Spine #7 - Answers

1. Posterior ligamentous disruption, L4-5.

Select the image to see a more detailed view.

The inferior facet of L4 is "perched" atop the superior facet of L5. Notice how all of the other facets are located normally with respect to each other. A grade I/IV spondylolisthesis of L4 on L5 is also noted. Minimal anterior wedging of T12 and L1 is also noted, which could possibly represent mild anterior compression fractures of unknown age.

The eagle-eyed observer noted the relative position of the facets at the L4-5 facet joint. Unlike the normal joints above, these two facets do not articulate properly. Instead, the inferior facet of L4 is "perched" on top of the superior facet of L5. What does this signify?

There is a handy rule that I find very useful when dealing with subluxations and dislocations: don't perseverate on the malalignment itself -- concentrate instead on the structures that normally keep things together. Soooo.... what normal structures have to be zapped in order for L4 and L5 to assume this perched position? Well, if you move from posterior to anterior, we see that the supraspinous ligament, the interspinous ligaments, the ligamenta flava, the capsular ligaments, the posterior longitudinal ligament, and possibly the posterior annulus fibrosus have to be torn to allow this new positional relationship between L4 and L5. This injury is one of a spectrum called seat belt injuries, which are produced when a hyperflexion force is applied to a subject wearing a seat belt but no shoulder belt. Usual hyperflexion injuries in unrestrained passengers flex the spine around a fulcrum through the anterior column of the spine, and typically result in an anterior compression fracture of the body. However, the presence of a seatbelt moves the fulcrum forward to the anterior abdominal wall. As far as the spine is concerned, this converts the hyperflexion force into a distraction force. This makes all the difference in the world to the spine. Whereas a typical hyperflexion force crushes the anterior vertebral bodies to produce a compression fracture, a distraction force essentially pulls the spine apart. The injuries thus produced fall into these categories:

• Posterior ligament disruption
• This may occur as a purely ligamentous injury, or may be associated with avulsion fractures of parts of the posterior vertebral body or of the articular processes. These injuries tend to occur more commonly in younger patients with more resilient spines. A rotational force is often part of the mechanism of injury. These injuries may be clinically unstable, and may require internal fixation.
• Posterior distraction fracture of the vertebral arch
1. Chance fracture
• The fracture line extends through the spinous process, pedicles, transverse processes, and up through the posterior superior corner of the vertebral body in the classic fracture described by Chance in 1948.
2. horizontal fissure fracture (sometimes mistaken for Chance fracture)
• The fracture line extends horizontally through the vertebral body as well. Older patients with more brittle bones are more likely to get this fracture type. The posterior ligamentous complex remains essentially intact, other than the supraspinous ligament. This has also been termed a "fulcrum fracture".
3. Smith fracture
• The superior articular processes and a small posterior fragment of the vertebral body are included with the arch fracture. The spinous process is intact, but the interspinous and supraspinous ligaments are torn.

This type of injury typically occurs in a passenger of an automobile following a head-on collision. These fractures may be difficult to appreciate on axial CT images, as they are usually oriented parallel to the scanning plane. Reformatted images in other planes may help. Plain radiographic clues to these injuries include:

1. a "vacant" or "empty" appearance of the vertebral body on the AP film, due to angulation of the fracture so that the posterior elements no longer lie over the vertebral body.
2. a break or discontinuity in the cortex of the pedicles or spinous process on the AP view.
3. fracture or dislocation seen on the lateral view -- as shown in the case presented above, this may be subtle.

Neurological injury to the spine occurs in about 15 % of seat belt injuries. This is in contrast to the much higher prevalence noted in spinal fracture-dislocations in general.

Other injuries

The abdomen and its contents are interposed between the spine and the fulcrum of injury, and may be injured in up to 15 percent of these spinal injuries. These injuries include ruptures or tears of the duodenum, distal small bowel, mesentery, stomach, colon, spleen, pancreas, aorta, the gravid uterus, and the musculature of the anterior abdominal wall (bruising or ecchymosis here should suggest the possibility of a seat belt injury to the wary clinician). In some cases, these soft tissue injures be more dangerous to the patient in the short term than the spinal fracture-dislocation itself.

References:

• Gehweiler JA, Osborne RL Jr, Becker RF. The radiology of vertebral trauma. Philadelphia, WB Saunders, 1980.
• Rogers LF. Radiology of skeletal trauma. 2nd ed. New York, Churchill Livingstone, 1992.
• Rogers LF. The roentgenographic appearance of transverse or Chance fractures of the spine: the seat belt fracture. AJR 1971;111:844-849.
• Chance GQ. Note on a type of flexion fracture of the spine. Br J Radiol 1948;21:452.
• Smith WS, Kaufer H. Patterns and mechanisms of lumbar injuries associated with lap seat belts. J Bone Joint Surg 1969;51A:239-254.

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