Introduction
Atlantoaxial instability and atlantoaxial dislocation often lead to spinal cord compression, spinal cord injury, increased rates of paraplegia, and effects on the respiratory centre, and in severe cases, it can endanger the lives of patients [
1,
2]. The key to solving this problem is to fuse the atlantoaxial complex by surgical procedures so that the atlantoaxial joint can acquire a stable structure, and the nerve compression can be relieved. At present, the methods for atlantoaxial fusion can be classified as posterior atlantoaxial fusion and anterior atlantoaxial fusion. Posterior atlantoaxial fusion can achieve good biological stability and a very high fusion rate [
3]; however, improper screw implantation may cause complications such as vertebral artery injury and spinal cord injury [
4], and in posterior atlantoaxial fusion, the injury to the neck muscles and posterior ligament complex of the patient are considerable and may cause extensive soft tissue injury, especially in the elderly [
5]. Percutaneous minimally invasive anterior transarticular screw (AATS) fixation is an effective surgical approach to address atlantoaxial instability [
6,
7], and this approach results in smaller surgical wounds, less damage to muscle ligaments, and less risk than posterior atlantoaxial surgery. However, the immediate stability of anterior minimally invasive surgery is not clear, and the amount of bone graft and the position of bone grafting are not easy to control. When the amount of bone graft is too large, patients are prone to dysphagia [
8], and when the amount of bone graft is too small, long-term atlantoaxial complex fusion will be affected [
6]. Many studies have shown that a porous metal structure can effectively promote the bone fusion rate, which has been verified in a variety of models [
9‐
11]. By taking advantage of this characteristic of the porous metal structure, a new type of AATS was designed, threaded structures at both ends, and a porous metal structure in the middle. The diameter of the improved AATS is significantly larger than that of the traditional AATS; thus, it can provide better immediate stability. The bone fusion-promoting characteristic of the porous metal structure can compensate for the shortcoming of the difficulty of bone fusion of traditional AATS fixation. In addition, finite element analysis was used to verify the biomechanical stability of the new AATS.
Discussion
The surgical treatment of atlantoaxial disorders is a high-risk area in spinal surgery [
21]. Atlantoaxial instability and atlantoaxial dislocation often lead to severe spinal cord injury, and in severe cases, they can endanger the lives of patients. The key to solving this problem is to fuse the atlantoaxial complex by surgery [
22] so that the atlantoaxial structure can be stabilised and nerve compression can be relieved.
At present, the atlantoaxial fusion methods widely used in and outside China are classified as atlantoaxial posterior surgery and atlantoaxial anterior surgery. Atlantoaxial posterior surgery was first proposed by Gallie et al. [
23] in 1939, and it has become the current method of atlantoaxial posterior pedicle screw fixation due to its continuous development and innovation [
24]. This surgical procedure is a commonly used method for atlantoaxial fusion with a relatively mature technique and reliable stability. However, this surgical procedure is difficult and involves high risks, which are unavoidable for many spinal surgeons. Posterior surgery is prone to damage of the vertebral artery and nerves [
25], resulting in severe consequences such as massive haemorrhage of the vertebral artery, cerebral infarction and hemiplegia caused by vertebral artery embolism, and dyspnoea resulting from central nervous system damage; in severe cases, the life of the patient may be in danger [
26,
27]. Some cases involve difficulties and risks when using the posterior approach, such as the abnormal development and pathway of the vertebral artery [
28‐
30] and congenital or iatrogenic absence of the bony structure of the atlantoaxial vertebra pedicle [
31,
32], and the atlantoaxial complex can be more safely fused through the anterior approach.
The atlantoaxial anterior approach is divided into atlantoaxial transoral fusion and fixation [
33,
34] and atlantoaxial transarticular fusion [
13]. Atlantoaxial anterior transoral fusion and internal fixation can better expose the atlantoaxial joint and provide a better surgical field, but its disadvantages are also obvious, including the inability to perform normal orotracheal intubation and the high susceptibility to postoperative infection [
35,
36]. In severe cases, a patient’s life could be in danger; therefore, the surgeon should use atlantoaxial anterior transoral fusion and internal fixation with caution. Traditional atlantoaxial anterior transarticular fusion was first proposed by Lesion et al. [
37] in 1987; subsequently, some scholars continuously developed and perfected open AATS fixation [
38], and all achieved good therapeutic results. Fong et al. [
39] reported an anatomical study of minimally invasive surgery of the upper cervical vertebrae and demonstrated the feasibility of anterior minimally invasive surgery for upper cervical vertebrae. Wang et al. completed seven cases of minimally invasive AATS fixation [
6], and except for two patients with transient dysphagia due to difficulty in controlling the amount and position of the bone graft, the remaining patients achieved good therapeutic results, confirming the feasibility of the minimally invasive AATS surgical approach. Minimally invasive AATS fixation does not damage any of the important muscles and ligaments of the cervical vertebrae, causes less trauma to the patient, and results in less intraoperative bleeding than posterior screw and rod fusion and fixation, and it preserves the cervical vertebral posterior ligamentous complex and greatly increases the stability of the cervical vertebrae. The volume of the atlantoaxial lateral mass is large, and it is not easy for the AATS to penetrate this mass, thus injury to the vertebral artery and central nervous system can occur [
40,
41]. Its disadvantage is that the immediate stability during atlantoaxial fusion by anterior minimally invasive surgery is still unclear. Moreover, it is not easy to control the amount and position of the bone graft. If the bone graft amount is too large, it tends to wear the oesophagus and causes dysphagia and other discomfort; if the amount is too small, unstable fusion or even fusion failure tends to occur [
6,
8]. In some cases, the efficacy of primary implantation and fusion is not certain, and a secondary surgery is required for treatment [
42] and not only increases the risk of the surgery but also increases the cost of patient treatment. A study by Kim et al. [
43] showed that the biomechanical stability of traditional AATS fixation was poor during flexion and extension. We performed relevant measurements of the screw path of AATS fixation, and the results showed that the AATS path can safely accommodate screws with a diameter of 6–7 mm, and that increasing the diameter of screws can improve the stability of fixation. The measurement results demonstrate that when a point at 4 mm above the junction between the inferior edge of the lateral arch of the axis and the lateral edge of the vertebral body of the axis is used as the entry point and the posterior edge of the superior articular process of the atlas is used as the implantation direction, the space of the lateral mass of the atlas can be used most effectively [
12,
13]. This technique can be safely used in anterior minimally invasive surgery for most normal atlantoaxial structures. By increasing the diameter of the screw, we increased the ability of the screw to immediately stabilise the atlantoaxial complex, while the porous metal facilitated bone fusion, thus long-term atlantoaxial bony fusion could be achieved.
In recent years, many studies have shown that a porous metal structure can effectively facilitate the occurrence of bone fusion, and in spinal surgery, porous metal has been shown to fuse the vertebral body to achieve good stability and thus achieve the purpose of the surgery [
9,
44‐
46]. Based on the above information, we designed a new AATS. Both ends of the screw contain a threaded structure, and the middle part contains a porous metal structure. The design of the porous metal structure can increase cell adhesion and promote cell differentiation, and the porous metal structure can also facilitate bone ingrowth so that bone fusion can be completed without the implantation of autologous or allogeneic bone, and related complications in bone harvesting area can be reduced [
11,
47,
48]. The pore size of the porous metal is 150 μm. Some scholars have shown [
49] that such a porous metal structure can not only achieve the best strength but can also fully facilitate bone ingrowth. After bone growth, a firm and stable structure is formed between the bones; this formation is similar to a “reinforced concrete” structure and can significantly increase the stabilisation effect [
50‐
52]. Following continuous research by scholars, the processing technology of porous metals has been improved, and the osteoconductive of porous metals has been enhanced [
10,
53], enabling the integration of anterior atlantoaxial transarticular internal fixation and fusion.
To increase the mechanical strength of the screws, the outer structure of the middle part of the new AATS consists of porous titanium, and the inner part is a solid cylinder with a diameter of 3 mm. We performed a biomechanical analysis by establishing a finite element model of the upper cervical vertebrae and preliminarily compared the effects of the three internal fixation methods on the stability of the atlantoaxial model and the stress distribution of the screw. The establishment of the finite element model of the upper cervical vertebrae was proposed by Puttlitz et al. [
54], and Brolin et al. [
16] and Zhang et al. [
55] improved the model. The current model included bones and ligaments, but muscles are not included in the current model. Although muscles are assumed to be capable of stabilizing the spinal column in vivo, they are seldom tested in vitro. When their study was compared with the in vitro model of the upper cervical vertebrae developed by Panjabi et al. [
19,
20], the rotation angles of flexion, extension, lateral bending, and axial rotation were the same, and the stress on the vertebral body was the same when a physiological load of 1.5 Nm was applied, validating the effectiveness of the finite element model of the upper cervical vertebrae. Therefore, it can be used clinically to replace the in vitro experiment. Several scholars have completed biomechanical analysis of cervical spine by using the above model and obtained reliable results [
17,
56,
57]. Within the limits of the model, the results of finite element-based mechanical analysis showed that the immediate stability of the new AATS after implantation was superior to that of the traditional atlantoaxial posterior screw. In terms of atlantoaxial rotation and l, it can be seen that immediate fixation stability plays an important role in long-term bony fusion lateral bending, and the stability of the new AATS was superior to that of the atlantoaxial posterior screw; whereas in terms of flexion and extension, the stability of the atlantoaxial posterior screw was superior to the new AATS. This result is also the same as Kim et al. [
43]. A review shows that the most commonly encountered perioperative complications were related to mechanical failure with rates as high as 7% during occipitocervical fusion and 6.7% during atlantoaxial fusion [
58]. In addition, instability of implantation may cause failure of atlantoaxial fusion [
59], and some scholars believe that the biomechanical stability of anterior atlantoaxial fixation is insufficient. In order to achieve long-term bony fusion, they performed one-stage anterior release and then posterior instrumentation and fusion [
60,
61]. It can be seen that immediate fixation stability plays an important role in long-term bony fusion, and the more stable internal fixation, the better the outcome [
62]. When compared with the stability of traditional atlantoaxial anterior screw fixation, the stiffness of the new AATS fixation was superior in terms of flexion, extension, lateral flexion, and rotation. Therefore, it improves the immediate stability of atlantoaxial and provides a better condition for the ultimate fusion of atlantoaxial and porous metals
.Within the limits of the model, the results show that the increased diameter can not only increase the immediate stability, reduce the local stress of the screw, and reduce the possibility of screw withdrawal and deformation, but it can also increase the area of bone ingrowth into the porous metal, which is expected to achieve better fusion effects.
There are still some limitations of this study. In this study, only the computer software and atlantoaxial model were used to simulate the implantation of the new ATS in the selected cases in order to prove its feasibility and safety. We need to implant the new AATS into the cadaver model to further verify its safety. The finite element analysis only simulated the stress and displacement of the atlantoaxial model after force was applied, and the results still need to be verified by animal models. In the subsequent experiments, the device will be implanted into animal models to verify its feasibility in fusing the atlantoaxial complex.
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