Biomechanical Analysis of a Novel 3D Printing Cervical Spine Prosthesis : A Finite Element Study

Background: Our group have developed a new 3D printing cervical composite joint system prosthesis. The corresponding biomechanical effect should be evaluated in the treatment of cervical spondylosis. The purpose of this study was to evaluate the biomechanical properties of the prosthesis using a three-dimensional nite element model. Methods: CT data were extracted by mimics16.0 software to reconstruct the C3-C7 model of cervical spine. The model was divided into three groups: normal cervical spine group, prosthesis implantation group and ACCF group. The three groups of models in six different motion states (exion, extension, left and right lateral bending, left and right rotation) were simulated by three-dimensional nite element simulation. The RoM of lower cervical vertebra, RoM between each vertebrae, and the stress of intervertebral disc were compared and analyzed. Results: Compared with the normal cervical spine, the RoM of C3-7 in the prosthesis implantation group showed a downward trend in six different motion states, with a decrease range of 5.27%. RoM of each segment in the prosthesis implantation group was decreased, and the decrease range was less than 7.34%. In the six motion states, the stress of each intervertebral disc in the prosthesis implantation group and ACCF group was increased. Compared with the prosthesis implantation group, the stress increased signicantly at at the C3-C4 level in ACCF group. Conclusion: The novel 3D printing cervical composite joint system prosthesis can not only retain the range of motion of the cervical spine, but also decrease the stress of the upper adjacent intervertebral disc.

segment range of motion of the cervical spine. Through the detailed three-dimensional FE simulation of its biomechanical characteristics, it provides a certain theoretical basis and mechanical guidance for the further experimental research and clinical application of 3D printing cervical composite joint system prosthesis.

Validation of normal lower cervical spine model
The C3-C7 three-dimensional FE model of normal cervical spine was successfully established (Fig. 3). The mesh generation of C3-C7 model of lower cervical spine was 46086 nodes and 202973 tetrahedral elements. The solid element mesh was used for each vertebral body, intervertebral joint and intervertebral disc of the lower cervical spine. 1D two node nonlinear element was used for each ligament, which was de ned as spring element attribute. Each structure keeps the grid nodes coordinate and contact with each other completely.
The network nodes of each structure coordinate and contact with each other and the appearance is lifelike. The structure is complete and the accuracy is high. The ROM of each segment of normal lower cervical spine FE model were calculated in this study compared with those of Zhang et al and Panjabi [20,24]. Compared with the published biomechanical experimental data in vitro and the nite data, it was found that the relative range of motion and the change trend of the lower cervical vertebrae in this study were basically consistent with those reported in the literature (Fig. 3). It is believed that the cervical spine model is correct and effective, which can be further used in subsequent experimental studies.

Comparative analysis of ROM data of C3-C7
The ROM data of C3-C7 in each group in the 6 loading directions were obtained by FE simulation calculation. The results were summarized into Table 2, with normal lower cervical spine data as control. Compared with the control group, the ROM loss of ACCF group was as high as 30%, especially in exion-extension and rotation. However, there was little effect on the ROM change in the prosthesis group, with a decrease range of 5.27%, which was close to the overall ROM value of normal cervical spine.

Comparative analysis of ROM in operative segments
The percentage of ROM relative to normal condition was fed back by histogram of each group, as shown in Fig. 4.
Compared with the control group, ROM of C4-C5 and C5-C6 in the prosthesis implantation group decreased in the 6 loading directions. The exion-extension decreased by 4.95%, lateral bending decreased by 1.64%, rotation decreased by 2.02% at the C4-C5 level. The exion-extension decreased by 3.33%, lateral bending decreased by 2.32%, and rotation decreased by 2.14% at the C5-C6 level. The ROM loss was obvious in the ACCF group compared with the control group. The exion-extension decreased by 94.40%, lateral bending by 91.79% and rotation by 91.70% at C4-C5 level. While at the C5-C6 level, the exion-extension decreased by 94.74%, lateral bending by 89.78% and rotation by 91.34% (Table 2 and Fig 4).

Comparative analysis of ROM in adjacent segments
Compared with the control group, the ROM of C3-C4 and C6-C7 levels in the prosthesis implantation group decreased. The exion-extension decreased by 2.13%, lateral bending decreased by 1.36%, rotation decreased by 3.29% at the C3-C4 level. At the C6-C7 level, the exion-extension decreased by 1.14%, lateral bending decreased by 2.32%, and rotation decreased by 3.43%. The ROM of C3-C4 and C6-C7 in ACCF group were increased signi cantly compared with the control group. The exion-extension increased by 36.33%, lateral bending increased by 50.54%, rotation increased by 34.88% at the C3-C4 level. The exion-extension increased by 26.90%, lateral bending increased by 42.56%, and rotation increased by 33.73% at the C6-C7 level ( Table 2 and Fig 4).
Comparative analysis of the stress of intervertebral disc in surgical segment Compared with the control group, the stress changes of C4-C5 and C5-C6 levels in prosthesis implantation group were as follows. The stress of C4-C5 increased in different degrees in the 6 loading directions, the stress increased 147.01% during exion-extension, increased 338.15% during lateral bending, increased 160.08% during rotation. While the change of C5-C6 intervertebral stress was less obvious. The stress increased 44.96% during exion-extension, increased 41.64% during lateral bending, increased 4.81% during rotation (Table 3 and Fig. 5).
In the ACCF group, the stress of C4-C5 increased with different degrees in the 6 loading directions, the stress increased 75.53% during exion-extension, increased 25.19% during lateral bending, increased 37.45% during rotation. While the change of C5-C6 intervertebral stress was less obvious. The stress increased 6.16% during exion-extension, increased 35.59% during lateral bending, increased 4.05% during rotation (Table 3 and Fig. 5).

Comparative analysis of the stress of intervertebral disc in adjacent segment
Compared with the control group, the stress increased 57.37% during exion-extension, increased 156.36% during lateral bending, increased 9.39% during rotation at the C3-C4 level in prosthesis implantation group. The stress increased 63.11% during exion-extension, increased 38.69% during lateral bending, increased 65.60% during rotation at the C6-C7 level in prosthesis implantation group. Compared with the prosthesis implantation group, the stress increased signi cantly at at the C3-C4 level in ACCF group. The stress increased 89.30% during exion-extension, increased 243.90% during lateral bending, increased 39.60% during rotation at the C3-C4 level. The stress increased 26.51% during exion-extension, increased 26.13% during lateral bending, increased 70.64% during rotation at the C6-C7 level (Table 3 and Fig. 5).

Results of prosthesis fatigue analysis
The fatigue analysis results of prosthesis are shown in Fig. 2B-D. The fatigue analysis of the prosthesis internal xation device was carried out by FE method in the 6 loading directions: exion and extension, left-right lateral bending and leftright rotation. The maximum stress was 188 MPa, which was obviously less than 817 MPa of titanium alloy (Ti-6Al-7Nb). Three kinds of fatigue analysis results showed that the life span of prosthesis was at least 106.28 cycles, and the maximum was 10 7 cycles, which could be regarded as in nite life.

Discussion
The concept of physiological reconstruction of spine promotes the emergence of non-fusion surgery, such as arti cial cervical disc replacement. Some ACDF operations have been replaced by non-fusion surgery. However, the non-fusion technology for ACCF operation is still a blank. Therefore, our research group has developed a new 3D printing cervical composite joint system prosthesis. In order to verify the biomechanical effect of our newly developed prosthesis in the treatment of cervical spondylosis, we conducted a detailed biomechanical study on the prosthesis by using FE analysis method.
Experimental biomechanics and theoretical biomechanics are two of the most common spinal biomechanics research methods. Compared with experimental biomechanics, theoretical biomechanics has its unique advantages: it can not only avoid high experimental costs and limited material sources, but also realize the prediction of the internal stressstrain eld of the spine [25][26]. As a theoretical biomechanical research method, FE analysis has been used for many years in the research of spinal mechanics [27][28]. As an e cient and commonly used algorithm, the basic principle of the FE analysis method is discretized into a nite aggregate, and the whole motion is decomposed into the motion of FE [29][30]. The FE biomechanical research of cervical spine has been paid more and more attention by spine surgeons.
The use of computer technology to explore cervical spondylosis has gradually become one of the focuses of researchers.
After the establishment of the FE model, in order to determine whether the model is reliable and whether it can truly re ect the normal cervical function, it is a very necessary process to verify its validity. At present, the more common method to verify the validity of the model is to use authoritative in vitro biomechanical experimental data to simulate the biomechanical characteristics of the reconstructed model. Then compare whether the difference is within the allowable range. After the validation of the newly established FE model, the continuous improvement of the model and nally reaching the experimental application standard is an essential key link of FE analysis [31][32][33].
In this study, we veri ed the ROM of each segment, the maximum value of disc stress and the stress of each segment in the normal lower cervical We simulated subtotal resection of C5 vertebral body and performed prosthesis replacement or ACCF respectively.
Through the comparative analysis of the experimental data, we found that although the overall ROM decreased in the six motion states. The impact on the overall ROM value was not obvious compared with the normal cervical spine. We found that the new prosthesis can maintain a certain degree of motion, which is closer to the biological characteristics of the real intervertebral disc movement from the physiological and mechanical aspects. However, compared with the normal cervical spine, the ROM values of ACCF decreased signi cantly in exion-extension, lateral bending and rotation. Titanium plate and screws are strongly xed on the vertebral body, which results in greater overall stiffness of cervical spine and loss of local motion after operation. Compared with the normal cervical spine, the loss of ROM after ACCF was signi cantly than that after prosthesis implantation, especially in exion, extension and rotation. Therefore, the prosthesis implantation can maintain the ROM of the whole and each segment more effectively than the ACCF operation, and better retain the postoperative range of motion of the cervical spine.
We also compared the stress of adjacent segments of intervertebral disc in the operation level. The results showed that the implant group and ACCF group showed different degrees of increase in six motion states. The average increase of prosthesis group was about 75% at C3/4 disc , which was signi cantly lower than that in ACCF group (average increase was 130%); the average increase was about 60% at C6/7 disc, slightly higher than that in ACCF group (average increase was about 40%). The results showed that there is no signi cant difference between the stress of lower adjacent intervertebral disc caused by prosthesis implantation and ACCF surgery.
In addition, the fatigue analysis of the new 3D printing cervical composite joint system prosthesis was carried out by FE method under three kinds of repeated motion states: exion and extension, left-right lateral bending and left-right rotation. The maximum stress was 188 MPa, and the yield strength of the model was less than 817 MPa of titanium alloy (Ti-6Al-7Nb). According to the three kinds of fatigue analysis results, the life of prosthesis structure is at least 106.28 cycles. The maximum is 107 cycles, which can be considered as in nite life (the life in blue position is the worst, which is in line with the actual situation). Therefore, from the perspective of FE theory, the prosthesis used in this study can be regarded as having very reliable performance in terms of durability and wear resistance.
There are some limitations in this study. This study is based on the FE method, which belongs to pure theoretical research. The boundary conditions constraints, material properties and other assumptions are different with the actual force and experiment of cervical spine. Therefore, the results of theoretical research should be more from the perspective of trend and qualitative. If more accurate quantitative research or improvement of the existing prosthesis device is needed, some experimental and clinical veri cation should be carried out combination with theoretical research.

Conclusion
In a word, this experiment carries out quantitative and qualitative biomechanical research on the prosthesis through FE method. The overall range of motion of cervical spine, the relative mobility of each vertebral, the stress of intervertebral disc, and the fatigue strength of prosthesis were systematically analyzed. Based on the mechanical principle and structural engineering idea, the biomechanical characteristics of each structure of lower cervical internal xation were simulated in detail. The results show that the new 3D printing cervical composite joint system prosthesis can not only retain the range of motion of the cervical spine, but also decrease the stress of the upper adjacent intervertebral disc.

FE model establishment and validation
The computed tomography (CT) data of the lower cervical spine (C3-C7) were obtained from a healthy man (age 23 years, weight 70 kg and height 172 cm) without a history of cervical disc disease at 1mm intervals. The intact cervical model consists of ve vertebrae, four intervertebral discs, and associated ligaments. This study was approved by the Ethics Committee of Xijing Hospital of the Air Force Medical University (KY20202040-F-1 ). Written informed consent was obtained from the volunteer. of 0.6 mm [17][18].The annulus brosus occupied 60% of the total volume of the intervertebral disc and the nucleus pulposus occupied 40% of the volume [19][20]. The material properties of cervical spine structures were set (Table 1).  Establishment of 3D FE model after C5 segment prosthesis implantation and ACCF surgery

Consent for publication
Not applicable Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no con ict of interest.

Funding
This study was not supported by any research fund. Titanium alloy 110000 0.34 Table 2 The ROM data of C3-C7 in each group (°)  Table 3 The stress of intervertebral disc in each group MPa     The percentage of ROM relative to normal condition of each group.