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SPINE Volume 24, Number 22, pp 2377–2382
©1999, Lippincott Williams & Wilkins, Inc.
Biomechanical Evaluation of Five Different Occipito-
Atlanto-Axial Fixation Techniques
Itaru Oda, MD,*† Kuniyoshi Abumi, MD,†, Laura C. Sell,* Charles J. Haggerty, MHS,*
Bryan W. Cunningham, MSc,* and Paul C. McAfee, MD*‡
Study Design. The stabilizing effects of five different
occipitocervical fixations were compared.
Objectives. To evaluate the construct stability pro-
vided by five different occipito-atlanto-axial fixation tech-
niques.
Summary of Background Data. Few studies have ad-
dressed occipitocervical reconstruction stability and no
studies to date have investigated anterior-posterior trans-
lational stiffness.
Methods. A total of 21 human cadaveric spines were
used. After testing intact spines (CO-C2), a type II dens
fracture was created and five different reconstructions
were performed: 1) occipital and sublaminar wiring/rect-
angular rod, 2) occipital screws and C2 lamina claw
hooks/rod, 3) occipital screws, foramen magnum screws,
and C1-C2 transarticular screws/rod, 4) occipital screws
and C1-C2 transarticular screws/Y-plate, and 5) occipital
screws and C2 pedicle screws/rod. Biomechanical testing
parameters included axial rotation, flexion/extension, lat-
eral bending, and anterior-posterior translation.
Results. Pedicle screw fixation demonstrated the high-
est stiffness among the five reconstructions (
P
,0.05).
The two types of transarticular screw methods provided
greater stability than hook or wiring reconstructions (
P
,
0.05). The C2 claw hook technique resulted in greater
stability than sublaminar wiring fixation in anterior-pos-
terior translation (
P
,0.05). However, the wiring proce-
dure did not significantly increase the stiffness levels be-
yond the intact condition under anterior-posterior
translation and lateral bending (
P
.0.05).
Discussion. C2 transpedicular and C1-C2 transarticular
screws significantly increased the stabilizing effect com-
pared to sublaminar wiring and lamina hooks. The im-
proved stability afforded by C2 pedicular and C1-C2 trans-
articular screws offer many potential advantages
including a high rate of bony union, early ambulation, and
easy nursing care.
Conclusion. Occipitocervical reconstruction tech-
niques using C1-C2 transarticular screws or C2 pedicle
screws offer biomechanical advantages compared to sub-
laminar wiring or lamina hooks. Pedicle screw fixation
exhibited the highest construct stiffness among the five
reconstructions. [Key words: occipitocervical reconstruc-
tion, biomechanics, spinal instrumentation, transarticular
screw, pedicle screw] Spine 1999;24:2377–2382
Since Foester’s description of occipitocervical recon-
struction using a fibular strut graft in 1927,
13
many sta-
bilizing procedures with and without internal fixation
have been reported for craniocervical reconstruc-
tion.
5,8,11,12,16,17,19,21,23,27,29,33,35
Most conventional
instrumentation techniques using posterior wiring re-
quire long fusions and/or postoperative rigid external
support due to insufficient initial stability. Recently, new
occipito-atlanto-axial fixation techniques, using C1-C2
transarticular screws or C2 pedicle screws as fixation
anchors, have been employed to improve reconstruction
stability.
2,3,14,15,24,32,34
However, few studies have ad-
dressed the relative biomechanical advantages, particu-
larly with regard to anterior translational stability, of
these new techniques.
7
Moreover, unstable occipitocer-
vical lesions, which need reconstruction frequently, in-
clude anterior translational instability. Therefore, fixa-
tion methods resisting anterior shear force are important
in occipitocervical fixation.
In this study, the stability of five different types of
occipito-atlanto-axial reconstructions were evaluated
under axial rotation, flexion/extension, lateral bending,
and anterior-posterior translation loading modes.
Materials and Methods
Specimen Preparation and Biomechanical Testing. A to-
tal of 21 fresh-frozen human cadaveric specimens (occiput
through C4) were utilized in this investigation. In preparation
for biomechanical testing, the specimens were thawed to room
temperature and cleaned of all residual musculature, with care
taken to preserve all ligamentous structures. The specimens
were harvested from 10 women and 11 men cadavers with an
average age of 71.8 611.1 years. To document abnormalities
and degenerative changes, anteroposterior and lateral radio-
graphs were taken. No fractures or other abnormalities were
found beyond the normal degenerative changes.
All biomechanical testing was performed using an MTS 858
Bionix Test System (MTS System Inc., Minneapolis, MN). A
5.2 mm diameter screw (Grip-Rite Fasteners™, Dallas, TX)
and a 2.0 mm diameter K-wire were longitudinally inserted
from the C4 to C2 vertebral body to immobilize C2-C4 motion
segments. The C3 and C4 vertebral bodies were secured in a
rectangular steel tubing container using four-point compres-
sion screws and the occiput (C0) was cast in polyester resin
molds (Dynatron/Bondo Corp., Atlanta, GA) reinforced by
two 3.0 mm diameter K-wires (Figure 1). To avoid screw pen-
From the *Orthopaedic Biomechanics Laboratory, The Union Memo-
rial Hospital, Baltimore, Maryland, †Department of Orthopaedic Sur-
gery, Hokkaido University School of Medicine, Sapporo, Japan, and
‡Scoliosis and Spine Center, Towson, Maryland.
Supported by Orthopaedic Associates Research Foundation, Inc.,
Towson, Maryland. The authors would like to thank Sven Olerud,
MD, PhD, for instructing his instrumentation techniques, Dieter Grob,
MD, AcroMed Corporation, and Showa Ika Kohgyo, Co. Ltd. for
providing spinal instrumentation.
Acknowledgment date: July 13, 1998.
First revision date: February 1, 1999.
Acceptance date: March 22, 1999.
Device status category: 7.
2377
etration into the polyester resin when inserting bicortical oc-
cipital screws, cotton was placed on the inner surface of the
occipital screw fixation region prior to casting. At the time of
specimen mounting on the testing table, the anterior wall of C3
was inclined anteriorly by 20°, and the base of the occiput was
oriented horizontally, as previously described by Panjabi
et al.
25,26
The remaining unconstrained segments (C0-C2) were first
tested under five nondestructive static loading conditions to
evaluate the intact stability of the operative motion segments
[Intact]. The loading modes included axial rotation (61.5 Nm
with 50 N compressive preload), flexion/extension (61.5 Nm),
lateral bending (61.5 Nm), and anterior-posterior translation
(650 N). Each testing mode was repeated over a period of five
ramp cycles at a rate of 10% full scale/second in axial rotation
and 20% full scale/second in the other loading modes. The first
three served as conditioning cycles with the data from the
fourth cycle used for computational analysis. In flexion/
extension, lateral bending, and anterior-posterior translation
tests, the specimen was placed horizontally using a specially
designed loading jig, achieving an application of pure moment
or translation.
19
For subsequent load applications, the axis of
rotation was determined according to previous investigations
as follows: upper third of the dens in flexion/extension, middle
third of the dens in lateral bending, and center of the dens in
axial rotation.
37
Destabilization and Reconstruction Techniques. Follow-
ing intact specimen analysis, a type II dens fracture
4
was cre-
ated anteriorly using an osteotome, and anterior translational
instability manually confirmed. The same loading tests were
Figure 1. The specimen orientation for axial rotation testing is
demonstrated. The C3 and C4 vertebral bodies were secured into
a rectangular steel tubing container using four-point compression
screws and the occiput (C0) was cast in polyester resin molds.
Figure 2. Five types of occipito-atlanto-
axial fixation systems are demonstrated. A,
Wiring: occipital and sublaminar wiring
with rectangular rod; B, O-H: two occipital
screws and C2 lamina claw hook/rod; C,
OF-TS: two occipital screws, two foramen
magnum screws, and C1-C2 transarticular
screws/rod; D, O-TS: two occipital screws
and C1-C2 transarticular screws/Y-plate; E,
O-PS: six occipital screws and C2 pedi-
cle screws/rod.
2378 Spine •Volume 24 •Number 22 •1999
then repeated on the destabilized specimens [Fracture] fol-
lowed by randomization of the 21 specimens into three groups
based on reconstruction procedure (Figure 2).
Group I (n 57):
1) Occipital and sublaminar wiring with rectangular rod
(n 57) [Wiring];
2) Two occipital screws - C2 lamina claw hooks with rod
(n 57) [O-H];
3) Two occipital screws - two foramen magnum screws -
bilateral C1-C2 transarticular screws with rod (n 57) [OF-
TS].
Group II (n 57):
1) Occipital and sublaminar wiring with rectangular rod
(n 57) [Wiring];
2) Two occipital screws - bilateral C1-C2 transarticular
screws with Y-plate (n 57) [O-TS].
Group III (n 57):
1) Occipital and sublaminar wiring with rectangular rod
(n 57) [Wiring];
2) Six occipital screws - C2 pedicle screws with rod (n 5
7) [O-PS].
For the wiring/rod reconstruction (Wiring), a combination
of 3.0 mm diameter rectangular rod (K-wire) and 0.9 mm di-
ameter double wire (Ethicon, Cincinnati, OH) were used.
33
For
O-H and OF-TS fixations, the Olerud Cervical System (Norel-
Opedic AB, Sweden) was employed
24
and included occipital
rods (3.5 mm in diameter) and unicortical occipital screws,
while transarticular screws penetrating the anterior cortex of
C1 (4.0 mm in diameter) were utilized for the OF-TS fixation.
22
The Y-Plus Plate (Endoplus, Switzerland), including unicortical
occipital screws (3.5 mm in diameter) and C1-C2 transarticular
screws penetrating the anterior wall of C1 (3.5 mm in diame-
ter), was used for O-TS fixation.
14,15
A combination of cervical
pedicle screw (CPS) (4.0 mm in diameter) (AcroMed Co.,
Cleveland, OH), newly designed CPS/rod connector (Ac-
roMed, Co., Cleveland, OH) and modified Spine System oc-
cipito-cervical rod (Aesculap, Tuttlingen, Germany) (4.75 mm
in diameter) were used for O-PS.
2,3
The C2 pedicle screw was
inserted in the manner previously described
3,30,31
and the C2
pedicular and occipital screws had bicortical purchase. A trans-
verse rod connector was utilized for O-H, OF-TS, and O-PS fix-
ations.
The same loading tests were repeated following each recon-
struction. To prevent desiccation during testing, specimens
were moistened with 0.9% NaCl sterile irrigation solution.
Data Analysis. Stiffness values of the reconstructed spines
were calculated using data from the fourth loading cycle. Ro-
tational stiffness (axial rotation, flexion/extension, and lateral
bending) was defined as a ratio of applied torque (Nm) to the
corresponding angular deformation (degrees). Anterior-
posterior translational stiffness was calculated as a ratio of
applied force (Ns) to the corresponding displacement (millime-
ters). In the context of this study, we equate the terms “stiff-
ness” and “stability.” Statistical significance was determined
using a one-way analysis of variance (ANOVA) combined with
a Student—Newman—Keuls test at 95% confidence.
Results
Biomechanical analysis of the intact, fracture, and wiring
constructs of Group I, II, and III (n 57 / each group)
demonstrated no statistical differences among the three
groups under all testing modes. Therefore, the three
groups were combined for further analysis. In other
words, intact, destabilized, and wiring had 21 specimens
each, while other reconstructions had 7 each.
Flexion/Extension Stability
Mean stiffness values as well as standard deviations
(STDs) are shown in Figure 3. Statistical difference was
indicated among the seven constructs (F 532.52, P5
0.000). The dens fracture decreased construct stiffness
compared to the intact specimen (P,0.05), and all
reconstruction methods provided significantly greater
stability than the intact spine (P,0.05). Pedicle screw
fixation (O-PS) demonstrated statistically higher stiffness
levels than all other reconstruction methods (P,0.05),
and reconstructions using C1-C2 transarticular screws
(OF-TS, O-TS) significantly improved construct stiffness
compared to wiring (P,0.05). No statistical difference
was demonstrated among O-H, OF-TS, and O-TS fixa-
tions (P.0.05); however, O-H was not significantly
different from wiring (P.0.05).
Anterior-Posterior Translational Stability
Figure 4 demonstrates mean stiffness values and STDs
under anterior-posterior translation. Statistical signifi-
cance was detected among the seven constructs (F 5
47.33, P50.000). The dens fracture decreased construct
stiffness compared to the intact specimen (P,0.05) and
pedicle screw fixation (O-PS) significantly increased stiff-
ness compared to other fixation techniques (P,0.05).
No statistical difference was observed between the two
transarticular screw fixations (OF-TS, O-TS) (P.0.05);
Figure 3. Mean stiffness values as well as STDs in flexion/
extension are shown. Intact: intact specimen; Wiring: rectangular
rod/wiring; O-H: occipital screws/C2 lamina claw hooks; OF-TS:
occipital and foramen magnum screws/C1-C2 transarticular
screws; O-TS: occipital screws/C1-C2 transarticular screws; O-PS:
occipital screws/C2 pedicle screws. NS5
P
.0.05.
2379Occipitocervical Reconstruction Techniques •Oda et al
however, transarticular screw fixation (OF-TS, O-TS)
provided statistically greater stiffness levels compared to
reconstructions using hook or wiring (O-H, Wiring)
(P,0.05). Hook instrumentation (O-H) revealed signif-
icantly higher stability than wiring (P,0.05), and
among the five reconstructions, only wiring was not sig-
nificantly different from the intact spine (P.0.05).
Lateral Bending Stability
Mean stiffness values as well as STDs are shown in Figure
5. Statistical difference was observed among the seven
constructs (F 550.60, P50.000). The stiffness of the
fractured spine was significantly lower than the intact
specimen (P,0.05). Pedicle screw fixation (O-PS) pre-
sented significantly greater stiffness levels than all other
reconstruction methods (P,0.05). Transarticular fixa-
tions (OF-TS, O-TS) provided statistically higher stabil-
ity than hook or wiring constructs (O-H, wiring) (P,
0.05). In addition, OF-TS revealed significantly greater
construct stiffness than O-TS (P,0.05). The hook in-
strumentation (O-H) did not significantly improve sta-
bility compared to the wiring fixation (P.0.05), and
among the five reconstruction techniques, the wiring
procedure did not significantly increase the stiffness lev-
els beyond the intact condition (P.0.05).
Axial Rotation Stability
Figure 6 indicates mean stiffness values and STDs under
axial rotation. Statistical significance was found among
the seven constructs (F 5397.18, P50.000). The dens
fracture decreased construct stiffness compared to the
intact specimen (P,0.05). All reconstruction methods
afforded significantly greater stability than the intact
spine (P,0.05), and pedicle screw instrumentation (O-
PS) indicated statistically higher stiffness levels compared
to all other reconstruction techniques (P,0.05). Trans-
articular fixation groups (OF-TS, O-TS) exhibited signif-
icantly greater stiffness than both hook and wiring recon-
structions (O-H, wiring) (P,0.05). Moreover, O-TS
instrumentation revealed statistically higher rigidity than
OF-TS (P,0.05), and once again, no statistical differ-
ence was found between O-H and wiring (P.0.05).
Discussion
Many fixation techniques have been reported for occipi-
tocervical reconstruction.
5,8,11,12,16,17,19,21,23,27,29,33,35
Among those techniques, wiring/strut-bone-graft and
wiring/rod were common; however, some required ex-
tended fusion and rigid postoperative external support
due to the mechanical weakness of the fixation systems.
To improve initial stability, new instrumentation sys-
Figure 4. Mean stiffness values as well as STDs in anterior-
posterior translation are illustrated. Intact: intact specimen; Wir-
ing: rectangular rod/wiring; O-H: occipital screws/C2 lamina claw
hooks; OF-TS: occipital and foramen magnum screws/C1-C2 trans-
articular screws; O-TS: occipital screws/C1-C2 transarticular
screws; O-PS: occipital screws/C2 pedicle screws. NS5
P
.0.05.
Figure 5. Mean stiffness values as well as STDs in lateral bending
are demonstrated. Intact: intact specimen; Wiring: rectangular
rod/wiring; O-H: occipital screws/C2 lamina claw hooks; OF-TS:
occipital and foramen magnum screws/C1-C2 transarticular
screws; O-TS: occipital screws/C1-C2 transarticular screws; O-PS:
occipital screws/C2 pedicle screws. NS5
P
.0.05.
Figure 6. Mean stiffness values as well as STDs in axial rotation
are indicated. Intact: intact specimen; Wiring: rectangular rod/
wiring; O-H: occipital screws/C2 lamina claw hooks; OF-TS: occip-
ital and foramen magnum screws/C1-C2 transarticular screw;
O-TS: occipital screws/C1-C2 transarticular screws; O-PS: occip-
ital screws/C2 pedicle screws. NS5
P
.0.05.
2380 Spine •Volume 24 •Number 22 •1999
tems using C1-C2 transarticular screw or C2 pedicle
screw for fixation have been developed.
3,14,15,24,29,33
In
1991, Grob et al first reported an occipitocervical plate
fixation technique utilizing occipital and C1-C2 transar-
ticular screws as fixation anchors.
14
Recently, Olerud
developed a new cervical screw/hook and rod-based in-
strumentation technique for occipitocervical reconstruc-
tion.
24
Moreover, in 1997, Abumi et al reported a
craniocervical fixation technique using occipital and C2
pedicle screws.
2,3
This in vitro study serves as the first
report to biomechanically evaluate these new techniques
compared to conventional rod/wiring fixation.
The, artificially produced, type II dens fracture signif-
icantly destabilized the specimens in all testing modes
(P,0.05). However, axial compressive instability was
not created in this study, although it is frequently found
in patients with rheumatoid arthritis as an upward mi-
gration of the dens. Furthermore, the stabilizing influ-
ence of the neuromuscular system was not considered in
current study; therefore, there are some limitations when
applying the acquired data to clinical situations.
The reconstruction techniques using screws and rod/
plate (O-PS, OF-TS, O-TS) achieved significantly greater
stability than the rod/wiring fixation (wiring) in all test-
ing parameters. In addition, one advantage of these tech-
niques is that the laminae is not required for fixation;
therefore, these can be used after posterior decompres-
sion.
1,3,14,15,32,34
Furthermore, using screw and rod/
plate reconstruction techniques, there is nothing to be
inserted into the spinal canal, while sublaminar wiring
may be hazardous in patients with a narrow canal or an
already compromised cord. Consequently, these screw
and rod/plate techniques have a significant advantage
over rod/wiring fixation. The enhanced stability afforded
by C1-C2 transarticular and C2 pedicle screw fixation
may provide many potential benefits including a high
rate of bony union, early ambulation, and easy nurs-
ing care.
A recent clinical survey has demonstrated that al-
though up to 20% of the population have normal ana-
tomical variations that place them at risk for complica-
tions associated with bilateral transarticular screw
placement, the risk of vertebral artery injury is 2.2% per
screw inserted, and the risk of subsequent neurological
deficit from iatrogenic vertebral artery injury is small.
38
Abumi et al reported his clinical experience and stated
that of the 24 screws inserted into the C2 pedicle, none
were found to be at risk for causing neurovascular in-
jury.
3
A recent anatomic study by Ebraheim et al sug-
gested that pedicle screws insertion guided directly by the
medial and superior aspect of the C2 pedicle can be safely
performed.
10
In terms of neurovascular complications,
C2 pedicle screws are as safe as C1-C2 transarticular
screw fixation.
The combination of occipital screws and C2 pedicle
screws connected by 4.75 mm diameter rod (O-PS) dem-
onstrated statistically greater rigidity than any other re-
construction methods, suggesting the C2 pedicle pro-
vides an excellent cervical fixation anchor for
occipitocervical reconstruction. Using an in vitro cervical
spine injury model, Kotani et al reported that transpe-
dicular screw fixation provides higher stability than con-
ventional wiring and lateral mass screw/plate fixation.
18
Moreover, Jones et al reported that the cervical pedicle
screw exhibited greater pull-out resistance than the lat-
eral mass screw.
20
These studies are very consistent with
the high stabilizing effect of the C2 pedicle screw dem-
onstrated in the present study.
It is inferred that the stabilizing effect of C1-C2 trans-
articular screw fixation is higher than that of C2 pedicle
screw because C1-C2 transarticular screw penetrates
four layers of cortical bone, while C2 pedicle screw pur-
chases two layers. In this investigation, however, pedicle
screw fixation (O-PS) provided higher construct stiffness
compared to transarticular screw fixation (OF-TS,
O-TS). For pedicle screw fixation, 4.75 mm diameter
rods and newly designed screw/rod connectors were
used. This connector shares the basic design as the
ISOLA system (AcroMed Co., Cleveland, OH), which
has been reported as quite secure.
6
On the other hand,
the Y-plate system, used for transarticular fixation (O-
TS), has unconstrained screw/plate junctions. In addi-
tion, 3.5 mm diameter rods were used for the transartic-
ular fixation with the Olerud Cervical System. These
differences in hardware design probably account for the
higher construct stiffness of the pedicle screw fixation
compared to that of the transarticular fixation.
Fixation using a C2 lamina claw hook (O-H) demon-
strated significantly lower stability than reconstructions
using C1-C2 transarticular (OF-TS, O-TS) or C2 pedicle
screw (O-PS), suggesting the C2 lamina claw hook tech-
nique is not as effective as C1-C2 transarticular and C2
pedicle screw anchors. On the other hand, the combina-
tion of occipital screws and C2 lamina claw hooks (O-H)
provided higher rigidity than rectangular rod fixation
(wiring) in anterior-posterior translation. From the bio-
mechanical point of view, the combination of occipital
screw and C2 lamina claw hooks are better than rectan-
gular rod fixation. The combination of rod and wiring
(wiring) did not significantly improve stability beyond
the level of the intact spine in anterior-posterior transla-
tion and lateral bending. Hence, this reconstruction
method should probably not be employed in the cases
who have severe anterior translational instability.
For occipital fixation, studies of occipital bone thick-
ness and screw pull-out testing have been per-
formed.
9,28,36
According to those studies, the bone of the
cranium in the posterior midline is more than twice as
thick as the lateral parts, and the strength of screw fixa-
tion is proportional to the bone’s thickness. In this study,
two midline screws and two foramen magnum screws
were used with C1-C2 transarticular screw (OF-TS),
while six para-midline screws utilized with C2 pedicle
screw (O-PS). Currently, the best screw fixation method
for the occiput remains unknown; hence, further biome-
2381Occipitocervical Reconstruction Techniques •Oda et al
chanical studies investigating the stability of occipital
fixation are required for certainty.
Conclusions
The biomechanical stability afforded by five types of oc-
cipito-atlanto-axial reconstruction techniques was eval-
uated in a human cadaveric spine model. The rod/wiring
combination did not significantly improve stability be-
yond the level of the intact spine in anterior-posterior
translation and lateral bending. The combination of oc-
cipital screws and C2 lamina claw hook exhibited
greater stiffness than the rod/wiring technique under an-
terior-posterior translation loading, and reconstruction
methods using C1-C2 transarticular and occipital screws
revealed significantly higher stability compared to the
wiring or hook procedures. The combination of six oc-
cipital screws and C2 pedicle screws provided the highest
stability among the five reconstructions.
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Address reprint requests to
Itaru Oda, MD
Department of Orthopedic Surgery
Hokkaido University School of Medicine
Kita-15, Nishi-7, Kita-ku
Sapporo, 060-0015, Japan
E-mail: odaitr@aol.com
2382 Spine •Volume 24 •Number 22 •1999
