Dr. Hayeri is a former Research Fellow, Division of MSK Imaging at University of California, Irvine Medical Center, Orange, CA, and Dr. Tehranzadeh is
Chief of Radiology at Long Beach VA and Professor Emeritus and Vice
Chair of Radiology, Department of Radiological Sciences, University of
California, Irvine Medical Center, Orange, CA.
Back pain is the most common cause of limited activity in people
younger than 45 years in the United States. It is the second most
frequent reason for visits to a physician and ranks fifth as the reason
for hospital admission.1 It is estimated that 18% of the U.S.
population experience low-back pain each year. Fortunately, in most
cases, the underlying pathology is benign and the pain is self-limited.
Noninvasive methods of treatment such as physical therapy and
pharmacotherapy typically resolve such pain.
Treatment of back
pain is the third most common indication for surgical procedures in the
nation. Decompression and occasional arthrodesis with frequent
instrumentation are the main surgical procedures performed in the U.S.2 It
is a common belief that immobilization and/or removal of the painful
segment decreases pain. Failed back surgery syndrome (FBSS) is defined
as failure to relieve lower back pain symptoms following surgery. In the
best of all situations, this syndrome occurs after a minimum of 20% of
spine fusion surgeries. The syndrome can result from: mistaken
diagnoses, technique error, poor application, inappropriate indication,
pseudarthrosis or continued natural progression of disease. This
syndrome can be prevented to a large extent by meticulous pre- and
intraoperative radiologic examination.3,4 Since the initial
description of spinal instrumentation by Harda in 1889 and subsequent
spinal fusion surgery by Fred Albee and Russel Hibbs in treatment of
spinal tuberculosis in 19115–7 there have been a great many
advances in surgical methods and instrumentation, as well as many more
indications for fusion. Among current indications are scoliosis,
spondylolisthesis, congenital deformities, spinal instability in trauma
or by iatrogenic causes (e.g. extensive laminectomy), infection and
neoplasm. The current indication for spinal arthrodesis is broad and it
includes the category of degenerative disc diseases.3
imaging is used to assess disease progression, positioning of
instrumentation, possible complications and the extent of bone-graft
fusion. Knowledge of the advantages and limitations of different imaging
modalities is necessary for optimal evaluation of patients with spinal
instrumentation. Radiologists should also be familiar with different
surgical methods used in spinal fusion, types of instrumentation and
potential complications to properly appraise postoperative images.
Stability is described as resistance of the spine to deformation under physiologic stress. Mulholland8 in
a recent review of instability and low-back pain hypothesized that the
cause of low-back pain could be due to abnormal disc loading. Currently,
the most widely accepted cause of low-back pain and the underlying
concept promoting the use of spinal fusion is nonphysiologic movement of
the degenerated segment. Most appliances are placed to provide
stability during bone fusion, and their function is complete when this
has occurred. Be-cause of the morbidity associated with repeated
surgery, intact implants are generally left in place for life. Fractured
and dislodged implants are often removed because of the need for
revision and the potential for migration of the components, leading to
substantial soft-tissue or neural injury.
implants in spinal surgeries are used to stabilize the spine, replace
the defective parts and maintain anatomic reduction. Internal spinal
instrumentation has undergone considerable advances during the last
century. Radiologists should be able to identify the devices most
commonly used and understand their biomedical principles and
Rods, plates and rectangles
Rods can extend to
single or multiple spine segments. They can be single or double,
straight, L-shaped or can be cut and fashioned as required. They are
attached to the spine by hooks, pedicle screws or sublaminar or
interspinous wires or cables. Rods are usually preferred over plates for
multisegment fusion because of their ability to span a long segment.
The Hartshill rectangle is seldom used today. It is a stainless-steel
rectangle that attaches to the spine by sublaminar wires and
occasionally interspinous wires. Various shapes of plates in different
sizes have been developed for anterior or posterior spine fusion.9–11 Some of the commonly used instruments and systems, and their specifications, are summarized in Table 1.
Translaminar or facet screws
These devices can be used when posterior spinal elements are intact. They attach the lamina of 2 adjacent vertebrae.
spacers could be solid (ramp) or hollow (cages). Cages are filled with
bone-graft material and inserted into the intervertebral space or
replace a vertebra after its removal (i.e. corpectomy). Cages are
usually made of titanium carbon fibers, polyetherether ketan (PEEK) or
of cortical bone graft. Most cages contain 2 radiopaque markers to
identify their position in radiographs and to enable their assessment.
They are made in different shapes based on the method of approach to the
In anterior interbody fusion (AIF), cages
are more round in shape, while in posterior interbody fusion (PIF) they
are more rectangular. Transforaminal interbody fusion (TIF) cages are
more crescent-shaped. Expandable cylindrical or mesh cages are used in
vertebral body replacement procedures.
Cages are usually
supported by additional posterior, anterior or lateral instrumentation
(i.e. screw with plates or rods) to increase stability. For a standalone
interbody fusion cage, the interbody spacer is fixed to the adjacent
vertebral body with screws to eliminate the need for additional
instrumentation support. Retropulsion of the cage is a possible
complication, but is more common in PIF.12 A distance of ≤2
mm between the cage’s posterior marker and the posterior margin of the
vertebra should exist to provide reassurance that the cage is not
invading the spinal canal.11 Cage subsidence (defined as
migration of >3 mm into the adjacent vertebra) and lateral
displacement is a disadvantage of using mesh and standalone cages.13–15 The
incidence of subsidence is reported from 18% to ≤62.5% in patients who
undergo spinal procedures with standalone cervical cages. Expandable
cages have broader surface area and duller edges at both ends, which
minimize their subsidence and also allow immediate load bearing and
stability after corpectomy.16
stabilization devices are a new category of instruments that are in
various stages of development. They can be used alone or in conjunction
with other instrumentation. They act by controlling the abnormal motion
and uneven load in segments adjacent to the level of fusion in order to
minimize progressive degeneration. Artificial ligaments (e.g. Dynamic
Stabilization System [Dynesys], Zimmer Inc., Warsaw, IN), interspinous
decompression systems (e.g. X-STOP Spacer, Medtronic Spine, Memphis, TN;
and the Wallis Dynamic Posterior Stabilization System, Zimmer Inc.,
Bordeaux, France), and posterior element replacement systems (e.g. Total
Facet Arthroplasty System, Archus Orthopedics, Redmond, WA) are
examples of such devices.11
Surgical techniques can be divided on the basis of perceived patient
morbidity into minimally invasive or traditional-open procedures
performed via either an anterior or posterior approach. In interbody
fusion, the intervertebral disc or a complete vertebra is removed and
replaced with bone graft. Interbody fusion of the spine can be
approached anteriorly or posteriorly.
Anterior interbody fusion
(AIF) has the advantage of a broader access to the disc space. However,
it is limited by potential injury to major vessels and sympathetic nerve
chain.17 Oskouian and Johnson reported a 5.8% incidence (12
of 207 patients) of vascular complications in patients who underwent
anterior thoracolumbar spine reconstruction procedures.18
lateral interbody fusion (XLIF) is a newer surgical approach to fuse L1
to L5 and to minimize disadvantages of AIF. Extreme lateral interbody
fusion approaches the anterior spine from the flank.
interbody fusion (PIF) bilateral laminectomies are performed and
bone-graft material is inserted into the disc space after the disc is
removed. Posterior interbody fusion has the disadvantage of potential
injury to nerve roots. Retrograde migration of the graft or cage is also
more common with the posterior approach.19
interbody fusion (TIF) is a modified PIF that uses a more lateral
approach and thus leaves the midline bone structures intact. Min et al.
showed both AIF and PIF can produce good outcomes in treating lumbar
spondylolisthesis, but AIF is more advantageous in preventing the
development of adjacent segment degeneration.20
Lemcke et al. reported that, with regard to the indications and
contraindications, AIF and PIF are unquestionably accepted as up-to-date
methods.21 The decision to use AIF or PIF is mainly based on
the patient’s presenting pathology, spine anatomy, the surgeon’s
experience, history of previous surgery and other conditions that may
favor one approach over another (e.g. AIF is difficult in the presence
of marked vascular calcification).11,22 Laparoscopic
interbody fusion can also be performed; however, compared with open
surgery, the overall complication rate is higher (19% vs. 14%,
Posterolateral fusion is an alternative for
interbody fusion. In posterolateral fusion, adjacent vertebrae are fused
together by placing the bone-graft material between the transverse
processes. In comparison, interbody fusion provides a greater surface
area of bone contact and produces a more favorable fusion compared to
the posterolateral method.23 Addition of instrumentation to
interbody fusion increases success rates to nearly 100%. Using cages in
interbody fusions provides more immediate stability during bone graft
Imaging of postoperative spine fusion
imaging plays an important role in the assessment of fusion and bone
formation. It is also helpful to detect instrument failure and other
suspected complications. It is necessary to compare current images with
previous studies to identify any subtle changes and disease progression.
Evaluation of the postoperative spine usually begins with
conventional radiographs in AP and lateral projections. It usually takes
6 to 9 months for a solid bone fusion to be established
radiographically. Conventional radiographs are capable of detecting
instrument failure, infection and other causes of failed fusion (Figures
1 through 7). Additional views in lateral flexion and extension are
sometimes used to evaluate the presence of motion and the integrity of
the fusion.17 Ray defined 6 criteria to radiographically verify a solid fusion:
- no motion or <3 degrees of intersegment position change on lateral flexion and extension views,
- lack of a lucent area around the implant,
- minimal loss of disc height,
- no fracture of the instrument, bone graft or vertebrae,
- no sclerotic change in the graft or adjacent vertebrae, and
- visible osseous formation in or around the cage.26
Sometimes radiographs are nondiagnostic and, based on clinical
suspicion and the type of the applied instrument, additional imaging
with other modalities may be applied. Currently, computed tomography
(CT) with multiplanar reconstruction (MPR) is considered the modality of
choice for imaging bony detail and assessing osseous formation and
hardware position despite artifact formation. CT is also useful in
demonstrating the spinal canal and its alignment and is capable of
detecting infection and pseudarthrosis12 (Figure 8). Cook et
al. evaluated the extent of bony fusion in an animal model and reported
that CT was capable of detecting fusion in 83% of cases, but coincidence
of CT image results with histological findings was present in only 14%
of specimens and CT significantly overestimated the extent of fusion.27
another study, Heithoff et al. compared CT images with reoperation
findings in symptomatic pseudarthrosis patients and reported that CT was
not reliable in identifying these patients.28 Artifacts are
the primary disadvantage of CT although artifacts are seen less commonly
with titanium implants compared with stainless steel because of the
lower beam attenuation coefficient of titanium implants.11
resonance imaging (MRI) has been used increasingly in recent years
since introduction of titanium-based implants with reduced artifact
compared to formerly used stainless-steel devices. These artifacts could
be decreased even more by changing imaging parameters such as reducing
echo time, increasing bandwidth and decreasing voxel size. Aligning the
implant along the axis of the magnetic field also reduces artifact
although it is often not completely achievable due to the
multidirectional configuration of most hardware. Spin echo sequences are
less vulnerable to magnetic susceptibility artifact and give better
quality images compared with gradient echo sequences. MRI is useful in
detecting infection (Figure 9) and assessing recurrent tumor. MRI is the
modality of choice in assessing intraspinal contents. Myelography
(Figure 6) is an alternative when MRI is contraindicated or is
nondiagnostic because of artifact.
Radionuclide scans are mainly used to detect infection.29 Early
stages of pseudarthrosis can also be assessed by increased radionuclide
uptake, although this may appear indistinguishable from remodeling.
Sonography is used to detect fluid collections and abscesses in the
postoperative spinal fusion.17
Spinal fusion instrumentation and complications
complications of spinal surgery vary based on the site of surgery,
surgical approach, underlying disease, applied instrumentation, surgeon
skill and other clinical factors. Besides the common complications
associated with spinal fusion procedures; there are some additional
complications based on site, procedure and type of instrumentation.
fracture (Figures 1 through 4) occurs most commonly as a result of
metal fatigue from the repeated stress in spinal movements. The
fractured appliance may not be displaced, making its detection
difficult. A dislodged or fractured appliance does not necessarily
indicate instability or clinical failure of the fusion but is most
frequently associated with motion, instability and pseudarthrosis.30 The
prominence of the instruments can cause chronic tissue irritation
leading to pain, bursa formation and even pressure sores with tissue
necrosis. This is an occasional indication for hardware removal.30 There
is also a risk of bone resorption around screws or under the implants
that are in direct contact with the bone (Figures 5 and 7). This will
cause the bones to weaken and predisposes them to fracture and it leads
to hardware failure. A loose appliance repeatedly moves and produces
bone resorption or erosion. Fused bones are less mobile, which makes the
bones vulnerable to fractures above or below the implants if subjected
to trauma (Figure 10). Unsuccessful fusion may have other causes such as
development of facet arthritis (Figure 6C) or disc disease above or
below the fusion level.3 Premature degenerative changes at
the disc levels above and below the fused segment can occur due to the
reduced number of mobile segments. This complication is reported in
10.2% of patients with posterior fusion and instrumentation.31
the cervical spine, potential complications of the posterior approach
are mainly neurological and include dural, nerve root or cord injury.
The anterior approach is associated with risks of injuring the main
vascular structures (carotid and vertebral arteries, jugular vein),
causing recurrent damage to the laryngeal nerve or soft tissue, such as
the esophagus, trachea or lungs (Figure 11). Postoperative complications
include hematoma, pseudomeningocele, infection and instability as a
result of laminectomy or incorrect hardware placement. Wires and cables
are used as a primary or supplementary instrument in stabilizing the
posterior cervical spine (Figure 6). Complications include breakage and
slippage of skeletal attachments. Cables (e.g. Songer cable) are much
more resistant to fatigue fracture and failure. Plates are used for the
anterior and posterior cervical spine. They are also prone to fracture
and failure (Figure 2). Screws may break or dislodge or may be misplaced
and impinge the cord or nerve root when placed posteriorly.17 In
a retrospective study of 1015 patients who underwent anterior cervical
discectomy for cervical radiculopathy and/or myelopathy due to
degenerative disc disease and/ or cervical spondylosis, Fountas et al.
reported the most common postoperative complications to be dysphagia
(9.5%), postoperative hematoma (5.6%) and recurrent laryngeal nerve
Screws should approach the opposite
cortex but should not breach it. In anterior-plate screw fixation, the
screws may back out and impinge soft tissue (e.g. great vessels, trachea
and esophagus) or overpenetrate the posterior cortex and impinge on the
cord. These complications can be prevented by using a cervical-spine
locking plate with screw caps (e.g. Morscher). This device prevents the
screws from backing out and provides increased holding power removing
the need for transcortical purchase with the risk of over penetration.
of the fused segment causes additional stress on adjacent levels of the
vertebral column. Ossification of anterior longitudinal ligament and
facet disease are common complications of anterior plate and screw
fixation (Figure 6).9,17
In anterior fusion of the
thoracic or lumbosacral spine, the devices should be laterally located
in the anterior column. Neurologic deterioration is the most-feared
complication of surgery and may be caused by hardware movement or
malpositioned screws (Figures 12 through 15). Incorrect use and later
dislodgment or fracture of instruments may also contribute to
complications such as instability, fusion failure or pain—with possible
resultant neurologic damage. Postoperative neurologic complication due
to lumbar instrumentation has been reported in 3% to 11% of patients
undergoing spinal procedures. Postoperative neurologic injuries can also
be due to cord edema or hematoma and are often self-limited.30 Bone graft material can migrate or hypertrophy resulting in impingement on the spinal canal or neural foramen.17,33,34 Radiographs
often show the failed instrument that may have caused neurologic
deterioration. Rare but life-threatening complications such as delayed
aortic rupture due to instrumentation have also been reported.35
is reported in 1% to 2.4% of patients undergoing lumbar
instrumentation. Infection leads to bone destruction and resorption
around the implant. On imaging, a lucent area around an implant implies a
loose appliance and potential infection (Figures 8 through 9).
CT-guided aspiration can be used to isolate the microorganism. Unlike
superficial infections that can even be diagnosed clinically, deeper
infections such as discitis are sometimes more challenging.
Osteomyelitis in adjacent vertebrae, disc collapse and destruction
indicate discitis radiographically. Radionuclide-labeled white blood
cell scintigraphy and MRI can be helpful to detect infection in early
Failed fusion with the development of
pseudarthrosis is a common end result of implant failure or improper
surgical technique (Figures 5 and 6). Its incidence in lumbar
instrumentation is reported in 5% to 32% of patients. CT is the optimal
method for evaluating a bone graft. A failed fusion with pseudarthrosis
formation results in continued stress on the implant, and hardware
fracture is inevitable. Suda et al. described radiological risk factors
for pseudarthrosis and/or instrument breakage after PLF with pedicle
screws to be related to preserved disc height and the presence of
The risk of pseudarthrosis
escalates with increased patient age and smoking. Pseudarthrosis is more
common using external braces than internal fixation. The rate of
pseudarthrosis is decreased with meticulous surgical technique,
including careful facet excision and adequate graft placement. Repair is
necessary if the patient presents with implant failure or pain. In
asymptomatic patients, intervention may be deferred and the patient’s
condition should be followed.38,39
face new challenges as the number of, and indications for, spinal
surgery grow. Adequate understanding of various surgical techniques and
instruments, coupled with improved awareness of the possible
complications, are vital when interpreting postoperative studies.
Radiologists should carefully compare these critical points with
baseline studies to develop a targeted assessment of grafts and
hardware. With more familiarity of postoperative spinal images obtained
on various modalities and the knowledge of how certain situations (e.g.
surgical technique and hardware) contribute to failed back surgery
syndrome, radiologists can quickly arrive at a precise diagnosis,
permitting appropriate treatment and minimizing patient suffering.
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- Deyo RA, Gray DT, Kreuter W, et al. United States trends in lumbar fusion surgery for degenerative conditions. Spine. 2005;30:1441-1445; discussion 1446-1447.
- Tehranzadeh J, Ton JD, Rosen CD. Advances in spinal fusion. Semin Ultrasound CT MR. 2005;26:103-113.
- Rosales-Olivares LM, Miramontes-Martínez V,Alpízar-Aguirre A, et al. Failed back surgery syndrome. Cir Cir. 2007;75:37-41.
- Hadra BE. The classic: Wiring of the vertebrae as a means of immobilization in fracture and Pott’s disease. Berthold E. Hadra. Med Times and Register. 1891;Vol 22. Clin Orthop Relat Res. 1975; 112:4-8.
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- Hibbs RA: A report of 59 cases of scoliosis treated by fusion operation. By Russel A. Hibbs, 1924. Clin Orthop Relat Res. 1988;229:4-19.
- Mulholland RC. The myth of lumbar instability: The importance of abnormal loading as a cause of low-back pain. Eur Spine J. 2008;17:619-625.
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- Berquist TH. Imaging of the postoperative spine. Radiol Clin North Am. 2006;44:407-418.
- Bartels RH, Donk RD, Feuth T. Subsidence of standalone cervical carbon fiber cages. Neurosurgery. 2006;58:502–508.
- Gercek E, Arlet V, Delisle J, Marchesi D. Subsidence of stand-alone cervical cages in anterior interbody fusion: Warning. Eur Spine J. 2003; 12:513–516.
Barsa P, Suchomel P. Factors affecting sagittal malalignment due to
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Riaz S, Fox R, Lavoie MV, Mahood JK. Vertebral body reconstruction for
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- Oskouian RJ Jr, Johnson JP. Vascular complications in anterior thoracolumbar spinal reconstruction.J Neurosurg. 2002;96(1 Suppl):1-5.
- McAfee PC. Interbody fusion cages in reconstructive operation on the spine. J Bone Joint Surg [AM].1999;81-A:859-878.
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- Blumenthal SL, Ohnmeiss DD; NASS. Intervertebral cages for degenerative spinal diseases. Spine J. 2003; 3:301-309.
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- Ray CD. Threaded fusion cages for lumbar inter-body fusions: An economic comparison with 360 degrees fusions. Spine. 1997;22:681–685.
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