Stress analysis of the disc adjacent to interbody fusion in lumbar spine. Med Eng Phys

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1 See discussions, stats, and author profiles for this publication at: Stress analysis of the disc adjacent to interbody fusion in lumbar spine. Med Eng Phys Article in Medical Engineering & Physics October 2001 Impact Factor: 1.83 DOI: /S (01) Source: PubMed CITATIONS 125 READS 40 4 authors, including: Chen-Sheng Chen National Yang Ming University 46 PUBLICATIONS 730 CITATIONS Cheng-Kung Cheng National Yang Ming University 253 PUBLICATIONS 2,850 CITATIONS SEE PROFILE SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately. Available from: Cheng-Kung Cheng Retrieved on: 10 May 2016

2 Medical Engineering & Physics 23 (2001) Stress analysis of the disc adjacent to interbody fusion in lumbar spine Chen-Sheng Chen a, Cheng-Kung Cheng a,*, Chien-Lin Liu b, Wai-Hee Lo b a Orthopaedic Biomechanics Laboratory, Institute of Biomedical Engineering, National Yang-Ming University, 155 Sec 2, Li-Nung St, Shih-Pai, Taipei, Taiwan b Department of Orthopaedic Surgery, Veteran General Hospital-Taipei, Taipei, Taiwan Received 25 January 2001; received in revised form 27 April 2001; accepted 20 June 2001 Abstract After anterior interbody fusion in the lumbar spine, the accelerated degeneration of the disc adjacent to the fusion levels was clinically observed. To understand the stress distribution of the adjacent disc, this study created a finite element model of the lumbar spine from L1 L5 vertebral body. The fusion model modified from the intact model was used to simulate the anterior interbody fusion. Various loading conditions, which included flexion, extension, lateral bending, and torsion, were applied to the finite element model to study the corresponding stress distribution. From the finite element model calculation, at a lower fusion site or more fusion levels, the stress of the disc adjacent to interbody fusion increased more than upper fusion site or single fusion level under flexion, torsion and lateral bending. Larger stress increase was estimated at the upper disc adjacent to interbody fusion than the lower disc adjacent to interbody fusion. In stress distribution, the upper disc adjacent to interbody fusion had a little alteration under torsion IPEM. Published by Elsevier Science Ltd. All rights reserved. Keywords: Disc degeneration; Finite element model; Anterior interbody fusion 1. Introduction The functions of the spine are to provide flexibility, support of the upper body weight, and protect the spinal cord and nerve roots. Some idiopathic diseases and severe external loads may cause compression on nerves and make the spine become unstable. Spinal fixation devices can be used to form a rigid construct with the spine to replace bone, restore alignment, maintain position, and prevent motion in the treatment of fractures, degenerative disease, and congenital deformities. Most fixation devices are mainly used to promote fusion by bone graft. For permanent stabilization, spinal bony fusion is indispensable. After bony fusion, the biomechanical behavior of the spinal structure is altered. The redistribution of stress among the unfused segments has been correlated * Corresponding author. Tel.: ; fax: address: ckcheng@bme.ym.edu.tw (C.-K. Cheng). with and led to subsequent degeneration of the adjacent disc. Numerous clinical studies indicate that these changes at one level may lead to degenerative changes at adjacent levels [1 5]. A long-term follow-up of lower lumbar fusion patients was conducted by Lehmann et al. s study [6]. They reported accelerated degeneration of adjacent segment and segmental instability above the fusion in 45% of their patients. Lee and Langrana [7] conducted a biomechanical study of lumbosacral fusion and demonstrated that stress increased on the adjacent unfused segments. Frymoyer et al. [8] in a long-term follow-up of disc excision with and without fusion, reported that symptomatic adjacent disc disease in 5 of their 143 fusion patients. All of their radiographic findings suggested that fusion imposed additional stresses on the motion segment above the fusion site [9,10]. However, little information is known about the load sharing of the adjacent disc under different fusion levels. Logically, one would expect that the amount of extra stress should be related directly to the stiffness of the motion segment after performing anterior interbody /01/$ - see front matter 2001 IPEM. Published by Elsevier Science Ltd. All rights reserved. PII: S (01)

3 484 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) fusion. Because the disc was replaced by fusion mass, the overall stress distribution has been changed, including the adjacent level. To determine the effect in various fusion levels, it is difficult to conduct experimental studies. The experimental difficulties include the anterior interbody fusion specimens collecting and the question of control with regard to the repeated testing of the same spine under various loads. In contrast, the finite element modeling technique mitigates these problems because of its reproducibility and repeatability characteristics. It could show stress distribution under different loading modes or experience any structure change. Regarding biomechanical applications of the finite element method, Goel et al. [11 13] and other investigators [14,15] have used the technique to perform many related spinal researches which included load sharing among spinal components, stress analysis of spinal instrumentation, effects of muscle on lumbar spine and so on. However, little information has focused on stress alteration of the disc adjacent to interbody fusion. Therefore, in order to help explain the structural changes that accompany the degenerative process, this study proposes to analyze stress alteration of the disc adjacent to the anterior interbody fusion using the finite element method. 2. Materials and methods 2.1. Intact model To understand the stress alteration of the disc adjacent to interbody fusion, the present study created a five-level finite element model (FEM) of the lumbar spine which included the five vertebral body and four discs from the L1 to L5 (L1 L5) vertebral body. The kinematics data of the present L1 L5 FEM were compared with the in vitro experiment [16] under the same loading conditions, which included flexion, extension, torsion, and lateral bending for the validation. The commercially available finite element program, ansys 5.5 (Swanson Analysis System Inc., Houston, TX) was used to model the spinal segments. Computerassisted tomographic images of the normal ligamentous lumbar spine of a 19-year-old male subject were obtained using transverse slices at every 3-mm interval. Each CT slice was acquired from the coronal plane and enlarged in order to identify the different regions of the tissues. At the same time, its position in a global coordinate system was calculated from roentgenograph, as well as its sagittal inclination due to lordosis. The coordinates of the nodal point were digitized in terms of the X- and Y- coordinates in each CT slice. The X-axis was defined as the mediolateral direction of the vertebral body and the Y-axis as the anteroposterior direction of the vertebral body. The orientation of each articular facet was modified to ensure the geometric congruence between adjacent articular facets which were connected with intervertebral discs and the ligaments. These ligaments were defined using lines to join their approximate attachment points on adjacent vertebrae. The edge of the spinal disc was obtained from the enhanced CT images. But the geometry of the disc nucleus was difficult to distinguish from the CT image and the study referred to Panagiotacopulos et al. s study [17]. The 30 50% of the total disc area in cross-section was defined as the disc nucleus of the FEM, and the rest of the region was assumed as the disc annulus of the FEM. The FEM of the ligamentous lumbar spine consisted of vertebrae, intervertebral discs, superior and inferior facet articulating surfaces, and a number of ligaments: supraspinous, interspinous, ligamentum flavum, transverse, posterior longitudinal, anterior longitudinal, and capsular. The material properties adopted from literature [12,13,18 20] are listed in Table 1. The cable elements were used to simulate ligaments and annulus fiber of disc, which were active only in tension. Three-dimensional contact elements were used to simulate the contact characteristics of the facet articulation. The rest of the portions assigned as the solid elements included the vertebral body, posterior element, the disc annulus, and the disc nucleus [13]. The whole model of the lumbar spine contained 8870 elements and 7603 nodes as listed in Table 2. The typical views of the reconstructed threedimensional model are shown in Fig Fused model The intact FEM described above was modified to simulate the anterior interbody fusion. To mimic the anterior bony fusion, the disc was totally removed and then replaced by the interbody bone graft. The assumption of the study was that the patients have gained solid fusion, so the stress distribution within the model was determined by assuming that the interbody bone grafts would be able to transmit loads in compression as well as in tension. After the modification of the model, this study further simulated interbody fusion on different levels L2 L3, L3 L4, L4 L5, L2 L3 L4, and L3 L4 L5 to understand its influence on the adjacent disc. The volume and area of each bone graft are listed in Table Boundary and loading condition In the five-level FEM, the degrees of freedom of inferior surfaces of the inferiormost vertebral body were completely fixed in all directions. To validate the model, same loading conditions as given in Yamamoto et al. s study [16] were applied. Therefore, the 10 N m flexion, 10 N m extension, 10 N m torsion, and 10 N m lateral bending moment under the 150 N pre-load were imposed on the L1 vertebral body, respectively. The maximum load was achieved in five load steps in the FEM. Because

4 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) Table 1 The material properties specified in the finite element models (ε: strain) Material Young s modulus (MPa) Poisson s ratio Cross-section (mm 2 ) Vertebra Cortical bone 12, Cancellous bone Posterior elements Disc Nucleus Ground substance Fiber Ligament ALL (anterior longitudinal ligament) 7.8(ε 12%) 20(ε 12%) 63.7 PLL (posterior longitudinal ligament) 10(ε 11%) 20(ε 11%) 20.0 LF (ligamentum flavum) 15(ε 6.2%) 19.5(ε 6.2%) 40.0 TL (transverse ligament) 10(ε 18%) 58.7(ε 18%) 1.8 CL (capsular ligament) 7.5(ε 25%) 32.9(ε 25%) 30.0 IL (interspinous ligament) 10(ε 14%) 11.6(ε 14%) 40.0 SL (supraspinous ligament) 8(ε 20%) 15(ε 20%) 30.0 Bone graft Table 2 Total element numbers of each component of the whole lumbar spine Structure Element type Whole model Vertebral body Cortical bone 8-node Solid 1030 Cancellous bone 1320 Posterior elements 8-node Solid 678 Disc Ground substance 8-node Solid 2048 Nucleus 8-node Solid 384 Fiber 2-node Cable 3070 Facet joint Contact 192 Ligaments ALL (anterior longitudinal ligament) 2-node Cable 40 PLL (posterior longitudinal ligament) 40 TL (transverse ligament) 16 LF (ligamentum flavum) 20 IL (interspinous ligament) 12 SL (supraspinous ligament) 4 CL (capsular ligament) 16 Total element numbers 8870 Total node numbers 7603 of the stress accumulation caused by different loads it was necessary to consider the overall stress variation in the estimated results of the FEM. Consequently, the stress results were expressed in terms of von-mises stresses. The estimation of stress increase rate of the adjacent disc is shown in the following equation Stress increase rate (S fused S intact )/(S intact ) (%) (1) where S fused and S intact represent the maximum von-mises stress of the adjacent disc in the fused model and the intact model, respectively. 3. Results 3.1. Model validation The kinematics data of the lumbar spine in Yamamoto et al. s study [16] were compared to the results of the FEM under the act of the same load as listed in Table 4. In our study, the total range of motion was 14.4 in flexion, 10.0 in extension, 9.1 in torsion, and 11.6 in lateral bending. In flexion, the lower motion segment had the larger range of motion. The pattern was consistent

5 486 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) Kinematics analysis of fused models Under the different loading conditions, the decrease rate of range of motion of the L1 L5 lumbar spine is shown in Fig. 2. Comparing the results of the three single fusion levels, the decrease rate of range of motion in the L4 L5 fusion was larger than that of the L2 L3 fusion and the L3 L4 fusion. The decrease rate of range of motion in L4 L5 fusion reached 26% in flexion, 36% in extension, 29% in torsion, and 10% in lateral bending. Comparing the results of the double fusion levels, the decrease rate of range of motion in the L3 L4 L5 fusion was larger than the L2 L3 L4 fusion. The decrease rate of range of motion in L3 L4 L5 fusion reached 51% in flexion, 46% in extension, 52% in torsion, and 42% in lateral bending Kinetics analysis of fused models Fig. 1. The FEM of the entire lumbar spine included ligaments, disc, vertebral body, and posterior element. Table 3 The area and volume of bone graft used in different fusion levels Area (mm 2 ) Volume (mm 3 ) L1 L ,352 L2 L ,995 L3 L ,765 L4 L ,460 with Yamamoto et al. s study. In extension, torsion and lateral bending, the range of motion in each motion segment, the least and the largest difference between Yamamoto et al. s study and FEM results were 0.17 and 1.72, respectively. The stiffness of this FEM was larger than that of cadaver specimens except for the torsion mode. Regarding the contact of the facet joint of this FEM, the study estimated that the contact force was 121 N in L1 L2, 157 N in L2 L3, 161 N in L3 L4, and 155 N in L4 L5. Comparison of contact force of facet joint between the present study and Shirazi-Adl s study [15] are listed in Table 5. This FEM had larger contact force than the Shirazi-Adl s model Flexion The stress increase rate of the adjacent disc in flexion is shown in Fig. 3. The stress increase rate of the upper disc was larger than that of the lower disc. For the same adjacent disc L2 L3, the stress increase rate of the L3 L4 L5 fusion was larger than that of the L3 L4 fusion. The more fusion levels had more stress increase rates on same adjacent discs. Additionally, the disc adjacent to the lower fusion site had larger stress increase rate than the disc adjacent to the upper fusion site. The adjacent disc in the fusion level L4 L5 had the largest stress increase rate and reached 9.23%. In all the models, the location of the maximum stress in adjacent disc occurred in the posterior region as shown in Fig. 4. The high stress region is demonstrated in the anterior and posterior border. Compared with the intact model, the stress distribution of the adjacent disc had no alteration. For the stress distribution of the overall fused model, the spinous process and lamina in the fusion site displayed lower stress compared with the intact model Extension The stress increase rate of the adjacent disc in extension is shown in Fig. 5. The stress increase rate of the lower disc was larger than that of the upper disc. The largest stress increase rate of the adjacent disc compared with intact models was 5% and occurred in the L2 L3 L4 fusion. The location of the maximum stress in the adjacent disc was in the posterior region, and had not changed between the fused model and intact model. For the overall stress distribution, the spinous process displayed much lower stress than lamina Torsion The stress increase rate of the adjacent disc in torsion is shown in Fig. 6. The stress increase rate of the upper disc was larger than that of the lower disc. For the same

6 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) Table 4 Comparison between the FEM results and the in vitro experimental study [16] in the three-dimensional angular motion and the stiffness of the L1 L5 lumbar spine Flexion Extension Torsion Lateral bending Present study Yamamoto Present study Yamamoto Present study Yamamoto Present study Yamamoto et al. (1989) et al. (1989) et al. (1989) et al. (1989) L1 L2 (degree) L2 L3 (degree) L3 L4 (degree) L4 L5 (degree) Stiffness (N m/degree) Table 5 Comparison of total contact force (N) in facet joint between the present study and Shirazi-Adl s study [15] (unit: newton) Loading condition L1 L2 L2 L3 L3 L4 L4 L5 Present study Shirazi- Adl s study 10 N m with 150 N axial force 10 N m adjacent disc L2 L3, the stress increase rate of the L3 L4 L5 fusion was larger than that of the L3 L4 fusion. The more fusion levels had more stress increase rate. Besides, the disc adjacent to the lower fusion site had a larger stress increase rate than the disc adjacent to the upper fusion site. The fusion site L4 L5 had the largest stress increase rate in the upper disc and reached 9.8%. As to the stress distribution of the adjacent disc, the Fig. 3. The results of stress increase of the disc adjacent to interbody fusion under 10 N m flexion moment are depicted. Larger stress increase was estimated at the upper level compared with the lower level. Fig. 2. The kinematics results in different fusion levels are illustrated.

7 488 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) Fig. 4. Under 10 N m flexion, the stress contours of the adjacent disc in the L2 L3 L4 fusion model are displayed. The location of the maximum stress (MX) between the intact model and the L2 L3 L4 fusion is not changed by anterior interbody fusion. (A) the L1 L2 disc of the intact model, (B) the L4 L5 disc of the intact model, (C) the L1 L2 disc of the L2 L3 L4 fusion model and (D) the L4 L5 disc of the L2 L3 L4 fusion model. Fig. 7. Under 10 N m torsion, the stress contours of the adjacent disc in the L2 L3 L4 fusion model are displayed. Compared with the intact model, the more stress increased around the rim of disc as indicated by the arrows. The location of the maximum stress (MX) of disc moved posteriorly in the L2 L3 L4 fusion model. (A) the L1 L2 disc of the intact model, (B) the L4 L5 disc of the intact model, (C) the L1 L2 disc of the L2 L3 L4 fusion model and (D) the L4 L5 disc of the L2 L3 L4 fusion model Lateral bending The stress increase of the adjacent disc in lateral bending is shown in Fig. 8. The disc adjacent to the lower fusion site had a larger stress increase rate than the disc adjacent to the upper fusion site. The largest stress increase rate reached 10% and occurred at the L4 L5 fusion and L3 L4 L5 fusion. The location of the maximum stress in adjacent disc was occurred at lateral region. Fig. 5. The results showed the stress increase of the disc adjacent to interbody fusion under 10 N m extension moment. Larger stress increase was estimated at the lower level compared with the upper level. 4. Discussion To understand the stress increase rate of the disc adjacent to interobdy fusion, this study established a five-level lumbar spine FEM to perform analysis. Although this FEM obtained a similar trend of biomechanical behavior compared with a previous study [16], there were still some differences between the in vitro tests and the FEM computation. Fig. 6. The results showed stress increase of the disc adjacent to interbody fusion under 10 N m torsion moment. Larger stress increase was estimated at the upper level compared with the lower level. stress contour gradually increased from the nucleus to the outer rim of the disc annulus as shown in Fig. 7. In these fused models, the location of the maximum stress in the upper disc moved posteriorly and the high stress region had a little expansion around the rim of the disc, but the lower disc had no alteration of stress distribution. Fig. 8. The results showed stress increase of the disc adjacent to interbody fusion under 10 N m lateral bending moment. Larger stress increase was estimated at the upper level compared with the lower level.

8 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) Yamamoto et al. [16] measured inter-segmental axial rotation which varied between 1.1 and 2.4. The present study estimated that the inter-segmental axial rotation varied between 2.08 and 2.56 and found that the torsional stiffness of the FEM was less than that of the cadaver specimen. This may due to the numbers of annulus fibers of the FEM being less than that of the in vitro disc. The annulus fibers could resist the torsional load. Marchand and Ahmed [21] addressed that the average space between fibers was 0.22 mm in the actual lumbar spinal disc. However, in the disc of the FEM, the space between fibers was defined to be about 0.9 mm. Therefore the numbers of fibers of the FEM were not as compact as those of the in vitro disc. Compared with the Yamamoto et al. s study, the stiffness of this FEM in flexion, extension and lateral bending modes was larger than that of the cadaver lumbar spine. The reasons may be attributed to the different geometries and material properties used. In the in vitro test, the older cadaver specimen revealed disc degeneration [16], such as the fissure formation of disc, bulging gel of disc nucleus or discrete fibrous lamellas of disc annulus. These factors probably led to the stability of older cadaver specimen being less than that of the younger spine model. Additionally, the in vivo or in vitro spinal disc has high flexibility due to its fluid containing disc nucleus and fibrous tissue of the disc annulus. The biomechanical characteristics of the disc showed viscoelastic behavior and had less stiffness. In the finite element analysis, the solid elements were applied to simulate the disc. It was consequently difficult to mimic the nonlinear characteristics and resulted in the disc of the FEM having higher stiffness than that of the cadaver specimen. Regarding the geometry of the FEM, our FEM lacked the L5 S1 motion segment compared with the cadaver specimen of Yamamoto et al. [16]. As the result, the length of the FEM of the lumbar spine was shorter than that of the cadaver specimen. The short column of the L1 L5 FEM should have larger stiffness than the long column of the L1 S1 cadaver specimen. Besides, the lordotic curvature of the FEM was probably less than that of the cadaver lumbar spine. Because the FEM had no self-weight, the cadaver specimen had self-weight and resulted in the increase of the lordotic curvature. Therefore, the shorter FEM with less lordosis might cause less angular motion than the cadaver specimen. As to the contact force of the facet joint, this FEM varied between 121 and 161 N. In Shirazi-Adl s FEM [15], the contact force of facet joint varied between 98 and 154 N. These force differences were because of the differences of gap of facet joint and imposition of preload of FEM. In our FEM, the gap of the facet joint was less than 1 mm and had pre-load 150 N. In Shirazi-Adl s study, the gap distance of their FEM was 1.25 mm and the loading condition had no pre-load. Therefore, in our model, the smaller gap distance with pre-load resulted in the larger contact force than Shirazi-Adl s model. Except for these differences in kinematics data and the contact force of the facet joint, this FEM also had some assumptions and limitations. The structure of the vertebral body was assumed as the isotropic and homogenous property. The bone graft was assumed to completely occupy the disc space, which may not be true in the in vitro test or clinical practice. The loading conditions were not under physiological condition, because this FEM had no mechanical effect of muscle contraction. Although this FEM had these assumptions and limitations, this FEM results had similar kinematics pattern and same order with the range of motion of the in vitro test. Therefore, this FEM should have enough reliability to undergo the qualitative analysis in the stress of disc adjacent to interbody fusion. Regarding the fused model, the longer fused level had larger stress increase on adjacent disc. This situation was probably due to the increase of stiffness of the motion segment. In the L2 L3 L4 fusion and the L3 L4 L5 fusion, the largest decrease of range of motion compared with intact model reached 50.2 and 51.7%, respectively. In the L2 L3 fusion, the L3 L4 fusion and the L4 L5 fusion, the largest decrease of range of motion compared with intact model only reached 27.1, 23.5 and 36.3%, respectively. One could predict that increase of stiffness of motion segment induced more stress on the adjacent disc, because the adjacent disc was located between the fused segment and normal motion segment. The stress concentration would be enhanced by a tremendous alteration of different rigidities. The disc adjacent to the lower fusion site had more stress increase. The possible reason was that the lower fusion site was close to the fixed point of the boundary. Before the lumbar spine flexed 60, the sacrum was not moved as the fixed point. The high rigidity of the fused segment was like a fixed point. The motion segment of the lower lumbar spine usually had more mobility as shown in FEM results or Yamamoto et al. s study, especially in the L4 L5 motion segment. From high mobility to complete fixation, the disc adjacent to the lower fusion site experienced more deformation than other discs. Therefore, the L4 L5 fusion or the L3 L4 L5 fusion generated more stress on the upper disc than other fusion levels under flexion, torsion and lateral bending. These situations were consistent with the clinical report [22], which Penta et al. addressed that the patients who had the upper disc degeneration adjacent to interbody fusion had 16.6% with L4 L5 fusion, 29.2% with L5 S1 fusion and 38.4% with L4 S1 fusion. Additionally, the study compared with Maiman et al. s study [23] about the stress increase of the disc adjacent to the cervical spine fusion. The results of their study also had a similarities with our results, where stress increase in the lower fusion level was larger than that

9 490 C.-S. Chen et al. / Medical Engineering & Physics 23 (2001) in the upper fusion level. Therefore, the results of our study confirmed the similar trends that the longer fused segment or the lower fused segment was easy to increase stress and it was likely to cause the disc early degeneration change. In the flexion, torsion and lateral bending, the largest stress increase rate could reach 9.2, 9.8 and 10%, respectively. Under the extension, the largest stress increase rate of the adjacent disc only had 5.13% and the lower disc had the largest stress increase. The stress increase on the adjacent disc was likely to be influenced by the facet joint, because the facet joint had considerable contribution in extension [24]. In the stress distribution of FEM, some stresses passed through the posterior element under extension. Consequently, in extension, the load sharing in the adjacent disc was influenced by the contact of the facet joint and exhibited the different stress increase with other loading modes. Regarding the stress distribution of the adjacent disc, the results showed that location of the maximum stress had no change under flexion, extension and lateral bending compared with the intact model. However, under the torsion, the high stress region had a little expansion around the edge and moved to the posterior region in the upper disc. Moreover, the stress contour displayed that the entire high stress region of the adjacent disc concentrated on the rim of the annulus. In the autopsy study [25], Pearce reported that the rim lesion of the degenerated disc appeared at the periphery of the annulus and endplate. Obviously, the edge between the annulus and endplate was a weak point for the degenerated disc. Besides, Nachemson et al. [26] also found no differences in disc deformation behavior in flexion, extension and lateral bending for specimens of different ages. However, the influence of age was apparent in torsion. Krismer et al. [27] reported that fissure formation of the annulus in the degenerated disc was associated with the torsion. Thus, the stress increase probably accelerated fissure formation of the disc in torsion. Although this study could not completely identify that the disc early degeneration was attributed to the fusion procedure, one could predict that the stiffness of the motion segment and stress of the adjacent disc had been altered because of implantation of fusion mass. When the adjacent disc experienced the aging process, high stress probably accelerated the fracture of the disc fiber, bulged gel of the nucleus, fissure formation, and separation of the fibrous lamellas of disc annulus [28]. As results, the disc height was reduced and it was possible to induce juxtafusion problems such as spinal stenosis or degenerative spondylolisthesis [3]. Therefore, the stresses on the adjacent disc probably accelerate degeneration, especially in the aging process. To further identify the disc early degeneration change after bony fusion, the more complicated physiological condition or more detail structure of lumbar spine could be considered in future studies. 5. Conclusion A three-dimensional nonlinear FEM of the lumbar spine was established to simulate the anterior interbody fusion. Under flexion, torsion and lateral bending, at a lower fusion site or more fusion levels, the stress of the disc adjacent to interbody fusion increased more than that of the upper fusion site or single fusion level. Additionally, the upper disc adjacent to anterior interbody fusion had more stress increase than the lower disc adjacent to anterior interbody fusion. 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