Whitmore et al., J Spine Neurosurg 2013, 2:5
http://dx.doi.org/10.4172/2325-9701.1000120
Journal of Spine &
Neurosurgery
Case Report
a SciTechnol journal
Advanced Radiographic
Imaging Results in Patients
with Trabecular Metal™ Spinal
Implants
Robert G Whitmore1, Dara Bakar1*, Paul Saiz2, Ira M
Goldstein3, Regina Konz4 and William C Welch1
especially when used to assess the extent of bone fusion, may be
compromised by metal spinal instrumentation due to the inherent
properties and construction of these implants. Compared to stainless
steel, titanium has a far more favorable imaging profile and highquality MRI studies may be obtained with titanium instrumentation
[9,10]. Although tantalum-based spinal implants have been used
since 2002, the imaging properties of these devices have not been
fully studied. The purpose of this study is to review the specific
characteristics of commercially available TM spine implants (Zimmer
Trabecular Metal Technology, Parsippany, NJ, USA) and compare
their imaging profiles with titanium spinal implants.
Case Report
Abstract
Trabecular Metal implants (TM) mimic desirable bone qualities
of porosity and elasticity while maintaining excellent compressive
strength. Despite these biomechanical advantages, metal spinal
implants often cause metal artifact on postoperative computed
tomography (CT) or magnetic resonance imaging (MRI). Although
tantalum-based spinal implants have been in use since 2002, the
imaging properties of these devices have not been fully studied.
The purpose of this study is to review the specific characteristics
of commercially available tantalum implants (TM) (Zimmer
Trabecular Metal Technology, Parsippany, NJ, USA) and compare
their imaging profiles with titanium spinal implants.
Keywords
Tantalum; Trabecular Metal implant; Spinal fusion; Magnetic
resonance imaging; Computed tomography
Porous tantalum Trabecular Metal implants, designed for use
in the lumbar, thoracic, and cervical spine, were used in this study
(TM-S, VBR-21, Zimmer Trabecular Metal Technology, Parsippany,
NJ, USA) (Figure 1a). Postoperative images from specific cases
using TM were reviewed and compared to the imaging profile of a
titanium expandable spinal implant (Synex expandable vertebral
body replacement, Synthes, Inc, West Chester, Pennsylvania, USA)
(Figure 1b).
MRI images were acquired on a 1.5 Tesla MRI machine with
routine clinical settings for T1-weighted and T2-weighted images
[11,12]. CT images were acquired using 512 matrix scanner with 2-3
mm axial images parallel to the disc space or TM device. Sagittal and
coronal reconstructions were performed with similar thickness.
Case 1
Introduction
Tantalum is a biologically-inert, dense elemental metal which
has been shown to be highly stable as an orthopedic implant [13]. This metal has been developed for use as a Trabecular Metal
implant (TM). The configuration of tantalum is processed in TM
so as to achieve increased porosity, allowing for osteoblastic cellular
infiltration and direct bone conduction [1,4,5]. The elasticity of
TM and micro-architecture is similar to cancellous bone, with high
fatigue strength and biochemical properties that allow it to bend prior
to fracturing [6]. While these properties may facilitate bone growth
and arthrodesis, the flexibility of TM also allows load sharing with
surrounding bone. Finally, TM has a high coefficient of friction,
providing excellent initial stability and fixation in bone [4]. These
properties are felt to be highly beneficial in orthopedic devices and
Trabecular Metal implants have been commercially available for
vertebral body reconstruction since 2002 and for cervical interbody
fusion since 2011.
45 year old female with C7 radiculopathy, underwent anterior
cervical discectomy and fusion at C6/7 with placement of TM
interbody spacer. Figure 2 demonstrates plain radiographs showing
bone remodeling and growth around the implant. The patient
developed new onset C8 symptoms at 1 year postoperatively,
prompting an MRI. Figure 3 demonstrates limited artifact created
by a TM device on routine clinical MRI, with excellent resolution of
neural structures.
Case 2
71 year old male with multiple myeloma, erosion and destruction
a
b
Metal implants are known to cause significant artifact and image
distortion, particularly on image modalities such as CT or MRI, in
which multiplanar data is combined [7,8]. Postoperative imaging,
*Corresponding author: Dara Bakar, Department of Neurosurgery,
Hospital of the University of Pennsylvania, 3400 Spruce Street, Silverstein
3, Philadelphia, PA 19104, USA, Tel: 410-459-0560; Fax: 215-829-6645;
E-mail: [email protected]
Received: May 16, 2013 Accepted: September 07, 2013 Published:
Spetember 11, 2013
International Publisher of Science,
Technology and Medicine
Figure 1: a) Trabecular Metal implant (VBR-21) for use in lumbar spine. b)
Synex expandable titanium cage.
All articles published in Journal of Spine & Neurosurgery are the property of SciTechnol, and is protected by copyright laws.
Copyright © 2013, SciTechnol, All Rights Reserved.
Citation: Whitmore RG, Bakar D, Saiz P, Goldstein IM, Konz R, et al. (2013) Advanced Radiographic Imaging Results in Patients with Trabecular MetalTM
Spinal Implants. J Spine Neurosurg 2:5.
doi:http://dx.doi.org/10.4172/2325-9701.1000120
a)
b)
Figure 2: 45 year old with C7 radiculopathy; treated with C6-7 anterior
cervical discectomy and fusion (ACDF). a) Trabecular Metal implant at
2 weeks post-op shows a gap between the device and the endplate. b)
The same Trabecular Metal implant at 3 months post-op is shown with no
evidence of interface gap.
tendency to interact with an applied magnetic field, of metal differs
from that of human tissue, image artifact is produced. Ferromagnetic
metals, such as stainless steel, produce local magnetic fields during
MRI imaging resulting in severe metallic artifact and with the
theoretical possibility of movement of the steel implant [7]. For this
reason, non-ferromagnetic metals such as titanium are now preferred
since their magnetic susceptibility is less than steel. However,
titanium still produces artifact in MRI studies due to local electric
current induced by the alternating magnetic fields in the MRI scanner
[15]. Tantalum has a lower magnetic susceptibility than titanium and
therefore would be expected to cause less MRI artifact [16].
b)
a)
of L5 vertebral body, with posterior retropulsion of bone fragments
causing cauda equine syndrome. This patient was treated with a
posterior decompression and stabilization from L3 to S1, followed by
an anterior corpectomy, placement of TM device, and L4 to S1 fusion.
Figures 4a-4e demonstrates CT imaging characteristics of L5 TM
device with adequate bone resolution in the immediate postoperative
setting. The most severe artifact occurs on the axial images.
Case 3
25 year old female with T11/12 compressive lesion, presenting
with severe back pain and incontinence. She underwent T11/12
corpectomy with placement of TM implant, followed by posterior
decompression and stabilization from T9 to L2. Figure 5a and 5b
demonstrates an axial T2-weighted MRI and a sagittal T2-weighted
MRI of the TM device, both showing very good resolution of the
neural structures. These images were obtained six days following
surgery.
Figure 3: 1 year following the ACDF, new onset C8 symptoms prompted a
magnetic resonance imaging (MRI) study. a) Axial T1 images. b) sagittal T2
images both show excellent resolution of neural structures.
a)
b)
c)
Case 4
67 year old female with metastatic colorectal carcinoma to L3 who
presented with unrelenting back pain and epidural tumor extension.
She underwent L3 corpectomy, placement of titanium expandable
vertebral spacer with a posterior L1 to L5 fusion. Figures 6a-6c shows
CT imaging in this patient, demonstrating appreciatively less artifact
than in the CT images of the TM device (Case 2). Conversely, Figures
6d-6g depicts extensive image distortion on MRI in this patient, and
the extent of artifact is clearly more than in MR images of the TM
devices in Cases 1 and 3.
d)
e)
Discussion
Trabecular Metal implants have several advantages for application
in spinal fusion including osteoconductivity, elasticity similar
to bone, stability, and strength. There has been limited literature
regarding the postoperative imaging characteristics of these implants,
although tantalum Trabecular Metal implants have been in use since
2002 [9,10,13,14].
All metal spinal implants will distort MRI imaging and potentially
limit the interpretation of such imaging due to the magnetic
susceptibility of the material. Since the magnetic susceptibility or
Volume 2 • Issue 5 • 1000120
Figure 4: 71 year male with multiple myeloma at L5 who presented with
cauda equina syndrome; treated with anterior-posterior fusion using a
Trabecular Metal implant. a) Sagittal computed tomography (CT) image
after decompression with posterior stabilization. L5 shows extensive
involvement. b) X-ray 6 weeks post op. c) Sagittal CT of the TM implant. d)
Coronal CT of the TM implant. e) Axial CT of the TM implant.
• Page 2 of 4 •
Citation: Whitmore RG, Bakar D, Saiz P, Goldstein IM, Konz R, et al. (2013) Advanced Radiographic Imaging Results in Patients with Trabecular MetalTM
Spinal Implants. J Spine Neurosurg 2:5.
doi:http://dx.doi.org/10.4172/2325-9701.1000120
that TM spinal implants were associated with less MRI artifact than
titanium spacers, and therefore, improved visualization of neural
structures [14]. Our results are consistent with this finding that TM
devices allows better MR imaging compared to titanium. However,
other studies have reported that the postoperative MRI of Trabecular
Metal implants does not differ greatly from those with titanium,
despite the lower magnetic susceptibility of tantalum [10,19].
b)
a)
Figure 5: 25 year old female with T11/12 compressive lesion, treated with
T11/12 corpectomy with placement of TM device, followed by posterior
decompression and stabilization from T9 to L2. a) Axial T2-weighted MRI in
immediate postoperative period. b) Sagittal T2-weighted MRI.
a)
b)
d)
e)
c)
f)
g)
Figure 6: 67 year old female with colorectal metastasis to L3, treated with
anterior-posterior fusion using an expandable titanium implant. a) Sagittal
CT of titanium implant in immediate postoperative period. b) Coronal CT
of titanium implant. c) Axial CT of titanium implant. d) Sagittal T1-weighted
MRI of titanium implant in the immediate postoperative period. e) Sagittal
T2-weighted MRI of titanium implant. f) Axial T2-weighted MRI of titanium
implant. g) Axial T1-weighted MRI of titanium implant.
As evidenced by the cases presented in this study, the MR images
with TM implants have excellent resolution, particularly with T1weighted studies. Other studies have reported that optimization of
MRI parameters, such as use of fast spin echo techniques, minimizing
the echo time and decreasing the field of view, may lessen the degree
of artifact [11,12,17,18]. T2-weighted MR images exhibit a greater
degree of artifact than T1-weighted images, but the results are still
sufficient for interpretation of neural element compromise. A recent
randomized trial comparing cervical TM device to allograft reported
that MRI of the TM device was successful in determining whether
decompression of neural structures had been achieved, but not for
radiographic assessment of bone fusion [13]. Levi et al. demonstrated
Volume 2 • Issue 5 • 1000120
Computed tomography is also an important tool for postoperative
spinal evaluation. X-rays will be absorbed by metallic implants,
creating missing data that results in severe artifact after the images
are reconstructed. The degree of image distortion depends on the
degree of x-ray attenuation or the attenuation coefficient. Metals such
as titanium have a lower attenuation coefficient, thus resulting in less
image distortion, than stainless steel [20]. CT imaging is particularly
useful for determining the extent of bone fusion or detecting a
pseudoarthrosis. CT is superior to x-rays for showing lucencies
surrounding spinal instrumentation, but lucencies are more visible
with nonmetallic implants compared to metallic devices [21,22].
However, recent work by our group indicates that the extent of fusion
may be under represented radiographically in Trabecular Metal
implants compared to polyetheretherketone (PEEK) intervertebral
spacers when histologic bone staining is also performed [23].
Bone fusion of TM implants has also been studied in a prospective
randomized study by evaluating intervertebral range of motion
(angular & translational) and radiolucencies on plain x-ray [24].
Porous tantalum devices such as Trabecular Metal implants
facilitate host bone attachment through in growth into the pores,
thereby improving mechanical support and limiting motion [25].
Bobyn et al. showed that porous tantalum devices are effective
scaffoldings for complete incorporation with new bone by 16 weeks,
with little change after one year in a simple transcortical canine model
[4,26]. Although the host bone in-growth through Trabecular Metal
implants results in solid fusion, it is difficult to image this in-growth
with conventional imaging modalities. In our cases, CT imaging
demonstrated adequate resolution of the bone detail surrounding the
TM implant. Sagittal and coronal reconstructions from 2-3mm slices
provide the highest quality CT images. However, the resolution of
CT imaging was not fine enough to visualize actual osteointegration
of bone into the TM implant. As shown in Figure 2, plain x-rays
allow greater visualization of the TM-bone interface and is often
the primary imaging technique to evaluate both the fusion and the
TM implant position after surgery. The extent of CT image artifact
produced by TM devices was greater than titanium, especially in
the axial slices. This finding is consistent with that of other studies
in which tantalum spinal implants produced CT distortion similar
to that of stainless steel, and far greater than titanium [9,14]. In our
study, the increased artifact seen on CT imaging is more likely the
result of greater metallic density of the spinal TM implant compared
to the titanium expandable mesh cage, which has an open interior,
rather than an intrinsic characteristic of tantalum. However, more
studies are required to fully investigate this question.
Conclusion
Trabecular Metal spinal implants have improved imaging
characteristics compared to titanium on postoperative MRI, but have
slightly greater artifact on CT imaging. Bone fusion occurs through
host in-growth into the Trabecular Metal implant, although assessment
of this in-growth is difficult to visualize on CT reconstructions. The
• Page 3 of 4 •
Citation: Whitmore RG, Bakar D, Saiz P, Goldstein IM, Konz R, et al. (2013) Advanced Radiographic Imaging Results in Patients with Trabecular MetalTM
Spinal Implants. J Spine Neurosurg 2:5.
doi:http://dx.doi.org/10.4172/2325-9701.1000120
biomechanical advantages of these implants and osteointegration
when used as spacers in the intervertebral space suggest that these
implants may be of benefit for spinal fusion procedures. Additional
studies are required to further investigate the imaging properties of
these implants and optimize MRI and CT parameters to minimize
artifact and image distortion.
technique in anterior cervical fusion for degenerative disease: a prospective,
randomized, controlled study with 2-year follow-up. Eur Spine J 19: 464-473.
14.Levi AD, Choi WG, Keller PJ, Heiserman JE, Sonntag VK, et al. (1998) The
radiographic and imaging characteristics of porous tantalum implants within
the human cervical spine. Spine (Phila Pa 1976) 23: 1245-1250.
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Top
Author Affiliation
Department of Neurosurgery, Hospital of the University of Pennsylvania,
Philadelphia, USA
2
Las Cruces Orthopedic Associates, Las Cruces, New Mexico, USA
3
Department of Neurological Surgery, New Jersey Medical School, UMDNJ,
Newark, NJ, USA
4
Zimmer Spine, Minneapolis, MN, USA
1
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