Dr. Hancock is a Neuroradiology Fellow, and Dr. Quencer is
Chairman, Department of Radiology, and The Robert Shapiro M.D.
Professor of Radiology, Jackson Memorial Hospital, University of Miami
Hospital and Clinics, Miami, FL. Dr. Falcone is the Executive Clinical Dean, Florida Atlantic University, Boca Raton, FL.
interpretation of the postoperative spine presents signiﬁcant difﬁculty
because of the various types of surgery performed, new surgical devices
employed, different imaging techniques used, and limited clinical
information available at the time of image interpretation. The
radiologist must understand the many different postoperative spine
imaging presentations in order to accurately convey clinically important
but often subtle ﬁndings.
In this article, we will discuss and
illustrate some of the common and uncommon imaging ﬁndings in the
postoperative spine, with a particular emphasis on the uncommon
cases—speciﬁcally, retained surgical material with associated clinical
sequelae, including gel foam, dural autograft, and catheters. The
article will also address magnetic susceptibility artifacts and
dislodged surgical hardware, as well as how to distinguish between
inﬂammation and infection and between scar and recurrent disc
herniation. Another topic covered is the common dilemma of delineation
between postoperative ﬂuid collections (including seroma, abscess, and
pseudomeningocele). This article reviews several manifestations of
postprocedure and postoperative spine imaging with a focus on the
challenges and pitfalls that are often encountered.
Spine surgery was ﬁrst contemplated by the ancient Egyptians and was later advanced by the Greeks and Romans.1 Many
of their works were preserved during the Dark Ages by Arabic and
Byzantine translators and were rediscovered during the Renaissance.1 Major
impediments to progress from Hippocrates’ era until the 19th century
included the lack of antiseptics, inadequate anesthetics, and the
absence of medical imaging. With the development of radiography, it was
possible to visualize the effects of surgery and the consequences of
intervention, and to document the presence of disease. The ﬁrst
successful spinal fusion was performed in 1911 to reduce
pseudoarthrosis. This was followed by autologous interbody bone grafting
in 1933; transfacet fusions in the 1940s; distraction Knodt rods in the
1950s; Harrington rods, methylmethacrylate, and transverse process
plate fusion in the 1960s; Luque rods and sublaminar wire in the 1970s;
and intrapedicle screws with rod ﬁxation in the 1980s.2 A
medical industrial complex surrounding spinal surgery has arisen as a
direct result of the introduction of computed tomography (CT) and spinal
magnetic resonance imaging (MRI).3 This growth in surgical
intervention has in turn created an increase in the demand for advanced
imaging for preoperative planning, intraoperative evaluations, immediate
postsurgical assessment, and postoperative care. Postoperative
evaluation of potential complications (such as retained surgical
material, infection, pseudomeningocele, seroma, hemorrhage, loose
hardware, compromised hardware, fracture, and pseudoarthrosis) plays an
increasingly important role in the radiologist’s daily interpretation of
the postoperative spine. Multiple artifacts may complicate the
interpretation of spine imaging, and these artifacts can arise from
chemical shift, motion, magnetic ﬁeld nonuniformity, magnetic
susceptibility and incomplete fat saturation.4 A textbook
would be required to provide a complete compendium of postoperative
imaging ﬁndings; therefore, this article includes several of the more
interesting and challenging presentations.
Clinical considerations and imaging
Retained foreign material
of surgical hardware to obtain spinal stability typically leads to a
successful postoperative outcome; however, surgical material may be
unintentionally malpositioned or inadvertently retained, which may have
signiﬁcant patient care implications (Figure 1). Gel foam, which is used
for hemostasis and postoperative scar reduction in spinal surgery, has
been associated with signiﬁcant pain.5
myelographic agent that was used prior to the availability of
water-soluble agents (Pantopaque, Lafayette Pharmaceutical, Inc.,
Lafayette, IN) could be retained within the spinal canal for years or
even decades, depending on the volume used.6 Adhesive
arachnoiditis may result as a complication of persistent Pantopaque
(Figure 2), which can cause meningeal thickening and hyalinization of
the arachnoid.7 The nerve roots may adhere to themselves and/or the dural margins.8 Arachnoiditis may occur in the postmyelographic state, particularly if bloody cerebrospinal ﬂuid (CSF) is present.
Catheters and stents
decompression (with or without dural grafting), cyst shunting, and
lysis of intradural adhesions is the most commonly performed surgical
treatment in the Chiari malformations and associated syringohydromyelia.
Suboccipital decompression relieves stenosis at the foramen magnum,
which corrects associated ﬂow-related abnormalities and thus prevents
syrinx development or enlargement of an existing syrinx. Some
researchers advocate intradural exploration to lyse commonly associated
intradural adhesions.9 Stents and shunts diverting CSF from
the syrinx to the adjacent CSF or into the peritoneal or pleural cavity
are favored in cases of isolated syrinx rather than cases associated
with Chiari I malformations10 (Figure 3). A multicenter study
that evaluated the surgical preferences and outcomes reported that
suboccipital decompression was a better treatment option for Chiari I
malformations while stents and shunts were more efﬁcacious in treating
cases of isolated syrinx.10 Several shunting procedures have
been suggested for the correction of symptomatic syringohydromyelia. One
multicenter study determined surgical preferences in the following
order: syringosubarachnoid shunt, syringopleural shunt,
syringoperitoneal shunt, and syringocisternal shunt.11-13
susceptibility artifacts are frequently encountered in the
postinstrumented spine; particularly when GRE sequences are used.
Gradient-recalled echo allows for a signiﬁcantly decreased acquisition
time; however, T2* ﬁeld inhomogeneities are worsened with GRE because of
lack of a refocusing RF pulse.14 Therefore,
gradient-recalled echo imaging can be used in certain applications to
not only limit acquisition time but also to search for otherwise subtle
foci of magnetic susceptibility such as paramagnetic blood products,
air/water interfaces, or calcium deposition.14 Magnetic
susceptibility due to neurosurgical hardware is increased at higher ﬁeld
strengths and, therefore, the ferromagnetic properties of spinal
hardware are ampliﬁed as the ﬁeld strength is increased. The use of fast
spin-echo (FSE) T2-weighted (T2W) imaging is preferred in these
patients. Phased-array coils with parallel imaging, high bandwidth, and
relatively long echo train FSE sequences can be used to further reduce
magnetic susceptibility.14 This fact should be taken into special consideration when comparing scans from 1.5T and 3T magnets (Figure 4).15 Artifacts
may be signiﬁcant enough, even with the use of FSE techniques, to limit
the evaluation of the spinal canal and the surrounding structures.
Failure of surgical hardware
spine fracture may occur from osteoporotic insufﬁciency, neoplasia,
subsequent trauma, osteomyelitis, pseudoarthrosis, Charcot joint
formation, repetitive stress reaction, hardware failure, or a
combination of these entities. Spinal hardware may fracture or migrate
from mal-positioning, misconstruction, excessive stress, acute trauma,
or metal fatigue.16,17 This potential for hardware compromise
requires the radiologist to precisely identify and report the
conﬁguration of hardware placement (Figure 5). Transfacet, lateral mass,
or transpedicular screws may fracture from metal fatigue, may cause
fractures in vertebral body margins, or may enter neural foramen, the
spinal canal, adjacent vertebral body levels, or proximate soft tissues.18
Disc versus scar
removal of herniated discs is one of the most common indications for
spinal surgery. Knowledge of the patient’s detailed surgical history is
essential, as there may be little evidence of postprocedure changes to
indicate any prior operations or interventions. Various terms have been
used to describe disc disease in an attempt to improve understanding of
the conﬁguration, degree, and location of disc pathology.19-24 Postsurgical
ﬁndings may include recurrent disc herniation, residual disc material,
scar formation, infection, hemorrhage, or a combination of these
entities. The hallmark sign distinguishing postoperative scar from
recurrent or residual disc herniation is the pattern of enhancement.
Scar tissue tends to enhance homogeneously, while disc herniation tends
to enhance peripherally (Figure 6). In the early postoperative period
(less than 3 to 6 months), it may be impossible to distinguish
peripherally enhancing scar type changes from recurrent/residual disc
Postoperative ﬂuid collections
ﬂuid collections are frequent and must be differentiated from seroma,
hemorrhage, abscess, and/or pseudomeningocele. Identiﬁcation is critical
because of adjacent vulnerable neural structures and potentially
serious sequelae of delayed or inappropriate treatment25–27 Hemorrhage
may be identiﬁed by observing a blood/serum level on cross-sectional
imaging, measuring HU on CT, and/or evaluating the MRI sequence signal
characteristics that are indicative of blood products.28 A
hemorrhage may be complicated by a superimposed infection, which may be
difﬁcult to distinguish from inﬂammatory scar formation.
may enhance peripherally in a manner that may simulate an inﬂammatory
or infectious process (Figure 7). It may be possible to differentiate
abscess from pseudomeningocele by observing T1W pre- and postgadolinium
and T2W sequences to determine the enhancement characteristics and
whether the internal signal exactly follows the signal of CSF.29,30 Pseudomeningoceles
do not regularly show the degree of adjacent inﬂammation and
enhancement characteristics that are associated with most abscesses.
Pseudomeningoceles exhibit signal that should virtually match CSF
intensity. Abscesses, conversely, are expected to display lower signal
intensity on T2W sequences and higher signal on T1W sequences because of
intermixed proteinaceous products. Infrequently, pseudomeningoceles
reveal the site of a leak through the dura and are seen as low signal
that represents dephasing of ﬂuid due to ﬂow.
Abscess or phlegmon
A focal region of infectious inﬂammation, also known as a phlegmon,
often precedes frank abscess formation and may enhance with
characteristics that are difﬁcult to differentiate from recently formed
scar tissue or tumor. An abscess typically presents in an epidural
location; however, it may also present in a subcutaneous or
intramuscular position or in the intradural space or disc space, and/or
it may involve the ligamentum ﬂavum or lie within the cord. An abscess
often appears with a thick rind or capsule of inﬂammatory tissue that
enhances avidly.31-33 Spinal epidural abscesses have been
shown to occur more commonly in the lumbar spine. In these abscesses,
72% of cultures reveal gram-positive organisms with Staphylococcus aureus as the most common culprit (45%).33 Figure
7F depicts a peripherally enhancing epidural lesion compressing the
cord that was culture-positive, consistent with abscess.
processes such as arachnoiditis, ﬁbrosis, inﬂammatory pseudotumor, and
tethered cord may complicate the postoperative state (Figure 8). MRI
evaluation of the lumbar spine following myelomeningocele repair shows
ﬁndings that are consistent with tethered cord in virtually all
Spinal canal decompression
of the spinal canal may be congenital or the result of spondylosis,
including facet hypertrophy, ligamentum ﬂavum hypertrophy, ossiﬁcation
of the posterior longitudinal ligament, and posterior endplate spurring,
which may be exacerbated by intervertebral disc herniation and
congenital spinal canal stenosis. Scoliotic deformities,
spondylolisthesis, and compressive fractures may also complicate
spondylotic stenosis. Spinal stenosis is most common in the cervical and
lumbar areas, and the pain associated with neural compression may be
ameliorated by decompressive surgical procedures such as partial disc
resection, laminectomy, unilateral laminotomy, bilateral laminotomy,
laminoplasty, and other surgical variations.35,36 Laminoplasty
of the cervical spine has become an accepted procedure for alleviating
focal or multilevel spinal canal stenosis (Figure 9). Hydroxyapatite,
ceramics, and other material have been used to promote structural
stability. Osseous autografts, allografts, and xenografts (including the
ﬁbula, iliac crest, vertebral bodies, and other sources) have been used
extensively for spinal reconstruction.37-39
grafts are used for the repair of some congenital malformations,
following dural resection, or in cases requiring enlargement of the
dural sac. It is important to recognize the expected presentation of a
dural graft (Figure 10) and to differentiate it from other potentially
associated complications, such as scar tissue, complicated
pseudomeningocele, infection, or CSF leak.40 It is essential
that the radiologist is familiar with the patient’s surgical history,
since this presentation may be inadvertently interpreted as an
inﬂammatory or infectious process.
Complicated postoperative spine
complicated postoperative spine may include multiple surgical hardware
revisions, hardware compromise, complicated ﬂuid collections, tumor
recurrence, subsequent osteoporotic insufﬁciency fractures, a mixture of
the above, or other complications. Figure 11 displays one example of
the plethora of potential complicated postoperative spine presentations.
Interpreting postoperative spine
imaging requires a systematic approach and knowledge of the patients’
surgical history. This approach will allow the radiologist to separate
the many possible confounding ﬁndings. Obtaining surgical reports and
detailed clinical history can be invaluable in understanding and
properly reporting on the postoperative spine.
The authors would like to thank Jennifer I. Hui for her assistance with the preparation of this manuscript.
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