27 Basilar Invagination: Role of Craniovertebral Realignment on the Basis of a New Classification

10.1055/b-0034-81404

27 Basilar Invagination: Role of Craniovertebral Realignment on the Basis of a New Classification

Goel, Atul

In Chapter 26, we discussed the subject of basilar invagi-nation and presented a classification for these anomalies based on the presence or absence of Chiari I malformation.1,2 On the basis of the possible pathogenetic factors, we suggested a specific treatment protocol for each of the two groups. With our improved understanding of the subject, we reclassified basilar invagination into two groups (groups A and B) based on parameters that determined an alternative treatment strategy. In group A, there is a “fixed” atlantoaxial dislocation, and the tip of the odontoid process invaginates into the foramen magnum and above the Chamberlain line,3 the McRae line of the foramen magnum,4 and the Wackenheim clival line.5 The definition of basilar invagination, or prolapse of the cervical spine into the base of the skull, as suggested by von Torklus and Gehle,6 is suitable for this group of patients ( Fig. 27.1 ). In group B, alignment of the odontoid process and clivus remains normal despite the presence of basilar invagination and other associated anomalies. In this group, the tip of the odontoid process is above the Chamberlain line but below the McRae and Wackenheim lines. Radiological findings suggest that the odontoid process in group A patients results in direct compression of the brainstem. Analysis on the basis of the Chamberlain line and on the distance from the odontoid tip to the pontomedullary junction shows that basilar invagination is only mild or moderate in these cases. Modified omega angle measurements suggest that the odontoid process tilts horizontally rather than rostrally.1 Despite the clinical evidence of instability of the craniovertebral joint, abnormal atlantoaxial mobility can be identified on dynamic radiology only in a minority of cases. Essentially, in group A basilar invagination, there is an element of instability of the region that is manifested by the tip of the odontoid process distancing itself from the anterior arch of the atlas or the lower end of the clivus. Some group A patients have a Chiari malformation, and this feature differentiates the present classification from the earlier classification. In this group, the atlantoaxial joints are “active,” and their orientation is oblique, as shown in Fig. 27.2a , instead of the normal horizontal orientation. We have found similarities in C1–C2 facets with spondylolisthesis seen in the subaxial spine ( Fig. 27.2b ). It appears to us that the atlantoaxial joint in such cases is in an abnormal position as a result of a mechanical problem rather than a congenital abnormality of the bones, and progressive worsening of the dislocation is probably secondary to increasing “slippage” of the facets of the atlas over the facets of the axis. The slip of the atlas over the axis appears to be accentuated by trauma. With our experience in handling atlantoaxial joints, we have realized that the joint in these cases is not “fixed” or “fused” but is mobile, and in some cases hypermobile, and is probably the prime cause for basilar invagination. The history of trauma preceding the clinical events, the predominant complaint of pain in the neck, and the improvement in neurological symptoms following institution of cervical traction suggest “vertical” instability of the craniovertebral region. In group B, the atlantoaxial joints are either fused or normally aligned ( Fig. 27.3 ).

**Fig. 27.1a, b** a T2-weighted magnetic resonance imaging (MRI) showing indentation of the odontoid process into the brainstem. There is group A basilar invagination. b Computed tomography (CT) scan showing group A basilar invagination.

**Fig. 27.2a, b** a CT scan showing spondylolisthesis of the C1 facet over the C2 facet. This is probably the cause of basilar invagination. b Illustration showing L5 over S1 spondylolisthesis.

**Fig. 27.3a, b** a T2-weighted MRI showing invagination of the odontoid along with the clivus. The alignment of the odontoid tip with the clivus is not changed. b T1-weighted MRI showing group B basilar invagination.

**Fig. 27.4a, b** a Illustration showing the plates and screws used for atlantoaxial fixation. b Photo showing a variety of titanium spacers used for distraction.

We recently studied the treatment of group A basilar invagination and discussed the feasibility of manipulation and distraction of the facets of the atlas and axis, as well as reduction of the basilar invagination and fixation of the atlantoaxial joint.7–10 Our current experience with the technique in over 200 cases makes us convinced that distraction and direct lateral mass fixation of the atlantoaxial joint is the ideal form of treatment in group A basilar invagination, and transoral surgery can be avoided entirely. The technique results in realignment of the facets into a horizontal position and in realignment of the entire craniovertebral junction ( Fig. 27.4 ).

In our series, the majority of patients with group A basilar invagination (58%) had a history of minor to major head injury prior to the onset of the symptoms. The pyramidal symptoms formed a dominant component. Kinesthetic sensations were affected in 55% of cases. Spinothalamic dysfunction was less frequent (36%). Neck pain as a major presenting symptom was seen in 77% of cases. Torticollis was present in 41% of cases. The analysis of radiological and clinical features suggests that the symptoms and signs were a result of brainstem compression by the odontoid process.

Craniovertebral Realignment for Group A Basilar Invagination

The standard form of treatment of group A basilar invagi-nation is transoral decompression.1,7,11 Most authors recommend a posterior occipitocervical fixation following the anterior decompression.

Earlier, we had attempted to reduce basilar invagination by performing occipitocervical fixation following institution of cervical traction.1,11 However, all four patients in this series who were treated in this manner subsequently needed transoral decompression, as the reduction of the basilar invagination and of atlantoaxial dislocation could not be sustained by the implant. Wide removal of the atlantoaxial joint capsule and the articular cartilage by drilling and subsequent distraction of the joint by manual manipulation provided a unique opportunity to obtain reduction of the basilar invagination and atlantoaxial dislocation. Maintenance of the joint in a distracted and reduced position with the help of bone graft with or without hydroxyapatite or metal spacers and subsequent fixation of the joint with interarticular screws and a metal plate provided a biomechanically firm fixation and sustained distraction. Holes in the titanium metal spacer provided space for bone fusion. The fixation was seen to be strong enough to sustain the vertical, transverse, and rotatory strains of the most mobile region of the spine.

Surgical Experience and Technique

All patients in our series underwent joint manipulation surgery ( Figs. 27.5, 27.6, 27.7, 27.8, 27.9, and 27.10 ). No patient underwent a transoral decompression as a first stage operation. The basic surgical steps of the joint manipulation surgery are the same as discussed in our papers on lateral mass plate and screw fixation of the atlantoaxial joint. The exposure of the atlantoaxial joint in cases with basilar invagination is significantly more difficult and technically challenging when compared with a normally aligned atlantoaxial joint encountered during the treatment of posttraumatic instability. The joint is significantly rostral in location and the microscope needs to be appropriately angled. Due to bony abnormalities of the region and frequently encountered rotation, the orientation can easily be lost. Cervical traction is given prior to induction of anesthesia and the weights are progressively increased to approximately one fifth of the total body weight. The patient is placed prone, with the head end of the table elevated to ~35°. Use of neuronavigation assistance facilitated the dissection and added safety to screw implantation.

**Fig. 27.5a–f** a Photo of a wheelchair-bound 15-year-old boy. The patient’s neck is remarkably reduced in size. b Axial CT scan showing the occipital bone riding up to the level of the petrous bone. c Preoperative T1-weighted MRI showing basilar invagination, fixed atlantoaxial dislocation, and indentation of the brainstem. d Preoperative CT scan showing marked basilar invagination and fixed atlantoaxial dislocation. e Postoperative CT scan showing reduction of the basilar invagi-nation and atlantoaxial dislocation. Note the change in the bony alignments of the craniovertebral junction. The alteration in the relationship of the anterior arch of the atlas and the clivus to the C2 body and odontoid process can be appreciated. Metal artifacts can be observed. f Picture taken 4 months after surgery. The child can now stand with support.

**Fig. 27.6a, b** a Preoperative CT scan showing the C2 body and odontoid and its relationship with the anterior arch. b Postoperative scan after distraction surgery. The alignment of the craniovertebral junction is now nearly normal.

**Fig. 27.7a–d** a CT scan of a 16-year-old boy showing basilar invagination and fixed atlantoaxial dislocation. b CT scan showing the oblique alignment of the atlantoaxial joint. c Postoperative CT scan showing the changes in the alignment of the craniovertebral region. Note the changes in the relationship of the odontoid process with the clivus. d Postoperative CT scan through the atlantoaxial joint. Note the partial realignment of the facets and the spacer.

**Fig. 27.8a, b** a Preoperative CT scan showing marked basilar invagination. b Postoperative CT scan showing significant realignment.

**Fig. 27.9a–d** a Preoperative CT scan showing basilar invagination, fixed atlantoaxial dislocation, and assimilation of the atlas. b Postoperative CT scan showing realignment of the craniovertebral junction. c CT scan showing oblique angulation of the facets of the atlas and axis. d Postoperative CT scan showing realignment of the facets, fixation with plates and screws, and distraction with the help of spacers.

**Fig. 27.10a–d** a CT scan showing basilar invagination. b CT scan with sagittal cut passing through the joint showing its inclined orientation. c Postoperative CT scan showing realignment of the craniovertebral junction. d Postoperative sagittal section of the CT scan showing the spacer. Distraction of the joint and its realignment can be observed.

The atlantoaxial facet joints are widely exposed on both sides after sectioning of the large C2 ganglion. The exposure of the facet of the atlas was significantly difficult as in several patients there was an assimilation of the atlas resulting in a rostrally located C1 facet. The joint capsule is excised and the articular cartilage is widely removed using microdrill. The joint on both sides is distracted using an osteotome. The flat edge of the osteotome is introduced into the joint and it is then turned vertical to effect distraction. The status of the dislocation and of basilar invagination is evaluated by intraoperative radiographic control. Corticocancellous bone graft harvested from the iliac crest is stuffed into the joint in small pieces. Specially designed titanium spacer or hydroxyapatite blocks are used in selected cases as strut graft and stuffed into the joints to provide additional distraction and stability. Plate and screw fixation of the region is subsequently performed by the interarticular technique. A two-holed stainless steel plate is used measuring 15 to 20 mm in length. The screws are 2.4 to 2.8 mm in diameter and measure 20 to 26 mm in length. Screws are passed bilaterally through the holes in the plate into the lateral mass of the atlas and axis ( Figs. 27.5, 27.6, 27.7, 27.8, and 27.9 ). In cases where lateral mass plating cannot be completed because of anatomical or surgical/technical limitations, a C1–C2 transarticular screw fixation method described by Grob and Magerl15 or an occipitoaxial fixation11,14 can be done. The point of entry and the direction of the screw for the transarticular fixation are altered to suit the complex local anatomy in these cases. Additional bone graft is placed between the posterior elements of C1 and C2 after decorticating the host bone area with a burr. Postoperatively, the traction is discontinued, and the patient is placed in a four-post hard cervical collar for 3 months, with all physical activities involving the neck restrained during this period.