DOI:10.2214/AJR.05.0145
AJR 2006; 187:859-868
© American Roentgen Ray Society
Chance-Type Fractures of the Thoracolumbar Spine: Imaging Analysis in 53 Patients
Mark P. Bernstein1,2,
Stuart E. Mirvis1 and
K. Shanmuganathan1
1 Department of Radiology, University of Maryland School of Medicine, Maryland
Shock Trauma Center, Baltimore, MD 21201.
2 Present address: NYU Medical Center/Bellevue Hospital, 560 First Ave., New
York, NY 10016.
Received January 28, 2005;
accepted after revision August 19, 2005.
Address correspondence to M. P. Bernstein
(mark.bernstein{at}med.nyu.edu).
CME
This article is available for 1 CME credit. See
www.arrs.org
for more information.
Abstract
OBJECTIVE. Chance-type fractures are subtle unstable injuries that
are often associated with intraabdominal injuries. CT-based observations made
during routine interpretations revealed involvement of a burst component to
this fracture pattern and a clue on the transaxial images to its presence. The
purpose of this review was to determine how often these features occurred in a
retrospective study of a large sample because these findings influence
diagnosis and management.
MATERIALS AND METHODS. A retrospective review of all patients
identified from the University of Maryland Shock Trauma Center trauma registry
and IDXRad system diagnosed with flexion-distraction injuries of the
thoracolumbar spine over an 8-year period was performed. Three trauma
radiologists assessed the admission spinal radiographs, CT studies with
multiplanar images, and available MRI examinations. Imaging findings were
confirmed by consensus. Abdominopelvic CT studies and surgical reports were
reviewed for evidence and type of intraabdominal injury. A literature review
of previous similar series was performed.
RESULTS. Fifty-three patients were identified for inclusion in the
study. Associated intraabdominal injury occurred in 40% and most commonly
involved the bowel and mesentery. A close examination of the fracture patterns
on CT revealed that a burst-type fracture with posterior cortex buckling or
retropulsion was a common finding (48%). Also, serial transaxial CT images
often (76%) showed a gradual loss of definition of the pedicles that we refer
to as the "dissolving pedicle" sign. The study showed that the
horizontally oriented fracture planes through the posterior elements can often
be recognized radiographically, but these fractures can be very subtle.
CONCLUSION. Intraabdominal injuries occurred in 40% of
flexion-distraction thoracolumbar fractures in our study cohort, which is
slightly lower than previously reported. About half of the patients with this
injury displayed a burst-type component that could have a significant
influence on surgical management. The dissolving pedicle sign can assist in
recognition of this often subtle injury on transaxial CT.
Keywords: fracture • spine • trauma
Introduction
The flexion-distraction fracture of the spine was first described in 1948
by G. Q. Chance [1]. This
pattern of injury had not been recognized before the use the lap seat belt.
Since the late 1960s, with the common placement and use of seat belts in
automobiles across North America, case reports describing these transverse
fractures began to emerge sporadically in the medical literature. Soon, small
cohorts of cases were analyzed, and associations between lap belt use and
spinal injuries were made. At the same time, and often in independent studies,
the relationship between lap belt use and intraabdominal injuries was
recognized.
Approximately 160,000 cases of thoracolumbar spine fractures occur annually
in the United States. Most victims are young males involved in motor vehicle
collisions. Other mechanisms include falls, sporting events, and assaults.
Thoracolumbar spine fractures often result in long-term disability.
Although thoracolumbar spine fractures are common in major blunt trauma,
they are often missed or diagnosed in a delayed fashion, thereby potentially
increasing morbidity. Flexion-distraction, or Chance-type, fractures are often
subtle on both radiography and CT and are unstable injuries that usually do
not present with a neurologic deficit. In addition, these fractures are
frequently associated with significant intraabdominal injuries. For this
retrospective review, we analyzed imaging findings in 53 patients with
flexion-distraction fractures of the thoracolumbar spine to determine the
spectrum of radiologic findings and diagnostic findings that may be
subtle.
Materials and Methods
A retrospective review of all patients from the University of Maryland
Shock Trauma Center trauma registry and IDXRad system (GE Healthcare)
diagnosed with flexion-distraction injuries or Chance fractures of the
thoracolumbar spine over an 8-year period was performed after institutional
review board approval was obtained. Three trauma radiologists assessed
admission spinal radiographs, CT studies with multiplanar images, and
available MRI examinations using anonymous hard-copy films in a blinded
fashion. Patients without adequate or available radiographs were excluded from
the study. Patient demographic information, including age, sex, injury level
or levels, injury pattern, and radiographic and CT signs were evaluated.
Anteroposterior (AP) and cross-table lateral radiographs were independently
assessed, as were transaxial and sagittal CT images, with a prepared
checklist. Imaging findings were confirmed by consensus.
Abdominopelvic CT studies and surgical reports were reviewed for evidence
and type of abdominal injury. Patient treatment and follow-up were evaluated
by chart review. A literature review of previous similar series was
performed.
Fifty-three patients with a Chance-type fracture of the thoracolumbar spine
were identified using radiographs and CT examinations. All 53 patients had
concurrent CT evaluation of the abdomen and pelvis. CT examinations were
performed on an MX8000 4-MDCT scanner (Philips Medical Systems) from April
2001 to April 2003 or a single-detector Somatom Plus 4 scanner (Siemens
Medical Solutions) from May 1995 to April 2001. Spinal protocols on 4-MDCT
were unenhanced 1.3-mm images with 50% overlap and 3-mm sagittal and coronal
reformations. On the single-detector helical scanner, acquisition consisted of
3-mm-thick images with 50% overlap and 3-mm sagittal and coronal reformations.
Abdominopelvic CT examinations were performed on the scanners mentioned with
the administration of oral and IV contrast material. Images were acquired at
5-mm intervals and reviewed on hard copy by consensus of the authors.
MRI examinations were performed in eight patients on either a 1.5-T Picker
Eclipse scanner (Marconi Medical Systems) from 1997 to 2005 or a 1.5-T
EchoSpeed scanner (GE Healthcare) from 1995 to 2004. Acquisition sequences
were as follows for the lumbar spine using the Picker Eclipse scanner:
sagittal turbo spin-echo T1, proton density, and T2; and axial turbo spin-echo
T1 and turbo fast spin-echo T2. In addition, the acquisition sequences for the
thoracic spine were as follows: T1 and proton density fast spin echo, axial
T2* with magnetization transfer, and axial T2 fast spin echo. On
the EchoSpeed scanner, the MRI sequences for the lumbar and thoracic spine
included sagittal spin-echo T1, proton-density, and T2; and axial T1 and
gradient-echo. Also a sagittal inversion recovery sequence was performed for
the thoracic spine.
Results
Demographics
Fifty-three patients were identified with 55 separate Chance-type fractures
of the thoracolumbar spine. Thirty-eight males (72%) and 15 females (28%) were
evaluated. The mean age for the study group was 26 years, ranging from 9 to 54
years. The mechanism of injury was motor vehicle crash in 38 (72%); fall in
nine (17%); struck by a falling object in two (4%); and assault, sporting
event injury, and bicycle crash in one each (2%). The Glasgow coma score, a
measure of best motor, verbal, and eye-opening responses on a scale from 3 to
15, in our series ranged from 10 to 15, with 88% of patients presenting with a
normal Glasgow coma score of 15; the Glasgow coma score for two patients was
not available. The injury severity score (ISS), an anatomic severity scale to
evaluate multitrauma patients and predict probability of survival, ranged from
2 to 43, with a mean of 19; the ISS for seven patients was not available. Two
patients were paraplegic, one secondary to a noncontiguous burst fracture. A
significant diagnostic delay was identified in only one case with 3 weeks
between admission and surgical fixation.
Fractures
Radiography—All patients had AP and lateral radiographs of
the thoracolumbar spine. The distribution of the Chance-type fractures is
outlined in Table 1 and ranged
from T4 to L5. Among the injuries, 78% occurred at the thoracolumbar junction,
between T12 and L2. Two patients had two-level Chance-type fractures, one
contiguous (T11, T12) and the other noncontiguous (T12, L4). On AP spinal
radiographs, increased interspinous distance was seen in all patients, which
is indicative of the flexion-distraction mechanism of injury. The resulting
relative radiolucency over the involved vertebral body is referred to as the
"empty vertebral body" sign (Figs.
1A and
1B). A transverse fracture
through the pedicles was seen in 66%, and increased intercostal spacing was
noted in 50% of the thoracic spine injuries. Other radiographic signs
supporting the diagnosis of flexion-distraction injury included horizontally
oriented fractures across the transverse processes, laminae, and articular
processes. Lateral radiographs showed fanning or distraction of the spinous
processes, indicative of a hyperflexion mechanism, in 80% of patients and
pedicle radiolucency in 73% (Fig.
2).
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Fig. 1A —Chance fracture. Anteroposterior lumbar spine radiograph
obtained after laparotomy in 28-year-old woman and corresponding line diagram
show horizontal fracture line through L2 vertebral body across both pedicles
and transverse processes. "Empty body" sign is present secondary
to displaced L1 and L2 spinous processes.
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Fig. 1B —Chance fracture. Anteroposterior lumbar spine radiograph
obtained after laparotomy in 28-year-old woman and corresponding line diagram
show horizontal fracture line through L2 vertebral body across both pedicles
and transverse processes. "Empty body" sign is present secondary
to displaced L1 and L2 spinous processes.
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Fig. 2 —Chance fracture in 28-year-old woman. Cross-table lateral
radiograph of lumbar spine shows fanning of spinous processes
(double-headed arrow) and fracture extending through pedicle (between
arrowheads) and into L2 vertebral body (single-headed
arrow).
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CT—Transaxial CT images of the thoracolumbar spine revealed
uncovering of articular facets secondary to the vertical distraction of the
posterior elements, previously described as the naked-facet sign
[2] in 40% of patients (Figs.
3A and
3B). In evaluating the
horizontally oriented Chance fracture, serial transaxial CT images revealed a
gradual loss of definition of the pedicles, a sign that we refer to as the
"dissolving pedicle" sign (Figs.
4A,
4B,
4C, and
4D). This sign was the most
common finding on transaxial CT images, identified in 76% of patients.
Whenever transaxial images displayed the "dissolving pedicle,"
reformations in the sagittal plane always confirmed a Chance-type fracture
pattern. Chance fracture imaging findings on CT and conventional radiography
are summarized in Table 2.
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Fig. 3A —Chance fracture in 18-year-old man. Transaxial CT image of L2
vertebral body shows superior articular processes of L2 are seen without their
normal articulation with L1 inferior articular processes (arrows).
Note left transverse process fracture (arrowhead). Findings represent
naked-facet sign and signify distraction of posterior elements.
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Fig. 4A —"Dissolving pedicle" sign in 28-year-old woman.
Three serial transaxial CT images of lumbar spine reveal progressive loss of
definition of L2 left pedicle (arrow, B and C). Note
right transverse process fracture (arrowhead, B).
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Fig. 4B —"Dissolving pedicle" sign in 28-year-old woman.
Three serial transaxial CT images of lumbar spine reveal progressive loss of
definition of L2 left pedicle (arrow, B and C). Note
right transverse process fracture (arrowhead, B).
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Fig. 4C —"Dissolving pedicle" sign in 28-year-old woman.
Three serial transaxial CT images of lumbar spine reveal progressive loss of
definition of L2 left pedicle (arrow, B and C). Note
right transverse process fracture (arrowhead, B).
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MRI—Eight of fifty-three patients were examined on MRI.
Three of these patients had separate burst fractures, one contiguous and two
noncontiguous. MRI revealed marked soft-tissue damage through the posterior
elements and surrounding soft tissues in all cases. The sandwich sign
[3], which is characterized on
T2-weighted images by low-signal hemorrhage along the fracture line with
flanking high-signal marrow edema, was also shown in all cases and could be
traced through the pedicles (Figs.
5A and
5B). Neither epidural
hematomas nor disk herniations were seen in this series.
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Fig. 5A —Inversion recovery sagittal MR images of lumbar spine in
31-year-old man. Chance fracture of L1 shows marked high signal in
interspinous ligaments and soft tissues (arrowheads). Bone marrow
edema (arrows) is seen in pedicle and vertebral body.
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Fig. 5B —Inversion recovery sagittal MR images of lumbar spine in
31-year-old man. Low-signal fracture line seen centrally in posterior
vertebral body (arrow) with surrounding edema represents MRI sandwich
sign.
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Fracture Patterns
A close examination of the fracture patterns on CT unexpectedly revealed
that a burst-type fracture with buckling or retropulsion of the posterior
cortex was a common finding, seen in 48% of the patients in our series (Figs.
6A,
6B,
6C, and
6D). Results of the anterior
and middle column injury classification
[4] are seen in
Table 3. No purely ligamentous
Chance-type fractures were identified in this series, although 26% were
asymmetric, with bone involvement of one pedicle and a contralateral facet
distraction.
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Fig. 6C —Chance-burst fracture in 28-year-old man. Midsagittal
(C) and parasagittal (D) CT reformations of thoracolumbar spine
confirm Chance-type fracture of T12 with fracture line through pedicle
(arrow, C). Note associated buckling and retropulsion of T12
posterior vertebral body cortex along with loss of vertebral body height
anteriorly. Together, these findings represent Chance-burst combination
injury. Note is also made of compression fracture of T11 (arrowhead,
C).
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Fig. 6D —Chance-burst fracture in 28-year-old man. Midsagittal
(C) and parasagittal (D) CT reformations of thoracolumbar spine
confirm Chance-type fracture of T12 with fracture line through pedicle
(arrow, C). Note associated buckling and retropulsion of T12
posterior vertebral body cortex along with loss of vertebral body height
anteriorly. Together, these findings represent Chance-burst combination
injury. Note is also made of compression fracture of T11 (arrowhead,
C).
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Treatment Modalities
The majority of patients in our series (60%) underwent posterior spinal
fusion. Management results are presented in
Table 4.
Abdominal Injuries
Associated intraabdominal injuries were seen in 21 (40%) of 53 patients;
this increases to 43% (20 of 47) if the six patients with thoracic fractures
above T12 are excluded. Organ injury distribution is seen in
Table 5 and included liver,
spleen, pancreas, adrenal glands, bowel, and mesentery. Bowel and mesentery
injuries were most common (48% and 38%, respectively) and often coexisted
(Fig. 7). Neither renal nor
aortic injuries were seen in this series.
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Fig. 7 —Transverse CT image of mid abdomen in 9-year-old girl with
Chance fracture. Small-bowel loops have diffuse mural thickening consistent
with jejunal contusion. Foci of free intraperitoneal air are present
signifying perforation. Hemoperitoneum is seen in right paracolic gutter
(arrow).
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Literature Review
Results of similar reported series
[5-22]
describing Chance-type fractures and seat belt-related abdominal visceral
injury are presented in Tables
6 and
7.
Discussion
Chance [1] described a
unique hyperflexion injury of the lumbar spine resulting in a
"horizontal splitting of the spine and neural arch" without
"...any cord damage." Seventeen years later, Howland et al.
[5], unaware of Chance's
earlier report, presented a similar case report of a "splitting
apart" transverse fracture of a lumbar vertebra.
It was not until the 1960s that a spectrum of abdominal injuries specific
to motor vehicle collision occupants wearing lap belts was observed. Garrett
and Braunstein [23] first
coined the term "seat belt syndrome" in their landmark article on
seat belt safety in 1962. Doersch and Dozier
[24] in 1968 discovered a
disproportionate number of victims with severe mesenteric and intestinal
injuries, although no specific connection with vertebral injuries was
established.
The following year, Smith and Kaufer
[6] published their series of
24 lumbar spine injuries related to lap belt use. Twenty patients had unusual
transverse-type fractures characterized by disruption and longitudinal
separation of the posterior elements with minimal or no vertebral body
compression or displacement. They termed this injury pattern the
"Chance" fracture. Three patients in their series were noted to
have sustained intraabdominal injuries.
By 1970, the connection between hyperflexion injuries of the spine and
intraabdominal visceral injuries was made. Both injuries were deemed the
result of the same forces set in motion by rapid deceleration. Ritchie et al.
[7] broadened the term
"seat belt syndrome" to include the characteristic transverse
fracture of the thoracolumbar spine. In their review of the previously
published 37 cases, a delay in diagnosis of 24 hours or more was noted in more
than 50% of cases. Delay in diagnosis contributed significantly to mortality.
Intraabdominal injuries most commonly involved perforation or transection of
small bowel and mesenteric lacerations
[17,
25-30],
but reports also included colonic perforation
[30,
31], splenic rupture
[32-34],
pancreatic rupture [23],
kidney rupture, liver rupture
[34], and gravid uterine
rupture [35]. Two mechanisms
for intraabdominal injuries have been proposed: first, direct compression
between the seat belt and spine; and second, entrapment of bowel above or
below the level of flexion effectively generating a closed-loop obstruction in
the setting of increased intraabdominal pressure.
Case reports of patients with both Chance-type fractures and intraabdominal
injuries began to be published. Tables
6 and
7 summarize the literature
regarding the frequency of coexisting intraabdominal injury and thoracolumbar
flexion-distraction or Chance fracture.
Radiographic Diagnosis
We present, to our knowledge, the largest series of Chance-type fractures
of the thoracolumbar spine yet reported. In our patient population, 78% of
fractures occurred at the thoracolumbar junction, but were identified as
craniad as T4 and as caudad as L5.
The thoracolumbar junction accounts for approximately 50% of all spinal
injuries occurring outside the cervical spine. Biomechanically, this
transition zone is susceptible to injury for a number of reasons: first,
transition from the relatively rigid thoracic spine to the more mobile lumbar
spine; second, change of facet orientation from coronal in the thoracic spine
to sagittal in the lumbar spine; third, shift in spinal alignment from a
kyphotic thoracic spine to a lordotic lumbar spine; and, fourth, loss of
inherent stability in the thoracic spine provided by articulation to the rib
cage and sternum (T1 to T10 only, as T11 and T12 are floating ribs).
Of all thoracolumbar spine fractures, Chance-type fractures are a
relatively small percentage. A review of the literature reveals an incidence
of 5% among 412 thoracolumbar fractures in the series of Denis
[36,
37], 11% among 179 in the
series of Gumley et al. [11],
and 15% of 271 in the series of Gertzbein and Court-Brown
[4].
These fractures are considered unstable, with disruption of the posterior
and middle columns, and often extend into the anterior column as well.
However, our study confirms the rarity of neurologic injury with these
fractures. Only one patient had a neurologic deficit attributable to a
Chance-type fracture of T11. No purely ligamentous injuries were identified in
our series. Because the study is a retrospective review, perhaps this injury
pattern was misdiagnosed or misclassified, such as a fracture-dislocation.
Alternatively, the purely ligamentous variant may be very uncommon.
Spine injuries are common in major blunt trauma and may be missed or
diagnosis may be delayed in polytrauma patients. This delay can lead to grave
neurologic sequelae and has been reported in 24% of all thoracolumbar
fractures, with 77% of these occurring in mechanically unstable patients
[38]. In a review by Ritchie
et al. [7], 50% of diagnoses
were delayed by 24 hours or more. A delay in diagnosis contributes to
neurologic deficit in 10.5% of spinal fractures compared with only 1.4%
diagnosed at initial screening
[39]. In addition,
noncontiguous injuries can be present 15% of the time. One patient in our
series had a delayed diagnosis of 20 days. In that patient, admission CT
showed bowel contusion. Retrospective review of the images revealed the
presence of a dissolving pedicle sign. Neither sagittal nor coronal
reformations were obtained at the time of scanning, as this had not been the
standard protocol at that time. The patient returned to the trauma service
with persistent back pain, and follow-up radiographs of the thoracolumbar
spine showed the overlooked Chance-type fracture.
Intraabdominal injuries are common with Chance-type fractures: 40% of our
entire series and 43% of those patients with fractures between T12 and L5
sustained internal injuries. Our results are supported by review of all
identified prior Chance fracture and flexion-distraction injury series in the
literature [1,
5-15],
which reveal a total of 51 intraabdominal injuries present in 122 patients for
a frequency of 42%. The addition of the results from the current series brings
the incidence to 41% (72 of 175).
Imaging findings in Chance-type fractures are often subtle. At the
University of Maryland Shock Trauma Center, all trauma patients with suspected
fracture of the thoracolumbar spine undergo both AP and cross-table lateral
radiography of the thoracic and lumbar spine, although this practice is
declining in favor of CT screening of the thoracolumbar spine. At many
institutions, the cross-table lateral view may be omitted, making the
diagnosis of these fractures on the AP projection critical. The key to
diagnosing these flexion-distraction injuries lies in the status of the
posterior elements. Fortunately, in this series the empty vertebral body sign
was identified in 100% of cases from the AP radiograph. The empty vertebral
body sign results from the vertical separation of the posterior elements
displacing the spinous processes or spinous process fracture fragments off the
vertebral body on the AP projection. In addition, one may see a horizontal
fracture through one or both pedicles. There may be associated widening of the
interpedicular distance, suggesting a burst component, as well. Other
supporting radiographic signs may include transverse fractures across the
transverse processes, laminae, and articular processes. There may be widening
of the facet joints and increased intercostal spacing.
Distraction of the posterior elements is best characterized on the lateral
view. In Chance-type fractures, the fracture line is seen extending from
posterior to anterior through the spinous process with fanning, or spreading
apart, of the fracture fragments. The fracture then propagates into the
pedicles and variably, if at all, into the vertebral body. The
ligamentous-only variant appears with fanning of the intact spinous processes
and facet joints. There may be increased vertical distance across the
posterior intervertebral disk, signifying disruption of the posterior annulus
fibrosis associated with focal kyphosis.
Relatively little has been reported describing the CT and MRI appearances
of flexion-distraction thoracolumbar fractures. CT is superior to conventional
radiography for depicting fractures of the thoracolumbar spine by removal of
overlapping osseous structures. CT accurately shows vertebral column
damage—specifically, the integrity of the critical middle
column—and helps identify patients at risk of acute neurologic
compromise. Evaluation of our current series with single-detector helical CT
and MDCT revealed the transverse fracture on transaxial images as a gradual
radiolucency through the pedicles. From anterior to posterior, there is
progressive loss of definition of the pedicle, a sign that we refer to as the
"dissolving pedicle," observed in 76% of our patients. The
dissolving pedicle sign was seen almost twice as often as the naked-facet sign
(40%).
Confirmation of the CT diagnosis, however, lies in the sagittal and coronal
reformations. Sagittal CT reformations more clearly and accurately depict the
radiographic signs as seen on the lateral projection. Thus, the progression of
injury from the posterior column of the thoracolumbar spine can be followed
anteriorly. Coronal reformations similarly improve on the AP radiograph by
more clearly depicting the transverse fracture through the posterior
elements.
MRI evaluation provides information regarding the integrity of the
posterior and middle column ligaments, annulus fibrosis, and spinal cord in
neurologically injured patients. MRI may also reveal the presence of disk
herniations and epidural hematomas. T2-weighted and inversion recovery images
are best to assess soft-tissue edema, a bone bruise, and spinal cord injury.
Ligamentous integrity is optimally seen on proton density-weighted images.
The Chance-Burst Fracture
Almost half the patients in our series showed buckling or frank
retropulsion of the posterior vertebral body cortex, consistent with an
associated burst component to their Chance-type fracture. Gertzbein and
Court-Brown [4] noted the
occurrence of this combination in 15% of patients in their 1988 series. Since
that publication, no additional reports of this Chance-burst subtype have
appeared in the English-language literature, to our knowledge. The majority of
the 20 patients Gertzbein and Court-Brown evaluated underwent radiography
only, with CT reserved for only those with suspected spinal canal compromise.
Screening CT of all thoracolumbar fractures may indeed have found more cases
of this type—in particular, those with subtle posterior cortex buckling
that is less evident on radiography. Furthermore, case reports and smaller
series of Chance fractures suggest the presence of unrecognized burst
components. In his review of six such fractures, Rogers
[8] reported that a burst
component was depicted on a radiograph in one patient, similar to the findings
in the series of Rennie and Mitchell
[10]. A recent T3 Chance
fracture case report by Davis et al.
[40] clearly shows
retropulsion of fracture fragments into the spinal canal. Interestingly, in
each of these examples, the patient had a spinal cord injury.
The diagnosis of a burst component to the Chance fracture may be of major
import on injury management. Application of an extension cast in a patient
with a retropulsed burst fracture component may cause further posterior
displacement with the potential for spinal cord injury or progression
[41]. The degree of
instability will dictate management, and Gertzbein and Court-Brown
[4] recommend that distraction
rods, rather than Harrington rods, are more appropriate. They further add that
patients with spinal canal compromise may benefit from anterior
decompression.
Pathogenesis and Mechanism of Injury
In his original publication, G. Q. Chance
[1] declared a flexion
mechanism responsible for this horizontal fracture pattern, although he could
not "think of any anatomic explanation of the peculiar site and
direction of the fracture." Smith and Kaufer
[6] explained that the lap seat
belt serves as the fulcrum, or axis of rotation, at the anterior abdominal
wall, thus subjecting all spinal elements to flexion and distraction or
tension forces. This theory explains the high frequency of associated
intraabdominal injuries, but it does not account for the presence of
compression or burst components. Rennie and Mitchell
[10] placed the axis of
rotation in line with the posterior longitudinal ligament to account for
anterior compression and posterior distraction. Begeman et al.
[42] conducted a cadaver study
examining sudden deceleration effects on spinal loading while seat belted.
Increasing axial loads occurred in direct proportion with higher-velocity
sudden decelerations. The spinal column underwent an initial axial load phase,
followed by flexion, and finally axial loading again as the body was thrown
back into the seat. Gertzbein and Court-Brown
[4] postulate that the
compression fracture component of the Chance fracture probably occurs at the
initial axial load but that the burst is the result of the final axial load,
after the spine has already suffered the Chance fracture during the flexion
phase of injury.
We hypothesize that the fulcrum, or axis of rotation, begins at the site of
the lap belt or other object pressed against the anterior abdominal wall at
the time of impact and sudden deceleration. As the spinal column begins to
fail in tension, from posterior to anterior, the axis of rotation migrates
from anterior to posterior. That is, once the interspinous ligaments tear or
the spinous process fractures such that the posterior column fails in
distraction and a flexion deformity results, the biomechanics begin to change.
The initial flexion and distraction forces begin to invoke an axial loading,
or vertical component, driving the effective fulcrum posteriorly from the
anterior abdominal wall. With enough focal kyphosis, the axis of rotation
migrates behind the anterior vertebral body cortex into the anterior column
allowing anterior compression. With greater deceleration forces, the axial
load further increases and the axis of rotation migrates beyond the posterior
longitudinal ligament beyond the middle column allowing the vertebral body to
burst (Figs. 8A,
8B,
8C,
8D, and
8E). In this manner, all three
columns fail by initial flexion and distraction and subsequently axial
loading, resulting in an unstable injury. With greater forces, the Chance
fracture becomes the Chance-burst combination injury, increasing the
likelihood of bone retropulsion and spinal cord injury.
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Fig. 8A —Diagrams show proposed mechanism of injury for Chance-type
fractures. White arrow shows point of contact of seat belt and abdomen, and
straight black arrows depict forces. Black circle represents axis of rotation.
Representative segment of lumbar spine is illustrated in abdomen restrained by
seat belt. On sudden deceleration, point of contact of seat belt and abdomen
serves as fulcrum, or axis of rotation, about which all structures posterior
are subject to flexion and distractive forces.
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Fig. 8B —Diagrams show proposed mechanism of injury for Chance-type
fractures. White arrow shows point of contact of seat belt and abdomen, and
straight black arrows depict forces. Black circle represents axis of rotation.
With enough force, bone integrity is overcome and horizontal Chance fracture
results. Curved arrows show axis of rotation.
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Fig. 8C —Diagrams show proposed mechanism of injury for Chance-type
fractures. White arrow shows point of contact of seat belt and abdomen, and
straight black arrows depict forces. Black circle represents axis of rotation.
With weakening of fractured spine, initial flexion-distraction force begins to
involve an axial load component, driving axis of rotation (curved
arrows) posteriorly.
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Fig. 8D —Diagrams show proposed mechanism of injury for Chance-type
fractures. White arrow shows point of contact of seat belt and abdomen, and
straight black arrows depict forces. Black circle represents axis of rotation.
Once axis of rotation (curved arrows) moves posterior to anterior
vertebral body cortex, compression begins.
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View larger version (47K):
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Fig. 8E —Diagrams show proposed mechanism of injury for Chance-type
fractures. White arrow shows point of contact of seat belt and abdomen, and
straight black arrows depict forces. Black circle represents axis of rotation.
With ongoing force, axis of rotation (curved arrows) continues to
migrate posteriorly in further weakened spine allowing greater axial loads
that ultimately cause vertebral body to burst.
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Conclusion
Chance-type fractures are unstable injuries that may be subtle
radiographically, and patients may present without neurologic deficit.
Intraabdominal injuries occurred in 40% of these flexion-distraction
thoracolumbar fractures, which is slightly lower than previously reported, and
almost half of the patients in our study with this injury display a burst-type
component that could have a significant influence on surgical management.
Evaluation of the posterior elements on radiographs and the dissolving pedicle
sign on transaxial CT can assist in recognition of this injury.
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Abstract
Introduction
