SYMPOSIUM: CURRENT CONCEPTS IN CERVICAL SPINE SURGERY
Operated and Adjacent Segment Motions for Fusion versus
A Pilot Study
Tomoya Terai MD, PhD, Ahmad Faizan PhD,
Koichi Sairyo MD, PhD, Vijay K. Goel PhD
Published online: 30 October 2010
? The Association of Bone and Joint Surgeons1 2010
(ACDF) represent the standard treatment for cervical
spondylolytic radiculopathy and myelopathy. To achieve
solid fusion, appropriate compressive loading of the graft
and stability are essential. Fusion may lead to adjacent
segment degeneration. Artificial discs have been intro-
duced as motion-preserving devices to reduce the risk of
We therefore asked: (1) Does the use
of a plate reduce motion at the operated level and bone
graft compression compared to fusion with bone graft
alone; and (2) is adjacent-segment motion higher after
fusion with a plate?
Motions and compressive loads in the graft were
quantified for intact, C4–C5 ACDF without and with a
plate, and total disc arthroplasty in human cadaver spines.
At the surgery level all motions decreased for
ACDF with a plate. The motions were similar to intact
Anterior cervical discectomy and fusion
motions after total disc arthroplasty. The motions across
the adjacent segment increased after fusion in all loading
modes except lateral bending and were closer to the intact
for the total disc arthroplasty case. The plate maintained a
compressive load on the graft with a maximum increase in
Unlike fusion, the arthroplasty can restore
motion to normal at the surgery and adjacent segments,
compared to fusion cases. A cervical plate with a pre-
compression of the graft provides enhanced stability and
fusion due to improved compression.
Our findings support the clinical
observations that fusion may lead to the degeneration of the
adjacent segments. Disc arthroplasty may be able to cir-
cumvent the adjacent segment degeneration.
Anterior cervical discectomy and fusion (ACDF) repre-
sents a widely accepted surgical procedure to manage
cervical spondylolytic radiculopathy and myelopathy
symptoms. Cloward  and Robinson and Smith 
originally described noninstrumented cervical arthrodesis,
but these approaches reportedly had nonunion rates ranging
from 8.3% to 12% [22, 35, 36]. Bohler  in 1967 reported
what was likely the first use of anterior cervical plate and
screw fixation in a patient with cervical spinal trauma. In
the early 1980s, Caspar et al.  popularized anterior
cervical plating. The constructs of the procedure had lim-
ited fixation at the screw-plate interface, leading to early
screw backouts. However, the concept facilitated graft
compression, allowing for a better chance of bony fusion.
ACDF is a reliable procedure with a fusion rate of
between 85% and 95% [2, 26]. Cervical plates are used to
One or more of the authors (VKG) has received funding from
DePuy Spine, Inc, Raynham, MA.
Each author certifies that his or her institution approved the human
protocol for this investigation and that all investigations were
conducted in conformity with ethical principles of research, and that
informed consent for participation in the study was obtained.
This work was performed at The University of Toledo.
T. Terai, A. Faizan, V. K. Goel
Engineering Center for Orthopaedic Research Excellence
(E-CORE), Departments of Bioengineering and Orthopaedic
Surgery, Colleges of Engineering and Medicine, University
of Toledo, Toledo, OH, USA
T. Terai, K. Sairyo (&)
Department of Orthopedics, University of Tokushima, School
of Medicine, 3-18-15, Kuramoto, Tokushima 770-8503, Japan
Clin Orthop Relat Res (2011) 469:682–687
stabilize the segment(s), affording solid fusion of the
grafted bone, probably due to enhanced compression of
the graft . However, some complications have been
reported, such as nonunion graft migration and kyphotic
malunion [1, 4, 9, 19]. Reported nonunion rates range from
4.0% to 9.7% after ACDF with different types of cervical
plate systems [22, 35, 36]. The reduced physiologic motion
of the spine after fusion may lead to a compensatory
increase in motion at the adjacent level, leading to
an increase in adjacent-segment degeneration [18, 21].
Adjacent-segment degeneration with new, symptomatic
radiculopathy reportedly occurs after ACDF in 2% to 3%
of patients per year on a cumulative basis . Spine
surgeons are now pursuing alternatives to fusion, such as
total disc arthroplasty (TDA) to address fusion-related
complications [7, 27, 33, 37]. The goal in using these
devices is to alleviate pain by replacing the diseased disc
while preserving and/or restoring motion at the surgery and
We therefore asked the following questions: (1) Does
the use of a plate reduce motion at the operated level and
bone graft compression in all loading modes compared to
fusion with bone graft alone?; (2) Is adjacent-segment
motion higher after fusion with a plate?
Materials and Methods
We conducted a series of experiments to study the motion
at the surgery and adjacent levels after fusion and TDA
using six fresh-frozen ligamentous human cervical spines
(Fig. 1). The loads across the bone graft after fusion using
a load cell were also quantified. The rotations across var-
ious levels were compared among the different conditions.
The specimens were prepared by fixing the T1 vertebra
to the base of the test frame and fixing a frame to the C3
vertebra for the load applications. Three light-emitting
diodes (LEDs) were affixed to L-shaped brackets rigidly
attached to each vertebral body. The motions of the LEDs
in response to the applied loads were tracked using the
Optotrak1system (Northern Digital, Waterloo, Canada).
The specimens were tested under 0 to 2 Nm of moment in
flexion, extension, lateral bending, and axial rotation [5, 6].
These moment values produce physiologic ROM in vivo
. The load displacement curves of the cadaver spines
and the load passing through the load cell were obtained.
The load cell was connected to an amplifier and the
amplifier’s signal was recorded using the Optotrak1data
The testing was conducted in the following sequence:
(1) intact spine; (2) load cell placement at the C4–C5 level;
(3) load cell and cervical plate at the C4–C5 level; and
(4) TDA at the C4–C5 level. The load cell (Honeywell, Inc,
Morristown, NJ) was custom made for the cervical spine
with proper dimensions to fit the C4–C5 disc space (6-mm
height, 10-mm diameter). To further ensure a tight fit of the
load cell, bone chips were placed along with the load cell.
The plate (DePuy Spine, Inc, Raynham, MA) was screwed
to the adjacent vertebral bodies such that the bone graft
was under compression before testing. For TDA at the
C4–C5 level case, a standard anterior cervical discectomy,
including removal of the anterior longitudinal ligament,
was performed at the surgery level in all specimens. The
posterior longitudinal ligament, lateral annulus, and unci-
nate processes were preserved. An appropriately sized
artificial disc (DiscoverTM, DePuy Spine) was implanted at
C4–C5 in the specimens (6-mm prostheses in four cases;
8-mm prostheses in two cases). The device consists of two
Fig. 1A–D Photographs illustrate the four conditions included in the
study. (A) Intact C3–T1 specimens were first tested on the kinematic
profiler. (B) A load cell was placed at the C4–C5 level. (C) A plate
was placed along with the load cell at the C4–C5 level. (D) A
DiscoverTMcervical artificial disc was placed at the C4–C5 level.
Volume 469, Number 3, March 2011Biomechanics of the Cervical Spine 683
cobalt-chromium-molybdenum endplates with a metal-on-
polyethylene bearing surface; the polyethylene insert is
fixed to the inferior endplate.
The motion levels at the surgery and adjacent sites were
quantified for all cases, intact, stabilized, and TDA. The
load cell data were also recorded for the fusion cases.
Statistical analysis was performed using one-way analysis
of variance (ANOVA) followed by a post hoc Fisher test
for multiple comparisons (p\0.05). All data are shown as
mean ± 1 SD.
With the load cell and plate implanted at the C4–C5 level,
the mean flexion motion decreased (p = 0.005) by about
80% (Fig. 2). Plating at the C4–C5 level restricted
(p = 0.0001) the motion during extension as well. The
C4–C5 level motions were closer to intact values after
TDA, except in extension. The mean extension motion at
the C4–C5 level increased by 22% after TDA (Fig. 2). In
ACDF with plate fixation, the compressive load increment
during cervical motions showed variations, although it was
always in compression (Table 1). The graft load decreased
in axial rotation, compared to the precompression load. In
lateral bending, the graft load was similar to the initial
precompression value. On the other hand, graft load
increased by approximately a factor of two during exten-
sion. In all loading modes, compression of the graft was
higher in the presence of the plate, except in flexion. In
flexion, graft load was higher in the graft-alone case,
illustrating the protective role of the plate in this mode.
The mean motion after fusion at the adjacent level
increased all loading modes except lateral bending, while
the motion was closer to intact for the TDA case (Fig. 3).
The mean motion at the C5–C6 level with the load cell and
plate at the C4–C5 level was similar in flexion (p =
0.4089) and in extension (p = 0.7867). The mean motion
at the adjacent level with TDA at the C4–C5 level was near
normal in flexion and extension.
ACDF is the standard to treat patients with cervical
spondylolytic radiculopathy and myelopathy. To achieve
solid fusion of the grafted bone, the graft should be under
compression and undergo very little motion during the
Fig. 2 A graph shows surgery-level (C4–C5) motions (mean and SD)
at the final loading step (2 Nm). The motion decreased after fusion
with a plate compared to the intact. TDA restored the motion to the
intact value. The p value using one-way analysis of variance
(ANOVA) is shown on the graph. *The asterisks show significant
difference using post-hoc Fisher multiple comparison analysis.
Table 1. Compressive loads on the graft (load cell) at the C4–C5
level under 2-Nm moments
Motion Mean compressive load (N)
Load cell Load cell + plate
Initial value (no moment)17 86
Left bending 4998
Right bending 7092
Left rotation53 71
Right rotation47 63
Data are presented for a total of six cases tested with a load cell
placed at the C4–C5 level to study the load passing through the
Fig. 3 A graph shows adjacent-level (C5–C6) motions (mean and
SD) at 2 Nm. The motion was similar for the bone graft with the plate
case in flexion and in extension, compared to the intact case. The
motion was close to the intact case for the TDA case. No difference
among any groups was seen by one-way analysis of variance
684 Terai et al. Clinical Orthopaedics and Related Research1
healing process. Fusion also leads to adjacent-segment
degeneration, due to a compensatory increase in motion
across this level as compared to intact values, as well as to
other factors. More recently, artificial discs have been
developed as motion-preserving devices to address these
issues. Besides restoring motion at the surgery level, TDA
should lessen the degeneration of the adjacent segment.
Using a cadaver model, we asked the following two
questions: (1) Does the use of a plate reduce motion at the
operated level and bone graft compression in all loading
modes compared to fusion with bone graft alone? (2) Is
adjacent-segment motion higher after fusion with a plate?
We acknowledge limitations to our experiments, as with
any other in vitro motion study reported in the literature.
First, in our cadaver studies, simulation of the stabilizing
effects of in vivo muscle forces was not feasible, although
it is desirable to test the spine in the presence of an
appropriate preload. The application of muscular forces
tends to create a highly unstable spine, with cervical
specimens tending to go into extension upon the applica-
tion of a preload [12, 25]. Second, the literature
recommends undertaking in vitro biomechanical studies
without the preload so long as the applied moments pro-
duce a physiologic ROM [5, 6, 15, 17, 23]. Thus, the
experimental studies described here were carried out using
pure moments of up to 2.0 Nm since these produce phys-
iologic ROM in a cadaver cervical specimen without a
preload . Third, while fresh freezing does not affect the
material properties of ligaments, bone, and annulus and the
motion of the spine , the question does arise of how
representative the specimens are of the general population.
Fourth, the cervical specimen donors used in this study had
an average age of 66 years, which has implications for
bone quality, ligament strength, and disc degeneration. All
specimens were screened for abnormalities and only those
that appeared ‘‘normal’’ were selected for biomechanical
testing. Therefore, even though the average donor age was
advanced, care was taken to include only specimens of
good quality for the present investigation. However, being
a cadaver model and despite these precautions and agree-
ment with other cadaver studies in the literature [10, 13–16,
23, 24, 31] (Table 2), it is crucial to show our findings are
We found, after ACDF with bone graft and a cervical
plate, motion at the surgery level decreased in all direc-
tions. The increase in flexion motion across the adjacent
segment observed in our study is likely to contribute to
adjacent-segment degeneration, in line with clinical
observation. The implant-level and the adjacent-level
motions were similar to intact after TDA at the C4–C5. The
near-normal motions after TDA suggest the adjacent-
segment degeneration should be the lesser value. In a meta-
analysis of the literature through a MEDLINE search,
Harrop et al.  reported adjacent-segment degeneration
in 34% of the fusion patients as compared to 9% in the
TDA patients. Ordway et al. , using a stereoradio-
graphic technique, found motions at the TDA level were
similar to those of the control group. Likewise, based on
plain radiographic analyses, several investigators have
reported preservation of motion at the surgery level after
Table 2. A summary of published in vitro biomechanical studies on cervical arthroplasty
StudyStudy designProsthesis* Mechanical variables Results
Chang et al.  (2007) TDA versus fusionProDisc IDP at the treated and adjacent
Adjacent level IDP was similar to
DiAngelo et al. 
TDA versus fusion PrestigeMotion at index and adjacent levelsTDA preserved motion and yielded
similar to intact
DiAngelo et al. 
TDA versus fusionProDisc Motion at index and adjacent levelsTDA was able to mimic kinematics
of the intact spine
Dmitriev et al. 
TDA versus fusionPCM Motion and IDPTDA preserved motion and IDP at
Duggal et al. 
TDABryan Extension, flexion, and axial rotation
motion until failure
Remaining ligaments and annulus
sufficient to provide stability with
Kotani et al.  (2002)TDA versus fusion 3D fabric disc Motion at index and adjacent levelsIncrease in extension-flexion motion,
no change at adjacent level
McAfee et al. 
TDA versus fusion PCMMotion, PLL contribution TDA preserved motion, resection
of PLL decreases stability
Puttlitz et al. 
TDAProDisc MotionTDA preserved motion and coupling
* Prostheses included: ProDisc1(Synthes, Inc, West Chester, PA), Prestige1(Medtronic Sofamor Danek, Memphis, TN), Porous Coated
Motion (PCM; Cervitech, Inc, Rockaway, NJ), Bryan1(Medtronic Sofamor Danek); TDA = total disc arthroplasty; IDP = intradiscal pressure;
3D = three-dimensional; PLL = posterior longitudinal ligament.
Volume 469, Number 3, March 2011 Biomechanics of the Cervical Spine 685
TDA at various time intervals postoperatively [30, 32,
33, 37]. Our data support these findings. The near-normal
motion in our TDA group is in agreement with the in vivo
data. The increase in adjacent-level motion observed in our
study after fusion is high compared to the TDA case. This
should lead to higher incidences of adjacent-segment
degeneration in the fusion group compared to TDA
patients, in line with the meta-analysis of Harrop et al. .
For the load cell-graft-alone case, the graft experienced
lesser compression during extension, since the C4 vertebral
body lost contact with the load cell with increasing
extension moment. In the other loading modes, the load
cell provided as much stability as the intact spine, sug-
gesting additional stabilization is needed. The plate
enhanced stability in extension and ensured compressive
load on the graft in all loading modes including extension.
The data also suggest, to enhance bone graft healing, the
plate should be placed with precompression on the graft.
These observations are in agreement with the clinical
findings that the fusion rates are higher in patients who
receive a cervical plate to augment the bone graft fusion
[22, 35, 36].
In summary, use of a cervical plate may increase the rate
of fusion but is likely to lead to adjacent-segment degen-
eration. Under the limited conditions of our experiment, the
DiscoverTMartificial disc replacement system at the sur-
gery level was able to produce motion similar to that of the
intact spine. The artificial disc may be able to circumvent
adjacent-level disease since it mimics the motion of the
native spine at the adjacent level. The long-term clinical
implications of these findings remain to be seen, although
short-term clinical followups do show a decrease in adja-
cent-segment degeneration after TDA, compared to fusion
Orthopaedic Surgery, Boston Medical Center, Boston, MA), Steven
Garfin (University of California, San Diego, San Diego, CA), Ashok
Biyani and Hossein Elgafy (University of Toledo, Toledo, OH), and
Hassan Serhan (DePuy Spine, Inc, Raynham, MA) for their guidance
in this study.
We thank Christopher Bono (Department of
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