Atlantoaxial dislocation (AAD) can result from several causes, including congenital abnormalities, trauma, inflammatory, or iatrogenic injury . Recently, the treatment of IAAD has been reported by many researchers, where the traditional treatment procedure is posterior pedicle internal fixation after anterior transoral release . Lv et al.  demonstrated rotating rod techniques to spare occipital-C1 motion of patients, which resulted in neurologic symptoms and congenital odontoid aplasia. Liu et al.  applied a cable-dragged reduction/cantilever beam internal fixation technique for the treatment of old irreducible atlantoaxial subluxation. Nonetheless several approaches of posterior cervical fusion could inevitably lead to the loss of atlantoaxial joint mobility. Wang et al.  demonstrated that the one-stage anterior transoral release and posterior internal fixation were safe and reliable for treating IAAD. Ren et al.  demonstrated the efficacy of anterior submandibular retropharyngeal release and posterior reduction and fixation as the optimal treatment for patients with IAAD. In their study, all patients achieved satisfactory clinical outcomes without apparent complications including, pharyngeal or chest infection, deep vein thrombosis, or dyspnea postoperatively. Yin et al. demonstrated a case series of 31 patients undergoing anterior release with TARP internal fixation for treating IAAD . In their study, they determined that the majority of IAAD cases could become reducible after anterior release without odontoid resection. Thus, the clinical outcome of traditional treatments for IAAD was relatively satisfactory [18–20].
However, the complications of these approaches are still inevitable and have also been reported by other researchers; for example, complications including cerebrospinal fluid leakage, infection, and abscess formation. In addition, some patients with no posterior arch of the atlas were not suitable for posterior cervical fusion . Additionally, posterior cervical fixation and fusion require occipitocervical fusion that can lead to the loss of occipitocervical joint function. In order to solve such problems, the retropharyngeal approach was described and was regarded as an alternative treatment for IAAD in recent years . Given the advancements in surgical equipment, a clear surgical field can be achieved, with the assistance of a cold light source headset lamp, through the retropharyngeal approach .
The retropharyngeal approach is an effective method for cervical discectomy and fusion, and its efficacy was reported by other surgeons. Cai et al.  indicated that their anterior atlantoaxial trans-articular locking plate system using the retropharyngeal approach provided biomechanical stability for those patients with atlantoaxial instability. Inspired by TRAP, we designed our retropharyngeal reduction plate system. Our findings indicated that our retropharyngeal reduction plate system might provide a new option for the treatment of IAAD with a low incidence of complications. Compared to the traditional treatment procedure for IAAD, one-stage effective reduction was achieved using our plate system by providing the atlantoaxial joint with forward and downward traction. In some cases, patient’s anterior C1 arch may be structurally weak and lateral mass screws can be an alternative choice to fix plate through upper round holes. The risk of infection is low because the transoral release is no longer needed. Meanwhile, our retropharyngeal reduction plate was performed in a fresh cadaveric atlantoaxial joint specimen to verify its feasibility.
In 1972, the finite element analysis was used by Brekelmans et al.  for the first time for biomechanical analysis in orthopedics. Recently, the finite element analysis can successfully analyze the inner stress and strain, which is hard to achieve by traditional experimental methods [25, 26]. The finite element analysis can not only provide surgeons with vital clinical data for further instruction in making individual operation plans, but also simulates the operation to better predict clinical outcomes. The publicity and presentation of retropharyngeal approach for IAAD is our ultimate purpose of this paper by introducing our newly designed reduction plate. Cadaver experiment showed that the newly designed reduction plate could be suitable to achieve placement on atlantoaxial joint through retropharyngeal approach. We compared the maximum stress of two different internal fixations using the 3D finite element analysis to make further verification of their biomechanical properties. The mean stress of two internal fixations was far below the maximum yield strength (795–827 MPa) and ultimate strength (860–896 MPa) of titanium alloy . Our finite element analysis also indicated that the stress was distributed evenly in the retropharyngeal reduction plate system.
In our study, the data and clinical outcomes cannot reflect upper cervical vertebra movements comprehensively because some data would be lost during modeling by using 3D finite element analysis. In addition, further verification of our retropharyngeal reduction plate system’s biomechanical properties is still needed because of limited cases allotted.