A new tool in percutaneous anterior odontoid screw fixation

Odontoid fractures are common, which represent approximately 9 to 20% of all cervical spine fractures [1‐4]. Its treatment remains challenging due to the complex anatomy of the craniocervical junction [5]. It is generally accepted that type I and type III odontoid fractures based on the classification of Anderson and D’Alonzo can be treated by conservative strategies such as cervical orthoses, halo vests, and rigid cervical collars [5‐7]. Type II fractures are mechanically unstable injuries [5, 8]. Conservative treatment for this type is associated with high bony nonunion rate, accordingly, surgical treatment is recommended [1, 5]. Over the past few decades, the applications of several surgical strategies (odontoid screw fixation, Magerl technique, and Harms technique) have achieved satisfying clinical results [3, 9‐13]. It was reported that posterior approaches can achieve high bone union rate [14, 15], but they inevitably sacrificed atlantoaxial rotational motion [5, 15‐17]. Anterior odontoid screw fixation can preserve normal atlantoaxial rotation [5, 8, 15, 16, 18] with union rate comparable to posterior approaches [17]. It is considered as the preferred treatment for odontoid fractures [17]. Percutaneous odontoid screw fixation is minimally invasive, which can shorten operation time and reduce blood loss compared with open technique [8].

The odontoid screw is percutaneously placed with the help of the guide tube, guide wire, and protection tube, which requires an experienced surgeon. The precise insertion of the guide wire into the ideal position is crucial and challenging. We have developed a new guide instrument to help with the placement of guide wire, which have achieved satisfying surgical results. In this report, the authors discuss the clinical application of this new tool.

Fractures at the odontoid base, classified as Type II injuries by Anderson and D’Alonzo system, are the most common type (more than 60%) of all odontoid fractures [8, 11, 14, 15]. They were considered relatively unstable and had a high incidence of nonunion [5]. Anterior odontoid screw fixation can achieve a high union rate (86–100%) as found in numerous studies [5, 8, 15, 19‐23]. It has a number of advantages, including immediate stability, less postoperative pain, and direct fracture fixation without bone graft [16]. More importantly, normal physiologic atlantoaxial rotation can be preserved [5, 8, 16]. However, the application of anterior odontoid screw fixation has three prerequisites: suitable anatomy of the odontoid without atlantoaxial dislocation, suitable fracture morphology, and suitable bone quality without osteoporosis [9]. Contraindications of anterior odontoid screw fixation include comminuted fracture, severe osteoporosis, severe cervicothoracic kyphosis, late fractures, ligament transverse rupture, posterior oblique fracture line, non-reducible fractures, and nonunion for more than 3 months [4, 14, 16, 17, 24‐26]. Posterior cervical spine fixation should be performed in patients with these contraindications [14, 15, 17].

In our case series, all patients had no contraindications of anterior odontoid screw fixation. The age, cause of injury, fracture type, associated injury, and time from trauma to operation showed a similar pattern in the traditional guide instrument group and the designed guide instrument groups. Grauer et al. [7] further classified type II fractures into IIA, IIB and IIC fractures, representing nondisplaced fractures, anterior superior to posterior inferior or displaced transverse fractures, and anterior inferior to posterior superior or comminuted fractures based on the fracture line obliquity, displacement and comminution to guide treatment, respectively. In our case series, type IIA and IIB odontoid fractures were treated with anterior odontoid screw fixation.

Open technique for odontoid screw fixation requires intraoperative extensive exposure and placement of drainage postoperatively [5]. Kazan et al. [27] developed a telescopic tube system and demonstrated the feasibility of percutaneous odontoid screw fixation technique on the cadavers, which was the first report of percutaneous odontoid screw fixation. Similarly, Horgan et al. [28] developed an endoscopic approach with a soft tissue dilator. Wu et al. [29] developed a two-hole guide tube to facilitate the making of a second optimal kirschner wire trajectory when an initial suboptimal kirschner wire hole was drilled. However, in our opinion, the distance between the two holes was fixed, and the position of the first Kirschner wire would affect the position of the second Kirschner wire. Chi et al. [5] developed a system of tools with a blunt tip-guide tube and achieved satisfying results in the clinical application of 10 patients. Wang et al. [8] compared percutaneous and open anterior screw fixation for odontoid fractures prospectively and found that the former can significantly shorten operation time, reduce surgical exposure and blood loss. Umana et al. [4] used a ruler in the X-ray monitor to evaluate the final trajectory of the kirschner wire and to make the adjustments needed. They also developed a soft tissue dilator to protect the soft tissue using an endotracheal tube with an internal diameter of 6 mm. However, without a guide instrument, there was no positioning and fixing function of the guide instrument to the kirschner wire. The guide instruments used by the above three authors [5, 8, 29] have a similar structure - a guide tube with a blunt tip. The guide tube we used before also had a blunt tip structure. We considered this guide tube with a blunt tip was prone to sliding on the anterior surface of the vertebra, especially when the power-drill was working. Compared to these guide instruments, the top of our designed guide instrument (guide tube and protection tube) has a wedge-shaped tip with 30° inclination, which has a larger contact area with the anterior surface of the cervical vertebra. It is more stable when positioning. It is helpful to guide the guide wire into the odontoid accurately and quickly. Moreover, it can cover the tip of the guide wire to prevent it from stabbing important tissues such as the esophagus and blood vessels. Compared with the traditional guide instrument with a blunt tip, our guide instrument significantly decreased operation time and radiograph times. The short operation time reflects the effectiveness of our new guide instrument. The less radiograph times reduce the injury to clinicians. Regardless of the magnitude of the difference, the reduction of these two parameters is of great significance for the clinic. However, this is a retrospective study. And the sample size is small. A higher level of research design such as large-sample-size randomized controlled trials should be conducted to further study.

Fracture reduction before anterior odontoid screw fixation is critical. Displacement of the fracture should be less than 3 mm in our series before surgery if the fractures were failed to anatomic reduction. After traction, most of our patients with fracture displacement met the surgical requirements. The length of the screw is also critical. Ideally, the screws should just break through the apical cortical bone of the odontoid to compress the fracture line when the screw is tightened. If the screw is too short, its pull on the odontoid will not be enough, which will easily cause reduction failure. If the screw is too long, it may damage the spinal cord or lose its compression.

In this study, we used a single screw for percutaneous procedure. Studies have shown that there were no significant differences in bony union rate [30] and biomechanical stability [31, 32] between a single screw and two screws fixation. In order to ensure sufficient screw trajectory and enough cortical bone in front of the screw, many surgeons described that the ideal entry point was mostly at the C2/3 disc level [8]. In our series, most screws entered through the anterior edge of the C2/3 disc. The impact of slight injury in the anterior edge of C2/3 disc on patients needs further study.

Due to the complex anatomy of the craniocervical junction and the high precision requirements of odontoid screw fixation, percutaneous anterior odontoid screw fixation may be technically challenging, especially for novice surgeons without minimally invasive spinal surgery experience [8]. Our guide instrument reduces the difficulty and learning curve of the surgery to a certain extent because it facilitates the placement of screw. However, this study has certain limitations. Firstly, the time span of the study is long. The proficiency of the surgeon affects the surgical outcomes. In addition, the follow-up time varies, some patients lack long-term follow-up results. Thirdly, there are only 13 samples in the traditional group and 16 samples in the designed group. The small sample size may affect the results. It is also a single-center, retrospective study. In view of these limitations, prospectively planned, multicenter and larger-sample trials in the future are essential for clinicians to treat and manage patients.

According to this retrospective study, our guide instrument significantly decreased operative time and radiograph times compared with the traditional guide instrument with a blunt tip. It has potential clinical value, which needs further testing with a higher level of research design such as large-sample-size randomized controlled trials.