacral fat (arrow). (b) Coronal T2-weighted MR image shows a hyperintense sacral cyst devoid of neural contents. The sacral nerve roots are displaced laterally by the cyst (arrows).
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Figure 10. Incidentally detected sacral meningeal cyst in a 76-year-old woman. Contrast material-enhanced axial CT scan shows an expanded, remodeled anterior neural foramen (white arrows). The cortical margins are intact. The sacral canal appears slightly enlarged (black arrow).
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Neoplastic Lesions
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Because the sacrum has an architecture similar to that of the rest of the spine, mass lesions may involve primarily the bone structure or the sacral canal. A variety of benign and malignant neoplasms occur in the sacrum (Table). Metastatic lesions of the sacrum are far more common than primary malignancy (13,14).
The diagnosis of sacral tumors is often delayed. The sacral canal can accommodate large, slowly growing tumors that become symptomatic only when they become large enough to compress adjacent nerves or pelvic organs. Patients often present with nonspecific low back pain. Plain radiography is notoriously insensitive for detection of sacral lesions (15). CT, MR imaging, and scintigraphy are much more helpful (13,14). Specific imaging findings such as calcification in cases of chondrosarcoma, complex cysts in cases of teratoma, or fluid-fluid levels may help limit the differential diagnosis, but image-guided biopsy is often required to make a specific diagnosis (16–19).
Sacral Canal Tumors
A minority of sacral neoplasms develop in the sacral spinal canal. Schwannomas and neurofibromas that occur in this location arise from the lower lumbar and sacral dorsal sensory nerve roots. They are generally benign lesions but may attain gigantic proportions (Figs 11, 12) (20). Schwannomas are rare in the sacrum. They may grow along nerve segments and expand the sacral canal and neural foramina (20). It is often not possible to reliably distinguish between schwannomas and isolated neurofibromas within the sacrum solely on the basis of imaging criteria. However, the presence of multiple nerve sheath tumors suggests the diagnosis of neurofibromatosis. In the latter case, the potential exists for malignant transformation of a neurofibroma. Meningiomas can occur within the sacral canal or foramina but are also very rare (21).
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Figure 11a. Sacral nerve neurofibroma in a 68-year-old woman with low back pain. (a) Coronal T2-weighted MR image shows increased signal intensity within an expansile neural foraminal mass (arrows). (b) Axial T1-weighted MR image shows a hypointense mass (white arrows) just distal to the right sacral foramen (black arrow). There is slight erosion of the sacral vertebral body. (c) Gadolinium-enhanced fat-suppressed axial T1-weighted MR image shows heterogeneous enhancement within the mass (arrows).
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Figure 11b. Sacral nerve neurofibroma in a 68-year-old woman with low back pain. (a) Coronal T2-weighted MR image shows increased signal intensity within an expansile neural foraminal mass (arrows). (b) Axial T1-weighted MR image shows a hypointense mass (white arrows) just distal to the right sacral foramen (black arrow). There is slight erosion of the sacral vertebral body. (c) Gadolinium-enhanced fat-suppressed axial T1-weighted MR image shows heterogeneous enhancement within the mass (arrows).
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Figure 11c. Sacral nerve neurofibroma in a 68-year-old woman with low back pain. (a) Coronal T2-weighted MR image shows increased signal intensity within an expansile neural foraminal mass (arrows). (b) Axial T1-weighted MR image shows a hypointense mass (white arrows) just distal to the right sacral foramen (black arrow). There is slight erosion of the sacral vertebral body. (c) Gadolinium-enhanced fat-suppressed axial T1-weighted MR image shows heterogeneous enhancement within the mass (arrows).
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Figure 12a. Schwannoma in a 75-year-old woman with low back pain. (a) Nonenhanced axial CT scan shows a large soft-tissue mass (straight arrows) with a cystic or necrotic component (curved arrow). (b) Contrast-enhanced sagittal T1-weighted MR image shows moderate enhancement of the solid elements of the tumor (arrows).
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Figure 12b. Schwannoma in a 75-year-old woman with low back pain. (a) Nonenhanced axial CT scan shows a large soft-tissue mass (straight arrows) with a cystic or necrotic component (curved arrow). (b) Contrast-enhanced sagittal T1-weighted MR image shows moderate enhancement of the solid elements of the tumor (arrows).
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Ependymomas are malignant tumors that arise from ependymal cells within the central canal of the spinal cord or ependymal cells within the filum terminale. These tumors, which rarely involve the distal filum terminale, usually consist of the myxopapillary subtype and can manifest as a large, destructive sacral mass (22). Drop metastases from malignant primary central nervous system neoplasms such as primitive neuroectodermal tumors, germinomas, choroid plexus tumors, and glioblastoma multiforme can accumulate within the dependent portion of the sacral spinal canal. Carcinoid tumor involving the sacral spinal canal has been reported and may be metastatic or primary, possibly developing from congenital tailgut cysts in the latter case (23).
Benign Bone Lesions and Tumors
Only 7% of giant cell tumors involve the spine, but with respect to spinal involvement, the sacrum is the most common site. Giant cell tumor is the second most common primary sacral tumor after chordoma (13–15). Giant cell tumors are most common in the 2nd–4th decades of life. There is a female predominance with spinal giant cell tumors (15). Giant cell tumor is locally aggressive and rarely may metastasize. Approximately 5%–10% of giant cell tumors are malignant. The giant cells, which are multinucleated macrophages, are not specific to giant cell tumor, since they generally appear to be reactive cells that develop in response to foreign material. These neoplasms manifest as a lytic, expansile, and destructive process that is often eccentrically located (Fig 13). At CT, giant cell tumor manifests as a soft-tissue mass that may contain a thin sclerotic rim. These very vascular neoplasms show intermediate signal intensity on both T1- and T2-weighted MR images, demonstrate significant enhancement at CT and MR imaging, and may contain areas of hemorrhage or necrosis. Angiography allows confirmation of the prominent vascularity of these tumors and is often performed along with embolization prior to attempted surgical resection. Giant cell tumor generally has a subchondral location in both long and flat bones, which may result in transarticular spread (13,14,24). In the sacrum, giant cell tumor may show growth across the sacroiliac joint to the ilium (24).
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Figure 13a. Giant cell tumor in a 32-year-old woman with left leg pain. (a) Axial CT scan of the pelvis shows a lytic soft-tissue mass within the left side of the sacrum. The mass obliterates the left sacral foramen and extends across the left sacroiliac joint (arrow). (b) Sagittal T1-weighted MR image shows a hypointense sacral mass (straight arrows) that extends into the sacral canal (curved arrow). (c) Frontal scout radiograph from an angiographic study shows an expansile, lytic lesion within the left side of the sacrum (arrows). (d) Frontal projection from arterial-phase angiography performed with selective left iliac artery injection shows a very prominent tumor stain (arrows), which suggests the hypervascular properties of the lesion. The mass was embolized by means of an endovascular approach with polyvinyl alcohol microparticles. Tumor devascularization with endovascular embolotherapy reduces intraoperative blood loss and facilitates more complete and efficient surgical resection.
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Figure 13b. Giant cell tumor in a 32-year-old woman with left leg pain. (a) Axial CT scan of the pelvis shows a lytic soft-tissue mass within the left side of the sacrum. The mass obliterates the left sacral foramen and extends across the left sacroiliac joint (arrow). (b) Sagittal T1-weighted MR image shows a hypointense sacral mass (straight arrows) that extends into the sacral canal (curved arrow). (c) Frontal scout radiograph from an angiographic study shows an expansile, lytic lesion within the left side of the sacrum (arrows). (d) Frontal projection from arterial-phase angiography performed with selective left iliac artery injection shows a very prominent tumor stain (arrows), which suggests the hypervascular properties of the lesion. The mass was embolized by means of an endovascular approach with polyvinyl alcohol microparticles. Tumor devascularization with endovascular embolotherapy reduces intraoperative blood loss and facilitates more complete and efficient surgical resection.
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Figure 13c. Giant cell tumor in a 32-year-old woman with left leg pain. (a) Axial CT scan of the pelvis shows a lytic soft-tissue mass within the left side of the sacrum. The mass obliterates the left sacral foramen and extends across the left sacroiliac joint (arrow). (b) Sagittal T1-weighted MR image shows a hypointense sacral mass (straight arrows) that extends into the sacral canal (curved arrow). (c) Frontal scout radiograph from an angiographic study shows an expansile, lytic lesion within the left side of the sacrum (arrows). (d) Frontal projection from arterial-phase angiography performed with selective left iliac artery injection shows a very prominent tumor stain (arrows), which suggests the hypervascular properties of the lesion. The mass was embolized by means of an endovascular approach with polyvinyl alcohol microparticles. Tumor devascularization with endovascular embolotherapy reduces intraoperative blood loss and facilitates more complete and efficient surgical resection.
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Figure 13d. Giant cell tumor in a 32-year-old woman with left leg pain. (a) Axial CT scan of the pelvis shows a lytic soft-tissue mass within the left side of the sacrum. The mass obliterates the left sacral foramen and extends across the left sacroiliac joint (arrow). (b) Sagittal T1-weighted MR image shows a hypointense sacral mass (straight arrows) that extends into the sacral canal (curved arrow). (c) Frontal scout radiograph from an angiographic study shows an expansile, lytic lesion within the left side of the sacrum (arrows). (d) Frontal projection from arterial-phase angiography performed with selective left iliac artery injection shows a very prominent tumor stain (arrows), which suggests the hypervascular properties of the lesion. The mass was embolized by means of an endovascular approach with polyvinyl alcohol microparticles. Tumor devascularization with endovascular embolotherapy reduces intraoperative blood loss and facilitates more complete and efficient surgical resection.
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Aneurysmal bone cysts are expansile lesions with blood-filled cystic areas. Over 80% of aneurysmal bone cysts are detected by the age of 20 years, and there is a slightly higher prevalence in females (15). These lesions are not actual neoplasms but are probably reactive lesions. They may result from trauma or coexist with other bone lesions, both benign and malignant (13,14,25). They are relatively rare in the sacrum (25). The presence of fluid-fluid or hematocrit levels suggests the diagnosis but is not pathognomonic, since fluid-fluid levels have been reported in giant cell tumors as well as other lesions (16,26). Aneurysmal bone cyst is one of several hypervascular lesions that may cross disk spaces to involve adjacent vertebral levels, others being chordoma and giant cell tumor, thereby sometimes engendering confusion with infection or aggressive malignancies.
Despite being the most common primary tumor of the spine, cavernous hemangiomas rarely involve the sacrum. Although these lesions are typically small and asymptomatic, aggressive spinal hemangiomas have been recognized (27). The lesions are usually hypervascular and are associated with an epidural soft-tissue component that encroaches on the adjacent neural structures within the spinal canal or neural foramina. An unusual hemangioma at S2–S3 causing pain has been reported (28).
Osteoid osteoma and osteoblastoma are rarely found within the sacrum (14,29). Although 10% of osteoid osteomas involve the axial skeleton, only 2% of these tumors are found within the sacrum (15). These lesions usually occur in males in the 2nd decade of life. Patients may present with the classic clinical history of night pain, which is relieved by salicylates. Osteoid osteoma can be recognized at CT if it demonstrates a high-attenuation nidus surrounded by a lower-attenuation zone, which in turn may be surrounded by reactive sclerosis (30). The nidus reflects the fibrovascular component of this benign tumor. Osteoblastoma, another unusual benign bone tumor, is rarely found in the sacrum. Only one case in the sacrum was found in a series of 98 osteoblastomas (31), although 33% were found in the remainder of the vertebral column, evenly divided between the cervical, thoracic, and lumbar levels. Although specific subtypes of osteoblastoma share pathologic features with osteoid osteoma, osteoblastomas manifest as neurologic symptoms and have a different radiographic appearance. Osteoblastoma tends to be expansile and involve the posterior vertebral elements.
Malignant Bone Lesions and Tumors
The distribution of hematopoietic marrow plays an important role in the distribution of bone malignancy. The sacrum, as a site of hematopoietic or red marrow in the adult, is a common site for metastatic disease as well as hematologic malignancies including myeloma, Ewing sarcoma, and lymphoma (Fig 14). Metastases are the most common sacral neoplasm, with lung, breast (Fig 15), kidney, and prostate carcinoma the most frequent causes (13). Metastatic lesions are usually osteolytic, although sclerotic lesions can be observed, especially from prostate or breast carcinoma. Metastatic renal cell carcinoma is noted for its hypervascularity at angiography. Contiguous spread from advanced pelvic neoplasms to the sacrum is not uncommon and may be seen with rectal, uterine, prostate, and bladder carcinoma. Plasmacytoma and myeloma most commonly produce lytic, destructive lesions, often with an expansile component (32). The presence of multiple lesions involving the sacrum and the remainder of the spine suggests the diagnosis of metastatic disease or multiple myeloma.
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Figure 14a. Lymphoma in a 33-year-old man with bilateral leg pain. (a) Axial CT scan of the sacrum shows slight expansion of the sacral canal and foramina (curved arrows) by a soft-tissue mass. There is subtle extension of the lesion into the posterior paraspinal musculature (straight arrow). (b) Sagittal T1-weighted MR image shows an intermediate-intensity mass within the sacral canal (arrows).
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Figure 14b. Lymphoma in a 33-year-old man with bilateral leg pain. (a) Axial CT scan of the sacrum shows slight expansion of the sacral canal and foramina (curved arrows) by a soft-tissue mass. There is subtle extension of the lesion into the posterior paraspinal musculature (straight arrow). (b) Sagittal T1-weighted MR image shows an intermediate-intensity mass within the sacral canal (arrows).
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Figure 15. Metastatic breast cancer in a 77-year-old woman. Sagittal T1-weighted MR image of the lumbosacral spine shows multiple hypointense foci within the sacrum and lumbar vertebrae. These lesions remained hypointense with all of the MR imaging sequences and did not exhibit enhancement. Plain radiography revealed sclerotic metastases.
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Chordomas arise from notochordal rests and therefore almost always occur in a midline or paramedian location in relation to the spine. Chordoma is the most common primary malignant sacral tumor and accounts for 2%–4% of malignant osseous neoplasms (13–15). These tumors are found at all ages but most commonly occur in the 4th–7th decades of life. Fifty percent to 60% of chordomas develop in the sacrococcygeal region. A 2:1 male-to-female ratio is observed in cases of spinal chordoma. A chordoma manifests as a destructive, lytic lesion, commonly with internal calcifications, at both plain radiography and CT (Fig 16). Thus, when a large destructive sacral lesion is encountered in an adult, chordoma should be considered in addition to other primary tumors (sarcomas, giant cell tumor, and, rarely, ependymoma) or metastatic tumor. A large presacral soft-tissue component is usually present, as are soft-tissue components within the sacrum and sacral canal. These tumors are capable of extending across the adjacent disk space and the sacroiliac joint. Chordomas demonstrate low to intermediate signal intensity on T1-weighted MR images and prominent increased signal intensity on T2-weighted MR images (Fig 17). Enhancement of the soft-tissue components is variable, yet often moderate, on both CT and MR images. Chordomas demonstrate a prominent vascular stain at angiography. Although chordomas are relatively low-grade malignancies that metastasize infrequently, they are locally aggressive and develop in locations, such as the sacrum and clivus, that do not permit easy surgical cure (15).
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Figure 16. Sacral chordoma in an elderly man. Contrast-enhanced axial CT scan of the pelvis shows a destructive soft-tissue mass (white arrows) centered within the sacrum. The mass extends across the right sacroiliac joint (black arrow).
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Figure 17a. Sacral chordoma in a 71-year-old man with low back pain. (a) Sagittal T2-weighted MR image shows a heterogeneously hyperintense sacral mass with a presacral soft-tissue component (arrow). Note the signal drop-off within the caudal part of the lesion and the adjacent structures due to the location of the lesion at the inferior margin of the routine lumbosacral spine field of view. (b) Contrast-enhanced sagittal T1-weighted MR image shows moderate enhancement within the solid components of the mass (arrows). Repositioning the field of view to center the lesion corrected the signal drop-off problem.
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Figure 17b. Sacral chordoma in a 71-year-old man with low back pain. (a) Sagittal T2-weighted MR image shows a heterogeneously hyperintense sacral mass with a presacral soft-tissue component (arrow). Note the signal drop-off within the caudal part of the lesion and the adjacent structures due to the location of the lesion at the inferior margin of the routine lumbosacral spine field of view. (b) Contrast-enhanced sagittal T1-weighted MR image shows moderate enhancement within the solid components of the mass (arrows). Repositioning the field of view to center the lesion corrected the signal drop-off problem.
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The sacrococcygeal region is the most common location of teratomas discovered in infancy, with a prevalence of 1 in 35,000–40,000 births. Rarely, sacrococcygeal teratomas occur in adults (Fig 18) (17). Sacrococcygeal teratomas are composed of a variable mixture of solid and cystic components (17,18). Although the majority of teratomas in infancy and childhood are benign, there is a tendency toward malignant transformation as the child gets older. However, in adults benign tumors are more common (17). Another difference between the two age groups is that 90% of sacrococcygeal teratomas are externally visible in infants, but they are generally not externally apparent in adults (17).
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Figure 18a. Sacrococcygeal teratoma in a 65-year-old woman. (a) Frontal radiograph from a myelographic study shows a large soft-tissue mass within the pelvis (straight arrows). The mass causes partial destruction of the sacrum and attenuates the column of contrast material (curved arrow) within the lower lumbar spine. (b) Nonenhanced axial CT scan shows a large, septated cystic mass with small calcifications (solid arrows). A fluid-fluid level (open arrow) is seen within the dependent portion of one of the cysts. (c) Sagittal T2-weighted MR image shows the mass, septa, and fluid-fluid levels (arrows). The mass had a sacral component and a presacral component.
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Figure 18b. Sacrococcygeal teratoma in a 65-year-old woman. (a) Frontal radiograph from a myelographic study shows a large soft-tissue mass within the pelvis (straight arrows). The mass causes partial destruction of the sacrum and attenuates the column of contrast material (curved arrow) within the lower lumbar spine. (b) Nonenhanced axial CT scan shows a large, septated cystic mass with small calcifications (solid arrows). A fluid-fluid level (open arrow) is seen within the dependent portion of one of the cysts. (c) Sagittal T2-weighted MR image shows the mass, septa, and fluid-fluid levels (arrows). The mass had a sacral component and a presacral component.
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Figure 18c. Sacrococcygeal teratoma in a 65-year-old woman. (a) Frontal radiograph from a myelographic study shows a large soft-tissue mass within the pelvis (straight arrows). The mass causes partial destruction of the sacrum and attenuates the column of contrast material (curved arrow) within the lower lumbar spine. (b) Nonenhanced axial CT scan shows a large, septated cystic mass with small calcifications (solid arrows). A fluid-fluid level (open arrow) is seen within the dependent portion of one of the cysts. (c) Sagittal T2-weighted MR image shows the mass, septa, and fluid-fluid levels (arrows). The mass had a sacral component and a presacral component.
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Ewing sarcoma, an aggressive malignant round cell tumor of bone, manifests between the ages of 5 and 30 years in 90% of cases. Primary Ewing sarcoma of the spine is uncommon, but the sacrum is the most common site of origin in the spine. The tumor is usually lytic and destructive, often with an accompanying soft-tissue mass, but sclerotic Ewing sarcoma may occur (33). Ewing sarcoma may spread to adjacent bones; in the spine, it may spread across a disk to the adjacent vertebra, a feature that may mimic infection.
Primary tumors that uncommonly or rarely affect the sacrum include chondrosarcoma, fibrosarcoma, osteosarcoma, and primitive neuroectodermal tumor. For example, chondrosarcoma, which generally occurs in adults, may manifest as a lytic lesion with an associated soft-tissue mass and calcifications (Fig 19) (19).
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Figure 19. Sacral chondrosarcoma in a 72-year-old man with left lower leg pain and lower back pain. Nonenhanced axial CT scan shows a lytic lesion within the lateral mass of the left sacrum (arrows). The mass extends into the pelvis and contains calcifications.
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Infectious Lesions
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Infection of the sacrum or sacroiliac joint is most often due to contiguous spread from adjacent infection. Pelvic abscesses can disrupt the anterior articular capsule of the sacroiliac joint or the periosteum and cortex of the ilium or sacrum (34). In addition, infection may spread hematogenously via the Batson plexus from the bladder, intestines, or genitourinary tract (Fig 20) or from intravenous injections in drug abusers (34–36). Soft-tissue infections in the pelvis, as occur in decubitus ulcers, can also lead to sacral infection, as may traumatic injuries to the sacrum and osseous pelvis (34). Other causes of sacral or sacroiliac joint infection include iatrogenic injury from gluteal injections, sacral biopsy, sacroiliac joint injections (Fig 21), or diskography at the L5-S1 level (34).
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Figure 20. Tuberculous sacroiliitis in a 71-year-old woman. Frontal radiograph of the abdomen and pelvis shows erosion, expansion (arrowhead), and sclerosis (open arrow) of an infected left sacroiliac joint. A previously infected, small right kidney (solid arrows) with calcifications ("putty kidney") is also seen.
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Figure 21a. Sacroiliac joint infection in a 65-year-old man with right-sided low back pain and fever 3 weeks after a steroid injection into the right sacroiliac joint. (a) Axial T1-weighted MR image shows low signal intensity within the bone marrow adjacent to the right sacroiliac joint (short arrows). Intermediate signal intensity is present within the right sacroiliac joint (long arrow). (b) Axial inversion-recovery MR image shows high signal intensity on each side of the sacroiliac joint (short arrows), a finding consistent with marrow edema. Edematous changes are also seen within the right gluteal muscles (arrowheads). A fluid collection is seen beneath the right iliacus muscle (long arrow). (c) Contrast-enhanced fat-suppressed axial T1-weighted MR image shows enhancement within the right sacroiliac joint (arrow) and adjacent structures.
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Figure 21b. Sacroiliac joint infection in a 65-year-old man with right-sided low back pain and fever 3 weeks after a steroid injection into the right sacroiliac joint. (a) Axial T1-weighted MR image shows low signal intensity within the bone marrow adjacent to the right sacroiliac joint (short arrows). Intermediate signal intensity is present within the right sacroiliac joint (long arrow). (b) Axial inversion-recovery MR image shows high signal intensity on each side of the sacroiliac joint (short arrows), a finding consistent with marrow edema. Edematous changes are also seen within the right gluteal muscles (arrowheads). A fluid collection is seen beneath the right iliacus muscle (long arrow). (c) Contrast-enhanced fat-suppressed axial T1-weighted MR image shows enhancement within the right sacroiliac joint (arrow) and adjacent structures.
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Figure 21c. Sacroiliac joint infection in a 65-year-old man with right-sided low back pain and fever 3 weeks after a steroid injection into the right sacroiliac joint. (a) Axial T1-weighted MR image shows low signal intensity within the bone marrow adjacent to the right sacroiliac joint (short arrows). Intermediate signal intensity is present within the right sacroiliac joint (long arrow). (b) Axial inversion-recovery MR image shows high signal intensity on each side of the sacroiliac joint (short arrows), a finding consistent with marrow edema. Edematous changes are also seen within the right gluteal muscles (arrowheads). A fluid collection is seen beneath the right iliacus muscle (long arrow). (c) Contrast-enhanced fat-suppressed axial T1-weighted MR image shows enhancement within the right sacroiliac joint (arrow) and adjacent structures.
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At imaging, sacral osteomyelitis appears as a destructive lesion of the sacrum associated with soft-tissue swelling. Unlike neoplastic processes in this location, the lytic lesion associated with osteomyelitis is often ill defined and poorly marginated. Sclerotic foci or osseous sequestra may also be observed in chronic infection. Another imaging finding that can be used to distinguish infection from neoplasm is the presence of extensive soft-tissue inflammatory change, which is often seen with infectious processes. Finally, fluid collections within the presacral soft tissues, adjacent articulations, or sacral epidural space are suggestive of abscess formation. Involvement of the adjacent disk space or sacroiliac joint can be observed in both infectious and neoplastic processes but tends to occur with greater frequency in the former. Sacroiliac joint infection is usually unilateral. Plain radiography or CT shows extensive erosion of the affected sacroiliac joint, which is followed by repair and ankylosis (34). Radionuclide examinations including skeletal scintigraphy and indium-111–labeled white blood cell scanning can be used in the imaging work-up of a suspected sacral or sacroiliac joint infection. Image-guided biopsy is useful in identifying the causative organism in these infectious processes.
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Noninfectious Arthropathies
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A number of inflammatory conditions can involve the sacrum by affecting the sacroiliac joints as part of a systemic or local inflammatory process. Most are arthropathies and include ankylosing spondylitis, inflammatory bowel disease, psoriatic arthritis, Reiter syndrome, osteoarthritis, rheumatoid arthritis, and crystal deposition arthropathy (gout, calcium pyrophosphate deposition disease) (37). These conditions are often imaged with plain radiography and are sometimes incidentally observed during radiographic evaluation of the abdomen or lumbar spine. Additional radiographic projections including oblique and modified Ferguson (x-ray beam angled 23° cephalad) views are often required to show the sacroiliac joints (38). Sacroiliac joint abnormalities are also often initially seen during radionuclide studies. The presence or absence of symmetry, the status of the joint space, the presence or absence of periarticular erosions or subchondral sclerosis, and the presence or absence of joint ankylosis allow characterization of sacroiliac joint arthropathies (37,38). In all of the above entities, findings predominate on the iliac side of the joint due to its thinner and more permeable articular cartilage (37,38). Osteitis condensans ilii is included in this section because the imaging findings may simulate the aforementioned arthropathies. Paget disease is also included in this category due to the inflammatory component associated with the initial lytic phase.
Both ankylosing spondylitis and inflammatory bowel disease cause bilateral symmetric sacroiliac joint involvement (Fig 22). Initial erosive changes are followed by repair, which leads to subchondral sclerosis and subsequent ankylosis (38–40). Psoriatic arthritis and Reiter syndrome can result in bilateral symmetric, bilateral asymmetric, or unilateral sacroiliac joint involvement. However, bilateral asymmetric involvement is considered to be most characteristic (38). Findings include erosions with subsequent repair, resulting in subchondral sclerosis but rarely ankylosis. Osteoarthritis can cause bilateral symmetric, bilateral asymmetric, or unilateral sacroiliac joint involvement (38,40). Unilateral disease is commonly seen when there is ipsilateral paralysis or contralateral hip disease. Findings include absence of erosions, joint space narrowing, subchondral sclerosis, and osteophyte formation (Fig 23). Thus, of the noninfectious arthropathies, psoriatic arthritis, Reiter syndrome, and osteoarthritis can all manifest as unilateral sacroiliac joint involvement. In addition to the aforementioned entities, the radiographic differential diagnosis for unilateral sacroiliac joint involvement includes sacroiliac joint infection, trauma, and, less commonly, gout, pigmented villonodular synovitis, and osteitis condensans ilii.
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Figure 22. Ankylosing spondylitis in a 44-year-old man. Axial CT scan of the sacrum shows bilateral symmetric sacroiliac joint erosions (arrows). Moderate sclerosis predominates along the iliac side of the sacroiliac joints.
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Figure 23. Osteoarthritis in an 85-year-old woman with breast carcinoma and abnormal left sacroiliac joint uptake on a radionuclide bone scan. Nonenhanced axial CT scan of the sacrum shows a small sacral osteophyte (small arrow) and subchondral sclerosis of the sacral side of the sacroiliac joint (large arrows) due to osteoarthritis.
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Osteitis condensans ilii typically causes bilateral symmetric sacroiliac joint involvement (41). Findings consist of a triangular area of subchondral sclerosis on the anteroinferior aspect of the iliac side of the joint, unassociated with erosions or joint space narrowing (Fig 24). The cause is believed to be chronic stress on this region from instability of the pubic symphysis, and the condition is seen most commonly in young multiparous women (41).
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Figure 24. Osteitis condensans ilii in a 38-year-old woman. Frontal radiograph of the pelvis shows a triangular area of bone sclerosis involving the anterior and inferior aspects of the iliac side of both sacroiliac joints (arrows). The joints themselves appear otherwise normal.
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Paget disease may involve the sacrum, and when it does so it is commonly polyostotic, but the sacral involvement can be isolated (38). Patients with Paget disease have elevated levels of serum alkaline phosphatase. The radiographic findings in Paget disease reflect the stage (lytic, mixed lytic-sclerotic, sclerotic) of disease activity. In the early lytic or active phase, an osteolytic lesion is observed. In the sacrum, with respect to the sclerotic phase, cortical thickening most noticeably affects the sacral foramina and the sacral side of the sacroiliac joints (Fig 25). The cortical thickening results in osseous expansion of the sacrum with encroachment on the sacral canal and foramina. This thickening can be quite subtle during the earlier lytic phase, when intracortical tunneling visually undermines the actual thickening. Owing to the dynamic pathologic nature of this disease, the MR imaging findings can be quite variable. In the lytic phase, the underlying fibrovascular matrix is hypointense on T1-weighted images and hyperintense on T2-weighted images. In the sclerotic phase, areas of sclerosis are hypointense on both T1- and T2-weighted images (42). Making the diagnosis of Paget disease on the basis of MR imaging findings is difficult in the absence of plain radiographs or CT scans. On MR images, the lytic phase of Paget disease can resemble metastatic disease or osteomyelitis. The sclerotic phase of Paget disease can be simulated by sclerotic metastases from prostate or breast cancer, by the sclerosing forms of lymphoma or myeloma, or by chronic osteomyelitis. An interval change in sequential radiographic studies or the presence of a new soft-tissue mass indicates the likely possibility of malignant transformation into an osteosarcoma or, much less frequently, a chondrosarcoma or fibrosarcoma.
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Figure 25a. Paget disease of the sacrum in a 70-year-old woman. (a) Frontal radiograph of the pelvis shows thickened trabecular bone within the sacrum and thickened cortical margins of the neural foramina (arrows). (b) Axial CT scan of the sacrum shows thickened cortical margins (arrows) throughout the sacrum. The iliac bones are spared.
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Figure 25b. Paget disease of the sacrum in a 70-year-old woman. (a) Frontal radiograph of the pelvis shows thickened trabecular bone within the sacrum and thickened cortical margins of the neural foramina (arrows). (b) Axial CT scan of the sacrum shows thickened cortical margins (arrows) throughout the sacrum. The iliac bones are spared.
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Traumatic Lesions
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Sacral fractures are difficult to diagnose and treat and may be associated with significant acute and chronic morbidity. Plain radiographs of the sacrum are particularly difficult to interpret in the setting of trauma owing to overlying soft tissues. In addition, concurrent fractures of the pelvis, which are likely present, may distract the radiologist and the trauma physicians from recognizing sacral fractures and dislocations. Supplementary views, such as oblique radiographs, are useful in diagnosing sacral injuries, but CT is the examination of choice for imaging suspected sacral fractures and dislocations.
The most widely used system for classifying sacral fractures is based on a retrospective review of 236 sacral fractures and was described by Denis et al (43) in 1988. Zone 1 fractures are lateral to the sacral foramina (Fig 26) and are uncommonly associated with significant neurologic deficits. In a minority of cases, there may be injury to the L5 nerve root. Zone 2 fractures involve one or more of the foramina and may or may not extend to zone 1. Neurologic deficits are seen in the minority of cases and are usually unilateral lumbar or sacral radiculopathies. Zone 3 fractures involve the central sacral canal with or without involvement of the other two zones (Fig 27). Significant bilateral neurologic damage is frequently present, often with bowel or bladder incontinence (44). Transverse zone 3 fractures may be isolated injuries due to a severe direct blow to the sacrum. True lateral radiographs are very useful for diagnosing this specific fracture (43). Lumbosacral junction injuries, which occur when sacral fracture lines pass cranially through or medial to the S1 articular process, are rare (45). The sacral plexus can be injured as a result of sacral fractures. Also, as occurs with the brachial plexus, the sacral plexus may be subject to isolated "stretch" injuries (Fig 28).
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Figure 26a. Zone 1 sacral fractures in a 76-year-old woman after a fall. (a) Axial CT scan of the sacrum shows bilateral fractures (arrows) lateral to the sacral foramina. (b) Sagittal two-dimensional reconstruction image of the sacrum shows the fracture (arrow) extending into the body of S1 with anterior displacement (zone 3 component).
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Figure 26b. Zone 1 sacral fractures in a 76-year-old woman after a fall. (a) Axial CT scan of the sacrum shows bilateral fractures (arrows) lateral to the sacral foramina. (b) Sagittal two-dimensional reconstruction image of the sacrum shows the fracture (arrow) extending into the body of S1 with anterior displacement (zone 3 component).
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Figure 27. Zone 3 sacral fracture in a 17-year-old boy after a motor vehicle accident. Frontal radiograph of the pelvis shows a vertical fracture line through the center of the sacrum (arrows).
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Figure 28a. Sacral plexus injury in a 23-year-old woman with bilateral foot-drop 2 days after vaginal delivery of an 11-lb (5-kg) newborn. (a) Contrast-enhanced fat-suppressed axial T1-weighted MR image of the sacrum shows prominent enhancement within the sacral nerve roots (arrowheads). (b) Contrast-enhanced fat-suppressed coronal T1-weighted MR image of the sacrum shows enhancement of the sacral nerve roots (arrows).
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Figure 28b. Sacral plexus injury in a 23-year-old woman with bilateral foot-drop 2 days after vaginal delivery of an 11-lb (5-kg) newborn. (a) Contrast-enhanced fat-suppressed axial T1-weighted MR image of the sacrum shows prominent enhancement within the sacral nerve roots (arrowheads). (b) Contrast-enhanced fat-suppressed coronal T1-weighted MR image of the sacrum shows enhancement of the sacral nerve roots (arrows).
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The treatment of sacral fractures is controversial. Up to one-half of patients with sacral fractures or sacroiliac joint injury experience chronic disability. In a series of fractures reported by Templeman et al (46), the need to assess pelvic stability as well as associated nerve injury in triaging patients to conservative versus surgical therapy was stressed. The authors suggested that open reduction and fixation of displaced sacral fractures led to better results, especially if nerve injury was present.
Insufficiency Stress Fracture
Insufficiency fractures of the sacrum have been increasingly recognized in the past 2 decades as a relatively common cause of lower back pain in elderly patients, particularly osteopenic women (47–51). Insufficiency fractures may also occur as a result of irradiation of the pelvis (52). Signs and symptoms are often nonspecific, and the diagnosis is usually not initially considered. There may be a history of minor recent low-impact trauma. Pain, which worsens with weight bearing, occurs in the lower back and sacroiliac joints and may radiate to the buttocks, hips, and legs. The pain may be so severe that the patient requires hospitalization (47–49,51).
As in the setting of acute trauma, plain radiographs are very inaccurate for diagnosis of sacral insufficiency stress fractures, particularly because the bones are osteopenic (47,49,50). Often, the findings are completely occult or very subtle on plain radiographs and if present may be detected only retrospectively (49). Unilateral or bilateral sclerotic bands or fracture lines parallel to the sacroiliac joints may be identified in a minority of patients. Bone scans are extremely helpful and are often diagnostic, with the classic H-shaped area of increased uptake (ie, corresponding to the alae and body of the sacrum); this finding is best appreciated on posterior views (Fig 29) (49). "Incomplete" patterns are also quite common and should still be distinguished from metastatic disease, which is typically patchier in distribution (47,48,52). CT and MR imaging are usually reserved for atypical or problem cases (47). CT reveals a sclerotic band or discrete fracture lines, commonly with disruption of the anterior cortex of the sacral ala, and MR imaging reveals bone marrow edema with or without discrete fracture lines. CT and MR imaging also help exclude a destructive lesion. Concurrent fractures in the pelvis, especially in the symphysis pubis, are common (47,49,51). Most sacral insufficiency fractures improve or resolve over weeks to months after rest, pain medications, and physical therapy (50). In some patients, it may be difficult to distinguish radiation-induced sacral insufficiency fractures from residual or recurrent metastatic disease with any imaging modality. If other pelvic insufficiency fractures are not present and if there is a questionable sacral lesion, then the patient can undergo percutaneous biopsy or close clinical and imaging surveillance.
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Figure 29a. Sacral insufficiency fracture in a 76-year-old woman with lower back pain. (a) Posterior projection from the static phase of bone scanning shows an H-shaped pattern of increased uptake within the sacrum (arrows). (b) Axial CT scan of the sacrum shows fractures through the lateral masses (arrowheads). (c) Sagittal T1-weighted MR image of the sacrum shows decreased signal intensity at the fracture site (arrow). (d) Sagittal T2-weighted MR image shows the fracture line (arrow) surrounded by increased signal intensity, which reflects marrow edema.
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Figure 29b. Sacral insufficiency fracture in a 76-year-old woman with lower back pain. (a) Posterior projection from the static phase of bone scanning shows an H-shaped pattern of increased uptake within the sacrum (arrows). (b) Axial CT scan of the sacrum shows fractures through the lateral masses (arrowheads). (c) Sagittal T1-weighted MR image of the sacrum shows decreased signal intensity at the fracture site (arrow). (d) Sagittal T2-weighted MR image shows the fracture line (arrow) surrounded by increased signal intensity, which reflects marrow edema.
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Figure 29c. Sacral insufficiency fracture in a 76-year-old woman with lower back pain. (a) Posterior projection from the static phase of bone scanning shows an H-shaped pattern of increased uptake within the sacrum (arrows). (b) Axial CT scan of the sacrum shows fractures through the lateral masses (arrowheads). (c) Sagittal T1-weighted MR image of the sacrum shows decreased signal intensity at the fracture site (arrow). (d) Sagittal T2-weighted MR image shows the fracture line (arrow) surrounded by increased signal intensity, which reflects marrow edema.
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Figure 29d. Sacral insufficiency fracture in a 76-year-old woman with lower back pain. (a) Posterior projection from the static phase of bone scanning shows an H-shaped pattern of increased uptake within the sacrum (arrows). (b) Axial CT scan of the sacrum shows fractures through the lateral masses (arrowheads). (c) Sagittal T1-weighted MR image of the sacrum shows decreased signal intensity at the fracture site (arrow). (d) Sagittal T2-weighted MR image shows the fracture line (arrow) surrounded by increased signal intensity, which reflects marrow edema.
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Fatigue Stress Fracture
Sacral fatigue stress fractures are very uncommon, representing less than 2% of all stress fractures in one series of 320 athletes (53,54). Most often, they have been reported in long-distance runners or military recruits. In female long-distance runners, the osteopenia due to the amenorrhea commonly seen in these women undoubtedly contributes to these fractures (Fig 30) (54,55). Approximately 25 cases have been reported in the literature (54,55). However, sacral fatigue stress fractures may be underreported in the literature. This underreporting is likely in part due to the initial physical examination findings, which are often not localizing to the sacrum (54,56).
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Figure 30a. Sacral fatigue fracture in a female member of a collegiate cross-country team. (a) Axial T1-weighted MR image of the sacrum shows decreased signal intensity within the right lateral mass (arrow). (b) Contrast-enhanced fat-suppressed axial T1-weighted MR image of the sacrum shows focal enhancement at the fracture site (arrow).
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Figure 30b. Sacral fatigue fracture in a female member of a collegiate cross-country team. (a) Axial T1-weighted MR image of the sacrum shows decreased signal intensity within the right lateral mass (arrow). (b) Contrast-enhanced fat-suppressed axial T1-weighted MR image of the sacrum shows focal enhancement at the fracture site (arrow).
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Fatigue stress fractures are usually unilateral. It has been postulated that this unilaterality is due to leg length discrepancy, with the longer side usually being affected. As with any other sacral condition, plain radiographs may not be revealing but should initially be obtained. Bone scans are almost always positive in cases of sacral fatigue stress fractures. MR imaging is diagnostic, revealing decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images with or without a discrete fracture line. The former findings are due to the presence of edema within the bone marrow (54,55).
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Conclusions
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The sacrum, often ignored as an important site of spinal pathologic conditions, is optimally imaged with cross-sectional techniques. The development of the sacrum, or failure thereof, reflects subsequent imaging manifestations of congenital and neoplastic sacral lesions. In terms of anatomy, sacral lesions are related to the tissues that make up and surround the sacrum (ie, bone, cartilage at the sacroiliac joint, meninges, and neural elements). Unusual entities that have a predisposition for affecting the sacrum include sacral agenesis; primary tumors, particularly teratoma, chordoma, giant cell tumor, and chondrosarcoma; sacroiliitis; and stress fractures. Perineural cysts, metastases, and fractures of the sacrum are relatively common lesions. Familiarity with all of the entities that may affect the sacrum will enhance the ability of subspecialty radiologists and general radiologists not only to arrive at a specific diagnosis or appropriate differential diagnosis but also to contribute to the management of these conditions.
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