The congenital or developmental bony anomalies at the CVJ are complex and primarily consist of AAD and BI [1, 18, 19]. These abnormalities may cause compression of the spinal cord or medulla, leading to myelopathy, intractable neck pain, and increased risk of worsening neurologic function due to trauma [18, 20]. The main aims of surgical treatments are relieving the cervicomedullary compression, restoring the stability of the CVJ, and restoring normal cerebrospinal fluid flow [18, 21].
Treatment of AAD with BI is still challenging because of abnormal osseous anatomy, and deformities of the facet joint make the reduction of AAD difficult with cervical traction, even under general anesthesia [20, 21]. After the introduction of the posterior distraction and fixation technique, the surgical treatment algorithm had changed from anterior transoral odontoidectomy followed by posterior instrumentation to posterior surgery only [1, 4, 13, 20, 22]. With intraoperative manipulation, spinal cord compression from the odontoid process could be alleviated by the reduction and distraction of the odontoid away from the brainstem and spinal cord. However, there were still cases with persistent neurological symptoms even after posterior reduction and indirect decompression. Possible reasons for inadequate decompression are as follows: (1) the complex bony abnormality hinders reduction of AAD or/ and BI, (2) severe obliquity of facet joint makes the placement of intrafacet spacer difficult or impossible, or (3) abnormal bony auto-fusion secondary to arthritis makes reduction impossible [6, 9, 20, 22]. Under such circumstances, a second stage transoral odontoidectomy could be considered for patients with residual medulla and cervical spinal cord compression.
The anterior approach through the posterior pharyngeal wall is the anatomically shortest path to the ventral aspect of the craniocervical junction [8, 10, 21]. The transoral odontoidectomy was once considered to be the gold standard for decompression at the craniovertebral junction, and this approach has its obvious advantages, such as providing direct access to the craniovertebral junction and avoiding critical anatomic structures ( e.g., Eustachian tube, internal carotid artery, and pterygoid nerve). In the past few decades, transoral odontoidectomy has proven to be a safe and effective surgical technique with low mortality in patients with irreducible AAD associated with Chiari malformation, displaced odontoid fractures, or BI [21]. In congenital anomalies, extremely abnormal osseous anatomy can make it difficult to confirm the midline of the odontoid process which could lead to incomplete odontoidectomy [22]. In BI/AAD patients, local distorted osseous anatomy could also lead to disorientation and asymmetrical odontoid drilling during the transoral procedure. Several studies have reported the use of navigation in the upper cervical spine to overcome this situation, showing good results and safety [23,24,25]. However, in our series, 3 patients (3/11) had incomplete odontoidectomy even with the help of navigation. Asymmetrical bony drilling toward the dural sac or oblique drilling of the odontoid process may lead to an erroneous estimation of the completeness of the odontoidectomy. In order to address this problem, the intraoperative CT can be a very useful tool to precisely identify the position of the odontoid.
The endoscopic transnasal odontoidectomy, however, only requires an incision in the upper part of the pharynx, and the risk of infection may be less compared with the transoral approach. However, the endoscopic transnasal approach has obvious limitations. The range of exposure to the surgical target area is less, and the surgical instruments can only reach the upper edge of the axis, whereas C3 can be reached through the transoral approach (Fig. 5) [23,24,25]. Moreover, the surgical window of the transnasal approach is longer and narrower, thus making it more difficult in the manipulation of surgical instruments and demanding more skills from the surgeon [26]. Also, suturing the nasopharynx incision through nasal approach can be more difficult [27, 28]. In addition, if a cerebrospinal fluid leak occurs during surgery, the primary closure is very challenging or even impossible.
With regard to the CMA, Wang et al. reported the range of CMA in normal patients ranging from 139.0° to 175.5°, with an average of 158.5° [17]. The CMA is considered to be an effective index to assess the grade of anterior spinal cord compression. In CVJ malformation, especially BI/AAD, the CMA is usually lower than normal [5, 14]. In our series, the mean pre-operative CMA was 119.3° ± 14.2°, the medulla and the spinal cord were severely compressed. After odontoidectomy, the CMA improved to 129.1° ± 11.1° (p < 0.01). Because of the alleviation of ventral bony compression and the reduction of medullary-spinal cord kinking, the patient’s neurological symptoms also improved after surgery.
We previously reported a series of 135 AAD patients with a 3-year follow-up, and we proposed a treatment algorithm [13]. For both reducible and irreducible dislocation patients, direct posterior reduction and fixation procedure would first be attempted. For patients who had no effective decompression after attempted posterior reduction because of anatomic constraints, a postoperative MRI showed that there was still anterior spinal cord compression, and the patient’s clinical symptoms still persisted, a transoral odontoidectomy should be performed.
The application of navigation has been useful in both cranial and spinal surgeries because it provides three-dimensional spatial orientation during surgery [28,29,30]. When dealing with BI and AAD with abnormal anatomy, navigation affords higher precision and accuracy to help with the odontoidectomy. Navigation also decreases the need for intraoperative fluoroscopy, hence decreases the amount of radiation exposure both to the patient and to the operating room staff.
This study had several shortcomings and limitations such as lacking control group, having a relatively small sample size, and single-institution data. Due to the clinical complexity of this pathology, the heterogeneity of this patients population is relatively large, and the treatment options are also relatively complex. In many cases, the initial posterior surgery was performed at different hospitals, and some patients even have undergone multiple surgeries. Although these are limiting factors, this disease entity tends to be rare, especially in patients who have failed posterior distraction and stabilization.
One patient died suddenly of unknown causes 7 days after the surgery while asleep. We speculated that the patient may die of pulmonary embolism or respiratory failure, which could be caused by long-term compression of the medulla oblongata.