Cyclic Loading of Sacroiliac Screws
in Tile C Pelvic Fractures
C.M.A. van Zwienen
E.W. van den Bosch
G.A. Hoek van Dijke
A.B. van Vugt
Submitted to Clinical Orthopaedics and Related
To investigate the stiffness and strength of completely unstable pelvic frac-
tures fixated both anteriorly and posteriorly under cyclic loading conditions.
Materials and Methods.
In 12 specimens a Tile C1 pelvic fracture was created. We compared the
intact situation to anterior plate fixation combined with one or two sacroiliac screws.
In 2000 measurements, each pelvis was loaded with a maximum of 400N. The
stiffness, the number of cycles before failure and the load to failure of the fixations
were measured using a 3-dimensional video system.
Both translation and rotation stiffness of the intact pelvis were superior to
the fixated pelvis. No difference in stiffness was found between the techniques
with one or two sacroiliac screws. However a significantly higher load to failure
and significantly more loading cycles before failure could be achieved using two
sacroiliac screws compared to one screw.
Although the combination of anterior plate fixation combined with two sac-
roiliac screws is not as stable as the intact pelvis, in this study embalmed aged
pelves could be loaded repeatedly with physiological forces. Given the fact that the
average trauma patient is younger, this suggests that further clinical research into
direct postoperative weight bearing can be undertaken safely.
In Tile C fractures both anterior and posterior pelvic ring are disrupted which
leads to both translation and rotation instability. Because conservative treatment
leads to a high percentage of complications and long term disability, operative treat-
ment is advocated. However, with external fixation direct postoperative weight
bearing is not possible1-3. Greater stability can be achieved by internal fixation,
consisting of a combination of posterior and anterior fixation3,4. Despite the supe-
rior stability obtained by internal fixation several institutions still limit weight bear-
ing after internal fixation for considerable time5-10. Although ideally internal fixa-
tion would provide enough stability to allow early mobilisation of the patient, most
biomechanical studies have shown inferior stability compared to the intact situa-
Although several authors studied the effect of various methods of (internal)
fixation for unstable pelvic ring fractures, only a few reports have studied cyclic
loading. Pohlemann loaded one specimen in which a sacral fracture was fixated
with small fragment AO plates 10,000 cycles with 60% of the body weight, after
which it showed no sign of loosening of the implants13. Meissner loaded isolated
symphyses fixated by plate and banding techniques with a force equalling 50% of
the physiological load over 55,500 cycles14. Banding techniques led to early failure.
Plate fixation showed better stability provided adequate grip of the screws could
be obtained initially. Loading with 100% of the body weight caused early failure.
They concluded that patients with open book fractures treated by plate fixation
could tolerate early half weight bearing, although no information was obtained
about the role of the injury to the posterior pelvic ring.
In this study we investigated the combination of an anterior plate with poste-
rior sacroiliac screw fixation in Tile C fractures. In order to simulate weight bear-
ing the pelvis was loaded 2000 times with a maximum of 400N, which equals the
upper body weight in adults15. A 3-dimensional video system was used to measure
the displacement between the various fracture parts in order to determine the
stiffness, strength and endurance of the fixation16.
Cyclic Loading in Pelvic Fractures
MATERIALS AND METHODS
We used 12 embalmed cadaveric pelves, which were dissected, removing
the femora, lumbar vertebrae and all muscles. The ligamentous structures, includ-
ing the sacrospinous and sacrotuberous ligaments, were left intact. However, in
three specimens these ligaments were damaged in previous experiments. All speci-
mens were over 60 years old. A Tile C1 fracture was created by disruption of the
pubic symphysis and a sacral fracture in the lateral mass was made using a saw.
All pelves were stabilised anteriorly with a 4-hole self compression plate (3.5
mm x 50 mm) across the symphysis (Biomet®, Warsaw, In., U.S.A.). Posteriorly
one or two 70 mm canulated partially threaded, cancellous lag screws (Biomet®,
Warsaw, In., U.S.A.) with washer were inserted. The screw(s) were placed
through the posterior ilium and into the first sacral vertebral body across the sacro-
iliac joint, according to the technique of Matta and Saucedo4. The fixation quality
was scored based on the grip of the screws and we made a clinical estimation of
the bone quality during dissection on a three point scale.
To enable the application of load to the pelvic ring, the sacrum was fixed
between two plates with screws and methylmethacrylate-polymere resin
(Demotec®, Demotec Siegfried Demel, Nidderau, Germany). The pelvis was ori-
ented with anterior superior iliac spines and the symphysis in the frontal plane
Figure 1. Laboratory setting for loading pelves. On the foreground right the
computer required for measuring displacements with on top the view from the
camera. On the left the computer required for loading the pelvis. In the middle
the pelvis mounted in the frame. Clearly visible on the pelvis are the markers.
which is approximately comparable to the physiological position during standing13,15.
The load was applied by introducing a force to a plate attached to the ilium. Through
an extension device the pelvis was loaded along a vertical line of action passing
through the sacroiliac joint. This approximates forces during weight bearing.
With a 3D video system displacements were measured in all 6 degrees of
freedom (3 dislocations and 3 rotations). To enable the computerized video regis-
tration of bone displacements, clusters of four infrared light reflecting markers
were attached to the cranioventral edge of the first sacral vertebral body and to
both superior anterior iliac spines. Two markers were placed bilaterally, about 2
cm from the sacroiliac joint and two markers were positioned on both superior
rami of the pubic bone close to lateral edges of the plate (figure 1). The markers
were illuminated by an infrared light source mounted on the cameras. The image
coordinates from the two cameras were combined to three-dimensional spatial
coordinates using Direct Linear Transformation17,18. From previous tests the reso-
lution of the system proved to be about 0.1 mm. For each pelvis baseline measure-
ments of the intact situation were obtained. After a unilateral Tile C1 fracture was
created and the pelvis was fixated with combined anterior and posterior fixation, it
was loaded 2000 times with a maximum load of 400N and with an increment of
100 N11,15,19. The pelves were randomized in two groups: 6 were posteriorly fix-
ated with one sacroiliac screw and 6 with two screws. If the pelves were intact
after cyclic loading, the load to failure, which was defined as the force required to
produce 5 mm displacement at the sacral fracture or as 10 mm at the symphysis,
We investigated the stiffness of the fixation, the load to failure and the number
of cycles until failure. We defined the translation stiffness (in N/mm) of the fixa-
tion as the slope of the load displacement curves of the ilium with respect to the
sacrum. The rotation stiffness was defined as the applied load divided by the ob-
served rotation in N/degree because the exact moment was not known. For the
statistical calculations we used SAS version 6.12 of the SAS institute Inc, Cary,
NC, USA. and SPSS version 9.0 of SPSS Inc., Chicago, IL, USA. In order to
compare the translation stiffness, the rotation stiffness and the load to failure of the
two fixation methods we performed both univariate and multivariate analyses. As
baseline we examined the translation/rotation stiffness of the intact pelvis. As co-
variables we used the fixation technique, bone quality, and fixation quality. Be-
cause the distribution was skewed we applied a log transformation to the data and
median and range were provided instead of mean and standard deviation. The log
rank test was used to calculate the difference in cycles until failure.
Cyclic Loading in Pelvic Fractures
In the intact situation generally less than 1 mm displacement was observed
between the two pubic rami when loaded to 400N. In most pelves the ipsilateral
marker moved in ventral and to a lesser degree in cranial direction. At the sacro-
iliac joint in most cases less than 1 mm displacement was seen. In three cases
more than 1 mm was observed with a maximum of 4.3 mm (median 0.7 mm).
While hardly any diastases was found, most movement was in cranial direction.
No significant effect of the damage of sacrotuberal ligaments was observed.
After fixation most displacement at the symphysis was seen in cranial and
dorsal direction (median 2.7 mm and 1.4 mm respectively). Diastases was less
prominent (median 0.4 mm). At the sacral fracture most displacement was seen in
cranial direction (median 6.5 mm), lesser movement was found in ventral and
lateral direction (median 0.6 mm and 0.9 mm).
The median and range of the translation and rotation stiffness of the ilium
with respect to the sacrum, when loaded up to 400N, are summarized in table 1.
Rotation was generally seen around an axis which ran approximately through the
symphysis and the medial tip of the sacroiliac screws. The loaded hemipelvis ro-
tated upwards and medially around this axis. No significant differences were found
for translation or rotation stiffness between the techniques with one or two sacro-
iliac screws. The intact situation was significantly superior to the fixated situation
(p < 0.022). In multivariate analysis the effect of the fixation quality was signifi-
cant for the translation stiffness (p=0.047). The other covariables were not signifi-
)mm/N( s s e n f f i t sn o i t a l s na rT
t c a t n iw e r c sc a i l i o r c a s e nosw e r c sc a i l i o r c a sowt
n a i deM07214061
) e e rge d /N( s s en f f i t sn o i t a t oR
n a i deM669482624
Table 1. Movements of the ilium versus the sacrum: translation and rotation stiffness
The load to failure as previously defined is shown in table 2. The technique
with two sacroiliac screws was significantly superior when measured at the sym-
physis (p= 0.047) and showed a strong trend at the sacral fracture (p = 0.088).
In graph 1 a survival curve for number of cycles which could be completed
without failure is shown. Failure occurred significantly later for the technique with
two sacroiliac screws for both definitions of failure. With a log rank test p = 0.027
and p = 0.017 were found for measurement of the load to failure at the sacral
fracture and at the symphysis respectively. Quality of the fixation was a significant
covariable for longer endurance (p=0.018 and p=0.026).
For the pelves that completed the entire loading protocol without failure the
stiffness over the first and the last 250 measurements were compared in order to
examine weakening of the osteosynthesis over time. Although the difference be-
tween the initial stiffness and the final stiffness was not significant (p = 0.067), the
median overall decrease in stiffness was 23.1 %.
Graph 1. Kaplan-Meier survival curve of the endurance of
the fixation until failure measured as 10 mm displacement
at the symphysis.
Cyclic Loading in Pelvic Fractures
In the literature several authors have used sacroiliac screws11-13,19,21-24. How-
ever, no study has examined the sacroiliac screw fixation under cyclic loading
conditions. Most authors found similar results for sacroiliac plate and screw fixa-
tion 19,23 but the fixation remained inferior compared to the intact pelvis1,11,12,15,21,22.
The aim of this study was to compare the stability of completely unstable pelvic
fractures, fixated with a symphyseal plate and one or two sacroiliac screws, ver-
sus the intact situation under cyclic loading conditions. In 12 embalmed pelves we
determined the stiffness, the load to failure and the endurance using a 3D video
system measuring displacement of the fracture parts.
The smooth fracture surface, created by the use of a saw, and the lack of
muscle support represent a worst case scenario11,25,26. Despite this fact, the use of
combined anterior and posterior pelvic ring fixation allowed us to apply a physi-
ological force (representing the upper body mass).
Routt recommends the use of anterior fixation in Tile C fractures to be lim-
ited to fractures in which there is over 1 cm displacement in any direction27,28. In a
comparison between the load to failure in this study and in a previous experiment in
which isolated sacroiliac screw fixation was performed, we found a significantly
e r u t ca r f l a r cas t a t nem e c a l ps i dmm5
w e r c s c a i l i o r c a se nosw e r c s c a i l i o r c a sowt
n a i deM552017
s i s yhpmys t a t nem e ca l p s i dmm01
n a i deM893757
Table 2. Load to failure, measured as 5 mm displacement
at the sacral fracture or 10 mm displacement at the
higher load to failure (median 168 N versus 400 N) in combined anterior and pos-
terior pelvic ring fixation (p<0.0001)29. In contrast with our previous study physi-
ological forces could be used to determine stiffness under cyclic loading condi-
tions. Stiffness could not be compared due to the different protocol.
Similar to other studies the translation and rotation stiffness of the intact
pelvis were superior to the fixated pelvis. No difference in stiffness was found
between the techniques with one or two sacroiliac screws. However a signifi-
cantly higher load to failure and significantly more loading cycles before failure
could be achieved using two sacroiliac screws compared to one screw. In the
pelves which completed the protocol a decrease of 23% between the initial and
final stiffness was seen, although this was not significant.
This study shows again that the intact pelvis is superior to any method of
fixation. The combination of anterior plate fixation and two sacroiliac screws is
superior to plate fixation and one sacroiliac screw in Tile C fractures. Even the
usually osteoporotic bone of aged embalmed pelves can withstand cyclic loading
up to 400 N. The quality (or grip) of the fixation was a significant covariable for
longer endurance of the fixation. This suggests that further clinical research into
direct postoperative weight bearing for the average (young) trauma patient with
both anteriorly and posteriorly fixated Tile C fractures can be undertaken safely.
Cyclic Loading in Pelvic Fractures
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Cyclic Loading in Pelvic Fractures