Anterior reduction and C1-ring osteosynthesis with Jefferson-fracture reduction plate (JeRP) via transoral approach for unstable atlas fractures

Background To introduce a novel transoral instrumentation in the treatment of unstable fractures of the atlas. Methods From January 2008 to May 2018, 22 patients with unstable C1 fractures who received Jefferson-fracture reduction plate (JeRP) via transoral approach were retrospectively analyzed. The case history and the radiographs before and after surgery were noted. The type of fracture, the reduction of the fracture, and position of the internal fixation were assessed through preoperative and postoperative CT scans. Results All 22 patients successfully underwent anterior C1-ring osteosynthesis using the JeRP system, with a follow-up of 26.84 ± 9.23 months. Among them, 9 patients had transverse atlantal ligament (TAL) injury, including 3 in Dickman type I and 6 in type II. The preoperative lateral mass displacement (LMD) decreased from 7.13 ± 1.46 mm to 1.02 ± 0.65 mm after the operation. Bone union was achieved in all patients without implant failure or loss of reduction. There were no surgery-related complications, such as wound infection, neurological deficit, or vertebral artery injury. However, atlantoaxial dislocation occurred in 3 patients with Dickman type I TAL injury 3 months postoperatively without any neurological symptoms or neck pain. Conclusions Transoral C1-ring osteosynthesis with JeRP is an effective surgical strategy to treat unstable atlas fractures with a safe, direct, and satisfactory reduction. The primary indication for the JeRP system is an unstable fracture (Gehweiler type I/III) or/ and TAL injury (Dickman type II). Supplementary Information The online version contains supplementary material available at 10.1186/s12891-021-04628-4.


Background
The atlas has a unique ring structure with no vertebral body or spinous processes,which provides the exibility and maximum range of motion (ROM) compared to any other vertebra of the spine [1] . As a transitional structure, it is an indispensable part of craniovertebral junction,which allows axial loading transferred from the occiput to axis. The junctions of the lateral mass connected to both the anterior and posterior arches are relatively thin, which are the weakest points of C1 and the most likely sites of fracture [2] . Thus, the fracture with two or more breaks commonly occurred in the C1-ring [3] . In general, the type of atlas fracture can be divided into stables and unstable according to the integrity of transverse atlantal ligament (TAL) and adjacent vertebrae.Unstable C1 fractures are characterized by high-grade transverse spread of lateral mass on open-mouth radiograph.
Until now, posterior atlantoaxial and occipitocervical fusion techniques have been widely used in the treatment of it and have achieved satisfactory results [4] . However, posterior fusion techniques decrease the ROM of the craniocervical junction. As to this problem, anterior C1-ring osteosynthesis is an appropriate option to perform ORIF via transoral approach with safety and effectiveness [5] .
However,surgical site infection is a worrying complication,making surgeons preferred the posterior C1-ring osteosynthesis. Nevertheless, posterior C1-ring osteosynthesis can't completely reduce and stabilize C1 anterior arch fractures.In 2004,Ruf [6] rst reported a C1-C2 rotation function-preserving technique of C1 osteosynthesis via transoral approach, which not only enables an anatomic reconstruction of anterior arch of C1, but also achieves bony union. The study by Ma [2] demonstrated anterior technique of C1-ring ORIF with direct manipulation at the lateral masses was safe and effective for treatment of highly unstable cases. However, the internal xation device used in C1-ring osteosynthesis is not a special instrumentation for atlas fractures,and it is unlikely to achieve near anatomical reduction for C1 fracture in direct and safety especially for the C1 anterior arch fractures. To our knowledge,there is no report on special apparatus for C1 osteosynthesis via transoral approach.
In this study, we described a novel Jefferson-fracture reduction plate (JeRP) system (Wego Corporation, China) via transoral approach. It is originally designed and clinically applied to treat unstable atlas fracture, for preserving motion-function of atlantoaxial joint.

Methods
This research program was approved by the institutional review committee of PLA General Hospital of Southern Theatre Command: People's Liberation Army General Hospital of Southern Theatre Command.
Each patient signed a medical informed consent documents preoperatively.
From Jan 2008 to May 2018, 22 patients with unstable C1 fractures were retrospective analyzed, who received JeRP via transoral approach. Patients with fractures of the adjacent vertebrae of the axis or occipital condyle were excluded. All patients suffered neck pain, stiffness and limited motion of neck movement. None of the them had neurological de cit.On admission,all the patients underwent x-ray lm,CT scans and MRI preoperatively ( Fig. 1, 2). The lateral mass displacement(LMD) of atlas was measured from the coronal reconstructed view of the CT scans. Integrity of the TAL was evaluated by preoperative MRI. All patients underwent 4-6 kg skull traction preoperatively and intraoperatively in the hope that the displaced lateral mass might close at least partially.
Design of the JeRP system The JeRP-system consists of titanium alloy plate,self-tapping screws,specialized repositor and corresponding surgical instruments (Fig. 3). The JeRP-plate shapes like an "L",and is consistent with the anatomical morphology in front of the atlas, and the lateral mass end of it is provided with two circular xing holes xed to the lateral mass of C1. On the side views,the plate has a slight arc, tting the physiological curvature of C1.Additionally, there are two types of L-shaped plates, left and right, which are used for C1 fractures with the fracture line closed to the left or right lateral mass,respectively. The middle of it is a long-elliptic hole in which a provisional reduction screw can slide. Specialized repositor is worth noting. With the help of it,compression force is gently applied to pull together the fractured ends for reduction. The working principle of JeRP-system in the treatment of C1 fracture is shown in Fig. 4.

Surgical Technique
All surgeries were performed by senior spine surgeons. Under general anesthesia with transnasal endotracheal intubation,the patient was placed in a supine position with skull traction with a weight of 4-6 kg. Codman oral retractor used to expose the oropharynx,a 3-4 cm longitudinal midline incision along the posterior pharyngeal wall was made,and soft tissue was dissected to fully exposed the surgical eld, including anterior arch, fragment bone and lateral mass of atlas (Fig. 5A). According to the preoperative CT measurement,the ideal entry point, the trajectory of the screw and the length of the JeRP-plate were determined. The anterior tubercle of C1 was polished with high-speed drill. After removing soft tissue between bony fragments,the JeRP -plate was placed in anterior aspect of atlas (Fig. 5B).By palpating the medial edge and lower margin of the lateral mass,we could determine the optimal entry point of the screws. First, one lateral mass screw close to the fracture line was installed and tightened for stabilized the plate. Then,a provisional reduction screw was placed in the anterior arch of C1, which could be moved in the runner of the plate,and the nut was about 3 mm away from the bone surface of the anterior arch. The specialized repositor was installed between a provisional screw and the complex comprised with the lateral mass and the plate (Fig. 5C, D). Compression force was gently applied via the special repositor to pull together the fractured ends and to make it t tightly. Next, the other lateral mass screws were tightened subsequently, the temporary screw removing, and the entire reduction xation process was complete. Finally, the JeRP-plate was a xed to the anterior arch,and satisfactory reduction of fracture and position of internal xation were con rmed by intraoperative uoroscopy.After JeRP-implant procedure, the wound was soaked with povidone-iodine for 15 minutes and was closed in muscular and mucosal layers respectively with interrupted sutures.

Postoperative treatment
Transnasal endotracheal catheter was maintained for 2 days after the operation, and a nasogastric feeding tube was kept for 1 week. After the removal of the nasogastric tube, uid diet was started and normal diet was started 3 weeks postoperatively. Antibiotics were given intravenously as preventive interventions in all patients for 5-7 days after surgery and the neck collar was kept for 3 months.
Postoperative CT were used to assess the e ciency of the C1 reduction and the accuracy of the screw placement. What's more, CT scans and X-ray lm of cervical spine were performed at three, six and twelve months after surgery to evaluate bony healing, reduction and stability of the C1-C2 (Fig. 6). At the nal follow-up, the atlanto-dens interval (ADI) was calculated based on exion-extension radiographs. Also, visual analogue scale (VAS) was performed to evaluate the degree of neck pain.

Results
The demographic data of patients were listed in Table 1. In all patients, the incision of the posterior pharyngeal wall healed well, and no infection or dehiscence occurred. No vertebral artery or spinal cord injury occurred during the operation. According to Gehweiler's classi cation system, 7 of 22 patients presented with type fractures and 15 of 22 patients had type fractures. C1-rings was violated into few parts, including 2 separate fragments (7 patients), 3 fragments (13 patients), and 4 fragments (2 patients). 9 patients had certain TAL injury (3 of type I and the other of type II based on Dickman's classi cation), while other 13 patients had TAL injury or not were uncertain. The surgical and clinical outcomes can be listed in Tables 2 and 3.All operations were completed successfully completed without surgery-related complications,such as wound infection, neurological de cit,or vertebral artery injury.The preoperative LMD decreased from 7.13 ± 7.08 mm to 1.02 ± 0.65 mm after operation. Bone union was achieved in all patients without implant failure or loss of reduction. At the nal follow-up, the mean VAS scores was 0.28 ± 0.13 points. What's more, all patients' upper cervical spine rotation range was well preserved. However, 3 patients had an ADI of more than 4 mm at the last follow-up without any neurological symptoms or neck pain. 1 patient had limited cervical movement due to the penetration of the atlanto-occipital facet with lateral mass screws, but no obvious pain and no special treatment.

Discussion
Most of stable atlas fractures can be treated conservatively [7] . However, as to unstable C1 fracture, surgical strategies have become the standard of care. Unstable atlas fracture usually accompanies TAL injury, behaving as separation of the lateral masses, and subluxation or dislocation of atlantoaxial joint [5] .There is also a point that atlas fractures, not C1 posterior arch fractures, destroyed the stability of the upper cervical spine. The spinal cord is under danger since the displacement of bone fragments or atlantoaxial dislocation, which resulting in severe complications like paraplegia even death [8,9] . Treatment aspires to reduce the fracture, correct dislocated fracture, stabilize the atlantoaxial joint, and preserve the maximum ROM of the upper cervical spine [10,11] . However, the surgical strategies of unstable C1 fractures is still controversial.
Value of C1-ring osteosynthesis in unstable atlas fracture Conservative therapy in unstable atlas fracture for several months may lead to severe discomfort and high incidence of bony nonunion [12] .Simultaneously,the inconsistency and mechanical instability of occipitocervical junction may restricted motion and cause continuous neck pain [13] .To date,the posterior atlantoaxial or occipitocervical fusion is supposed to be the main surgical method,but the motion of upper cervical spine was sacri ced.The ideal treatment method is limited xation without restricting the ROM of upper cervical spine [14] . For this, many spine surgeons gave up the posterior fusion and recommend C1-ring osteosynthesis [15,16] . The question regarding of relationship between C1-ring osteosynthesis and integrity of the TAL is still controversial.Traditionally, the integrity of TAL is key in determining the stability of C1 fractures. Rule of Spence [17] showed that total LMD over 6.9 mm on openmouth radiographs correlated with rupture to TAL has important clinical value in determining whether surgical intervention is needed and is currently being questioned.What's more,there are other tissues that help maintain C1-C2 stability and restrict motion notwithstanding rupture of TAL.Some scholars' founding shown that the signi cance of axial ligamentous tension of craniocervical junction has been underrated [18] .Because of its unique anatomical structure and biomechanical environment, occipitocervical junction mainly stabilized by the ligamentous complex of C0-C1-C2 [13] . Previous literature had showed that C1 burst fractures are axial load entities and better to maintain the integrity of secondary stabilizers comprised the alar ligaments, facet capsule and neck musculature. C1-ring osteosynthesis techniques is able to restore the axial tension of ligamentous complex of C0-C1-C2 through reduction of fracture. Studies have shown that even with the rupture of TAL,C1-ring osteosynthesis can provide su cient stability under physiological load. Thus, incompetence of TAL may not be a contraindication to C1-ring osteosynthesis.The conventional de nition of C1 instability based on the integrity of TAL underestimates the number of fractures requiring surgical intervention and overestimates the number requiring C1-C2 fusion.
Advantage of anterior C1-ring osteosynthesis using JeRP system C1-ring osteosynthesis using both posterior approach and transoral approach has been published [18] .With posterior C1-ring osteosynthesis techniques,posterior arch fracture and the lateral mass displacement could be satisfactorily reduced by the compression force on the end of bilateral mass screws.However,it makes the front of the lateral mass screw swing laterally, leading to insu cient reduction of anterior arch fracture of C1 [19] .
Anterior direct reduction of the atlas fractures promotes the rate of bony union of fractures by improving the integrity of C0-C1-C2 complex structure.This approach has a good safety pro le,avoiding fusion of important motion segments, and restoring the C0-C2 height. Over last decade, results of transoral C1-ring osteosynthesis for unstable atlas fracture have been veri ed. However, many surgeons are hesitant about this technique,because of unfamiliarity with the transoral approach,the theoretical increased risk of infection.The universal shortcoming of transoral C1 osteosynthesis published previously is that reduction of C1 fracture is only an acceptable repair rather than anatomical reconstruction.The transoral C1-ring osteosynthesis is technically challenging, and there is no speci c instrumentation devices or spinal implants designed for treating unstable atlas fracture.The main problem with current techniques is that the posterior pharyngeal soft tissue which is not thick enough to cover the plate or rod,thus increasing the risk of wound complications.Additionally,it is di cult to implement satisfactory reduction of atlas fracture in the deep and narrow space. Simultaneously,the end of the lateral mass screw via transoral approach is too high,which may easily cause the wound of the posterior pharyngeal wall to crack or postoperative dysphagia.In this study,the use of JeRP-system is introduced for anterior C1-ring osteosynthesis.The advantage of it lies in the use of a dedicated reduction instrument, which not only satis es the reduction of the fracture end,but also can place the xation screws in the fracture reduction state and the reduction instrument will not affect the screw placement. Generally,it is acceptable to follow the principle that the screws don't penetrate the edge of the lateral mass into the atlanto-occipital joint, and don't enter atlantoaxial joint.The lateral mass is wedge-shaped, with higher outside and lower inside.The insertion point of the screw is deviation inward, which easily leads to the screw entering joint. In our paper,1 lateral mass screws were observed to enter atlanto-occipital joint.In the series, 22 patients had bone fusion, and wound infection and dehiscence had not been observed.The main advantage of JeRP-system is not only ideally reduction of C1 fracture performed via anterior approach, but also inserted plate and screws could not interfere with midline wound closure.The incongruency of the C0-C1 and C1-C2 joints is recti ed, and the ligamentous tension band of craniocervical junction is regained as well.As far as we know,this new technology can minimize lateral mass displacement.JeRP-system appears to be an effective and safe method to deal with unstable C1 fractures, which achieve ideal bone fusion and motion-preserving of craniocerivcal junction.
The original intention of the JeRP-system designed is to be used for unstable C1 fractures with TAL intact,and its indication is very narrow. In the actual process, we also applied the JeRP-system to C1 fractures with TAL rupture,and achieved satisfactory results. Among them,3 patients with Dickman type I TAL injury occurred atlantoaxial dislocation postoperatively,while the patients with Dickman type II TAL injury had good effect. Hence, the primary indication for JeRP system is an unstable C1 fracture (Gehweiler type I/III) with or without TAL injury (Dickman type II).

Conclusions
Transoral C1-ring osteosynthesis with JeRP-system is a valid option of unstable C1 fractures, achieving direct and satisfactory reduction safely. Even if the TAL is ruptured,the atlantoaxial joint can remain relatively stable.TAL injury may not necessarily be an absolute contraindication for ORIF for altas fracture.However, further studies should be made to investigate the long-term effect of atlantoaxial instability.
Limitations of this paper are the lack quanti ed range of motion-preserving of atlantooccipital and atlantoaxial joints,small sample size,retrospective design and possibility of selection bias.