Summary of Key Points
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The upper cervical spine presents many unique challenges to successful stabilization, including complex anatomy and limited access corridors.
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Because of the high degree of mobility at these levels, internal screw fixation is often required to achieve successful bony fusion and stability.
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Odontoid screw fixation is the preferred motion-preserving option for repair of odontoid fractures, although dorsal fixation techniques without fusion are being adopted.
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Dorsal C1-2 fixation through the transarticular approach provides immediate stabilization and is especially useful in patients with atlantoaxial instability, such as those suffering from rheumatoid arthritis.
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The Goel-Harms construct is an alternative to transarticular screws that offers similar levels of stability and may have a superior complication profile, especially in relation to vertebral artery injury.
Internal fixation is often used to provide immediate stabilization to protect the vital neural and vascular elements rendered vulnerable by instability produced by trauma, disease processes such as rheumatoid arthritis or neoplasms, and surgical procedures such as transoral odontoidectomy. Immediate stabilization is especially important in the highly mobile cervical spine. The occipitocervical junction and atlantoaxial complex comprise a transitional region connecting the rest of the spinal column to the cranium. The vertebrae and joints in this region are anatomically unique, differing from those in the subaxial spine because of their special adaptations to allow additional degrees of motion. Arguably, the most important of these are at the C1-2 complex, where the flat lateral articulations, absence of an intervertebral disc, and lax ligaments permit appreciable rotation (about 50% of total head rotation) ( Fig. 55-1 ). This motion is safely tolerated because the spinal canal is more capacious, the instantaneous axis of rotation is located close to the spinal cord (minimizing distortion of that structure), and the vertebral arteries loop laterally (allowing for at least one to remain patent, even at the extremes of rotation). Potentially catastrophic translational movements are prevented by the strong transverse component of the cruciate ligament (usually 8 to 10 mm in diameter in adults) that contains the odontoid process of the axis in the ventral compartment of the atlas. Disruption of this ligament, with or without bursting of the ring of C1 (Jefferson fracture), or disruption of the odontoid process results in gross instability. The remaining ligamentous structures, if intact, may provide some support, but they are too weak intrinsically to protect the spinal cord from even relatively minor trauma.
The restoration of structural integrity is critical. If the instability is caused by bone disruption, healing can occur with proper external immobilization. Instability caused by ligamentous disruption, however, requires surgery to achieve a bony fusion between previously hypermobile motion segments to protect the spinal cord. For bone healing or fusion to occur, two criteria must be met: (1) the bone graft (or bone fragments) must be touching or in proximity, and (2) motion must be eliminated or minimized.
Internal fixation can provide immediate stabilization to optimize bone graft and fragment healing. It accomplishes this more effectively than rigid external immobilization (such as a halo vest or Minerva jacket), while avoiding the cost, discomfort, and complications associated with these devices. To achieve the degree of stability necessary to protect the neural elements, screw fixation is often helpful because most wiring techniques alone do not constrain rotation and are therefore inadequate.
General Considerations
Protection of the neural elements when instability exists is paramount. Before surgery, the patient must be properly immobilized. Depending on the degree of instability, this may be achieved with a rigid cervical collar or it may require skeletal traction, a halo vest, or a Minerva jacket. Ongoing spinal canal compromise, if present, should be corrected before fusion is attempted by restoring alignment with cervical traction via cranial tongs or by surgical removal of intraspinal masses. Once the nature of the pathologic condition has been fully investigated and restoration of the spinal canal and spinal realignment have been planned, strategies for surgical stabilization can be considered. Because some techniques require anatomic integrity in specific regions, careful assessment during the planning stage is necessary. The patient’s general medical condition should be optimized and other associated injuries and traumatic situations evaluated and treated as appropriate.
Anesthetic Considerations
The degree of cervical spine instability and direction of movement that produce subluxation can impact the choice of anesthesia. For example, an odontoid process fracture is often unstable in both flexion and extension and requires an awake, fiber-optic intubation, whereas a transverse ligament rupture may be unstable only in flexion, in which case routine laryngoscopic techniques can be used. If C-arm fluoroscopy is planned for intraoperative guidance, it can be set up before anesthesia is induced to monitor spinal alignment during intubation and positioning.
Patients with spinal cord injury who have significantly reduced vasomotor tone may require substantial intravenous fluid volume replacement or vasopressors to maintain adequate circulatory volume and blood pressure.
Although it is uncommon, significant airway swelling after operative intervention that necessitates emergent reintubation can provide significant challenges to both the neurosurgeon and the anesthesiologist. Some authors have suggested delayed extubation for this reason, and the anesthesia team must be aware that a difficult airway should be expected in this population.
Ventral Approaches
Indications
Ventral techniques are primarily indicated for direct screw fixation of odontoid process fractures. C2-3 ventral fusion and plating is an option for treatment of a hangman’s fracture. It is no different from ventral cervical fusion and plating at lower levels other than the difficulty associated with the angle of approach to C2.
Odontoid process fractures, classified by Anderson and D’Alonzo as types I, II, and III, have been associated with distinctive treatment algorithms. Type I fractures, involving the apical part of the odontoid process, are usually believed to be stable and may be treated with external orthosis; however, one report suggests otherwise, and dynamic imaging may be used to assess stability. Type II fractures involve the neck of the odontoid process and are the most common. Type III fractures extend into the body of C2 and generally heal well with external immobilization. In a comprehensive review of fractures of the C2 vertebral body, Benzel and colleagues noted, however, that the type III fracture described by Anderson and D’Alonzo is not an odontoid fracture at all. They have proposed a classification of C2 body fractures that is more comprehensive and more meaningful in regard to mechanisms of injury.
Debate continues regarding the optimal treatment of type II fractures. Nonunion rates of 21% to 45% have been reported frequently, and there are many other reports of nonunion in the 50% to 63% range. A meta-analysis found that halo vest immobilization produced a fusion rate of 65%, which was only slightly better than traction alone at 57%. The variable success of immobilization led some authors to try to define parameters that would predict failure with external immobilization. Extent of dislocation (approximately 65% to 90% nonunion if dislocation is greater than 4 to 6 mm), patient age (higher failure rate in older patients), and direction of subluxation (higher failure rate with dorsal subluxation) have all been suggested as predictors of failure, as has a comminuted fragment of bone at the base of the odontoid process (type “IIA”). Among these, age appears to be the most valid indicator of the propensity for nonunion. In a randomized controlled prospective study, Lennarson and colleagues found the nonunion rate in patients treated with halo immobilization was 21 times greater in those over the age of 50 years than in younger patients. This study was a key factor leading to a recommendation for surgery in the guidelines for management of acute cervical spine and spinal cord injuries published by the Joint Section of Disorders of the Spine and Peripheral Nerves of the American Association of Neurological Surgeons and Congress of Neurological Surgeons.
Because nonoperative treatment of type II odontoid fractures clearly has a high nonunion rate, several methods of surgical fixation have been developed, each with distinctive advantages and pitfalls.
Contraindications
Absolute contraindications include comminuted fractures of the C2 body and transverse ligament disruption as defined by MRI or suggested by a C1 lateral mass fracture with extensive lateral displacement (greater than 7 mm total on anteroposterior radiographs) ( Fig. 55-3 ), pathologic fractures, and nonunions of more than 6 to 8 months duration that do not meet the aforementioned criteria. Severe osteoporosis is a relative contraindication. In addition, an oblique fracture of the odontoid process, angled caudally and ventrally so that it is parallel to the planned screw trajectory, may not be as suitable for ventral screw fixation because the odontoid process may slide down the fracture plane as the screw is tightened. Such anterior oblique fractures, while accounting for only 16% of the cases in one published series, had a significantly higher failure rate.
A barrel-shaped chest and short neck or an immobile or kyphotic spine because of cervical spondylosis can render the surgical approach more difficult, but these are relative contraindications that may be compensated for using specialized instrumentation. Finally, access to two quality C-arms during surgery is preferred, and the procedure should not be attempted without at least one.
Direct Odontoid Screw Fixation
Direct screw fixation of the odontoid process was first described in 1980 in the Japanese literature by Nakanishi, who began using this technique in 1978. This was followed in 1981 and in 1982 by publications from Böhler, who reported his experience dating back to 1968. Although others also described their experiences with various approaches to achieve direct odontoid screw fixation, the procedure was not widely accepted. With the development of specialized instrumentation, facilitating accurate screw placement with minimal trauma to the patient, the procedure has gained in popularity. The technique has the advantages of (1) decreased postoperative pain resulting from lack of extensive muscle dissection, (2) avoidance of bone graft harvest, and (3) maintenance of normal anatomy and rotation at the C1-2 joint. Furthermore, many patients require no postoperative immobilization.
Direct odontoid screw fixation ( ) can be used as the primary approach to treat acute type II fractures or it can be used after failure to heal with external immobilization. Patients with type II dens fractures with concomitant C1 ring fracture may also be candidates for odontoid screw fixation; however, assessment of transverse ligament integrity by magnetic resonance imaging (MRI) preoperatively ( Fig. 55-2 ) and by flexion fluoroscopy postoperatively is essential. If the latter demonstrates continued C1-2 instability, either a ventral or a dorsal C1-2 fusion is necessary. The direct screw fixation technique may also be used in some patients with chronic nonunion of type II odontoid fractures. Candidates should have a relatively small gap between the odontoid process and the C2 body and a reasonably sized odontoid fragment that has not autofused to C1 and does not have sclerosis of the surface opposing the body of C2. Chronic malunions that do not meet these criteria rarely fuse and will ultimately fracture the hardware and become unstable. The chance of successful bony union in one small series of such patients with fractures over 18 months of age was only 25%. This sharply contrasts with an 88% fusion rate for type II and high type III fractures that are less than 6 months of age. For this reason, we generally recommend posterior C1-2 fusion for chronically nonunited fractures. Unstable type III odontoid fractures that do not extend too far into the body of C2 are also candidates for direct screw fixation.
Patient Positioning
The patient is placed supine with the neck extended for proper screw trajectory. Padding is placed under the shoulders. If the neck cannot be initially extended, as judged by careful lateral fluoroscopic monitoring, the head is supported on folded towels in neutral neck alignment. Holter traction with a light weight (5 lbs.) hung over the Mayfield U-bar attachment to the operative table is useful for stabilizing the head.
For odontoid or ventral C1-2 screw placement, biplanar fluoroscopy is necessary. The anteroposterior view is obtained transorally. A wine bottle cork, notched for the teeth or gums, is an ideal radiolucent mouth prop. A single fluoroscope, swung back and forth frequently from the lateral to the anteroposterior position, can be used if necessary. It is much easier, however, to use a second C-arm fluoroscope if one is available. One C-arm unit is placed laterally with the arc horizontally or up to 45 degrees above the horizon. The other can be brought in at a 45-degree angle from the head of the table and positioned for the transoral view. Some adjustments may be needed to optimize the views, but once this is achieved, the remainder of the procedure is greatly facilitated. The C-arm should be positioned for optimal viewing by the surgeon, who stands on one side of the patient with the assistant on the opposite side. The anesthesiologist may remain at the head of the table. This provides optimal access to the patient’s head and airway. Alternatively, the patient can be positioned 180 degrees from the anesthesiologist to facilitate optimal positioning of the two C-arm fluoroscopes. In this scenario, either nasotracheal intubation is planned or the endotracheal tube is secured to the left side of the patient’s mouth and run along the left side of the body to the anesthesia.
Operative Technique
All ventral odontoid fixations begin with the same exposure. The initial approach to the spine is the same as for an anterior cervical discectomy. The spine is approached at about the C5 level through a unilateral natural skin crease incision ( Fig. 55-4 ). We use a local injection with epinephrine (1 : 200,000) to minimize skin bleeding and complete hemostasis with bipolar cautery. The platysma muscle is elevated and divided with monopolar cautery. The sternocleidomastoid muscle fascia is opened along the medial side of the muscle with sharp dissection. Blunt dissection then opens the deeper tissue planes medial to the carotid sheath and lateral to the trachea and esophagus to expose the prevertebral space. Dividing the longus colli fascia and the anterior longitudinal ligament in the midline with electrocautery allows the bellies of the longus colli muscle to be elevated bilaterally over approximately 1.5 vertebral segments. Sharp-bladed Caspar retractor blades are set in place below the muscle and attached to the Caspar retractor.
The loose areolar tissue in the prevertebral space ventral to the longus colli muscles is easily opened with a Kittner or “peanut” dissector held in a curved tonsil clamp. It is swept from side to side while advancing up to the C1-2 level (monitored with lateral fluoroscopy). Several screw systems are available. The Apfelbaum system (Aesculap Instrument Corporation, Center Valley, PA) has an angled retractor blade that reaches into this space under the mandible and holds open the working tunnel. It attaches to one side of the previously placed modified Caspar retractors (see Fig. 55-4 ). Other systems use different retractors, such as a curved handheld retractor (Synthes) or small metal hook-shaped handheld Hohmann retractors that lock over the shoulders of C2 bilaterally alongside the dens, as initially described by Böhler. The key to the retraction is to create a working tunnel up to the caudal edge of C2, without having any device caudally in the wound that restricts the low trajectory needed for proper screw placement.
At this point, the various instrument systems use somewhat different approaches for placing the screws. The Apfelbaum system consists of an outer guide tube with spikes that anchor it to C3 and that can be used to optimize spinal alignment. An inner guide tube, within the outer tube, guides the drilling. Once the pilot hole is drilled, the inner guide is removed, the hole is tapped, and the screws are placed through the outer guide tube. First, under biplanar fluoroscopic control, an entry site on the ventral caudal edge of C2 is selected and a K-wire is impacted into C2 ( Fig. 55-5A ). If one screw is to be placed, a midline location is chosen. If two are to be placed, a paramedian location is selected 2 to 3 mm from the midline. Care and patience in selecting the entry site and setting the K-wire will be rewarded by the remainder of the procedure being expedited. Once the K-wire is set, a 7-mm hollow drill is placed over the K-wire and is rotated by hand to create a shallow trough in the face of C3 and in the C2-3 annulus ( Figs. 55-5B–D ). No bone is removed from C2. The two guide tubes are then mated together, passed over the K-wire, and walked up the ventral face of the spinal column until the spikes on the outer tube are over the body of C3. The inner guide tube is then advanced in the trough to the ventral caudal edge of C2 ( Fig. 55-6 ), and the K-wire is removed. Having the guide tube at the entry site prevents the drill from skipping off the edge of the bone and walking up the ventral face of C2. With the guide tube system firmly engaged in C3, the surgeon can then optimize the C2 alignment on the fluoroscopic images by either pushing C2 and C3 dorsally relative to the odontoid-C1 complex or pulling C2 and C3 ventrally. In the case of a retrolisthesed odontoid process, this realignment can be performed while gradually extending the patient’s head and removing the supporting towels beneath it to obtain an ideal working trajectory.
A pilot hole is then drilled from the ventral caudal edge of C2 to the apex of the odontoid process, advancing the drill slowly under biplanar fluoroscopic control ( Fig. 55-7 ). The dense cortical shell of the odontoid must be pierced to engage the screw properly and avoid splitting. Because the odontoid process is firmly held in position by its periosteum and attached supporting ligaments, it is not displaced as the drill enters from the soft cancellous fracture site. The angle of drilling is such that the drill can penetrate a substantial distance beyond the apex of the odontoid process into the apical ligaments without threatening the dural or neural structures. If, however, a more dorsal trajectory is needed, greater care must be taken not to penetrate too far into the spinal canal. This is controlled by visualizing the drill’s progress on the fluoroscope.
Once the drill is into the distal odontoid cortex, its depth of penetration is read on the calibrated shaft, and the anteroposterior and lateral fluoroscopic images are saved on the monitor screens. Comparison of future live images with these saved images allows reestablishment of the identical alignment in successive steps.
The drill is then withdrawn, and the inner guide tube is removed. A tap is placed through the outer guide tube, and the pilot hole is tapped. This cuts threads in the bone, allowing a more precise bone–screw junction that may reduce bone absorption around the screw caused by pressure necrosis if a self-tapping screw is used. The tap is then removed, and a screw is placed through the guide tube ( Fig. 55-8 ). A screw that is a few millimeters shorter than the measured drill depth may be chosen to allow for reduction at the fracture site, but it is important that the screw fully engages the apical cortex. Extending the screw a few millimeters beyond the cortex into the apical ligaments is safe and preferable to having one too short, as the latter may back out. To achieve some fracture reduction, a partially threaded screw (lag screw) is used to pull the odontoid back toward the body of C2.
If a second screw is to be placed, the identical series of steps is followed on the contralateral paramedian site, except that either a partially threaded lag screw or a fully threaded screw can be used, because no further lagging action would be expected to occur.
After removal of the guide tube, bleeding from C3 can be controlled with bone wax or a slurry of Gelfoam powder and thrombin. Lateral fluoroscopy in flexion and extension confirms stability. Closure is routine and is performed in layers, closing the platysma muscle and subcutaneous tissue with absorbable sutures and the skin with sterile tape strips or Dermabond. No drains are placed. External collars are not usually recommended unless there is concern about the patient’s bone quality, and patients are allowed to return to work and resume nontraumatic activities promptly.
Several alternative systems have been proposed that are based on existing long-bone screw fixation techniques. These use a K-wire to drill the pilot hole and then pass a hollow overdrill over this, followed by a cannulated screw. Theoretically, once the K-wire is placed, it does not have to be removed so that precise reentry into the same trajectory is assured; however, a drawback of these systems is that they do not appear to have any provision for optimizing alignment with the drill guide. Furthermore, K-wires are suboptimal drills because they lack the torsional rigidity of drill bits and can be deflected by irregular densities within the bone. To redirect them, one must remove the K-wire and select a new starting point. In addition, great care must be taken when drilling over the K-wire because the drill can bind to the K-wire and advance it into the spinal canal or cut the K-wire.
Controversies
One-Screw versus Two-Screw Constructs.
Theoretically, with one screw, the odontoid process could rotate on C2, although the interdigitation of the irregular fracture surfaces may prevent this with fresh fractures. This may explain why constructs with either one or two screws have been reported to have similar clinical success. Laboratory studies also show no greater resistance to screw fracture from bending with one- or two-screw constructs. A more recent clinical study by Dailey and coworkers, however, found a significant difference in fusion rates among 57 patients over the age of 70 with odontoid fractures: a 56% success rate was seen if one screw was placed, whereas a 95% fusion rate was achieved when two screws were placed.
The demographics of patients with odontoid fractures are changing. Elderly patients who sustain odontoid fractures after relatively minor trauma, often a ground-level fall, now constitute the majority of patients with odontoid fractures, highlighting the importance of considering placement of two screws in older and possibly middle-aged patients.
When two screws are placed, the entry site for each is located paramedially a few millimeters off the midline and the screws are angled toward each other at the odontoid apex. The diameter of the odontoid process should be assessed on the preoperative computed tomography scan to ensure adequate bone volume for a second screw. Some patients may not have a sufficiently wide odontoid process to accommodate two screws side by side, but the odontoid process may be deep enough that the screws can be placed is such a way that they end up in front of and behind each other to achieve the same fixation.
Screw Size and Type.
Biomechanical data suggest that cannulated screws are only about 5% to 10% weaker than solid screws. Screw diameters usually range from 3.5 to 4 mm. The initial experience was with cancellous threaded screws, which have a deeper thread (smaller minor diameter or core) and are better at resisting pullout. However, pullout forces on odontoid screws are minimal. An odontoid screw has to primarily resist bending and translational forces. Screw failure, if it occurs, is almost always due to fracture at the level of the bone fracture. Cortically threaded screws (4-mm outer diameter) with a larger minor diameter (2.9 mm versus the previously used 2 mm) would therefore seem optimal. An additional benefit is that the pilot hole is drilled larger (3 mm versus 2 mm). This makes the drill much more directionally stable, allowing precise correction of pilot hole trajectory to optimize screw placement.
Results
Type II odontoid fractures less than 6 months old treated with this technique have a high rate of fusion. The combined published series of Veres in Hungary and Apfelbaum and colleagues in Salt Lake City, Utah, encompassed 147 patients whose ages ranged from 15 to 92 years. Successful bony union was achieved in 88% of patients, with an additional 3% achieving stability via fibrous union. These results agree with those of other published series with fewer patients. Failures usually occur in elderly patients with poor quality bone. If this complication is recognized early, manipulative realignment and external immobilization has been successful. If not, additional surgery is required.
Other series have shown similar low complication rates; however, there has been a high incidence of dysphagia from the retropharyngeal approach in elderly patients, and some of these patients required temporary feeding tubes.
Although they are rare, serious and even fatal complications can result from the K-wire being driven beyond the odontoid tip as mentioned in the alternate technique. One published report also describes K-wire bending and eventually breaking within a cannulated system, necessitating abandonment of the fractured tip within the patient.
The high degree of success achieved by using this straightforward, easily mastered technique, usually with minimal complications, merits its consideration as the primary treatment for many type II odontoid fractures. By allowing the patient to quickly resume normal activity while avoiding the cost and medical and social morbidity of a halo vest, screw fixation appears to be a cost-effective treatment for this problem.
Ventral C1-2 Transarticular Screw Fixation
Ventral C1-2 transarticular screw fixation may provide an alternative if odontoid screw fixation is not possible or if successful odontoid screw fixation fails to stabilize C2 because of unrecognized concomitant transverse ligament incompetence. This approach also allows for resection of odontoid lesions. Stabilization is accomplished by inserting two screws through the lateral masses of C2 into the lateral mass of C1. The entry site is just medial to the vertebral artery, which is placed at risk by this approach. The screws angle laterally approximately 20 degrees and dorsally at a similar angle ( Fig. 55-9 ). The entry site is selected by following the lateral edge of the vertebral body rostrally from the C2-3 interspace to its junction with the lateral mass, staying as medial as possible in that structure. The drill guide system used for odontoid screw fixation can be used for this as well, although the screw length is considerably shorter.
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