M. GOREN[EK et al.: TOWARDS THE OPTIMUM SPINAL FUSION DEVICE
99–102
TOWARDS THE OPTIMUM SPINAL FUSION DEVICE
V ISKANJU OPTIMALNEGA UMETNEGA MEDVRETEN^NEGA
VSADKA
Matev` Goren{ek1, Monika Jenko2, Bo{tjan Kocjan~i~3, Drago Dolinar3, Urban Brulc1
1MD Medicina, Bohori~eva 5, 1000 Ljubljana, Slovenia
2Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
3University Medical Centre Ljubljana, Department of Orthopaedic Surgery, Zalo{ka 9,1000 Ljubljana, Slovenia
urban.brulc@gmail.com
Prejem rokopisa – received: 2017-11-22; sprejem za objavo – accepted for publication: 2018-01-04
doi:10.17222/mit.2017.197
Spinal interbody fusion (spondylodesis) remains a gold standard for the treatment of instability, deformity and degenerative
disease of the spine. Over the past 40 years surgical techniques and implant-device (cage) technology for spinal fusion have
changed significantly. Design and materials have evolved with one common goal, to develop suitable implants that would
address all three major issues of the procedure: stability, restoration of lordosis and osteointegration. Historically, two main
materials have been utilized in the creation of cages: titanium (Ti) and polyetheretherketone (PEEK). The focus of spinal
surgeons is constantly shifting from one material to another, because of the aggressive, commercial drive from the industry. The
choice of optimal spinal interbody fusion device was always a matter of controversy. Therefore, this article aims to provide an
overview of the different materials and designs from the biomechanical and also clinical aspect. A systematic review of the
literature was made. The inadequate available clinical trials and lack of comparisons between different models have prevented
definitive conclusions; therefore, further prospective randomized studies are necessary in the future to define one cage as a
mainstay of clinical practice.
Keywords: spondylodesis, cage, titanium, polyetheretherketone
Zatrditev hrbteni~nega segmenta (spondilodeza) predstavlja zlati standard zdravljenja nestabilnosti, deformacij in degene-
rativnih bolezni hrbtenice. V zadnjih 40 letih so se kirur{ka tehnika in umetni medvreten~ni vsadki (kletke) ob~utno spremenili.
Oblika in materiali so se razvijali z enim skupnim ciljem, to je odkriti vsadek, ki bi omogo~al stabilnost hrbteni~nega segmenta,
vzdr`evanje lordoze ter osteointegracijo. Zgodovinsko gledano sta bila dva najpogosteje uporabljena materiala titan in
polietereterketon. Dandanes med hrbteni~nimi kirurgi ostaja nesoglasje glede optimalne izbire umetnega medvreten~nega
vsadka. Namen tega ~lanka je predstaviti prednosti in slabosti najpogosteje uporabljenih materialov. Na podlagi aktualne stro-
kovne literature je bila opravljena primerjava biomehanskih in klini~nih rezultatov razli~nih vsadkov. Glede na pomanjkljivost
naklju~nih prospektivnih multicentri~nih {tudij ostaja izbira optimalnega umetnega hrbteni~nega vsadka {e naprej problema-
ti~na.
Klju~ne besede: spondilodeza, umetni medvreten~ni vsadek, titan, polietereterketon
1 INTRODUCTION
Spinal interbody fusion (spondylodesis) remains a
"gold standard" for the treatment of spinal instability,
deformity, degenerative disc disease, infection and failed
decompressive spinal surgery.1 Substantial effort has
been made to optimise surgical techniques and even
more substantial to develop suitable implants that would
address all three major issues of the procedure: stability,
restoration of lordosis and osteointegration. The era of
interbody fusion began with mesh cages, manly pro-
duced from titanium.2 This first generation of cages was
met with great expectations, but little knowledge of
anatomical postulates such as sagittal balance and the
importance of segmental lordosis. Results were relatively
favourable in the early post-surgical period, with good
interbody fusion, but this deteriorated with years.3 The
beginning of the 21st century brought up the advent of
implants made of polyetheretherketone or popular
PEEK. Its biomechanical properties were closer to bone
then those of the titanium implants. Therefore, the hopes
were high, but again short-lived. PEEK implants have
indeed improved the possibility of the anatomical rest-
auration of diseased segment, but failed to persuade with
its limited osteointegration.4 Currently, there is a lack of
consensus among spinal surgeons. The focus is con-
stantly shifting from one material to another, because of
aggressive, commercial drive from the industry, which
develops and promotes new and new products. Con-
sequently, the choice of optimal spinal interbody device
has remained a matter of controversy. This article
therefore aims to provide an overview of the different
materials and designs from biomechanical and also
clinical aspect.
2 TITANIUM CAGES
The history of spinal interbody fusion devices stems
back to the original cage implant developed by Bagby. It
was made from a stainless steel and used for the
treatment of cervical instability in thoroughbred horses.5
Titanium cages were first introduced in 1986 by Harms
Materiali in tehnologije / Materials and technology 52 (2018) 1, 99–102 99
UDK 620.1./.2:616.711:616-089 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)99(2018)
and Biederman. The design of these cages involved mesh
that has been rolled into cylindrical shape and reinforced
with rings at each end (Figure 1). They were tradition-
ally filled with autograft (spongiose bone).6
Titanium was chosen because of excellent corrosion
resistance, low density (approximately 4700 kg/m3) and
biocompatibility. Due to its ability to enhance osseointe-
gration the early results of first generation implants were
very promising.3,7 Biomechanically, Ti-alloy implants
demonstrate close approximation with surrounding bone.
It was reported that Ti-alloy surfaces with complex
micron/submicron scale roughness promote osteoblastic
differentiation and foster a specific cellular environment
that favors bone formation whereas other materials
favours fibrous tissue formation.8,9 Clinically this means
higher fusion rates, which is the main objective of spon-
dylodesis.
However, the use of titanium and its alloys also poses
several issues for interbody fusion. Firstly, there is a
mismatch between the elastic modulus of titanium (110
GPa) and that of vertebrae trabecular bone (2.1 GPa) and
cortical bone (2.4 GPa). This elastic modulus mismatch
results in reduced stress shielding around the implant
which can precipitate graft subsidence. The result is
kyphotic alignment of fused segment and aggravation of
pain.10–12 A second major issue with the use of titanium
is its high radio-density. Therefore, the accurate assess-
ment of radiographic fusion status after operation is
much harder to achieve.7
3 POLYETHERETHERKETONE CAGES
To address the before-mentioned disadvantages of
titanium and titanium alloys, polyetheretherketone
(PEEK) cages were developed (1990s, AcroMed). PEEK
is a hydrophobic, organic and thermoplastic polymer.13
The initial interbody devices constructed of plastic mate-
rial were either round or hexagonal shape designed as a
spacer with a central cavity for bone graft placement.
The hopes were high again because of their biome-
chanical properties. PEEK is perfectly inert and its
modulus of elasticity (3.5 GPa) is similar to that of
cortical bone. This may promote even load sharing and
stress distribution which can be translate into lower
subsidence rates.12,14 Some studies have suggested that
PEEK materials are relatively resistant to microbial
adhesion and hence associated with lower infection rates
than their titanium counterparts.15 Continuously, its mini-
mal radiographic signature allows optimal evaluation of
bony fusion on plain radiographs and computed tomo-
graphy (Figure 2). There is also no risk of metal allergy.4
PEEK implants have indeed improved possibility of
anatomical restauration of diseased segment, but its
chemical inertness limits an ability to osseointegrate into
the surrounding bone environment.14,16 The material does
not allow protein absorption and cell adhesion, which
leads to poor bone contact. With in vitro studies was
demonstrated that implants fabricated from PEEK are
often encapsulated by fibrous tissue. This can be due to
reduced osteoblastic differentiation of progenitor cells
and production of an inflammatory environment that
favours cell death via apoptosis and necrosis.8,17
4 TITANIUM-POLYETHERETHERKETONE
COMPOSITE CAGES
An ideal cage design would restore healthy alignment
and disc height and achieve immediate post-operative
stability, high-fusion rates and low complication rates.
To combine advantages of both materials, titanium and
PEEK, composite fusion devices were developed (Fig-
ure 3). It was shown that PEEK implants coated by a
M. GOREN[EK et al.: TOWARDS THE OPTIMUM SPINAL FUSION DEVICE
100 Materiali in tehnologije / Materials and technology 52 (2018) 1, 99–102
Figure 2: X-ray image of thoracic spine after spondylodesis. An
accurate assessment of radiographic fusion status is possible due to
the radiolucency of PEEK cage
Figure 1: Titanium mesh cage
thin layer of titanium improves bioactivity of material
and consequently enhance osseointegration.18 Recently,
biomechanical and histologic analyses were performed
in an ovine lumbar interbody fusion model. Ti-Peek con-
struct demonstrated significant increase in bone
on-growth. It was concluded that Ti-Peek inter-body
devices could potentially lead to a more robust intever-
tebral fusion relative to a standard PEEK device (Fig-
ure 4).19 On the other hand, we have studies which
contradict to a good results of composite spacers. Pre-
dominant problem may be wear debris and delamination
caused by shear loading. With a biomechanical study the
impaction process in clinical practice was simulated. It
was confirmed that titanium coated implants are suscept-
ible to impaction related wear debris and delamination.20
5 COMPARISON BETWEEN DIFFERENT
MATERIALS AND DESIGNS- CLINICAL
ASPECT
Current literature suggests comparable radiographic
and clinical outcome of titanium and PEEK devices. No
statistical significant difference was found between this
two groups in the rate of patient performance. Minimal
evidence are present for increased radiographic sub-
sidence with titanium cages, however not statistically
important.
R. F. M. R. Kersten et al.21 performed a study to
assess the clinical aspect of PEEK cages in the treatment
of degenerative disc disorders in the cervical spine.
Systematic review of all randomized controlled trials and
prospective nonrandomized comparative studies was
done. The primary outcome variable was clinical perfor-
mance of patients, and secondary was consisted of radio-
graphic scores. They found minimal evidence for better
clinical and radiographic outcome for PEEK cages
compared with bone grafts in the cervical spine, but no
differences were found between PEEK, titanium, and
carbon fibre cages. Another review of the literature was
done in June 2017 by S. Seaman et al.22 This time
titanium and PEEK cages have been evaluated in the
cervical and lumbar spine. Six large studies were
included in meta-analysis with a total of 410 patients
(Ti-228, PEEK-182) and 587 levels (Ti-327, PEEK-
260). No statistically significant difference was found
between groups in the rate of fusion (OR 1.16, 95% C.I
0.59–2.89, p = 0.686) but there was an increased rate of
subsidence with titanium cage.
The last group of cages which stays to be compared
are titanium coated PEEK cages. The novelty of this
intervention translates into a paucity of clinical trials.
Nevertheless, it was possible to find a published study
which summarise radiological and clinical outcomes at
this early stage. In the meta-analysis combined of seven
reports, authors concluded that Ti-PEEK implants are
safe and efficacious, exhibiting similar fusion rates and
clinical outcomes compared to the current standard
PEEK. There is clinical evidence substantiating the im-
proved radiographic fusion of Ti-PEEK, albeit the diffe-
rences were not significant.23,24
6 CONCLUSION
The aim of this article was to present the advantages
and disadvantages of different spinal interbody fusion
devices. On the basis of novel literature, each material
was discussed separately and at the end compared to one
another. All in all, multiple promising designs are
currently available; however, a completely optimal
material does not exist. The inadequate available clinical
trials and lack of comparisons between different models
have prevented definitive conclusions; therefore, further
prospective randomized studies are necessary in the
future to define one cage as a mainstay for clinical
practice.
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M. GOREN[EK et al.: TOWARDS THE OPTIMUM SPINAL FUSION DEVICE
Materiali in tehnologije / Materials and technology 52 (2018) 1, 99–102 101
Figure 4: X-ray image of lumbar spine after spondylodesis. Ti-PEEK
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Figure 3: Titanium endplates on a PEEK cage
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M. GOREN[EK et al.: TOWARDS THE OPTIMUM SPINAL FUSION DEVICE
102 Materiali in tehnologije / Materials and technology 52 (2018) 1, 99–102