A Lambotte

A Rudisch

AR Vaccaro

AS Malik

CA Petersilge

CB Henk

Gabert Heidrich

Gottfried Buchhorn

H Trouillier

JC Wang

JF Debatin

JW Brantigan

JW Goethem Van

M Thomsen

MJ Rauzzino

MP Staiger

MS Znamenskii

O Diedrich

O Ortiz

PC McAfee

R Rupp

T Ernstberger

TA Walde

Thorsten Ernstberger

TR Kuklo

U Schreiner

VV Troitskii

English

PubMed

Springer - Publisher Connector

Artifacts in spine magnetic resonance imaging due to different intervertebral test spacers: an in vitro evaluation of magnesium versus titanium and carbon-fiber-reinforced polymers as biomaterials

DIAGNOSTIC NEURORADIOLOGY
Artifacts in spine magnetic resonance imaging due
to different intervertebral test spacers: an in vitro evaluation
of magnesium versus titanium and carbon-fiber-reinforced
polymers as biomaterials
Thorsten Ernstberger & Gottfried Buchhorn &
Gabert Heidrich
Received: 19 April 2009 /Accepted: 12 May 2009 /Published online: 26 May 2009
# Springer-Verlag 2009
Abstract
Introduction Intervertebral spacers are made of different
materials, which can affect the postfusion magnetic imaging
(MRI) scans. Susceptibility artifacts especially for metallic
implants can decrease the image quality. This study aimed to
determine whether magnesium as a lightweight and biocom-
patible metal is suitable as a biomaterial for spinal implants
based on its MRI artifacting behavior.
Materials and methods To compare artifacting behaviors,
we implanted into one porcine cadaveric spine different
test spacers made of magnesium, titanium, and carbon-fiber-
reinforced polymers (CFRP). All test spacers were scanned
using two T1-TSE MRI sequences. The artifact dimensions
were traced on all scans and statistically analyzed.
Results The total artifact volume and median artifact area
of the titanium spacers were statistically significantly larger
than magnesium spacers (p<0.001), while magnesium
and CFRP spacers produced almost identical artifacting
behaviors (p>0.05).
Conclusion Our results suggest that spinal implants made
with magnesium alloys will behave more like CFRP devices
in MRI scans. Given its osseoconductive potential as a metal,
implant alloys made with magnesium would combine the
advantages to the two principal spacer materials currently
used but without their limitations, at least in terms of MRI
artifacting.
Keywords Magnesium alloys . Innovative biomaterials .
Interbody test implants . MRI artifacting
Introduction
Spinal fusion devices such as implantable interbody spacers
are well-established and routinely used by spine surgeons to
keep adjacent vertebrae spaced apart while bone ingrowth
and fusion take place. Such spacers also provide weight-
bearing support between adjacent vertebrae. In this context,
titanium alloys as well as nonmetal materials like carbon-
fiber-reinforced polymers (CFRP) are commonly used [1, 3,
8, 11, 14, 17, 21]. These biomaterials have enjoyed clinical
success and rapid widespread use by improving patient
outcomes.
However, these materials have clinical and radiological
limitations. Titanium is an excellently bioinert material that
exhibits high biocompatibility. Titanium spacers produce
good bone ingrowth without bone grafting. However, in
magnetic resonance imaging (MRI) studies, titanium-based
implants tend to cause distortion of the magnetic field which
may obscure normal regional anatomy. These properties
pose difficulties in the postoperative MRI follow-up and
evaluation of the fusion process due to the artifacting it
causes [4, 6, 7, 12, 15].
Neuroradiology (2008) 51:525–529
DOI 10.1007/s00234-009-0537-4
T. Ernstberger (*)
Center for Spinal Surgery, Klinikum Bad Bramstedt,
Oskar-Alexander-Straße 26,
24576 Bad Bramstedt, Germany
e-mail: ernstberger@klinikumbb.de
G. Buchhorn
Biomaterial Laboratory, Department of Orthopedic Surgery,
University of Gottingen,
Robert-Koch-Straße 40,
37075 Göttingen, Germany
G. Heidrich
Department of Diagnostic Radiology, University of Gottingen,
Robert-Koch-Straße 40,
37075 Göttingen, Germany
The other principal material used for spacers consists of
CFRP. Spacers made of this nonmetallic biomaterial are not
associated with the postoperative diagnostic problems of
titanium because carbon produces a very low rate of artifact
reactions which allows easier evaluation of the fusion process
byMRI [4]. Carbon’s modulus of elasticity affords good load
bearing with sufficient hardness. But unlike titanium, carbon
spacers undergo poor osteointegration because a soft tissue
interface develops around the material surface that prevents
direct ingrowth of bone. As a result, carbon spacers have to
be filled with bone allografts to achieve long-term stability
[1, 4]. CFRP implants have therefore been reviewed very
critically in the literature [17].
Surgeons, over a century ago, recognized the potential
of the lightweight metal magnesium as a biocompatible,
osteoconductive, and degradable implant material [9]. In
1907, Lambotte was the first to introduce magnesium-based
orthopedic devices; using a pure magnesium plate, he secured
a bone fracture of the lower leg with gold-plated nails [9]. A
half a century later, magnesium-based metals were reported
to have osteoconductive bioactivity and produce a more
rapid formation of hard callus when used to support fractures
in humans [20, 27]. The large amount of evidence supporting
the clinical advantages of magnesium has been summarized
in a recent review paper [18]. None of the studies to date
have yet investigated the diagnostic behavior of magnesium
in MRI. This situation motivated us to determine whether
magnesium is a suitable biomaterial for spinal implants by
studying its MRI artifacting behavior.
Material and methods
To evaluate the behavior of spacers made with a magne-
sium alloy, we compared their artifacting in diagnostic MRI
scans with that of spacers made of a conventional titanium
alloy and of CFRP. We consecutively implanted three
spacers made of each of the three biomaterials dimensioned
in small, medium, and large sizes in one cadaveric spine of
a Gottingen minipig (Figs. 1a–c and 2). The three spacers in
group I were made of a magnesium–aluminum–manganese
alloy (MgAlMn50), the three in group II of a titanium–
aluminum–vanadium alloy (TiAl6V4), and those in group
III of a CFRP.
Table 1 presents the implant characteristics. A cylinder
was chosen as for spacer shape because cylinders have
demonstrated lowest rate of MRI artifacting behavior [4].
Fig. 1 a–c Cylindrical EST implants. aMagnesium (implant group I).
b Titanium (implant group II). c CFRP (implant group III)
Fig. 2 Cadaveric porcine spine model with an implanted medium
titanium test cylinder
526 Neuroradiology (2008) 51:525–529
The spacer sizes—small, medium, and large—were dimen-
sioned the same for each group (height in centimeter×base
area in square centimeter); and their implant volume (IV) in
cubic centimeter and cross-sectional area (CSA) in square
centimeter were calculated for each size (Table 1). The spacer
sizes were dimensioned as listed after a Newman–Keuls
multiple-comparison analysis showing that the selected sizes
would produce significantly different artifacting behaviors
(p<0.001). Thus, a total of nine individual spacers were
implanted, scanned by MRI, and evaluated for their artifact-
ing behavior on the scans.
Spacer implantation
For each serial MRI study, the cylindrical implant was
placed exactly between two adjacent vertebrae of the
cadaveric porcine spine. The spine with implant was then
completely packed in a soft tissue mass and placed in a plastic
container [5]. To create comparable trial conditions, mark-
ings were drawn on the container wall to demarcate the
vertebrae and implant positions. These demarcations were
used to define the median artifact area (MAA). The container
with the spine implanted with each spacer was examined by
serial MRI.
Magnetic resonance imaging
MRIwas performedwith a 1.5-TMRI (MagnetomSymphony,
Siemens AG Medical Solutions, Erlangen, Germany). The
T1w-TSE sequences were used to acquire a slice thickness of
3 mm (Fig. 3a–c) which included a first sequence (TR 600;
TE 14; flip angle 15; band width 150) and a second sequence
(TR 2,260, TE 14, flip angle 15, band width 150). We selected
a matrix of 512×512 pixels combined with a field of view of
500 mm. The T1w-TSE sequence has been established to
produce best imaging results for implants and the least
amount of intrinsic artifacting [4, 7, 10, 12, 14, 16, 25].
Using a current version of DICOM reader software, one
author (TE) measured the artifact area on the scan of each
of the nine implants six times, i.e., a total of 54 individual
tracings were recorded and analyzed. The measurements
started with the slice with the first artifacting reaction and
ended with the last slice exhibiting an artifact reaction.
Corresponding to the respective implant CSA, the middle
slice of all slices exhibiting artifact reactions was defined as
the MAA for each implant. To calculate the total artifact
volume (TAV) for each spacer, all artifact areas measured for
that spacer were added and multiplied by the slice thickness
of 3 mm according to the multisection slice technique
described by Debatin [2]. The ratio of CSA to MAA and the
ratio of IV to TAV were calculated and presented in tables
(Table 1).
Statistical analysis
Newman–Keuls multiple comparisons were used to calcu-
late intragroup differences in TAV and MAA (Table 2).
t test correlations were performed to determine any
intergroup differences regarding the implant materials
Table 1 Spacer dimensions.
Sizes for
all groups
Dimensions height×
base area, cm×cm2
Cross-sectional
area (CSA), cm2
Implant
volume (IV),
cm³
Small 1.5×0.78 1.5 1.2
Medium 2.0×1.13 2.4 2.3
Large 2.5×1.54 3.8 3.5
Fig. 3 a–c Median MRI artifact
range depicted in a selection of
three large test implants
Neuroradiology (2008) 51:525–529 527
(Table 2). A p value<0.05 indicated a significant difference
between the means of any two groups.
Results
Table 1 presents the spacer dimensions. Table 2 shows the
intragroup comparisons of target variables. Table 3 lists the
results of the intergroup t test correlations between TAVand
MAA in relation to spacer material. Mean artifacting
behavior increased with spacer size. When magnesium was
compared with titanium, there were significant differences
in both MAA and TAV. When magnesium was compared
with carbon, the differences were not significant. In fact,
magnesium produces an artifacting behavior very similar to
that of CFRP.
Discussion
Spinal surgeons have not stopped searching for the
optimum spacer material that combines high biocompati-
bility with artifact-free MRI imaging behavior in the
implant environment. This study was conducted to deter-
mine whether cylindrical spacers made of the biomaterial
magnesium are suitable as spinal implants by comparing
their MRI artifacting with that of identically dimensioned
spacers made of a titanium alloy and a carbon-fiber-
reinforced polymer.
In radiological spinal diagnostics, MRI is highly effec-
tive for clarifying postfusion questions regarding osseus
and soft tissue structures in relation to implant position. A
comparative in vitro study shows that MRI has a higher
sensitivity than CT in detecting osseus changes in the
implant’s direct surroundings [24]. Moreover, MRI is well
suited to demonstrate myelopathies, inflammatory and
infectious processes, and any neurodegenerative changes.
The MRI imaging behavior of metallic spinal implants is
obviously well documented in the literature [6, 10, 13–16,
19, 22, 23, 25]. However, the aims of the published studies
differed in that most focused on determining sequence-
related artifact size. In a comparative cadaveric artifact
studies, Wang et al. [26] described the MRI behavior of
intervertebral spacers made of titanium and tantalum. The
authors concluded that T1- and T2-weighted spin echo
images were suitable for both metals to visualize the neural
structures of the spine with comparable amounts of artifact.
The artifact rate of the titanium spacer as well as the
tantalum spacer was primarily limited to the implant’s
direct surroundings and anatomic neighboring structures
were clearly distinguishable.
In a phantom study by Rudisch et al. [15], the relevance
of metallic artifacts and implant-related characteristics, such
as implant material and position, was demonstrated in
addition to effects caused by the selected MRI sequence. In
materials with a higher magnetizability like titanium alloys,
implant shape additionally has an effect on the range of
MRI artifacts [5].
The results of this comparative study showed that
implant material and volume both affected the MRI
Table 2 Intragroup comparison of target variables.
Spacer material Size MAAa, cm2 (mean±SD) Ratio CSA to MAA TAVa, cm2 (mean±SD) Ratio IV to TAV
Group I MgAlMn50 (n=3) Small 1.91±0.04 1:1.3 1.83±0.09 1:1.5
Medium 3.26±0.06 1:1.4 4.17±0.09 1:1.8
Large 4.06±0.07 1:1.2 5.08±0.15 1:1.3
Group II TiAl6V4 (n=3) Small 3.26±0.04 1:2.2 5.71±0.09 1:4.8
Medium 4.61±0.23 1:1.9 9.32±0.10 1:4.1
Large 5.54±0.04 1:1.6 10.84±0.13 1:2.9
Group III CFRP (n=3) Small 1.89±0.07 1:1.3 1.81±0.07 1:1.5
Medium 3.18±0.06 1:1.3 4.09±0.11 1:1.7
Large 4.06±0.13 1:1.2 5.08±0.13 1:1.3
CSA cross-sectional area, MAA median artifact area, IV implant volume, TAV total artifact volume, SD standard deviation
a Newman–Keuls multiple-comparison analysis p<0.001
Table 3 Intergroup comparisons of artifacting behavior by t test
correlation.
Spacer material Size MAA P valuea TAV P valuea
Group I vs. group II Small ≤0.001 ≤0.001
Medium ≤0.001 ≤0.001
Large ≤0.001 ≤0.001
Group I vs. group III Small 0.59 0.61
Medium 0.09 0.26
Large 1.0 0.96
MAA median artifact area, TAV total artifact volume
a Significance level p<0.05
528 Neuroradiology (2008) 51:525–529
artifacting behavior of our cylindrical test spacers. It was
also noted that the smaller the implant size, the smaller was
the range of susceptibility artifacts produced. The ratios
calculated in Table 2 prove that the magnesium metal alloy
exhibited behavior artifacting that was more like a nonmetal.
In this context, the magnesium test spacer produced
significantly less artifact compared to the titanium implant.
Given its osseoconductive potential as a metal [9, 18],
implant alloys made with magnesium would combine the
advantages to the two principal spacer materials currently
used, but without their limitations, at least in terms of MRI
artifacting
Acknowledgements The authors thank Dr. Mark Riner of MedTech
Composites GmbH, Switzerland, the Peter BrehmCompany,Weisendorf,
Germany, and the Material Science at the Technical University of
Hannover, Germany for producing the respective test implants.
Conflict of interest statement We declare that we have no conflict
of interest.
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Magnetic resonance imaging-cardiac ejection fraction measurements. Phantom study comparing four different methods.

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MR parameters for imaging titanium spinal instrumentation.

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Posterior lumbar interbody fusion (PLIF) with cages and local bone graft in the treatment of spinal stenosis.

Postoperative magnetic resonance imaging with titanium implants of the thoracic and lumbar spine.

Review article: MRI of the postoperative lumbar spine.

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Single-stage treatment of pyogenic spinal infection with titanium mesh cages.

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Artifacts in spine magnetic resonance imaging due to different intervertebral test spacers: an in vitro evaluation of magnesium versus titanium and carbon-fiber-reinforced polymers as biomaterials

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