AJR 2001; 176:1161-1164
© American Roentgen Ray Society
The Vacuum Phenomenon
A CT Sign of Nonunited Fracture
Bernard Stallenberg1,
Afarine Madani1,
Franz Burny2 and
Pierre Alain Gevenois1
1
Department of Radiology, Hôpital Erasme,
Université Libre de Bruxelles, Route de Lennik
808, 1070 Brussels, Belgium.
2
Department of Orthopedic Surgery, Hôpital
Erasme, Université Libre de Bruxelles, 1070
Brussels, Belgium.
Received September 7, 2000;
accepted after revision October 16, 2000.
Address correspondence to B. Stallenberg.
Abstract
OBJECTIVE. The purpose of this study was to describe on CT scans the
presence of a gas collection within a bone fracture reflecting the vacuum
phenomenon as a sign of nonunited fracture.
CONCLUSION. A gas collection between fractured bone fragments
suggests a nonunited fracture.
Introduction
The vacuum phenomenon appears as a radiolucent area visible in synovial
joints, intervertebral disks, and vertebrae
[1]. This phenomenon is
explained by gas accumulation, mostly nitrogen, produced by the surrounding
soft tissues [2]. In synovial
joints, this phenomenon is related to the distraction of the articular
surfaces [1]. In intervertebral
disks, the vacuum phenomenon is, in most cases, related to degenerative
processes, but this phenomenon has also been reported in rare cases of tumors
and infections [3,
4]. In vertebrae, the
phenomenon has been described in cases of collapse, usually resulting from
osteonecrosis [5]. The vacuum
phenomenon detected on CT was recently reported in a case of a nonunited
fracture of the public bone
[6]. The aim of this report is
to describe the vacuum phenomenon as a CT sign of delayed or nonunited
fracture.
Materials and Methods
Patients
Between November 1997 and September 1999, CT with multiplanar
reconstructions were performed in 19 successive patients referred by
orthopedic surgeons to evaluate the continuity of the callus, the bone cortex,
or both in suspected delayed or nonunited bone fractures of the upper and
lower extremities. This suspicion was based on the clinical criteria proposed
by Resnick et al. [7] and on
findings from frontal, lateral, and oblique radiographic views.
The study group consisted of 13 men and six women, with a mean age of 45
years (age range, 16-79 years). The fracture sites were the humeral diaphysis
(n = 6), the radial diaphysis (n = 1), the waist of the
scaphoid (n = 1), the diaphysis of the fifth metacarpal (n =
1), the femoral neck (n = 1), the femoral diaphysis (n = 1),
and the tibial diaphysis (n = 8). The time from injury to CT
examination ranged from 6 months to 18 years.
Either the nonunion or the consolidation of the fracture was verified by
different approaches. Nine patients were treated surgically, and all nine had
a nonunited fracture. In these patients, the median delay between the CT
examination and the surgical procedure was 43 days; the delay ranged from 15
to 242 days. Ten other patients were treated conservatively and followed up by
clinical examination and repeated conventional radiographic examinations of
the fractured bone. This follow-up period ranged from 5 to 16 months after the
CT examination. In seven of these 10 patients, the consolidation was
considered complete because the following three criteria were present
simultaneously: the conventional radiographs showed cortical continuity; the
patient was asymptomatic; and no refracture occurred with normal use of the
limb. In two of these 10 patients, the consolidation was considered probable
because these two patients were asymptomatic, and no refracture occurred;
however, neither cortical nor callus continuity was visible on radiographs.
Finally, in one of these 10 patients treated conservatively, the nonunion was
suggested by the association of an increased technetium uptake on a nuclear
bone scan, obtained 5 years after the initial trauma; pain; mobility of the
bone fragments; and neither cortical nor callus continuity was detectable on
conventional radiographs. Thus, 10 and nine patients had a nonunited or a
united fracture, respectively. In all patients, associated neoplasm or
infection was excluded by clinical follow-up or at surgery, when
performed.
CT Evaluation
CT was performed on Somatom Plus scanners (Siemens Medical Systems,
Erlangen, Germany). Axial helical CT scans (189-171 mA, 120-140 kV) were
obtained through the fracture site, extending 1-2 cm proximally and distally
from the fracture site. Data were obtained with a 2 mm/sec table feed and a
2-mm collimation (pitch of 1). Images were reconstructed at 1-mm intervals
with a 180° linear interpolation algorithm. A software package (Siemens
Medical Systems) for multiplanar reconstruction was used to generate
sequential images through the fracture site in sagittal, coronal, and oblique
planes. All images were photographed with two ranges of window width settings
(350-450 H and 2000-2500 H) and levels (45-75 H and 200-250 H). Axial and
multiplanar reconstructed CT images were interpreted by a skeletal radiologist
who looked for a lack of bone or callus bridging across the fractured site on
the multiple CT views. For the purpose of the present study, the CT images
were reviewed retrospectively, and the presence of a gas collection within the
fracture site as well as within the surrounding soft tissues was coded. In
addition, the appearance of the gas collection was characterized as linear or
round.
Results
In seven of the 10 patients with nonunion, between one and four round gas
bubbles, 1 mm in diameter, were detected on the CT images (Fig.
1A,1B)
and were located in the nonunion area between the bone segments without any
gas or liquid collection in the soft tissues surrounding the fracture. In
three of these seven patients, a linear gas collection was associated with
round gas bubbles (Fig.
2A,2B).
The gas collection was seen on from one to six adjacent CT sections. On the
other hand, in the nine patients with consolidation confirmed or probable, no
gas collection was detected. On the CT scans and on the multiplanar
reconstructed CT images, bone and callus bridging was absent in 14 patients,
doubtfully present in one patient, and obviously present in four patients. In
three and one of these 14 patients gas was absent, and the diagnosis was
nonunion and probable union, respectively. In the patient in whom present
callus bridging was doubtful, gas was absent and the union was considered
probable. Our CT findings are summarized in
Table 1.
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Fig. 2A. —52-year-old man with fracture of left tibial diaphysis.
Straight arrows = gas bubbles. Axial CT image photographed with window at 2050
H and level at 400 H shows linear gas collection (arrow) between bone
fragments.
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Fig. 2B. —52-year-old man with fracture of left tibial diaphysis.
Straight arrows = gas bubbles. Multiplanar reconstruction of A
generated sequential coronal image photographed with window at 2050 H and
level at 400 H shows linear gas collection (arrows) within
fragments.
|
|
Discussion
Our findings suggest that the vacuum phenomenon detected in a fracture site
is a sign of nonunion of this fracture. Furthermore, this sign appears to be
characteristic because in patients who eventually developed union despite the
absence of bony bridging on CT examination the presence of a gas collection
was detected on CT in none. Nevertheless, the absence of this sign cannot be
used to guarantee union if no bony bridging is seen. In bones other than
vertebrae, intraosseous gas has only been reported in osteomyelitis
[8], whereas in intervertebral
disks or vertebral bodies, the presence of gas makes the diagnosis of
infection highly unlikely [5],
although a few exceptions have been reported
[4]. The mechanisms of gas
production are different in osteomyelitis than in the vacuum phenomenon. In
osteomyelitis, gas is generated by microorganisms and is probably under high
pressure [4]. However, in the
vacuum phenomenon, the negative pressure between the bone fragments seems to
be mandatory for gas to be released from the surrounding tissues
[2]. In our study group, no
infections were diagnosed, and the gas collections were strictly limited to
the cleft of the fracture without any gas in the adjacent soft tissues. In
contrast, in two of the three patients with infection reported by Bielecki et
al. [4] and Ram et al.
[8] intraosseous gas was
associated with gas in the surrounding soft tissues.
In our study, three patients presented with a gas collection with a linear
shape, whereas in the other patients, it had a bubble appearance. This
particular appearance is frequently seen in the vacuum phenomenon, whereas the
gas collection has a bubble appearance when it occurs in infection
[4,
8,
9]. The duration and the
appearance of this phenomenon are variable and depend on the intensity of the
distraction applied to the fracture, on the amount of fluid within the cleft,
and on physiologic factors governing gases in the human body, including
partial pressures, solubility coefficients, and diffusion gradients
[2,
9].
The vacuum phenomenon may be accentuated by hyperextension and shown on
stress views [5]. In our study,
the position of the patient for the CT scan could induce stress on the
fractured site and thus favor the vacuum phenomenon. In the CT scanner, the
patients with humeral fractures were lying supine with the forearm positioned
over the head. The weight of the forearm could thus stress the fractured bone
and create or accentuate the vacuum phenomenon.
Several reports have suggested that the vacuum phenomenon observed in
insufficiency vertebral fractures could represent pseudarthrosis
[10]. This phenomenon was also
described in a patient with a posttraumatic nonunited fracture of the pubic
bone [6]. In the present study,
the vacuum phenomenon was present in the fracture site only in patients with
nonunion shown by the evolution of the fracture. Because of the retrospective
nature of this study and because our findings are based on a low number of
patients, further studies should be performed to evaluate whether the vacuum
phenomenon can predict the occurrence of a pseudarthrosis.
CT has been recommended to qualitatively or quantitatively evaluate the
fracture consolidation [11].
In routine clinical practice, the quantification of fracture healing by this
technique remains difficult, but CT is a simple and valuable method to assess
qualitatively the presence of bone or callus bridging
[12]. Nevertheless, bone
continuity between multiple fragments in comminuted fractures could be
difficult to detect on CT. Therefore, the presence of gas within the fracture
facilitates the diagnosis of an absence of consolidation.
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Abstract
Introduction
