Full-Endoscopic Anterior Odontoid Screw Fixation: A Novel Technique and Technical Report

Background: Odontoid fractures are common among cervical spine fractures and are categorized into three types. Unstable type II fractures are among the most challenging to treat, and the best treatment approach has been debated. Anterior odontoid screw xation, a surgical treatment option, yields a high union rate and helps preserve cervical motion; however, there are risks for approach-related complications. Here, we report a novel minimally invasive technique of full-endoscopic anterior odontoid xation (FEAOF). Methods: The authors introduce the technique and describe in detail the technical approach of FEAOF for the surgical treatment of type II odontoid fractures. Conclusions: FEAOF is a feasible and effective option for treating type II odontoid fractures. The procedure is less invasive than other techniques and provides clear direct visualization of the involved structures. Level of Evidence: Not applicable


Introduction
Odontoid fractures are the most common type of cervical spine fractures, accounting for 9-15% of all cervical spine fractures [1][2][3]. These injuries frequently lead to many complications, morbidities, and mortalities. According to the Anderson-D'Alonzo classi cation, high nonunion rates of 15-85% were reported in type II-odontoid fractures [1,4,6]. This type of fracture is associated with poor prognosis and is the most challenging for spine surgeons. In unstable type II fractures, the surgical treatment options vary and are currently debatable.
To stabilize these fractures, many techniques have been proposed, including posterior cervical instrumented fusion and anterior odontoid screw xation. Patients with reducible type II odontoid fractures with Grauer subclassi cation type II are considered good candidates for anterior odontoid screw xation [7]. This technique has many advantages over posterior procedures, including a high union rate, immediate stability, preservation of cervical spine motion, and less soft tissue injury [13]. However, there are many serious complications related to the anterior retropharyngeal approach for odontoid xation. These include possible injury to the pharynx, esophagus, airway, vascular, and neural structures [12].
Surgical approach-related and screw-related complications have been reported in both open and percutaneous techniques [8][9].
In order to minimize these risks, we initiated the use of full-endoscopic surgery for use in anterior odontoid screw xation procedures. The endoscopic system can help the surgeon visualize the appropriate screw entry point and surrounding structures. Therefore, there is increased screw placement accuracy and reduced soft tissue injury. Here, we report a novel minimally invasive technique of fullendoscopic anterior odontoid xation (FEAOF).

Preoperative planning
Certain prerequisites are necessary to utilize this technique. A recent fracture has a higher likelihood of fracture reduction. The oblique fracture pattern perpendicular to the screw trajectory results in the greatest biomechanical bene t. The patient's body habitus must allow proper screw trajectory. The presence of anomalies such as barrel-shaped chest, short neck, and cervical or thoracic kyphosis may have an impact on results [6,8].
Preoperative X-ray or computed tomography (CT) scans are used to measure the length of the screw and the proper angle of the syringe, which is used as a soft tissue protector. The syringe tip cutting angle ranges between 25° and 40° depending on the fracture line (Fig. 1a).
Under general anesthesia, the patient is placed in a prone position. The head is positioned over the end of the table and xed with a May eld clamp in an extended position to allow the appropriate trajectory for xation. A dual uoroscopic technique is used for anteroposterior (AP) and lateral X-ray images.
Anatomic reduction is performed and con rmed via uoroscopy. A radiolucent bite block is placed in the mouth to allow an unobstructed AP open-mouth view image. After prepping and draping, the tip of a 10-mL polyethylene syringe is cut at the exact degrees that were measured preoperatively. A 3-to 4 cm oblique skin incision is made at the sternocleidomastoid groove at the level of the C4/5 intervertebral disc. Gentle dissection is performed between the carotid sheath (laterally) and medial structures, which include the strap muscles, esophagus, and trachea. Narrow size Langenbeck retractors are used to guard the surrounding structures. The longus coli muscles are identi ed, and the anterior of the C2/3 intervertebral disc is approached. The beveled syringe is then introduced into the disc space and the endoscope applied through the syringe (Fig. 1b).

Entry point identi cation
An isotonic saline solution is used as the irrigation uid. The water pressure and ow are set at approximately 80 mmHg and 0.8 L/min. An appropriate entry point is visualized endoscopically and checked by biplanar uoroscopy. The superior part of the C2/3 intervertebral disc is cauterized using a 4-MHz bipolar radiofrequency electrocautery and partly removed using various types of endoscopic instruments (Fig. 2a-d).

Drilling and screws xation
Drilling is performed through an endoscope (Fig. 2e), which is closely monitored by biplanar uoroscopy. At this point in the procedure, the irrigation uid ow and pressure are increased for better visualization, which is compromised by bone bleeding. A partially threaded screw is tightened under both uoroscopic in the same manner. This system decreases bone bleeding from the drilling step. The skin is closed in subcutaneous fashion without retention of the surgical drain after a nal bleeding check. Postoperative images are obtained (Fig. 3a-b). A hard collar is applied for 4-6 weeks after surgery.

Discussion
A type II fracture is the most common type of odontoid fracture. However, these fractures have the poorest prognosis; the fracture line occurs through the waist of the odontoid process, often resulting in nonunion [2]. Greene et al. [4] reported that 28.4% of patients who received nonoperative treatment of type II odontoid fractures developed nonunion and required delayed surgical treatment. In fractures at risk for nonunion, such as those in individuals age > 50 years, those with fracture angulation, severe fracture gap, fracture comminution, fracture displacement, or instability, surgical intervention should be considered.
The use of an anterior odontoid screw, an osteosynthetic technique, is a surgical option for type II odontoid fractures with favorable fracture lines. The technique provides immediate stability, improves fracture union over nonoperative treatment, and preserves major cervical mobility [7]. Even with operative treatment using anterior odontoid screw xation, a nonunion rate of 10%, screw-related complications, suboptimal screw position, and approach-related complications such as dysphagia and hematoma have been reported [10]. To minimize approach-related complications and surrounding soft tissue injury, minimally invasive cervical spine surgery was introduced. Endoscopic-assisted surgery was reported in 2003 by Hashizume et al. [5]; however, the authors used a micro-endoscopic camera system. They found improved visualization allowing a smaller incision, decreased blood loss, and reduced surrounding soft tissue injury compared to the traditional open technique. To the best of our knowledge, our current technique was the rst to describe the use of a full-endoscopic system in performing anterior odontoid screw xation. Apart from its minimally invasive nature, full-endoscopic surgery has many advantages. A continuous uid irrigation system provides surgeons with a better eld of visualization and helps reduce bleeding by local vasoconstriction effect from the lower temperature of the irrigation uid. A channel of the camera unit is available for drilling or inserting a K-wire and tightening the screw through the instrument (Fig. 4). This technique should be performed by endoscopically trained surgeons. Converting to a traditional open technique must be prompted in any case of unexpected events. Using an orthosis for 4 to 6 weeks postoperatively helps reduce the load to the cervical spine and remains an important step in the anterior screw xation method. The dens has only half of its original strength immediately after screw xation and will gain its normal strength when the fracture is fully healed [11].

Conclusion
This novel FEAOF technique is a possible and effective option for treating type II odontoid fractures.
Owing to the minimally invasive nature of the full-endoscopic system, direct visualization and less soft tissue compromise are the main advantages of this technique. No funding was received to assist with the preparation of this manuscript Authors' contributions: VK analyze and performed all surgeries. JSK was a project consultant and inspiration for this work. All authors read and approved the nal manuscript.