In vitro biomechanical evaluation of four fixation techniques for distractive–flexion injury stage 3

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Upsala Journal of Medical Sciences

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In vitro biomechanical evaluation of four fixation
techniques for distractive–flexion injury stage 3 of
the cervical spine

Thomas Henriques, Bryan W. Cunningham, Paul C. Mcafee & Claes Olerud

To cite this article: Thomas Henriques, Bryan W. Cunningham, Paul C. Mcafee & Claes
Olerud (2015) In�vitro biomechanical evaluation of four fixation techniques for distractive–flexion
injury stage 3 of the cervical spine, Upsala Journal of Medical Sciences, 120:3, 198-206, DOI:
10.3109/03009734.2015.1019684

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Upsala Journal of Medical Sciences. 2015; 120: 198–206

ORIGINAL ARTICLE

In vitro biomechanical evaluation of four fixation techniques for
distractive–flexion injury stage 3 of the cervical spine

THOMAS HENRIQUES1, BRYAN W. CUNNINGHAM2, PAUL C. MCAFEE3 &
CLAES OLERUD4

1Stockholm Spine Center, Löwenströmska Hospital, Upplands Väsby, Sweden, 2Orthopaedic Spinal Research Institute,
The University of Maryland St. Joseph Medical Center, Baltimore, Maryland, USA, 3Scoliosis and Spine Center, The
University of Maryland St. Joseph Medical Center, Baltimore, Maryland, USA, and 4Department of Orthopaedics,
Uppsala University Hospital, Uppsala, Sweden

Abstract
Purpose. Anterior plate fixation has been reported to provide satisfactory results in cervical spine distractive flexion (DF)
injuries stages 1 and 2, but will result in a substantial failure rate in more unstable stage 3 and above. The aim of this
investigation was to determine the biomechanical properties of different fixation techniques in a DF-3 injury model where all
structures responsible for the posterior tension band mechanism are torn.
Methods. The multidirectional three-dimensional stiffness of the subaxial cervical spine was measured in eight cadaveric
specimens with a simulated DF-3 injury at C5–C6, stabilized with four different fixation techniques: anterior plate alone,
anterior plate combined with posterior wire, transarticular facet screws, and a pedicle screw–rod construct, respectively.
Results. The anterior plate alone did not improve stability compared to the intact spine condition, thus allowing considerable
range of motion around all three cardinal axes (p > 0.05). The anterior plate combined with posterior wire technique improved
flexion–extension stiffness (p = 0.023), but not in axial rotation and lateral bending. When the anterior plate was combined with
transarticular facet screws or with a pedicle screws–rod instrumentation, the stability improved in flexion–extension, lateral
bending, and in axial rotation (p < 0.05).
Conclusions. These findings imply that the use of anterior fixation alone is insufficient for fixation of the highly unstable
DF-3 injury. In these situations, the use of anterior fixation combined with a competent posterior tension band reconstruction
(e.g. transarticular screws or a posterior pedicle screws–rod device) improves segmental stability.

Key words: Biomechanical analysis, cervical spine, distractive–flexion injury, internal fixation, transarticular screws, pedicle
screws

Introduction

Distractive flexion stage 3 injury (DF-3) of the cervical
spine is characterized by rupture of the posterior soft
tissue elements causing instability in flexion (1). Both
facet joints are dislocated, and there is a translational
deformity not exceeding 50% (Figure 1). In order to
dislocate both facet joints all posterior structures
including the posterior annulus fibrosus and the pos-
terior longitudinal ligament (PLL) must be ruptured

(2). There still is a clinical debate on how best to
manage these injuries surgically. An anterior approach
has obvious advantages. However, when the posterior
annulus is ruptured, disk fragments may shift into the
spinal canal when the facet joints are reduced (3-9). An
anterior plate fixation is technically simple, and the
fusion is under compression, thus optimizing bone
healing. The anterior approach leaves the patient
with less pain and stiffness (10), and provides accept-
able clinical outcomes (11). Also, the patient does not

Correspondence: Professor Claes Olerud, MD PhD, Department of Orthopaedics, Uppsala University Hospital, SE751 85 Uppsala, Sweden.
Fax: +46 18 50 94 27. E-mail: claes.olerud@surgsci.uu.se

(Received 11 December 2014; accepted 10 February 2015)

ISSN 0300-9734 print/ISSN 2000-1967 online � 2015 Informa Healthcare
DOI: 10.3109/03009734.2015.1019684

http://informahealthcare.com/journal/ups
mailto:claes.olerud@surgsci.uu.se

have to be turned to the prone position with a highly
unstable injury, when placed on the operation table.
However, Koller et al. (12) reported a 31%

incidence of construct failure in patients treated
with an anterior plate alone, and Henriques et al.
(13) also reported high failure rates in patients with
DF-3 injuries and concomitant severe neurological
injury when managed with anterior reconstruction
alone. From a biomechanical standpoint, it is hypoth-
esized that anterior fixation alone is less than ideal for
a DF-3 stage injury—characterized by significant pos-
terior column disruption. When the posterior tension
band fails, the only remaining stabilizing structures
are located anteriorly. A stand-alone anterior plate
reconstruction technique will be positioned close to
these structures—providing suboptimal stability for a
posterior column injury. Angle stable screws that lock
to the plate will improve stabilization of the construct
(14); however, the addition of posterior fixation will
create a more ideal biomechanical situation (15-23).
Using an in vitro human cadaveric cervical spine
model with distractive–flexion stage 3 injury at
C5–C6, the present study aims to quantify the
multidirectional stability provided by four different
reconstruction techniques: anterior plate fixation
alone (A), and combined with one of three posterior
fixation techniques—triple wire technique (24) (AW),
transarticular facet joint screws (25) (AT), or a
posterior pedicle screw–rod device (AP).

Materials and methods

Preparation and experimental groups

Eight human cadaveric spines from C1 to T2 were
harvested from fresh cadavers (5 males and 3 females;

age range 49–82 years, mean 67 years), frozen imme-
diately in double-wrapped plastic bags and stored at
�20�C until testing. Pre-experimentation radiographs
were obtained to identify and exclude any specimen
that demonstrated spinal pathology. Prior to bio-
mechanical testing, the specimens were thawed to
room temperature and the surrounding soft tissue
and muscles were removed, with care being taken
to preserve osseous and pertinent ligamentous struc-
tures. Prior to biomechanical testing, all specimens
were sectioned at C2–C3 proximally and T1–
T2 interbody levels. The C3–C4 (proximal) and
C7–T1 (distal) motion segments were rigidly fixed
with bone screws, with care taken not to disrupt the
operative C5–C6 site and adjacent proximal (C4–C5)
and distal (C6–C7) intervertebral levels. The speci-
mens were mounted to the six-degree-of-freedom
spine simulator with transfixation pins and polyester
adhesive resin (Bondo�, 3M Corporation, St. Paul,
MN, USA) at C3–4 and C7–T1, respectively, leaving
the motion segments C4–C5, C5–C6, and C6–
C7 unconstrained (Figure 2). The specimens were
kept moistened with saline during the mechanical
testing, which never exceeded 8 hours (26).
To enable each specimen to serve as its own con-

trol, the specimens were first tested intact and then

Figure 1. Schematicdrawingofthedistractive–flexionstage3injury,
DF-3. The upper vertebra is dislocated in flexion in relation to the
lower one. Both facet joints are dislocated, but the overall anterior
displacement is less the 50%. An observation from the creation of the
injury in the specimen was that the posterior longitudinal ligament
had to be torn in order to allow dislocation of the joints.

Figure 2. The test setup in the six-degree-of-freedom spinal
simulator.

Evaluation of fixation techniques in cervical spine injury 199

tested after destabilization and reconstruction at the
C5–C6 level. The destabilization consisted of simu-
lating a distractive flexion injury stage 3 (DF-3) by
transecting the C5–C6 supraspinous ligament, inter-
spinous ligament, facet joint capsules, ligamentum
flavum, posterior longitudinal ligament, and the pos-
terior half of annulus fibrosus. The facet joints were
then manually dislocated and reduced.

Spinal constructs

The spinal specimens were reconstructed and tested
in the following order.
Anterior plate (A): Anterior plate reconstruction

with the cervical spine locking plate (CSLP, DePuy-
Synthes, Inc. Raynham, MA, USA) device and
tricortical interbody bone graft between C5 and
C6. The CSLP plate has monocortical angle stiff
locking screws. Angle stable screws improve stability
compared to plates with non-locking screws (20).
This necessitated transecting of the remaining annu-
lus fibrosus and the anterior longitudinal ligament.
The bone graft was harvested from the parietal bone
of the cranium of the same cadaver that was tested.
Anterior plate–wire (AW): The anterior plate was

left in place. A Bohlman triple wire reconstruction
using 1-mm stainless steel wire and bone graft from
the parietal bone of the cranium of the same specimen
was added (24). The wire was tightened manually
with pliers until just before the wire started to cut into
the bone substance. To standardize this procedure,
the same surgeon performed the wire application in all
specimens, trying to use the same force each time.
Anterior plate–transarticular facet screws (AT):

The posterior wire reconstruction was removed,
and 3.5-mm standard bone screws were applied as
transarticular facet joint screws bilaterally over the
C5–C6 facet joints. The screw holes were drilled with

a 2.5-mm drill bit. The screw hole was tapped with an
appropriate tap both in the proximal and distal facets.
Anterior plate–pedicle screw instrumentation (AP):

The OC Fixation System was utilized (Anatomica
AB, Göteborg, Sweden). The transarticular screws
were removed, and 4.0-mm pedicle screws were
implanted into C5 and C6 bilaterally. The screw holes
were prepared according to the recommendation by
the manufacturer by first probing the pedicles with a
blunt probe, then tapping the hole with the appropri-
ate tap. Longitudinal 3.5-mm rods were then applied
to complete the fixation device (Figure 3).

Multidirectional flexibility analysis

Multidirectional flexibility testing was performed uti-
lizing a custom-designed six-degree-of-freedom spine
simulator interfaced with an OptoTrak 3020 motion
analysis system (OptoTrak 3020, Northern Digital
Inc., Waterloo, Ontario, Canada) and Labview Soft-
ware (National Instruments Corporation, Austin,
TX, USA). The six-degree-of-freedom gimbal appa-
ratus contains three independent stepper motors,
harmonic drives, and electromagnetic clutches, which
apply pure, unconstrained rotational moments (±)
about three axes—X, Y, and Z. Unconstrained trans-
lations (±) are permitted using linear bearing guide
rails (X and Z) and 858 Bionix Materials Testing
System (MTS) servo-controlled linear actuator (Y
axis) (MTS Systems Corporation, Eden Prairie,
MN, USA) (Figure 2). The intact and reconstructed
cervical motion segments were evaluated under axial
rotation (Y axis, ±1.5 Nm), flexion/extension (X axis,
±1.5 Nm), and lateral bending (Z axis, ±1.5 Nm)
testing modes using a pure moment loading condi-
tion. Intersegmental motion was quantified using
specialized rig markers containing three non-co-linear
infrared light-emitting diodes (LEDs) rigidly attached

Figure 3. Various fixation methods as they were applied on the specimen mounted on plastic models. A = anterior plate alone; AW = anterior
plate combined with posterior wire; AT = anterior plate combined with transarticular screws; AP = anterior plate combined with a pedicle
screw–rod construct.

200 T. Henriques et al.

to the vertebral elements at C5 and C6 and oriented to
permit detection by an optoelectronic motion analysis
system. Each test was repeated for three loading and
unloading cycles at a rate of three degrees/second,
with data from the third cycle used for computational
analysis.

Statistical analysis

For non-destructive multidirectional flexibility anal-
ysis, the peak intervertebral range of motion (Euler
angles, degrees) for each loading mode was calculated
as the sum of motions [maximum ± rotation for
torsion, flexion–extension, and left + right bending
(degrees)] observed at the third loading cycle range-
of-motion (RoM). The raw data set consisted of angle
measurements around three axes (FE: flexion–exten-
sion, LB: lateral bending, and AR: axial rotation) for
the intact condition and following reconstruction
using four different preparations: A, AW, AT, and
AP. The analysis data set was created from the raw
data set by normalizing the angles dividing them by
the corresponding value for the intact specimen, e.g.
(FE, angular plate angle)/(FE, intact angle). The
resulting value was expressed as a percentage.
Thus, the value 100 indicates that the angle was
identical to the corresponding angle for the intact
condition. The three rotational axes (FE, LB, and
AR) were analyzed separately. For each axis, the four
stabilization methods were compared pairwise. As the
data failed to show normality when examined with the

Shapiro–Wilk W test, the Wilcoxon signed ranks test
for paired data was utilized. The target parameter is
the pseudomedian for the difference in percentage
discussed above. The pseudomedian of a variable x is
the median of (x–y)/2, where y is an independent copy
of x. For symmetric distributions, it coincides with the
ordinary median. Results are presented as 95% con-
fidence intervals. No correction has been made for
multiple testing. The missing value for specimen 1,
FE, AP is disregarded in these analyses. Thus, com-
parisons to this group have been made on seven pairs
only, as opposed to the other ones, performed on eight
pairs. One should note that the statistical power of
these analyses is limited due to the small sample size.
Hence, moderately large p values should not be inter-
preted as strong evidence against any difference
between the groups.
The data analysis was performed using R version

3.0.2.

Results

During the multidirectional testing of the reconstruc-
tion with anterior plate alone, the 95% CI of range-of-
motion (RoM) was 19–116 percentage points (pp) of
the intact spine in flexion–extension which was close
to but not significant (p = 0.15) (Table I; Figure 4).
A closer analysis of the data revealed that the anterior
plate alone provided stability mainly in extension
whereas the stability in flexion was poorer. The
95% CI for the RoM for lateral bending was 67–

Table I. The data set. Values are percentages (angle/ intact angle).

Flexion–extension Lateral bending Axial rotation

A AW AT AP A AW AT AP A AW AT AP

Spec 1 3.4 2.9 0.6 – 36.7 33.8 0.3 1.1 4.3 41.4 5.6 34.5

Spec 2 113.3 1.0 97.6 5.5 194.4 237.4 8.3 14.7 275.9 253.9 42.9 17.4

Spec 3 115.5 2.2 28.3 10.0 422.2 492.3 1.5 29.7 64.1 108.9 14.6 9.5

Spec 4 78.5 13.2 53.1 4.2 555.9 515.7 21.1 10.2 123.8 127.4 9.9 13.4

Spec 5 131.9 6.7 38.8 7.1 101.7 104.6 11.7 3.0 132.6 85.0 15.9 15.5

Spec 6 37.9 3.7 2.9 10.7 97.0 78.7 100.8 15.5 55.2 78.1 11.5 4.5

Spec 7 7.1 9.3 0.5 10.6 36.7 206.0 68.5 26.1 43.0 142.8 21.8 67.6

Spec 8 35.6 7.2 10.8 11.8 103.2 107.8 28.8 18.2 84.9 98.7 11.0 20.2

Median 58 5 20 10 102 157 16 15 75 104 13 16

Range 3–132 1–13 0–98 4–12 37–556 34–516 0–101 1–30 4–276 41–254 6–43 4–68

95% CI 19, 116 2, 10 2, 63 6, 11 67, 375 71, 377 4, 65 6, 24 34, 180 70, 181 9, 29 9, 43

p 0.15 0.008 0.008 0.016 0.38 0.15 0.016 0.008 0.55 0.74 0.008 0.008

For the test sequence flexion–extension—AP in specimen 1, a mechanical failure of the construct occurred, thus no data could be retrieved.
A = anterior plate alone; AP = anterior plate combined with pedicle-screw construct; AT = anterior plate combined with posterior transarticular
facet screws; AW = anterior plate combined with posterior wire.

Evaluation of fixation techniques in cervical spine injury 201

375 pp of intact (p = 0.38), and for axial rotation it was
34–180 pp of intact (p = 0.55). Thus, in none of the
tested modes did the anterior plate alone provide
improved stability compared to the normal mobility
of the intact spine.

Flexion–extension

When posterior instrumentation was added to the
anterior plate alone construct (A), the stability
improved for all the applied techniques: with Bohl-
man wire construct (AW) the segmental range of
motion decreased by 59 pp compared to A
(p = 0.023), with the transarticular screws (AT) it
decreased by 28 pp compared to A (p = 0.008), and
with the pedicle screw construct (AP) it decreased
by 66 pp compared to A (p = 0.031). There were no

significant differences between the different poste-
rior techniques in flexion–extension (p = 0.22)
(Table II).

Lateral bending

When the anterior plate stabilization was supplemen-
ted with the Bohlman wire construct (AW) there was
no improvement in stability in lateral bending com-
pared to the stability provided by the anterior plate
alone (p = 0.38). When the anterior plate was com-
bined with transarticular facet screws (AT) or a
pedicle screw construct (AP), the segmental range
of motion decreased by 121 pp compared to A
(p = 0.039) and 131 pp compared to A
(p = 0.008), respectively. The AT (–158 pp;
p = 0.016) and AP constructs (–171 pp; p = 0.008)
were significantly more stable when compared to the

The data set

R
a
n

g
e
o

f
m

o
ti

o
n

(
%

o
f

in
ta

c
t)

600

500

400

300

200

Intact

0
A AW AT AP

Flexion-extension Lateral bending Axial rotaton

Median

25%–75%

Non-outlier range

Outliers

A AW AT AP A AW AT AP

Figure 4. Boxplots of the data set for the different testing moments. Data have been normalized and expressed as the percentage of range-of-
motion for the intact specimen for each test sequence.

Table II. Analysis of flexion–extension. Reconstruction methods in the left column have been compared to methods in the top row. Thus,
values above zero mean that the method to the left gives larger values than the method above. Entries are: Estimate (CI) p.

A AW AT

AW –59 (–113, –13) 0.023 – –

AT –28 (–64, –9) 0.008 17 (–3, 61) 0.15 –

AP –66 (–115, –12) 0.031 3 (–4, 7) 0.30 –20 (–62, 8) 0.22

A = anterior plate alone; AP = anterior plate combined with pedicle-screw construct; AT = anterior plate combined with posterior transarticular
facet screws; AW = anterior plate combined with posterior wire.

202 T. Henriques et al.

AW construct. There was no difference between the
AT and AP constructs in lateral bending (p = 0.25)
(Table III).

Axial rotation

When the anterior plate stabilization was supple-
mented with the Bohlman wire construct (AW)
there was no improvement in stability in axial rota-
tion compared to the stability provided by the
anterior plate alone (p = 0.31). When the anterior
plate was combined with transarticular facet screws
(AT) or a pedicle screw construct (AP) the seg-
mental range of motion decreased by 71 pp com-
pared to A (p = 0.016) and 62 pp compared to A
(p = 0.039), respectively. The AT (–94 pp;
p = 0.008) and AP constructs (–86 pp;
p = 0.008) were significantly more stable when
compared to the AW construct. There was no
difference between the AT and AP constructs in
axial rotation (p = 0.64) (Table IV).

Discussion

Methodology—DF-3 injury procedure

In patients traumatized with a DF-3 injury, the extent
of insufficiency of the posterior tension band mech-
anism probably varies, resulting in a pronounced
instability. In order to dislocate both facets on the
specimen, all posterior ligaments including the PLL
and the posterior part of the intervertebral disc were

transected. In further preparation, the anterior longi-
tudinal ligament was sectioned and anterior disc
removed in order to place the bone graft, leaving
only the anterolateral remnants of annulus fibrosus
to stabilize the operative motion segment. Hence, the
tested specimens were more unstable than most clin-
ical cases where the muscles and fasciae may contrib-
ute to stability (27). Also, the tested specimens were
most likely older and with a decreased bone quality
compared to the average patient, which correlates
with inferior purchase of screws (28).
Due to the scarcity of human cadaver specimens,

we chose to perform non-destructive testing and
omitted cyclic loading in spite of the known impor-
tance of this as shown by Weis et al. in a bovine
DF-3 model. They described fatigue failure in exten-
sion–flexion in two out of six specimens with posterior
wire fixation, and in one specimen with anterior plate
alone (19). Thus, our study probably overestimates
the stabilization provided by anterior plate alone and
anterior plate combined with posterior wiring.
The purpose of this study was to evaluate how the

stability was affected by adding different posterior
fixations to an anterior fixation. In the clinical situa-
tion anterior exploration has certain advantages and
may therefore be some surgeons’ first choice,
although many would consider a ‘posterior alone’
fixation for these injuries, especially when a previous
MRI has shown that no disk fragment has been
dislodged to the spinal canal. We are also fully aware
that posterior wire techniques probably have very little

Table IV. Analysis of axial rotation. Reconstruction methods in the left column have been compared to methods in the top row. Thus, values
above zero mean that the method to the left gives larger values than the method above. Entries are: Estimate (CI) p.

A AW AT

AW 19 (–22, 61) 0.31 – –

AT –71 (–153, –21) 0.016 –94 (–153, –62) 0.008 –

AP –62 (–162, –10) 0.039 –86 (–157, –41) 0.008 3 (–13, 27) 0.64

Table III. Analysis of lateral bending. Reconstruction methods in the left column have been compared to methods in the top row. Thus, values
above zero mean that the method to the left gives larger values than the method above. Entries are: Estimate (CI) p.

A AW AT

AW 17 (–19, 87) 0.38 – –

AT –121 (–360, –2) 0.039 –158 (–362, –34) 0.016 –

AP –131 (–363, –46) 0.008 –171 (–364, –63) 0.008 –10 (–48, 10) 0.25

Evaluation of fixation techniques in cervical spine injury 203

clinical use nowadays, but we wanted to see the effect
of this technique mainly as a historical reflection.

Stability of the cervical reconstructions

Anterior plate alone. When reconstructed with A, the
range of motion equaled the intact spine in axial
rotation and lateral bending, whereas the stability in
flexion–extension was only marginally improved.
A detailed analysis of the flexion–extension data
revealed that the anterior plate stabilized foremost in
extension, and was poorer in flexion. In a DF-3 injury,
the posterior tension band mechanism is absent.
Therefore, a flexural moment will rotate the motion
segment anteriorly, separating its posterior structures.
Ananteriorplate will resistthemoment closetotheaxis
of segmental rotation. As a result, the fixation device by
itself has to withstand the flexion moment. In exten-
sion, on the other hand, the posterior structures are
compressing the bone graft, and the anterior plate will
serve to improve stability of the anterior tension band
mechanism. The fact that an anterior plate only stabi-
lizes themotionsegmentatonepoint probablyexplains
why the device performs comparably poorly in axial
rotation and lateral bending.
In agreement with the present results and clinical

experience (12,13), several studies have indicated poor
properties of an anterior plate alone for reconstruction
of a three-column ligamentous injury, in contrast to
some reports on good clinical results (11,29-32). One
explanation may be that in a series of patients the
severity of the injuries varies also within the same
fracture class. Incomplete disruption of the PLL may
provide enough posterior tension band to allow a suc-
cessful result with an anterior plate alone. Another
explanation may be that the stability provided by an
anterior plate, although weaker than the combined
reconstructions, may be sufficient for patients where
the bone stock is of good quality. Some support for this
latter hypothesis is provided by a biomechanical report
in a DF-3 injury model (33). However, their specimens
were all male and considerably younger (mean age
44 years, range 21–65) compared to those utilized in
the current study 67 years (range 49–82), which could
indicate better bone quality.

Anterior plate and Bohlman triple wire technique. This
two-point fixation (one anteriorly, one posteriorly) is
effective in restricting motions in extension–flexion
compared to when the spine is fixed with an anterior
plate alone, as the posterior tension band mechanism is
reconstructed. The wires connect to the spinous pro-
cesses providing a longer lever arm for the posterior
component compared to the other posterior fixation
techniques in the study, which may explain the slightly

higher stiffness in extension–flexion compared to these
techniques. However, the reduced stabilization dem-
onstrated in axial rotation loading may be secondary to
the inability to tighten a stainless steel wire, creating
enough compression, and thus friction, between the
bone surfaces. The wire resists tension loads, whereas
axial and lateral bending moments are poorly con-
trolled. In axial rotation, the geometry of the triple
wire construct is almost 90 degrees with respect to
therotationalmoment,thusexplainingthepoorresults.

Anterior plate and transarticular facet screws or pedicle
screw–rod fixation. The combination of an anterior
plate with transarticular facet screws or pedicle
screw–rod device rigidly stabilizes at three circumfer-
ential points around the operative motion segment.
This three-point fixation effectively controls the
applied rotational moments as no significant difference
could be detected between these two fixation techni-
ques. Hence, from a biomechanical point of view they
are similar, but they differ significantly from a clinical
point of view. The transarticular facet screw technique
is difficult to implant superior to C4 due to the prom-
inence of the occiput. It is often difficult to locate the
correct screw trajectory, without coming in conflict
with the cranium. Also, the facets have to be perfectly
reduced in order for this screw technique to be appli-
cable. If the facets are subluxed, too little bone may be
available for the screw canal, posing a risk for fracture
or nerve root compromise when the screw is inserted.
The cervical pedicle screw technique described by

Abumi and Kaneda (34), on the other hand, can be
applied superior to C4. It permits for segmental
compression over the rod, thus aiding in the facet
reduction. This technique has been demonstrated to
improve segmental fixation and stability (35) and
probably offers the best purchase in the cervical spine
(36). There are obvious neurovascular risks involved
in placing cervical pedicle screws, and from a clinical
point of view safer techniques such as lateral mass
screws have been shown to be sufficient (37). How-
ever, for the present study, pedicle screws were
included as they probably offer the best stabilization
that can be achieved in the cervical spine.
It is possible that other anterior constructs, e.g.

utilizing plates with bicortical screws and/or other
designs of the bone graft may have yielded better
results for the ‘anterior fixation alone’, but as we
do not have any data on such constructs we cannot
comment on this any further. Other fixation devices
primarily designed for anterior cervical fusion in
degenerative conditions have also been tested biome-
chanically in fracture models. Two low-profile
devices—intracorporal cages with locked and variable

204 T. Henriques et al.

angle screw anchorage, respectively—were compared
in a recent study (38). Both devices had very similar
features. The authors concluded that the stability
provided by the locked screw device probably was
sufficient for distractive-flexion stage 3 injury if com-
bined with an external immobilization, while the
device with variable angle screws was not suited for
patients with distractive-flexion injuries.

Conclusions

In this in vitro study using human spine specimens we
evaluated four fixation methods in a DF-3 injury of
the cervical spine. We found that anterior plate fixa-
tion alone is not sufficient to stabilize a DF-3, but by
combining it with a competent posterior tension band
reconstruction, i.e. transarticular facet screws or ped-
icle screw–rod instrumentation, segmental stability is
improved.

Acknowledgements

Funding was provided by Orthopaedic Associates
Research Foundation, Inc., Towson, Maryland,
USA. Cervical spinal implants were provided by
Anatomica AB, Göteborg, Sweden. The statistical
analysis was performed by Lars Lindhagen, PhD,
biostatistician at Uppsala Clinical Research Center,
Uppsala, Sweden.

Declaration of interest: None of the authors has any
conflict of interest with regard to the content of this
article.

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Abstract
Introduction
Materials and methods
Preparation and experimental groups
Spinal constructs
Multidirectional flexibility analysis
Statistical analysis

Results
Flexion&ndash;extension
Lateral bending
Axial rotation

Discussion
Methodology&mdash;DF-3 injury procedure
Stability of the cervical reconstructions
Anterior plate alone
Anterior plate and Bohlman triple wire technique
Anterior plate and transarticular facet screws or pedicle screw&ndash;rod fixation

Conclusions
Acknowledgements
Declaration of interest
References