Biomechanical effects of different approaches and titanium mesh in combined anterior cervical corpectomy decompression and fusion：a finite element study

doi:10.21203/rs.3.rs-4127773/v1

Background: Anterior Cervical Corpectomy and Fusion(ACCF), which is one of the common surgeries used to treat cervical spine diseases, has been widely applied in clinical practice. The commonly used internal fixation forms in ACCF surgery include the traditional anterior vertebral body screw-plate (AVBSP) structure and the anterior cervical pedicle screw-plate (APSP) structure, both of which are combined with titanium mesh to achieve support and bone fusion.

Objetives: The purpose was to investigate the effects of different surgical plans on cervical spine biomechanics and the interplay between internal fixation instruments after surgery.

Methods: In this study, a finite element model of the human lower cervical spine (C3-C7) after ACCF surgery was established. The surgical plan consisted of two internal fixation forms (AVBSP and APSP) and two titanium mesh forms (linear and curved), combined in different ways.
Results: The mechanical sensitivity of adjacent intervertebral disc nuclei to different surgical plans was significantly different. The stress concentration areas on the vertebral body entry surface varied with different entry methods, and the stress values were greatly affected by cervical movements. The related instrument studies showed that the choice of anterior fixation method would affect the stress level and distribution of the titanium mesh. Theoretically, the combination of curved titanium mesh and AVBSP is beneficial to reducing the overall stress level of the internal fixation instruments and titanium mesh.
Conclusion:The research provides theoretical basis for the selection of clinical surgical plans. It is advantageous in enhancing postoperative stability of cervical vertebrae while reducing the risk of recurrence or other complications such as adjacent segment disease. Clinically, when selecting the excision fusion surgical plan based on the condition of the patient's cervical lesion, consideration should also be given to the matching characteristics between internal fixation methods and titanium mesh forms, as well as their effects on the biomechanics of adjacent segments.

ACCF

Finite element

Anterior fixation

Titanium mesh

Biomechanics

Anterior Cervical Corpectomy Decompression and Fusion (ACCF) is commonly used in the treatment of diseases such as cervical spine fractures, cervical spine tumors, cervical disc herniation, and cervical spinal stenosis. With the continuous improvement of surgical techniques and instruments, the success rate and safety of ACCF surgery have been significantly improved. Now, this surgery has become one of the common surgeries for treating cervical spine diseases and is widely used in clinical practice. The commonly used internal fixation screw form in ACCF surgery is the anterior vertebral body screw-plate (AVBSP) structure[1–6].

Zhang ZX team evaluated the clinical efficacy of using a titanium mesh combined with anterior vertebral body screw-plate (AVBSP) for anterior cervical debridement fusion in the treatment of cervical spine diseases. All patients showed solid bone fusion postoperatively, with no recurrence of infection. The single-stage surgical treatment of cervical spondylitis was satisfactory. The implantation of titanium mesh and instrumentation did not increase the risk of recurrent or persistent infection[7].

In 2008, Koller et al. first reported an alternative anterior cervical fixation system for the lower cervical spine, known as the Anterior Pedicle Screw-Plate (APSP). They believed that the main advantage of fixing the anterior pedicle screw-plate (APSP) is that the screw bearing and locking area are longer, which may provide a certain advantage in terms of fixation stability[8]. Zhang L et al. elaborated on the clinical application of anterior pedicle screw fixation in unstable cervical spine diseases. They believe that the anterior pedicle screw fixation is a new technology applied to cervical spine fixation in recent years. Due to its strong stability, perfect mechanical performance, and satisfactory therapeutic effects for patients. Although this technology has been applied clinically, further clarification is needed on its long-term clinical outcomes and related theoretical studies[9]. Scholars such as Zhang Z also believe that the APSP method is a safe and effective treatment for cervical facet dislocation. However, it presents certain technical challenges[10]. H. H. Wu et al. measured the range of motion (ROM) postoperatively through experiments to assess the stability of cervical three-column injuries treated with anterior pedicle screw-plate (APSP) and anterior vertebral body screw-plate (AVBSP) fixation. The results showed that the ROM in all directions was significantly greater in the AVBSP group compared to the APSP group. The conclusion was that APSP fixation can provide sufficient stability for lower cervical spine injuries. Furthermore, their research model demonstrated that the primary stability of APSP fixation is superior to AVBSP fixation[11, 12].

In clinical practice, after partial vertebrectomy, titanium mesh is commonly used to support and fuse the vertebral body[13–19]. Research by the Lu team suggests that titanium mesh cages (TMC) can provide sufficient biomechanical stability and have high clinical efficacy. However, due to the structural design characteristics of TMC, the postoperative subsidence rate remains high, leading to various related complications. They believe that further optimization of the TMC structure is still needed. Research by Y. Tang et al. indicates that using excessively long TMC for single-level anterior cervical corpectomy and fusion (ACCF), excessive distraction, and overcorrection of cervical curvature may all lead to postoperative subsidence of TMC.

With the continuous optimization of fusion support devices and the iterative updates in technology, titanium mesh has evolved from its initial form of cut surface directly contacting the endplate of the vertebral body to a combination structure. And the new structure can be chosen according to the specific situation with two-sided end caps.

The research team led by K. R. Zhang completed a biomechanical evaluation of a novel anatomical titanium mesh cage (NTMC). The NTMC model consists of a titanium mesh and two spacers located on both sides, which together match the anatomical structure between the endplates. By comparing measurements, it was found that the use of NTMC can effectively reduce the risk of postoperative subsidence of the titanium mesh, postoperative hardware-related complications, and adjacent segment degeneration (ASD)[20]. H. Ji and colleagues compared the fusion outcomes and subsidence rates after using titanium mesh cages with no end caps, traditional end caps, and novel end caps as fusion devices in single-level anterior cervical corpectomy and fusion (ACCF) surgeries. The results indicate a correlation between postoperative subsidence and end caps: the larger the end cap area, the lower the rate of significant postoperative subsidence. Additionally, a design with end caps extending inwards is more conducive to the procedure[21, 22]. Y. Wang and colleagues studied the impact of dome-shaped titanium mesh cages on vertebral endplates under cyclic loading. They believe that under the same cyclic load, compared to traditional TMCs, dome-shaped TMCs exhibit smaller subsidence displacement and a more gradual subsidence trend. From a biomechanical perspective, dome-shaped TMCs, due to their unique structural design that closely matches the vertebral endplates, possess a stronger resistance to subsidence[13].

Jung-Woo Hur and colleagues conducted a comparative study on the effectiveness of end caps in titanium mesh cages. They believe that for patients undergoing single-level anterior cervical corpectomy and fusion (ACCF), using a TMC with end caps yields better clinical outcomes and similar fusion rates compared to TMC without end caps. The end cap reduces the severity of postoperative subsidence and related neck pain. Additionally, the preserved sagittal alignment indicates that it may contribute to cervical lordosis[23]. Z. J. Zhang and colleagues also believe that for patients with cervical spondylotic myelopathy combined with osteoporosis, maintaining intervertebral height and segmental lordosis angle postoperatively, a titanium mesh cage with end caps is superior to one without end caps. The use of an end cap titanium mesh cage can effectively reduce the postoperative subsidence rate[24, 25].

As research progresses, many scholars have proposed various new or improved forms of titanium meshes, including but not limited to straight titanium mesh, as well as a pre-bent titanium mesh based on the physiological curvature of the cervical vertebrae. Luo C developed a new type of curved belt endplate ring titanium mesh structure and conducted partial simulation analysis[26]. Yan TF studied the effects of the titanium mesh placement position and specifications on the outcomes of anterior cervical decompression and fusion surgery[27]. Cao GL analyzed the factors influencing titanium mesh subsidence after ACDF titanium mesh implantation[28]. The team led by Miao DC analyzed the clinical characteristics and efficacy of treating cervical spine fractures and dislocations with anterior cervical decompression and fusion (ACCF). The conclusion is that using the ACCF method for treating cervical spine fractures and dislocations can achieve complete decompression of the cervical spine, maintain the natural curvature and alignment of the cervical spine, provide immediate and long-term support for the anterior column of the cervical spine, and facilitate neurological functional recovery[29]. The team led by Kwon Ji-Won used finite element analysis to investigate the stress effects of different fixation methods on the same interbody spacer, vertebral endplates, and implants. They evaluated the risk of loosening of the relevant components[30, 31].

Based on the above viewpoints or related research foundations, this study intends to adopt two anterior cervical fixation methods (AVBSP and APSP) with different combinations of titanium mesh (straight and curved) with endplates for the anterior cervical decompression and fusion (ACCF) surgery. Using finite element analysis, the study will comprehensively analyze the biomechanical effects of different surgical approaches on the cervical spine. Additionally, the study will explore the mutual effects of different anterior fixation methods and various forms of titanium mesh on the forces involved.

Three-dimensional model establishment after Anterior Cervical Corpectomy Decompression and Fusion surgery

The study was based on a cervical anterior fixation kit from a medical device company. The specific parameters of each component used in this study are listed in Table 1. The study included two sets of titanium screws. Titanium screw 1 used in AVBSP was inserted horizontally. Titanium screws 2 and 3 used in APSP were inserted at an inclined angle. Titanium screw 2 was inserted from the vertebral body surface with a vertical angle of 78° and a horizontal angle of 50° towards the contralateral pedicle for superior tilt-in insertion with a depth of 35mm; Titanium screw 3 was inserted from the vertebral body surface with a vertical angle of 65° and a horizontal angle of 30° towards the contralateral pedicle for inferior tilt-in insertion with a depth of 14mm. The study included 2 sets of titanium mesh components. The straight titanium mesh component consisted of a straight titanium mesh and upper and lower end caps 1. The upper end cap was inclined at -7.5° (positive in the direction of cervical curvature, negative in the opposite direction), i.e., lower in the front and higher in the back; The lower end cap was inclined at + 7.5°. The upper and lower sections of the titanium mesh were parallel. The curved titanium mesh component consisted of a curved titanium mesh and upper and lower end caps 2. The upper end cap was inclined at -7.5°; The lower end cap was inclined at 0°. The curved titanium mesh was inclined at + 7.5°.

Table 1

Parameters of the anterior cervical partial corpectomy internal fixation system
No.	Component Name	Parameter 1	Parameter 2	Parameter 3
1	Titanium screw1(AVBSP)	Diameter 4mm	Length 14mm	Horizontal nailing
2	Titanium screw 2(APSP)	Diameter 3mm	Length 35mm	Lateral offset 12° upward inclination 50°
3	Titanium screw 3(APSP)	Diameter 3mm	Length 14mm	Lateral offset 25° downward inclination 30°
4	Titanium plate	Thickness 2.5mm	Length 50mm	6 holes/diameter 5mm
5	Straight titanium mesh	Diameter 12.5mm	Height 21mm	Thickness 2mm
6	Upper end cap 1	Diameter 12.5mm	Angle-7.5°	Height 2mm
7	Lower end cap 1	Diameter 12.5mm	Angle + 7.5°	Height 2mm
8	Curved titanium mesh	Diameter 12.5mm	Middle Height 21mm	Curvature + 7.5°
9	Upper end cap 2	Diameter 12.5mm	Angle − 7.5°	Height 2mm
10	Lower end cap 2	Diameter 12.5mm	Angle 0	Height 2mm

Using three-dimensional modeling software to create an equivalent simplified 3D model. The titanium plate model retains the anatomically curved design features of the original instrument. This allows for a more realistic simulation of the interaction between the postoperative internal fixation system and the anterior cervical spine. The screws retain their threaded features, enabling a more realistic simulation of the biomechanical impact of the fixed screws on the vertebral body. The titanium mesh retains its original bone growth mesh pore design, enabling a more accurate simulation of the transmission of cervical spine biomechanics and the stress distribution of the titanium mesh itself.

This study plans to use four combination methods of anterior fixation and titanium mesh. They are vertebral screw-plate (AVBSP) + Straight titanium mesh (S) and vertebral screw-plate (AVBSP) + Curved titanium mesh (C). These will be referred to as AVBSP-S and AVBSP-C respectively. Additionally, anterior pedicle screw-plate (APSP) + Straight titanium mesh (S) and anterior pedicle screw-plate (APSP) + Curved titanium mesh (C) will be referred to as APSP-S and APSP-C respectively.

Based on CT scan data from a normal individual, the cervical spine surface envelope model of the C3-C7 segments (there are seven cervical vertebrae from top to bottom, designated as C1-C7) will be extracted. The surface details of the cervical spine model will be optimized and closed using three-dimensional modeling software to create a three-dimensional solid model. The intervertebral disc of the cervical spine will be divided into four parts: nucleus pulposus, annulus fibrosus, and upper and lower endplates. This allows for a more refined material assignment and a more realistic biomechanical impact assessment. Additionally, this study will reconstruct an equivalent model of articular cartilage between the bilateral facet joints of the vertebral body, which improves the accuracy and effectiveness of the cervical spine finite element model. According to the four planned anterior fixation and interbody fusion combination methods, surgical simulation will be performed on the established cervical spine model, and the three-dimensional models created have been shown as in Fig. 1.

To facilitate the study of the biomechanical effects on the vertebral body after screw placement, the following definitions were specifically made. Figure 2a shows a schematic diagram of the division of the lower endplate region of the C4 vertebral body. This region directly contacts the upper end cap of the titanium mesh. The region is divided into four equally spaced paths in a counterclockwise direction. The paths are numbered A to D, representing the anterior, right, posterior, and left directions of the cervical spine. Similarly, Fig. 2b shows a schematic diagram of the division of the upper endplate region of the C6 vertebral body. This region directly contacts the lower end cap of the titanium mesh. It is divided into four segments a to d corresponding to the anterior, right, posterior, and left directions of the cervical spine in a clockwise direction. Figure 2c shows a schematic diagram of the screw insertion areas of the C4/C6 vertebral bodies. Path E to G divides the C4 region from left to right through two screw insertion holes. Path e to g divides the C6 region into three segments.

Establishment of finite element models for ACCF surgery

First, the established postoperative 3D solid model of C3-C7 is imported into the finite element mesh software. By using the 2D/3D mesh function, the model is subjected to finite element solid cutting and mesh partitioning. In this study, the model generated a total of 121,544 linear triangular elements of type S3 (triangular surface mesh) and 1,319,227 linear tetrahedral elements of type C3D4 (four-node linear tetrahedral mesh). Subsequently, each cervical vertebra, internal fixation mesh model, and assembly relations are imported into the finite element analysis software. Relevant model parameters are defined for each part separately. The cervical vertebrae and internal fixation materials and properties are assigned values based on the relevant parameters in Table 2.

Table 2

Material properties used in the cervical spine model.
Material type	Elastic modulus(MPa)	Poisson's ratio	Mesh type
Cortical bone	12000	0.29	S3
Cancellous bone	100	0.29	C3D4
End plate	500	0.4	C3D4
Annulus fibrosus	3.4	0.4	C3D4
Nucleus pulposus	1	0.49	C3D4
Cartilage	10.4	0.4	C3D4
Titanium plate	114000	0.35	C3D4
Titanium screw	114000	0.35	C3D4
Titanium mesh	114000	0.35	C3D4

The interaction relationships between intervertebral disc, adjacent segment vertebrae, facet joints and cartilage of vertebral bodies on both sides, internal fixation screws and vertebrae, as well as all internal fixation instruments included in the research model are simulated by setting action constraints. In order to more realistically reproduce the biomechanical effects and range of motion of the cervical spine, the finite element model introduces equivalent modeling of ligaments. The material parameters of the ligaments are obtained by consulting relevant studies and shown in curve in Fig. 3. It includes the anterior longitudinal ligament, posterior longitudinal ligament, ligamenta flava, interspinous ligament, and joint capsule ligament of the C3-C5 segment and C5-C7 segment, corresponding to curves A and B. The load-deformation curves of the ligaments of the lower cervical spine after fitting ignore material plasticity and failure zones.

According to the relevant parameters of cervical spine motion in normal adults, a head gravity load of 73.6N is equivalently applied to the surface of the C3 vertebra. At the same time, three motion axes are defined as the rotation center with 1Nm bending moment to simulate flexion-extension, lateral bending, and rotation movements. Fixed constraint boundary conditions are applied to the lower surface of the C7 vertebra to simulate the actual connection situation at the lower end of C7. With this, the establishment of the finite element model of the cervical spine after internal fixation surgery in this study is completed. Figure 4 shows the schematic diagram of the finite element model of the cervical spine after AVBSP-S and AVBSP-C surgery.

Range of Motion (ROM) of the model after ACCF

Finite element model validation

Based on the previous research in this paper, the established finite element model of the normal human C3-C7 cervical spine was solved and analyzed. The range of motion (ROM) values of the C3-C7 cervical spine were calculated based on simulation results and compared with the experimental data results of in vitro biomechanical measurements by Moroney[32], Panjabi[33] as shown in Table 3.

Table 3

Comparison of range of motion (ROM) values between c3-c7 cervical vertebrae. (°)
	Flexion and Extension				Lateral Bending				Rotation
	C3- C4	C4- C5	C5- C6	C6- C7	C3- C4	C4- C5	C5- C6	C6- C7	C3- C4	C4- C5	C5- C6	C6- C7
This study	7.7	8.8	8.7	8.0	7.3	6.0	5.8	4.9	8.6	8.6	7.5	6.5
Moroney	9.1	9.1	9.1	9.1	9.42	9.42	9.42	9.42	9.42	9.42	9.42	9.42
Panjabi	7.7	10.1	9.9	7.1	9	9.3	6.5	5.4	9	9.3	6.5	5.4

According to the relevant studies in the reference materials, the range of deviation in intervertebral motion during cervical spine flexion and extension is approximately ± 3.8 degrees, while during lateral bending and rotation it is within ± 6 degrees. Based on this, the range of motion established in the cervical spine finite element model in this study is within the normal deviation range. The validity of the C3-C7 cervical spine finite element model is confirmed.

Results of Model Displacement after ACCF

The finite element results of postoperative model displacement of AVBSP internal fixation at C4-C6 segments are shown in Fig. 5. Figure 5a shows the displacement cloud map results of flexion and extension movements. It can be seen that the maximum relative displacement of the vertebrae is 18.5mm, occurring at the upper end of C3. Figure 5b and Fig. 5c show the displacement cloud map results of lateral flexion and axial rotation movements, respectively. The maximum relative displacements of the vertebrae are 13.6mm and 10.5mm, respectively.

The finite element results of model displacement after C4-C6 segment anterior-posterior spinal fusion (APSP) are shown in Fig. 6. Figure 6a presents the displacement cloud map results during flexion-extension motion. It can be observed that the maximum relative displacement of the vertebrae is 18.7mm, located at the top of the C3 vertebra. Figure 6b and Fig. 6c show the displacement cloud map results during lateral bending and axial rotation motions, respectively. The maximum relative displacements of the vertebrae are 13.9mm and 10.7mm, respectively. Overall, the vertebrae displacement during flexion-extension motion is relatively larger. During lateral bending motion, the compensatory increase in intervertebral mobility of the C3-C4 segment is influenced by the C4-C6 segment combined internal fixation, while the intervertebral mobility of the C6-C7 segment remains relatively stable under various conditions. For all cervical spine motions, the impact of the two screw fixation methods on relative vertebral displacement remains consistent. Both fixation screws effectively stabilize the vertebrae. The relative displacement of the C4-C6 vertebrae is generally less than 3.5mm.

Biomechanical simulation results of four types of anterior fixation in C4-C6 segment after cervical spine surgery

The postoperative stress simulation results with C4-C6 as the target fixation segment are shown in Fig. 7. Figure 7a ~ d correspond to the equivalent stress cloud maps of cervical spine flexion movement after four anterior approach methods. It can be seen from the figures that there are significant differences in the maximum stress values of the models during cervical spine flexion movement. The stress distribution trends of the cervical spine and internal fixation systems are generally consistent across different approaches.

The stress distribution trends of the nucleus pulposus of the adjacent segments P3 and P6 after simulated postoperative four anterior fixation methods are basically consistent, as shown in Fig. 8. From the Fig. 8, it can be observed that under cervical spine flexion conditions, the equivalent stress of the nucleus pulposus at P3 is mainly concentrated in the front of the cervical spine. While the equivalent stress of the nucleus pulposus at P6 is mainly concentrated in the center of the cervical spine, with uniform stress distribution in other areas. During extension movements, the stress at P3 is mainly concentrated in the central region, showing a uniform decrease from the center towards the periphery. P6 appears on both sides of the cervical spine, with a gradual decrease from the sides towards the center. When the cervical spine moves to the left and right, there is a noticeable concentration of stress in the nucleus pulposus of P3 and P6, on the side corresponding to the direction of cervical spine movement. During axial rotation of the cervical spine, there is a more pronounced localized stress concentration on the side where the nucleus pulposus of P3 and P6 is opposite to the direction of movement.

The simulation results indicate that the distribution of equivalent stress cloud maps of nucleus pulposus at P3 and P6 for the other three internal fixation methods is basically consistent with that of the AVBSP-S method. Similarly, there is a strong correlation in the variation trend of stress concentration regions under different cervical spine movements. However, there are still significant differences in the specific maximum and average stress values among the methods. The stress value comparison of nucleus pulposus at P3 and P6 for the four methods under different working conditions is shown in Table 4.

Table 4

Maximum and average stress values of nucleus pulposus at P3 and P6 under different cervical spine movements. (MPa)
Condition	Type	AVBSP-S		AVBSP-C		APSP-S		APSP-C
		P3	P6	P3	P6	P3	P6	P3	P6
Flexion	MAX	1.518	0.752	1.406	0.704	1.521	0.753	1.423	0.715
Flexion	AVG	0.423	0.195	0.416	0.202	0.422	0.197	0.418	0.201
Extension	MAX	0.724	0.183	0.827	0.195	0.723	0.183	0.768	0.171
Extension	AVG	0.159	0.100	0.220	0.092	0.157	0.100	0.181	0.094
Left Bend	MAX	0.420	0.518	0.379	0.443	0.421	0.519	0.400	0.501
Left Bend	AVG	0.141	0.138	0.096	0.122	0.140	0.140	0.124	0.134
Right Bend	MAX	0.854	0.410	0.747	0.361	0.851	0.411	0.824	0.404
Right Bend	AVG	0.223	0.151	0.207	0.143	0.224	0.151	0.221	0.155
Left Rotation	MAX	0.692	0.498	0.343	0.451	0.697	0.501	0.374	0.480
Left Rotation	AVG	0.202	0.147	0.109	0.138	0.201	0.149	0.139	0.148
Right Rotation	MAX	0.452	0.556	0.422	0.546	0.458	0.555	0.441	0.545
Right Rotation	AVG	0.185	0.220	0.171	0.210	0.185	0.221	0.181	0.217

From the Table 4, it is evident that for both the AVBSP and APSP internal fixation methods, the maximum equivalent stress in the nucleus pulposus at P3 and P6 of adjacent segments occurs during cervical spine flexion. However, there is a difference in stress values between the AVBSP-S and APSP-S methods compared to the AVBSP-C and APSP-C methods, with an increase of approximately 7.42% for P3 nucleus and 5.92% for P6 nucleus. This indicates that for flexion movements, the straight titanium mesh results in higher maximum stress values in the nucleus compared to the curved titanium mesh solutions. Furthermore, this difference is slightly greater in the P3 nucleus compared to the P6 nucleus. Observing the average stress values, it is noted that flexion movements do not necessarily correspond to the highest average stress values. The average stress values for the four methods during flexion movements are relatively close, with the nucleus pulposus at P3 and P6 stabilizing at around 0.42 MPa and 0.2 MPa, respectively.

Under extension movements, the maximum stress value in the P3 nucleus corresponding to the AVBSP-S and APSP-S methods stabilizes at 0.72 MPa. Compared to AVBSP-C, there is a decrease of 12.9%, and compared to APSP-C, there is a decrease of 6.25%. The maximum stress value in the P6 nucleus corresponding to the AVBSP-S and APSP-S methods is approximately 0.18 MPa. Compared to AVBSP-C, there is a decrease of 7.69%, while compared to APSP-C, there is an increase of 5.26%. Similarly, the average stress value in the P3 nucleus corresponding to the AVBSP-S and APSP-S methods stabilizes at around 0.16 MPa, showing a decrease of 27.3% compared to AVBSP-C and a decrease of 11.6% compared to APSP-C. It is evident that during extension movements, the straight titanium mesh method has a significantly smaller impact on the maximum and average stress in the adjacent cervical spinal segments' nucleus pulposus compared to the curved titanium mesh method.

The minimum stress extreme value in the P3 nucleus occurs during left lateral bending of the cervical spine. Under this condition, the maximum stress value corresponding to the AVBSP-S and APSP-S methods is approximately 0.42 MPa, showing an increase of 10.8% compared to AVBSP-C and an increase of 5% compared to APSP-C. The stress average also shows the minimum values among the various cervical spine movements. The average stress value corresponding to the AVBSP-S and APSP-S methods is around 0.14 MPa, while for APSP-C, it is even lower at 0.096 MPa, showing a decrease of approximately 31.4% compared to the former two methods and a decrease of 11.4% compared to APSP-C. Conversely, the minimum stress extreme value in the P6 nucleus occurs during right lateral bending of the cervical spine. Under this condition, the maximum stress value corresponding to the AVBSP-S and APSP-S methods is about 0.41 MPa, showing an increase of 13.6% compared to AVBSP-C and an increase of 2.4% compared to APSP-C. However, the average stress value is slightly higher than in left lateral bending movements. These observations indicate that during left and right lateral bending movements of the cervical spine, the P3 and P6 nucleus both exhibit the minimum stress extreme values for all four surgical methods.

In left rotation, the maximum stress values corresponding to the P3 nucleus pulposus for the AVBSP-S and APSP-S methods are approximately 0.69 MPa, which is about double compared to the AVBSP-C and APSP-C scenarios. The corresponding average stress values also show a similar doubling pattern. However, under this action, the maximum stress values and averages corresponding to the P3 nucleus pulposus for the four methods are quite close, with no significant difference. During right rotation, the maximum stress values for the P3 and P6 nucleus pulposus corresponding to the four surgical methods are very similar. Additionally, the maximum stress value for the P6 nucleus pulposus increases by approximately 25% compared to P3. Similarly, the average stress value for the P6 nucleus pulposus increases by about 18.6% compared to P3. These findings suggest that during axial rotation, using a straight titanium mesh has a significant impact on stress in the P3 nucleus pulposus compared to a curved titanium mesh, potentially leading to a doubling of stress.

Based on the region division of Fig. 2a and Fig. 2b, we analyze and study the biomechanical impact on the vertebrae after the surgical nail placement. Regions A ~ D make direct contact with the upper end cap of the titanium mesh, while regions a ~ d make direct contact with the lower end cap of the titanium mesh. The four segments of the two vertebrae correspond to the anterior, right, posterior, and left sides of the cervical spine simultaneously. Based on the finite element model, data in Table 5 are obtained through selected path calculations. The table lists the stress values of the two target paths of C4 and C6 vertebrae corresponding to four surgical methods under six cervical spine motions. This includes the average stress value (AVG), maximum value (MAX) and Maximum Location (MAX Loc) of the path region, as well as the Average Value of Load Concentration Zone (LCZ AVG). LCZ AVG refers to the average stress value of the segment where the load is most concentrated within the path. This parameter serves as a supplement to explain the maximum value along the path, preventing stress singularities or key information of other load concentration areas from being masked by local edge effects.

Table 5

Stress values along paths of C4 and C6 endplates under various cervical spine motions. (MPa)
Condition	Type	APSP-S		AVBSP-S		APSP-C		AVBSP-C
		C4	C6	C4	C6	C4	C6	C4	C6
Flexion	AVG	12.93	6.20	20.57	5.88	9.17	11.32	13.82	6.92
	MAX	48.20	20.40	91.10	31.10	40.60	38.50	35.30	30.00
	MAX Loc	D	a	D	a	B	d	C	b
	LCZ AVG	15.97	7.41	40.83	9.89	16.49	21.46	23.42	10.88
Extension	AVG	11.72	3.44	16.36	4.60	3.89	10.47	8.02	4.25
	MAX	72.80	8.71	53.90	14.80	15.40	38.50	24.80	19.60
	MAX Loc	C	b	C	d	C	d	C	d
	LCZ AVG	31.29	5.42	31.05	7.40	5.87	15.11	17.07	10.34
Left Bend	AVG	12.14	5.54	15.60	5.98	9.13	6.45	4.90	5.14
	MAX	59.60	21.60	91.10	21.90	40.60	38.50	19.20	21.80
	MAX Loc	C	a	C	d	B	d	D	b
	LCZ AVG	26.14	12.23	40.42	10.89	18.24	16.21	8.07	8.84
Right Bend	AVG	13.44	4.14	8.08	8.99	6.27	7.23	7.30	4.93
	MAX	49.10	18.80	38.80	21.70	16.80	24.00	35.30	22.90
	MAX Loc	D	a	C	a	B	b	C	b
	LCZ AVG	21.00	4.61	14.99	11.30	8.76	15.07	10.88	6.63
Left Rotation	AVG	10.51	3.18	18.25	5.39	2.52	7.69	10.71	5.06
	MAX	48.50	20.10	103.0	14.80	10.70	34.00	27.00	22.90
	MAX Loc	A	a	C	d	C	d	A	b
	LCZ AVG	19.19	5.98	35.53	7.32	3.75	11.08	13.71	9.87
Right Rotation	AVG	13.26	4.32	9.89	3.53	7.11	6.01	10.03	4.01
	MAX	59.60	21.60	32.90	21.90	30.80	27.10	27.00	19.60
	MAX Loc	C	a	B	d	B	b	A	d
	LCZ AVG	12.14	5.22	14.15	8.07	14.97	13.35	15.01	9.33

In Table 5, it is observed that after simulating the implementation of the APSP-S surgical method, the maximum stress on the contact area of the C4 vertebral endplate occurs during the extension motion in the C region. The value is approximately 72.8 MPa, representing an increase of approximately 51.3% compared to the smaller value observed during the flexion motion. In this scenario, the maximum load concentration also appears during the extension motion. The average value is 31.29 MPa, representing an increase of approximately 95.9% compared to the flexion motion. Across all loading conditions, the average stress levels in the contact area are fairly consistent, ranging from 12 MPa to 13 MPa.

After simulating the AVBSP-S post-surgery, the maximum stress on the contact area of the C4 vertebral endplate occurs during the left rotation motion in the C region. And the value is approximately 103 MPa, representing a doubling compared to the smaller value observed during the right rotation motion. In this scenario, the maximum load concentration appears during the left rotation and flexion motions, showing a doubling of stress compared to other motions. After simulating the APSP-C post-surgery, the maximum stress on the contact area of the C4 vertebral endplate occurs during the flexion and left lateral motion in the B region. The value is approximately 40 MPa. In this scenario, the maximum load concentration appears during the flexion motion, significantly exceeding the impact of other motions on the vertebral endplate stress. Overall, the flexion motion is considered to have a greater impact on vertebral stress after APSP-C surgery. After simulating the AVBSP-C post-surgery, the maximum stress on the contact area of the C4 vertebral endplate occurs during the flexion and right lateral motion in the C region. And the value is approximately 35 MPa. In this scenario, the maximum load concentration appears during the flexion motion, significantly exceeding the impact of other motions on the vertebral endplate stress. Similarly, the flexion motion is considered to have a greater impact on vertebral stress after AVBSP-C surgery.

In Table 5, it can be seen that after simulating the implementation of the APSP-S surgical method, the maximum stress in the contact area of the C6 vertebral endplate remains consistent at approximately 20 MPa during various cervical spine movements. These stresses are primarily concentrated in the "a" segment area. The stress value for the extension movement is 8.71 MPa, concentrated in the "b" segment area. Compared to other movements, the stress is reduced by approximately 56.5%. In this case, the load distribution pattern is relatively consistent with an average range of 3 MPa to 5 MPa.

After simulating the AVBSP-S procedure, the maximum stress in the contact area of the C6 vertebral endplate is observed in the "a" segment area during the flexion movement. And the value is about 31.1 MPa, approximately doubling compared to the extension movement which had lower stress values. In this scenario, the maximum load concentration occurs during lateral bending movements. The stress during these movements increases by up to 57.1% compared to other actions.

Following the APSP-C simulation, the maximum stress in the contact area of the C6 vertebral endplate is found in the "d" segment area during flexion, extension, and left bending movements.And the value is around 38.5 MPa. In this case, the maximum load concentration is evident during flexion movements, surpassing the impact of other movements on the stress of the vertebral endplate. It is concluded that flexion movements significantly affect the stress on the C6 vertebral endplate after APSP-C surgery.

After simulating the AVBSP-C procedure, the maximum stress in the contact area of the C6 vertebral endplate is observed in the "b" segment area during the flexion movement, with a value of approximately 30 MPa. In this scenario, the maximum load concentration also occurs during flexion movements. It is determined that flexion movements have a significant impact on the stress on the C6 vertebral endplate after AVBSP-C surgery.

In summary, after simulating the implementation of 4 surgical methods, the stress on the C-section position of the C4 vertebral body endplate contact area is the most affected. Among them, the AVBSP-S method exhibits significant stress concentration, with the highest value reaching 103 MPa. The maximum stress in the C6 vertebral body endplate contact area occurs during cervical flexion. After the APSP-S and AVBSP-S procedures, the a-section position of the C6 vertebral body endplate contact area is the most affected by stress. After the APSP-C procedure, the d-section position of the vertebral body is most affected by stress, reaching 38.5 MPa. After the AVBSP-C procedure, the b-section position of the vertebral body is most affected by stress.

Based on the area division method shown in Fig. 2c, an analysis and study of the biomechanical effects on the vertebral body after nail placement during surgery is conducted. E-G and e-g represent the pathways near the nail insertion points of the C4 and C6 vertebral bodies, respectively. During the procedures of drilling and nail insertion, the cortical bone of the vertebral body in this area is subjected to compression and cutting effects. Additionally, there is a locking connection force acting on the surrounding bone region after the screw insertion. Using a finite element model, data in Table 6 are calculated based on the selected pathways. The table lists the stress values of the two target pathways of the C4 and C6 vertebral bodies corresponding to the four surgical methods under six cervical spine movements.

Table 6

Stress values table of the C4 and C6 nail insertion areas along pathways under various cervical spine movements. (MPa)
Condition	Type	APSP-S		AVBSP-S		APSP-C		AVBSP-C
		C4	C6	C4	C6	C4	C6	C4	C6
Flexion	AVG	3.21	1.23	1.78	0.73	1.26	7.69	3.62	0.68
	MAX	7.92	3.62	3.88	1.77	6.47	19.90	9.01	1.96
	MAX Loc	F	g	E	f	F	f	F	g
	LCZ AVG	5.49	2.53	2.07	1.08	2.30	12.30	5.72	1.40
Extension	AVG	3.24	2.00	1.30	1.00	2.27	1.83	1.15	0.92
	MAX	7.45	5.52	3.43	4.85	7.45	5.42	3.75	1.96
	MAX Loc	F	f	F	f	F	g	E	g
	LCZ AVG	3.57	3.02	2.41	1.63	4.42	3.24	2.11	1.94
Left bend	AVG	2.95	1.34	0.82	0.39	0.42	3.54	1.35	0.71
	MAX	6.86	3.62	4.58	1.47	4.52	13.90	4.19	1.96
	MAX Loc	E	g	F	e	F	f	F	g
	LCZ AVG	4.71	2.51	1.24	0.68	0.87	4.32	1.88	1.94
Right bend	AVG	3.53	0.99	1.74	0.49	2.48	5.19	2.01	0.42
	MAX	9.58	2.21	3.88	1.37	7.00	19.90	7.95	1.14
	MAX Loc	F	f	E	f	E	f	E	e
	LCZ AVG	5.14	1.44	1.86	0.76	4.21	9.42	3.39	0.73
Left rotation	AVG	2.96	0.31	1.34	0.73	3.87	2.93	2.60	0.52
	MAX	6.07	1.55	4.58	2.52	9.70	19.90	8.21	1.96
	MAX Loc	E	a	F	f	F	f	E	g
	LCZ AVG	4.52	0.67	1.96	0.94	4.37	5.46	3.65	1.00
Right rotation	AVG	1.47	0.94	1.15	0.19	1.81	3.58	1.85	0.56
	MAX	7.45	2.31	4.29	1.30	6.11	26.30	8.47	2.25
	MAX Loc	F	e	F	e	E	f	E	g
	LCZ AVG	2.37	1.37	1.64	0.35	3.13	7.85	4.15	0.74

In Table 6, it can be seen that after simulating the implementation of the APSP-S surgical method, the maximum stress in the nail insertion area of the C4 vertebra appears in the F segment area of the right rotation movement. Its value is approximately 9.58 MPa, which is about 57.8% higher than the relatively smaller value of the left rotation movement. In this case, the larger load concentration also occurs in the right rotation movement, with an average value of 5.14 MPa. Under various conditions, the average stress in the nail insertion area remains relatively consistent. After simulating the AVBSP-S procedure, the maximum stress in the nail insertion area of the C4 vertebra remains consistent under various cervical spine movements. And its value is approximately 4 MPa. In this case, the maximum load concentration is also consistent, with an average value of approximately 1 MPa to 2 MPa. After simulating the APSP-C procedure, the maximum stress in the nail insertion area of the C4 vertebra appears in the F segment area of the left rotation movement. The value is approximately 9.7 MPa. In this case, the maximum load concentration also occurs in the left rotation movement, with an average value of 4.37 MPa. This significantly exceeds the impact of other movements on the nail insertion surface of the vertebral body. After simulating the AVBSP-C procedure, the maximum stress in the nail insertion area of the C4 vertebra appears in the F segment area of the flexion movement and the E segment area of the left and right rotation movements. The values are approximately 8 MPa to 9 MPa. In this case, the maximum load concentration occurs in the flexion movement Its value is approximately 5.72 MPa, which is about 56.7% higher than the average stress in the concentrated area of the left and right rotation movements. Based on this, it is judged that the flexion movement should be the condition with the greatest impact on the stress of the vertebral body after the AVBSP-C procedure.

After simulating the implementation of the APSP-S surgical method, the maximum stress in the C6 vertebral body at the screw insertion site appears in the f segment region during extension movements. It is approximately 5.52 MPa, nearly doubled compared to the smaller values during axial rotation movements. In this scenario, a larger load concentration also occurs during extension movements, with an average of 3.02 MPa. Following AVBSP-S simulation postoperatively, the maximum stress in the C6 vertebral body at the screw insertion site occurs in the f segment region during extension movements, with a value of approximately 4.85 MPa. In this case, the largest load concentration situation is also relatively consistent, with an average value of around 1.63 MPa.

After simulating the APSP-C postoperatively, the maximum stress in the C6 vertebral body at the screw insertion site appears in the f segment region during flexion and left-right rotation movements. The value reaches up to 26.3 MPa. In this scenario, the greatest load concentration occurs during flexion movements. Its average value is 12.30 MPa, significantly exceeding the impact of other movements on the stress at the screw insertion site. Based on this, it can be inferred that flexion movements pose a greater stress impact on the vertebral body post APSP-C surgery.

In the simulation post-AVBSP-C surgery, the maximum stress in the C6 vertebral body at the screw insertion site remains relatively consistent across all cervical spine movements And the value is approximately 2 MPa. In this case, the largest load concentration situation is also relatively consistent, with an average value of around 1 MPa.

In conclusion, after simulating the implementation of the four surgical methods, the F segment position of the C4 vertebral body at the screw insertion site experiences the greatest stress impact. The maximum stress values corresponding to the different methods are relatively consistent, around 9 MPa. The F segment position of the C6 vertebral body at the screw insertion site experiences the greatest stress impact. In particular, post-APSP-S and post-AVBSP-S surgeries, extension movements of the cervical spine exert a significant stress impact on the vertebral body at the screw insertion site. Following APSP-C surgery, flexion movements of the cervical spine exert the greatest stress impact on the vertebral body at the screw insertion site.

Stress simulation results of four anterior fixation systems at the C4-C6 segments

The stress contour maps of internal fixation screws obtained from finite element models of four internal fixation schemes are shown in Fig. 9. The stress distribution of two sets of anterior vertebral body screws and two sets of anterior pedicle screws under six cervical spine movements is included. After implementing the AVBSP-S surgery, there are significant differences in the distribution of equivalent stress of the internal fixation screws under various cervical spine movements. During flexion, noticeable stress concentration appears at the fixed screws of the C4 vertebral body, mainly at the connection between the rod and the screw head. The fixed screws of the C6 vertebral body show a gradually decreasing and relatively uniform stress distribution from the root to the tip. And it has slightly higher average stress values than those of the C4 vertebral body screws. During extension movements, the stress distribution is more uniform, without any stress concentration. However, contrary to the flexion movements, the average stress of the C4 vertebral body screws is slightly higher than that of the C6 vertebral body screws. The fixed screws on both sides show a trend of decreasing stress from the sub-root to the tip. The difference lies in the occurrence of stress peaks on the right side for the C4 vertebral body screws during left movements. While the stress peaks appear on the left side for the right movements. The screws of the C6 vertebral body show no significant difference. During axial rotation movements, stress concentration is observed in the screws of the C6 vertebral body. And the peak stress appears on the left side screw for left rotation and on the right side for right rotation. After implementing the AVBSP-C surgery, there are localized changes in the stress distribution under each cervical spine movement. During flexion, compared to the AVBSP-S situation, the C6 vertebral body screws show a significant and expanded area of load concentration. While the C4 vertebral body screws show no significant change. The stress distribution trends of each screw during extension and left-right movements are basically consistent with the AVBSP-S situation. During left-right rotation motions, the stress concentration of the screws in the C4 vertebral body weakens. But there is a noticeable trend of expanded load concentration in the root area of the screws of the C6 vertebral body.

After the implementation of APSP-S, varying degrees of stress concentration were observed at the connection between the pedicle screw heads and rod under different cervical spine movements. During flexion and extension movements, there was a significant increase in load concentration at the root region of the pedicle screw of the C4 vertebral body. Conversely, the stress distribution of the short screw rod was more uniform; during left-sided movements, the stress was concentrated at the root region for the long screw. While during right-sided movements, the stress load was more evenly distributed along the length of the rod. This is a biomechanical distribution characteristic of unilateral pedicle fixation from the anterior route, which is a result of the structural design of entering the pedicle screws from the right to left side. The rotation movements on both sides showed similar patterns, with stress concentration in the direction of unilateral movement and uniform stress distribution in the opposite direction. It can be understood that when the direction of cervical spine movement aligns with the direction of the long screw penetration. Stress concentration occurs in the pedicle screw. When the direction of cervical spine movement is opposite to the direction of long screw penetration. The biomechanical force of the cervical spine acts more uniformly along the length of the long screw. However, overall, the stress peak in the former case is slightly smaller than in the latter. After the implementation of APSP-C, the stress distribution of long and short screws under different cervical spine movements was similar to the APSP-S situation. Of note, when the pedicle screw fixation was combined with a curved titanium mesh, the peak screw stress during flexion movements increased significantly compared to using a straight titanium mesh. This could be attributed to the curvature of the titanium mesh causing specific directional forces to have a cumulative effect under certain conditions. And it leads to load concentration in a specific direction and resulting in stress discontinuity at the screw junction area.

According to the finite element model of four internal fixation schemes, the stress contour plot of the titanium mesh is shown in Fig. 10. It is obvious that, with the circumferential direction of the titanium mesh as a reference, stress intensification or stress release occurs in different circumferential regions corresponding to different cervical spine movements. Local stress concentration appears at the edges of the upper and lower end caps of the titanium mesh. And its position is strongly correlated with the direction of cervical spine movement. Specifically, after implementing the AVBSP procedure, there is no significant difference in stress distribution between the straight titanium mesh and the curved titanium mesh. However, the stress peak of the curved titanium mesh is relatively lower than that of the straight titanium mesh overall. Similarly, after implementing the APSP procedure, there is no significant difference in stress distribution between the two types of titanium mesh. And the stress peak of the curved titanium mesh is lower than that of the straight titanium mesh overall. This situation is particularly evident during flexion and backward extension movements.

Biomechanical Impact Analysis of Four Anterior Fixation Combinations

The AVBSP (S) - APSP (S) series represents the influence of AVBSP and APSP internal fixation on the mean stress of the nucleus when using straight titanium mesh; the AVBSP (C) - APSP (C) series represents the influence of AVBSP and APSP internal fixation on the mean stress of the nucleus when using curved titanium mesh; the AVBSP (S) - AVBSP (C) series represents the influence of straight and curved titanium mesh on the mean stress of the nucleus when using the AVBSP method; the APSP (S) - APSP (C) series represents the influence of straight and curved titanium mesh on the mean stress of the nucleus when using the APSP method.

Figure 11a and Fig. 11b show the extreme difference curves of the average stress of the nucleus after four anterior internal fixation surgeries under different cervical spine movements. In Fig. 11a, AVBSP (S) - APSP (S) represents the result of subtracting the average stress value of the P3 nucleus using the AVBSP-S method from the corresponding average value of the APSP-S method. By subtracting, the changes in internal fixation methods or the effects of different titanium mesh forms on the biomechanical properties of the nucleus can be more clearly analyzed.

In Fig. 11a, it can be seen that the values of AVBSP (S) - APSP (S) tend to approach 0 under various cervical spine movements. This indicates that when using straight titanium mesh, whether using the AVBSP or APSP method, there is almost no impact on the mechanical characteristics of the postoperative P3 nucleus. However, when using curved titanium mesh, the choice between AVBSP and APSP has a more pronounced effect on the stress results of the P3 nucleus. During flexion and axial rotation movements, using the APSP method leads to a significant increase in the average stress of the nucleus compared to the AVBSP method. Its maximum vale increase approximately 0.07 MPa. Conversely, during extension and left-sided movements, using the APSP method leads to a maximum decrease in the average stress of the nucleus of about 0.05 MPa compared to the AVBSP method. The series of values from AVBSP (S) - AVBSP (C) indicate that when using the AVBSP internal fixation method, both straight and curved titanium mesh similarly have a significant impact on the nucleus. During cervical flexion and axial rotation movements, using straight titanium mesh results in the largest increase in the average stress of the P3 nucleus compared to curved titanium mesh. Its maximum vale increase approximately 0.11 MPa. Conversely, during extension and left-sided movements, using straight titanium mesh leads to the highest decrease in average stress compared to curved titanium mesh. The maximum value reduces about 0.1 MPa. The series of values from APSP (S) - APSP (C) indicate that when using the APSP internal fixation method, the effects of straight and curved titanium mesh on the nucleus follow a similar trend to the former. During cervical axial rotation movements, using straight titanium mesh leads to the largest increase in the average stress of the nucleus compared to curved titanium mesh, with a maximum increase of about 0.03 MPa. Conversely, during extension and left-sided movements, using straight titanium mesh results in the greatest reduction in average stress compared to curved titanium mesh, with a maximum decrease of approximately 0.05 MPa.

Similarly, Fig. 11b to some extent represents the biomechanical effects of choosing titanium mesh or internal fixation on the P6 nucleus pulposus. The values of AVBSP (S)-APSP (S) tend to approach 0 under various cervical spine movements. This indicates that when using straight titanium mesh, the two internal fixation methods have little effect on the mechanical characteristics of the postoperative P3 nucleus pulposus; While using curved titanium mesh, selecting the AVBSP method results in an average stress increase of approximately 0.07 MPa in the nucleus pulposus under flexion movements. During right lateral and axial rotation of the cervical spine, selecting the AVBSP method results in a maximum reduction of 0.05 MPa in the average stress compared to the APSP method. From the series of values of AVBSP (S)-AVBSP (C), it can be seen that when using the AVBSP internal fixation method, the impact of the straight and curved titanium mesh during cervical flexion-extension and left bend is relatively small. During right bend and axial rotation movements, the average stress of the nucleus pulposus is increased by approximately 0.08 MPa, with straight titanium mesh compared to curved titanium mesh; When using the APSP internal fixation method, the impact of straight and curved titanium mesh on the nucleus pulposus is consistent with the former trend, but with minor differences.

In conclusion, the choice of anterior fixation method or titanium mesh will have varying degrees of impact on the biomechanical characteristics of the P3 and P6 nucleus pulposus. Specifically, when using a straight titanium mesh, changes in the anterior fixation method have minimal impact on the average stress of the nucleus pulposus; Whereas when using a curved titanium mesh, the mechanical characteristics of the nucleus pulposus are more noticeably affected by changes in the internal fixation method. Similarly, when determining the anterior fixation method, both the straight and curved forms of titanium mesh are sensitive factors to changes in the average stress of the nucleus pulposus. In cases where the use of AVBSP is determined, changes in the form of titanium mesh have a more significant impact on the mechanical characteristics of the nucleus pulposus. On the other hand, in cases where the use of APSP is determined, the sensitivity of the nucleus pulposus stress to the different form of titanium mesh is somewhat weaker.

Based on the calculation results, the following theoretical statements can be made. For cervical flexion and axial rotation movements, the straight titanium mesh scheme corresponds to a higher maximum stress value in the nucleus pulposus. However, during extension movements, the curved titanium mesh scheme corresponds to a higher maximum stress value in the nucleus pulposus. This biomechanical theory can be used to make more suitable treatment choices based on the actual injury conditions of clinical patients. And it aims to achieve a faster postoperative recovery with fewer complications. Additionally, the stress-sensitivity theory of the nucleus pulposus of adjacent segment intervertebral discs to different internal fixation schemes can also provide a theoretical basis for the selection of clinical surgical schemes. And it may achieve a high postoperative stability of cervical fixation with reduced risks of recurrence or adjacent segment pathologies.

Analysis of the mutual influence between anterior fixation methods and titanium mesh

Analysis of the impact of titanium mesh forms on anterior fixation screws

In order to describe their mutual influence more clearly, this section uses the method of taking the difference to analyze the impact of different titanium mesh forms on the internal fixation method more clearly. The specific calculation results are shown in Fig. 12.

In Fig. 12, it can be observed that for the AVBSP anterior approach method, there is no significant change in the maximum stress of the internal fixation screws with the variation of the titanium mesh. The maximum change occurs during flexion and bending movements. The maximum stress difference does not exceed 12MPa; Similarly, the maximum difference in average stress for the screws is only 1.65MPa. The variation in the straightness and curvature of the titanium mesh has little effect on the stress of the AVBSP screws; On the contrary, for the APSP approach method, the maximum stress of the internal fixation screws shows a higher sensitivity to the form of the titanium mesh. Under various cervical movements, choosing a curved titanium mesh results in an increase of approximately 77.1MPa compared to the straight titanium mesh. The average stress of the screws shows a maximum increase of 4MPa.

Analysis of the impact of anterior approach internal fixation methods on titanium mesh

In Fig. 13, it can be seen that when using a straight titanium mesh, the changes in the two anterior approach internal fixation methods have a certain impact on the stress distribution of the titanium mesh. Comparing the AVBSP method to the APSP method, the maximum stress on the straight titanium mesh is reduced by approximately 9MPa. The average stress value increases by a maximum of 0.5MPa; And it is evident from the graph that the difference in maximum and average stress for the straight titanium mesh shows irregular trends. It is initially considered that variations in the selection of anterior approach internal fixation methods may have uncertain effects on the stress of straight titanium mesh.

When using a curved titanium mesh, changes in the two anterior approach internal fixation methods have a more pronounced impact on the stress distribution of the titanium mesh. Comparing the AVBSP method to the APSP method, the maximum stress on the curved titanium mesh is reduced by approximately 15MPa. The average stress value decreases by a maximum of 3MPa; It is initially judged that variations in the selection of anterior approach internal fixation methods will have a significant impact on the curved titanium mesh. The use of AVBSP in conjunction with curved titanium mesh may help reduce the overall stress level of the titanium mesh. This effect is more significant in cases of cervical spine lateral flexion with axial rotation. Conversely, using APSP with curved titanium mesh will result in a certain increase in both maximum and average stress values of the titanium mesh.

Based on the stress calculation results, this study indicates that variations in anterior approach internal fixation methods will affect the stress values and distribution of the titanium mesh. The impact on the straight titanium mesh is irregular. Theoretically, the use of curved titanium mesh in conjunction with AVBSP is beneficial for reducing the overall stress level of the internal fixation devices. Clinically, internal fixation schemes can be selected based on the patient's cervical spine lesion status. According to relevant clinical observations and studies, APSP is more advantageous than AVBSP in reconstructing the stability of the cervical spine system. However, it is also necessary to consider the compatibility of the two internal fixation methods with the titanium mesh and their impact on the biomechanics of adjacent segments. Of note, when using APSP in combination with curved titanium mesh, the stress peak of the screws increases significantly during cervical spine flexion and extension movements. While the stress value of the curved titanium mesh relatively decreases. According to this theoretical result, in clinical practice of selecting anterior approach for cervical spine internal fixation, it is necessary to focus on the patient's postoperative recovery and other complications. More attention should also be paid to the additional effects of patient's flexion and extension movements on postoperative cervical spine stability and biomechanical characteristics of adjacent segments.

This study established finite element models of human cervical spine C3-C7 with anterior approach(AVBSP, APSP), and two types of titanium mesh(straight and curved). Relevant verification and calculation result analysis were completed. The biomechanical conditions of the adjacent segment intervertebral disc nucleus were obtained. And the stress situation of the anterior approach internal fixation devices were acquired.

After anterior approach fixation surgery in the C4-C6 segments, there is relatively large vertebral body displacement during flexion and extension movements. The stress concentration areas on the vertebral body entry surface varied with different entry methods, and the stress values were greatly affected by cervical movements. Additionally, there is a significant difference in the mechanical sensitivity of the adjacent segment intervertebral disc nucleus to different internal fixation schemes combined with different titanium meshes. The aforementioned biomechanical theories can provide theoretical basis for the selection of clinical surgical techniques. Those may enhance postoperative stability of the cervical spine and reducing the risk of complications such as recurrence or adjacent segment pathologies.

In addition, regarding internal fixation devices, this study suggests that variations in the selection of anterior internal fixation methods can influence the stress values and distribution of titanium mesh. The impact on the straight titanium mesh does not follow a clear pattern. Theoretically, the curved titanium mesh in combination with AVBSP maybe beneficial for reducing the overall stress levels on the internal fixation devices and titanium mesh. Clinically, when selecting internal fixation strategies based on the condition of the cervical spine lesion in patients, it is also important to consider the compatibility between the two internal fixation methods and the form of the titanium mesh, as well as their impact on the biomechanics of adjacent segments.

Acknowledgement

None.

Author Contributions

The authors confirm contribution to the paper as follows:study conception and design: D.L. and Y.Y.; data collection: C.D., L.G., B.Z.; analysis and interpretation of results: D.L., C.D., and Y.Y.; draft manuscript preparation: D.L. and Y.Y. All authors reviewed the results and approved the final version of the manuscript.

Funding

This research was funded the National Vocational Education Teacher Teaching Innovation Team Project ,Grant Number ZI2021090303.

Availability of Data and Materials

The data and materials of this study are available from the corresponding author.

Ethics Approval and Consent to Participate

This study was reviewed and approved by the Ethics Committee of Hunan Railway Professional Technology College, and written informed consent was obtained from the participant, in accordance with ethical standards.

Conflicts of Interest

The authors declare that they have no conflicts of interest to report regarding the present study.

Competing interests

The authors declare no confict of interest.

Oliver JD, Goncalves S, Kerezoudis P, Alvi MA, Freedman BA, Nassr A, Bydon M: Comparison of Outcomes for Anterior Cervical Discectomy and Fusion With and Without Anterior Plate Fixation: A Systematic Review and Meta-Analysis. Spine (Phila Pa 1976) 2018, 43(7):E413-e422.
Williams J, D'Amore P, Redlich N, Darlow M, Suwak P, Sarkovich S, Bhandutia AK: Degenerative Cervical Myelopathy: Evaluation and Management. Orthop Clin North Am 2022, 53(4):509-521.
Iyer A, Azad TD, Tharin S: Cervical Spondylotic Myelopathy. Clin Spine Surg 2016, 29(10):408-414.
Tan W, Zhou C, Guo D, Sun J, Cao W, Yang LZ, Wu M: Treatment of Single-Level Cervical Spondylosis: Cervical Disk Arthroplasty Versus Anterior Cervical Decompression and Fusion. Orthopedics 2017, 40(1):e23-e34.
Ma Z, Ma X, Yang H, Guan X, Li X: Anterior cervical discectomy and fusion versus cervical arthroplasty for the management of cervical spondylosis: a meta-analysis. Eur Spine J 2017, 26(4):998-1008.
Iwanami A, Toyama Y: [Cervical spondylosis]. Nihon Rinsho 2014, 72(10):1755-1760.
Zhang ZX, Li T, Hao DJ: Single-stage Treatment of Osteomyelitis of the Cervical Spine Using Anterior Instrumentation and Titanium Mesh Cages. Spine (Phila Pa 1976) 2016, 41(16):E949-e954.
Koller H, Hempfing A, Acosta F, Fox M, Scheiter A, Tauber M, Holz U, Resch H, Hitzl W: Cervical anterior transpedicular screw fixation. Part I: Study on morphological feasibility, indications, and technical prerequisites. Eur Spine J 2008, 17(4):523-538.
Zhang L, Wang H: A novel segmentation method for cervical vertebrae based on PointNet++ and converge segmentation. Comput Methods Programs Biomed 2021, 200:105798.
Zhang Z, Mu Z, Zheng W: Anterior pedicle screw and plate fixation for cervical facet dislocation: case series and technical note. Spine J 2016, 16(1):123-129.
Wu HH, Tang T, Yu X, Pang QJ: Stability of two anterior fixations for three-column injury in the lower cervical spine: biomechanical evaluation of anterior pedicle screw-plate fixation. J Int Med Res 2018, 46(4):1455-1460.
Wu W, Yang Y, Liu H, Hong Y, Gong Q, Li T, Liu L, Song Y: New Dynamic Plate System Combined with Titanium Mesh Cage and Bone Graft in the Treatment of Cervical Spondylosis. Turk Neurosurg 2016, 26(5):750-757.
Wang Y, Lu T, He X, Wen Z, Gao Z, Gao Z, Liang H: Effect of Dome-Shaped Titanium Mesh Cages on Cervical Endplate Under Cyclic Loading: An In Vitro Biomechanics Study. Med Sci Monit 2019, 25:142-149.
Tang Y, Geng X, Li F, Sun Y, Jia L, Zhou S, Chen X: Factors affecting titanium mesh cage subsidence in single-level anterior cervical corpectomy and fusion for ossification of the posterior longitudinal ligament. J Orthop Surg Res 2022, 17(1):515.
Thalgott JS, Xiongsheng C, Giuffre JM: Single stage anterior cervical reconstruction with titanium mesh cages, local bone graft, and anterior plating. Spine J 2003, 3(4):294-300.
Tohamy MH, Osterhoff G, Abdelgawaad AS, Ezzati A, Heyde CE: Anterior cervical corpectomy and fusion with stand-alone cages in patients with multilevel degenerative cervical spine disease is safe. BMC Musculoskelet Disord 2022, 23(1):20.
Pescatori L, Tropeano MP, Visocchi M, Grasso G, Ciappetta P: Cervical Spondylotic Myelopathy: When and Why the Cervical Corpectomy? World Neurosurg 2020, 140:548-555.
Yu H, Li X, Chen S, Zhang L, Yang G, Welle K, Gathen M, Kabir K: Comparative Effectiveness and Safety of Anterior Cervical Corpectomy with Fusion, Laminoplasty, and Laminectomy and Instrumented Fusion for Ossification of the Posterior Longitudinal Ligament: A Systematic Review and Network Meta-Analysis. J Invest Surg 2022, 35(3):667-676.
Madan A, Thakur M, Sud S, Jain V, Singh Thakur RP, Negi V: Subaxial Cervical Spine Injuries: Outcomes after Anterior Corpectomy and Instrumentation. Asian J Neurosurg 2019, 14(3):843-847.
Zhang KR, Yang Y, Ma LT, Qiu Y, Wang BY, Ding C, Meng Y, Rong X, Hong Y, Liu H: Biomechanical Effects of a Novel Anatomic Titanium Mesh Cage for Single-Level Anterior Cervical Corpectomy and Fusion: A Finite Element Analysis. Front Bioeng Biotechnol 2022, 10:881979.
Ji H, Xie X, Zhuang S, Zhang C, Xie L, Wu X: Comparative analysis of three types of titanium mesh cages for anterior cervical single-level corpectomy and fusion in term of postoperative subsidence. Am J Transl Res 2020, 12(10):6569-6577.
Abudouaini H, Wu T, Liu H, Wang B, Chen H: The predictive value of Hounsfield units for titanium mesh cage subsidence after anterior cervical corpectomy and fusion. Front Surg 2022, 9:1012364.
Hur JW, Ryu KS, Ahn S, Kim JS, Chung HJ, Song MS: Comparative Analysis of 2 Different Types of Titanium Mesh Cage for Single-level Anterior Cervical Corpectomy and Fusion in Terms of Postoperative Subsidence and Sagittal Alignment. Clin Spine Surg 2020, 33(1):E8-e13.
Zhang ZJ, Lu YS, Chen H: [A comparative study between on-endcaps and non-endcaps titanium mesh cage for the treatment of elderly cervical spondylotic myelopathy complicated with osteoporosis approach for anterior cervical spine surgery]. Zhongguo Gu Shang 2018, 31(1):5-11.
Liu X, Chen Y, Yang H, Li T, Xu H, Xu B, Chen D: The application of a new type of titanium mesh cage in hybrid anterior decompression and fusion technique for the treatment of continuously three-level cervical spondylotic myelopathy. Eur Spine J 2017, 26(1):122-130.
Luo C, Ou J, Lu Z:Biomechanical Test on Novel Arc Cervical Titanium Mesh with Endplate Ring Journal of Medical Biomechanics, 2022, 37(01):85-90.
Yan TF, Li Y, Wu XY, Chen SL, Jin WM, Song PW, Shen CL, Dong FL: Imaging study of mesh placement on the postoperative effects of hybrid decompression and fixation for 3-level cervical spondylotic myelopathy” Chinese Journal of Spinal cord, 2022,32(05):418-425.
Cao GL, Chen Z, Shi J:Risk factors of titanium mesh subsidence after anterior cervical corpectomy and fusion.Chinese Journal of Spine and Spinal Cord, 2023, 33(07):602-609.
Miao DC, Zhang BY, Lei T, Shen Y: Clinical Efficacy of Anterior Partial Corpectomy and Titanium Mesh Fusion and Internal Fixation for Treatment of Old Fracture Dislocation of the Lower Cervical Spine. Med Sci Monit 2017, 23:5675-5682.
Kwon JW, Bang SH, Park TH, Lee SJ, Lee HM, Lee SB, Lee BH, Moon SH: Biomechanical comparison of cervical discectomy/fusion model using allograft spacers between anterior and posterior fixation methods (lateral mass and pedicle screw). Clin Biomech (Bristol, Avon) 2020, 73:226-233.
Koller H, Acosta F, Tauber M, Fox M, Martin H, Forstner R, Augat P, Penzkofer R, Pirich C, Kässmann H et al: Cervical anterior transpedicular screw fixation (ATPS)--Part II. Accuracy of manual insertion and pull-out strength of ATPS. Eur Spine J 2008, 17(4):539-555.
Moroney SP, Schultz AB, Miller JA, Andersson GB: Load-displacement properties of lower cervical spine motion segments. J Biomech 1988, 21(9):769-779.
Panjabi MM, Brand RA, Jr., White AA, 3rd: Mechanical properties of the human thoracic spine as shown by three-dimensional load-displacement curves. J Bone Joint Surg Am 1976, 58(5):642-652.

No competing interests reported.

Biomechanical effects of different approaches and titanium mesh in combined anterior cervical corpectomy decompression and fusion：a finite element study

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Three-dimensional model establishment after Anterior Cervical Corpectomy Decompression and Fusion surgery

Establishment of finite element models for ACCF surgery

Results

Range of Motion (ROM) of the model after ACCF

Finite element model validation

Results of Model Displacement after ACCF

Stress simulation results of four anterior fixation systems at the C4-C6 segments

Discussion

Biomechanical Impact Analysis of Four Anterior Fixation Combinations

Analysis of the mutual influence between anterior fixation methods and titanium mesh

Analysis of the impact of titanium mesh forms on anterior fixation screws

Analysis of the impact of anterior approach internal fixation methods on titanium mesh

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1