10 Atlantoaxial Fixation Using Lateral Mass Plate and Screws
The treatment of patients with atlantoaxial dislocation is a surgical challenge, and achieving a successful outcome for these patients is gratifying. The complications of surgery, however, are potentially lethal.
The techniques of fixation and stabilization of the craniovertebral region evolved during the 20th century as the anatomy and the biomechanical properties were evaluated and understood. Various methods of fixation have been described and used successfully in the treatment of atlanto-axial instability. In 1988, we described an alternative plate and screw technique of fixation of the lateral masses of the atlas and axis vertebrae ( Fig. 10.1 ).1,2 We also described simultaneously a technique of occipitocervical fixation, in which the occipital fixation was done with screws, and the cervical end of the fixation was done with either a C2 lateral mass screw alone or in addition to C1 lateral mass screws, as shown in Fig. 10.2. Our technique is gaining wide acceptance and is being used by most large units in which patients with craniovertebral anomalies are treated, although Magerl’s technique,3 which combines interspinous wiring with transarticular screw fixation and midline fixation techniques, are still used. Several clinical and bio-mechanical studies have been performed, and the lateral mass plate and screw fixation technique as discussed by us has been uniformly identified to be safer and stronger when compared with other similar fixation techniques. Recently, some authors have modified our technique and have recommended polyaxial screws instead of monoaxial screws, as well as rods instead of a plate.
Our method uses fixation with a plate and direct implantation of screws in the lateral masses of both the atlas and the axis. Fixation of the subaxial spine has been achieved with the use of similar interarticular plates and screws. The lateral masses of the atlas and axis are considerably larger and stronger than any other lateral masses of vertebrae in the rest of the spine and can be used effectively. Firm, multidirectional stabilization is possible with the use of four screws in addition to plates. With our experience now exceeding 650 cases, we are convinced that our technique of atlantoaxial fixation is biomechanically strong, technically easier, safer for the neural structures, and results in remarkable clinical and radiological improvement.
Indications
All cases having atlantoaxial instability can be suitable for operation by this technique. Although pathology and deformities of bones in the region may sometimes make the operation a little difficult, considering the remarkable stability that it provides, an attempt can always be made to perform this technique. The procedure can be performed safely even in the presence of torticollis and/or assimilation of the atlas. This technique can be used in cases with “fixed” and rotatory dislocation and in group A basilar invagination.
Contraindications
There are no specific contraindications to the performance of lateral mass plate and screw fixation, as long as the lateral masses of the atlas and axis are normal. We have observed that such a fixation is possible even in cases where there is lateral mass destruction or erosion or in cases where there is significant osteoporosis. Abnormal course of the vertebral artery within the facet of the axis and in the vicinity of the posterior arch of the atlas may rarely preclude the use of our technique.
Operative Technique
Traction and Positioning
Cervical traction is given prior to induction of anesthesia, and the weights are progressively increased to approximately one fifth of the total body weight, or 3 to 7 kg. The patient is placed prone with the head end of the table elevated to ~35° ( Fig. 10.3 ). The turning of the patient from the supine position to the operative prone position is done carefully by firmly stabilizing the neck by both hands and under continuous traction. This is because any abnormal movement of the neck under anesthesia, when the neck muscles are relaxed, can critically compromise the cord. 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 toward the direction of the traction, and pressure over the face or eyeballs by the headrest is avoided. Although the head is placed on the headrest, it is essentially “floating,” as the traction pulls the head away from the headrest. Elevation of the head end of the table, which acts as a countertraction, helps in reducing venous engorgement in the operative field. The body parts are adequately protected by the use of soft and firm cushioning pads placed on the operating table. Particular attention is given to avoid undue pressure over the penis, testicles, and breasts.
Incision and Dissection
The suboccipital region and the upper cervical spine are exposed through an ~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 C2 laminae are widely exposed subperiosteally, and the dissection is followed laterally over the pedicles. Use of an operating microscope facilitates the dissection and adds remarkable safety and precision to the entire surgical procedure. Actual vertebral artery exposure is unnecessary either lateral to the pars of the axis or superior to the arch of the atlas.
Sectioning of the C2 Ganglion
The C2 ganglion is placed transversely over the atlanto-axial joints. The large ganglion is widely exposed, then sectioned and resected. The ganglion is closely related to the vertebral artery on its lateral aspect, and all dissection in the region must be done under direct vision. On some occasions, the ganglion can even be mobilized superiorly or inferiorly, and sectioning can be avoided. However, sectioning of the ganglion provides a wide panoramic exposure of the lateral masses of the atlas and axis, along with the atlantoaxial joint region. The consequence of numbness related to the ganglion sectioning is marginal and easily tolerated by the patient ( Fig. 10.4 ).
Venous Bleeding
Large venous plexuses and sinuses in the region and in the extradural space can result in troublesome bleeding and on some occasions can be a cause of significant difficulty during the surgical procedure. Excessive coagulation in the region can easily be avoided. A relatively quick and sharp dissection in the region is helpful in minimizing exposure in the region and in reducing overall blood loss. Packing of the region with Surgicel and Gelfoam can assist in the control of venous bleeding. The packing is generally required in the extradural space of the spinal cord and in the region lateral to the ganglion. As packing can control the bleeding, it is sometimes helpful to pack the region and change the side of the dissection and return after a period of time. Such a strategy can assist in reducing blood loss.
Joint Opening, Distraction, and Graft Insertion
The joint capsule is cut sharply, and the articular surfaces of the joints are exposed. The articular surfaces are distracted with the help of varying sizes of osteotomes. The osteotome is introduced into the joint with its flat end and is then turned 90° to effect distraction. The articular end plate cartilage is denuded 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 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 facet bones ( Fig. 10.5 ). Drilling also helps in placing the plate with ease, in reducing the size of the plate, and in placing the screw more superiorly and almost directly into the lateral mass of the axis.
Hardware Insertion
Metal screws are implanted into the drilled guide holes in the lateral mass of the atlas and axis through a two-holed adequately selected and shaped double-compression stainless steel or titanium plate, ~2 cm in length.
Screw Insertion into the Atlas
First, a screw is placed into the facet of the atlas. It is directed at an angle of ~15° medial to the sagittal plane and 15° 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 posterior or inferior surface of the lateral aspect of the posterior arch of the atlas in relation to its lateral mass can provide additional space for placement of the plate and screw. The screw may even be implanted from the articular surface of the lateral mass of the atlas. Such a site is useful more frequently in children. We have also placed the screw directly into the posterior arch of the atlas that traversed into the facet. Elevation of the vertebral artery off the arch of the atlas may or may not be required for screw implantation.
Screw Insertion into the Axis
Screw implantation in the axis needs precise direction of insertion, because of the intimacy of the vertebral artery. As discussed in an earlier article on this subject,4 the pars interarticularis can be divided into nine quadrants ( Fig. 10.6 ).5,6 The superior and medial compartment can be used for the interarticular technique of screw implantation. The direction of screw implantation must be sharply me-dial and superior and should be toward the tubercle of the anterior arch of the atlas located in the midline. The medial surface of the pars of the axis is identified before implantation of the screw. The screw is directed at an angle ~25° medial to the sagittal plane and 15° superior to the axial plane. The angle of screw implantation 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 usually good, providing an excellent purchase of the screw.
Implant Specifications
The devices used to cut and mold the small plates are commercially available. Precut and premolded plates are also available. The plates and screws can be of stainless steel or titanium. The screws used in adult patients are 2.9 mm in diameter 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 preoperative radiological studies. The approximate lengths of the atlas screws in adults are 22–26 mm and in children, 18–22 mm. The screws in the atlas and axis are almost similar in 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. Although injury to the carotid artery and pharyngeal wall is possible in such cases, the chances of such injury are extremely low, and the tissues usually get displaced by the advancing screws.
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9 Congenital and Developmental Anomalies of the Craniovertebral Junction
16 Transoral Surgery
12 Occipitocervical Stabilization
20 Intraoperative Neurophysiological Monitoring in Surgery for the Craniovertebral Junction
19 Image-guided Surgery in the Craniovertebral Junction and Upper Cervical Spine
22 Techniques of Occipitocervical Fixation
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