11.1 Introduction

Accessing the anterior aspects of the upper cervical spine in patients with ventral cervical spine pathology has historically presented a challenge to surgeons. Classic approaches include the transoral and transoral–transpalatal, as well as more expanded approaches including the transoral–transmandibular and the transoral-extended maxillotomy (open door) approaches (Fig. 11‑1).

Credit for popularizing these techniques may be given to many such as Sherman (1935), Fang and Ong for their work for infection (1962), Greenberg (1968), and many others including Crockard (1985) who designed the well-known Crockard retractor. ^{1 ,​ 2} More recently, the endoscopic transnasal approaches have been popularized either as independent approaches or in combination with the transoral approach. This chapter will analyze the utility and limits of these approaches to the upper cervical spine when coupled with intraoperative navigation techniques.

Nearly a century of experience has increased the success of the transoral or transoral–transpalatal approaches for pathology of the upper cervical spine while simultaneously simplifying and standardizing the optimal preoperative and postoperative care. The challenges of operating in a small corridor have been partially addressed through the development and implementation of modern technology including microscopes, endoscopes, and malleable or angled instruments. Furthermore, these new technologies have helped limit the approach-related damage to normal anatomy not directly involved with the pathology. This is particularly true of the endoscopic transoral and transnasal approaches. Additionally, as with other surgical disciplines, the use of two-dimensional (2D) or three-dimensional (3D) intraoperative image-guided navigation has dramatically increased surgical precision and improved outcomes in the anterior approach to ventral pathologies of the cervical spine. ¹

11.2 Indications

Indications for the transoral approach to the anterior cervical spine include, but are not limited to, accessing the following pathologies which may involve the craniovertebral junction, also known as the craniocervical junction (CCJ):

• 
  

  Pathology of the bone:

  
  
  
  
  • 
    

    Traumatic bone fracture.

    
    
  • 
    

    Removal of foreign objects (e.g., ballistics).

    
    
  • 
    

    Basilar impression/invagination or upward migration of the occipital condyles.

    
    
  • 
    

    Pannus related to rheumatoid arthritis. (This is more historic given the frequency of posterior instrumented arthrodesis in these patients.)

  
  
• 
  

  Extradural mass:

  
  
  
  
  • 
    

    Abscess.

    
    
  • 
    

    Chordoma.

    
    
  • 
    

    Metastasis (e.g., renal).

    
    
  • 
    

    Primary bone or central nervus system (CNS) tumor.

  
  
• 
  

  Intradural extramedullary mass (e.g., schwannoma, meningioma, neurofibroma).

11.3 Anatomy

The CCJ includes the inferior clivus at the junction of the foramen magnum, the atlas and axis, as well as the associated ligaments such as the anterior atlanto-occipital ligament (continuation of the anterior longitudinal ligament [ALL]), the apical and alar ligaments, and tectorial membrane (Fig. 11‑2). The pharyngeal wall consists of several layers which have been studied and described by Wang et.al. ³

The posterior pharyngeal wall was found to range in thickness from 2.9 to 4.3 mm at the C1 tubercle and 5.2 to 7.1 mm at the C1 lateral mass, and 4.3 to 6.5 mm at the central portion of the C2 vertebral body (Table 11‑1). Through these dissections, five layers were identified and described: mucosa, muscularis mucosae, prevertebral fascia, prevertebral muscles (longus capitis and longus cervices), and the anterior longitudinal ligament which drapes the dens anteriorly. ³

Several major veins and arteries (i.e., pharyngeal branches of carotid, palatine, and pharyngeal arteries and veins) were noted to be in the space between the muscularis mucosae and the prevertebral fascia (retropharyngeal space). The internal carotid artery (ICA) is most susceptible to injury at the level of C1 because it resides anterolaterally to the C1 arch before it enters the skull base. The C2 segment lies more anteromedially than the C1 segment.

When discussing the safe exposure zone in the transoral approach, it is important to consider that an expected width ranges 35.5 to 43.7 mm and expected length ranges 44.3 to 62.0 mm. Additional exposure from soft palate splitting may provide nearly 1.5 cm more length ^{3 ,​ 4} (Fig. 11‑3).

With microscopic visualization, it is recommended that the entrance to the oral cavity must be at least 2.5 to 3.0 cm between incisors in order to have adequate visualization and working room. This may be circumvented by the use of endoscopic transoral visualization and angled instruments. ⁴

11.4 Navigation

Intraoperative computed tomography (CT) image-guided navigation for the treatment of anterior cervical pathologies has been well studied and shown to improve accuracy and decrease morbidity. It can aid the surgeon in creating a safe exposure with minimal dissection (i.e., minimal access) as well as preventing injuries to neural and vascular structures. The use of image-guided navigation also limits the use of fluoroscopy, thereby decreasing radiation exposure to the patient and operating room staff and it allows a similar or greater degree of accuracy which is critical when working in this high-risk location. Indications for use and, furthermore, 2D versus 3D may include complex and disrupted anatomy.

The use of navigation, or image guidance, for anterior approaches to the upper cervical spine has been increasingly studied with continued refinement of the registration process, given that the transoral approaches to the anterior cervical spine have been complicated by the lack of anatomically consistent or characteristic landmarks for obtaining registration. ^{5 ,​ 6 ,​ 7} There are several systems that utilize 2D or 3D image guidance for intraoperative assistance. The 2D navigation systems are similar to those used for cranial cases which are registered through point system, fiducials, or another form of tracker designed for placement on the soft tissue such as a face mask. An example of this is the commonly used CranialMask Tracker (Stryker) which is paired with LED-based instrumentation and the compatible software system. This software, like other systems, provide image fusion between image modalities including CT and magnetic resonance imaging (MRI; Fig. 11‑4).

The 3D options require the use of a reference frame and trackers which may be attached to the headrest, bone, or skin. These are often referred to as “real-time” navigation and include the Airo Mobile Intraoperative CT-Based Spinal Navigation (Brainlab, Westchester, IL) system, the Stealth Station Spine Surgery Imaging and Surgical Navigation with O-arm (Medtronic Inc, Minneapolis, MN), Stryker Spinal Navigation with SpineMask and SpineMap Software (Stryker, Kalamazoo, MI), and Ziehm Vision FD Vario 3D with NaviPort (Ziehm Imaging, Orlando, FL). ⁸

Both 2D and 3D image guidance system and intraoperative CT scanning systems have anatomical reference clamps, pins, or surface attachments that require placement at various points before or after exposure. The most significant challenge with using 3D image guidance navigation is ensuring that the dynamic reference frame remains fixed during the case. ⁹ In cases of the anterior cervical spine approaches including transoral, it is difficult to find a bony attachment and novel approaches have been discussed including attachment to the forehead. ⁹ In most cases, the 2D systems are adequate for achieving the goals of both accuracy and minimal exposure.