Surgical Treatment for Atlantoaxial Instability

41 Surgical Treatment for Atlantoaxial Instability

Abstract

Chronic or longstanding atlantoaxial instability is usually of central or axial variety. In such form of dislocation dynamic imaging does not show abnormal alteration of atlantodental interval and there is no direct neural or dural compression by the odontoid process. Basilar invagination, Chiari malformation, syringomyelia, bone fusions, assimilation of atlas, bifid anterior and posterior arches of atlas, os odontoideum, platybasia, and several such musculoskeletal and neural alterations are secondary natural protective processes to primary atlantoaxial instability. All these alterations when present in a cohort or in isolation indicate the presence of atlantoaxial instability and guide the further treatment.

Keywords: atlantoaxial instability, basilar invagination, chiari formation, syringomyelia

41.1 Introduction

The last few decades have witnessed remarkable advances in the understanding of the craniovertebral junction instability related issues. Surgery on craniovertebral junction needs highest level of understanding of anatomical issues and radiological subtleties in addition to biomechanical information and technical expertise. While successful surgery can be a gratifying surgical procedure, any complication can be unforgiving and can severely threaten the function and life of the patient.

Craniovertebral junction is a complex of occipitoatlantal and atlantoaxial joints. It is supremely designed and immaculately structured to cater for both stability and for mobility and to provide for smooth and unrestricted traverse to the most critical neural and vascular structures. Occipitoatlantal joint is the most stable and atlantoaxial joint is the most mobile joint of the body. Atlantoaxial joint is specially fashioned to provide for an unrestricted circumferential movement. The articular surfaces are flat and round unlike ball and socket and hinge joints. Although the joint is most mobile, it is most prone for instability. It may not be incorrect to state that the term craniovertebral junction instability is synonymous to atlantoaxial instability. In the same tone the term craniovertebral junction stabilization refers to atlantoaxial stabilization. This is because, due to the strength of its ligaments, occipitoatlantal instability is extremely uncommon and is rarely identified in cases with high intensity trauma or in cases with syndromic multiple joint affections that are usually identified in pediatric population. Advancements in the understanding of the biomechanics of the region have disclosed that the incidence of atlantoaxial dislocation is much more than what is usually understood. The scope of treatment has thus expanded.

41.2 Atlantoaxial Dislocation

Atlantoaxial dislocation can be of mobile and reducible type or fixed or irreducible type. The conventional parameter that has been used to diagnose atlantoaxial instability is an increase in atlantodental interval that can be seen on dynamic plain radiographs or on CT scan and MRI. In the mobile variety the atlantodental interval increases on flexion of the head and reduces on extension of the head 1 ^, ² (Fig. 41.1). This parameter has been the sole measure to identify atlantoaxial dislocation for over 50 years. The most important advance in the understanding of the craniovertebral junction has been the identification of the fact that the entity of fixed or irreducible atlantoaxial dislocation is almost nonexistent³ (Fig. 41.2). The joint in such cases is not only mobile but it is pathologically hypermobile and more importantly, the dislocation can be reduced by manual manipulations. This understanding has led to the treatment by joint manipulation, craniovertebral junction, or facetal realignment and subsequent fixation. The treatment that was based on the understanding that the dislocation is of fixed nature and was focused on bone resection by anterior transoral or by posterior foramen magnum decompression has now changed to joint reduction and stabilization.

Fig. 41.1Images of a 46-year-old male patient. ( a ) CT scan with the head in flexed position showing atlantoaxial instability. (b) CT scan with the head in extended position showing reduction of atlantoaxial dislocation. (c) T2-weighted MRI showing atlantoaxial dislocation and cord compression. (d) Postoperative scan showing reduction of the atlantoaxial dislocation. (e) Postoperative CT scan with the cut through the facets showing the plate and screws in the lateral mass of atlas and axis.

Fig. 41.2Images of a 32-year-old female patient. (a) MRI showing cord compression at the craniovertebral junction. (b) CT scan with the head in flexion showing atlantoaxial instability. (c) CT scan with the head in extension showing incomplete reduction of the dislocation. (d) Postoperative image showing reduction and fixation of atlantoaxial instability. (e) Sagittal cut passing through the facets showing the implant.

Atlantoaxial dislocation can be of vertical variety, wherein on flexion of the head the odontoid process travels abnormally superiorly, and returns to position on head extension⁴ (Fig. 41.3). Vertical atlantoaxial dislocation is related to incompetence of the atlantoaxial joint. Basilar invagination is a result of vertical dislocation of the odontoid process.

Fig. 41.3Images of a 28-year-old male patient. (a) T2-weighted MRI shows basilar invagination, atlantoaxial instability, Chiari formation, and syringomyelia. (b) CT scan with the head in flexion shows basilar invagination, assimilation of atlas, and anteroposterior atlantoaxial dislocation. (c) CT scan with head in extension shows mild reduction of vertical atlantoaxial instability. (d) CT scan with the cut through the facets shows Goel Type 1 atlantoaxial facetal instability. (e) Postoperative CT scan shows fixation in reduced position. (f) Postoperative CT scan showing the implant.

41.3 Basilar Invagination

In the year 2004, we identified a subgroup of patients having basilar invagination where there was radiological evidence of instability of the region that was manifested by distancing of the odontoid process away from the anterior arch of the atlas/clivus or the atlantodental or clivodental interval was abnormally increased.⁵ We labeled such a subgroup of patients as having Group A basilar invagination (Fig. 41.4). The radiological findings suggested that the odontoid process in Group A patients resulted in direct compression of the brainstem. In this group, the atlantoaxial joints were “active” and their orientation was oblique instead of the normally found horizontal orientation. Similarities of such a position of the C1–C2 facets with the lie of vertebral bodies as observed in spondylolisthesis of the lumbosacral spine can be observed.⁶ The atlantoaxial joint in such cases is in an abnormal position and progressive worsening of the dislocation is probably secondary to increasing “slippage” of the facets of atlas over the facets of axis. Essentially, it was identified that Group A basilar invagination was a consequence of atlantoaxial instability. As Group A basilar invagination was considered to be related to instability, fixation of the atlantoaxial joint and craniovertebral junction realignment were proposed as treatment. We introduced the concept of reduction of basilar invagination by atlantoaxial facetal distraction and direct atlantoaxial fixation. Transoral decompression of the odontoid process was considered to be a suboptimal form of treatment.

Fig. 41.4Images of a 26-year-old male patient. (a) T2-weighted MRI showing Group A3 basilar invagination. Internal and external syringomyelia and external syringobulbia can be observed. (b) CT scan with the head in flexed position showing basilar invagination. Assimilation of atlas and C2–C3 fusion can be seen. (c) CT scan cut passing through the facets showing facetal malalignment. (d) CT scan with the head in extended position showing mild vertical reduction of basilar invagination. (e) Postoperative CT scan showing reduction of basilar invagination. (f) Postoperative CT scan cut passing through facets showing the implant.

In Group B basilar invagination, the entire craniovertebral junction complex was rostrally positioned (Fig. 41.5). As per the understanding in 2004, the pathogenesis appeared to be congenital dysgenesis of the region.⁵ The atlantoaxial joint was considered to be stable or fixed and instability was not identified to be an issue in this group of patients. Foramen magnum decompression was described as treatment for Group B basilar invagination, as small posterior cranial fossa volume was identified to be the pathological issue in this group.

Fig. 41.5Images of a 27-year-old male patient. (a) T2-weighted sagittal MRI showing Group B3 basilar invagination, Chiari formation, syringomyelia, and external syringomyelia. (b) CT scan showing Group B basilar invagination. (c) CT scan cut passing through the atlantoaxial facets showing Type 2 atlantoaxial dislocation. (d) Postoperative CT scan showing atlantoaxial fixation. (e) Postoperative CT scan sagittal image with cut passing through the facets showing atlantoaxial fixation implants. (f) Postoperative MRI 6 months after surgery showing reduction of syrinx cavity.

In the year 2015, we identified that atlantoaxial instability is the nodal point of pathogenesis of all types of basilar invagination, including the cases with Group B basilar invagination.7 The structural malformations in basilar invagination are based on the nature of atlantoaxial instability. Atlantoaxial instability was identified by facetal malalignments (on sagittal imaging with the head in neutral position) to be of three types.⁸ ^, ⁹ In Type 1 facetal instability, wherein the facet of atlas is displaced anterior to the facet of axis (more frequently identified with Group A basilar invagination), the basilar invagination is usually identified in younger patients and is associated with more acute clinical symptoms. The odontoid process is displaced posteriorly and compresses the neural structures. In basilar invagination related to Type 2 facetal instability, the facet of atlas is displaced posterior to the facet of axis. In basilar invagination related to Type 3 facetal instability, the facets of atlas and axis are in alignment and only direct bone handling and facetal manipulation can identify instability during surgery. Basilar invagination with Type 2 or Type 3 facetal instability is more frequently identified with Group B basilar invagination. Atlantodental interval, a classically described parameter to determine atlantoaxial instability, may not be affected in both Type 2 and Type 3 facetal instabilities. Accordingly, Type 2 and Type 3 facetal instabilities are labeled as central or axial atlantoaxial instability. Types 2 and 3 instability related basilar invagination is identified in relatively older patients and is associated with more chronic or long-standing structural malformations.

Musculoskeletal changes that include short neck, torticollis, platybasia, Klippel-Feil abnormality, and neural alterations like Chiari 1 malformation and syringomyelia are frequently identified in cases with basilar invagination that is associated with long-standing atlantoaxial instability that is usually associated with Type 2 and Type 3 atlantoaxial facetal instabilities.10 On the basis of our understanding, it was identified that atlantoaxial fixation formed the basis of treatment in all types of basilar invagination. An attempt for craniovertebral realignment could be made in cases with Type 1 facetal instability, wherein odontoid process invaginates into the neural structures. Decompression of bone either by transoral route or by posterior foramen magnum decompression is not necessary. Stabilization of an unstable atlantoaxial joint is the treatment.

41.4 Chiari 1 Malformation and Syringomyelia

It was recently identified that Chiari 1 malformation with or without the presence of basilar invagination or any other bone anomaly in the region of the craniovertebral junction may not be a primary pathology but a secondary and protective natural response to atlantoaxial instability. Chiari 1 malformation simulates an air-bag and is placed in position by Nature in presence of manifest or potential atlantoaxial instability to provide a soft cushion for the critical neural structures and protect them from direct pinching between bones.11 ^, ¹² The atlantoaxial instability can be identified by facetal malalignment (Types 1 and 2). Even in the absence of facetal malalignment (Type 3), presence of Chiari 1 in itself is an indicator of atlantoaxial instability. The concept has therapeutic relevance and suggests the need for atlantoaxial fixation in these cases, rather than the conventional surgery of foramen magnum decompression (Fig. 41.6).