Pure endoscopic endonasal odontoidectomy: anatomical study

Different pathological disorders may produce atlanto-axial translocation with ventral compression of the brain stem or spinal cord. The most common are congenital malformations, such as Arnold Chiari malformation type II, chronic inflammation, such as rheumatoid arthritis, genetic transformation, such as Down’s syndrome and trauma, such as type II odontoid fracture. Some of these patients are candidate to the resection of the odontoid process for anterior decompression. The indication for odontoid resection is irreducible atlanto-axial subluxation, associated with severe spinal cord compression causing progressive myelopathy.

The most favoured approach to the odontoid process is the transoral approach. This approach provides a direct route to the surgical field, without any neurovascular manipulation and passing through the oropharynx, without injuring major neurovascular structures. The main limitation to this approach is the difficulty of dural closure and the subsequent higher risk of CSF leak and meningitis. For this reason the trans-oral approach is mainly used for extradural lesions [7, 8, 11‐13, 15, 22‐24], although some studies have reported its application for the surgical treatment of intradural pathology of the lower clivus and ventral cranio-cervical junction [6, 9, 16, 25, 26]. Other minor disadvantages are, however, related to this approach: the split of the soft palate and even of the hard palate is often performed in the case when rostral extension of the approach is required; tongue swelling may occur for prolonged compression; there is risk of damaging the teeth with retractors; velopharyngeal insufficiency may develop; and there is the necessity of nasal feeding in the postoperative stay.

Recently, increased diffusion in the use of the endoscope for transsphenoidal pituitary surgery [3, 17] led some studies to explore the possibility of applying the endoscopic endonasal approach in the surgical treatment of skull base lesions other than pituitary tumors. In recent years some works have reported anatomical studies and surgical experience in the endoscopic endonasal approach to different areas of the midline skull base, from the olfactory groove to the cranio-vertebral junction [1, 2, 4, 5, 10, 14, 18‐21].

Thanks to the properties of the endoscope itself, the endonasal approach provides a wider view of the surgical field and a close-up vision, when compared with the transoral microscopic approach. Furthermore, the minimal invasiveness of the endoscopic endonasal route may reduce some morbidities related to the transoral approach. In fact, it is no longer necessary to use mouth retractors, prolonged compression of the tongue or split of the soft palate, and even considering the necessity of a middle turbinectomy or removal of the posterior portion of the nasal septum to enlarge the surgical corridor, these adjunctive manoeuvres do not usually produce morbidity to the patient. These manoeuvres are often performed in the endonasal extended approaches to the area around the sella in live patients and do not cause any respiratory problems.

The possibility of performing an odontoidectomy through the nose is strictly related to the level of the C1–C2 junction. In fact, in the case of a low junction, below the level of the hard palate, it is virtually impossible to remove the odontoid process with an endonasal approach. On the contrary, in the case of a high position of the atlas-axis junction, the dens is more easily reached and removed through the nasal cavities.

Odontoidectomy may be considered one of the most complicated manoeuvres for the transoral approach, in which the split of the soft and even hard palate is often necessary. Thus, this approach could be evaluated for those cases in which a transoral removal is considered more difficult.

However, this kind of approach still presents some of the main problems of the transoral approach. The first problem concerns the risk of CSF leak and subsequent meningitis. Although the endoscope, thanks to its close-up and multi-angled vision, has a greater chance of detecting an occasional CSF leak, it is quite hard to suture the dura and the nasopharynx mucosa with conventional suturing tools through the nose. For this reason, in our anatomical study, we have created a muscle-mucosal flap, comprehensive of the entire muscular and mucosal tissue covering the ventral cranio-vertebral junction. This flap, as shown, is transposed into the oral cavity during the bone’s removal and replaced in its original site at the end of the procedure. Due to the difficulty of anchoring the flap with suture, it is only distended on the defect and its borders are apposed on the corresponding lines of incision. The mucosa of the inferior wall of the sphenoid sinus could be stripped to favour the adherence of the flap and fibrin glue could be used to seal the edges. The creation of a peduncolated muscle-mucosal flap permits a more physiological reconstruction of the surgical corridor and, furthermore, the vascularization of the flap that directly continues with the oropharynx, facilitates a rapid healing. An endoscopic control should be performed one month after surgery to check the recreating integrity of the rhinopharynx mucosa.

The second problem of the transoral approach concerns the stability of the cranio-vertebral junction. The removal of the odontoid process with its ligaments can destabilize the cranio-vertebral junction.

The third problem concerns the haemostasis that is often difficult in the extended endonasal approaches. Bleeding control may become difficult with bipolar coagulation because the endonasal approach presents a long and narrow corridor, with a limited working space between the tips of the bipolar forceps. Nevertheless, specific bipolar forceps (TAKE-APART bipolar forceps; Karl Storz GmbH, Tuttlingen, Germany) have been used to work through the nose in a safe and effective way as well.