Polyetheretherketone (PEEK) Spacers for Anterior Cervical Fusion: A Retrospective Comparative Effectiveness Clinical Trial.

Abstract

Anterior cervical decompression and fusion (ACDF) is the standard surgical treatment for radiculopathy and myelopathy. Polyetheretherketone (PEEK) has an elasticity similar to bone and thus appears well suited for use as the implant in ACDF procedures. The aim of this study is to examine the clinical and radiographic outcome of patients treated with standing alone PEEK spacers without bone morphogenic protein (BMP) or plating and to examine the influence of the different design of the two spacers on the rate of subsidence and dislocation.
This retrospective comparative study reviewed 335 patients treated by ACDF in a specialized urban hospital for radiculopathy or myelopathy due to degenerative pathologies. The Intromed PEEK spacer was used in 181 patients from 3/2002 to 11/2004, and the AMT SHELL spacer was implanted in 154 patients from 4/2004 to 12/2007. The follow-up rate was 100% at three months post-op and 82.7% (277 patients) at one year. The patients were assessed with the Japanese Orthopedic Association (JOA) questionnaire and radiographically.
At the one-year follow-up there were 118/277 patients with an excellent clinical outcome on the JOA, 112/277 with a good outcome, 20/277 with a fair outcome, and 27/277 with a poor outcome. Subsidence was observed in 13.3% of patients with the Intromed spacer vs 8.4% of the patients with the AMT SHELL. Dislocation of the spacer was observed in 10 of the 181 patients with Intromed spacers but in none of the 154 patients with Shell spacers.
The study demonstrates that ACDF with standing alone PEEK cages leads to excellent and good clinical outcomes. The differences we observed in the subsidence rate between the two spacers were not significant and cannot be related to a single design feature of the spacers.

Full text

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348 The Open Orthopaedics Journal, 2011, 5, 348-353


1874-3250/11 2011 Bentham Open
Open Access
Polyetheretherketone (PEEK) Spacers for Anterior Cervical Fusion: A
Retrospective Comparative Effectiveness Clinical Trial
Johannes Lemcke*,1, Ferass Al-Zain1, Ullrich Meier1 and Olaf Suess2
1Department of Neurosurgery, Unfallkrankenhaus Berlin, Germany
2Department of Neurosurgery, Charité-Universitätsmedizin Berlin, Germany and Departement of Neurotraumatology
and Spine Surgery, German Red Cross Hospital Westend, Berlin, Germany
Abstract: Background: Anterior cervical decompression and fusion (ACDF) is the standard surgical treatment for
radiculopathy and myelopathy. Polyetheretherketone (PEEK) has an elasticity similar to bone and thus appears well suited
for use as the implant in ACDF procedures. The aim of this study is to examine the clinical and radiographic outcome of
patients treated with standing alone PEEK spacers without bone morphogenic protein (BMP) or plating and to examine
the influence of the different design of the two spacers on the rate of subsidence and dislocation.
Methods: This retrospective comparative study reviewed 335 patients treated by ACDF in a specialized urban hospital for
radiculopathy or myelopathy due to degenerative pathologies. The Intromed PEEK spacer was used in 181 patients from
3/2002 to 11/2004, and the AMT SHELL spacer was implanted in 154 patients from 4/2004 to 12/2007. The follow-up
rate was 100% at three months post-op and 82.7% (277 patients) at one year. The patients were assessed with the Japanese
Orthopedic Association (JOA) questionnaire and radiographically.
Results: At the one-year follow-up there were 118/277 patients with an excellent clinical outcome on the JOA, 112/277
with a good outcome, 20/277 with a fair outcome, and 27/277 with a poor outcome. Subsidence was observed in 13.3% of
patients with the Intromed spacer vs 8.4% of the patients with the AMT SHELL. Dislocation of the spacer was observed
in 10 of the 181 patients with Intromed spacers but in none of the 154 patients with Shell spacers.
Conclusion: The study demonstrates that ACDF with standing alone PEEK cages leads to excellent and good clinical
outcomes. The differences we observed in the subsidence rate between the two spacers were not significant and cannot be
related to a single design feature of the spacers.
Keywords: Anterior cervical decompression and fusion, ADCF, PEEK spacer, subsidence, dislocation.
INTRODUCTION

for degenerative disc diseases in the cervical spine [1, 2].
Today, many options are available, including bone grafts,
bone cement, and spacers made of titanium, carbon, and
synthetic materials.
Anterior cervical fusion is a routine surgical procedure

the area of cervical implants, there are today still no
consensus recommendations for the choice of the most
suitable implant materials. Such a material should have
mechanical properties similar to that of bone and should also
be highly biocompatible.
Despite continual refinements and new developments in

thermoplast with long-term stability. It serves as a matrix
material for composite material implants. Previous studies
have shown that PEEK demonstrates a high degree of
biocompatability and thus is permissible as a material for
medical implants. PEEK possesses an EM comparable to
that of bone and thus seems quite suitable for use as an

Polyetheretherketone (PEEK) is a ductile, crystalline

*Address correspondence to this author at the Department of Neurosurgery,
Unfallkrankenhaus Berlin, Warener Straße 7, D-12683 Berlin, Germany;
Tel: +49 (0)30 5681 3701; Fax: +49 (0)30 5681 3703:
E-mail: johannes.lemcke@ukb.de
intervertebral implant. Additionally, PEEK distinguishes
itself through radiotransparency, high durability, high
chemical and thermal resiliency, and favorable friction and
wear properties. Because PEEK cages are radiotransparent,
they are produced with embedded titanium pins, in order to
enable radiological examination of the cage position.

PEEK spacers for the past 7 years. The aim of this study was
to examine the differences between two cervical PEEK
spacers concerning the clinical and radiographic outcomes.
After experience with other spacers, we have been using
METHODS
Study Design

analysis of two different PEEK spacers we used between 29
January 2002 and 25 October 2007 in patients undergoing
anterior cervical diskectomy and fusion (ACDF) for
degenerative disc disease.
This study is a retrospective comparative effectiveness
Patients

radiculopathy with paresthesia, hypesthesia, radicular
pareses, or radicular pain. The patients had not improved
sufficiently from conservative therapy. On MRI, all patients
showed a disc prolapse and/or an osteochondrosis in the
All patients had either cervical myelopathy or cervical

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Polyetheretherketone (PEEK) Spacers for Anterior Cervical Fusion The Open Orthopaedics Journal, 2011, Volume 5 349
segment corresponding to the neurological symptoms. The
level was specified in some cases by myelography with
intrathecal contrast medium and a post-myelo-CT of the
cervical spine. The exclusion criteria from the present
statistical analysis were trauma or prior surgery on the
cervical spine.
Surgery

Smith-Robinson technique with the Caspar instruments. The
segment was identified preoperatively and verified
intraoperatively by an image intensifier. The intervertebral
space was emptied, and osteophytes were removed with a
high speed drill. The cartilage portion of the end plate was
removed. The PEEK spacer was placed into the
intervertebral space under control of the image intensifier.
Nothing was placed inside any of the cages. No patient
received plate fixation. Intraoperative fluoroscopy ensured
the correct placement of spacers.
All patients underwent ACDF according to the modified
Implants

Heidesee, Germany) was implanted between 3/2002 and
11/2004 (plus one further case in 8/2006) (Fig. 1A). The
AMT SHELL spacer (Advanced Medical Technologies;
Nonnweiler, Germany) was implanted between 4/2004 and
11/2007 (Fig. 1B). In the overlapping period of time four
instrument containers – two of each brand – were in use. The
chosen spacer brand depended to the availability of the
containers. The height of the spacer was individually chosen
by the surgeon according to the subjective impression of
stability after testing with a “dummy spacer”.
The Intromed PEEK spacer (Intromed Medizintechnik;

the Intromed PEEK spacer to the AMT SHELL spacer was
the hypothesis that the retaining pins of the AMT spacer
provide more security against dislocation.
The underlying rationale for the decision to change from

contact surface area between 109 mm2 (14-mm implant) and
129 mm2 (18-mm implant) [3]. The Intromed PEEK spacer
has a contact surface area between 159 mm2 and 189 mm2.
The AMT SHELL cage with a width of 14 mm has a
Data Collection

day postoperatively, three months postoperatively, and one
year postoperatively. The clinical symptoms were docu-
mented according to the Japanese Orthopedic Association
We examined the patients one day preoperatively, one
(JOA score) [4]. We calculated the Recovery rate (RR)
according to Hirabayashi et al. [5], in order to make
comparisons between patients. Following the suggestion of
Yoshida [6], we define a recovery rate (RR) of 75% or more
as an excellent outcome, a RR of 50-74% as a good
outcome, a RR of 20-49% as a fair outcome, and a RR of
less than 20% as a poor outcome.

cervical spine in two planes was made at every follow-up.
The subsidence of the spacers was assessed by two
experienced neuroradiologists looking at the radiography of
each patient independently and then comparing their
assesments. Penetration of the spacer into the vertebral body
by 1 millimeter or more was defined as implant subsidence.
Any movement of the cage in sagittal direction exceeding
the margin of one of the vertebra of the fused segment was
defined as dislocation. The results of the bony fusion were
not reviewed systematically by a CT scan. Postoperative CT
scans were performed in cases of cage dislocation or signs of
instability.
In addition to the clinical examination, radiography of the
Statistical Analysis

sample and summarize the clinical findings. The Mann-
Whitney U-Test and chi-square test were used to compare
groups.
Descriptive statistics were used to characterize the study

time in the study, and other factors on the clinical outcomes,
three forward stepwise regression analyses were performed.
The dependent outcome variables were RR post-op, RR 3M,
and RR 12M. The independent co-variables for all regression
analyses were: sex, age, case number, week of the study,
spacer brand, mean spacer height, number of levels, C3/C4,
C4/C5, C5/C6, C6/C7, and C7/T1. Additionally, a logistic
regression analysis was performed with subsidence as the
dependent outcome variable and the same independent co-
variables just mentioned.
In order to assess the independent effect of cage brand,

operated on between 1 April 2004 and 30 November 2004,
as this was the period when both brands of spacers were in
use with comparable frequency. The data in this subgroup
was not normally distributed according to Kolmogorov-
Smirnov and Shapiro-Wilk tests, so non-parametric tests
were used. The patients were compared by brand of spacer,
using the Mann-Whitney U test for RR and chi-square test
for subsidence.
A subgroup analysis was performed on all patients

Fig. (1). The Intromed PEEK spacer and the amt SHELL spacer.

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350 The Open Orthopaedics Journal, 2011, Volume 5 Lemcke et al.
RESULTS
Study Sample and Implants

179 (53.3%) men. The mean (range) age was 50.6 (26-84)
years. The surgery was single-level in 228 patients (68%),
two-level in 104 patients (31%), and three-level in 3 patients
(1%). Surgery was performed at C3/C4 in 18 patients, at
C4/C5 in 55 patients, at C5/C6 in 221 patients, at C6/C7 in
152 patients, and at C7/T1 in 2 patients.
The study sample of 335 consecutive patients contained

follow-up data available. Sixteen of these patients were at
the end of the study. Thus the ratio of Intermed to SHELL
spacers was higher among the completers (156 : 121) than
among the non-completers (25 : 33), p=0.066. There was
also a greater proportion of males among the completers
(56%) than the non-completers (43%), p=0.083. Completers
had better mean clinical outcomes (RR) than non-completers
at post-op (49 vs 43, p=0.087) and at 3 month follow-up (64
vs 54, p=0.01). Completers and non-completers did not differ
on other variables. Data collection was complete for all other
variables at earlier timepoints.
There were 58 patients who did not have 12 month

SHELL spacer, the age distributions appeared comparable
(Fig. 2A) and the operated levels were essentially the same
(Fig. 2B). The distribution of implant height was consistently
1mm less for the AMT SHELL spacer than for the Intromed
spacer (Fig. 3).
Comparing the patients who received the Intromed vs
Complications

any superficial wound infections, but one deep infection
(0.3%) was observed and treated conservatively. Three
patients (0.9%) showed a transient unilateral paresis of the
N. laryngeus recurrens. Two patients (0.6%) had paresis of
the right N. laryngeus recurrens that persisted at the one year
follow-up.
There was no perioperative mortality. We did not observe
Clinical Outcomes

postoperative 36/335 patients (10.8%) had an excellent
Following the categories defined above, directly
outcome, 180/335 (53.7%) a good, 80/335 (23.9%) a fair and
39/335 patients (11.6%) a poor outcome. Three months after
surgery we found 125/335 patients (37.3%) with an excellent
outcome, 133/335 (39.7) with a good, 40/335 (11.9%) with a
fair and 37/335 (11.1%) with a poor outcome. At the one
year-follow up we registered 118/277 patients (42.6%) with
an excellent outcome, 112/277 (40.4%) with a good, 20/277
(7.2%) with a fair and 27/277 (9.8%) with a poor outcome.

post-op (p=0.38), 61.1 vs 64.0 at 3 months (p=0.36), and
65.8 vs 69.2 at 12 months (p=0.39). Thus, there were no
significant differences detected in the responder rate between
patients with the Intromed vs SHELL spacer (Fig. 4).
The mean RR in Interomed vs Shell was 47.2 vs 48.9 at
Radiographic Outcomes

patients (11%). We did not judge that any of these patients
needed surgical revision. Comparing the two types of
spacers, there appeared to be a possible difference: 13.3% of
patients with the Intromed spacer vs 8.4% of the patients
with the AMT SHELL spacer had subsidence (p=0.16) (Fig.
5). There was no difference in the mean clinical outcome
(RR 12M) between patients who did vs did not show
subsidence (67.5 vs 67.2, p=0.94).
Subsidence as defined above was observed in 37/335

patients with 245 Intromed spacers but in none of the 154
patients with 203 Shell spacers, (p=0.008 with Yates
correction, Fig. 5). There was no statistically significant
difference in the mean clinical outcome (RR 12M) between
patients who did vs did not have a dislocated spacer (58.5 vs
67.5, p=0.374).
Dislocation of the spacer was observed in 10 of the 181
Subgroup Analysis

November 2004: 18 with Interomed, 22 with SHELL. The
sex, age, and number of levels were similar in the two
groups. There was no statistically significant difference
between the two groups for RR post-op, RR 3M, RR 12 M,
or subsidence. However, this subgroup analysis was severely
underpowered; (e.g. at the measured standardized difference
for RR 3M, in this subgroup sample size, the study had a
power of 25% at a p-value of 0.05, and all others had even
There were 40 patients operated between April and

Fig. (2). Histogramms of the age distribution (left) and operated levels (right).

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Polyetheretherketone (PEEK) Spacers for Anterior Cervical Fusion The Open Orthopaedics Journal, 2011, Volume 5 351
less power). Thus, no conclusions can be drawn from the
subgroup analysis.

Fig. (3). Histogramm of the heights of the implanted amt spacers
(with retaining pins) and of the Intromed spacer (without retaining
pins).

Fig. (4). The Recovery-Rate directly postoperative, after 3 and 12
months for patients with the Intromed and the amt spacer shown as
box-and-whisker plots.
Regression Analyses

as significant predictors of RR at post-op: age (p<0.001),
operation at C4/C5 (p=0.027), and spacer height (p=0.034).
However, this model explained only a very small amount of
the variance in the data (R2=0.056 (whereby R2 can range
from 0 [accounts for none of the data variance] to 1
[accounts for all data variance])), meaning that these factors
could barely explain much of the difference in outcomes.
The forward stepwise regression identified three factors

(p=0.038). But again, this model explained a negligible
amount of the data variance (R2=0.010). In other words, age
had a statistically reliable influence on the 3 month outcomes
(such that older patients did worse), but this was a negligible
influence in comparison to other factors not captured in the
database.
Only age was still a significant predictor for RR 3M

Fig. (5). The rates of subsidence and dislocation compared between
Intromed spacer and amt spacer.

the number of levels operated (p=0.039), but again this
model explained a negligible amount of the data variance
(R2=0.012).
At 12 months, the only significant predictor of RR was

predictor of subsidence was age (p=0.009). However age had
minimal influence on whether or not subsidence occurred
(OR=1.05, 95%CI of OR=1.01-1.09). Spacer brand
(Intromed) was implicated as a predictor of subsidence but
the results were far from statistically significant (OR=2.14,
95%CI of OR = 0.55 – 8.31, p=0.27). Case number and week
of study were devoid of influence or statistical significance
(case number: OR=0.99, p=0.98; week: OR=1.00, p=0.99);
therefore, experience and timeperiod were absolutely not
confounding factors for subsidence. Spacer height was also
not a significant predictor or subsidence (OR=1.19, p=0.52).
In the logistic regression analysis, the only significant
DISCUSSION

the standard surgical treatment for radiculopathy and
myelopathy. It aims to decompress the affected nerves by
restoring the physiological height of the intervertebral space
and the initial width of the neuroforamina. Thus, subsidence
of the spacer into the end plate of an adjacent vertebra might
result in renewed clinical deterioration.
Anterior cervical decompression and fusion (ACDF) is

patients with full one-year follow-up (14%). This is
somewhat higher than two previous studies (3.8% in Hwang
et al. [7] and 9% in Lin et al. [8]), and somewhat lower than
in two other studies (19% in Niu et al. [9] and 33% in
Zevgarides et al. [10]). It seemed that the earlier patients
implanted with the Intromed spacer had a higher rate of
subsidence than did the more recent patients implanted with
the AMT SHELL spacer, though the statistical significance
was unreliably weak. The regression analysis proved that
experience and timeperiod were not confounding factors.
Although the regression analysis implicated the Intromed
brand as more prone to subsidence, this finding was not
statistically reliable.
In our study, subsidence occurred in 39 of the 277

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352 The Open Orthopaedics Journal, 2011, Volume 5 Lemcke et al.

responsible for the occurrence of subsidence and dislocation.
Several studies have asserted that the preparation of the
endplates has an influence on subsidence [7, 11-13]. And at
least one study has stated that excessive distraction of the
intervertebral space has an influence on the probability of
subsidence [14, 15]. In our study, all patients were operated
on by the same surgical team, which would argue against the
hypothesis that differences in endplate preparation or
distraction were responsible for the differences in the rates of
subsidence and dislocation.
Factors unrelated to the implants themselves could be

could conceivably explain the difference in their rates of
subsidence and dislocation. First, although both spacers were
made of PEEK, the titanium pins of the AMT SHELL spacer
could have somehow contributed to preventing subsidence.
Alternately, several aspects of the design of the spacers

of contact surface area, whereby less surface area means that
more force per area is being transmitted by the spacer to the
vertebral endplate. If this was so, we would expect the spacer
with less surface area to have a higher rate of subsidence, but
in fact we observed the opposite. The AMT SHELL spacer
has a smaller surface area, due to its larger central fusion
hole, but nonetheless it had a lower rate of subsidence.
Similarly one would expect a spacer with less surface area to
be less able to grip the vertebral endplate and therefore to
dislocate more often. Despite this, the Intromed spacer
dislocated more often.
geometric/mechanical factors may play a role, they do not
explain our clinical observations.
Second, subsidence could also be related to the amount
So although these

subsidence rates is the difference in the spacer heights used,
which is indeed due in turn to a design difference, as we will
explain in a moment. We found a linear correlation between
spacer height on the one hand and the rates of subsidence
and dislocation on the other (Fig. 6). The Intromed spacers
we used were consistently 1mm taller than the AMT SHELL
spacers we used (Fig. 3). Thus the higher rate of subsidence
and dislocation of the Intromed spacers (Fig. 5) is apparently
due most likely to the fact that they were consistently taller
than the AMT SHELL spacers. Of course, the reader will
first think that the systematic selection of Intromed spacers
that were 1mm larger than the AMT Shell spacers has
nothing to do with the cage design per se, but instead is the
human error of our surgical team.
Third, the most likely explanation for the difference in

spacer is indeed what leads the surgeons to systematically
choose a spacer that is a bit too big. All operating surgeons
in this study checked the stability of the implanted spacers
intraoperatively when the spacer was already placed in its
final position. Then the Caspar retractor was released and the
spacer was still fixed in the application instrument. So when
the spacer had no pins (Intromed), a slightly taller spacer
was required to create the same feeling of stability in the
hands of the surgeon. In other words, the existence of pins in
the AMT Shell spacer makes it feel stable with 1mm less
spacer height. The different rates of dislocation (5.5% for
Intromed vs 0% AMT SHELL) support this view. We
believe that the x-ray pins of the AMT SHELL spacer
accomplish the fixation effectively. By contrast, the
Intromed spacer only feels stable by using a slightly taller
But we believe that a design feature of the Intromed
spacer, which then increases the rates of subsidence and
dislocation.

Fig. (6). The rate of subsidence is drawn up against the height of
the spacers.

effects is responsible for the difference, as the cage types
differ on several design features, the alleged motivation of
the surgeons to use different spacer heights are not
confirmable, and the difference between the two spacers is
not statistically significant.
Concluding, we are not able to quote which of these three

dislocation, we did not observe any differences in the clinical
outcomes between the two groups. This is consistent with
several other studies that have also not found any influence
of subsidence on the actual clinical improvement [7, 14, 16-
21]. Furthermore, none of the patients with a subsided or
dislocated cage needed to be reoperated for this.
Nonetheless, we would still consider the Intromed spacer’s
higher rates of subsidence and dislocation to be an increased
safety risk. Studies with larger sample sizes might indeed
find more extreme cases of subsidence or dislocation that do
negatively impact on clinical outcomes and/or require
reoperation.
Despite this difference in the rates of subsidence and

majority of patients with cervical radiculopathy or
myelopathy. Complications were rare. There were no
differences in clinical outcomes between patients receiving
the Intromed or AMT SHELL spacer. The AMT SHELL
spacer had lower rates of subsidence and dislocation than the
Intromed spacer. We believe the reason for this is that the
retaining pins of the AMT SHELL spacer make it feel more
secure at slightly lower spacer heights, and the use of a lower
spacer height leads mechanically to less subsidence and
dislocation. Although subsidence and dislocation did not
affect the clinical outcomes, it could do so. Thus all other
factors being equal, we believe patients are a bit safer with
the spacer with retaining pins than with the spacer without.
In summary, ACDF led to substantial recovery in the
ACKNOWLEDGEMENTS

the Introduction; Ayce Atalay, MD, and Michael Hanna,
We would like to thank: Martin Schomacher for drafting

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Polyetheretherketone (PEEK) Spacers for Anterior Cervical Fusion The Open Orthopaedics Journal, 2011, Volume 5 353
PhD, (both Mercury Medical Research & Writing) for
performing data quality-control and contributing to statistical
analysis; and Michael Hanna, PhD, for revising the
manuscript. The study was supported by "Deutsche Arthrose
Hilfe e.V.".
CONFLICT OF INTEREST
None declared.
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Received: April 11, 2011

Revised: July 5, 2011 Accepted: July 12, 2011
© Lemcke et al.; Licensee Bentham Open.

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