Cervical traction is set up prior to induction of anesthesia and the weights are progressively increased to approximately 5 to 8 kg or one sixth of the total body weight. The patient is placed prone with the head end of the table elevated to about 35 degrees (see Fig. 29.2). Cervical traction stabilizes the head in an optimally reduced extension position and prevents any rotation. The traction also ensures that the weight of the head is directed superiorly toward the direction of the traction and pressure over the face or eyeball by the headrest is avoided. The head is in a “hanging” position, the headrest being placed only for additional or minimal support. Elevation of the head end of the table, which acts as a countertraction, helps in reducing venous engorgement in the operative field. The suboccipital region and the upper cervical spine are exposed through an approximately 8-cm longitudinal midline skin incision centered on the spinous process of the axis. The spinous process of the axis is identified, and the attachment of paraspinal muscles to it is sharply sectioned. The large second cervical ganglion is closely related to the vertebral artery on its lateral aspect (see Fig. 29.3). It is first exposed widely and then sectioned sharply. This procedure provides a wide exposure of the lateral masses of the atlas and axis. Bleeding from the large venous sinuses in the region and in the extradural space can be troublesome. Packing of the region with Surgicel and Gelfoam can assist in the control of venous bleeding. The joint capsule is cut sharply, and the articular surfaces of the joint are exposed. The adjacent synovial articular surfaces of the atlantoaxial joint are decorticated widely with a microdrill, and pieces of bone harvested from the iliac crest are stuffed into the joint space. The lateral aspect of the lamina and a part of the pars of the axis are drilled to make the posterior surface of the lateral mass of the axis relatively flat so that the metal plate can be placed snugly and parallel to the bone. Drilling also helps in reducing the length of the plate and in placing the screw more superiorly and almost directly into the lateral mass of the axis. Actual vertebral artery exposure is unnecessary either lateral to the pars of the axis or superior to the arch of the atlas.

FIGURE 29.1 A, This drawing shows the Goel technique of atlantoxial fixation using plates and screws. The atlantoaxial joint is opened, articular cartilage is widely denuded, and the joint space is filled with bone graft. B, In the occipitocervical fixation, the occipital end of the plate is fixed with the help of screws. The cervical end of the plate is fixed with lateral mass of C2 alone or of both C1 and C2.

FIGURE 29.2 Position of the patient under cervical traction. The head end of the table is elevated by approximately 35 degrees.

FIGURE 29.3 Drawing showing the relationship of the C2 ganglion, vertebral artery, and atlantoaxial joint.

FIGURE 29.4 In the “double insurance” method of fixation the screw in C2 is placed in a transarticular method of fixation.

FIGURE 29.5 A, Lateral radiograph shows mild basilar invagination and atlantoaxial dislocation. The pathological change is related to degenerative arthritis of the atlantoaxial joint. B, Postoperative computed tomography (CT) scan shows reduction of the atlantoaxial dislocation. C, CT scan reconstruction image through the atlantoaxial joint shows the “double insurance” plate and screw fixation.

FIGURE 29.6 A, Computed tomography (CT) scan with the head in flexed position shows atlantoaxial dislocation. B, CT scan with the head extended shows reduction of the dislocation. C, Coronal image of CT scan shows spiked spacers within the atlantoaxial joint. D, Sagittal image shows the spacer within the joint. E, Lateral radiograph shows the atlantoaxial fixation using spacers.

Screws are implanted into the previously created guide holes in the lateral mass of the atlas and axis through a two- holed (approximately 2 cm in length) metal (stainless steel or titanium) plate. First, a screw is placed into the atlas. It is directed at an angle of approximately 15 degrees medial to the sagittal plane and 15 degrees superior to the axial plane. The preferred site of screw insertion is at the center of the posterior surface of the lateral mass, 1 to 2 mm above the articular surface. Whenever necessary, careful drilling of the inferior surface of the lateral aspect of the posterior arch of the atlas in relation to its lateral mass can provide additional space for the placement of the plate and screw implantation. The screw may even be implanted by choosing an insertion point on the articular surface of the lateral mass of the atlas. Such a site is useful more frequently in children than in adult patients. Screws can also be implanted into the facet of the atlas through the lateral aspect of the posterior arch of the atlas. Owing to the intimacy of vertebral artery relationships, screw implantation in the axis is relatively unsafe and needs to be precise. It was observed that the screw implantation in the superior facet of the C2 vertebra has to be sharply medial and directed toward the anterior tubercle of the C1 for transarticular fixation and toward the vertebral body of C2 for interarticular fixation. The pars interarticularis can be divided into nine quadrants;⁹ the superior and medial compartments can be used for the interarticular technique of screw implantation. Neuronavigation can be of assistance in identifying the best and the safest trajectory of the screw. The direction of screw implantation must be sharply medial and superior and should be toward the superior aspect of the body of the axis vertebra toward the midline. The medial surface of the pedicle of the axis is identified before the implantation of the screw. The screw is directed at an angle approximately 25 degrees medial to the sagittal plane and 15 degrees superior to the axial plane. The angle of screw insertion varies, depending on the local anatomy and the size of the bones. The quality of cancellous bone in the lateral masses of the atlas and axis in the proposed trajectory of screw implantation is generally good, providing an excellent purchase of the screw, and avoids the vertebral artery. The screws used are 2.9 mm in diameter in adult patients and 2.7 mm in diameter in pediatric patients. The length of the required screw is calculated on the basis of the size of the lateral masses observed on the preoperative radiological studies. The approximate lengths of the atlas screws are 26 to 28 mm in adults and 22 to 26 mm in children. The screws in the atlas and axis are almost similar in their length. The lateral masses of the atlas and axis are firm and cortical in nature, and, although preferable, it is not mandatory that the screws engage both the posterior and anterior cortices. If the screw traverses beyond the anterior cortex, it will lie harmlessly in the anteriorly displaced soft tissue. Injury to the critical neurovascular and pharyngeal wall is possible in such a situation but is extremely rare. Intraoperative fluoroscopic control and navigation were found to be helpful but not essential in determining the state of the screws. Large pieces of corticocancellous bone graft from the iliac bone are then placed over the adequately prepared posterior elements of atlas and axis. After the wound is closed, cervical traction is discontinued. The patients are mobilized as soon as possible and advised to wear a hard cervical collar for 3 months.

Basilar Invagination

Basilar invagination forms a prominent component of the craniovertebral anomalies. Chiari malformation and syringomyelia are commonly associated with basilar invagination and are the soft tissue components of the dysgenesis. Plain radiological and tomographic parameters have been principally used to diagnose basilar invagination for many years. There has been a renewed interest in the normal anatomy and pathological lesions of the craniovertebral junction with the development of imaging by high-resolution CT scan and MRI. Dynamic MRI and CT scans have helped in the evaluation of the pathology of basilar invagination, in the assessment of biomechanics of the joints, and in the formulation of a rationale for the surgical strategy. Despite the clarity of imaging, controversy regarding the management of basilar invagination continues. Even the natural history has not been clearly elucidated in the literature. The surgical indications for a given approach together with the timing of the surgical stages are still under discussion.

Pathogenesis

Several theories have been suggested to elucidate the probable cause and origin of basilar invagination. Most of these theories point toward embryological dysgenesis, genetic abnormalities, and viral infections.¹³^–¹⁶ Several authors, for over a century, have thought that deformation had a mechanical cause and therefore applied the name: “impressio baseos cranii” (Berg and Retzius, 1855, cited by Virchow, 1876),¹⁷ or basilar impression.¹⁸ Grawitz (1880)¹⁹ believed that basilar invagination was often a result of under- or maldevelopment of the craniovertebral transition region. A range of malformations is frequently associated with anomalies of the atlas and axis, some of which may be quite atypical, and with fissures or defects and bone projections from the spinal column in the craniovertebral transition zone. The latter anomalies were grouped together by von Torklus and Gehle (1970) as suboccipital dysplasias.¹⁸ Basilar invagination can be secondary to abnormally inclined alignment of the facets of the atlas and axis.²⁰ The progressive slippage of the atlas over the axis secondary to this malalignment, a process similar to spondylolisthesis in the lumbosacral spine, results in invagination of the odontoid process into the craniocervical cord.^20,²¹ Short neck, low hairline, web-shaped neck muscles, torticollis, reduction in the range of neck movements, and several such physical variations have been described as hallmarks of basilar invagination for over a century. A number of bone fusion deformities and platybasia have also been recorded. Neck pain, muscle spasms, and restriction of neck movements are frequently noted and suggest instability of the region. Degeneration of the atlantoaxial joint and primary or secondary destruction of the facets of atlas or axis can lead to superior and posterior migration of the odontoid process and result in basilar invagination and fixed or irreducible atlantoaxial dislocation. Such involvement of the lateral masses can result in laxity of the posterior atlantoaxial or retro-odontoid ligaments that result in retro-odontoid pannus or “tumor-like” or “osteophyte-like” formation.