Imaging The Sacrum and Coccyx

Imaging of the sacrum and coccyx is a common procedure in radiology. This article discusses various protocols for plain-film and CT imaging.

Author: Joseph, Nicholas
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Imaging The Sacrum and Coccyx

Written by Nicholas Joseph Jr. RT(R)(CT) ARRT, B.S., MHA


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Anatomy of the Sacrum/coccyx

Pelvic Instability

Imaging Low Back Pain

Plain Film Imaging of the Sacrum and Coccyx

Critique Sacrum/coccyx Radiographs

The Sacroiliac Joints

CT of the Sacrum and SI Joints

Pathology of the Sacrum/Coccyx

Summary Points


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The sacrum is imaged in general radiology as well as in subspecialties such as computed tomography (CT), magnetic resonance (MR), nuclear medicine (NM), positron emission tomography (PET) and other modalities. Evaluating the sacrum is a component of pelvic imaging; however, specific imaging of the sacrum may not include the entire pelvis. Some common reasons to image the sacrum and sacroiliac (SI) joints include trauma, joint pain, and sacroiliitis. Other conditions such as gout, bone tumors, and Bechter’s syndrome are also common reasons for selective imaging of the sacrum or sacroiliac joints. Finding the best imaging modality when the clinical history is "low back or sacral pain" can be challenging since sacroiliac joint pain affects approximately 15 to 25% of patients presenting with low back pain. The source of pain that radiates to the lower back and SI joints can be challenging to diagnose for clinicians. Although physical examination and radiological studies help uncover the elusive etiology of SI joint pain, the most common method for diagnosing chronic low back pain originating from the SI joint is one's response to a small-volume local anesthetic joint block (Stevenson, 2015). In other words, when anesthetic SI joint block successfully alleviates low back pain, it also proves diagnostic for the source of low back pain. Radiologic studies do confirm conditions like sacroiliitis and other inflammatory conditions. Correct diagnostic imaging and diagnosis are essential to early detection of ankylosing spondylitis. Some sacral lesions are difficult to visualize with plain film imaging and are best demonstrated using a cross-sectional modality such as CT or MRI. Imaging modalities that offer thin axial slices through the sacrum and ability to reformat image data into coronal and sagittal images provides the best diagnostic information. Magnetic resonance imaging is very useful for diagnosing congenital disorders like meningocele, sacral agenesis, and other congenital lesions. Selective imaging protocols display optimal sacral anatomy and pathology. The radiographer’s skills should include anatomy, pathology, and a basic understanding of mechanisms causing most sacral injuries. The goals of this self-learning article are to discuss these points and specific radiographic imaging protocol objectives when imaging the sacrum and SI joints.

Anatomy of the sacrum

Anatomically, the sacrum is part of the vertebral column. It articulates superiorly with the fifth lumbar vertebra at the lumbosacral junction. The sacrum and coccyx are the distal portions of the spinal column and are also structurally part of the pelvis. The sacrum and coccyx are classified as irregular bones and are also atypical vertebrae. A typical vertebra is a single bone having distinct parts like the spinous process, transverse processes, vertebral body and foramen, pedicles and lamina. The sacrum is a composite bone formed by the fusion of its five sacral vertebrae into a single bone at about age 20 years. This fusion presents distinct anterior and posterior sacral landscapes. On the whole, these fused sacral segments can still be identified on the adult sacrum allowing for accurate localization numbering. The main functions of the sacrum are to strengthen the pelvis, which stabilizes it, and transmitting the weight of the body to the pelvic girdle through the sacroiliac joints (SI joints). To perform these functions the sacrum forms five main articulations: 1-2) two with the inferior articular processes of the fifth lumbar vertebra, 3-4) bilaterally with the ilium to form right and left sacroiliac joints, and 5) inferiorly with the coccyx.

The anterior or pelvic surface of the sacrum is concave, which is a shape that adds depth to the pelvic cavity. This surface forms the posterior ring of the pelvis. Four pairs of foramina are on the anterior surface. They are the remnants of what would be intervertebral foramen of a typical vertebra seen in other parts of the spine. Structures seen on the anterior surface include prominent sacral wings (called the ala), superior articular processes, sacral promontory, transverse ridges (lines), vertebral bodies, and anterior (pelvic) sacral foramina. Sacral vertebrae are numbered from top to bottom so that the superior vertebra is the first sacral vertebra. It articulates with the 5th lumbar vertebra. Distally, the 5th sacral vertebra articulates with the 1st coccygeal segment. The 1st sacral vertebra articulates with the 5th lumbar vertebra through their articular processes forming bilateral zygoapophyseal joints at the lumbosacral junction. Thus, the lumbosacral junction occurs at the L5/S1 articulations.

These two 3D CT images show the relationship of the sacrum to the pelvis. The sacrum is a shovel-shaped structure with the apex pointing downward. Its anterior portion forms the posterior ring of the pelvis and plays a major role in its stability. The sacrum provides stability to the pelvis by forming articulations with the 5th lumbar vertebra above, coccyx below, and sacroiliac joints laterally. There are various ligament attachments of the sacrum to the hip bones to stabilize the pelvis further.

The anterior surface of the sacrum portion of the pelvic surface is concave, which is a shape that adds depth to the pelvic cavity. Four pairs of foramina are seen on the anterior sacral surface, which are the remnants of the intervertebral foramen viewed on other vertebrae of the spine. Structures seen on the anterior surface include the ala (wing), superior articular processes, sacral promontory, transverse ridges (lines), vertebral bodies, and anterior (pelvic) sacral foramina. The superior vertebra is the first sacral vertebra, which articulates with the 5th lumbar vertebra; the fifth sacral vertebra is most distal and articulates with the 1st coccygeal segment. The 1st sacral vertebra articulates with the 5th lumbar vertebra through their inferior and superior articular processes forming bilateral zygoapophyseal joints at the lumbosacral junction. Thus, the lumbosacral junction occurs at the L5/S1 articulations.

The body’s weight transmits from the trunk and upper extremities to the vertebral column and to the first sacral vertebra, which is large to receive these vectors. It bears the body's weight on its large vertebral body supported by strong pillars called wings or ala. Sacral vertebrae become progressively smaller distally fusing into a single triangular bone in the adult. The smaller fifth sacral vertebra articulates with the first coccygeal vertebra and may also fuse with it. Four transverse lines representing the developmental separations of the five sacral vertebrae which are visible on the anterior surface. The body of an individual sacral vertebra is that portion between two transverse lines. Transverse lines give superior and inferior boundaries to the sacral body, which standardized the accurate counting of sacral vertebra. Four pairs of anterior sacral foramina (aka pelvic sacral foramina) align along the lateral anterior walls. Anterior sacral foramina are somewhat rounded modified remnants of intervertebral foramina; they project forward and laterally. Anterior sacral foramen serve as conduits through which anterior divisions of sacral nerves and blood vessels pass in route to structures in the pelvis and lower extremities. Sacral foramina have communications with the sacral canal, which is continuous with the longitudinal vertebral canal that runs the length of the spine. However, the vertebral canal does not extend into the coccyx.

The posterior surface of the sacrum is convex presenting a prominent bony midline ridge called the median sacral crest. It is a remnant of fused spinous processes of vertebrae 1 through 3. Only sacral vertebrae 1-3 have spinous processes, sacral vertebrae 4 and five do not. Instead, they form a U-shaped structure called the sacral hiatus from which two small bony projections called the sacral horns (cornua) protrude. Anesthetic agents are sometimes administered into the back through the sacral hiatus to relieve pain (called an epidural anesthetic). Two large wedged shaped surfaces called auricular surfaces (shaped like the auricle of the ear), form the lateral borders of the sacrum. Auricular borders of the sacrum articulate with corresponding auricular surfaces on the ilium to form the two sacroiliac joints (SI). Each SI joints open posteriorly and obliquely at an angle of about 30-degrees. Also presented on the posterior lateral surface are four corresponding pairs of irregularly shaped sacral foramina. Like the anterior sacral foramen, they transmit the posterior sacral nerves and blood vessels to the sacrum and pelvis.

These two 3D surface rendered CT images show the major features of the anterior sacrum. The large prominent wing or ala of the first sacral vertebra (A), anterior sacral foramina (SF), numbered are the individual partially fused sacral vertebra from proximal to distal (1-5), and the transverse lines (SL) that indicate the body of the individual vertebra.
These two 3D surface rendered CT images show the distinguishing features of the posterior sacrum: superior articular process (A) that articulate with the inferior articular processes of the 5th lumbar vertebra. Also demonstrated is the auricular surface (B), posterior sacral foramen (C), sacral cornua(D), sacral hiatus (E), and median sacral crest (F).
These two radiographs show the complex articulation of the sacrum with the ilium forming the paired sacroiliac joints. On the left is a 3D surface rendered CT image of the pelvis that is 30 degrees oblique to show an opened sacroiliac joint (A). A coronal 2D CT image on the right shows symmetrical sacroiliac joints; the right one is labeled (B). Sacroiliac joints stabilize the pelvis by the sacrum's articulation with the ilium along with a complex arrangement of fortifying supporting ligaments.

When imaging the sacrum it is important to remember that the spinal cord is not the same length as the vertebral column, for instance, an infant’s spinal cord terminates at L2 or L3. However, because of linear growth, the adult spinal cord terminates at or near L1/L2 junction. Since the spinal cord does not extend the entire length of the spinal column, there is room below L2 for insertion of a spinal needle into the subarachnoid space (lumbar puncture). Needle puncture into the subarachnoid space in the lumbar region of the spinal cord to withdraw cerebrospinal fluid (CFS) or inject anesthetic drug is called a spinal tap, or lumbar puncture procedure. The risk of injuring the spinal cord itself from an approach below L2 is minimal. Also, the spinal cord is anatomically and functionally segmented. For example, the area of the spinal cord corresponding to L4 is not at the level of the fourth lumbar vertebra because the cord ends at L1/L2. Nerves for the lower lumbar segments, sacrum, and coccyx arise from their shorter cord segment to descend within the vertebral canal. They form a bundle of nerve roots in the distal subarachnoid space caudal to the termination of the spinal cord. Nerve roots extend distally from the cord at about L2 to form a tail-like collection called the caudal equina (L. horse’s tail). At termination, the spinal cord tapers to a cone shaped structure called the conus medullaris. Extending from the conus is a thin fibrous thread of meninges (mostly dura mater), the filum terminale, which anchors the cord to the dorsum of the coccyx.

These two CT myelogram images demonstrate the subarachnoid space within the vertebral canal. The coronal image on the left shows contrast material in the space that appears non-homogenous because of nerve roots that form the caudal equine. The sagittal image on the right demonstrates the relationship of the vertebral canal to the spinal cord. The spinal cord ends at about L1/L2 in the adult. Spinal fluid fills the distal portion of the sac.
These CT axial images are of the lumbar spine taken following a myelogram. The subarachnoid space and nerve roots within the sac are visible in both radiographs. The image on the right shows the exit site of the intervertebral foramen (white arrow) where spinal nerves exit the vertebral column. The distal nerve roots form the caudal equina (yellow arrow) as they descend to their respective vertebral level to exit intervertebral foramina.

Spinal nerves exit the vertebral canal through intervertebral and sacral foramina to innervate peripheral structures in the body. Spinal nerves are mixed nerves carrying both sensory and motor stimuli to and from the brain. Information from the body to the brain is sensory because these stimuli convey information about the environment. Examples of sensory information include sight, touch, proprioception (ability to know what your body is doing without looking at it, for, example, not looking at your hands when driving), two-point discrimination, pressure such as gripping the steering wheel, hearing, taste, balance, and others. Motor responses are those nerve impulses from the brain that coordinated muscle movements, like steering a car, accelerating or decelerating a vehicle, swinging a golf club, dancing movements, pulling one's hand back from a hot stove, and the like. There are five pairs of sacral nerves and one coccygeal nerve that travel down the vertebral canal in the sacral canal to exit sacral foramina. Injury to sacral foramen can occur from trauma or degeneration, or be congenitally malformed. Therefore, the sacral canal and sacral foramina must be demonstrated on sectional images of the sacrum. It is especially important to do so when imaging the pelvis following high impact trauma with suspected pelvic instability.

These coronal CT images demonstrate the sacral canal and sacral foramina (yellow asterisks) through which the sacral nerves exit. The sacral foramina form by fusion of the sacral vertebra. Typically vertebrae do not fuse but do form intervertebral foramen between adjacent vertebrae. The intervertebral foramen is where spinal nerves exit the vertebral canal. The yellow arrow indicates the intervertebral foramen of L5/S1 on the coronal image on the left. Notice the shape of sacral foramina formed by sacral vertebrae that have fused (right CT image).

The coccyx, also called the tailbone marks the most distal portion of the vertebral column. It is highly regressed in humans and has no resemblance of a vertebra at all. It too is a composite bone formed by 3 to 5 fused coccygeal segments. The lateral sacrum/coccyx view demonstrates the dominant curve of the sacrum and forward projection of the coccyx. These curves are important when imaging the sacrum or coccyx because they determine how much to angle the central ray for a true anterior to posterior (AP) projection of these structures. The frontal plain film radiograph of the sacrum requires a cephalic tube angle, whereas the coccyx is best demonstrated using a caudal angle. One of the more understood roles of the coccyx is that it provides an attachment for the meninges; particularly the dura mater is anchored to it distally. By attaching to the coccyx, the meningeal sac suspends the spinal cord reducing unrestricted movement within the vertebral canal.

This sagittal CT view of the sacrum (left) shows its major structures. The sacral promontory labeled (A), and the concave curvature of the sacrum (B). The forward curvature of the coccyx (C ), the superior articular processes of the first sacral segment labeled D on the right CT image, and the sacral canal, which is continuous with the vertebral canal of the spine (yellow arrows).
This sagittal view of the sacrum/coccyx shows the distal spinal column. Shown is the 5th lumbar vertebra superiorly through the last coccygeal segment. The vertebrae of the sacrum are numbered beginning with L5 articulating with the first sacral vertebra. Sacral vertebrae are numbered from superior to inferior as are the coccygeal vertebrae. Numbering the coccygeal segments are from proximal to distal too, i.e. (Cx 1 through Cx4/Cx5).

Pelvic Stability

A system of tightly woven muscles and ligaments provide support to the inherent anatomical weakness of the bony pelvis, stabilizing it. Strong ligaments arranged transversely, oblique, and horizontally resist forces that can externally rotate the pelvis, thereby opening it. Among these are the short posterior SI ligament, the anterior SI ligament, the iliolumbar ligament, sacrospinous ligaments and others. Their function is to counter opposing forces such as AP compression. These ligaments fail when forces exceed their ability causing compression type injury. Vertical stability is provided primarily by the short and long posterior SI ligaments. Other interosseous ligaments within the sacroiliac joints also provide additional vertical stability.

These stabilizing ligaments help perform one of the main functions of the pelvis, to transmit weight from the trunk and lumbar vertebrae to the lower extremity. Weight bearing forces from the body are transmitted primarily along vectors to the posterior pelvis, then to the sacrum and sacroiliac joints. During weight-bearing forces are conveyed to the acetabula for distribution to the femurs. During weight bearing movements of the body, the anterior pelvic arch functions like a strut maintaining the shape of the pelvic ring. During passive weight bearing such as sitting the vector is transmitted downward to the ischial tuberosities.

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These three 3D volume rendered CT images depict some basic patterns of those dense, broad ligaments (blue lines) that support the bony pelvis. The CT image on the left demonstrate the arrangement of ligaments from the AP perspective; the middle CT image illustrates how these ligaments attach and support the posterior pelvis. The far right CT image demonstrates how these ligaments provide support laterally to stabilize the sacrum/coccyx and close the true pelvis inferiorly. Many ligaments and corresponding muscles support and provide short stabilizing struts for the pelvis. The greater sciatic notch forms an opening bridged by ligaments that form a soft tissue entrance into and out of the pelvis. The lesser sciatic notch provides an opening to the perineum. While it is not our purpose to name all of these ligaments it is important to understand the general distribution of ligaments and their role in stabilizing the pelvis. For example, the sacrotuberous ligament is a long band of fibrous tissue that extends from the posterior superior and posterior inferior iliac spines and the back and sides of the sacrum/coccyx down to the ischial tuberosities. Their role is to resist posterior rotation of the lower sacrum. We can see some of the muscles and their tendons and ligament that stabilize the pelvis on MR images below.
Some ligaments that stabilize the pelvis
  • Short posterior S.I. ligaments
  • Long posterior S.I ligaments
  • Anterior S.I. ligaments
  • Iliolumbar ligaments
  • Sacrospinous ligaments
  • Interosseous ligaments
These three coronal MR images show a portion of the extensive network of ligaments and muscles that stabilize the bony pelvis. They also can cause the pelvis to shift due to unopposed pull when high impact forces separate bone or joints causing pelvic opening. Pelvic compression can also separate bones and often results in rupture of the main blood vessels causing profound life-threatening intrapelvic hemorrhaging.
The schematic lines on the 3D CT image on the left (blue) represent some of the ligaments supporting and closing the true pelvis. Many ligaments of the pelvis are named according to their origins and insertions. Blue lines show some of the well-known ligaments that stabilize the pelvis. From superior to inferior is the posterior SI ligaments, sacrospinous ligaments, sacrotuberous ligaments, and sacrococcygeal ligaments. These ligaments also provide a soft tissue passageway for blood vessels and nerves to enter and exit the true pelvis and perineum. The greater sciatic notch (A) guards the entrance into and out of the true pelvis. The lesser sciatic notch (B) provides an opening into the perineum. The coronal MR image on the right shows some of the muscle and ligaments supporting the pelvis.

Accompanying pelvic fractures structural ligaments can be disrupted. Separation of the sacroiliac joints and severe fractures of the sacrum can cause instability of the pelvis. Crush injuries and rotational injuries that “open” the pelvis can have life-threatening consequences, in particular, uncontrolled hemorrhaging and shock. Of particular radiographic interest is the iliospinous ligaments that attach to the transverse processes of the fifth lumbar vertebra and the ilium. An avulsion type fracture of the fifth lumbar transverse process may be a single indicator of pelvic instability when seen on plain film x-ray. For this reason, the AP pelvis view should include the entire fifth lumbar vertebra and iliac crests.

These two CT images show a fracture of the transverse process of the fifth lumbar vertebra (arrows). Often because of fecal material in the colon and a filled bladder it is not easy to see the posterior elements of the pelvis on plain film. It is particularly the case of portable made radiographs of the pelvis with a history of trauma. A subtle fracture of the transverse process of the fifth lumbar may be the only indicator of pelvic injury and instability. Theses CT images show the fracture of the fifth lumbar vertebra and profound fractures of the ilium and sacrum. Less obvious fractures can be obscure but indicated by a transverse process fracture. The CT image demonstrates the posterior aspect of the sacrum and L5, which is an area that 's hard to show with the plain film AP x-ray view of the pelvis.

  • The Tile classification system provides a descriptive appraisal of fractures affecting pelvic stability. The Tile classification system can also be used to describe pelvic fractures that do not appreciably disrupt the pelvic ring (Tile type A).