The results of our long-term follow-up of nine patients undergoing EEO with partial preservation of C1 anterior arch showed no need of posterior fusion.
Over the years, different authors have demonstrated the effectiveness of the endoscopic endonasal approach and the lower morbidity compared to the traditional transoral approach [1, 2]. Compared to the nasopalatine line (Kassam line or K line), some recent studies proposed other lines, such as the rhinopalatine line, as more accurate predictors to define the caudal limit of craniocervical lesions that can be reached via EEA [18,19,20].
However, it is currently thought that the surgical approach must be tailored to the unique anatomical characteristics of each patient and the features of the lesion. Drilling the posterior nasal spine, located between the soft and the hard palate, could be useful to widen the route of access to the CVJ, as already reported .
Once the decompression is obtained, the necessity to perform an instrumented fixation of the craniovertebral joints complex has to be reasonably weighed. On the one hand, there is the risk of instability, especially if no bony structures are preserved. For instance, the odontoidectomy followed by the partial or complete excision of different primary and secondary elements of stabilization of a complex and high degree of ROM joints, brings, inevitably, a certain risk of instability of the CVJ [4, 21, 22]. On the other hand, fixation extremely reduces the range of movements (ROMs) of wide movable joints, limiting daily activity like eating. Nevertheless, these limitations are less impacting on the elderly; in fact, in this age group, there is a decrease in mobility of the cervical spine, as demonstrated by several studies [23,24,25].
Indeed, the CVJ is characterized by a great degree of motion compared to the rest of the spine. ROMs in the physiologic range are guaranteed by bony and ligamentous structure, avoiding joint stress and muscular effort to maintain the erect posture. The primary movement of Oc-C1 is flexion–extension of the skull (23° to 24.5°), and an additional 10.1° to 22.4° is provided by C1–C2 joints. Less degree of lateral bending and axial rotation is guaranteed by bony structures. The axial rotation is the primary movement of C1–C2 (23.3° to 38.9°), and other movements are constrained by ligamentous structures, namely the ipsilateral transverse and the contralateral alar ligaments, with support from the joint capsules of the occipitoatlantal and atlantoaxial junctions [26, 27].
Dickman et al. was the first to describe the biomechanical consequences of the endoscopic odontoidectomy performed trans-orally. Seeking the impact of surgery on animal and cadaveric models, they found a significant increase in anteroposterior, lateral and vertical subluxations, promoting the so-called cranial settling; additionally, the odontoidectomy led to unconstrained and hypermobile motion, greatly increasing the susceptibility to clinical instability . Primary and secondary (including the dens, the alar ligaments, and the transverse ligament) stabilizers of the Oc–C1–C2 complex are usually detached or completely removed during surgery. A way to reduce the postoperative risk of spinal instability is to preserve as much normal osseous architecture as possible . It was demonstrated that the complete or partial anterior C1 arch preservation reduces the lateral displacement of the C1 lateral masses. When not preserved, the axial or vertical load—that is the main mode of loading at the CVJ in the upright neutral position—increases the lateral mass offset and the horizontal spreading of C1, causing a movement of the occiput toward C1 (namely cranial settling) . Hence, the complete or partial preservation of the anterior C1 arch, acting as cross-link between lateral masses, may decrease the incidence of postoperative instability, avoiding a posterior fixation. Moreover, capsular ligaments, paraspinal muscle, tectorial membrane, anterior longitudinal ligament, and ligamentum flavum play an important role of second stabilizers, whose action can be negatively affected by the loss of integrity of the C1 ring.
The iatrogenic nature is not the only cause of Oc–C1–C2 complex impairment. For instance, it can be observed in systemic inflammatory state, like rheumatoid arthritis, which most commonly affects the cervical spine . An uncontrolled inflammatory process of the cervical spine could lead to ligaments erosion, and even the unique involvement of the transverse ligament affects the stability of the joint complex, with anterior atlantoaxial subluxation. Because of an increased instability of the joints complex, a posterior instrumentation is indisputable after odontoidectomy with C1 arch complete resection.
Posterior fixation leads to a great limitation in ROMs which have been calculated: In cadaveric models, normal axial rotation, flexion–extension and lateral bending were reduced to 90% after posterior fixation, where occiput-C1 constructs (OCF) mainly limit the flexion–extension and C1 lateral mass screws and C2 pedicle or transarticular screws in atlantoaxial fusion (AAF) limit the axial rotation .
More specifically, a C1–C2 construct with transarticular screws was found to decrease 61.4% in flexion, 82.7% in lateral bending and 94.8% during axial rotation which is less affected when a different type of construct, such as the one that was used in our cases, is implanted [30, 31].
The selection of either OCF or AAF depends on the degree of spine instability.
Again, in patients with rheumatoid arthritis, other structures, both ligamentous and bony elements, could be affected by a degenerating process as well, causing ventral encroaching and spinal cord compression. If, additively, other elements of CVJ (i.e., lateral masses of occipitus and atlas) are progressively deteriorated, an atlantoaxial impact occurs due to a complete collapse of the lateral masses determining craniocervical instability and promoting, occasionally, spine ankylosis . This process leads to a spontaneous stabilization of C1 and C2 and paradoxically reduces the instability, influencing the decision process for posterior instrumentation. In the current study, anterior decompression was never jeopardized by trying to preserve C1 anterior arch; indeed, when decompression was not satisfactory, the anterior C1 arch was sacrificed and then posterior fixation was required.
Both in those patients where a posterior fixation was performed and in those which were spared of a second surgical procedure by preserving C1 arch, the duration of the follow-up is still a matter of debate. Analyzing the literature, the low number of cases reported makes it difficult to reach an agreement. Twenty-eight cases of documented anterior C1 arch integrity without posterior or anterior fixation are currently reported [9, 33,34,35,36,37,38,39,40,41]. Only four studies, along with the current one, reported a follow-up longer than 24 months [9, 37, 40, 41] (Fig. 3).
In the current study, every patient in which a part of C1 anterior arch was preserved has been followed with clinical and radiological evaluation to detect early possible instability (mean FU 61,9 months, range 98–13). So far, no patient needed a delayed posterior instrumentation. This finding strongly supports the strategy of preserving C1 arch in every case where it is possible. Indeed, avoiding the second surgery not only means a shorter anesthesiological time but also removes all the risks connected with the posterior approach leading to a faster postoperative recovery. The drawback from posterior fixations, such as decreased cranio-cervical ROM, increased risk of adjacent segment disease and construct failure are virtually canceled. However, it has been postulated that the area of decompression is greater in case of posterior fixation. However, it can be argued that decompression must be correlated with clinical outcomes more than volume of decompression. Furthermore, an intra-operative CT scan, where it is available, has emerged as useful tool along with the neuronavigation system to actively guide and control the endoscopic odontoidectomy. In this way, it is possible to provide a real-time image guidance and a double-check before closing the rhino-pharyngeal flap, evaluating the extent of bony decompression and possibly pushing C1 arch drill to the maximum safe resection.
It must be noted that in all the non-fixated cases, clinical and radiological follow-up should not be stopped. Even in the longest series, where the follow-up is only about 5 years, there is not enough data that allow the definition of definitive stability .
As described by several authors and reported in this series, the endoscopic endonasal route allows a rapid removal of the orotracheal tube with a prompt resumption of oral feeding. The approach spares the soft and hard palate integrity and avoids the risk of velopharyngeal insufficiency.
In this series, 18 patients were extubated in the immediate postoperative period. 17 patients were able to ingest liquid and fluid from the third post-operative day.
The safety and effectiveness of this approach has been highlighted in different papers. In the review by Morales et al., indications, outcome, and complications of the endoscopic endonasal approach in the treatment of CVJ pathology have been analysed . As they pointed out, in accordance with the complication rate reported for this series, only 1 of 72 patients developed postoperative meningitis, and although there was 18% rate of intraoperative CSF leak, the rate of postoperative CSF leak was only 4.2%, probably because this approach allows the surgeon to perform an effective multilayer dural repair.
Despite the wide spread of endoscopic endonasal approaches in the last 20 years, there are still few centers performing relatively high volume endoscopic endonasal odontoidectomies [42,43,44,45]. Although there are several studies focusing on surgeons’ learning curve in endoscopic endonasal approach, to the authors’ best knowledge, there are no specific studies on EEO learning curve. The main differences regard the use of longer instruments and the extremely caudal working angle. However, there are no differences in the learning curve of the reconstructive surgical steps: On the one hand, the risk for CSF leak in EEO is relatively low considering the extradural nature of the approach, while on the other hand, even if intraoperative CSF leak ensues, the reconstruction should be performed following the same rules of clival reconstruction, preferring the reverse U-shaped rhinopharyngeal flap ever the Hadad nasoseptal flap .
In our experience, the endoscopic endonasal approach, with its shorter surgical working distance, a wide and panoramic view, and the vertical trajectory, promotes the attempt of anterior C1 arch preservation. Still the corridor brings to a very deep field where only few instruments allow good maneuverability.
In the recent years, the use of intraoperative CT scan helped the surgeons to perform a customized, case by case, surgery and to push the limits of C1 drill while preserving its integrity and achieve a complete odontoid resection.
Nevertheless, endoscopic endonasal odontoidectomy remains a challenging procedure even when performed by a well-trained surgeon; therefore, a preoperative selection of suitable cases is paramount.