The biomechanical impact of facet tropism on the intervertebral disc and facet joints in the cervical spine

The biomechanical impact of facet tropism on the intervertebral disc and facet joints in the cervical spine

Accepted Manuscript Title: The biomechanical impact of the facet tropism on the intervertebral disc and facet joints in the cervical spine Author: Xin...

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Accepted Manuscript Title: The biomechanical impact of the facet tropism on the intervertebral disc and facet joints in the cervical spine Author: Xin Rong, Beiyu Wang, Chen Ding, Yuxiao Deng, Hua Chen, Yang Meng, Weijie Yan, Hao Liu PII: DOI: Reference:

S1529-9430(17)30327-3 http://dx.doi.org/doi: 10.1016/j.spinee.2017.07.009 SPINEE 57393

To appear in:

The Spine Journal

Received date: Revised date: Accepted date:

17-3-2017 22-6-2017 6-7-2017

Please cite this article as: Xin Rong, Beiyu Wang, Chen Ding, Yuxiao Deng, Hua Chen, Yang Meng, Weijie Yan, Hao Liu, The biomechanical impact of the facet tropism on the intervertebral disc and facet joints in the cervical spine, The Spine Journal (2017), http://dx.doi.org/doi: 10.1016/j.spinee.2017.07.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

The biomechanical impact of the facet tropism on the intervertebral disc and facet

2

joints in the cervical spine

3 4

Xin Rong, M.D.a, Beiyu Wang M.D. a, Chen Ding M.D. a, Yuxiao Deng, M.D. a, Hua

5

Chen, M.D. a, Yang Meng, M.D. a, Weijie Yan M.S.M.T.b, Hao Liu, M.D., Ph.D. a

6 7

a

8

Sichuan Province, China, 610041.

9

b

10

Department of Orthopedics, West China Hospital, Sichuan University, Chengdu,

Department of Radiology, West China Hospital, Sichuan University, Chengdu,

Sichuan Province, China, 610041.

11 12

Correspondence to: Hao Liu, M.D. PhD., Department of Orthopedic Surgery, West

13

China Hospital, No. 37, Guo Xue Xiang, Chengdu, Sichuan Province, China, 610041.

14

Fax number: 86-28-85423438

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Telephone Number: 86-28-85422430

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E-mail: [email protected]

17 18

Abstract

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BACKGROUND CONTEXT: The facet tropism is defined as the angular difference

20

between the left and right facet orientation. The facet tropism was suggested to be

21

associated with the disc degeneration and facet degeneration in the lumbar spine.

22

However, little is known about the relationship between the facet tropism and

23

pathological changes in the cervical spine and the mechanism behind.

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PURPOSE: This study was conducted to investigate the biomechanical impact of the

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facet tropism on the intervertebral disc and facet joints.

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STUDY DESIGN: A finite element analysis study.

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METHODS: The CT scans of a 28 year-old male volunteer was used to construct the 1

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finite element model. Firstly, a symmetrical cervical model from C2 to C7 was

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constructed. The facet orientations at each level was simulated using the data from our

3

previously published study. Secondly, the facet orientations at the C5-C6 level was

4

altered to simulate the facet tropism with respect to the sagittal plane. The angular

5

difference of the moderate facet tropism model was set to be 7 degrees, whereas the

6

severe facet tropism model was set to be 14 degrees. The inferior of the C7 vertebra

7

was fixed. A 75 N follower loading was applied to simulate the weight of the head. A

8

1.0 N·m moments was applied on the odontoid process of the C2 to simulate flexion,

9

extension, lateral bending and axial rotation.

10

RESULTS: The intradiscal pressure (IDP) at the C5-C6 level of the severe facet

11

tropism model increased by 49.02%, 57.14%, 39.06% and 30.67%, under flexion,

12

extension, lateral bending and axial rotation moments, in comparison to the

13

symmetrical model. The contact force of the severe facet tropism model increased by

14

35.64%, 31.74%, 79.26% and 59.47% from the symmetrical model under flexion,

15

extension, lateral bending and axial rotation, respectively.

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CONCLUSIONS: The facet tropism with respect to the sagittal plane at the C5-C6

17

level increased the IDP and facet contact force under flexion, extension, lateral

18

bending and axial rotation. The results suggested that the facet tropism might be the

19

anatomical risk factor for the development of cervical disc degeneration or facet

20

degeneration. Future clinical studies are in need to verify the biomechanical impact of

21

facet tropism on the development of degenerative changes in the cervical spine.

22 23

Key words: facet joint; cervical spine; facet contact force; intradiscal pressure.

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Introduction

2 3

Diseases due to the degenerative changes of the cervical spine posed great burden on

4

the society. Knowing the risk factors that promotes the disc or facet joint degeneration

5

is therefore of great clinical importance, for the prevention or delay of the

6

degenerative changes in the cervical spine.

7 8

The intervertebral disc and the facet joints formed the three-joint complex [1]. Any

9

abnormality of one joint could affect the others. Facet tropism, the angular asymmetry

10

between the right and left facet joint orientation, was suggested to be the anatomical

11

factor related to the disc or facet degeneration in the lumbar spine [2-5]. It was

12

demonstrated in previous studies that the facet tropism in the cervical spine was a

13

common anatomical feature [6, 7]. However, little is known about the relationship

14

between the facet tropism and cervical degenerative changes in the disc or the facet

15

joints. Therefore, it is imperative to explore the biomechanical impact of the facet

16

tropism on the cervical spine.

17 18

Cervical disc degeneration is most common at the C5-C6 level. A 10-year longitudinal

19

radiographic observation stated that the most frequent sites of degeneration was at the

20

C5-C6 level [8]. Teraguchi et al. [9], in a population-based study including 975

21

participants, reported that the prevalence of disc degeneration at the C5-C6 level was

22

51.5% in men and 46% in women. Therefore, we conducted this finite element study

23

to investigate the biomechanical impact of the facet tropism on the intervertebral disc

24

and facet joint at the C5-C6 level.

25 26

Materials and methods

27 28

The finite element model of the C2-C7 cervical spine was developed on the basis of

29

the CT data of a 28 healthy male volunteer (165 cm, 65 kg), who was willing to

30

provide the written informed consent. The CT scans were obtained with the slice 3

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thickness of 0.75 mm and the slice increment of 0.69 mm using a CT scanner

2

(SOMATOM Definition AS+, Siemens, Germany). The finite element models were

3

constructed using Mimics 17.0 (Materialize Inc., Belgium), Geomagic Studio 12.0

4

(3D System Corperation, Raindrop, USA), CATIA v5r21 (Dassault Systems

5

Corporation, Velizy-Villacoublay Cedex, France), Hypermesh 12.0 (Altair, Troy, USA)

6

and ABAQUS 6.9.1 (Dassault Systems Corporation, Velizy-Villacoublay Cedex,

7

France).

8 9

Geometrical reconstruction of the cervical spine

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The symmetrical model

11

The CT images were imported into the Mimics 17.0 software for the reconstruction of

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the C2 to C7 vertebrae by thresholding and dynamic region growing methods. The

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intervertebral discs were constructed filling the intervertebral space and connecting

14

the adjacent vertebral bodies. Initially the facet joint space was filled with the same

15

mask as bone, treating as bony fusion. Then the reconstructed model was processed in

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the Geomagic Studio 12.0 to create the symmetrical model. Thereafter, the

17

symmetrical model was imported into the CATIA v5r21 software. Using the disc

18

plane as the reference plane, the “fused” facet joint was “cut” with predetermined

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angle to simulate the facet orientations at each level (Table 1) [7]. The facet joint

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space was simulated to be 0.5 mm [10]. A 0.2 mm thick articular cartilage layer was

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added to both the superior and inferior articular process, respectively [10].

22 23

The cortical bone and bony endplates of the vertebra were constructed as a shell with

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the thickness of 4 mm [11]. The intervertebral disc was subdivided into annulus

25

fibrosus and nucleus pulposus with the volume ratio to be 6:4 [11]. A total of 6 layers

26

of annulus fibers were embedded in the annulus fibrosus substance, with the

27

inclination to the disc plane between 15° to 30°, consisted 19% of the volume of the

28

annulus fibrosus [11, 12]. The annulus fibers within each layer were parallel, whereas

29

fibers between adjacent lamina were in alternated orientation. Finally, the ligaments

30

were inserted into the model. The transection area was 6.1 mm2 for the anterior 4

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longitudinal ligament (ALL), 5.1 mm2 for the posterior longitudinal ligament (PLL),

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50.1 mm2 for the ligamentum flavum (FL), 13.1 mm2 for the interspinous ligament

3

(IL), 13.1 mm2 for the supraspinous ligament (SL) and 46.6 mm2 for the capsular

4

ligament (CL), respectively.

5 6

The facet tropism model

7

The moderate facet tropism model and severe facet tropism model were developed on

8

the basis of the symmetrical model. The only difference was the facet orientations at

9

the C5-C6 level. At the C5-C6 level, the left and right “fused” facet joints were “cut”

10

with planes of different angles. Based on our previous study, the average raw

11

difference between the left and right facet orientation with respect to the sagittal plane

12

was 0.08 degree, and the standard deviation was 7.28 degree. As described by

13

Vanharanta et al. [13], the facet joints were classified as symmetrical (within 1SD),

14

moderate facet tropism (between 1 SD and 2 SD) and severe facet tropism (larger than

15

2 SD). In this study, the moderate facet tropism model was simulated with the angular

16

difference of 7 degree with respect to the sagittal plane, whereas the severe facet

17

tropism model was 14 degree.

18 19

Material properties assignment and meshing

20

The meshing and material properties assignment were accomplished Hypermesh 12.0

21

and ABAQUS 6.9.1. The material properties and mesh types are listed in Table 2.

22 23

Boundary condition

24

The intervertebral discs and vertebral bodies, the insertion of the ligaments to bones

25

were assigned as tie connection. The interaction between the facet cartilages was

26

assigned with frictionless sliding contact formulation.

27 28

Experimental condition

29

The cervical model was fixed at the inferior endplate of the C7 vertebra at six degrees

30

of freedom. A 75 N follower loading was applied to simulate the weight of the head. A 5

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1.0 N·m moments was applied on the odontoid process of the C2 to simulate flexion,

2

extension, lateral bending and axial rotation. The range of motion (ROM) under all

3

moments were compared with previously published data to validate the model. The

4

intradiscal pressure (IDP) and facet contact force were tested under all experimental

5

moments.

6 7

Results

8 9

Validation of the symmetrical model

10

The predicted ROMs at different moments were compared with previous

11

biomechanical and finite element studies [14-16]. The ROM of the symmetrical

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model for flexion was 4.53° at C2-C3, 6.83° at C3-C4, 8.11° at C4-C5, 7.86° at

13

C5-C6 and 5.57° at C6-C7. The ROM of the symmetrical model for extension was

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3.45° at C2-C3, 5.13° at C3-C4, 6.61° at C4-C5, 5.94 at C5-C6 and 4.76° at C6-C7.

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The ROM for unilateral bending was 4.07° at C2-C3, 6.15° at C3-C4, 8.17° at C4-C5,

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5.73° at C5-C6 and 3.41° at C6-C7. The ROM for axial rotation was 5.16° at C2-C3,

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5.36° at C3-C4, 5.28° at C4-C5, 3.94° at C5-C6 and 3.24° at C6-C7. The ROMs of the

18

present study were within the range of the results from previous studies (Fig. 1).

19 20

Comparison of IDP between symmetrical, moderate facet tropism and severe

21

facet tropism model

22

The IDP of the moderate facet tropism model increased by 33.33%, 38.78%, 18.75%

23

and 18.67% under flexion, extension, lateral bending and axial rotation moments, as

24

compared with the symmetrical model. The IDP of the severe facet tropism model

25

increased by 49.02%, 57.14%, 39.06% and 30.67%, under flexion, extension, lateral

26

bending and axial rotation moments, in comparison to the symmetrical model. (Fig. 2)

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Comparison of facet contact force between symmetrical, moderate facet tropism

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and severe facet tropism model

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Under the flexion moments, the facet contact force of the moderate facet tropism 6

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model and severe facet tropism model increased by 8.51% and 35.64% in comparison

2

to the symmetrical model, respectively. Under the extension moments, the contact

3

force increased by 6.88% and 31.74%, respectively. Under the latera bending

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moments, the increase were 26.73% and 79.26%, respectively. And under the axial

5

rotation moments, the increase were 39.02% and 59.47%, respectively. (Fig. 3)

6 7

Discussion

8 9

Facet tropism was defined as the angular asymmetry between the left and right side

10

facet joint with respect to the sagittal plane. It was reported to contribute to abnormal

11

stress distribution and abnormal motion pattern [17, 18]. Some clinical observations

12

suggested that facet tropism might predispose the disc degeneration, facet

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degeneration in the lumbar region [2, 5, 19].

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Different from the lumbar spine, facet orientation of the cervical spine was reported to

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be more complex. In the lumbar spine, the facet joints are inclined to a nearly vertical

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orientation. Whereas in the cervical region, the facet joints inclination with the

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transverse, sagittal and coronal planes changed from level to level [6, 7, 20]. A

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previous study demonstrated that facet tropism was commonly seen in the sub-axial

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cervical spine [7]. Results of this finite element study showed that the facet tropism

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caused imbalanced stress distribution at both the intervertebral disc and facet joints.

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Besides, under the same experiment moments of 1.0 N·m, cervical model with facet

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tropism demonstrated higher pressure at both the facet joint and intervertebral disc in

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comparison to the symmetrical model. Therefore, with regard to the abnormal stress at

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the disc or the facet joints, it seemed to be reasonable to hypothesize that the facet

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tropism might predispose to the degenerative changes of both the intervertebral disc

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and facet joints.

28 29

Several studies were conducted trying to investigate the risk factors for the

30

development of the cervical disc degeneration. Okada et al. [8] conducted a 10-year 7

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longitudinal MRI study on 223 asymptomatic healthy subjects and reported no

2

significant correlation between progression of disc degeneration and gender, smoking,

3

alcohol, sport or BMI, except for older age. Teraguchi et al. [9], in their

4

population-based MRI study, stated that overweight (modified BMI>23) was a

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significant associated factor for the development of cervical disc degeneration.

6

However, in these studies, no intrinsic anatomical factors were investigated. In this

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finite element analysis, the IDP of the severe facet tropism model increased by 49.02%

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under flexion, 57.14% under extension, 39.06% under lateral bending and 30.67%

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under axial rotation moment, comparing to the symmetrical model. Previous studies

10

suggested that increased IDP could be the predisposing factor for the development of

11

disc degeneration [21]. Therefore, it is reasonable to speculate that the facet tropism

12

might be an anatomical risk factor for the development of cervical disc degeneration.

13

However, long-term longitudinal studies are needed to verify this hypothesis.

14 15

The facet joints are synovial joints in the sub-axial spine, composed of articular

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cartilage, subchondral bone, capsular ligament and synovial folds. The facet joints

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play important roles in guiding the cervical motion and transmitting the axial loading

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[22]. Like any other joints, the facet joints are also affected by degenerative changes

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with aging, characterized by thinning of the articular cartilage, erosion of the

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subchondral bone and laxity of the capsular ligaments [23, 24]. The radiographic

21

findings are characterized as decreased of the joint space, irregularity of the articular

22

surface, formation of osteophytes and sclerosis of the subchondral bone [25, 26].

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Except for older age, no other definite risk factors were identified to be associated

24

with the development of facet joint degeneration. It was suggested that abnormal

25

loading or motion pattern would contribute to the initiation or acceleration of the

26

process of facet degeneration [27, 28]. The results of this study demonstrated that

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facet contact force increased in the facet tropism models under all experimental

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moments, especially lateral bending. The increased loading on the facet joint surface

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could cause micro-injury to the facet joints. The accumulation effect of these

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repetitive micro-injury could therefore result in initiating or accelerating the 8

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degenerative process. Therefore, the hypothesis that facet tropism could be a

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predisposing factor for the facet degeneration is reasonable, yet requires further

3

clinical observations to verify.

4 5

This study has several shortcomings. First, cautions should be taken when interpreting

6

the results of this study, because they were based on a single finite element model.

7

However, this study aimed to provide the trend rather than the actual data. By altering

8

the facet orientation, the present study was able to demonstrate the biomechanical

9

impact of facet tropism on both intervertebral disc and facet joints. Second, the

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articular surface of the facet joint was simulated as a flat plane, which was not the

11

perfect representative of the real world scenario. Although the shape of the articular

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surface varied, the flat articular surface was most common. Besides, the superior

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articular surface was depicted to slide upon the inferior articular surface when the

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flexion-extension motion occurred. Therefore, it is reasonable to simulate the articular

15

surface as a flat plane.

16 17

In this study, the impact of facet tropism on the stress distribution in both facet joint

18

and intervertebral disc were investigated. Previous studies suggested that facet

19

tropism could be the risk factor for the development of degenerative changes of the

20

spine. Further, the facet tropism was reported to cause accelerated facet degeneration

21

after lumbar total disc replacement [5]. This post-operative accelerated facet

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degeneration could be resulted from the increased facet contact force endured by

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patient with facet tropism. Besides, finite element analyses demonstrated that artificial

24

disc replacement could increase the facet contact force, especially when the prosthesis

25

had unconstrained design [12, 29]. Therefore, patient with facet tropism seemed to be

26

contraindicated to the artificial disc replacement. However, 1) the relationship

27

between facet tropism and facet degeneration after artificial disc replacement needs to

28

be evaluated; 2) to what extent the facet tropism be should be considered as the

29

contraindication is needed to be investigated.

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In summary, this study demonstrated that facet tropism with respect to the sagittal

2

plane at the C5-C6 level increased the IDP and facet contact force under flexion,

3

extension, lateral bending and axial rotation. The results suggested that facet tropism

4

might be the anatomical risk factor for the development of cervical disc degeneration

5

or facet degeneration. Future clinical studies are in need to verify the biomechanical

6

impact of facet tropism on the development of degenerative changes in the cervical

7

spine.

8 9 10

Acknowledgements The authors state that there are no conflicts of interests.

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3 4 5 6 7

Fig. 1. Comparison of the predicted range of motion to previous published data.

8

Fig. 2. Normalized intradiscal pressure (IDP) of the three finite element model under

9 10 11

flexion, extension, lateral bending and axial rotation moments. Fig. 3. Normalized facet contact force of the three finite element model under flexion, extension, lateral bending and axial rotation moments.

12 13

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Page 13 of 14

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Table 1. The facet orientations of the symmetrical model at each cervical level C2-C3 C3-C4 C4-C5 C5-C6 C6-C7

2 3 4 5

T-angle (°)

C-angle (°)

S-angle (°)

56.25 56.25 54.95 56.68 63.86

40.27 35.45 36.37 34.70 27.61

72.86 84,38 90.99 92.46 89.62

T-angle: facet orientation with respect to the transverse (disc) plane C-angle: facet orientation with respect to the coronal plane S-angle: facet orientation with respect to the sagittal plane

6 7

Table 2. Material properties and mesh types of the cervical finite element model Young’s Modulus (MPa) Cortical bone Cancellous bone Annulus fibrosus substance Annulus fibers Nucleus pulposus Facet joint cartilage ALL PLL LF IL SL CL

8 9 10 11 12 13 14 15 16

Poisson Ratio

12000 200 1 450 4.2 10.4 30 20 10 10 1.5 10

0.29 0.29 0.49 0.45 0.45 0.4 0.4 0.4 0.4 0.4 0.4 0.4

Element Type C3D4 C3D4 C3D4 T3D2 C3D4 C3D4 T3D2 T3D2 T3D2 T3D2 T3D2 T3D2

Cross (mm2)

Section

6.1 5.4 50.1 13.1 13.1 46.6

ALL: anterior longitudinal ligament PLL: posterior longitudinal ligament LF: ligamentum flavum IL: interspinous ligament SL: supraspinous ligament CL: capsular ligament C3D4: tetrahedron T3D2: truss, tension only

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