Stabilization of the Craniocervical Junction Following Resection of Chordomas and Chondrosarcomas of the Skull Base and Spine

Abstract

Until the last decade of the 20th century, craniocervical junction (CCJ) stabilization was achieved largely through simple instrumentation such as spinous process and lateral mass wiring. The introduction of rigid fixation anchors, including lateral mass and pedicle screws and improved occipital fixation, ushered in marked improvements for stabilization following resection of CCJ lesions, such as chordomas and chondrosarcomas. Tumor characteristics and the extent of resection impact CCJ stability and call for careful preoperative decision making. This chapter details our surgical technique for CCJ stabilization and summarizes the literature investigating this area.

Keywords

Atlantoaxial instability, CCJ fusion, Craniocervical junction, Halo fixation, Retractors, Spinous process

Outline
Introduction 271
Surgical Technique 272
- Preoperative Decision Making 272
- Anesthesia Concerns 272
- Patient Positioning 272
- Operative Approach 272
- Implant Options 274
- Complications 274
Important Considerations 274
- Extent of Fusion 274
- Condylar Resection 275
Representative Case 275
References 277

Introduction

Tumors involving the skull base demonstrate diverse histology and distinct clinical implications, including chordomas and chondrosarcomas. Advances in operative techniques enable resection of these tumors. The underlying pathology and tumor growth, as well as the extent of bony resection during surgery, may produce significant instability of the craniocervical junction (CCJ). The anatomic relationships that comprise the CCJ are unique. Bony resection may produce both acute and long-term implications for atlanto-occipital stability.

Biomechanical studies have demonstrated that the extent of condylar resection directly influences CCJ stability. The bony and ligamentous structures of the CCJ provide stability, and the articulations enable a broad range of motion, accounting for 25% of neck flexion–extension and 40 degrees of head rotation. This substantial amount of motion both explains the propensity of this spinal segment to develop pseudoarthrosis when a fusion attempt is made and illustrates the significant change in cervical range of motion produced in an occipitocervical fusion. Each of these factors should be weighed in operative decision making.

Instability of the CCJ can lead to marked pathological translation, longitudinal displacement, and/or basilar invagination. Occipitocervical stabilization may be required when pathological instability occurs secondary to tumor invasion or develops secondary to treatment. Clinical and cadaveric studies have established that more than 50% resection of the condyle promotes hypermobility at the occipitoatlantal junction.

The primary goals of stabilization include preservation of alignment, encouragement of arthrodesis, and an expedited return to daily activities with minimal postoperative immobilization. Instability of the CCJ is addressed primarily via posterior surgical approach and may include the placement of instrumentation, allograft/autograft, and other stabilization techniques.

Surgical Technique

Early approaches to posterior stabilization involved subperiosteal dissection, followed by the onlay of autograft to encourage fusion of the targeted spinal levels. Rates of pseudoarthrosis, especially at the CCJ, prompted improvement in operative techniques. A variety of techniques, such as the use of iliac crest struts, methylmethacrylate, or allograft rib, have been described. Instrumentation, such as plates, loops, and rods, may immediately stabilize the CCJ and may improve the rates of fusion at this highly mobile spinal segment.

Preoperative Decision Making

Stabilizing the CCJ is often accomplished through a posterior approach. Anterior approaches have been reported, but may pose higher complication rates. Next, we detail a posterior approach for stabilization of the CCJ.

The decision to perform CCJ fusion depends on the degree of junction stability, which is often determined by either the location and size of the surgical lesion or the extent of planned surgical resection. When the degree of bony destruction/resection of the occipital condyles or the degree of bony replacement at the skull base threatens the structural stability at the CCJ, operative stabilization may be necessary.

CCJ fusion may be performed prior, during, or after the surgical resection of a skull base tumor. If fusion is not performed prior to resection, patients are often placed in a halo for external immobilization, pending the stabilization stage of treatment.

Anesthesia Concerns

Patients with CCJ instability may require fiber-optic intubation. Similarly, previous operative approaches in the CCJ may complicate airway management. These concerns should be discussed with the anesthesia team. The operative technique will begin with the administration of a general anesthetic in the supine patient. We routinely use neuromonitoring in this patient population. From the time of anesthesia onward, somatosensory evoked potentials (SSEPs), brainstem auditory evoked potentials (BAEPs), and motor evoked potentials (MEPs) are collected. Preoperative baseline measurements of these parameters are used as a reference to compare the subsequent changes. Baseline neurophysiological recordings are obtained prior to patient positioning.

Patient Positioning

Patient positioning is a crucial step. A substantial portion of the patient’s axial rotation and cervical flexion and extension will be eliminated by the occipitocervical fusion procedure. Hence adequate time must be devoted to ensuring that the patient is positioned in a neutral, physiologic alignment. This should be confirmed by direct visualization and also by intraoperative imaging with fluoroscopy or X-ray visualization.

Preoperative placement of a halo brace may reduce the risk of exaggerating spinal malalignment. In patients in whom a halo brace is not used, a three-pin Mayfield head holder is secured to the cranium. The patient is placed in prone position, with a firm cervical collar, head/neck in neutral position, and upper limbs secured adjacent to the thorax to enable surgeons to stand on either side of the patient. A fluoroscopic C-arm is utilized to verify the alignment of the CCJ, specifically regarding the position of the occiput over the atlas. Alternately, lateral X-ray films may be procured for confirmation. Clear communication between team members as well as extreme caution must be employed to prevent adverse effects during turning, intubating, and transferring the patient.

Operative Approach

Once the SSEPs, BAEPs, and MEPs are evaluated in the context of baseline recordings, a radiograph of the lateral occipitocervical junction is visualized and the occipital/posterior cervical regions are shaved. Lateral radiographs ensure optimal reduction of subluxation. Further manipulation to ensure anatomic alignment may be completed. Exposure of the CCJ is then completed by planning an incision from the midcervical region to the inion. This region is infiltrated with 0.5% lidocaine with epinephrine (10–20 mL, 1:100,000) and sterilely prepped and draped. The posterior superior iliac crest is separately prepped to provide autograft bone for arthrodesis.

A vertically oriented skin incision is performed from the midline inion to the lower cervical spine. Retractors can be used to stabilize the wound opening, while monopolar electrocauterization deepens the wound until the surgeon reaches the supraspinous ligament. D’Errico cerebellar retractors enable dissection of the occiput and C1, C2, and C3 posterior processes.

The underlying soft tissue along the posterior arch of C1, and lamina of C2 and C3, are laterally displaced with unipolar electrocauterization and periosteal elevators. During this procedure, the vertebral artery and venous plexus located superolaterally to the C1 posterior arch must be deliberately exposed to avoid accidental destruction. Dissection along the lateral borders of C1 should proceed with blunt separation of tissue planes and then bipolar electrocautery and sharp dissection, to minimize the risk of injury to the vertebral artery as it passes along the lateral and superior border of C1.

Patients with a history of craniotomy require exquisite care in identifying any bony defect to prevent injury. Instrumentation choices may be limited due to previous bony resection of the occiput and should be selected accordingly.

At this point, attention is shifted to the occipitocervical region. C1 posterior elements are exposed with bipolar electrocautery and sharp dissection, similarly from midline to the lateral aspect of C1. After dissection, self-retraining retractors are secured and entry points for subaxial lateral mass screws are determined.

At this point, attention is shifted to the occipitocervical region. C1 posterior elements are exposed with bipolar electrocautery and sharp dissection, similarly from midline to the lateral aspect of C1. If C1 fixation is planned, the C1 lateral mass is visualized via inferior retraction upon the C2 nerve root.

Fixation in the lateral masses of the subaxial cervical spine is completed first. After dissection, self-retraining retractors are secured and standard entry points for subaxial lateral mass screws are determined. Entry points are selected around the midportion of the lateral mass, and a small burr scores the bone. After drilling and tapping, small amounts of bone wax at each screw site at and inferior to C3 may help maintain hemostasis. At the C2 level, the pars interarticularis is directly palpated through elevating the C2 nerve root superiorly. A small burr scores the bone over the C2 pars interarticularis. The bone is drilled, tapped, and applied with bone wax. We use a small bit on the Midas Rex drill to generate our screw entry sites, although an awl may be used as well.

At the time of C2 screw insertion, a ball-tipped pedicle probe is inserted to confirm the bony integrity of the screw tract. A C2 pars screw is inserted into the C2 lateral elements. Lateral mass screws are inserted at involved subaxial spinal levels; each screw is oriented 30 degrees lateral and 10–15 degrees superior at each site. Confirmation of screw insertion, including the point of insertion and direction, is done with lateral C-arm projection.

During the placement of screws, surgeons should be mindful of the possibility of thinned pedicle cortex near the vertebral artery. Tapping the pedicle is known to cause a shift parallel to the sagittal plane, thereby damaging the areas of thin cortex. To avoid this result, tap direction should be parallel to the insertion angle through application of force against the direction of force stemming from the paravertebral muscle.

Once the lateral mass screws are inserted, a burr is utilized to proceed with decortication of the posterior cortex and residual laminae. For reducing anterior translation of the atlas and spacing the bone grafting only between rod and lateral mass, 2.5- or 3.5-mm washers may be positioned on C2 pedicle screws.

We plan placement of the occipital plate based on patient anatomy and subaxial screw placement. Usually, rods are bent to bridge from the cervical screws to the base of the occiput. Based on where the rods lie in relation to the occiput and the possible options for plate placement on the occiput, we choose a spot for placement of the occipital plate.

There are a variety of occipital fixation devices available, most using bone screws to achieve fixation on the occiput ( Fig. 23.1A–C ). The surgeon should remember that the occiput has its area of maximal thickness just inferior to the inion, with thickness of the occipital bone decreasing radially from this point. Hence the strongest points of fixation will be the midline screws fixated under the occiput.

Care should be taken to achieve bicortical fixation at each screw site. Sequential drilling, with extension of drill depth accomplished with minor incremental change, allows for safe bicortical drilling of the bone. The screw holes are tapped, and final screws are positioned.

The occipitocervical rods are contoured at the plate–rod junction using a French bender or hand bending of the rod prior to cutting the rod to length. The junction of the cervical spine and the occiput/occipital plate usually requires a very acute bend of the rod, unless an occipital plating system with extended options for fixation to the subaxial screws are utilized. In systems in which an acute rod bend is necessary, using a rod with greater thickness at the bend site or using prebent rods may decrease the likelihood of construct failure. Cap nuts are used to attach the rods; the nuts should be tightened and torqued following manufacturer guidelines. Offset torqueing methods may be required for the occipital cap nuts.

The choice of whether or not to incorporate a laminectomy is based on patient pathology. Resection of posterior bony elements may decrease the available substrate for fusion. If necessary, laminectomies are subsequently performed. At the C2 level, troughs are drilled at the junction of the lamina and lateral mass. A curette helps gently lift away the C2 posterior elements. Subsequently C1 troughs are cut bilaterally to elevate the lamina specimen and morselize it for later use. If required, 2- and 3-mm Kerrison punches were then utilized to undercut the posterior elements of the foramen magnum to both decompress the occipitocervical junction and ensure the lack of fibrous bars or constricted elements posteriorly at the occiput of C1 junction. Decompression is confirmed by passing a nerve hook through the foramen magnum with relative ease.

We routinely use iliac crest autograft to improve the fusion rates in these patients. An incision is made in the skin superficial to the posterior superior iliac crest. Dissection proceeded down to the fascia, and the fascia is opened with the Bovie. Electrocautery is utilized to reflect paraspinal muscles off of the crest. The tricortical strips of iliac crest are extracted using the oscillating saw as well as straight and curved osteotomes. An adequate amount of cancellous autograft as well as bone marrow aspartate is collected. The wound is irrigated and closed in standard fashion.

Bone graft is packed in and around the screw and rod construct from the occiput downward. Morselized allograft helps augment the local bone graft and iliac crest autograft.

A drain is positioned if necessary. The patient is gently turned to the supine position off the operating room table into a hospital bed, with the neck immobilized using a hard collar. The patient’s head is freed from the Mayfield three-pin head holder.

Implant Options

Craniocervical stabilization can be accomplished with a variety of instrumentation techniques, which encompass implant or autograft fixation to the occiput and facets or cervical laminae.

Other implants include Wertheim Bohlman, modified Brattstrom, Hartshill rectangle, Ransford loop, and titanium frame implants.

The Wertheim Bohlman approach was conceived to replace high-failure subperiosteal dissection with onlay grafts. This approach involves fixing separate pieces of autograft bone to the occiput and posterior arches of C1 and C2 utilizing wire.

The modified Brattstrom technique was designed to improve on onlay techniques, but unfortunately it has also been associated with relatively high nonfusion rates. In this technique, a first piece of bone is positioned between the posterior arches of C1 and C2. Then, a second piece of bone is placed at the superior surface of C2 spinous process, extending to the occiput. Occipital screws secure the wire, which runs underneath the C2 spinous process in the epidural space, to hold the graft in place.

Complications

Adverse events chiefly result from three main types of events: perioperative, postoperative, and failed instrumentation ( Table 23.1 ). In a study by Winegar et al. comparing the different types of adverse events, surgical complications were found in 24%, postoperative complications in 16.4%, and instrumentation failure in 22.3% of all cases.

Table 23.1

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