J. Biomedical Science and Engineering, 2011, 4, 599-608 JBiSE
doi:10.4236/jbise.2011.49076 Published Online September 2011 (http://www.SciRP.org/journal/jbise/).
Published Online September 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Conventional versus minimally-invasive cervical discectomy
for treatment of severe degenerative disease at C5-C6: a
biomechanical comparison using a model of the full cervical
spine and finite element analysis
Yuan Li, Gladius Lewis
Department of Mechanical Engineering, The University of Memphis, Memphis, USA.
Email: glewis@memphis.edu
Received 19 February 2011; revised 7 April 2011; accepted 5 June 2011.
ABSTRACT
The purpose of this study was to determine the dif-
ferences in biomechanical responses of tissues in the
cervical spine when pain and other problems secon-
dary to severe disc degeneration disease are surgi-
cally treated by conventional discectomy (CONDIS)
compared to minimally-invasive discectomy (M IV D I S ).
A validated three-dimensional model of an intact,
healthy, adult full cervical spine (C1-C7) (INT) was
constructed. This model was then modified to create
two models, one simulating each of the above-men-
tioned two techniques for discectomy of the severely
degenerated C5-C6 disc. For each of these three
models, we used the finite element analysis method to
obtain four biomechanical parameters at various tis-
sues in the model, under seven different physiologi-
cally relevant loadings. For each of the biomechanical
parameters, the results were expressed as relative
change in its value when a specified combination of
simulated discectomy model and applied loading was
used, with respect to the corresponding value in the
intact model. We then computed the value of a com-
posite biomechanical performance index (CBPI) for
CONDIS and MIVDIS models, with this value in-
corporating all of the aforementioned relative changes.
We found that CBPI was marginally lower for
MIVDIS model. This trend is the same as that re-
ported for the relative complications rate and out-
come measures following conventional and mini-
mally-invasive discectomies in the lumbar spine.
From a healthcare perspective, one implication of our
finding is that minimally-invasive cervical discectomy
should be considered an attractive option provided
that detailed patient selection criteria are clearly de-
fined and strictly followed.
Keywords: Finite Element Analysis (FEA); Cervical
Spine; Disc Degeneration; Discectomy
1. INTRODUCTION
When conservative treatments, such as analgesics and
braces, do not provide relief from the pain due to/asso-
ciated with severe degenerative disc disease (DDD) in
the cervical spine, there are a host of surgical methods
that may be employed. These are anterior cervical dis-
cectomy followed by fusion (ACDF) (the most popular
method) [1,2]; discectomy without fusion (DISWF)
(used in a modest number of cases) [1,3-9]; percutane-
ous nucleotomy (used in a few cases) [10]; total disc
replacement (recently approved by regulatory bodies,
such as the US Food and Drug Administration) [11-13];
and nucleus pulposus (NP) replacement [14] and in-
tervertebral disc transplant [15] (each being an emerging
option).
There are two variants of DISWF. One is referred to
as the conventional, standard, or open approach [1,3,4]
while the other is a family of minimally-invasive proce-
dures, namely, percutaneous cervical discectomy, percu-
taneous endoscopic cervical discectomy, percutaneous
cervical laser decompression, percutaneous microcom-
pressive endoscopic cervical discectomy microdiscec-
tomy [5-9]. Arguably, the two most important driving
forces for the introduction and increasing popularity of
the latter techniques are to utilize a narrower surgical
entry window and to reduce the amount of soft tissue
that is disrupted and/or excisedin addition to the af-
fected parts of the disccompared to the former proce-
dure [16]. A biomechanical comparison of conventional
and minimally-invasive techniques for discectomy in the
cervical spine is lacking. The purpose of the present
study was to perform such a comparison. To this end, 1)
we used a validated three-dimensional solid model of the
Y. Li et al. / J. Biomedical Science and Engineering 4 (2011) 599-608
Copyright © 2011 SciRes. JBiSE
600
full cervical spine, in which severe degeneration of the
C5-C6 disc was simulated, modifications of this model
to simulate both conventional and minimally-invasive
discectomy techniques, and the finite element analysis
(FEA) method; and 2) we determinedan assortment of
biomechanical parameters in a large number of hard and
soft tissues in each of the models, when subjected to a
number of different physiologically relevant loadings.
2. FINITE ELEMENT ANALYSIS
We constructed three three-dimensional (3D) solid
C1-C7 models, one representing an intact, healthy, adult
spine and the other twobeing modifications of this
spineto simulate conventional and minimally-invasive
techniques for discectomy of a severely degenerated
C5-C6 di sc .
2.1. Model of Intact, Healthy, Adult Spine
This model (hereafter, INT Model) comprised the verte-
bral bodies, the bony posterior elements (transverse
processes, pedicles, laminae, spinous processes, and
facet joints), the discs, the endplates, and the ligaments
at each level (Figure 1). It was built from digitized
quantitative axial computed tomogr aphy scans/images of
an adult male cadaver that were imported from the Visi-
ble Human Project® dataset (National Library of Medi-
cine, Bethesda, MD, USA). The values of the material
properties of all the tissues in the model are given in
Ta b l e 1 . In our previous report [25], we gave details of
the procedures used to construct this model; showed that
the model had geometric fidelity vis a vis the cervical
spine of healthy, male adults; highlighted the fact that
the natural lordosis of all the functional spinal units
(a) (b)
Figure 1. Two views of the converged finite element mesh of
the INT model.
(FSUs) was preserved in the model; described all of the
steps used to generate the finite element mesh from the
model; provided details of the convergence test; and
provided d etails of the valid ation exercise. In the v alida-
tion work, we compared our FEA results obtained using
INT model to those from two experimental studies,
namely, those by Panjabi et al. [26] and by Wheeldon et
al. [27].
2.2. Models of Severely Degenerated Spine and
Simulated Discectomies
We created a model of a spine in which there was severe
degeneration of the disc at C5-C6 (corresponding to
grade4 in a classification system that is used in clinical
assessment of cervical disc degeneration [28]). Specifi-
cally, to do this, we: 1) put the anterior and posterior
heights of the disc = 3.5 mm and 2.0 mm, respectively,
each being 75% of the value in the healthy disc [29]; 2)
assigned the modu lus of elasticity (E) of the ground sub -
stance in the annulus fibrosus (AF) to be 8.4 MPa, this
being two times that for the ground substance in the AF
in the healthy disc [29]; 3) took the volume fraction of
the elastic fibers in the ground substance in the AF =
15%, this being 75% that for a healthy disc [30]; and 4)
used E for the NP = 8.4 MPa, this being two times that
for the ground substance in the AF in the healthy disc
[29].
Conventional discectomy was simulated using the
method described by Nandoe-Tiwari et al. [3]. Thus, the
inferior endplate on C5, the C5-C6 disc, the ALL at
C5-C6, and the superior endplate on C6 were all deleted
and then the inferior surface of the C5 vertebral body
and the superior surface of the C6 vertebral body were
contoured so as to form an exact bond; that is, they were
fully bonded. The resulting model is herein designated
CONDIS Model.
Minimally-invasive discectomy was simulated by us-
ing aspects of the technique described by Chiu et al. [5]
and by Ahn et al. [6]. This meant shaving all around the
periphery of the AF, such that the final volume of the AF
was 85% of that in INT model, and reducing the vo lume
of the NP to be 17% of that in INT model. The resulting
model is herein designated MIVDIS Model.
For the simulated discectomy models, a validation ex-
ercise was not conducted because, to the best of our
knowledge, there are no published appropriate experi-
mentally-obtained results.
2.3. Boundary Conditions and Loadings
For each of the three models (INT, CONDIS, and
MIVDIS Models), the loading was applied to the supe-
rior surface of C1 vertebral body while the inferior sur-
face of C7 vertebral body was fully constrained in all
Y. Li et al. / J. Biomedical Science and Engineering 4 (2011) 599-608
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Table 1. Element type and elastic properties of the tissues in the finite element model.
Tissue Element type Elastic property Reference(s)
Cortical bone 3-noded trian gular
general-purpose
shell
E11 = 9600 MPa; E22 = 9600 MPa
E33 = 17,800 MPa; G12 = 3097 MPa
G13 = 3510 MPa; G23 = 3510 MPa
12 = 0 .55; 13 = 0.30; 23 = 0.30
Rho [17]; Cowin [18 ]
Cancellous bone 4-noded tetrahedral E11 = 144 MPa; E22 = 99 MPa
E33 = 344 MPa; G12 = 53 MPa
G13 = 45 MPa; G23 = 63 MPa
12 = 0 .23; 13 = 0.17; 23 = 0.11
Ulrich et al. [19]
Posterior elements 4-noded tetrahedral E = 3500 MPa; = 0.29 Kumaresan et al. [20]
Annulus fibrosus 4-noded tetrahedral Ground substance: E = 4.2 MPa; = 0.45
Elastic fibers: E = 450 MPa; = 0.30 Ha et al. [21]
Nucleus pulposus 8-noded brick E = 1.0 MPa;  = 0.499 Ha et al. [21]
Brolin and Halldin [22]
Endplates 4-noded tetrahedral
E = 500 MPa; = 0.40 Yoganandan et al. [23]
ALL 2-noded tension-only link E = 30.0 MPa; = 0.30 Zhang et al. [24]
PLL 2-noded tension-only link E = 20.0 MPa; = 0.30 Zhang et al. [24]
ISL, LF (C1- C 2 ) 2-noded tension-only link E = 10.0 MPa; = 0.30 Zhang et al. [24]
SSL, ISL, LF (C2-C7) 2-noded tension-only link E = 1.5 MPa; = 0.30 Zhang et al. [24]
CL (C1-C3) 2-noded tension-only link E = 10.0 MPa; = 0.30 Zhang et al. [24]
CL (C3-C7) 2-noded tension-only link E = 20.0 MPa; = 0.30 Zhang et al. [24]
A1L 2-n oded tension-only link E = 5.0 MPa; = 0.30 Zhang et al. [24]
TL 2-noded tension-only link E = 20.0 MPa; .30 Zhang et al. [24]
ApL 2-n oded tension-only link E = 20.0 MPa; = 0.30 Zhang et al. [24]
Table 2. Values of R and S, as computed from the % changes in the
MVM,MSED, LF, and ROM results.
Discectomy method
Simulated
(model designation) R1 S1 R2 S2 R3 S3 R4 S4 CBPI
CONDIS 66 126 87 126 35 35 3 18 2.38
MIVDIS 82 154 107 154 35 42 3 18 2.23
degrees of freedom.
For each of the models, seven different loadings were
applied, these being 1) 1 Nm sagittal plane (flexion)
moment +73.6 N axial compressive pre-load; 2) 1 Nm
sagittal plane (extension) moment +73.6 N axial com-
pressive pre-load; 3) 1 Nm left lateral bending moment
+73.6 N axial compressive pre-load; 4) 1 Nm right lat-
eral bending moment +73.6 N axial compression pre-
load; 5) 1 Nm clockwise-acting axial rotational moment
+73.6 N axial compression pre-load; 6) 1 Nm counter-
clockwise-acting axial rotational moment +73.6 N axial
compression pre-lo ad; and 7 ) an axial compression force
of 250 N only. Note that (a) for each of the loadings
1)-6), the pre-load was appliednormal to the surface of
the superior endplate of the C1 vertebral body, as a uni-
form pressure of 0.13 MPa (the area of this superior sur-
face556 mm2was obtained using the “Query Area
Propertie s” fe atur e in ABAQU S®6.4); (b) loading 7) was
applied as a uniform pressure of 0.45 MPa; and (c) each
of the moments was applied as a coupled load to the
aforementioned surface. Note that 1) the axial compres-
sive pre-load corresponds to the weight of the head of an
adult male [1,31,32], and 2) the magnitudes of each of
the moments and the axial compression force are within
the rangestelemetrically measured during a variety of
activities of daily living in a group of patients [33].
2.4. Parameters Determined
For each combination of model (INT, CONDIS, and
MIVDIS Models) and applied loading, the parameters
determined were 1) maximum von Mises stress (
MVM)
in a series of hard tissues at the C5-C6 level;2) maxi-
mum strain energy density (MSED) in each of the
aforementioned tissues; 3)
MVM and MSED in each of a
series of hard tissues at the C4-C5 level; 4) MVM and
MSED for the superior endplate at C7, the an nu lus of the
C6-C7 disc, and the nucleus of the C6-C7 disc; 5) the
force in each of the ligaments (LF) at the C5-C6 level;
and 6) the total principal rotation angle or range of mo-
tion (ROM) at each of the six intersegmental positions.
We used items 3) and 4) as examples of the biome-
chanical response at levels above and below the level of
simulated discectomy, respectively. We point out that
MVM and MSED were not determined for the bony is-
Y. Li et al. / J. Biomedical Science and Engineering 4 (2011) 599-608
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602
sues at the C7 level because it is very likely that the
results will be artifactual; that is, they will not be true
measures of responses to the applied loading because the
model was fully constrained at the inferior surface of C7.
For a given tissue, we used
MVM and MSED as indices
of the stress-strain state in it, while ROM provides an
indication of how much compromise the patient may
experience in terms of fulfilling the kinematics require-
ments for performing normal activities of daily living.
For each combination of simulated discectomy model
and applied loading, we calculated the % change of each
of the biomechanical parameters determined, relative to
the correspondin g valu e wh en IN T model was used. A %
change may thus be considered as ameasure of the extent
to which the biomechanical functions of the spine are
affected by the variant of discectomy technique simu-
lated.
For a given simulated discectomy model, we calcu-
lated the value of a parameter, which we call the com-
posite biomechanical performance index (CBPI) [25],
thus: CBPI = R1/S1 + R2/S2 + R3/S3 + R4/S4, (1)
where R1 is the number of times the % change in MVM
exceeds 10%, S1 is the total number of results of %
change in
MVM, R2 is the number of times the % chang e
in MSED exceeds 10%, S2 is the to tal number of results
of % change in MSED , R3 is the nu mber of times the %
change in the force in a given ligament exceeds 10%, S3
is the total number of results of % change in ligament
force, R4 is the number of times the % change in ROM
exceeds 10%, and S4 is the total number of results of %
change in ROM.
In other words,CBPImay be regarded as a compre-
hensive biomechanical parameter in that it incorporates
changes in stress and strain of both hard and soft tissues
as well as in kinematics of all the intersegmental posi-
tions, under a specified loading. Thus, CBPI is a plausi-
ble parameter to use in comparing the relative sensitivi-
ties of the biomechanical responses of the two simulated
discectomy models to the collection of applied loadings
imposed.
3. RESULTS
Because of the large volume of these results (specifically,
7sets of
MVM results, 7 sets of MSED results, 7 sets of
ligament force results, and 3 sets of ROM results, with
each set containing results for the three models), we only
present a small sample (Figures 2-5). We point out that
the three main trends seen in these results are the same
as those seen in the rest of the whole collection of re-
sults.The first trend is that 1) with four exceptions, MVM
in a given tissue is about the sa me when CONDIS model
or MIVDIS Model is used. The exceptions are in the AF
in the disc at C4-C5 (7 times higher when the MIVDIS
Model was used); the posterior elements on the C5 ver-
tebral body (5 times higher when CONDIS Model was
used); the cortical bone of the C6 vertebral body (8
times higher when MIVDIS Model was used); and the
posterior elements on the C6 vertebral body (2 times
higher when MIVDIS Model was used); 2) with three
exceptions, MSED in a given tissue is about th e same for
both CONDIS and MIVDIS models. The exceptions are:
the posterior elements on the C5 vertebral body (9
times higher when CONDIS Model was used); and cor-
tical bone of the C6 vertebral body (2 times higher
when MIVDIS Model was used); and the posterior ele-
ments on the C6 vertebral body (2 times higher when
MIVDIS Model was used); and 3) with the exception of
the ROM at C5-C6, ROM at a given intersegmental po-
sition is about the same for both CONDIS and MIVDIS
models. The second trend, which derives from the first,
is that three hard tissues (the posterior elements on the
C5 vertebral body, the cortical bone of the C6 vertebral
body, and the posterior elements on the C6 vertebral
body) are sensitive to the simulated discectomy tech-
nique used. This outcome is not surprising given that the
simulated discectomy was of the disc at the C5-C6 level.
The third trend is that the change in the force in each of
the ligaments at C5-C6 was markedly lower when
MIVDIS Model was used compared to when CONDIS
Model was us ed.
A principal finding is that the difference in CBPI
value when CONDIS Model was used compared to
when MIVDIS Model was used is small (7%) (Table
2).
4. DISCUSSION
There are two variants of cervical discectomy that are in
clinical use; namely, the standard or open method and
the minimally-invasive or percutaneous method [1,3-9].
There are no biomechanical comparisons of these two
variants. The purpose of t he present work was to conduct
such a study, and we have done so by 1) simulating se-
vere degeneration of the disc at the C5-C6 level (a
common site of severe degeneration seen clinically), 2)
utilizing a collection of applied loadings that are
physiologically relevant, and 3) obtaining a wide collec-
tion of biomechanical parameters, including ROM. In
other words, the present study may be regarded as being
comprehensive, raising the possibility that the results
obtained could be treated in such a way as to provide
insights that may have clinical relevance.
Although both conventional and minimally-invasive
techniques for cervical discectomy have been shown to
be safe, efficacious, and with low rates of postoperative
surgical site infections [1,3,5-8,34], to the best of our
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knowledge, there are no literature reports (case studies,
prospective data from surgical registries, or prospective,
randomized, controlled trials) on comparison of these
two variants on the basis of, for example, duration of
surgery and clinical outcomes. This situation contrasts
with that for lumbar discectomy, in which there are
comparisons of conventional and minimally-invasive
techniques on the basis of, for example, length of oper-
ating time, blood loss, duration of hospital stay, surgical
complications, and outcome measures (such as changes
in Visual Analogue Scale, Oswestry Disability Index,
and the Core Outcome Measures Index after the surgery)
and duration of hospital stay [35-37]), These compari-
sons found no significant difference in, for example, rate
Figure 2. Summary of the % change in the maximum von Mises stress in a simulated discectomy model,
under combined loading of 1 Nm left lateral bending moment +73.6 N axial compression pre-load, relative
to the value when INT model was used.
Y. Li et al. / J. Biomedical Science and Engineering 4 (2011) 599-608
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604
Figure 3. Summary of the % change in the maximum strain energy density in a simulated discectomy
model, under combined loading of 1 Nm left lateral bending moment +73.6 N axial compression pre-
load, relative to the value when INT model was used.
of surgical complications, duration of hospital stay, and
any outcome measures between the two variants [35-37].
In the present work, we utilized some simplifications,
two key assumptions, and one caveat. Some of these
simplifications are common to FEA studies of models of
spine sections that contain a degenerated disc; for exam-
ple, treating the geometry of the cancellous bone as a
continuum [29,38-40], neglect of time-dependent effects
in the AF and the NP [29,38-40], and neglect of the ef-
fect of tears in the AF [39,40]). Other simplifications are
specific to the present study. We highlight two of these.
First, our MIVDIS model does not, strictly speaking,
correspond to any specific minimally-invasive technique
used for cervical discectomy. This is because there is a
variety of surgical approaches that are taken to accom-
plish minimally-invasive cervical discectomy [5-9]. The
Y. Li et al. / J. Biomedical Science and Engineering 4 (2011) 599-608
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Figure 4. Summary of the % changes in forces in various ligaments at the C5-C6 level
in a simulated discectomy model, under a combined loading of 1 Nm left lateral bending
moment +73.6 N axial compression pre-load, relative to the value when INT model was
used.
Figure 5. Summary of the % changes in range of motions in a simulated discectomy
model, under a combined loading of 1 Nm left lateral bending moment +73.6 N axial
compression pre-load, relative to the value when INT model was used.
key point to note is that our approach is consistent with
the essential elements of each of these surgical ap-
proaches [5-9]. The second specific simplification is that
we modeled each of the two constituents of the AF (the
ground substance and the reinforcing fibers) as well as
the NP as isotropic, elastic solids. Although this consti-
tutive material behavior model has been used for these
tissues in a number of previous FEA studies of the cer-
vical spine [31,41], many other material models have
also been used, such as hyperelastic (Mooney-Rivlin) or
hyperelastic (neo-Hookean) for the ground substance
[42], no n linear stress -s train relation sh ip for the AF f ibers
[43]; incompressible fluid for the NP [29]; and poroelas-
ticity for both the AF and the NP [44]. The issue of the
influence that the material models used for the AF and
the NP exerts on the results obtained using the present
models was outside the scope of the present study. The
first assumption used was that, in INT model, the only
degenerated tissue is the disc at C5-C6. It is possible,
however, that discs at other levels may be degenerated.
The second assumption made was that the line of action
of the pre-load passes through the instantaneous center
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606
of rotation of C1 and, hence, the load does not generate
an additional moment abo ut that axis. Most likely, this is
what happens except, perhaps, when axial rotation mo-
ment is applied [45]. The caveat is that we used 10%
change as the cutoff value in obtaining the value of the
parameter, R. Although this cutoff value is arbitrary, it is
plausible because it is based on a clinical result. For a
group of 29 healthy, adult volunteers, Watanabe et al.
[46] found an increase of 11% in the mean value of
axial relaxation time for the AF in lumbar discs, obtained
using magnetic resonance imaging, in going from a
healthy disc to a severely degenerated one.
Four important limitations of our study are discussed.
First, the facet joints were not explicitly included in the
solid model; rather, they were modeled as part of the
posterior elements. For a C4-C6 model, the principal and
coupled motions, under a loading of either 1.8 Nm flex-
ion + 73.6 N compression pre-load or 1.8 Nm extension
+ 73.6 N compression pre-load, ranged from being 11%
lower to 28% higher when facet joints were not explic-
itly included in the model [42] compared to correspond-
ing results when they were (Ha et al. [21]). We expect
this trend to be the same when a full cervical spine
model is used. Second, the muscles of th e cervical spine
were not included in the solid model, but it is realized
that muscle forces have an important influence on spinal
motions. Third, shear forces were not included in the
suite of loadings used. We are, however, not aware of
any literature reports in which the response to the cervi-
cal spine under shear load is documented, which means
it would not have been possible to validate our FEA
model (INT model) under shear load.
The aforementioned simplifications, assumptions, and
limitations do not undermine the validity of our work
because the principal trend in our study, namely, the
small difference in CBPI values for the two simulated
discectomy models, is consistent with relevant clinical
observations [35-37].The significance of our study is
that its principal finding may be interpreted as buttress-
ing the clinical view, which is that, at the current stage of
development, a minimally-invasive technique for cervi-
cal discectomy has a number of benefits but the decision
to use it should be made by the surgeon on a case-
by-case basis, guided by very strict patient inclusion
criteria, especially the specifics of the disc degenera-
tive disease [35-37].
5. CONCLUSIONS
On the basis of the value of an index of the biome-
chanical performance of a model of the full cervical
spine that has been shown to be plausible, we found
that the overall performance of a model in which
minimally-invasive discectomy at C5-C6 was simu-
lated was about the same as when a conventional
discectomy technique was simulated.
The aforementioned trend is the same as that reported
on the comparative complications rate and outcome
measures of these two variants of discectomy in the
case of the lumbar spine.
From a healthcare perspective, one implicatio n of our
principal finding is that minimally-invasive cervical
discectomy should be considered an attractive option
provided that detailed patient inclusion criteria are
clearly defined and strictly followed.
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