Journal of Medical and Bioengineering Vol. 5, No. 2, April 2016

Effect of Cage Insertion Orientation on Stress
Profiles and Subsidence Phenomenon in Posterior
Lumbar Interbody Fusion
Muhammad Hazli Mazlan
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan
Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, Malaysia
Email: hazli.010@s.kyushu-u.ac.jp

Mitsugu Todo
Research Institute for Applied Mechanics, Kyushu University, Japan
Email: todo@riam.kyushu-u.ac.jp

Hiromitsu Takano and Ikuho Yonezawa
Department of Orthopedic Surgery, Juntendo University, School of Medicine, Japan
Email: {hrtakano, yoza}@juntendo.ac.jp

Posterior lumbar interbody fusion (PLIF) is a surgical
technique that involves removing a disc and fusing
vertebrae together in the lower back (lumbar region). It
has become a widely accepted surgical procedure in thefield of spinal surgery to stabilize unstable segment due
to disc degenerations or could be used as a postoperative
assistive devices [1]. It was reported that 80,000 lumbarinterbody fusion were implanted worldwide from 1995 to1999 [2]. Currently, numerous cage designs have beencommercialized by taking into account its primaryfunction as a load-bearing structures that is capable towithstand post-operative spine motions, to avoid bone
graft collapse and subsequently to promote biologicalformation of a full thickness of bone bridges. However,its unavoidable clinical implications such as cagesubsidence, cage migration and cage failures were still
occurred, which forcing the used of posterior
instrumentation (PI) to mitigate the impacts.
The use of two cages combined with PI seems to be an
ideal solution to limit segmental movement and to
facilitate fusion. However, it will be likely to incur higher
cost and more risky [3]. In addition, inserting bilateral
PLIF cages required a wider laminectomy andfacetectomy. Moreover, the risk of neurologic injury as
well as risk of dural tear is increased due to bilateralnerve root manipulation. Based on these considerations,
there is a strong argument on the effectiveness of using
two cages for a successful PLIF. For that reason, single
oblique cage insertion surgical procedure has been
introduced as one of the viable solution that has the samepotential as two cages did [4]-[6].
In the present study, finite element analysis was used
to evaluate a subsidence phenomenon based on thefracture risks evaluation and cage-endplate interface
stress distributions in two different cage insertion
orientations namely as double cages and single oblique

Abstract—Posterior lumbar interbody fusion has been
widely accepted as one of the surgical procedure to treat
clinical problems. However, vertebral endplate subsidence
failure has been detected as one of its major problems thatmight increase the potential of pain and mechanicalinstability. Therefore, posterior instrumentation (PI) has
been introduced alongside with double fusion cages implant
to limit segmental movement and to facilitate fusion.
Nevertheless, the use of two interbody fusion cages will belikely to incur higher cost and more risky. Hence, single
oblique cage insertion surgical procedure has been reported
as one of the reliable solution. In the present study, animage-based finite element analysis was used to evaluate asubsidence phenomenon based on the fracture risks
evaluation and the stress profiles at cage-endplate interface
in two different cage insertion orientations namely as double
cages and single oblique cage. Apparently, the single obliqueinserted cage with PI has significantly produced lower stressthan the double inserted cages at the cage-endplateinterfaces. At higher impact loading (2000N), the total
number of compressive deformations of the double cages
outnumbered the single oblique cage at the cage-endplateinterface junctions and the deformations were more
uniformly distributed. Obviously, there was a trade-off
between the stress generation, the implant stability and therisk of vertebral bone failures. The single oblique cage
insertion method could be considered as one of the bestalternative for the posterior lumbar interbody fusion
surgical procedure due its structural symmetry that could
provide similar stability as two cages did. 
Index Terms—interbody fusion, cage insertion, posterior
instrumentation, cage-endplateI.

INTRODUCTION

Manuscript received July 1, 2015; revised September 12, 2015.
©2016 Journal of Medical and Bioengineering
doi: 10.18178/jomb.5.2.93-97

93Journal of Medical and Bioengineering Vol. 5, No. 2, April 2016bone fracture sites for each of the model was also
evaluated based on the Newton-Raphson nonlinear
fracture analysis method [11].

cage. Five spine motions were considered namely ascompression, flexion, extension, axial rotation and lateral
bending movements [7].
II.

MATERIALS AND METHODS

A. FE Modelling
The FE models were constructed in MECHANICALFINDERTM software (Research Center of Computational
Mechanics Co. Ltd. Japan). Written informed consent,
permission and cooperation of 29-year-old Japanese male
healthy subject (78kg weight and 176cm height) was
obtained. To create the FE models, CT scan images of thehealthy subject (Juntendo University) was taken andtransferred to FE software. From the obtained CT scan
images, the FE model was then constructed based on theextracted bone edges of the region of interests (ROI)
around the outer region of the cortical bone to obtain the
anatomical structure of the spinal bone. The FE model
was then modelled with 1mm linear tetrahedral andtriangular elements (thickness of 0.4mm) to represent the
inner portion of the cortical and the cancellous bone, andthe outer cortex, respectively.
To reflect the heterogeneity of the FE models, themechanical properties for each element was calculated
from the Hounsfield Unit (HU) values. Young’s modulus
was obtained using the relationship as reported by Keyaket al. [8]. Poisson’s ratio was set to a constant value of 0.4[8]-[10]. Facet joints and intervertebral discs were createdbased on the approximation and visualization of theiractual structure and position, which were verified byorthopaedic surgeons. Poisson’s ratio for theintervertebral disc and the facet joint were set at 0.45 and
0.2, correspondingly [7]. Meanwhile, Young’s modulus
were set at 8.4MPa and 11MPa for the intervertebral discand the facet joint, respectively [7].
A 23mm long ogival interbody PLIF cage (OIC) madeof titanium alloy (Ti-6Al-4V) was simulated. In this study,
we simulated decompression surgery by deleting someannulus fibrosus as well as nucleus pulposus, which werenecessary to insert bilateral cages and adding posterior
pedicle screws-rods system with the diameters being
6.2mm (rods and screws) and the screw length being
51.8mm. Thus, a complete model of post-PLIF was built(Fig. 1a). Two types of cage insertions surgicalprocedures were simulated namely as: (a) traditional
bilateral cages; (b) unilateral oblique cage (Fig. 1a).

a.

b.
Figure 1. (a) Simulated PLIF model with PI; (b) Loads and boundary
condition.

III.

The Drucker-Prager stress distributions of the different
cage orientations under the different loading conditions
were shown in Fig. 2. In most of the cases, the maximal
Drucker-Prager stresses were detected and concentrated
on the interface between the cage and the endplate of the
fourth and fifth lumbar vertebrae. Based on these stress
distributions the maximal Drucker-Prager stresses wereplotted as depicted in Fig. 3. The maximal DruckerPrager stress was used as a criterion of failed constructwhen maximal distortion energy theory was applied. For
the double cages orientation, the maximal Drucker-Prager
stresses were 11.6MPa with compression of 1000N,
26.3MPa with compression of 2000N, 0.7MPa withflexion, 2.3MPa with extension, 3.0MPa with lateral
bending and 2.0 with axial rotation. For the single oblique
cage orientation, the maximal Drucker Prager stresses
were 7.2MPa, 12.7MPa, 0.6MPa, 2.7MPa, 1.9MPa and
1.9MPa for compression of 1000N, compression of
2000N, flexion, extension, lateral bending and axial
rotation, respectively.

B. Analysis
The FE models were loaded with compressive and four
rotational loads (flexion, extension, lateral bending andaxial rotation) to stimulate the physiological motions of
the spine. The loads were applied on superior surface of
L2. The inferior surface of L5 was fixed in all directions(Fig. 1b). Maximal Drucker-Prager stress from different
cage orientations and loading activities were compared toevaluate the subsidence phenomenon. The prediction of©2016 Journal of Medical and Bioengineering

RESULTS AND DISCUSSIONS

94

Journal of Medical and Bioengineering Vol. 5, No. 2, April 2016

Figure 2. Drucker-prager stress distributions for (a) double and (b) single cage(s).

priority and consideration in surgery [1]. Theoretically,placing the cage sagittally in the midline could provide
the best symmetry, but it requires excessive retractions on
the nerve roots and might results in nerve root damage,especially at higher lumbar levels. A big portion of loadsof the single oblique cage was believed had beentransferred through the stiff structure of the PI as
indicated by the higher number of tensile failure elements
around the attachment point between the pedicle screws
and the vertebrae as depicted in Fig. 5. The used of the PI
itself could reduce the stress of the cage-endplateinterface by 50-60% compared to non-instrumented cage[1]. Moreover, the risks of the cage migration and thecage failures were assume reduced.

The relative maximal Drucker-Prager stresses
difference of the different implant modalities were also
calculated and plotted in Fig. 4. In most of the spine
activities, the double cages group has significantly
produced higher stress than the single oblique cage groupin compression of 1000N (38.3%), compression of 2000N(51.6%), flexion (14.9%), lateral bending (35.6%) andaxial rotation (4.4%) activities. However, in extension
activity the single oblique cage group outnumbered the
double cages group by 16.7%. The possible explanation
for this condition was due to better structural symmetry
exhibited by the single oblique cage position has
effectively diminished the stress concentration within the
structure and therefore, it must be given the highest

Figure 3. Maximal Drucker-Prager stress distributions at cage-endplate interface.

Figure 4. Relative maximal Drucker-Prager stress difference between two cage orientations.

©2016 Journal of Medical and Bioengineering

95

Journal of Medical and Bioengineering Vol. 5, No. 2, April 2016

TABLE I.

Fig. 5 shows the distributions of failure and yielding
elements in the cancellous bone and the inner portion of
the cortical bone in the Newton-Raphson loop under theapplication of the compressive load of 2000N to simulate
the worst loading condition. In general, for the both cageorientations almost all of the failure and the yielding
elements could be detected at the cage-endplate interface
and around the posterior-lateral region of 4th and 5th
lumbar vertebrae. The posterior-lateral region was thearea in which pedicle screws were inserted, and theregion is important in terms of supporting andtransferring loads between the vertebra and the screw[11]. The tensile failure elements were densely
distributed widely along the axis of the inserted screw,
while few and no compressive yielding and failure
elements were found, respectively. The compressive
yielding and failure elements only could be found at thecage-endplate interface, which was highly related tosubsidence phenomenon that was prevalently happened ininterbody fusion surgery method. Even though themodels did not reach a state of whole fractures, theexistence of the failure and the yielding elements on thatareas indicating that these regions still faces higher risks
of fracture.
In Table I, the total number of the failure and theyielding elements were comparably higher for the single
oblique cage (107 elements) than the double cages (84elements). In addition, the failure and the yielding
elements for the single oblique cage were distributed
more uniformly than the double cages group. However,
the total numbers of compressive deformations wereslightly surpassed the double cages group by 12 elements
and these conditions were highly correlated with theexistence of the subsidence phenomenon (cagesubsidence) created at the cage-endplate interfacejunctions. Less compressive deformation and lower
distortion stress generation was a signed of higher
structural stability offered by the single oblique cage thanthe double cages PLIF. Lower distortion stress generationof the single oblique cage (12.7MPa) was accompaniedwith higher number of tensile failure elements (59elements), while higher distortion stress generation of the
double cages (26.7MPa) was accompanied with lower
number of tensile failure elements (27 elements). These
compensatory mechanisms were inevitable and ultimatum
in considering the best combination of the implantmodalities. The occurrence of the tensile failuressuggested that the stress concentration might cause thescrew to slip and loosen and this problem is believed
could be overcome by increasing the rod sizes or used
flexible fixation for load dispersions and to provide anappropriate load path in the vertebrae and screws.

(a)Number of element
Double Cages
Single oblique cage31
9Failure type
Compressive failure
Compressive yielding

26

39

Tensile failure

27

59

IV.

CONCLUSION

These results suggested that the used of unilateral
oblique PLIF cage could be considered as one of theoptional alternative to replace the bilateral PLIF cages bythorough consideration of its stress distortion, structural
stability and subsidence effect. Obviously, single oblique
cage would produce lower maximal Drucker-Prager
stresses at cage-endplate junction, more stable and could
potentially reduce the subsidence (compressive
deformation) effect on that areas. However, its inevitableeffect of higher tensile deformation seems unavoidable.
Hence, single oblique cage PLIF has the same potential
as double cages did and last but not least, it may also
reduce medical cost and surgical risk withoutcompromising its structural stability.
REFERENCES[1]

Y. H. Tsuang, Y. F. Chiang, C. Y. Hung, H. W. Wei, C. H. Huang
and C. K. Cheng, “Comparison of cage application modality inposterior lumbar interbody fusion with posterior instrumentationA finite element study,” Medical Engineering & Physics ,vol. 31,
pp. 565-570, 2009.
[2] C. Adam, M. Pearcy, and P. M. Combe, “Stress analysis ofinterbody fusion-finite element modelling of intervertebral implantand vertebral body,” Clinical Biomechanics, vol. 18, pp. 265-272,
2003.
[3] R. W. Monilari, J. Sloboda, and F. L. Johnstone, “Are two cages
needed with instrumented PLIF?” A Comparison of 1 Versus 2Interbody Cages in A Military Population, Am J orthop, vol. 32,
pp. 337-343, 2003.
[4] J. Zhao, Y. Hai, N. R. Ordway, C. K. Park, and H. A. Yuan,“Posterior lumbar interbody fusion using posterolateral placementof a single cylindrical threaded cage,” Spine, vol. 25, pp. 425-430,
2000.
[5] S. T. Wang, V. K. Goel, C. Y. Fu, S. Kubo, W. Choi, C. L. Liu, etal. “Posterior instrumentation reduces differences in spine stability
as a result of different cage orientations: An in vitro study,” Spine,
vol. 30, pp. 62-67, 2005.
[6] M. F. Chiang, Z. C. Zhong, C. S. Chen, C. K. Cheng and S. L.
Shih, “Biomechanical comparison of instrumented posterior
lumbar interbody fusion with one or two cages by finite element
analysis,” Spine, vol. 31, pp. 682-689, 2005.
[7] M. H. Mazlan, M. Todo, H. Takano, and I. Yonezawa, “Finite
element analysis of osteoporotic vertebrae with first lumbar (L1)vertebral compression fracture,” International Journal of Applied
Physics and Mathematics, vol. 4, pp. 267-274, 2014.
[8] J. H. Keyak, S. A. Rossi, K. A. Jones, and H. B. Skinner,“Prediction of femoral fracture load using automated finite
element modeling,” Journal of Biomechanics, vol. 31, pp. 125-133,
1998.
[9] D. T. Reilly and A. H. Burstein, “The elastic and ultimate
properties of compact bone tissue,” Journal of Biomechanics, vol.
8, pp. 393-405, 1975.
[10] W. C Van Buskirk and R. B. Ashman, “The elastic moduli ofbone,” Trans. American Society of Mechanical Engineers (Applied
Mechanics Division,), vol. 45,pp. 131-143, 1981.
[11] D. Tawara, K. Noro, T. Tsujikami, Y. Okamoto, and H. Murakami,“Nonlinear mechanical analysis of posterior spinal instrumentationfor osteoporotic vertebra: effects of mechanical properties of therod on the failure risks around the screw,” Journal of
Biomechanical Science And Engineering (Advanced Publication),
2014.

(b)

Figure 5. Distribution of failure and yielding elements: (a) Single
oblique cage and (b) Double cages©2016 Journal of Medical and Bioengineering

NUMBER OF FAILURE AND YIELDING ELEMENTS

96

Journal of Medical and Bioengineering Vol. 5, No. 2, April 2016

Muhammad Hazli Mazlan was born in Johor,
Malaysia and obtained his Bachelor Degree ofElectrical Engineering from Kolej Universiti Tun
Hussein Onn and Master of Biomedical
Engineering from University of Malaya,
Malaysia. Currently, he is a PhD candidates at
Kyushu University, Japan. He is a tutor at Facultyof Electrical and Electronic Engineering,
Universiti Tun Hussein Onn Malaysia andinvolved in medical electronics research group.

Hiromitsu Takano, MD, obtained his Doctor of
Medicine (MD) Degree from Juntendo University,
School of Medicine, JAPAN (2010). He gained 2
years surgery resident at NTT Medical Center. He
is currently a medical doctor at Department ofOrthopedic Surgery, Juntendo University, Schoolof Medicine, Tokyo, JAPAN. He is a PhD
candidates at Juntendo University, Tokyo,
JAPAN.

Mitsugu Todo obtained his Bachelor and Master’s
degrees of Engineering from Kyushu University,
Japan, and PhD from The Ohio State University,
The United States of America. He is currently an
Associate Professor of Research Institute forApplied Mechanics, Kyushu University, andworking on biomaterials for osteochondral tissue
engineering and biomechanics of orthopedic
implant.

Ikuho Yonezawa MD, PhD, obtained his Doctorof Medicine (MD) Degree from Gunma
University, JAPAN (1990). He is currently an
Associate Professor of Juntendo University
Hospital, Tokyo, JAPAN and serving as a top
surgeon of the Spinal Surgery Division. He isserving as a board member of three academic
leading society, namely as The Japanese Society
for Spine Surgery and Related Research, JapaneseScoliosis Society and Japanese Spinal Instrumentation Society.

©2016 Journal of Medical and Bioengineering

97