Thoracic spine injuries may be caused by hyperflexion, hyperextension, rotation, lateral tilt, shearing, or any combination of these mechanisms of injury.  Because the inherent thoracic kyphosis mutates axial loading force into a hyperflexion injury, bursting fractures do not occur in the thoracic spine.  However, the thoracolumbar spine (T10-L2), being transitional between the thoracic and lumbar spines, is capable of voluntary straightening.  Consequently, bursting fractures do occur at this level.

All thoracic dislocations (fig. T04, T03) and fracture dislocations are considered mechanically unstable.  Neurologic instability is dependent on the presence of neurologic findings.

On the lateral projection, the upper 3-4 thoracic segments are typically not visible because of the superimposed density of the shoulders.  The thoracolumbar junction is typically obscured by the density of superimposed subdiaphragmatic structures with thoracic spine technique.  Therefore, a collimated (“coned down”) lateral radiograph, centered on T12 and using lumbar spine technique, is commonly required for optimum visualization of the segments.

The swimmer’s view provides only limited visualization of the upper thoracic segments, particularly the posterior elements, because of superimposition of ribs.  Oblique views of the thoracic spine have been replaced by CT.

If the upper thoracic or thoracolumbar vertebrae are not adequately visualized by conventional radiography, and a clinical or radiologic reason exists for further imaging, CT is indicated.

Even displaced thoracic fractures or fracture dislocations (fig. T06, T05) may be subtle on both AP and lateral radiographs.  On the AP radiograph a paraspinal hematoma, which may be unilateral (fig. T06) or bilateral (fig. T02) and is typically focal, may be the most obvious or only sign of injury.

THORACIC ANATOMY

AP:  The AP radiograph (fig. T07) shows the superior and inferior endplates and lateral cortex of the vertebral bodies, intervertebral disk spaces, pedicles which should be equidistant from the midline and aligned on the same vertical plane, transverse processes, spinous processes which normally are aligned in the midsagittal plane and vertically equidistant, and the costovertebral joints.  The left para-spinal (mediastinal) stripe represents the paraspinal fat-mediastinal pleural interface. It is normally visible in 96% of patients [37] and extends from the level of T10 to the aortic arch at which point it disappears due to the impression of the aortic arch upon the mediastinal pleural surface of the left lung.  The left paraspinal stripe lies parallel to the vertebral bodies medial to the inferiorly oblique course of the descending thoracic aorta.  The paraspinal stripe has a white Mach edge while that of the descending thoracic aorta is dark.

Lateral:  The lateral projection (fig. T01) is characterized by a gentle, continuous kyphotic curve of the vertebra.  The upper 3-4 vertebrae are completely obscured by the shoulder density.  The lower 2-3 segments are partially obscured by the density of the upper abdominal, subdiaphragmatic soft tissues.  The obliquely downward oriented ribs are superimposed on the vertebral bodies and their interspaces.

Because the lumbar spine can be voluntarily straightened and is not encumbered by costovertebral joints, any of the mechanisms of injury capable of causing acute injury in the lower cervical spine can, and do, cause similar injuries in the lumbar spine.  A fracture peculiar to the thoracolumbar spine (T10-L2) is the Chance fracture [38] which is unique in its site of occurrence in the spinal column, its traumatic pathology, and most important clinically, by its 15-20% incidence of intra-abdominal injury to primarily, the pancreas, duodenum and proximal small bowel.  The intra-abdominal injuries are either clinically silent or masked by the fracture.  Consequently, it is incumbent on the radiologist to urge abdominal CT with oral and IV contrast medium to assess for these injuries in patients with radiographic signs of a Chance fracture.

Transverse process fractures of L2-3, when the result of major trauma, must raise the question of renal vessel injury because of their proximity to the renal pedicles.

Unilateral and bilateral interfacetal dislocation may rarely occur in the lumbar spine.  Minor anomalies & variants common to the thoracolumbar spine should, by virtue of their location in the vertebra and their parallel sclerotic margins, not be misinterpreted as acute fractures.  The limbus vertebra (fig. L17), resulting from extrusion of nucleus pulposis material into the physis between the ununited ring apophysis and the vertebral body preventing apophysis-body union, is easily identified by its characteristic location and sclerotic, parallel margins.  Schmorl’s nodules (fig. L01), caused by herniation of nucleus pulposis material through clefts in the fibrocartilagenous endplates into the vertebral body, are characterized by a hemispherical lucent defect with thick, irregular sclerotic margins.  The nodules typically occur in the mid third of the body but are also commonly located anteriorly.   Regardless of location and size, the characteristics of the nodule are the same on both conventional radiography and CT.

LUMBAR ANATOMY

AP (fig. L08):  The AP radiograph of the lumbar spine shows the lumbar vertebral bodies, their superior and inferior endplates and lateral cortical margins.  The pedicles are equidistant from the spinous process.  Other posterior elements include the transverse processes, the superior and inferior articular processes and facets and interfacetal joints, the pars interarticularis, and the interlaminar space.

Lateral (fig.L09):  The lateral radiograph of the lumbar spine shows the lumbar vertebrae in typical lordotic curve, pedicle, superior and inferior articular processes, interfacetal joints, the pars interarticularis, and disk spaces including the lumbosacral space which is typically narrower posteriorly than anteriorly.

Oblique (fig. L06):  The oblique projection is obtained with the patient’s entire body rotated into 45 degrees of obliquity to the central x-ray beam.  With the patient in the right anterior oblique position, the posterior elements of the left side are seen en face and include the vertebral body and the elements of the “Scottie dog” – i.e., the transverse process (nose), pedicle (eye), superior articulating process (ear), pars interarticularis (neck) and inferior articular process (front leg) and the apophyseal joint space.  Spondylolysis, congenital absence of the pars (fig. L07), is best seen in the oblique projections.

CT:  The CT anatomy of the lumbar spine is identical to that described and illustrated in the cervical spine, with the only difference being larger posterior elements and the absence of the foramen transversarium.  Axial CT display of lumbar anatomy is shown in fig. L02 and parasagittal CT anatomy on fig. L03.

 

LUMBAR SPINE INJURIES

Transverse Process Fracture may occur following relatively minor blunt trauma, are frequently subtle, may be single or multiple, and if not specifically looked for, are easily missed.

Simple wedge (compression) fractures are the most common lumbar spine injuries after transverse process fractures.
MOI:   Hyperflexion
Pathology:  distraction of the posterior and middle and compression of the anterior columns.
Radiographic Signs
     AP:  buckling, discontinuity or normal appearance of the superior end-plate depending on the degree of impaction.  Buckling or discontinuity of the lateral cortex of the vertebral body (fig. L04).
     Lateral:  In minimally impacted simple wedge fractures the interfacetal joints may not be radiographically subluxated (fig. L05).  In all cases, the superior cortex is disrupted, and wedged anteriorly.  A transverse band of increased density representing impacted trabeculae is commonly present below the superior end-plate.  The anterosuperior cortex of the vertebral body is disrupted, angulated, impacted or shows a “step-off.”
Additional Imaging
     CT:  Not necessary, but commonly obtained.  Axial images (fig. L02) show disruption of the anterior margin of the superior end-plate and subcortical trabecular fractures and impaction.  Axial images through the lower half of the body are normal.  The interfacetal joints are normal.  Midsagittal reformation (fig. L03) shows disruption and impaction of the superior end-plate, impaction of the anterosuperior aspect of the anterior cortex and normal interfacetal joints at the level of injury.
     MRI:  not necessary.
Stability:  mechanically and neurologically stable.

NB:  More forceful hyperflexion results in subluxation of the corresponding interfacetal joints (fig. L15) in addition to the anterior column changes previously described.  Recognition of interfacetal joint subluxation is critical because it is an important factor in patient management decisions.

Burst Fracture:  The burst fracture can occur in any of the thoracolumbar or lumbar vertebrae, including L5.
MOI:  Axial load while the thoracolumbar or lumbar spine is straight.
Pathology:  Identical to that described for burst (bursting, dispersion) fracture of the lower c. spine.
Radiographic Signs
     AP:  Disruption, comminution and impaction / concavity of each endplate; fracture of one or each lateral cortex of the vertebral body; vertical fracture line through vertebral body; widened interpedicular distance (fig. L11).
     Lateral:  Concave deformity, with fracture lines, of each endplate; displacement of body fragments anteriorly and posteriorly; diffuse loss of vertical height (fig. L12).
Additional Imaging
     CT:  Necessary axial images show comminution of vertebral body and endplates with displacement of fragments anteriorly and varying degrees of retropulsion into the spinal canal; fracture of either a lamina or the spinous process (fig. L10).
            Sagittal Reformation:  Images show endplate fractures and loss of body height; anterior displacement of body fragments; retropulsion of fragments into the spinal canal (fig. L14).
     MRI:  Required to assess status of disk, spinal cord, and the presence of other intra-spinous traumatic pathology.

 

SOFT-TISSUE INJURIES

Both unilateral and bilateral interfacetal dislocation occur rarely in the lumbar spine.  The MOI of each is identical to that described for the same injuries in the cervical spine.  BID of L2 or L3 is often referred to as a “soft-tissue” Chance which carries the same incidence of intra-abdominal injury as the Chance fracture.

 

SACRUM

Most sacral fractures are a component of pelvic ring disruption, which will be discussed in the Pelvis chapter.

Isolated Sacral Fractures
MOI:  Fall, landing in sitting position.
Pathology:  Typically a transverse fracture of the sacral segments caudal to the sacro-iliac joints.
Radiographic Signs
     AP, including angled views:  Usually of little help.  Transverse fracture line may be visible.
     Lateral:  Transverse fracture best seen through the relatively smooth anterior cortex.  Commonly associated with a pre-sacral hematoma soft-tissue mass.
Additional imaging
     CT:  Only for confirmation; best seen on sagittal reformations.
     MRI:  Not indicated.