Biomechanical Effect of C5 /C6 Intervertebral Reconstructive Height on Adjacent Segments in Anterior Cervical Discectomy and Fusion - A Finite Element Analysis.

OBJECTIVE: To investigate the biomechanical effect of different intervertebral reconstructive heights on adjacent segments following C5 /C6 anterior cervical discectomy and fusion (ACDF) through finite element analysis.
METHODS: A finite element model of intact C4 -C7 segments was developed and validated for the present study. Five additional C4 -C7 postoperative models were constructed with 100%, 125%, 150%, 175%, and 200% of the benchmark height of C5 /C6 on the basis of the intact model. The changes in intradiscal pressure (IDP) and range of motion (ROM) of adjacent segments before and after reconstruction of C5 /C6 were analyzed.
RESULTS: For the upper adjacent segment (C4 /C5 ), the IDPs under the different loading conditions all increased after reconstruction. The maximum IDPs were 0.387, 0.489, 0.491, and 0.472 MPa under flexion, extension, axial rotation, and lateral bending, respectively, observed at the reconstructive height of 200%. The minimum IDPs were observed at 150% reconstructive height under all loading conditions except extension, and were 57, 86 and 81% of the maximum IDPs under flexion, axial rotation, and lateral bending, respectively. The minimum IDP under extension occurred when the reconstructive height is 125% of the benchmark height. For the lower adjacent segment (C6 /C7 ), the IDPs of postoperative models under all loading conditions also increased compared to the preoperative model. The maximum IDPs after reconstruction under flexion, extension, axial rotation, and lateral bending were 0.402, 0.411, 0.461, and 0.497 MPa, respectively, when the height of the reconstruction was 200% of the benchmark. The minimum IDPs were observed after a reconstruction at 150% of the benchmark, and were 59%, 85%, 82%, and 81% of the maximum IDPs under flexion, extension, axial rotation, and lateral bending loading conditions.
CONCLUSIONS: The reconstructive height is an important factor affecting the IDP and the ROM of adjacent segments after ACDF. To delay the adjacent segment disease, an intervertebral reconstructive height of 150% is an appropriate height in C5 /C6 ACDF.

Introduction

Cervical degenerative disc disease is arguably the most common pathology of the cervical spine, mainly including cervical radiculopathy, cervical myelopathy, and a combination . Cervical radiculopathy is pain in unilateral or bilateral upper extremities, often in the setting of neck pain, secondary to compression or irritation of nerve roots in the cervical spine. Cervical myelopathy is a degenerative change in the vertebral column that results in symptoms of a spinal cord disorder that range from dexterity or balance disturbances to quadriparesis and incontinence . In addition, cervical myelopathy patients may also complain of atypical symptoms, including vertigo, blurred vision, nausea, and hypomnesia , . For patients unresponsive to appropriate nonsurgical measures for at least 6 months, surgical treatment should be considered . Primary aims of surgery are to relieve radiating arm pain in case of radiculopathy and to prevent progression of neurological deficit in case of myelopathy.

First introduced by Smith and Robinson, anterior cervical discectomy and fusion (ACDF) is currently the gold standard surgical treatment for affected patients , , . ACDF can achieve stabilization and solid arthrodesis with good clinical outcomes and lower complications. However, many studies have found that cervical fusion can lead to the degeneration of adjacent segments, eventually result in adjacent segment disease (ASD) , , . ASD is defined as a new clinical finding that corresponds to radiographic signs of the degeneration of adjacent segments. As a complication of ACDF, ASD has led to 5.6% of ACDF patients having to undergo a second surgery, which significantly increases the economic burden on society .

There have been a few studies suggested that intervertebral reconstructive height , , plate‐to‐disc distance , and post‐operative segmental alignment are factors that affect the incidence of ASD. Chung et al. reviewed 177 patients who underwent ACDF using cervical plates, with follow‐up periods of at least 10 years. In this study, they found most clinical adjacent‐segment degeneration appeared on the patients with a plate‐to‐disc distance less than 5 mm. Hence, they considered that to prevent ASD, the plate‐to‐disc distance should be 5mm or more if possible. Katsuura et al. found that for patients with ACDF, postoperative kyphotic change in the fused segment is a factor of ASD.

In addition, reconstructive height is considered to be an important factor affecting the development of ASD. However, the most suitable reconstructive height is controversial. Smith and Robinson indicated that the 10–15 mm reconstructive height is suitable. White and Panjabi proposed that the intervertebral space should be 4–5 mm. Olsewski et al. concluded that the stress significantly decreased when the reconstructive height was in excess of 3 mm. Kawakami et al. concluded that 2–5 mm reconstruction is an appropriate reconstructive height, with a lower rate of axial neck pain. Li et al. reported that excessive disc space distraction is a risk factor for the development of radiographic ASD.

Finite element analysis (FEA) has been widely used to investigate the external changes, as well as analyze the internal status of the structural components quantitatively , . Through FEA, Sun et al. found that noncontinuous cervical disc arthroplasty could preserve intradiscal pressure (IDP) and facet joint forces at the adjacent and intermediate levels. Ren et al. found the cervical vertebrae after percutaneous posterior endoscopic cervical discectomy showed good biomechanical performance and stability. Although some scholars have used FEA to calculate the stress state of adjacent segments after ACDF , , , quantitative analyses of the influence of different reconstructive heights on adjacent segment mechanic bearing by FEA have not been reported. Hence, the objective of the present study was to gain a better understanding of the ASD after ACDF by quantitatively analyzing the effect of different reconstructive heights of C5/C6 on the IDP and the cervical vertebrae range of motion (ROM) of adjacent segments after ACDF by FEA.

Material and Methods

Preoperative Model Construction

To establish a finite element (FE) model, we selected a 27‐year‐old healthy male who had no history of cervical disc disease. We performed a computed tomography (CT) scan of his spine and imported the images into the MIMICS 21.0 (Materialise, Leuven, Belgium) modeling program to obtain the preliminary C4‐C7 vertebral model. The preliminary model was then smoothed and corrected in Geomagic Studio 14.0 (Geomagic, Research Triangle Park, NC, USA).

Other anatomical structures such as intervertebral disc and cartilage were built using Solidworks 2019 (Solidworks, Waltham, MA, USA). The intervertebral disc is composed of nucleus pulposus, annulus fibers and annulus ground substance. We constructed annulus fibers surrounded the ground substance with an inclination to the transverse plane between 15° and 45°, accounting for approximately 19% of the entire annulus fibrosus volume in Hypermesh 14.0 (Altair, Troy, MI, USA).

Five groups of ligaments, including the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), ligamentum flavum, interspinous ligament and capsular ligament were established using tension‐only spring elements and attached to the corresponding vertebrae. Then, the geometry was imported into Abaqus 6.13 (Simulia Inc., Providence, RI, USA) to set the loading and boundary conditions and output the IDP.

The bony structures of the complete model consisted of the cortical bone, the cancellous bone and the posterior bony elements. The 1.5 mm‐thick shell was separated from each cervical vertebra to act as the cortical bone. The 0.2 mm–thick endplates were inserted on the upper and lower sides of the intervertebral discs. The cartilages were inserted into the spaces of the bony articular process joints. All cartilages of the articular processes were subjected to a nonlinear face‐to‐face frictionless contact with each other , .

Finally, the whole preoperative C4‐C7 FE model was constructed as shown in Fig. 1. The entire FE model consists of 139,672 solid elements and 299,408 nodes. The material properties are listed in Tables 1 and 2 , .

Fig. 1

Finite element models of preoperative and postoperative C4–C7 cervical spine and other structures and details. (A) Frontal view of preoperative model; (B) Lateral view of preoperative model; (C) Intervertebral disc details; (D) Vertebral body details; (E) Lateral view of the cage; (F) Titanium plate and screws; (G) Frontal view of postoperative model; (H) Lateral view of postoperative model; (I) Loading and boundary condition.

TABLE 1

The material properties of the spinal soft tissues and hard tissues used in the finite element model

Description	Element type	Young's modulus (MPa)	Poisson's ratio
Cortical bone	C3D4	12,000	0.3
Cancellous bone	C3D4	100	0.2
Posterior elements	C3D4	3500	0.25
Facet cartilage	C3D4	10.4	0.4
End plate	C3D4	600	0.3
Nucleus pulposus	C3D4	1	0.49
Annulus ground substance	C3D4	3.4	0.4
Annulus fibers	T3D2	450	0.45
Titanium plate	C3D4	120,000	0.3
Titanium screw	C3D4	120,000	0.3

TABLE 2

The material properties of the ligaments

ALL		PLL		LF		ISL		CL
Displacement (mm)	Force (N)	Displacement (mm)	Force (N)	Displacement (mm)	Force (N)	Displacement (mm)	Force (N)	Displacement (mm)	Force (N)
0	0	0	0	0	0	0	0	0	0
1	35.5	0.9	1.33	1.7	2.2	1.2	0.75	1.7	2.452
2	64.9	2	29.0	3.74	45.9	2.7	16.9	3.9	53.6
4	89.7	3	51.4	5.61	82.9	4.0	24.4	5.8	87.9
5	108.6	4	71.38	7.48	119.6	5.4	29.5	7.7	109.4
6	119.6	5	85.8	9.35	133.7	6.7	32.9	9.7	125.8
		6	94.7	11.3	147.2	8.1	34.9	11.5	134.8

ALL, anterior longitudinal ligament; CL, capsular ligament; ISL, interspinous ligament; LF, ligamentum flavum; PLL, posterior longitudinal ligament.

Mesh Convergence Test

The FE model was tested for mesh convergence. Three mesh resolutions were generated consecutively (in the order of Mesh 1, Mesh 2, and Mesh 3) for this FE model. Mesh 1 had the smallest number of elements and nodes among the three mesh resolutions. Mesh 2 and Mesh 3 had approximately doubled numbers of elements and nodes than the previous mesh resolution. The number of elements and nodes for each mesh resolution are shown in Table 3. The three mesh resolutions were tested under the same rotation with a moment of 1.0 N·m. The von Mises stress was calculated and compared for different structures in the FE model. When the prediction results obtained by two consecutive mesh resolutions have differences smaller than 5%, the mesh was considered to be convergent , .

TABLE 3

Element and node numbers for three different mesh resolutions

	Element number	Node number
Mesh 1	63218	148526
Mesh 2	139672	299408
Mesh 3	232536	452768

Validation of the Model

Range of motion of the preoperative C4‐C7 finite element (FE) model was predicted with a pure bending moment of 1 N·m for flexion, extension, axial rotation, and lateral bending with 73.6 N of axial compression superior to C4 and compared to previous experimental results . To measure the ROM, we established a cross coordinate system on the superior plane of the target vertebral body, and then measured the ROM in different directions according to the changes in the position of the coordinate system after loading.

Postoperative Models Reconstruction

Generally, during a real procedure, the C5/C6 anterior longitudinal ligament, C5/C6 disc, inferior endplate of C5, superior endplate of C6 and C5/C6 posterior longitudinal ligament were resected. Hence, we deleted corresponding structures to simulate the surgery more precisely. In order to reduce the impact of individual differences in the height of the intervertebral discs, we chose the heights of 100%, 125%, 150%, 175%, and 200% of the benchmark height.

In addition, because cervical disc degeneration mostly occurs in the C5/C6 segment, we selected the C5/C6 segment for the simulation of the surgical segment so that the results could be suitable for more patients. The preoperative C5/C6 intervertebral height was 5.0 mm. Hence, five postoperative FE models were built with heights of 5, 6.25, 7.5, 8.75, and 10 mm at 100%, 125%, 150%, 175% and 200% of the benchmark height.

In the actual operation, the interbody fusion cage (Medtronic Sofamor Danek, Memphis, TN, USA) can increase cervical lordosis of 7.5°, so we simulated the reconstruction to increase the cervical lordosis by 7.5° in all postoperative models. All postoperative models were based on a validated model of the aforementioned preoperative C4–C7 model. One of the postoperative models was shown in Fig. 1.

In the present study, the C5/C6 anterior titanium alloy plate and self‐tapping screws (Medtronic Sofamor Danek, Memphis, TN, USA) were simulated. The length of the titanium alloy plate increases in response to an increase in the reconstructive height, so as to make the screws fixed in the same position of the vertebral body in all postoperative models. The five postoperative models were loaded in flexion, extension, axial rotation, and lateral bending, by imposing a pure moment of 1.0 Nm on C4 with 73.6 N of axial precompression superior to the upper endplate of C4. The lower endplate of C7 was firmly fixed in all degrees of freedom. All related connections were set to binding except the mutual contact between the cartilages of the articular processes.

Results

The percentage differences in von Mises stress of Mesh 1 vs. Mesh 2 and Mesh 2 vs. Mesh 3 are shown in Fig. 2. The differences of von Mises stress between Mesh 2 and Mesh 3 were less than 5% in the model. Hence, Mesh 2 was considered to be stress‐converged and was chosen for this study.

Fig. 2

The predicted percentage differences of von Mises stress between Mesh 1 and Mesh 2 and between Mesh 2 and Mesh 3 in different structures in the axial rotation.

FE Model Validation

To validate the FE model, we compared the predicted ROMs of our model with the data of previous specimen experiment under different loading conditions by linear regression analysis. The regression equation and correlation coefficient were obtained as follows: y = 1.014x − 0.089, R 2 = 0.766. The y‐axis represents the ROMs of the FE model and the x‐axis represents the ROMs of previous specimen experiment under different loading conditions. The R2 represents the correlation coefficient of the regression equation, which indicated that the results of the FE model had a correlation with the previous experimental results.

Meanwhile, the comparisons between in vitro data and predicted values in the FE models are shown in Fig. 3. All the predicted data in this study occurred within the standard deviation of the mean values of the previous literature , meaning the data was in a good agreement with published experimental results.

Fig. 3

The predicted ranges of motion (ROM) of the preoperative model are validated by previous published study.

Hence, the FE model can be regarded as validated and could be used in the present study.

Adjacent IDPs

The maximum von Mises stresses in upper adjacent segment (C4/C5) are shown in Figs 4A and 5 and Table 4. After reconstruction, the adjacent IDPs under the different loading conditions all increased. The maximum adjacent IDPs were 0.387, 0.489, 0.491, and 0.472 MPa under flexion, extension, axial rotation, and lateral bending, respectively, observed at the reconstructive height of 200%. The minimum IDPs were observed after a reconstruction at 150% of the benchmark under all loading conditions except extension, and were 57, 86 and 81% of the maximum IDPs under flexion, axial rotation, and lateral bending, respectively. The minimum IDP under extension occurred when the reconstructive height is 125% of the benchmark height.

Fig. 4

The intradiscal pressures distribution diagram of adjacent intervertebral discs. (A) C4/C5; (B) C6/C7.

Fig. 5

The intradiscal pressure (IDP) of C4/C5 levels under different loading conditions.

TABLE 4

The maximum von Mises stress (MPa) in C4/C5 segment under different loading conditions

	Baseline	100%	125%	150%	175%	200%
Flexion	0.208	0.242	0.236	0.221	0.307	0.387
Extension	0.313	0.383	0.327	0.340	0.467	0.489
Rotate	0.253	0.463	0.435	0.423	0.459	0.491
Lateral bending	0.272	0.469	0.460	0.384	0.455	0.472

The maximum von Mises stresses in lower adjacent segment (C /C7) are shown in Figs 4B and 6 and Table 5. The IDPs of postoperative models under all loading conditions also increased compared to the preoperative model. The maximum IDPs after reconstruction under flexion, extension, axial rotation, and lateral bending were 0.402, 0.411, 0.461, and 0.497 MPa respectively, when the height of the reconstruction was 200% of the benchmark. The minimum IDPs were observed after a reconstruction at 150% of the benchmark, and were 59%, 85%, 82%, and 81% of the maximum IDPs under flexion, extension, axial rotation, and lateral bending loading conditions.

Fig. 6

The intradiscal pressure (IDP) of C6/C7 levels under different loading conditions.

TABLE 5

The maximum von Mises stress (MPa) in C6/C7 segment under different loading conditions

	Baseline	100%	125%	150%	175%	200%
Flexion	0.215	0.289	0.283	0.239	0.275	0.402
Extension	0.263	0.393	0.372	0.351	0.369	0.411
Rotate	0.352	0.427	0.403	0.379	0.386	0.461
Lateral bending	0.363	0.483	0.475	0.401	0.479	0.497

Segmental Motion of Adjacent Levels

The results of the ROM of upper adjacent segment (C4/C5) are shown in Fig. 7 and Table 6. After the reconstruction, the ROMs all increased under the four loading conditions. The lowest ROM was found at the 150% reconstructive height under different loading conditions except extension among the five postoperative models.

Fig. 7

The range of motion (ROM) of C4/C5 levels under different loading conditions.

TABLE 6

The ROM (°) in C4/C5 segment under different loading conditions

	Baseline	100%	125	150%	175%	200%
Flexion	4.8	7.3	6.8	6.0	7.0	7.9
Extension	4.1	8.3	7.3	7.9	8.0	8.5
Rotate	6.6	8.1	7.9	7.5	8.1	8.5
Lateral bending	7.8	10.3	9.7	9.0	10.2	11.0

ROM, range of motion.

The results of the ROM of the lower adjacent segment (C6/C7) were shown in Fig. 8 and Table 7. Similarly, the ROM of C6/C7 was lowest at the reconstructive height of 150% under all loading conditions among the five postoperative models.

Fig. 8

The range of motion (ROM) of C6/C7 levels under different loading conditions.

TABLE 7

The ROM (°) in C6/C7 segment under different loading conditions

	Baseline	100%	125%	150%	175%	200%
Flexion	3.7	6	5.6	5.1	6.1	6.5
Extension	4.8	8.3	7.7	7.3	7.9	9.1
Rotate	3	3.8	3.6	3.4	3.7	4
Lateral bending	4.8	5.7	6.3	5.5	6.1	7

ROM, range of motion.

For the fusion segment (C5/6), the ROM was significantly reduced and approached 0° in all loading conditions.

Discussion

Controversy in Reconstructive Height

ACDF is currently the gold standard surgical treatment for affected patients. However, many studies , have reported that ACDF can accelerate the degeneration of adjacent segments, but the specific mechanism is still not very clear.

The reconstructive height is considered to be an important factor affecting the development of ASD. Lack of distraction may cause insufficient decompression or cervical kyphosis. However, excessive distraction may lead to increased mechanical stress on adjacent segments, eventually resulting in ASD. Hence, the most suitable reconstructive height is still controversial.

Smith and Robinson indicated that the 10–15 mm reconstructive height is suitable. White and Panjabi proposed that the intervertebral space should be 4–5 mm. Olsewski et al. concluded that the stress significantly decreased when the reconstructive height was in excess of 3 mm. Kawakami et al. concluded that 2–5 mm reconstruction is an appropriate reconstructive height, with a lower rate of axial neck pain. Li et al. reported that excessive disc space distraction is a risk factor for the development of radiographic ASD.

IDP after Reconstruction

In our study, compared with preoperative results, the IDPs after the fusion all increased. Eck et al. found the same result in a cadaveric experiment. Furthermore, lowest IDPs of adjacent segments were found at 150% reconstructive height compared with other models of height except C4/C5–extension‐condition after ACDF. It has been reported that excessive loading can induce degeneration of intervertebral discs . Hence, achieving the lower IDP is beneficial to delay the degeneration of the intervertebral disc.

ROM after Reconstruction

In our study, we also found the ROM of the cervical vertebrae for C4/C5 and C6/C7 both increased after reconstruction. Meanwhile, we found that when the reconstructive height is 150% of benchmark height, the ROM of adjacent segments under all loading conditions except the C4/C5–extension‐condition were lowest in the five postoperative models. Elsawaf et al. have reported that the increase in the ROM of the adjacent levels accelerates the degeneration of adjacent segments. Eck et al. proposed that adjacent ROM increases at both superior and inferior adjacent segments following C5/C6 ACDF during flexion and extension in cadaveric cervical spines and speculated that this increase is related to the incidence of ASD. White and Panjabi proposed that as the motion of adjacent vertebral body increases, the risk of developing ASD increases as well. Therefore, 150% intervertebral reconstructive height has the least influence on the ROM of adjacent cervical vertebrae that is beneficial to delaying the degeneration of adjacent segments.

Considering that the increase in IDP and ROM of cervical vertebrae can cause the degeneration of the adjacent segments, the intervertebral reconstructive height of 150% is most suitable compared to other heights in C5‐C6 ACDF, this may serve as a protective factor against ASD.

Limitations

The present study has a number of limitations: (i) the restoration of cervical lordosis by the reconstruction is affected by muscles, surgical technique and many other factors, which should be explored in a further study; (ii) muscles and other soft tissue were not constructed in the models, however, these structures are extremely important for spine biomechanics research; (iii) The screws were designed as solid cylinders bound to the cage or plate, and the threads on the screws were not modeled; (iv) the model was based on only one person, which may limit the present study's applicability to a wider population; and (v) some simplifications were carried out in the prosthesis geometry. For example, we simplify the cancellous bone as a solid structure which may affect the distribution and geometric deformation of the load. Although completely duplicating the result of in vivo studies in FE analysis was impossible, this study effectively shows the biomechanical differences among different intervertebral reconstructive height models.

Conclusion

The reconstructive height is an important factor affecting the IDP and the cervical vertebrae ROM of adjacent segments after ACDF. To delay the degeneration of adjacent segments, an intervertebral reconstructive height of 150% is an appropriate height in C5/C6 ACDF.